EASA Module4

Student Resource
Subject B2-4b
Integrated Circuits
Copyright © 2008 Aviation Australia

All rights reserved. No part of this document may be reproduced, transferred, sold, or
otherwise disposed of, without the written permission of Aviation Australia.
This page intentionally left blank
AA Form TO-19
Part 66 Subject B2-4b Electronic Fundamentals II
CONTENTS
TOPIC
Definitions ii
Study Resources iii
Introduction v
Semiconductors – Integrated Circuits 4.1.3.2.1
Printed Circuit Boards 4.2
Servomechanisms 4.3.2
Issue B – January 2008 Revision 1 B2-4B - i

AA Form TO-19
DEFINITIONS
Define
To describe the nature or basic qualities of.
To state the precise meaning of (a word or sense of a word).
State
Specify in words or writing.
To set forth in words; declare.
Identify
To establish the identity of.
List
Itemise.
Describe
Represent in words enabling hearer or reader to form an idea of an object or process.
To tell the facts, details, or particulars of something verbally or in writing.
Explain
Make known in detail.
Offer reason for cause and effect.
Issue B – January 2008 Revision 1 B2-4B - ii

AA Form TO-19
STUDY RESOURCES
JEPPESEN Sanderson Training Products:

AandP Technical General Textbook - 2000
Aircraft Maintenance Text 4 – Basic functional Devices and Systems – 1989
Electronic Devices – Thomas L. Floyd 4th edition (Electron Flow Version)– 2002
US Navy Electricity and Electronics Training Series – September 1998
Issue B – January 2008 Revision 1 B2-4B - iii

AA Form TO-19
Issue B – January 2008 Revision 1 B2-4B - iv

AA Form TO-19
INTRODUCTION
The purpose of this subject is to allow you to gain knowledge of aircraft systems utilising
digital and electronic applications.
On completion of the following topics you will be able to:
Topic 4.1.3.2 Semiconductors – Integrated Circuits
Describe the operation of logic circuits and linear circuits
Describe the operation and function of an operational amplifier used as the following:
 Integrator;
 Differentiator;
 Voltage Follower and
 Comparator.
Topic 4.2 Printed Circuit Boards

Identify printed circuit boards and describe their construction and applications.
Topic 4.3 Servomechanisms

Describe the following terms:
 Open and closed loop,
 Follow up,
 Servomechanism,
 Analogue transducer,
 Null,
 Damping,
 Feedback, and
 Deadband;
Describe the construction, operation and use of the following synchro system components:
 Resolvers,
 Differential,
 Control and torque,
 E and I transformers,
 Inductance transmitters,
 capacitance transmitters,
 synchronous transmitters;
Describe typical servomechanism defects including hunting and describe the effect of
reversal of synchro leads.
Issue B – January 2008 Revision 1 B2-4B - v

AA Form TO-19
Issue B – January 2008 Revision 1 B2-4B - vi

AA Form TO-19
TOPIC 4.1.3.2 – SEMICONDUCTORS – INTEGRATED CIRCUITS

DIGITAL ANALOGUE SYSTEMS
A digital system is a combination of devices designed to manipulate logical information or
physical quantities that are represented in digital form; that is, the quantities can take on only
discrete values. These devices are most often electronic, but they can also be mechanical,
magnetic, or pneumatic, Some of the more familiar digital systems include digital computers
and calculators, digital audio and video equipment, and the telephone system—the world’s
largest digital system.
An analog system contains devices that manipulate physical quantities that are represented
in analog form. In an analog system, the quantities can vary over a continuous range of
values. For example, the amplitude of the output signal to the speaker in a radio receiver can
have any value between zero and its maximum limit. Other common analog systems are
audio amplifiers, magnetic tape recording and playback equipment, and a simple light
dimmer switch.
ADVANTAGES OF DIGITAL TECHNIQUES
An increasing majority of applications in electronics, as well as in most other technologies,
use digital techniques to perform operations that were once performed using analog
methods. The chief reasons for the shift to digital technology are:
Digital systems are generally easier to design.
This is because the circuits that are used are switching circuits, where exact values of voltage
or current are not important, only the range (HIGH or LOW) in which they fall.
Information storage is easy
This is accomplished by special devices and circuits that can latch onto digital information
and hold it for as long as necessary, and mass storage techniques that can store billions of
bits of information in a relatively small physical space. Analog storage capabilities are, by
contrast, extremely limited.
Accuracy and precision are greater
Digital systems can handle as many digits of precision as you need simply 1w adding more
switching circuits. In analog systems, precision is usually limited to three or four digits
because the values of voltage and current are directly dependent on the circuit component
values and are affected by random voltage fluctuations (noise).
Operation can be programmed.
It is fairly easy to design digital systems whose operation is controlled by a set of stored
instructions called a program. Analog systems can also be programmed, but the variety and
the complexity of the available operations are severely limited.
Digital circuits are less affected by noise
Spurious fluctuations in voltage (noise) are not as critical in digital systems because the exact
value of a voltage is not important, as long as the noise is not large enough to prevent us
from distinguishing a HIGH from a LOW.
More digital circuitry can be fabricated on IC chips.
It is true that analog circuitry has also benefited from the tremendous development of IC
technology, but its relative complexity and its use of devices that cannot he economically
integrated (high-value capacitors. precision resistors, inductors, transformers) have
prevented analog systems from achieving the same high degree of integration.
B2-4.1.3.2 Integrated Circuits

Issue B: January 2008 Revision 1 Page 1 of 24
AA Form TO-19
LIMITATIONS OF DIGITAL TECHNIQUES
There is really only one major drawback when using digital techniques:
The real world is mainly analog.
Most physical quantities are analog in nature, and it is these quantities that are often the
inputs and outputs that are being monitored, operated on, and controlled by a system. Some
examples are temperature, pressure, position, velocity, liquid level, flow rate, and so on. We
are in the habit of expressing these quantities digitally, such as when we say that the
temperature is 64° (63.8° when we want to be more precise); but we are really making a
digital approximation to an inherently analog quantity.
To take advantage of digital techniques when dealing with analog inputs and outputs, three
steps must be followed:
• Convert the real-world analog inputs to digital form.
• Process (operate o) the digital information
• Convert the digital outputs back to real-world analog form.
The figure shows a block diagram of this for a typical temperature control system. As the
diagram shows, the analog temperature is measured and the measured value is then
converted to a digital quantity by an analog-to-digital converter (ADC). The digital quantity is
then processed by the digital circuitry, which may or may not include a digital computer. Its
digital output is converted back to an analog quantity by a digital-to-analog converter (DAC).
This analog output is fed to a controller which takes some kind of action to adjust the
temperature.
Another good example where conversion between analog and digital takes place is in the
recording of audio. Compact disks (CDs) have taken the recording industry by storm because
they provide a much better means for recording and playing back music. The process works
something like this:
 sounds from instruments and human voices produce an analog voltage signal in a
microphone;
 this analog signal is converted to a digital format using an analog-to-digital conversion
process;
 the digital information is stored on the CD’s surface;
 during playback, the CD player takes the digital information from the CD surface and
converts it into an analog signal which is then amplified and fed to a speaker where it
can he picked up by the human ear

AA Form TO-19
The need for conversion between analog and digital forms of information can be considered
a drawback because of the added complexity and expense. Another factor that is often
important is the extra time required to perform these conversions. In many applications, these
factors are outweighed by the numerous advantages of using digital techniques, and so the
conversion between analog and digital quantities has become quite commonplace in the
current technology
There are situations, however, where use of analog techniques is simpler or more
economical. For example, the process of generating and distributing electrical power to
homes and businesses is primarily done using analog circuitry.
It is common to see both digital and analog techniques employed within the same system in
order to profit from the advantages of each. In these hybrid systems, one of the most
important parts of the design phase involves determining what parts of the system are to be
analog and what parts are to be digital.
INTEGRATED CIRCUITS – CONSTRUCTION
Up to now the various semiconductors, resistors, capacitors, etc., in our discussions have
been considered as separately packaged components, called DISCRETE COMPONENTS. In
this section we will introduce some of the more complex devices that contain complete
circuits packaged as a single component. These devices are referred to as INTEGRATED
CIRCUITS and the broad term used to describe the use of these devices to miniaturize
electronic equipment is called MICROELECTRONICS.
With the advent of the transistor and the demand by the military for smaller equipment,
design engineers set out to miniaturize electronic equipment. In the beginning, their efforts
were frustrated because most of the other components in a circuit such as resistors,
capacitors, and coils were larger than the transistor. Soon these other circuit components
were miniaturized, thereby pushing ahead the development of smaller electronic equipment.
Along with miniature resistors, capacitors, and other circuit elements, the production of
components that were actually smaller than the space required for the interconnecting wiring
and cabling became possible. The next step in the research process was to eliminate these
bulky wiring components. This was accomplished with the PRINTED CIRCUIT BOARD
(PCB).

AA Form TO-19
After the printed circuit boards were perfected, efforts to miniaturize electronic equipment
were then shifted to assembly techniques, which led to MODULAR CIRCUITRY. In this
technique, printed circuit boards are stacked and connected together to form a module. This
increases the packaging density of circuit components and results in a considerable
reduction in the size of electronic equipment. Since the module can be designed to perform
any electronic function, it is also a very versatile unit. However, the drawback to this
approach was that the modules required a considerable number of connections that took up
too much space and increased costs. In addition, tests showed the reliability was adversely
affected by the increase in the number of connections. A new technique was required to
improve reliability and further increase packaging density. The solution was INTEGRATED
CIRCUITS.
An integrated circuit is a device that integrates (combines) both active components
(transistors, diodes, etc.) and passive components (resistors, capacitors, etc.) of a complete
electronic circuit in a single chip (a tiny slice or wafer of semiconductor crystal or insulator).
Integrated circuits (ICs) have almost eliminated the use of individual electronic components
(resistors, capacitors, transistors, etc.) as the building blocks of electronic circuits. Instead,
tiny CHIPS have been developed whose functions are not that of a single part, but of dozens
of transistors, resistors, capacitors, and other electronic elements, all interconnected to
perform the task of a complex circuit. Often these comprise a number of complete
conventional circuit stages, such as a multistage amplifier, logic circuits linear circuits and
operational amplifiers (in one extremely small component). These chips are frequently
mounted on a printed circuit board which plugs into an electronic unit.
Integrated circuits have several advantages over conventional wired circuits of discrete
components. These advantages include
 a drastic reduction in size and weight,
 a large increase in reliability,
 lower cost, and
 possible improvement in circuit performance.
However, integrated circuits are composed of parts so closely associated with one another
that repair becomes almost impossible. In case of trouble, the entire circuit is replaced as a
single component.

AA Form TO-19
Integrated circuits are being used in an ever increasing variety of applications. Small size and
weight and high reliability make them ideally suited for use in airborne equipment, missile
systems, computers, spacecraft, and portable equipment. They are often easily recognized
because of the unusual packages that contain the integrated circuit. These tiny packages
protect and help dissipate heat generated in the device. One of these packages may contain
one or several stages, often having several hundred components.
This integrated circuit was produced in about 1960 by Fairchild Semiconductor. It is a

bistable RS (Reset/Set) Flip-Flop constructed using four NPN bipolar transistors and two
resistors diffused into a single monolithic chip of silicon. The maximum operating clock speed
is 1 megahertz and the delay is 50 nanoseconds
LINEAR INTEGRATED CIRCUITS
The linear IC is an analogue type of circuit, as opposed to the digital type. An analogue
function has continuous values within a specified range, whereas a digital function has
discrete values or steps. Linear circuits can be broken down into several main categories:
 Operational Amplifiers
 Voltage regulators
 Communication circuits
 Interface circuits
o Comparators
o Sense Amplifiers
o Line drivers and receivers
o Analogue to Digital (A to D) and Digital to Analogue (D to A) converters
There is a multitude of types of linear integrated circuits and these will not be covered at this
stage. An example of such is a basic IC linear voltage regulator, as following.

AA Form TO-19
Voltage Regulators
The purpose of a voltage regulator is to provide a constant output voltage independent of
input supply voltage, output load current, and temperature. One basic type of linear
integrated circuit regulator is known as the three-terminal regulator. It has an input, an output,
and a ground connection.
The L7800 series of three-terminal positive regulators is available in TO-220 ISOWATT220

TO-3 and D2PAK packages and several fixed output voltages, making it useful in a wide
range of applications. These regulators can provide local on-card regulation, eliminating the
distribution problems associated with single point regulation. Each type employs internal
current limiting, thermal shut-down and safe area protection, making it essentially
indestructible. If adequate heat sinking is provided, they can deliver over 1A output current.
Although designed primarily as fixed voltage regulators, these devices can be used with
external components to obtain adjustable voltages and currents.

AA Form TO-19
OPERATIONAL AMPLIFIERS (OPAMPS)

The standard operational amplifier (op-amp) symbol is shown in the figure. It has two input
terminals, the inverting (-) input and the noninverting (+) input, and one output terminal.
The typical op-amp operates with two dc supply voltages, one positive and the other
negative, as shown in the left figure below. Usually these dc voltage terminals are left off the
schematic symbol for simplicity but are understood to be there. Some typical opamp IC
packages are shown in the right figure.

AA Form TO-19
The Ideal Op-Amp

To illustrate what an op-amp is, let’s consider its ideal characteristics. A practical opamp, of
course, falls short of these ideal standards, but it is much easier to understand and analyze
the device from an ideal point of view.
First, the ideal op-amp has

 infinite voltage gain and infinite bandwidth.
 Also, it has an infinite input impedance (open) so that it does not load the driving
source.
 Finally, it has a zero output impedance.
These characteristics are illustrated in the figure. The input voltage, Vin, appears between
the two input terminals, and the output voltage is as indicated by the internal voltage source
symbol.
The Practical Op-Amp
Although modern integrated circuit (IC) op-amps approach parameter values that can be
treated as ideal in many cases, the ideal device can never be made.
Any device has limitations, and the IC op-amp is no exception. Op-amps have both voltage
and current limitations. Peak-to-peak output voltage, for example, is usually limited to slightly
less than the two supply voltages. Output current is also limited by internal restrictions such
as power dissipation and component ratings.
Characteristics of a practical op-amp are

 very high voltage gain,
 very high input impedance,
 very low output impedance, and
 wide bandwidth.

AA Form TO-19
Internal Block Diagram of an Op-Amp
A typical op-amp is made up of three types of amplifier circuit: a differential amplifier, a
voltage amplifier, and a push-pull amplifier, as shown in the figure.
A differential amplifier is the input stage for the op-amp. It has two inputs and provides
amplification of the difference voltage between the two inputs. The voltage amplifier is usually
a class A amplifier that provides additional op-amp gain. Some op-amps may have more than
one voltage amplifier stage. A push-pull class B amplifier is used for the output stage.
INPUT SIGNAL MODES

Single-Ended Input
When an op-amp is operated in the single-ended mode, one input is grounded and the signal
voltage is applied only to the other input, as shown in the figure. In the case where the signal
voltage is applied to the inverting input as in part (a), an inverted, amplified signal voltage
appears at the output. In the case where the signal is applied to the noninverting input with
the inverting input grounded, as in the right hand figure, a noninverted, amplified signal
voltage appears at the output.

AA Form TO-19
Differential Input
In the differential mode, two opposite-polarity (out-of-phase) signals are applied to the inputs,
as shown in the figure. This type of operation is also referred to as double-ended. The
amplified difference between the two inputs appears on the output.
Common-Mode Input
In the common mode, two signal voltages of the same phase, frequency, and amplitude are
applied to the two inputs, as shown in the figure. When equal input signals are applied to both
inputs, they cancel, resulting in a zero output voltage.
This action is called common-mode rejection. Its importance lies in the situation where an
unwanted signal appears commonly on both op-amp inputs. Common-mode rejection means
that this unwanted signal will not appear on the output and distort the desired signal.
Common-mode signals (noise) generally are the result of the pick-up of radiated energy on
the input lines, from adjacent lines, the 60 Hz power line, or other sources.

AA Form TO-19
FEEDBACK
Negative feedback
Negative feedback is one of the most useful concepts in electronics, particularly in opamp
applications. Negative feedback is the process whereby a portion of the output voltage of an
amplifier is returned to the input with a phase angle that opposes (or subtracts from) the input
signal.
Negative feedback is illustrated in the figure. The inverting (-) input effectively makes the
feedback signal 180o out of phase with the input signal.
Why Use Negative Feedback?

As you have seen, the inherent voltage gain of a typical op-amp is very high (usually greater
than 100,000), Without negative feedback, a small input voltage drives the op-amp to its
output limits and it becomes nonlinear.
The usefulness of an op-amp operated without negative feedback is severely restricted and
is generally limited to comparator and other applications to be covered later. With negative
feedback, the closed-loop voltage gain (Acl) can be reduced and controlled so that the op-
amp can function as a linear amplifier. In addition to providing a controlled, stable voltage
gain, negative feedback also provides for control of the input and output impedances and
amplifier bandwidth.

AA Form TO-19
Positive Feedback
Positive Feedback is characterised by the condition wherein an in-phase portion of the output
voltage of an amplifier is fed back to the input with no net phase shift, resulting in a
reinforcement of the output signal. This basic idea is illustrated in the figure. As you can see
the feed back voltage, Vf, is amplified to produce the output voltage, which in turn produces
the feedback voltage. That is, a loop is created in which the signal sustains itself and a
continuous sinusoidal output is produced. This phenomenon is called oscillation.
OPAMPS With Negative Feedback

In this section, you will learn several basic ways in which an op-amp can be connected using
negative feedback to stabilize the gain and increase frequency response. The extremely high
open-loop gain of an op-amp creates an unstable situation because a small noise voltage on
the input can be amplified to a point where the amplifier is driven out of its linear region. Also,
unwanted oscillations can occur. In addition, the open-loop gain parameter of an op-amp can
vary greatly from one device to the next. Negative feedback takes a portion of the output and
applies it back out of phase with the input, creating an effective reduction in gain. This
closed-loop gain is usually much less than the open-loop gain and independent of it.
Closed-Loop Voltage Gain, Acl

The closed-loop voltage gain is the voltage gain of an op-amp with external feedback. The
amplifier configuration consists of the op-amp and an external negative feedback circuit that
connects the output to the inverting input. The closed-loop voltage gain is determined by the
external component values and can’be precisely controlled by them.

AA Form TO-19
Non-inverting Amplifier
An op-amp connected in a closed-loop configuration as a non-inverting amplifier with a
controlled amount of voltage gain is shown in the figure. The input signal is applied to the
non-inverting (+) input. The output is applied back to the inverting (-) input through the
feedback circuit (closed loop) formed by the input resistor Ri, and the feedback resistor Rf.
This creates negative feedback as follows. Resistors Ri and Rf form a voltage-divider circuit,
which reduces Vout, and connects the reduced voltage Vf to the inverting input.
The closed loop gain of a non-inverting amplifier is determined by the formula
Rf
A cl  1 
Ri
Voltage-Follower
The voltage-follower configuration is a special case of the non-inverting amplifier where all of
the output voltage is fed back to the inverting (-) input by a straight connection, as shown in
the figure. As you can see, the straight feedback connection has a voltage gain of 1 (which
means there is no gain). The closed-loop voltage gain of the voltage-follower is Acl = 1
The most important features of the voltage-follower configuration are its very high input
impedance and its very low output impedance. These features make it a nearly ideal buffer
amplifier for interfacing high-impedance sources and low-impedance loads.

AA Form TO-19
Inverting Amplifier
An op-amp connected as an inverting amplifier with a controlled amount of voltage gain is
shown in the figure. The input signal is applied through a series input resistor Ri to the
inverting (-) input. Also, the output is fed back through Rf to the same input. The non-inverting
(+) input is grounded.
The closed-loop voltage gain of the inverting amplifier (Acl(I)) is the ratio of the feedback
resistance (Rf) to the input resistance (Ri). The closed- loop gain is independent of the op-
amp’s internal open-loop gain. Thus, the negative feedback stabilizes the voltage gain. The
negative sign indicates inversion.
The formula for determining gain is
Rf
A cl(I)  
Ri

AA Form TO-19
Summing op-amps
Summing op-amps sum two or more input signals that feed the common junction through
separate input resistors. If the 3 input resistors and the feedback resistor are equal value,
amplifier voltage output will equal the sum of the separate voltages applied to the input
resistance. If the resistors are each 1000Ω
1 volt applied to Ri1 causes 1 mA
These three currents sum at the common junction to 6mA, and then flow through Rf. The
volts drop (IR) across Rf will be
0.006 × 1000 = 6V
and this 6V, which is the sum of the input voltages, is the output of the amplifier.

AA Form TO-19
Differentiating op-amps
Differentiating op-amps, like the basic RC ‘differentiator’, produce an output voltage that is
proportional to the rate of change of the input voltage. In figure shown, instead of an input
resistor there is an input capacitor Ci. With a steady voltage at the input there is no current
flow to or from Ci or the summing junction, and no current through Rf. With no current
through and therefore no voltage drop across Rf, there is zero output from the amplifier. With
a varying-voltage input the current to or from Ci, and therefore through Rf, depends on the
rate at which the input voltage changes. Amplifier output voltage is set by the volts drop
across Rf, which varies with Ci current, which varies with rate of change of input voltage.
Amplifier voltage output is therefore proportional to the rate of change of input voltage. The
graphs show the differentiated output relative to the input. Note from the graph that:
A positive or negative input that is going more positive at a steady rate causes a constant
value negative output A positive or negative input that is going more negative at a steady rate
causes a constant value positive output The amplitude of the output is proportional to the
slope (rate of change) of the input
And
A constant value causes a zero output. It follows from the above that a sawtooth input will
give a square-wave output

AA Form TO-19
Integrating op-amps
Integrating op-amps like any other electrical ‘integrator’, produce an output voltage that
changes at a rate proportional to the value of the input voltage. Instead of a feedback resistor
there is a feedback capacitor Cf. Amplifier output and the voltage across Cf always agree.
With a constant value voltage applied to the input, Cf charges at a constant rate. This raises
at a constant rate the voltage across Cf and at the amplifier output. With zero input voltage
there is no current flow, so the voltage across Cf and the amplifier output remain steady. If a
constant-value reverse-polarity voltage is applied to the input, Cf discharges at a constant
rate. This lowers at a constant rate the voltage across Cf and at amplifier output. The graphs
show the integrated output relative to the input. Note from the graph that
A constant value negative input causes an output that goes more positive at a constant rate
A constant value positive input causes an output that goes more negative at a constant rate
And
A zero input maintains output at a steady value.
It follows from the above that a square wave input causes a sawtooth output.
Integrator operation is explained with reference to the part the capacitor plays in the feedback
loop. Rather than explaining capacitor charge building up opposing inputs signals, or current
flow through op-amps, use the analogy that the capacitor appears as a short to initial voltage
change, but increases in resistance to appear as an open circuit when fully charged.

AA Form TO-19
An integrator is designed so the capacitor should never be fully charged. Designers need
simply design the circuit so the capacitor can never be charged, eg if an accelerometer
outputs 5 volts when an aircraft is accelerating at 20 meters/sec2, and that aircraft is only
capable of accelerating to that speed for a maximum of 10 seconds (then the engines are
incapable of continuing to accelerate), the time constant of the capacitor needs to be set so it
takes longer than 10 seconds to fully charge when 5 volts is applied – the capacitor can never
be fully charged because the accelerometer will be incapable of providing 5 volts for in
excess of 10 seconds. Obviously decelleration characteristics would have to be considered
as well in set the time constant for the circuit. As long as the capacitor charge & discharge
rate keeps the gain of the op-amp within its linear region, the integrator will accurately
perform its function linearly throughout the entire range of aircraft operation.
Replace feedback resistor with capacitor
With initial voltage input – uncharged capacitor reacts as a short circuit – zero ohms – therefore
op-amp gain is zero
As capacitor is gradually charged by op-amp output – resistance to current flow increases –
increasing resistance increases op-amp gain
With input signal applied as illustrated – gain is initially low – op-amp output is low
Input signal remains constant, but as capacitor charges effective resistance of feedback loop
increases – op-amp gain & output signal increase proportionally
Rate at which output increases controlled by varying capacitance – circuit designed to remain
within linear portion of op-amp – linear output
When the aircraft cruises (accelerometer input drops to zero), the capacitor holds its charge
because it has nowhere to discharge through (ideal Op-amp input impedance is infinite
presenting a virtual open circuit). Therefore op-amp output remains constant, mirroring
aircraft velocity. The capacitor will not begin to discharge until the signal from the
accelerometer is negative, applied to the inverting input, this will oppose the capacitor voltage
(Op amps amplify the difference between the inverting & non-inverting input – so when
accelerometer input goes negative {& capacitor voltage felt at the inverting input is negative
as shown on the slide}, the signal being amplified is effectively diminished), therefore causing
the Op-amp output to begin to drop: indicating deceleration. The capacitor will begin to
discharge as the output voltage diminishes (voltage felt at inverting input will diminish & op-
amp output will drop accordingly mirroring the aircraft velocity decrease).
Therefore, the op-amp output will continue to increase whilst ever the accelerometer signal is
applied. The output voltage will remain constant when the accelerometer signal; drops to
zero, & will not begin to diminish until a deceleration signal (negative accelerometer signal) is
applied.

AA Form TO-19
Comparator op-amps
Comparator op-amps compare two input signal voltages at the inverting and noninverting
inputs. If the compared inputs are equal there is no output. If they are unequal there is a large
output because with no feedback the op-amp gives the high gain that is characteristic of open
loop operation. The output may be positive or negative depending on the polarity of the
imbalance at the differential inputs.
Zero-Level Detection
One application of an op-amp used as a comparator is to determine when an input voltage
exceeds a certain level. The figure shows a zero-level detector. Notice that the inverting (-)
input is grounded to produce a zero level and that the input signal voltage is applied to the
noninverting (+) input. Because of the high open-loop voltage gain, a very small difference
voltage between the two inputs drives the amplifier into saturation, causing the output voltage
to go to its limit. For example, consider an op-amp having Aol = 100,000. A voltage difference
of only 0.25 mV between the inputs could produce an output voltage of (0.25 mV)(100,000) =
25 V if the op-amp were capable. However, since most op-amps have maximum output
voltage limitations of ±15 V because of their dc supply voltages, the device would be driven
into saturation.
The right hand figure shows the result of a sinusoidal input voltage applied to the
noninverting (+) input of the zero-level detector. When the sine wave is positive, the output is
at its maximum positive level. When the sine wave crosses 0, the amplifier is driven to it
opposite state and the output goes to its maximum negative level, as shown. As you can see;
the zero-level detector can be used as a squaring circuit to produce a square wave from a
sine wave.

AA Form TO-19
The zero-level detector in the previous figure can be modified to detect positive and negative
voltages by connecting a fixed reference voltage source to the inverting (-) input, as shown in
the figure, where Vref =Vz
555 Timer
The 555 timer consists basically of two comparators, a flip-flop, a discharge transistor, and a
resistive voltage divider, as shown in the figure. The flip-flop (bistable multivibrator) is a two-
state device whose output can be at either a high voltage level (set, S) or a low voltage level
(reset, R). The state of the output can be changed with proper input signals (Covered in Topic
4A).
The resistive voltage divider is used to set the voltage comparator levels. All three resistors
are of equal value; therefore, the upper comparator has a reference of 2/3Vcc, and the lower
comparator has a reference of 1/3Vcc The comparators’ outputs control the state of the flip-
flop. When the trigger voltage goes below 1/3Vcc, the flip-flop sets and the output jumps to its
high level. The threshold input is normally connected to an external RC timing circuit. When
the external capacitor voltage exceeds 2/3Vcc, the upper comparator resets the flip-flop,
which in turn switches the output back to its low level. When the device output is low, the
discharge transistor (Qd) is turned on and provides a path for rapid discharge of the external
timing capacitor. This basic operation allows the timer to be configured with external
components as an oscillator, a one-shot, or a time-delay element. (The numbers in
parentheses are the pin numbers)

AA Form TO-19
A Comparator Application: Over-Temperature Sensing Circuit
The figure shows an op-amp comparator used in a precision over-temperature sensing circuit
to determine when the temperature reaches a certain critical value. The circuit consists of a
Wheatstone bridge with the op-amp used to detect when the bridge is balanced. One leg of
the bridge contains a thermistor (R1), which is a temperature-sensing resistor with a negative
temperature coefficient (its resistance decreases as temperature increases). The
potentiometer (R2) is set at a value equal to the resistance of the thermistor at the critical
temperature. At normal temperatures (below critical), R1 is greater than R2, thus creating an
unbalanced condition that drives the op-amp to its low saturated output level and keeps
transistor Q off.
As the temperature increases, the resistance of the thermistor decreases. When the
temperature reaches the critical value, R1 becomes equal to R2, and the bridge becomes
balanced (since R3 = R4). At this point the op-amp switches to its high saturated output level,
turning Q1 on. This energizes the relay, which can be used to activate an alarm or initiate an
appropriate response to the over-temperature condition.

AA Form TO-19
AMPLIFIER COUPLING
RC Coupling
RC coupling is very common. The capacitors perform the role of blocking unwanted DC
voltages that may vary the transistor biasing set up by the resistor voltage divider networks. It
has a moderately flat frequency response (constant gain over a range for frequencies) that
drops off at the lower and higher frequencies. Common emitter stages are usual. Impedance
matching is not good and this detracts from the overall gain. However this is tolerable
because with the low cost of RC coupled amplifiers more stages can be used to compensate
for the lower gain per stage.
LC Coupling
LC coupling is rarely used. It is more efficient than the RC method in that the coils (L1 and
L2) resistances to dc current are lower than the load resistors (RL1 and RL2) in the RC case,
and they therefore dissipate less power. However, the frequency response of amplifiers with
LC coupling is not flat because a coil’s inductive reactance to ac signals increases with
frequency.

AA Form TO-19
Transformer Coupling
Transformer coupling is fairly frequently used. It can improve gain because of good
impedance matching and power transfer characteristics. Like LC coupling it has poor
frequency response. Even though the transformer T in the figure blocks dc between the two
stages, capacitor C2 is still necessary to prevent the base bias voltage developed across R4
from being shorted out by the low resistance secondary winding of T. Often, the last stage of
RC coupled amplifiers will be coupled to a low resistance load by a transformer.
Direct Coupled
Capacitors and transformers cannot be used in coupling circuits when dc, or ac frequencies
close to zero are to be amplified. Direct coupling is then used. Its big disadvantage is that any
instability of the dc current occurring for any reason in any stage is amplified by all the later
stages. This does not occur with capacitive or transformer coupling because the dc is blocked
between each stage.

AA Form TO-19
Page Intentionally Left Blank

AA Form TO-19
TOPIC 4.2 – PRINTED CIRCUIT BOARDS

Printed Circuit Boards
A printed circuit board is a flat insulating surface upon which printed wiring and miniaturized
components are connected in a predetermined design, and attached to a common base.
Figure shows a typical printed circuit board. Notice that various components are connected to
the board and the printed wiring is on the reverse side. With this technique, all
interconnecting wiring in a piece of equipment, except or the highest power leads and
cabling, is reduced to lines of conducting material (copper, silver, gold, etc.) deposited
directly on the surface of an insulating "circuit board." Since printed circuit boards are readily
adapted as plug-in units, the elimination of terminal boards, fittings and tie points, not to
mention wires, results in a substantial reduction in the overall size of electronic equipment.
Printed Circuit Board Construction
A basic printed circuit board has the copper circuit pattern or foil on one side of the board.
Holes are drilled through pads or terminals in the foil and board. The component leads are
pushed through the holes from the other side of the board. The leads are then soldered to the
copper foil to complete the circuit connections.
A PCB is found in almost every electronic device. If you have electronic components in a
device, they are mounted on a PCB, big or small. Besides keeping the components in place,
its purpose of a PCB is to provide electrical connections between the components mounted
on it. As electronic devices have become more complex, and require more components, the
PCB has become more populated, and dense with wiring and components.
Conductor Pattern
The substrate of the board itself is an insulating and non-flexible material. The thin wires that
are visible on the surface of the board are part of a copper foil that initially covered the whole
board. In the manufacturing process this copper foil is partly etched away, and the remaining
copper forms a network of thin wires. These wires are referred to as the conductor pattern or
the tracks and provide the electrical connections between the components mounted on the
PCB.
B2-4.2 Printed Circuit Boards

AA Form TO-19
Component Mounting
To fasten the components to the PCB their legs are soldered to the conductor pattern. On the
most basic PCBs (single-sided boards) the components are located on one side of the board
and the conductor pattern on the opposite side. This requires holes in the PCB for the
component legs to penetrate the board. Hence, the legs are soldered to the PCB on the
opposite side of where the components are mounted. The top and bottom side of a PCB is
therefore respectively referred to as the 'Component Side' and 'Solder Side.'
Component Sockets
If a component needs to be removable from the PCB after it is manufactured, it is mounted
on the board with the use of a Socket. The socket is soldered to the board while the
component can be inserted and taken out of the socket without the use of solder.
Edge Connector
To connect a PCB to another PCB an edge connector is often used. The edge connector
consists of small uncovered pads of copper located along one side of the PCB. These copper
pads are actually part of the conductor pattern on the PCB. The edge connector on one PCB
is inserted into a matching connector (often referred to as a Slot) on the other PCB. In a PC,
graphic cards, sound cards and other similar products are connected to the main board with
the use of edge connectors.
Solder Mask and Silk Screen

What gives the PCB its green or brown colour is the solder mask. This is an insulating and
protective coat that protects the thin copper wires and prevents solder from attaching outside
the connection points for the components. On top of this coloured mask a silk screen is
printed. This is text and symbols (often white) printed on the board to label the locations for
the different components that are to be mounted. The silk screen is also referred to as the
legend.

AA Form TO-19
TYPES OF PCBS
Single-Sided Boards
Most basic boards have the components mounted on one side of the board and the
conductor pattern on the opposite side. Since there only is a conductor pattern on one side,
this type of PCB is called 'Single-sided.' This type of board has severe limitations when it
comes to routing the wires in the conductor pattern (since there is only one side no wires can
cross, and they have to be routed around each other), it is only used in very primitive circuits.
Double-Sided Boards
These types of boards have a conductor pattern on both sides of the board. Having two
separate conductor patterns requires some kind of electrical connection between them. Such
electrical 'bridges' are called 'vias'. A via is simply a hole in the PCB that is filled or plated
with metal and touches the conductor pattern on both sides. Since the surface available for
the conductor pattern is twice as large compared to a single-side board, and that wires now
can cross (by routing them on opposite sides of the board), double sided PCBs are much
more suited for complex circuits than the single-sided.

AA Form TO-19
Multi-Layer Boards
To increase the area available for the wiring even more these boards have one or more
conductor pattern inside the board. This is achieved by gluing (laminating) several double-
sided boards together with insulating layers in between. The number of layers is referred to
as the number of separate conductor patterns. It is usually even and includes the two outer
layers. Most main boards have between 4 and 8 layers, but PCBs with almost 100 layers can
be made. Large super computers often contain boards with extremely many layers, but since
it is becoming more efficient to replace such computers with clusters of ordinary PCs, PCBs
with a very high layer count are less and less used. Since the layers in a PCB are laminated
together it is often difficult to actually tell how many there are, but if you inspect the side of
the board closely you might be able count them.
In multi-layer PCBs whole layers are almost always dedicated to Ground and Power. We
therefore classify the layers as Signal, Power or Ground planes. Sometimes there is more
than one of both Power and Ground planes, especially if the different components on the
PCB require different supply voltages.

AA Form TO-19
TECHNOLOGIES FOR COMPONENT PACKING
Through Hole Technology
The components that are mounted on one side on the board while its legs are soldered on
the opposite side are called 'Through Hole' (THT: Through Hole Technology). Such
components takes up a large amount of space and require one hole to be drilled in the PCB
for every leg. Hence, their legs occupy space on both sides of the board, and the connection
points for them are also fairly large. On the other hand, THT components are fairly good
mechanically connected to the PCB compared to Surface Mounted devices, which will be
discussed below. Connectors for cables and similar devises also have to withstand
mechanical stress and are usually THT.
Surface Mounted Technology
The legs of components that are made using 'Surface Mounted Technology' are soldered to
the conductor pattern on the same side of the PCB as the component is mounted. This
technology does therefore not require a hole in the PCB for every leg of the component.
Surface Mounted Components could even be mounted on both sides of the PCB directly
underneath each other.
SMT components are also much smaller than THT components. This makes PCBs with SMT
components much more dense compared to similar PCBs with THT components. Today SMT
components are also cheaper than THT components. It is therefore no surprise that most
components on main boards nowadays are SMT. Since the connection points and
component legs are so small it becomes very hard to solder on a SMT component manually.
Considering that machines do almost all assembly, this issue only becomes important when
repairs have to be done.
Printed Circuit Labels
The foil pattern takes various shapes, depending on the function of the circuit. Figure shows
an example of the shapes of circuit elements. Heat sinks are used to dissipate heat from
components and require large areas. Voltage and ground lines or planes to which
components are attached are long and slender in length and will follow a path so as to
provide power to all components on the board. Terminals or pads are the points that are
drilled with holes to accommodate the component leads. Conductors or runs are thin foil
strips between components. Edge connectors are part of the foil that connects the circuits of
the board to a special plug, which in turn connects to other boards to form a system.

AA Form TO-19
Base or Substrate Material
The base material of a printed circuit board, referred to as the substrate or laminate, is made
of phenolic paper, epoxy paper, and epoxy glass. The metal foil, referred to as the cladding,
is usually made of copper, but other types of metal may be used. The substrate board is
produced first and then the cladding is bonded to it.
Photoresist
There are several methods for producing the conductive pattern on a printed circuit board.
The circuit pattern connecting the components must be placed on the copper foil. The pattern
may be drawn with a chemical, applied with special circuit tape, then produced by silk-
screening or photographic means. Photographic methods are normally employed where
many of the same types of board must be produced. Figure illustrates a general method for
producing a printed circuit board.
The copper side of the board must first be cleaned of the oxide layer with chemical cleaners
and then rinsed in water. A clear- to-amber-colored material that resembles and handles
similar to varnish or lacquer is then painted or sprayed on top of the copper. When exposed
to light of the proper wavelength, this material, called photoresist or simply resist, is
chemically changed in its solubility to certain solvents or developers. The next step is to
photo-expose the circuit pattern onto the board.
With a negative-acting resist the portions beneath the pattern that are not exposed to light
become hardened and are not soluble in the developing solution. Positive-acting resist is also
available, where the portion exposed to light will be soluble in solutions.

AA Form TO-19
Etchant Solution
The exposed board is now placed into a solution called the etchant, which dissolves the
copper and resist exposed to light. The etchant solution used most often is ferric chloride
(FeCI3). This solution removes the exposed areas of the copper, leaving the substrate
material underneath. The copper under the hardened resist remains, producing the circuit
pattern. The board is then placed into a cleaning solution to remove any traces of resist that
may interfere with soldering.
Finished Printed Circuit Board
The copper pattern surfaces are usually covered with a thin coating of solder (a tin-lead
composition) to reduce the effects of oxidation and improve the soldering process of
components. The finished printed circuit board is then dried and ready for assembly.
From circuit design to printed circuit board design depends largely on the complexity of the
circuit. It could be done manually or, for complex circuits, with CAD programs specifically for
printed circuit board track layout.

AA Form TO-19

AA Form TO-19
TOPIC 4.3.2 – SERVOMECHANISMS

Error Detector Systems
Servo mechanism systems are used for measurement and control in a range of aircraft
systems including pressurisation, flight controls, transducer monitoring, etc
Error detecting devices can take many forms:
 Differential transformers
 Pendulous monitors
 Inductive transducers
 Capacitive transducers
 Resistive networks
Linear Variable Differential Transformer – LVDT
The LVDT has one primary and two secondary windings. The two secondary windings are
connected so that the voltages induced in each coil are in opposition. The LVDT has a
magnetic core (also called an armature) that is positioned by a linear motion. When the
armature is in the central position, the secondary windings have an equal voltage induced in
each coil. The LVDT output is sum of both coils. As the coils are wired in opposition, one coil
will be in phase with the primary voltage and the other coil will be 180 degrees out of phase.
With the voltage in each coil being equal. the result is that they cancel each other out, giving
an output of zero. This is shown in Figure.
With the two coils in opposition, the smaller voltage is cancelled out leaving the residual of
the higher voltage. The phase would be that of the higher voltage. If the armature is moved
the same amount in the opposite direction the output voltage would be the same magnitude,
however the phase of the output would be opposite. Therefore the direction of the armature
movement detem1ines the phase of the output and the amount of movement determines the
magnitude of the output.
B2-4.3.2 Servomechanisms
AA Form TO-19
The main advantage of the LVDT transducer over other types of displacement transducer is
their high degree of robustness. This is derived from their very principle in which there is no
physical contact across the sensing element and so there is zero wear in the sensing
element. This also means that RDP Electronics LVDTs can be made waterproof and in a
format suitable for the most arduous applications.
The LVDT principle of measurement is based on magnetic transfer which also means that the
resolution of LVDT transducers is infinite. The smallest fraction of movement can be detected
by suitable signal conditioning electronics.
The combination of these two factors plus other factors such as accuracy and repeatability
has ensured that this technology is still at the forefront of displacement measurement after
over 90 years.
An LVDT comprises a coil former or bobbin onto which three coils are wound. The first coil,
the primary is excited with an a.c. current, normally in the region of 1 to 10kHz at 0.5 to 10V
rms. The other two coils, the secondaries are wound such that when a ferritic core is in the
central linear position, an equal voltage is induced into each coil. However, the secondaries
are connected in opposition so that in the central position the outputs of the two secondaries
cancel each other out.
A linear variable differential transformer (LVDT) is an electromechanical device which
translates straight line motion into a linear Alternating Current (AC) signal (proportional to
amount of movement).
Transformer theory explains how an alternating current passed through a coil induces a
current into a coiled conductor in the vicinity. The expanding and contracting magnetic field in
the primary coil induces a current into the secondary coil.
If the magnetic flux is concentrated in an iron (ferrite) core, in lieu of just a hollow air gap the
transformer is more efficient and a stronger signal is induced into the secondary winding. The
strength of the signal induced into the secondary winding is therefore variable by inserting
and removing the ferrite rod core.
This is the basis of operation of an LVDT
AA Form TO-19
LVDT Operation
By incorporating two secondary coils (or a single coil with a centre-tap) whenever one end of
the secondary is positive the other end will be negative. If the signals from each end of the
coils are measured and compared to earth, the two signals will be of equal amplitude and
frequency, but of opposite phase. If the two signals are combined the resultant will be zero
volts because the two signals will cancel each other out.
The two signals will only be of equal amplitude when the ferrite rod is in the centre of the
secondary coil. If the rod is displaced in either direction, one of the secondary coils signals
will be stronger than the other, and the resultant signal will be indicative of direction (the
phase indicates this) and amount of movement (amplitude is proportional to amount of
movement).
The AC signal produced by the LVDT can then be rectified and combined with the initial error
signal applied to the transfer valve, nulling it out – this is called feedback.
AA Form TO-19
Force Transducers
In an E and I bar force transducer, the principle of operation is virtually the same as for an
LVDT.
The AC input signal is applied to the centre winding on the E bar and the outer legs support
the secondary windings. In the force transducer pictured, any input on the left hand end will
move the I bar with respect to the E Bar because the outer case of the transducer is designed
to expand and contract as force is varied. The magnetic relationship between the E and I bar
will vary, producing output signals in the same manner as the LVDT.
AA Form TO-19
E and I Bar Transformer
This device is so called by the shapes of the components. The transformer coils are wound
on the legs of the E core with the primary on the centre core and the secondaries on the outer
cores.
The I bar may be pivoted at the centre. It is generally actuated by linear devices, although it
can be adapted to limited circular movement. When it is moved toward one end, the reduced
air gap will create a stronger magnetic linkage with that end, giving an output signal relative
to the end in contact. The amplitude of the error signal will depend on the amount of rotation
of the I bar.
AA Form TO-19
E and I Bar Acceleration Sensor
An application of the E and I bar is in an acceleration and side slip sensor. When an aircraft
maintains an attitude change which is less than one which can be sensed by the gyros, an
acceleration sensor can provide an output in a direct relationship to the attitude change.
An I bar, suspended on springs in the sensing axes, is able to sense acceleration in that
plane. Under constant velocity, the I bar will maintain its position giving a zero output from the
secondary. If acceleration or deceleration forces are detected, the I bar will be displaced as a
function of the acceleration forces acting upon it. This will induce an EMF in the secondary in
the way we have already described. This EMF will be a signal, which will carry details of the
displacement. After application to an amplifier, it will provide power to the relevant
servomotor to correct for the change of attitude.
AA Form TO-19
C and Y Core Differential Transformer
The operation of the C and Y core differential transformer is similar to that of the E and I core
and the L VDT devices. The C and Y core has one primary and two secondary windings. The
primary winding is on the bottom yoke of the "y'. The two secondary windings are wound on
the branches of the "Y' and are connected so that the voltages induced in each coil are in
opposition. A "c" shaped armature surrounds the "Y' shaped stator. The C and Y core is used
to detect rotary motion. When the armature is evenly distributed around the stator, the
secondary windings have an equal voltage induced in each coil. As shown in Figure.
The C and Y core output is sum of both coils. As the coils are wired in opposition one coil will
be in phase with the primary voltage and the other coil will be 180 degrees out of phase. With
the voltage in each coil being equal, the result is that they cancel each other out, giving an
output of zero. If the armature is moved from centre, a higher voltage is induced in one coil
and a lower voltage in the other coil. The larger the movement the greater the difference in
voltage between each coil.
With the two coils in opposition the smaller voltage is cancelled out leaving the residual of the
higher voltage. The phase would be that of the higher voltage. If the “C" armature is moved
the same amount in the opposite direction the output voltage would be the same magnitude,
however the phase of the output would be opposite. Therefore the direction of the armature
movement determines the phase of the output and the amount of movement determines the
magnitude of the output. Again, you will see that when the stator and armature are centered,
there is no output. When the armature is moved, an output is generated, the phase being
dependent on the direction of 'C bar movement.
AA Form TO-19
Capacitance Transmitters
Capacitive sensor can be connected into a bridge circuit or an oscillator circuit. Movement of
the plate will vary the capacitive reactance and give either an output from the bridge or a
change in frequency of oscillation, which is then converted into a measure of the mechanical
position of the capacitor plate.
A capacitance transmitter has a rotor and stator of intermeshing plates which is shown in
Figure. The relative position of the rotor and the stator plates determines the capacitance
value.
Therefore if you vary the plate area, you are varying the capacitance. The anode and cathode
of a capacitance transmitter is the rotor and stator. The amount of stator and rotor
intermeshing is controlled by the rotation of a shaft by a mechanical input. When the stator
and rotor plates are fully intermeshed the capacitance is high. When the stator and rotor
plates are partly meshed the capacitance is low.
AA Form TO-19
Pendulous transmitters
The figure illustrates the operation of another transformer type of transmitter that has only
one secondary winding.
In (a), with the core centered in the frame the flux of the primary winding does not intersect
with the secondary, which therefore has no emf induced in it.
With the frame displaced around the core as in (b), or in the opposite direction as in (c), there
is a low reluctance path for some of the primary flux that intersects with the secondary
winding.
Note that for any same instant in time, the flux intersecting with the secondaries in (b) and (c)
are in opposite directions. The secondary emf’s induced in (b) and (c) are therefore of
opposite phase. If in (b) the secondary emf is opposite phase to the primary emf, then in (c)
the secondary and primary emf’s would be in phase.
The transmitter in the figure can be used to detect shifts from the vertical (or horizontal). If the
frame is secured vertically to the aircraft structure and the aircraft is level, the core will hang
in a central position. If the aircraft tilts from level in the same plane as the page, the
pendulous core will still hang vertically as in (b) and (c), but it will be displaced relative to the
frame.
AA Form TO-19
Inductance Transmitters
An inductive transmitter has a coil supplied with alternating current set against two vanes. A
portion of this voltage will be induced into the secondary coil depending on the inductance of
the vane next to the coils. The amount of inductance depends on the type of vane material
used. An example of an inductive transmitter is shown in Figure.
A common inductive transmitter is made of aluminium and a ferrite material. The null position
is when the inductive coil is positioned on the join between the ferrite and the aluminium
vanes. A displacement of the inductor from the join either increases or decreases the
inductance.
In the centre position you can see that the coil is centered over the join between the
aluminium and the ferrite vanes. If the vane is moved so that more of the aluminium vane is
beside the coils, less inductance results due to the vane's properties.
AA Form TO-19
AUTOSYN
This system uses 26 V AC 400 HZ power from the aircraft’s instrument power supplies. The
term Selsyn is often used to describe this self synchronous system, which can be used to
measure and indicate such things as fuel flow, oil pressure, and flap position. The name
autosyn is derived from automatic synchronism.
The units are of similar construction, the transmitter and indicator being variable
transformers, the rotors being the primaries and the stators being secondaries. They are
connected in parallel.
Operation
In this system the transmitter has its rotor physically positioned by the medium to be
measured, whilst the rotor of the indicator moves because of magnetic action. When power is
applied, the current in the rotors sets up an alternating flux that induces a voltage into the
stators. The position of the rotors determines the value of voltage induced into each segment
of the stators.
Whenever two rotors have the same physical position, both stators will have the same
voltages induced into their corresponding segments, and since they are connected in parallel,
no potential difference exists, and no current will flow between the units. This position is
called the in correspondence condition and no pointer movement will take place.
When the two rotors do not have the same physical position, the voltages induced into the
stators will not be the same. This will now cause a potential difference to exist and current will
flow through the connecting wiring. The current flow will create a motor action that moves the
indicator rotor until both rotors are again aligned. Whenever the two rotors are out of
alignment, their voltages differ, and they are said to be out of correspondence.
Main disadvantage of this system is that the pointer will remain on scale when the power
fails, which could give the crew misleading information about the system being monitored.
Many of these systems incorporate a power off flag to alert the crew to a power failure
situation.
AA Form TO-19
MAGNESYN
This system makes use of 26 V AC 400 HZ single phase AC power from the aircraft’s supply.
It can be used wherever a mechanical movement is available. The system consists of a
transmitter and indicator connected electrically and is more compact, lighter, and simpler
than an autosyn.
The transmitter consists of the mechanical actuating mechanism and the transmitter, which
can be either a rotary type or a linear type. The theory of operation is the same for both, and
the rotary type is described here.
The rotor is a permanent magnet attached to, and positioned by, the actuating shaft. The
stator consists of a circular laminated core, upon which is wound the excitation coil, and a
tapping is made at each 120 degrees on the coil away from the input. Outer laminations
within the housing encircle the outside of the stator, and provide a return path for the
magnetic flux.
The indicator is of the same construction, except that the rotor is attached to a pointer which
indicates the medium which is being measured.
Operation
When the permanent magnet rotor is placed inside the ring or stator of soft iron, the flux lines
will establish a flux within the ring. If a coil is wound around the ring and connected to an AC
supply, the ring will become magnetically saturated twice each cycle when the current
reaches its peak. The rotor flux is forced out of the ring because the ring now has a higher
reluctance than the air surrounding the rotor.
When the excitation current is at zero, the rotor flux cuts across the excitation coil inducing an
EMF, this is generated in all three sections of the stator windings. The amount and phase of
the EMF in each section is dependent on the position of the permanent magnet rotor.
When the indicator rotor corresponds with the transmitter rotor, identical changes to the
EMF’s will take place to their respective stators. There will be no difference in potential at
each tapping, and therefore no current flow.
When the mechanical mechanism moves the transmitter rotor, the EMF will differ in the stator
windings creating a difference in potential, and current will flow in the interconnecting wires.
As the transmitter is mechanically held, the receiver rotor will turn to align itself with the
transmitter rotor thus moving the pointer around the scale.
AA Form TO-19
SYNCHROS
Purpose
The use of synchros in position sensing and data transmission, is very common in aircraft,
especially in automatic pilot systems. It is a fast accurate method of transmission and control,
and provides an accuracy of approximately 0.5%. The synchro is essentially a rotary
transformer whose secondary output voltage depends upon the primary input voltage, and
upon the position of the rotor. The simplest system consists of two synchros connected
together electrically, one is called a transmitter, the other a receiver. The purpose of the
receiver is to take up the same position as the transmitter.
Null point
If the transmitter and receiver rotor position are identical the rotor fields produced are
identical both in magnitude and direction. The voltages induced in the corresponding stator
coils will be equal in both magnitude and phase, and zero current will flow through the stator
coils. No current flow, no stator field produced, no torque developed, null point.
TORQUE SYNCHRO SYSTEM
To convert a mechanical movement into electrical signals and then transmit the signals to
another location, a system of torque syncros is used. The system consists of two items, a
torque transmitter (TX) and a torque receiver (TR). Both items are similar except that the
receiver will contain some form of damping to prevent oscillations in the rotor. The markings
on the terminals are the same for both, S for stator R for rotor and the symbols used in
electrical drawings are the same for both. Sometimes the word indicator is used instead of
torque receiver.
AA Form TO-19
A circuit will be created if the three stator windings of a TX synchro are connected to the
same connections of a TR synchro. When a voltage is applied to the TX rotor, the magnetic
field generated by the current in the rotor, will induce a voltage in each of the stator windings
by transformer action. The current flowing in the three windings will create three magnetic
fields which will combine to produce one field. The TR rotor and the TX rotor are now
connected in parallel, creating magnetic fields in both rotors which are in phase, therefore
their fields will always be in the same direction. If the TX rotor is turned 30 degrees clockwise,
the stator field of the TR will follow it and move 30 degrees away from its rotor field. The two
magnetic fields in the TR will be out of line, and an attraction will exist between the two. This
will cause the TR rotor to turn and bring the two fields into line
Symbols
Synchro transmitters and receivers are virtually the same, and so the schematic symbols for
them are the same. Figure shows three examples of the way in which synchros are drawn,
(a) is the most commonly used, (b) is usually used when the operation is explained, and (c) is
usually on wiring diagrams.
AA Form TO-19
TORQUE TRANSMITTER
Notice that the internal construction looks very much like an ordinary motor or generator that
you have seen in previous training. The TX is made up of the following two major parts:
 Stator
 Rotor
AA Form TO-19
Stator
The stator consists of an upper end cap, a shell, and a lower end cap. The shell and the
lower end cap are shown in figure.
The inner surface of the shell is slotted, to hold the stator winding. The stator winding is
actually three separate windings physically spaced 120º apart and star connected. Three
leads, one from each winding, are connected to form a common connection inside the shell,
as shown.
Torque Rotor
The rotor of a Torque Transmitter is made up of two coils connected in series to form one
continuous winding. These coils are wound on a laminated bobbin or H shaped iron core, to
form the complete rotor assembly as shown.
The rotor core is laminated to reduce eddy currents. The ends of the rotor assembly are
mounted in ball bearings to allow almost friction-less rotation of the shaft. The ends of the
rotor windings are connected to two slip rings on the rotor shaft.
AA Form TO-19
DIFFERENTIAL TRANSMITTER (TDX)
A torque differential synchro can be used to transmit either:
 an electrical signal which is the sum or the difference of two inputs, one mechanical,
the other electrical
 a mechanical signal which is either the sum or the difference of electrical inputs from
two synchro transmitters
 a corrective signal to compensate for errors in various parts of a system.
This means that they can be either a transmitter TDX or a torque receiver TDR.The principle
of operation is the same as the torque transmitter, however the rotor is designed with three
separate windings which are placed electrically 120 degrees apart. In this case the stator
acts as the primary of the transformer, and the rotor the secondary.
Figure shows a three component synchro system, it consists of a torque transmitter, a

differential synchro, and a torque receiver. The stator leads of the torque transmitter are
connected to the stator leads of the differential synchro. The rotor leads of the differential
synchro are connected to the stator leads of the torque receiver.
AA Form TO-19
The differential synchro is not directly connected to the AC supply.

The torque differential transmitter (TDX) and the torque differential receiver (TDR) are
ELECTRICALLY IDENTICAL. The only difference in their construction is that the receiver
(TDR) has a damper, which serves the same purposes as the damper in the TR — it prevents
the rotor from oscillating.
The principle of operation is the same as the torque transmitter, however the rotor is
designed with three separate windings which are placed electrically 120 degrees apart.
AA Form TO-19
Torque Differential Receiver (TDR)
The real difference in the receiver lies in its application. It provides the mechanical output for
a differential synchro system usually as the sum or difference of two electrical inputs from
synchro transmitters. As in the case with the TDX, the TDR addition or subtraction function
depends upon how the units in the system are connected.
The rotor and stator of the torque differential receiver receive energizing currents from two
torque transmitters. These currents produce two resultant magnetic fields, one in the rotor
and the other in the stator. Each magnetic field assumes an angle corresponding to that of
the magnetic field in the transmitter supplying the signal. It is the interaction of these two
resultant magnetic fields that causes the rotor in the TDR to turn.
Torque Differential Transmitter (TDX)

In the torque differential transmitter, BOTH the rotor and stator windings consist of three Y-
connected coils. The stator is normally the primary, and receives its input signal from a
synchro transmitter. The voltages appearing across the differential's rotor terminals (R1, R2,
and R3) are determined by the magnetic field produced by the stator currents, the physical
positioning of the rotor, and the step-up turns ratio between the stator and the rotor. The
magnetic field, created by the stator currents, assumes an angle corresponding to that of the
magnetic field in the transmitter supplying the signal. The position of the rotor controls the
amount of magnetic coupling that takes place between the stator magnetic field and the rotor,
and therefore, the amount of voltage induced into the rotor windings. If the rotor position
changes in response to a mechanical input, then the voltages induced into its windings also
change. Therefore, the output voltage of the TDX varies as a result of either a change in the
input stator voltage or a change in the mechanical input to the rotor. This electrical output of
the TDX may be either the SUM or the DIFFERENCE of the two inputs depending upon how
the three units (the TX, the TDX, and the TR) are connected.
AA Form TO-19
DIFFERENTIAL TRANSMITTER OPERATION
When using the differential synchro as a transmitter, it can be used to add or subtract
information to the remote torque receiver.
Subtraction
The system is used to produce a difference output from the two inputs to the differential
transmitter. The two inputs come from the movement of the shaft of the TDX and an electrical
input from the TR stator. The signal transmitted to the TR is the difference between the
electrical signal A and the mechanical signal B. The shaft of the TR will position itself at an
angle equal to A to B.
The TDX rotor is held at 45 degrees and the TX rotor is turned to 15 degrees the TR rotor will
turn back from -45 degrees to 30 degrees. The 15 degrees signal from the TDX has
cancelled the signal from the TX.
Imagine if TDX were at 45 degrees as well, resultant field would be aligned to axis of R2,
resulting in resultant field in RT being aligned to S2, effectively subtracting one input from the
other with TR indicating 0 degrees
AA Form TO-19
Addition
The illustration shows the same set up as used for subtraction, except that both input and
output the leads of the TDX are changed. This will mean that the shaft of the TR will revolve
to a position whose angle will be equal to the angles travelled by the shafts of the TX and
TDX. If the rotor of the TDX is held at 40 degrees clockwise, and the rotor of the TX is turned
from -45 degrees through an angle of 90 degrees to a final position of 45 degrees clockwise,
the rotor of the TR will turn from -5 degrees to a position of 85 degrees clockwise.
AA Form TO-19
Differential synchro transmitter
The differential control synchro transmitter (CDX) is the same as the torque differential
transmitter mentioned earlier. However the rotor is always mechanically driven.
The figure shows a synchro transmitter connected to the stator of a differential synchro
transmitter and the rotor connected to a synchro receiver. the voltages appearing at the
receiver are modified by the angular position of the CDX. The resultant magnetic field in the
stator of the CR is also dependant upon the connections between all three items. The
resultant voltages can represent either the sum or the difference depending on the
connections.
Differential transmitter (TDX) Symbols
AA Form TO-19
CONTROL SYNCHRO SYSTEMS

It should be clear to you from our discussion of torque synchro systems that, since they
produce a relatively small mechanical output, they are suitable only for very light loads. Even
when the torque system is moderately loaded, it is never entirely accurate because the
receiver rotor requires a slight amount of torque to overcome its static friction.
When large amounts of power and a higher degree of accuracy are required, as in the
movement of heavy radar antennas and gun turrets, torque synchro systems give way to the
use of CONTROL SYNCHROS. Control synchros by themselves cannot move heavy loads.
However, they are used to "control" servo systems, which in turn do the actual movement.
Servo systems are covered in depth in the next chapter in this module.
There are three types of control synchros: the CONTROL TRANSMITTER (CX), the
CONTROL TRANSFORMER (CT), and the CONTROL DIFFERENTIAL TRANSMITTER
(CDX). The control transmitter (CX) and the control differential transmitter (CDX) are identical
to the TX and the TDX we discussed previously except for higher impedance windings in the
CX and CDX. The higher impedance windings are necessary because control systems are
based on having an internal voltage provide an output voltage to drive a large load. Torque
systems, on the other hand, are based on having an internal current provide the driving
torque needed to position an indicator.
Since we discussed the theory and operation of the TX and the TDX earlier, we will not
discuss their counterparts, the CX and CDX. However, we will cover the third control synchro,
the CT, in depth during this discussion.
In the previously described torque transmission system, the output element exerts a torque
which tends to align its rotor with the angular position of the input shaft. When positioning
heavy loads, for example a radar antenna, the torque synchro is inadequate, and a system
which provides an output in the form of an electrical signal is used.
This signal is then passed to an amplifier whose output can control a motor capable of
producing the correct amount of torque. The synchro system is still used, but the normal
synchro receiver is replaced by a unit called a control transformer, which takes the signal
from the transmitter and turns it into a control voltage.
AA Form TO-19
Synchro control transmitter (CX)
The synchro control transmitter, like the torque transmitter is wound with a three phase output
winding in the stator and a moveable rotor winding. When an AC voltage is applied to the
rotor winding a voltage will be induced into the stator winding, the phase and value being
dependant on the rotor position.
The stator of the control transformer is very similar to stators of other synchros, with one
exception. The windings of the control transformer's stator are made up of many more turns
of finer wire. The object of the increased number of turns is to make the impedance of the
stator high. This limits the flow of current through the stator increasing the accuracy of the
synchro.
Control Transformer Rotor
The rotor of the control transformer consists of one series winding, just like the rotor of the
transmitter or of the receiver. It is drum-shaped however, as shown in Figure not bobbin-
shaped as is the rotor of the synchro receiver.
The drum shape prevents the rotor from being attracted by the magnetic field in the stator. As
a result, the rotor does not turn to any particular position as the magnetic field in the stator
shifts. The only torque exerted on the rotor shaft is through a mechanical coupling to the
shaft.
AA Form TO-19
Synchro control transformer (CT)
 In Torque synchro system, TX/TR rotors are connected to same AC source.
 In Control synchro systems, ONLY the rotor of CX is connected to AC supply.
 CT rotor is the secondary, and provides an output to an amplifier for control.
Figure shows the circuit of a synchro control transmitter (CX) connected to a synchro control
transformer (CT). The rotor of both units are at electrical zero. The axis of the transmitter
rotor winding is aligned with the axis of S2 winding of its stator, the control transformer rotor
winding is at right angles to the axis of the S2 stator winding. In this situation the magnetic
field created by the current in the transmitter rotor winding gives rise to magnetic fields in the
two stators, the axis of these fields being in line with the axis of the S2 winding of each stator.
Rotating the transmitter rotor in either direction from the electrical zero position will produce a
corresponding angular movement of the axis of the magnetic field of both stators. The axis of
the transformer rotor winding and the axis of the transformer stator magnetic field are no
longer at right angles to each other. The flux of the stator field begins to induce an EMF
within the turns of the rotor winding.
The magnetic field of the stator induces an EMF in the rotor winding, the amplitude of which
will increase as the transmitter rotor is moved further away from the electrical zero position.
When the transmitter rotor has traveled through 90 degrees from electrical zero, the axis of
the transformer rotor winding and the axis of the transformer stator field are parallel. At this
point maximum flux transfer is achieved and so maximum EMF is induced in the rotor
winding.
AA Form TO-19
Control Synchro System Operation
Now that we have covered the basic operation of the control synchro system, let us see how
this system works with a servo system to move heavy equipment. Figure shows a block
diagram of a typical servo system that uses a control synchro system. Assume the shaft of
the CX in this system is turned by some mechanical input. This causes an error signal to be
generated by the CT because the CX and the CT rotors are now out of correspondence. The
error signal is amplified by the servoamplifier and applied to the servomotor. The servomotor
turns the load, and through a mechanical linkage called RESPONSE, also turns the rotor of
the CT. The servomotor turns the rotor of the CT so that it is once again in correspondence
with the rotor of the CX, the error signal drops to zero volts, and the system comes to a stop.
Control Transformer Symbols
AA Form TO-19
TROUBLESHOOTING SYNCHRO SYSTEMS
Troubleshooting
The following conditions can cause total or partial failure in a servomechanism.
Power failure
A suspected power supply failure can be checked quite easily by measuring the AC volts
across the rotor winding of the transmitter check the fuse.
Open circuit
An open circuit in either rotor winding will cause the receiver synchro to stick in one position.
If a stator winding had an open circuit the operation would be sluggish.
Short circuits
Will cause the fuse to blow or, at the worst, component and wiring burn out. Whichever, the
servomechanism will have ceased operation.
Incorrect wiring
This condition usually results in reversed direction. The following is a table of common
synchro system faults.
Rotor Leads Reversed
If the rotor leads are reversed to either the transmitter or receiver, the receiver will
have a constant error of 180°. In this case, when R1 of the transmitter goes positive,
R2 of the receiver is positive thereby causing the receiver to follow the transmitter in
the same direction, but with a 180° error.
AA Form TO-19
Stator Leads Reversed
Should any two of the stator leads be reversed, the receiver will turn in a direction
opposite to that of the transmitter.
Cyclic Rotation of Stator Leads

If the stator leads are rotated in order, instead of being reversed, the receiver will
track in the proper direction but it will be displaced 120° from the transmitter.
AA Form TO-19
Symptom Cause
Receiver rotor follows Transmitter rotor Open circuit between R1 and R2 – TR rotor
sluggishly following due to electrostatic charge on coil
and soft iron core aligning to field – may be
180° out
360° rotation of TX rotor causes TR rotor to Open circuit at S1
oscillate between 60° and 180° or 180° and
300°
60°
180°
TR rotor follows TX rotor but is out by 180° R1 and R2 leads reversed
TR rotor turns in opposite direction to TX 2 Stator leads reversed – when rotors aligned
rotor (2 points) indicates which stator coil is not
reversed
TR rotor turns in same direction as TX rotor All 3 stator leads rotated CW or CCW
but is 120° or 240° out in display between TX and TR
Ref voltage blows fuse – no synchro output Rotor leads shorted
Multiple errors will result in a combination of symptoms
AA Form TO-19
RESOLVER
Another type of synchro is the ‘resolver’. One of its functions is to convert alternating voltages
representing the ‘cartesian (rectangular) coordinates’ of a point, into ‘polar coordinates’
represented by an alternating voltage and the angular position of a shaft, as will be
explained. The resolver can also convert polar coordinates to cartesian ones, which is in fact
‘resolving’ The left figure shows the polar coordinates of the point P represented by the vector
r and the angle q it makes with the baseline x. The right figure shows the same point whose
location is now identified by the cartesian coordinates x and y. It is worth noting for later
considerations that the cartesian coordinate x of P = r cos q and its y coordinate = r sin q,
also that x2 + y2 = r2.
The stator and rotor in a common type of synchro resolver each have two windings with their
axes perpendicular to each other. The figure shows alternative schematics and the symbol
for synchro resolvers. The schematics shown are in the electrical zero position. In various
applications resolvers may have additional windings, or only one stator or rotor winding.
AA Form TO-19
The figure shows how a resolver and a follow-up servo can be used to convert from
Cartesian to polar coordinates. The two ac voltages representing x and y cartesian
coordinates are fed to the two stator windings. Let us say y = 3 V and x = 4 V. The two stator
fluxes combine to give a resultant that represents the polar angle coordinate.
The voltage induced in the R1R2 rotor winding is amplified and fed to the servo which drives
the resolver rotor until the R1R2 winding is in null, as at in the figure. The angle through which
the servo turns R1R2 to put that winding in null is the polar angle coordinate. If R1R2 is in null
then R3R4 is in its maximum voltage position, and the value of this voltage represents the
polar distance coordinate r.
AA Form TO-19
Assuming a unity transformation ratio the voltage induced in R3R4 would be 5 V. See the
vector diagram in the figure below.
To appreciate that the voltages fed to the stator windings can represent a point anywhere in
the four quadrants of a cartesian graph. For a point in the first quadrant both stator windings
would be fed with voltages in phase with the reference supply. For a point in the second
quadrant the y voltage to S1S2 would be in phase with reference supply, and the x voltage to
S3S4 would be 180° out of phase. In the third quadrant both the y and the x voltages would be
180° out of phase. In the fourth quadrant the y voltage would be out of phase, and the x
voltage would be in phase.
AA Form TO-19
To convert from polar to cartesian coordinates a voltage representing the polar distance
coordinate r is fed to one stator winding. If the rotor is turned from its electrical zero position
to the polar coordinate angle, the resulting voltage induced in one rotor winding will be
proportional to the y coordinate, and in the other rotor winding the induced voltage will be
proportional to the x coordinate.
ADF Application of a Resolver

The figure demonstrates an ‘Automatic Direction Finder’ (ADF) application for a synchro
resolver. 2 loop antennas are mounted perpendicular to each other. The radio signals
induced in them represent the x and y coordinates of where the station is relative to the
aircraft. The loops are connected to the stator winding of the resolver motor. The resulting
voltage induced in the resolver rotor (one winding) is amplified (by servo amp) and fed to the
servo motor which drives the rotor to a null. In doing this, the servo also turns the indicator
pointer to a direction that points to the transmitting antenna to which the ADF receiver is
tuned.
AA Form TO-19
Resolver synchro
The resolver syncro from the outside looks like any other synchro, but internally it is very
different. It has two rotor windings mounted on the one rotor, and has two stator windings.
Resolver Components
Connection to the rotor is made by the brushes and slip rings, or inductive coupling.
Resolvers using the inductive method are referred to as brushless types. The inductive
(brushless) resolvers offer up to 10 times the life of brush types and are insensitive to
vibration and dirt, therefore they are used in the majority of industrial applications.
The stator signals from a resolver are routed to a specialized type of analog-to-digital
converter system known as a resolver-to-digital (R/D) converter. Commercially available
models include both tracking and multiplexed types.
AA Form TO-19
SERVOMECHANISM - OVERVIEW
Servomechanism
Many of the closed loop control systems in aircraft require more power than is available from
command or error detector sources. Amplification of the error signal is then necessary.
Closed loop control systems in which an error corrector positions something in response to
an amplified error signal are known as ‘servomechanisms’.
Automatic device for the control of a large power output by means of a small power input or
for maintaining correct operating conditions in a mechanism. It is a type of feedback control
system.
The constant speed control system of a DC motor is a servomechanism that monitors any
variations in the motor's speed so that it can quickly and automatically return the speed to its
correct value. Servomechanisms are also used for the control systems of guided missiles,
aircraft, and manufacturing machinery.
Definitions
Open-Loop System
A control system that does not have a means of comparing input and output for control
purposes.
Closed-Loop System
Automatic control equipment in which the system output is fed back for comparison with the
input for the purpose of reducing any difference between input command and output
response.
Follow Up
A device or group of devices which serves to govern in some predetermined manner, the
electric power delivered to the apparatus to which it is connected.
AA Form TO-19
Servomechanism
An automatic feedback - control system in which one or more of its signals represent
mechanical motion.
A system in which output is compared to input to control error according to desired
relationship, or feedback.
A self-contained system (except for inputs) in which the feedback signal is subtracted from a
desired valve so that the difference is reduced to zero.
A control system that provides the following: a command instrument to control or program the
final process; amplification to strengthen and modify the command signal; work
instrumentation to manipulate the controlled process; and feed back provision to initiate
corrective action when needed.
Since feedback signals go from the controlled process back to the original command station,
a servo system is said to operate closed-loop.
Analogue
1. Pertaining to the general class of devices or circuits in which the output varies as a
continuous function of the input.
2. The representation of numerical quantities by means of physical variables, e.g. translation,
rotation, voltage, resistance, contrasted with “digital”.
Transducer
A device, component machine, system or combination of these that is used to convert one
form of energy into another. There are many different types of transducers: temperature
transducers, which convert temperature changes into electrical voltages (or mechanical
switching); pressure transducers, which change barometric pressure into electrical voltage;
and still others which convert mechanical movement into electrical signals and vice versa.
Null
1. A balanced condition which results in zero output from a device or system.
2. In a synchro system if “B” rotor was rotated to electrically align with “A” rotor, its error
voltage would fall to zero, i.e. it will be in a “NULL” position
Damping
Progressive diminution of a free oscillation due to expenditure of energy.
Types of Damping
 Viscous Damping - Is one in which a disc is free to rotate between the pole faces of
an electromagnet. Eddy currents are produced of a magnitude proportional to the field
strength and to the disc velocity and they establish magnetic fields and forces which
appose rotation of the disc and output shaft.
Disadvantages are:
o Wastes energy
o Velocity lag is increased
 Velocity feedback damping - Feeds back a voltage from a tachogenerator which is
proportional to the load velocity and in opposition to the error signal applied to the
amplifier unit of the servomechanism. Nett input to amplifier is the error signal voltage
(difference between input and output voltages) minus the velocity feedback voltage.
AA Form TO-19
 Error-Rate Damping
o A second tachogenerator at the input so that it feeds a signal forward into the
amp.
o A resistance-capacitance differentiating net work feeding forward into an
amplifier. (in parallel to error signal).
 Transient velocity damping - Utilises a differentiating network connected in the
velocity feedback signal line. It is effective only during the transient response period
(when a rate of change of load velocity exists).
 Phase Advance Damping - Inserts a resistance-capacitance network in the input to
the amplifier. With this arrangement, the output signal is 0 degrees in advance of the
input signal.
 Integral Control - This is required to compensate for lag and dead space due to
inherent friction. It is a differentiator as for Error-Rate and Transient Velocity Damping
but with the inclusion of an Integrator which feeds the “time integral” of the error signal
into the amplifier. When the error signal is zero, the subsequent constant integrator
output is just sufficient to counter the inherent friction.
To be classed as a servomechanism it must conform to the following.
 detect the difference between input and output (error detection)
 amplify the error signals (power amplification)
 provide feedback (closed loop)
 capable of continual operating (continuous operation).
Feedback
The signal or signals returned from a controller process to denote its response to the
command signal. Feedback is derived from a comparison of actual response to desired
response, and any variations is used as an error signal combined with the original control
signal to help obtain proper system operation. Systems employing feedback are altered
closed-loop systems; feedback closes the loop.
Deadband
Is the amount of error in a servomechanism that can exist without correction
Hunting
Continuous, cyclical searching by a control system for a desired or ideal value. Rapid hunting
usually is termed oscillation; slower cycling is called bird-dogging.
AA Form TO-19
Open Loop Control
Open loop control is one where the control action is independent of the output. The open loop
control does not self-correct when the PV (Process Variable) drifts , and this may result in
large deviations from the optimum value of the PV.Full manual control. NO system feedback.
An open loop control system does not use a comparison of the actual result and the desired
result to determine the control action. The primary advantage of open loop control is that it is
simpler to implement and less expensive than closed loop control. The disadvantage of open
loop control is that errors caused by unexpected disturbances are not corrected. This error
may be corrected by the intervention of a human operator.
AA Form TO-19
Closed Loop Control
Closed loop control differs from open loop control in that feedback is added to the system.
Feedback consists of measuring the difference between the actual result and the desired
result. By using the difference, the closed loop control system will drive the actual result
toward the desired result.
The advantage of a closed loop control system is that it gives more accurate control over the
process.
AA Form TO-19
DC Servo Motor Control Principle
Understanding the operating principles of servo motor systems is necessary for complete
understanding of avionics systems in general. The figure shows a DC servo motor loop. It is
called a loop because of the closed nature of the system operation. Whether or not an
operated item is attached to the servo motor is of no consequence to the operation of the
servo motor loop. The operated item could be anything ranging from an indicator readout to a
flight control surface of an airplane. Servo motors are also commonly used inside avionics
black boxes.
The 28 volt DC source is connected to the variable control potentiometer and to the follow-up
potentiometer. As long as the voltages seen by the wiper arms of the potentiometers are
equal, the servo motor does not run.
If the control potentiometer wiper arm is moved downward, there is a more positive voltage
on the right motor brush than on the left motor brush, and the motor runs so as to cause its
potentiometer also to move downward until wiper arm voltages are equal and the motor stops
running.
Moving the control potentiometer wiper arm upward would cause a more positive voltage to
appear on the left motor brush than on the right, and the motor would run in the opposite
direction, causing its potentiometer wiper arm also to move upward until the voltages are
equal.
The figure indicates another arrangement for DC servo motor loop. As in the previous
example, the servo motor is operating the wiper arm of a potentiometer connected to a
positive 28 volts DC.
The motor is connected so that if there is no control signal in, the potentiometer wiper arm is
driven to zero potential so that there is no signal into the amplifier. In this case, control
signals are always negative DC.
If we supply a negative 14 volt DC control signal to the amplifier, the servo motor will run until
its potentiometer wiper arm sees a positive 14 volts DC. At that time the amplifier has no net
input and the servo motor stops running.
AA Form TO-19
Any particular control signal voltage (down to a negative 28 volts DC) results in the servo
motor wiper arm being driven to a particular position. The operated item moves to a particular
position in response to a particular control signal.
Two-Phase Servo Motor

The figure shows a schematic symbol for what is called a two-phase inductance motor. This
is a typical servo motor. It may be very small, on a computer card, or it may quite powerful.
The construction of the stator is similar to the construction of the resolver stator. It has four
poles, paired, so that there are actually two fields. These two fields are represented
symbolically by two coils drawn at right angles to each other, because in the motor itself the
two fields do exist at right angles to each other.
If a 400 Hertz voltage of phase angle 0° is connected to one field and a 400 Hertz voltage of
phase angle 90° (a capacitor in series with the field can shift the voltage 90°) to the other
field, the instantaneous field, resulting from the addition of these two, rotates at 400
revolutions per second. You can visualise this rotation if you consider the instantaneous DC
voltages present across these fields.
For example, begin with the phase angle 0° voltage at a null, but rising. The phase angle 90°
voltage is at a positive peak, so that magnetic field is parallel to the fixed winding. As the
fixed winding voltage falls, the variable voltage rises.
Electrically 45° later, they will be equal and both positive. The magnetic fields will then be of
equal strength with a resultant that has rotated through 45°. If you pursue these
instantaneous DC voltages and the resulting magnetic fields through one complete electrical
cycle, you will see that the magnetic field has rotated through 360°.
Rotation of the magnetic field in the motor tends to drag the rotor after it in the same
direction. How fast the motor moves depends upon its load and the strength of the magnetic
field, which effectively is dependent upon the strength of the variable signal.
AA Form TO-19
In the figure the phase of the variable signal has been reversed. This reverses the direction of
rotation of the resultant field.
The direction of motor rotation depends upon the phase of the variable signal, and the speed
of rotation depends upon its amplitude
Often times it is desirable to apply an electrical brake to a two-phase servo motor (see
figure). This can be done by disconnecting either the variable field or the fixed field. If only
one field is left operative, the motor does not rotate because the field does not rotate. This
tends to hold the rotor of the motor in a fixed position.
The figure shows a two-phase servo motor loop using a control synchro input. It is typical of
many such loops used throughout aircraft systems. When ever the amplifier sees a signal of
a particular phase, it drives the motor in a particular direction until the synchro rotor comes to
a particular null. A signal of opposite phase from the synchro rotor drives the servo motor in
the opposite direction. The synchro rotor therefore always is driven to a particular null.
The operating signal will come from some remote source whose mechanical position we want
to duplicate in he operated item. For example, the remote source could be directional gyro
and the operated item could be a compass indicator.
AA Form TO-19
AC Servo Motor
The figure shows AC servo motor loops. If the potentiometer wiper arm is at its midpoint, its
voltage, referenced to ground, is zero. This is sometimes called a phantom ground because
there is no direct connection to ground. If the wiper arm is not at its midpoint, there is a
voltage at its contact. On one side of the mid position, the voltage is of one phase. On the
other side, the voltage is of the opposite phase (because opposite ends of the transformer
have opposite phase voltages).
With no control signal input, the servo motor will have driven the follow-up potentiometer to a
null. If an input control signal appears at the servo amplifier, it will be a voltage of one phase
or the other. The servo motor will run one direction or the other, driving the wiper in the
direction which will give a follow-up voltage of the phase opposite to that of the input signal.
The distance through which the wiper arm has to move is a function of the amplitude of the
input signal. The larger the amplitude of the input signal, the farther the wiper arm has to be
driven to cancel the input, and the greater the movement of the operated item.
Cancellation occurs when control signal and the follow-up voltages are equal in amplitude. If
the control signal becomes a null, the follow-up signal drives the servo motor until the follow-
up signal is also a null.
Follow-up voltage and signal presence monitor voltage are developed from the same synchro
in the figure. When the synchro rotor is in the position shown, the voltages across the X leg
and Y leg are equal. Since they are, there is no voltage difference between these X and Y
points.
The Y point is grounded; therefore, the X point also has a ground potential (phantom ground).
Movement of the rotor in either direction from this position develops a voltage (measured to
ground) from point X of one phase or the other.
A synchro connected in this way is never intended to have its rotor moved more than 20° or
so. An input signal of phase angle 0° causes the servo motor to run in the direction which will
develop an X leg output of phase angle 180°. It runs until the X leg voltage equals the control
signal input, If the amplitude of the input voltage is then decreased or eliminated, the servo
motor reverses because the follow-up signal exceeds the control signal. It runs until the
follow- up signal equals the control signal amplitude.
AA Form TO-19
As long as the synchro rotor remains within its normal limits of movement, the amplitude of
the voltage on the Z leg is always sufficient to satisfy the signal presence monitor.
The servo system in the figure is much more common than either of the two previous
examples. The servo loop is shown nulled out (synchro rotor perpendicular to its field).
Moving the rotor of the transmit synchro with the knob moves the field in the control synchro,
causing the servo motor to drive its rotor to a new null position. These servo motor loops
always drive the synchro rotor to a particular null, even if it has to drive the rotor 179.5°.
The figure illustrates a servo loop controlled by inputs from two different mechanical devices.
The transmit synchro on the left provides the original signal, which is fed through the
differential synchro to the control synchro in the servo loop. Movement of the rotor of the
transmit synchro by its knob rotates the field in the differential synchro and in the stator of the
control synchro. This causes the servo motor to drive the control synchro rotor to a new null
position.
Movement of the differential synchro rotor by itself also moves the field in the stator of the
control synchro, causing the servo motor to drive. If the transmit synchro rotor is moved 30°
clockwise, and the differential synchro rotor is also moved 30° clockwise, the servo motor will
have to drive its control synchro rotor 60° to find a new null.
AA Form TO-19
If we could move the transmit synchro rotor 30° clockwise and simultaneously, at the same
rate, move the differential synchro rotor 30° counter-clockwise, the servo motor would not run
because the control synchro field would not move.
AA Form TO-19
POSITION CONTROL SERVOMECHANISM
A block schematic diagram of a position control servomechanism is illustrated in the figure,
and from this it will be noted that it is one in which a load has to be rotated through an output
angle o corresponding to an input angle i of a controlling shaft. The controlling shaft is, in
this example, mechanically couples to the wiper arm of a potentiometer, the signal output of
which is fed to a servomotor via an amplifier. The output angle of the load is measured by a
second potentiometer whose wiper arm is mechanically coupled to an output shaft.
The potentiometers are electrically connected such that when their wiper arms occupy
corresponding angular positions the servomechanism is in a ‘null’ or zero signal condition.
When it is required to move the load to a particular angular position (o) the controlling shaft
is rotated through the appropriate number of degrees; thus the mechanism is no longer at
‘null’ and an error signal corresponding to angle i is produced and fed to the amplifier. The
amplifier has an amplification factor of K, and therefore the input to the servomotor is
increased to Ki. As the motor positions the load, the output shaft rotates the wiper arm of the
second potentiometer to produce a signal corresponding to an angle o. This signal is fed
back to the amplifier thereby reducing the input error signal to the amplifier so that the real
output from this unit to the servomotor is K(i- o). When the load finally reaches the position
required, the servomechanism will then be at a new ‘null’ condition.
The amplifier has an amplification factor of K, and therefore the input to the servomotor is
increased to Ki. As the motor positions the load, the output shaft rotates the wiper arm of the
second potentiometer to produce a signal corresponding to an angle o. This signal is fed
back to the amplifier thereby reducing the input error signal to the amplifier so that the real
output from this unit to the servomotor is K(i- o). When the load finally reaches the position
required, the servomechanism will then be at a new ‘null’ condition.
AA Form TO-19
SPEED CONTROL SERVOMECHANISM
A speed control servomechanism is one in which error signals are produced as a result of a
difference between voltages corresponding to input and output speeds, such signals being
used to control the speed of the servomotor and load. Referring to the figure, it will be noted
that the system differs from that used for position control in that the servomotor also drives a
device known as a tachogenerator.
When it is required to operate the load, the servomotor is drive by an amplified input error
voltage, Vi, and the motor accelerates the load towards the required speed. At the same time,
the motor drives the tachogenerator which produces an output voltage, Vo, in proportion to its
speed of rotation. The output voltage is fed back to the amplifier thereby reducing the input
error voltage and so producing a real output from the amplifier equal to K(Vi-Vo). The
servomotor in this class of servomechanism (sometimes called a velodyne) is therefore
controlled by differences in voltages, and will speed up or slow down until the difference is
zero.
Rate Generator And Servo
In the figure we show a servo motor loop whose servo drives a tachometer generator. The
tachometer generator output is fed into the servo amplifier. This illustrates a typical and very
common use of tachometer generators in servo motor systems.
Sometimes there tachometer generator is not drawn in the schematic, even though it exists
and is used in the servo system. Other times the fixed field portion of the tachometer
generator is omitted and only the output winding shown.
A tachometer generator is always connected to the servo amplifier so that when the motor
runs, the generator supplies an opposition signal. It can never stop the servo motor, of
course, because as soon as the servo motor stops, the tachometer generator has no output.
But, the faster the motor runs, the greater the opposition. In this way it provides inverse
feedback for speed limiting and smoothing.
AA Form TO-19
Response Of Servomechanism
The response to servomechanism is the pattern of behaviour of the load when a change is
made to the input condition, the most important factors being the form which the input change
takes and the various restraints, friction, etc, which act on the output. There are two types of
input change to be considered and these are referred to as step input, and ramp input.
Step Input
A step input is one whereby the input (e.g. the controlling shaft of the system shown in the
figure) is suddenly changed to a new angular position i, from a null position. Because of the
inertia of the load an angular change at the servomechanism output will not be able to follow
exactly that at the input, with the result that a large error signal is produced initially. This
causes the load to be accelerated to its required position, and thereby reduces the error to
zero.
AA Form TO-19
At this point however, and although the acceleration is zero, the load has reached a steady
rate of change, and so it overshoots resulting in an increase of error in the opposite sense to
decelerate the load until it comes to rest in the opposite direction. By this time the error signal
is equal to the original error signal but of opposite polarity, and so the load is accelerated
back towards the required position and produces another overshoot. If the frictional losses in
the system are negligible, a continuous oscillation is produced.
Ramp Input
A ramp input is one whereby the input is suddenly moved at a constant (rate), speed. In the
early stages of the input, and while the error signal is small, the load accelerates slowly and
lags behind the input. The signal increases as the lag increases, thereby building up the
acceleration.
AA Form TO-19
SERVOMECHANISM PERFORMANCE
An ideal servomechanism would maintain a condition of no error regardless of the speed and
direction of changes at the command input. At all times the output would exactly correspond
with the input. This ideal cannot be fully met because corrective action does not start until an
error occurs. In practice, close to the ideal can be achieved if the servomechanism is
 sensitive enough to respond to small errors fast in its response so that it quickly
corrects errors and
 stable in that it does not overshoot or hunt the null.
Deadbands, time lags, overshoots and hunting are unwanted in servomechanisms.
Deadband
A deadband is the amount of error that can exist without correction. It is caused by friction.
There may be insufficient amplification of small error signals to operate the servo against the
friction of itself, the reduction gearing and the load. The command signal may have to be
above a minimum value before motion starts, and when satisfying the command the motion
may stop when the error signal falls below that minimum value, and before the null is
reached. The region of no motion each side of null is the deadband. It can be narrowed by
reducing friction, increasing the amplification gain, or integrating the error signal.
Time Lag
‘Time lag’ is caused by friction and inertia of the load and servo. When a command signal is
originated or changed, the response of a servomechanism is not the instantaneous ideal. It
takes time for a servo and load to start moving and to accelerate up to operating speed.
Because of this, movement of the servo output shaft lags behind movement at the input shaft
of the command transmitter. This lag is known as ‘velocity error’. It has to exist to some
degree because without it there would be no error signal and therefore nothing to amplify to
continue driving the servo.
Velocity error increases the ‘response time’ of a system. This means that it takes longer to
satisfy the command.
In applications where commands are frequently changing, low velocity-errors and fast
response-times are essential. These can be improved by reducing friction and inertia (mass),
increasing amplifier gain, and by a powerful servo.
AA Form TO-19
Overshooting
‘Overshooting’ is caused by the momentum of the load and servo. Their momentum, which is
a function of speed and mass, carry them beyond the null at the commanded position. The
error signal then builds up in opposite sense to reverse the motion back towards null and
possibly to another overshoot in the opposite direction. See the figure.
Friction and any other retarding forces in the system diminish overshoots. But it is better if
retardation of the corrective action occurs only when the system is very close to null - ideally
so that the motion stops right on null. At other times, when the servo starts to move and
accelerates, retardation forces tend to increase the velocity error and slow the response time.
Damping
When overshoots are eliminated or diminished they are said to be ‘damped’. The figure
illustrates the effects of differing degrees of damping. With slight ‘underdamping’ the output
quickly reaches the commanded position followed by a slight overshoot before settling to the
null. ‘Critical damping’ has one overshoot but it has a longer time response. With
‘overdamping’ the time response is too slow.
AA Form TO-19
Hunting
Overshooting that persists with undiminished amplitude, as in the figure, is termed hunting. It
is unlikely that a system would be designed to hunt. It may occur however in a malfunctioning
system that has lost its designed damping.
Desired Servo Performance

The desired performance in a servomechanism is fast response time with little or no
overshoot, no hunting, and little or no dead band.
Response time is shortened by having
 high amplification,
 a powerful servo and
 low friction and inertia
All of the above except low inertia will narrow the deadband. High inertia, because it
increases momentum, could narrow the deadband by carrying the system more towards its
null.
Overshoots can be eliminated or much reduced by damping, and with little or no overshoot,
hunting does not occur.
AA Form TO-19
Frictional Damping Devices
Damping can be in the form of mechanical or ‘viscous frictional’ devices that add to a
system’s inherent friction. The latter are more common. There are oil and eddy current types.
An example of the oil type is the driving by the servo of a paddle wheel in a chamber of oil.
The viscous friction damps the servo.
The ‘eddy current damper’ is a cup or disc of copper or aluminium that rotates in a magnetic
field. When driven by the servo the cup or disc has eddy currents induced in it through its
interaction with the magnetic field. By Lenz’s law the flux of the induced eddy currents
creates a retarding force by opposing the original field.
Although frictional damping can reduce or eliminate overshoots, it has the following
disadvantages
 Lengthens the time response
 Causes or widens a deadband
 Reduces sensitivity
 Consumes power
 Produces heat.
These may not be significant disadvantages in some light-duty applications, but in the bigger
systems any method of damping at the output end has problems because that is the power
end, and damping applied there absorbs relatively large amounts of energy. This is not so
with damping applied at the input and where power levels are low. Damping can be
accomplished there by modifying the error signal with negative feedback from the output end.
Velocity Feedback Damping
Except of course for position follow-up, ‘velocity feedback’ by ‘rate generators’ is probably the
most common form of negative feedback used in aircraft electro-mechanical
servomechanisms. Rate generators are also called ‘tacho-generators’.
A rate ac or dc generator driven by the servo gives a voltage output proportional to servo
speed. The polarity or phase of the output voltage depends on the direction of rotation. It is
normally of opposite phase or polarity to the error signal except during that part of overshoot
before motion is reversed.
In the figure the system is static and in null, so there is no output from the rate generator, and
also there is no error signal. The feedback gain potentiometer allows adjustment for optimum
nulling with minimum overshoot.
AA Form TO-19
In the figures you can see that with either direction of rotation the feedback voltage is
identified as of opposite phase to the error signal.
Other than during overshoots the rate generator output subtracts from the error signal and
hence is negative feedback. During acceleration and steady-speed operation this serves no
useful purpose, in fact it has the disadvantage of increasing the velocity lag. However, when
the error signal starts to decrease when null is approached, if momentum maintains servo
speed, the negative feedback voltage from the rate generator will predominate. Since it is of
opposite sense to the error signal, servo torque will reverse to oppose the momentum and
stop the servo on null with little or no overshoot.
If there is overshoot the error signal reverses in sense to now assist the feedback signal to
reverse servo motion and return it to null. With the reversal of servo motion, the feedback
voltage reverses sense to retard motion as null is approached from the opposite direction.
The velocity lag caused by velocity feedback during acceleration and steady-speed operation
can be decreased by increasing the amplifier gain of the system. Systems using velocity
feedback are designed with a high amplification gain that would cause hunting except for the
damping provided by the velocity feedback.
AA Form TO-19
Error-Rate Damping
As already noted, in velocity control servomechanisms (ramp input) employing velocity
feedback damping the transient response is improved but velocity lag is increased. This can
be tolerated in certain applications, but where requirements for rotating a load at constant
speed are to be met the lag must be reduced to zero in the steady condition. This may be
achieved by adopting either of two methods which in each case produce the same result, i.e.
cancelling the velocity feedback signal when the input and output velocities are equal.
One method see figure is to fit a second tachogenerator at the input so that it feeds a signal
forward into the amplifier thus making the net input an error voltage plus a voltage
proportional to input shaft speed minus the velocity feedback voltage. During a ramp input a
steady state is eventually reached in which the tachogenerators apply equal and opposite
voltages to the amplifier, the net input is therefore zero. If any velocity lag exists at this stage,
the position error signal will establish torques at the servomotor to reduce it.
The foregoing method although eliminating velocity lag, presents the difficulty of ensuring
that the voltage outputs of both tachogenerators will remain constant over a long period of
time. Since the velocity of an error is equal to its rate of change, with respect to time, i.e. the
differential of the error, then by combining a differential signal with the actual error signal at
the amplifier input, the same final result will be obtained as when using two tachogenerators.
In the second method, therefore, the tachogenerators are dispenses with and are replaced by
a resistance- capacitance differentiating network as shown in the figure.
AA Form TO-19
Acceleration Feedback
That part of velocity lag caused by velocity feedback can be eliminated by differentiating the
velocity feedback. This changes it to ‘acceleration’ or ‘transient velocity’ feedback. A
differentiator in the feedback
 blocks steady-state output from the rate generator
 treats a rising velocity-signal during acceleration to make it into positive feedback that
assists the acceleration and
 treats a falling velocity-signal during deceleration to make it into negative feedback
that assists the deceleration.
In the figure is a block diagram of a servomechanism with a differentiator in the rate feedback
loop. Remember that it changes a velocity rate signal into an acceleration rate signal.
AA Form TO-19
Phase Advance Damping
A suitable transient response in a remote position control system and a good steady state
response in a velocity system can be obtained by inserting a resistance-capacitance network
in the input to the amplifier, as shown in the figure. With this arrangement, the output signal is
 degrees in advance of the input signal.
When a position control system is subjected to a step function input, the error rises
immediately to its maximum value because of the inertia of the system. Initially, therefore,
since the capacitor ‘C’ cannot charge instantaneously (due to its time constant) the full error
voltage is developed across R2 and is applied to the amplifier, causing the motor to
accelerate rapidly. As the capacitor is charges the voltage across it rises and the input to the
amplifier falls, thus reducing the motor torque.
As the load reaches the required position, the error voltage falls. However, if the values of the
components of the phase advance network have been carefully chosen, the charge acquired
by the capacitor during the initial period will cause the voltage across it to exceed the error
voltage. Thus the voltage applied to the amplifier is now negative despite the slightly positive
error voltage. This means that a retarding torque is applied to the load before it reaches its
required position; overshooting is therefore prevented and stability during the transient period
is improved.
For a ramp function input the phase advance network gives an almost zero error in the
steady state, i.e. it virtually eliminates velocity lag. In the steady state there is neither
acceleration nor deceleration and zero torque is required. For this condition to be satisfied,
the input to the amplifier must be zero, i.e. zero error.
AA Form TO-19
Integral Control
The methods so far described reduce velocity lag, but have no effect on lag and ‘dead space’
(dead band) caused by inherent friction. A commonly used method of dealing with these
residual steady state errors is known as integral control. The arrangement as used in
conjunction with feed forward of error rate is shown in the figure.
The differentiator operates in the same manner as that used for error rate and transient
velocity damping, but the conditions are modified by the inclusion of an integrator which
feeds the time integral of the error signal into the amplifier. differentiator output ensures
stability.
The effect on the transient response is negligible, but as the error settles to its steady state
so its integral increases, superimposing on the amplifier a signal which provides additional
torque at the load. The load is moved by this torque towards the correct position.
Adjustment of the proportion of the integrator output can be made to ensure, when the error
signal is zero, that the subsequent constant integrator output is just sufficient to counter the
inherent friction. Thus, velocity lag is zero. For a step input the dead space error signal is
integrated until large enough to zero the error, and adjustment of the damping differentiator
output ensures stability.
AA Form TO-19
DISPLACEMENT AND RATE CONTROLS
Servomechanisms in which the displacement or movement at the output is proportional to the
displacement or movement at the input. These are ‘displacement/displacement’ systems.
Other systems may require that the rate of change at the output be proportional to the rate of
change at the input. These are ‘rate/rate’ systems.
There are also ‘rate/displacement’ systems that have an amount of displacement at the
output proportional to the rate of change at the input, and ‘displacement/rate’ systems that
have a rate of change at the output proportional to the amount of displacement at the input.
Some control systems are various combinations of the above. A common arrangement on
aircraft is to have both displacement and rate control of a system. Such a system has
displacement feedback (follow-up), plus rate feedback. The rate feedback is proportional to
the velocity at the output, and it opposes the error signal. Its purpose is to eliminate
overshoot by reducing output velocity when the system is close to null.
Systems may also have both displacement and rate detection of error signals. The speed of
error correction may then be proportional not only to the amount of error, but also to the rate
at which the error or shift from null occurs.
ANALOGUE TRANSDUCERS
Closed loop control systems must include the means to measure the variables being
monitored or controlled in the system. These variables are many in number and might include
one or more of the following types:
• Force
• Level
• Pressure
• Flow rate
• Temperature
• Velocity
• Displacement
They are analogue devices in that their outputs in terms such as voltage, current, frequency,
pressure etc., are varying indications of the conditions they represent such as mechanical
displacement, volume, weight, speed, flow, temperature, pressure etc.
AA Form TO-19
Devices capable of translating physical variables into an equivalent electrical variable are
called electrical transducers, or for ease just transducers.
Most transducers are of the passive variety, requiring an external source of electrical
excitation for their operation. Active transducer develop an output without external excitation.
The thermocouple is such a device and is used for temperature measurement.
Most transducers are inherently analogue in nature. Commonly used analogue signals used
in industry are:
• 4 – 20 mA
• 0 – 10 volt
AA Form TO-19
Advantages of servomechanisms
 remote control of systems
 small command input signals used
 multiple input commands possible
 can be fully automated or integrated into computerised control systems
 can be used in explosive liquid or gas environments
 can be used underwater.
Limitations of servo systems
 electrical, hydraulic or pneumatic power required at all times to provide operation
 component failure can disable system
 difficult to override in the event of a system failure.
AA Form TO-19

EASA Module4

Uploaded by

Copyright:

Available Formats

EASA Module4

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

EASA Module4

Uploaded by

Copyright:

Available Formats

Student Resource

Copyright © 2008 Aviation Australia

Issue B – January 2008 Revision 1 B2-4B - i

Issue B – January 2008 Revision 1 B2-4B - ii

JEPPESEN Sanderson Training Products:

Issue B – January 2008 Revision 1 B2-4B - iii

This page intentionally left blank

Issue B – January 2008 Revision 1 B2-4B - iv

Topic 4.2 Printed Circuit Boards

Topic 4.3 Servomechanisms

Issue B – January 2008 Revision 1 B2-4B - v

This page intentionally left blank

Issue B – January 2008 Revision 1 B2-4B - vi

TOPIC 4.1.3.2 – SEMICONDUCTORS – INTEGRATED CIRCUITS

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

This integrated circuit was produced in about 1960 by Fairchild Semiconductor. It is a

B2-4.1.3.2 Integrated Circuits

The L7800 series of three-terminal positive regulators is available in TO-220 ISOWATT220

B2-4.1.3.2 Integrated Circuits

OPERATIONAL AMPLIFIERS (OPAMPS)

B2-4.1.3.2 Integrated Circuits

The Ideal Op-Amp

First, the ideal op-amp has

Characteristics of a practical op-amp are

B2-4.1.3.2 Integrated Circuits

INPUT SIGNAL MODES

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

Why Use Negative Feedback?

B2-4.1.3.2 Integrated Circuits

OPAMPS With Negative Feedback

Closed-Loop Voltage Gain, Acl

B2-4.1.3.2 Integrated Circuits

The closed loop gain of a non-inverting amplifier is determined by the formula

B2-4.1.3.2 Integrated Circuits

The formula for determining gain is

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

B2-4.1.3.2 Integrated Circuits

Page Intentionally Left Blank

B2-4.1.3.2 Integrated Circuits

TOPIC 4.2 – PRINTED CIRCUIT BOARDS

B2-4.2 Printed Circuit Boards

Solder Mask and Silk Screen

B2-4.2 Printed Circuit Boards

B2-4.2 Printed Circuit Boards

B2-4.2 Printed Circuit Boards

B2-4.2 Printed Circuit Boards

B2-4.2 Printed Circuit Boards