SENSORS

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 SENSORS

The Sensor in a control system is affected by the

phenomenon in the physical medium ( such as
Temperature, Pressure, Flow, Level etc.) either by direct or
physical contact or by indirect interaction of the
phenomenon with some portion of the sensing element.
Sensors give a signal to initiate a control system.
In case of temperature sensing by RTD the resistivity of the
sensing element is affected by the change in the
temperature of the medium.
Sensors may give either a Digital or Analogue signal
to actuate a control system
There are different kinds of sensors used to measure
different parameters in the physical medium

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 TYPES OF SENSORS
Analogue sensors give signals whose size is
proportional to the size of the variable being
monitored.
e.g. a thermocouple ,a RTD, strain gauge etc.

Digital sensors can give either discrete signals or

pulses.

Devices which give discrete signals are ones

where the signal is either on or off (either a
voltage or no voltage condition). e.g. a switch,
a P.B.

Devices which give pulses can be considered to

be essentially discrete devices which gives a
sequence of on-off signals.- e.g. Proximity
switches., encoders

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 Mechanical Switches

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 Proximity Switches

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 Photo-electric sensors & encoders

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 Thermal Sensors

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 A Bi-metal strip

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 Resistance-Based
Temperature Devices
Resistance of certain metals change with their temperature.
The two basic Resistance-base devices used for
measurement of temperature are
a. Resistance-temperature detector (RTD) based on the
variation of metal resistance with temperature.
b.The thermistor, based on variation of semiconductor
resistance with Temperature.
Thus the measure of the resistance of a metal can be used to
measure temperature.
Resistance variation of metals are approximately linear and
positive (increases with Temp)
Resistance variation of semiconductors are highly non-linear
and negative. (decreases with Temp)

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 RTDs Thermal Sensors
RTD is a temperature sensor, a metal whose resistance increases with
temperature. Materials used for these vary from Platinum ( sensitive, repeatable
but costly) to Nickel ( more sensitive, less repeatable but cheap).
Sensitivity linear fractional change in resistance with temperature
(Ohms/Degree C).
Response time Time taken by the device to come into thermal equilibrium with
the environment. Normally specified for free air condition (slow) or oil bath
(fast) conditions
Construction Wire- wound to achieve small size, improve thermal conductivity
and to decrease response time.
Signal Conditioning Since fractional change of resistance are very small,
generally used in a bridge circuit. Compensation leads are used to nullify effects
of thermal gradient on long leads.
Dissipation Constant Self heating of RTD can cause error in measurement. The
current in the RTD circuit must be kept low. Dissipation constant is normally
specified as mW/degree C.

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 RTD Signal conditioning

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 Thermocouple &
Connections

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 Thermocouple signal
with automatic reference correction

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 Linear Variable Differential Transformer
LVDT
A moving core is used to vary the magnetic flux coupling between two or
more coils.
A core of permeable material slides freely through the center of the form.
Inner Coil is the Primary and connected to an ac source. Flux formed by
the Primary is linked to the two secondary coils and induces voltage in two
secondaries.
If the secondaries are connected in Series opposition, a differential
voltage is formed.
When the core is central, the voltage is zero. When the core is moved to
one side, the net voltage will increase linearly.
There is also a change of Phase as the core is moved from one side to the
other.
The signal conditioning for LVDTs consists primarily of a circuit that
performs phase-sensitive detection of the differential secondary voltage
and gives a DC output whose magnitude and polarity varies with the
signal.

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 Displacement Sensor

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 Semiconductor Strain Gauges

As with metal strain gauge, semi-conductor such as

silicon can be used for strain measurement.
Contrary to metal sensors, the resistance of semi-
conductor varies negatively and non-linearly with
strain.
Signal conditioning is still a bridge circuit with
temperature compensation and additionally
linearization.
Typical application of Strain Gauges found in Load
Cells used for measuring large forces

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 Strain Gauges as sensors

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 Pressure Sensors-1

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 DIAPHRAGMS
A Diaphragm is used to convert pressure to deflection. In addition
to use in pressure sensors, diaphragms can serve as fluid barriers
in transmitters, as seal assemblies.

A diaphragm usually is designed so that the deflection versus

pressure characteristics are as linear as possible over a
specified pressure range, and with a minimum of hysteresis and
minimum shift in the zero point
As shown in Figure, if P1 is greater than P2 side of the diaphragm,
it is deflected toward the P2 side. The extent of this deflection
(i.e., the diaphragm displacement) is a measure of the pressure
difference

Metals commonly used for making diaphragms are trumpet brass,

phosphor bronze, beryllium copper, stainless steel,, titanium etc. In
many pressure measuring applications, the process fluid must not
contact ,in order to isolate the pressure element from corrosive
fluids
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 BOURDON TUBE
A hard metal tube, usually a type of bronze or brass, is
flattened. and one end is closed off. The tube is then
bent into a curve or arc, sometimes even a spiral.

The open end is attached to a header by which a

pressure can be introduced to the inside of the tube.
When this is done, the tube will deflect when the
inside applied pressure is different from the outside
pressure. The tube will tend to straighten out if the
inside pressure is higher than the outside pressure and
to curve more if the pressure inside is less than that
outside

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 BELLOWS
A Bellow is another device much like the diaphragm
that converts a pressure differential into a physical
displacement, except that here the displacement is
much more a straight-line expansion.

The accordion-shaped sides of the bellows are made

from thin metal.

When there is a pressure difference, a net force will

exist on the flat, front surface of the bellows. The
bellows assembly will then collapse like an accordion
if P2 is greater than PI or expand if P2 is less than PI
Again, we have a displacement which is proportional to
pressure difference
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 Pressure sensors - 2

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 Flow Sensors - 1

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 FLOW THROUGH RESTRICTION
One method of measuring the flow of liquids in pipes is by
introducing a restriction in the pipe and measuring the pressure
drop that results across the restriction.

When such a restriction is placed ,the velocity of the fluid through

the restriction increases, and the pressure in the restriction
decreases.

We find that there is a relationship between the pressure drop and

the rate of flow such that, as the flow increases, the pressure
drops.
We can find an equation of the form
Q=KP
Q=volume of flow rate
K=a constant for the pipe and liquid type
P =drop in pressure across the restriction
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DOPPLER FLOW METER

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DOPPLER FLOW METER

Doppler-effect flow meters use a transmitter that projects a
continuous ultrasonic beam at about 0.5 MHz through the
pipe wall into the flowing stream.

Particles in the stream reflect the ultrasonic radiation which

is detected by the receiver

The frequency of the radiation reaching the receiver is

shifted in proportion to the stream velocity.

The frequency difference is a measure of the flow rate.

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ELECTROMAGNETIC LOG

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ELECTROMAGNETIC LOG
The electromagnetic log is based upon the Faradays law.

A direct current through the windings of a coil generates a

magnetic field.

If the stationary conductors around the coil do not move

relative to the coil, they do not intersect the magnetic lines
of force, and no voltage is induced in them.

The direction of the voltage E depends on the direction of

velocity of the conductors

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ELECTROMAGNETIC LOG
The induced voltage is proportional to the
velocity V. Should the velocity have an opposite
direction, the direction of voltage will also
change. The coil of the electro-magnetic log is
inserted in a water-tight flow sensor, which
projects through the hull into the water below.
The conductors forming the loop are, in reality,
the surrounding water, which behaves as a
conductor, since it is saline.
The water moves relative to the coil, at the ship's
velocity V

ETHIOPIA MARITIME TRAINING INSTITUTE ELECTRICAL / CHAPTER 9 Slide No. 30 of 6

 MAGNETIC FLOW METER

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MAGNETIC FLOW METER

If charged particles move across a magnetic field, a

potential is established across the flow, perpendicular to
the magnetic field. If the flowing liquid is also a conductor
(even if not necessarily a good conductor) of electricity, the
flow can be measured by allowing the liquid to flow through
a magnetic field and measuring the transverse potential
produced

The pipe section in which this measurement is made must

be insulated, and a nonconductor itself, or the potential
produced, will be cancelled by currents in the pipe

ETHIOPIA MARITIME TRAINING INSTITUTE ELECTRICAL / CHAPTER 9 Slide No. 32 of 6

 LEVEL SENSING

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LEVEL SENSING
As shown in the figure, two concentric cylinders are contained in a
liquid tank. The level-of the liquid partially occupies the space between
the cylinders, with air in the remaining part.
***Substance between the plates is known as the dielectric and its
ability to affect the electrical charge storage capacity or capacitance of
the plates, is known as dielectric constant
***The capacitance will be least when the tank is empty and air fills
the space between the probe and the tube or tank side, and at a
maximum when the air has been displaced by the liquid and the
tank is full, since the dielectric constant of liquid is higher than that of
air. Thus, as the liquid rises and falls, the electrical charge storage
capacity of the probe and tube etc. will vary.
***Tube (sensor) connected into an a.c. bridge network, which
measures the degree of change in capacitance and hence level and
produces a signal representing this. The a.c. bridge is supplied with a
high frequency, low voltage current.

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MEASUREMENT OF TORQUE &

SHAFT HORSE POWER
The twisting or torsion of a rotating shaft can be measured
in a number of different ways to give a value of applied
torque.

Shaft power can then be calculated by multiplying the

torque by the rotational speed of the shaft.

Four strain gauges are mounted onto the shaft as shown

in figure. Twisting of the shaft as a result of applied torque
results in a change in resistance of the strain gauge
system. Brushes and slip rings are used to take off the
electrical connections and complete the circuit as shown

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MEASUREMENT OF TORQUE &

SHAFT HORSE POWER

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MEASUREMENT OF TORQUE &

SHAFT HORSE POWER
Since the shaft horse power is proportional to
torque and the rotation of the shaft per second, a
tachometer input is necessary to calculate the
shaft horse power of the main engine.

These two values are multiplied with suitable

constants in a calculator to get the horse power,
normally shown as a digital display in the control
room.

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MEASUREMENT OF SPEED BY
TACHOGENERATORS
Tachogenerators provide a voltage which is
proportional to the speed and may be AC or DC
instruments.

The DC tachogenerator is a small DC generator

with a permanent magnet field.

The output voltage is proportional to the speed

and may be measured on a voltmeter calibrated in
units of speed.

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PICKUP TACHOMETERS

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PICKUP TACHOMETERS
Various pick-up devices can be used in conjunction with a
digital counter to give a direct reading of speed.

An inductive pick-up tachometer is shown in Figure.

While its individual teeth pass the coil they induce an e.m.f.
These pulses which is appropriately modified are then fed
to a digital counter

A capacitive-type pick-up tachometer is shown in

Figure). As the rotating vane passes between the plates a
capacitance change occurs which is detected and then fed
to a digital counter.

ETHIOPIA MARITIME TRAINING INSTITUTE ELECTRICAL / CHAPTER 9 Slide No. 40 of 6