This document discusses displacement measurement in industries. It describes three main types of electromechanical transducers used for displacement measurement: 1) variable resistance type (potentiometer), 2) variable inductance type (LVDT), and 3) variable capacitance type. It then focuses on describing the resistive potentiometer and inductive LVDT transducers in more detail, covering their construction, working principles, advantages, and disadvantages.

1. Shroff S.R. Rotary Institute of Chemical Technology
Principal Supporter & Sponsor- UPL LTD./ Shroff family
Managed By Ankleshwar Rotary Education Society
Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated
Department of Electrical Engineering
Active Learning Assignment (ALA)
NAME OF TOPIC :- DISPLACEMENT MEASUREMENT
NAME OF COURSE :- INDUSTRIAL INSTRUMENTATION (2170913)
STUDENT NAME ENROLLMENT NO.
CHAUHAN SACHIN V. 170993109001
GAMIT SATISH C. 160993109005
PRAJAPATI DHAVAL R. 160993109014
VANSIYA DIVYARAJSINH K. 160993109023
Guided By: Mr. HIREN JARIWALA 1

2. Displacement Measurement
Introduction
 Displacement measurement is important in industries.
 Displacement means change in position for a body with respect to some reference point.
 Displacement can be measured by both mechanical and electrical methods, but electrical
methods are mostly used in industries.
 The various types of electromechanical transducer are used to convert the displacement
into equivalent voltage or current signal.
 Displacement is a vector representing a change in position of a body or a point with
respect to a reference.
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3. The various displacement transducers are;
1. Variable resistance type transducer (Resistive potentiometer)
2. Variable inductance type transducer
3. Variable capacitance type transducer
4. Synchros and resolvers
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4. 1. Resistive Transducers
• Potentiometer is one of most commonly used devise for measurement of the
displacement of the body.
• It is of resistive type because it works on the principle of change of resistance of the wire
with its length. The resistance of the wire is directly proportional to the length of the
wire, thus as the length of the wire changes the resistance of the wire also changes.
• The potentiometer is an electric circuit in which the resistance can be changed manually
by the sliding contacts. The typical potentiometer is shown in the figure below. Here the
voltage Vs is applied across the two points of the wire A and B. C is the variable contact
point between A and B and its position can be changed by the sliding contact. The
voltage Vo is measured between the points A and C.
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5. • As per the resistance law of the conductor, the resistance of the conductor AC changes
as the length of the wire AC changes.
• Accordingly the output voltage Vo between A and C also changes.
• The point C is the slider whose position is changed by the operator or by the motion of
the body whose displacement is to be measured. The relationship between the length of
the conductors and the voltage across them can be expressed as:
• Vo/Vs = AC/AB
Advantages
• Cost-effective
• Simple design and simple working
• Can be used for measuring even large displacements.
• The device produces a large output and hence can be used for control purposes
without further amplification steps. Thus the whole operation is bounded to a single
device.
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6. • Can produce a high electrical efficiency.
• Except wire wound, all other potentiometers can provide excellent resolutions.
Disadvantages
• A huge force may be required for the slider movement.
• Can produce unwanted noise due to alignment problems, wear and tear of the sliding
contact. This may also affect the total life of the device.
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7. 2. Inductive Transducers
This type of transducer is used for finding the linear displacement in terms of voltage or
other digital parameters.
Linear Voltage Differential Transformer (LVDT)
Like other inductive transducers, this transducer is also used for converting a linear motion
into an electrical signal. The basic construction of an LVDT is explained and shown in the
figure below.
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8. Construction
• The device consists of a primary winding (P) and two secondary windings named S1
and S2.
• Both of them are wound on one cylindrical former, side by side, and they have equal
number of turns. Their arrangement is such that they maintain symmetry with either
side of the primary winding (P).
• A movable soft iron core is placed parallel to the axis of the cylindrical former. An arm
is connected to the other end of the soft iron core and it moves according to the
displacement produced.
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9. Working
• As shown in the figure, an ac voltage with a frequency between (50-400) Hz is supplied
to the primary winding. Thus, two voltages VS1 and VS2 are obtained at the two
secondary windings S1 and S2 respectively. The output voltage will be the difference
between the two voltages (VS1-VS2) as they are combined in series.
• Let us consider three different positions of the soft iron core inside the former.
Null Position:
• This is also called the central position as the soft iron core will remain in the exact
center of the former. Thus the linking magnetic flux produced in the two secondary
windings will be equal. The voltage induced because of them will also be equal. Thus
the resulting voltage VS1-VS2 = 0.
Right of Null Position
In this position, the linking flux at the
winding S2 has a value more than the
linking flux at the winding S1. Thus, the
resulting voltage VS1-VS2 will be in phase
with VS2.
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10. Left of Null Position:
• In this position, the linking flux at the winding S2 has a value less than the linking flux at
the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS1.
• From the working it is clear that the difference in voltage, VS1-VS2 will depend on the
right or left shift of the core from the null position.
• Also, the resulting voltage is in phase with the primary winding voltage for the change of
the arm in one direction, and is 180 degrees out of phase for the change of the arm
position in the other direction.
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11. Advantages
• Maintains a linear relationship between the voltage difference output and displacement
from each position of the core for a displacement of about 4 millimeter.
• Produces a high resolution of more than 10 millimeter.
• Produces a high sensitivity of more than 40 volts/millimeter.
• Small in size and weighs less. It is rugged in design and can also be assigned easily.
• Produces low hysteresis and thus has easy repeatability.
Disadvantages
• The whole circuit is to be shielded as the accuracy can be affected by external magnetic
field.
• The displacement may produce vibrations which may affect the performance of the device.
• Produces output with less power.
• The efficiency of the device is easily affected by temperature. An increase in temperature
causes a phase shift. This can be decreased to a certain extent by placing a capacitor across
either one of the secondary windings.
• A demodulator will be needed to obtain a d.c output.
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