ELECTRONIC INSTRUMENTATION (EC)

(EC 452 A ) LECTURE 6

PROF O P VYAS
TOPIC

• Transducers: Construction, characteristics and circuits for common types of resistive,

capacitive, inductive, magneto-structive; piezo-electric. Photo-electric and thermo-
electric transducers for measurement of process physical variables. Various sensing
elements and transducers for measurement of Force, Pressure, Humidity, Moisture, strain,
Velocity, Acceleration and pH. Inductive and Capacitive proximity switches. Physical and
electrical loading of and by the transducer Systems.
INDUCTIVE TRANSDUCERS

• LVDT (Linear Voltage Differential Transformer) Most important in Inductive

transducers.
 THIRD B.E. EXAMINATION, 2018
(EC/EEE/ECC ENGINEERING) (VI SEMESTER) (EXCEPT EC/EEE/ECC)
EC-391 A: OPEN ELECTIVE-III: ELECTRONICS INSTRUMENTATION (EC/EEE/ECC)

•Ques 1.
(a) Explain the construction, principle and operation of LVDT. Show characteristic curves.
State advantages and disadvantages of LVDT. (10)
•Ques 5.
(a) Describe the working of capacitive transducer. Also discuss its advantages and
disadvantages. (10)
(b) A capacitive transducer uses two quartz diaphragms of area 750mm 2 separated by a distance
of 3.5mm. A pressure of 900 KN/M2 when applied to the top diaphragm produces a
deflection of 0.6mm. The capacitance is 370PF when no pressure is applied to the
diaphragm. Find the value of capacitance after the application of a pressure of 900KN/m².
(4)
• Q 5 (b)
• Sensitivity and linearity are two conflicting parameters in a resistance potential divider?
Explain. (6)
•Ques 7.
•Write short notes on any three:
(a) Inductive method of measurement of pressure.(7)
INDUCTIVE TRANSDUCERS

•VARIABLE INDUCTANCE TRANSDUCERS

•The variable inductance transducers work, generally, upon one of the following three
principles:
•(i) change of self inductance,
•(ii) change of mutual inductance, and
•(iii) production of eddy currents,
CHANGE OF SELF INDUCTANCE

•The self inductance of a coil is

• L = N2 / R
•Here N = number of turns and R = reluctance of the magnetic circuit.
•Also, the reluctance of the magnetic circuit is R = l/ µ A
•Therefor Inductance L = N2 µ (A / l) = N2 µ G
•Where µ = effective permeability of the medium in and around the coil; H/m
•G = A / l = geometric form factor, A = area of cross-section of coil; m 2 and
•l = length of coil; m
CHANGE OF SELF INDUCTANCE

•It is clear from the above discussion that the variation in inductance may be caused by:
•Change in number of turns, N,
•Change in geometric configurations, G, and
•Change in permeability, µ.

•Inductive transducers are mainly used for measurement of displacement. The displacement
to be measured is arranged to cause variation of any of these three variables and thus alter
the self inductance L by ∆L.
DIFFERENT TYPES OF INDUCTIVE
TRANSDUCERS
DIFFERENT TYPES OF INDUCTIVE
TRANSDUCERS
DIFFERENT TYPES OF INDUCTIVE TRANSDUCERS
DIFFERENT
TYPES OF
INDUCTIVE
TRANSDUCERS
DIFFERENTIAL OUTPUT INDUCTIVE TRANSDUCERS

• The transducers can be designed to provide two outputs, one of which represents increase
in inductance (self or mutual) and the other the decrease in inductance (self or mutual).
• The difference between these outputs i.e. 2 ∆L This is known as the differential output.
• The differential arrangement consists of a coil which is divided into two parts. In
response to a physical signal, which is normally a displacement, the inductance of one
part increases from L to L+ ∆ L while that of the other part decreases from L to L- ∆L.
The change is measured as the difference of the two resulting in an output of 2 ∆L instead
∆L when only a single winding is used.
ADVANTAGES OF DIFFERENTIAL OUTPUT

•[a] The sensitivity and accuracy are increased.

•[b] The output is less affected by external magnetic fields.
•[c] The effective variations due to temperature changes are reduced.
LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

• Principal of working:
• The most widely used inductive transducer to translate the linear motion into electrical
signals is the linear variable differential transformer (LVDT). The basic construction of
LVDT consists of a single primary winding P and two secondary windings S 1 and S2
wound on a cylindrical former. The secondary windings have equal number of turns and
are identically placed on either side of the primary winding. The primary winding is
connected to an alternating current source. A movable soft iron core is placed inside the
former. The displacement to be measured is applied to the arm attached to the soft iron
core.
CONSTRUCTIONAL DETAILS OF LVDT

• In practice the core is made of high permeability, nickel iron which is hydrogen annealed.
This gives low harmonics, low null voltage and a high sensitivity. This is slotted
longitudinally to reduce eddy current losses. The assembly is placed in a stainless steel
housing and the end lids provide electrostatic and electromagnetic shielding. The
frequency of a.c. applied to primary windings may be between 50 Hz to 20 kHz.
ARRANGEMENT OF LVDT
WORKING OF LVDT

• The two secondaries S1 and S2 are

connected in series opposition as
shown in Fig. Thus the output
voltage of the transducer is the
difference of the two voltages
• E0 = E1 – E2 which will be zero
when the core is in the centre (No
input force/displacement applied).
WORKING OF LVDT

•When the core is moved either left or right by applying the force/displacement, the output
voltage magnitude will change in proportion to the displacement of the core. It will be
positive in one direction and negative in the opposite direction.
•The amount of output voltage may be measured to determine the displacement.
•The output signal may also be applied to a recorder or to a controller that can restore the
moving system to its normal position.
CHARACTERISTIC CURVE OF LVDT

• The output voltage of an LVDT is a linear

function of core displacement within a limited
range of motion, say, about 5 mm from the null
position. Figure shows the variation of output
voltage against displacement of core. The curve
is practically linear for small displacements
(upto about 5 mm). Beyond this range of
displacement, the curve starts to deviate from a
straight line.
ADVANTAGES OF LVDT

• 1. High range. The LVDTs have a very high range for measurement of displacement. This
can be used for measurement of displacements ranging from 1.25 mm to 250 mm. With a
0.25% full scale linearity, it allows measurements down to 0.003 mm. However, the
dynamic response is considerably slower than the 2.5 kHz excitation signal.
• 2. Friction and Electrical Isolation. The LVDT has many commendable features that make
it useful for a wide variety of applications. The features arise from the basic fact that
LVDT is an electrical transformer with a separable non-contacting core.
• 3. Low Power Consumption. Most of LVDTs consume power which is less than 1 W.
ADVANTAGES OF LVDT

• Frictionless transducer: Ordinarily, there is no physical contact between the movable

core and coil structure which means that the LVDT is a frictionless device. This permits
its use in critical measurements that cannot tolerate the friction loading. The absence of
friction between coil and core of an LVDT means that there is no wear out. This gives an
LVDT essentially infinite mechanical life.
• The infinite mechanical life is also important in high reliability mechanisms and systems
found in aircrafts, missiles, space vehicles and critical industrial equipment.
ADVANTAGES OF LVDT

• Electric Isolation: The fact that the LVDT is a transformer means that there is complete
isolation between excitation voltage given to the primary winding and the output
produced by the secondary windings. This makes an LVDT an effective analog
computing element without the need of buffer amplifiers.
• Ruggedness: These transducers can usually tolerate high degree of shock and vibrations
especially when the core is spring loaded without any adverse effects. They are simple in
construction and by virtue of their being small and light in weight, they are stable and
easy to align and maintain.
DISADVANTAGES OF LVDT

•Relatively large displacements are required for appreciable differential output.

•They are sensitive to stray magnetic fields but shielding is possible. This is done by
providing magnetic shields with longitudinal slots.
•The receiving instrument must be selected to operate on a.c. signals or a demodulator
network must be used if a d.c. output is required.
•The dynamic response is limited mechanically by the mass of the core and electrically by
the frequency of applied voltage.
•Temperature affects the performance of the transducer.
APPLICATIONS OF LVDT

• The LVDT can be used in all applications where displacements ranging from fraction of a mm
to a few cm have to be measured. The LVDT acting as a primary transducer converts the
displacement direct into an electrical output proportional to displacement.
• In contrast, the electrical strain gauge requires the assistance of some form of a sensing
element to act as primary transducer to convert the mechanical displacement into strain which
in turn is converted into an electrical signal by the strain gauge acting as a secondary
transducer.
• Acting as a secondary transducer LVDT can be used as a device to measure force, weight and
pressure etc. The force measurement can be done by using a load cell as the primary transducer
while fluid pressure can be measured by using Bourdon tube which acts as primary transducer.
In these applications the high sensitivity of LVDTs is a major attraction.
APPLICATIONS OF LVDT

• In Fig. (a) four LVDTs are used for

measurement of weight or pressure
exerted by liquid in a tank. They
(LVTDs) are excited in parallel to
increase the sensitivity.
APPLICATIONS OF LVDT

• Figure (b) shows two LVDTs

which are used for measurement
and control of thickness of a metal
sheet being rolled. When the
thickness equals the desired value,
the two LVDTs are balanced out.
APPLICATIONS OF LVDT

• Figure (c) shows an LVDT

being used for measurement
of tension in a cord.
PROBLEM 1

• The output of an LVDT is connected to a 5 V voltmeter through an amplifier whose

amplification factor is 250. An output of 2 mV appears across the terminals of LVDT
when the core moves through 0.5 mm. Calculate the sensitivity of the LVDT and that of
the whole set-up. The milli-voltmeter scale has 100 divisions. The scale can be read to 1/5
of a division. Calculate the resolution of the instrument in mm.
•Solution:
•Sensitivity of LVDT = output voltage / displacement = (2 x 10-3) / 0.5 = 4 x 10-3 V / mm
•Or sensitivity = 4 mV / mm
CONTINUED

• Sensitivity of instrument = amplification factor x sensitivity of LVDT

• = 250 x 4 x 10-3 V / mm = 1000 mV / mm
• 1 scale division = 5 V / 100 = 50 mV
• Minimum voltage that can be read on the voltmeter = (1/5) x 50 mV = 10 mV
• Therefor resolution of the instrument = 10 mV x (1 / 1000 mV/mm)
• = 1 / 100 mm = 0.01 mm
FURTHER STUDY

• RVDT (Rotary Variable Differential Transformer)

• CAPACITIVE TRANSDUCERS
• PIEZO-ELECTRIC TRANSDUCERS (A piezo-electric material is one in which an
electric potential appears across certain surfaces of a crystal if the dimensions of the
crystal are changed by the application of a mechanical force.)
FURTHER STUDY

• MAGNETO-ELASTIC AND MAGNETO-STRICTIVE TRANSDUCERS (A change in

mechanical stress of a ferromagnetic material causes its permeability to change. This
phenomenon is known as the Villari effect or magnetostriction and is particularly strong
in Nickel Iron alloys. The permeability changes in response to dimensional changes, such
as compression, tension or torque. This magnetic-elastic effect can cause the induction of
voltages and hence can be used in mechanic-electric transducers.)
THANKS

PROF O P VYAS