Module 2: Resistive, Capacitive & Inductive Transducer

Resistance Potentiometer

What is a Potentiometer?

A potentiometer (also known as a pot or potmeter) is defined as a 3

terminal variable resistor in which the resistance is manually varied to
control the flow of electric current. A potentiometer acts as an adjustable
voltage divider.

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How Does a Potentiometer Work?

A potentiometer is a passive electronic component. Potentiometers work

by varying the position of a sliding contact across a uniform resistance.
In a potentiometer, the entire input voltage is applied across the whole
length of the resistor, and the output voltage is the voltage drop between
the fixed and sliding contact as shown below.

A potentiometer has the two terminals of the input source fixed to the
end of the resistor. To adjust the output voltage the sliding contact gets
moved along the resistor on the output side.

This is different to a rheostat, where here one end is fixed and the sliding
terminal is connected to the circuit, as shown below.

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This is a very basic instrument used for comparing the emf of two cells
and for calibrating ammeter, voltmeter, and watt-meter. The basic
working principle of a potentiometer is quite simple. Suppose we have
connected two batteries in parallel through a galvanometer. The negative
battery terminals are connected together and positive battery terminals
are also connected together through a galvanometer as shown in the
figure below.

Here, if the electric potential of both battery cells is exactly the same,
there is no circulating current in the circuit and hence the galvanometer
shows null deflection. The working principle of potentiometer depends

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upon this phenomenon.

Now let’s think about another circuit, where a battery is connected across
a resistor via a switch and a rheostat as shown in the figure below.

The resistor has the uniform electrical resistance per unit length
throughout its length.

Hence, the voltage drop per unit length of the resistor is equal throughout

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its length. Suppose, by adjusting the rheostat we get v volt voltage drop
appearing per unit length of the resistor.

Now, the positive terminal of a standard cell is connected to point A on

the resistor and the negative terminal of the same is connected with a
galvanometer. The other end of the galvanometer is in contact with the
resistor via a sliding contact as shown in the figure above. By adjusting
this sliding end, a point like B is found where there is no current through
the galvanometer, hence no deflection in the galvanometer.

That means, emf of the standard cell is just balanced by the voltage
appearing in the resistor across points A and B. Now if the distance
between points A and B is L, then we can write emf of standard cell E =
Lv volt.

This is how a potentiometer measures the voltage between two points

(here between A and B) without taking any current component from the
circuit. This is the specialty of a potentiometer, it can measure voltage
most accurately

Applications of Potentiometer

There are many different uses of a potentiometer. The three main

applications of a potentiometer are:

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Comparing the emf of a battery cell with a standard cell

Measuring the internal resistance of a battery cell

Measuring the voltage across a branch of a circuit

Strain gauge

A strain gauge is a passive transducer, that converts mechanical

displacement into the change of resistance. A strain gauge sensor is a
thin wafer-like device that can be attached to a variety of materials to
measure applied strain. These are used as a fundamental sensor in many
types of sensors like pressure sensors, load cells, torque sensors etc.

Strain Gauge Working Principle

The foil type strain gauges (Figure #1) are very common in which a
resistive foil is mounted on a backing material. These are available in a
variety of shapes and sizes for different applications. The resistance of
the foil changes as the material to which the gauge is attached undergoes

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tension or compression due to change in its length and diameter.

This change in resistance is proportional to the applied strain. As this

change in resistance is very small in magnitude so its effect can be only
sensed by a Wheatstone bridge. This is the basic strain gauge working
principle.

A circuit diagram is shown in Figure #2. In this circuit diagram, a strain

gauge is connected into a Wheatstone bridge. This circuit is so designed
that when no force is applied to the strain gauge, R1 is equal to R2 and
the resistance of the strain gauge is equal to R3. In this condition the
Wheatstone bridge is balanced and the voltmeter shows no deflection.

But when strain is applied to the strain gauge, the resistance of the strain
gauge sensor changes, the Wheatstone bridge becomes unbalanced, a
current flows through the voltmeter. Since the net change in the
resistance is proportional to the applied strain, therefore, resultant current
flow through the voltmeter is proportional to the applied strain. So, the

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voltmeter can be calibrated in terms of strain or force.

In the above circuit, we have used only one strain gauge. This is known
as ‘quarter bridge’ circuit. We can also use two strain gauges or even
four strain gauges in this circuit. Then this circuit is called ‘half bridge’
and ‘full bridge’ respectively. The full bridge circuit provides greater
sensitivity and least temperature variation errors.

Gauge Factor of Strain Gauge

The gauge factor of strain gauge is defined as the unit change in

resistance per unit change in length.

i.e. gauge factor Gf = (∆R/R)/( ∆l/l)

where, R = nominal gauge resistance,

∆R = change in resistance,

l = length of the specimen in an unstressed condition,

∆l = change in specimen length.

It can be proved mathematically,

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Gauge factor, Gf = 1 + 2v + (∆ρ/ρ)/(∆L/L)

If the change in resistivity due to strain is almost negligible, then

gauge factor of strain gauge, Gf = 1 + 2v

Where, v is Poisson’s ratio. It may be defined as the ratio of strain in the

lateral direction to the strain in the axial direction. The Poisson’s ratio for
most metals lies in the range of 0 to 0.5 and this gives a gauge factor of 2
approximately.

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10
Resistance thermometr/RTD:

"RTD" is an abbreviation for "Resistance Temperature Detector" An

RTD is a type of temperature sensor which can be utilised in the
manufacture of Variohms' temperature probe range.

They are available with different temperature / resistance values

depending on the application requirement.

How Does an RTD Work?

An RTD consists of a resistance element and insulated copper wires. The

most common number of wires is 2; however some RTDs have 3 or 4
wires. The resistive element is the temperature sensing element of the
RTD. It is usually platinum because as a material it is highly stable over
time, it has a wide temperature range, it offers an almost linear
relationship between temperature and resistance and it has a chemical
inertness. Nickle or copper are also other popular choices of material for
the resistive element.

An RTD works by using a basic principle; as the temperature of a

metal increases, so does the resistance to the flow of electricity. An

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electrical current is passed through the sensor, the resistance element
is used to measure the resistance of the current being passed through
it. As the temperature of the resistance element increases the electrical
resistance also increases. The electrical resistance is measured in Ohms.
The resistance value can then be converted into temperature based on the
characteristics of the element. Typical response time for an RTD is
between 0.5 and 5 seconds making them suitable to applications where
an immediate response is not required.

Benefits of using RTD:

RTDs are used within different industries including; automotive, white

goods, marine and industrial applications. Benefits of using RTDs over
other temperature sensors are;

· Highly accurate

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· Consistent

· Offer long term stability

· High repeatability

· Suitable for extreme environments

· Have a high temperature range (depending on resistance element

material)

Thermistor

The Thermistor or simply Thermally Sensitive Resistor is a temperature sensor that works on
the principle of varying resistance with temperature. They are made of semiconducting
materials. The circuit symbol of the thermistor is shown in the figure.

Construction of Thermistor

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A thermistor is made of oxides of metals such as Nickel, Manganese, Cobalt, Copper,
Uranium etc. It is available in a variety of shapes and sizes. Commonly used for
configurations are Disk type, Bead type and Rod type.

The disc type thermistor and rod type thermistor is used when greater power dissipation is
required. The rod type thermistor has high power handling capacity.

The smallest thermistor in these configurations is the bead type thermistor. its diameter is
low as 0.15 mm. The measurement element is typically encapsulated in a glass probe. It is
commonly used for measuring the temperature of liquids.

Working Principle of Thermistors

The thermistor works on the simple principle of change in resistance due to a change in
temperature. When the ambient temperature changes the thermistor starts self-heating its
elements. its resistance value is changed with respect to this change in temperature. This
change depends on the type of thermistor used. The resistance temperature characteristics
of different types of thermistors are given in the following section.

Types of Thermistors

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The two basic types of thermistors available are the NTC and PTC types.

NTC Thermistor

NTC stands for Negative Temperature coefficient. They are ceramic semiconductors that
have a high Negative Temperature Coefficient of resistance. The resistance of an NTC will
decrease with increasing temperature in a non-linear manner.

Circuit symbols of NTC and PTC thermistors are shown in the following figure.

The temperature resistance characteristics of an NTC is shown in the following figure.

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PTC Thermistor

PTC thermistors are Positive Temperature Coefficient resistors and are made of
polycrystalline ceramic materials. The resistance of a PTC will increase with increasing
temperature in a non-linear manner. The PTC thermistor shows only a small change of
resistance with temperature until the switching point(TR) is reached.

The temperature resistance characteristics of a PTC is shown in the following figure.

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Advantages of thermistors

⦁ Less expensive.

⦁ More sensitive than other sensors.

⦁ Fast response.

⦁ Small in size.

Dis-advantages of thermistors

⦁ Limited Temperature range.

⦁ Resistance to temperature ratio correlation is non-linear.

⦁ An inaccurate measurement may be obtained due to the self-heating effect.

⦁ Fragile.

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Applications of thermistors

NTC Thermistor Application

⦁ Digital Thermostats.

⦁ Thermometers.

⦁ Battery pack temperature monitors.

⦁ In-rush-current limiting devices

PTC Thermistor Application

⦁ Over-current protection

⦁ In-rush-current protection

LVDT (Linear Variable Differential Transformer):

It is an important type of inductive transducer; those transducers that

work on the principle of transduction mechanism are known as inductive
transducers. LVDTs are considered the most accurate inductive
transducer to measure the linear displacement from the polarity and
magnitude of the net induced electromotive force (emf), which is why
they are also known as the Linear Variable Displacement Transducer.
Basically, LVDT is a position sensor that can sense and convert the
linear motion or vibrations into electrical signals or a variable electrical
current in the circuit.

Construction of LVDT:

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The structure of LVDT is similar to the transformer; it consists of one
primary winding, i.e., P and two secondary windings, i.e., S1 and S2. The
primary and secondary windings are wounded on a hollow cylindrical
shaped structure, called former. The former is usually made of glass-
reinforced polymer wrapped in a highly permeable material and then
covered with cylindrical steel. The primary winding is at the centre of the
cylindrical former and the secondary windings are present on both sides
of the primary winding at an equal distance from the centre. Both the
secondary windings consist of an equal number of turns, and they are
linked with each other in series opposition, i.e., they are wounded in
opposite directions but are connected in series with each other. The
series-opposed connection ensures that the induced emf in both the
secondary coils opposes each other. The primary winding is connected
with the constant source of the AC power supply whose values ranges
from 50 Hz to 20 kHz. This whole coil assembly remains stationary
during the linear distance measurement process. The movable part of
LVDT is a separate arm that is made up of a magnetic material. It is
usually a soft iron core, which is laminated to reduce the losses due to
eddy current. The core can freely move within the hollow coil (former),
and the object whose displacement is to be measured is attached to the
core through a non-magnetic rod. The hollow former has a larger radial
diameter than that of the core to ensure zero physical contact between
them so that the coil can easily move within the former.

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Working Principle of LVDT:

The working of LVDT is based on the principle of Faraday’s law of

electromagnetic induction that states that “the net induced emf in the
circuit is directly proportional to the rate of change of magnetic flux
across the circuit, and the magnetic flux of the coil wounded with wires
can be changed by moving a bar magnet through the coil.”As the primary
winding of the LVDT is connected to the AC power supply, the
alternating magnetic field is produced in the primary winding, which
results in the induced emf the secondary windings. Let us assume that the
induced voltages in the secondary windings S1 and S2 be E1 and E2
respectively. Now, according to Faraday’s Law, the rate of change of
magnetic flux, i.e., dØ/dt is directly proportional to the magnitude of
induced emf’s, i.e., E1 and E2. Hence, the induced emf in the secondary
windings will be more if the value of ‘dt’ will be low (dØ/dt ∝ E1 and
E2), and the low value of ‘dt’ implies that the soft iron core present
inside the LVDT is moving faster. Thus, emf of large magnitude will
induce in the secondary windings S1 and S2 if the movement of the core

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is faster inside the LVDT.

As we have discussed in the construction above that both the secondary

winding S1 and S2 are connected in series with each other but in
opposite phases, due to this phase opposition connection, the total output
voltage (Eo) in the circuit will be given by,

E0= E1 -E2

Characteristics of LVDT with respect to the Position of the Core

The net emf induced in the circuit depends upon the position of the
movable core, let us discuss the three different cases according to the
position of the core.

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CASE 1: Core at the Null Position

As both of the secondary windings have an equal number of turns, and

they are placed at an equal distance from the primary winding, hence at
the normal position when the core is placed at the centre, the rate of
change of magnetic flux will be the same in both the secondary windings.
This implies that the induced emf’s E1 and E2 in the secondary windings
S1 and S2 respectively will be the same, i.e., E1=E2. Hence, the net
induced emf (Eo) in the circuit at the normal position of the core is zero
(E1-E2=0). The normal position of the soft iron core at which the net
induced emf is zero is called the ‘Null Position’ of the LVDT.

CASE 2: Core at the Left of Null Position

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When the position of the core is displaced from the null position, it will
result in the electromagnetic imbalance between the secondary windings,
and a differential AC voltage will generate across the output terminal of
the secondary coils. If the core is moved towards the left from the null
position, the magnetic flux associated with the secondary coil S1 will
become larger than the magnetic flux associated with the coil S2, i.e., the
induced emf in coil S1 will be larger than the induced emf in coil S2.

Hence, the tool output voltage (E0) of LVDT is given by,

E0= E1 – E2 = Postive (E1 > E2)

This implies that the total output voltage of the LVDT is positive, i.e., in-
phase with that of the primary voltage.

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CASE 3: Core at the Right of Null Position

If the core is displaced from the null position and moved towards the
right, the magnetic flux associated with the winding S1 will be more than
that of the winding S2, i.e., induced emf in winding S2 will become more
than the emf induced in winding S2.

Hence, the tool output voltage (E0) of LVDT is given by,

Eo= E1 – E2 = Negative (E2 > E1)

This implies that the total output voltage of the LVDT is negative, i.e.,
out of phase (Φ={180} with that of the primary voltage.

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From all the three cases discussed above, it can be concluded that the
displacement of the body is directly proportional to the output voltage,
i.e., the more the displacement of the body, the more will be the output
voltage of LVDT. Hence, the direction of the movement of the body
attached to the core of the LVDT can find out with the help of net output
voltage obtained across the output terminal of the LVDT. One can
analyse that the body is moving away from the null position towards the
left direction if the output voltage of LVDT is positive, and if the output
voltage of the LVDT is negative it means that the body is moving
towards the right from the null position. However, if we take the core out
of the hollow structure, the output voltage of the LVDT will become
zero. It is observed that when the core is displaced from the null position
either towards the left or towards the right, up to the 5 mm displacement,
the output voltage increases linearly but after 5mm, it becomes non-
linear. Let us understand the linear range and linearity error from the
following graph, which shows the variations of the output voltage with

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respect to the displacement of the body.

Graphical Representation of the Output Voltage of LVDT with

respect to the Displacement

The above graph indicates the transfer function of the linear variable
differential transducer. The x-axis represents the displacement of the
body, and the y-axis represents the magnitude of the output voltage of
LVDT. Ideally, when the displacement is zero, the output voltage should
also be zero, but there exists a small output voltage even when the core is
at the null position because of the residual magnetism of the soft iron
core, hence it is called residual voltage of LVDT. When the core is

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moved away from the null position to either right or the left the output
voltage increases linearly with respect to the displacement of the core to
a certain value, and after that non-linear increase of output voltage is
observed.

Linear Range: As shown in the graph above, LVDT shows the linear
increase in the output voltage only for a limited range of displacement of
the core, the range up to which linear transfer function is observed is
called the linear range of LVDT. Now, let us understand that why the
output voltage is observed non-linear after a certain range of
displacement. The maximum distance that can be travelled by the core
from the null position up to which the linear transfer function can be
observed is known as the full-scale displacement. When the core is
displaced further after full-scale displacement, the magnetic flux
associated with the core due to the primary winding P becomes low,
which eventually results in the reduction of the voltage across the
secondary windings S1 and S2.

Linearity Error: Linearity error is the maximum deviation of the output

voltage from the expected straight line in the output versus displacement
graph. It is observed from the graph that the variation of the output
voltage with respect to the displacement in the linear range does not give
a perfectly straight line. The reason behind the non-linear curve even in
the linear range is due to the saturation of the soft iron core, which
results in the third harmonic component even when the core is at the null
position. The harmonic components can be repressed by using the low-

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output filter at the output terminal of the LVDT.

Sensitivity: The sensitivity of the LVDT tells about the relation between
the output voltage of LVDT and the displacement of the core. It is also
known as the transference ratio of the LVDT. The sensitivity of the
LVDT is measured when the primary AC source is kept at the particular
voltage (3 Vrms) and when the core is displaced by the full-scale
displacement from the null position, and then the voltage across the
windings S1 and S2 is measured to find out the net output voltage of the
LVDT. The sensitivity of the LVDT is then calculated by substituting the
obtained values in the following equation.

Sensitivity = V output / (Vprimary × Core Displacement)

It is expressed in the terms of mV/V/mm or mV/V/in, i.e., millivolt

output per volt of excitation per displacement of the core in
millimetres/inches.

Applications of LVDT

⦁ Apart from the measurement of displacement, LVDT can also be

used to measure other physical quantities like force, pressure, and
weight if it is used as a secondary transducer. For example, a
Bourdon tube can be used as a primary transducer that can measure
the pressure by converting it into linear displacement, and then we

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 can get the pressure reading using LVDT that converts the linear
displacement into the voltage or electrical signals.

⦁ LVDT is used in civil engineering to test the strength of the

various soil samples and rocks to be used in the construction of
buildings or bridges and to measure other factors like spring
tensions, weight, and displacement.

⦁ It is also used in the medical field for the manufacturing of pills. A

computer-controlled hybrid mechanism consisting of primary and
secondary windings transducers are used for this purpose; it
reduces human errors and accurately measures the weight and
thickness of the pills.

⦁ They also find its applications in inspecting the quality of flat

display panels by monitoring the waveform at the output terminal
of LVDT.

⦁ It is used in ṭhe aerospace industry to monitor various mechanisms

like flight control and pilot control. Various mini-positon
transducers are mounted at the fixed positions, and the moving core
is attached to the moving parts, for example, landing gears. When
the landing gears are moved, the core also displaces from the null
position, and various output electrical signals will provide the
angles, lengths, motion, and rate of the moving depending upon the
sensitivity of LVDT and the mounting system.

⦁ It is used in hydraulics for the detection of any leaks or damage to

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 objects that are submerged in non-corrosive and non-conductive
fluids. LVDT sensors are also used in robotic manipulators.

Advantages of LVDT

⦁ LVDT is a frictionless device as there is no direct contact between

the moving core and the fixed coil structure (former). This reduces
the damage to the device due to the absence of wear and tear
because of friction. Hence, the mechanical life of LVDT is quite
longer than the other devices that have friction during the working
process.

⦁ It can be used to estimate the displacement of the object ranges

from a fraction of millimetres to a few centimetres. Modern LVDTs
that can measure the displacement of broad ranges (±100μm to ±
25 cm) are widely used in laboratories and for industrial purposes.

⦁ LVDT does not require the application of an amplifier to enlarge

the signals as LVDT provides a high output signal, and it is highly
sensitive to even small displacements.

⦁ They consume very low electricity, usually less than 1 W, and they
also show less hysteresis loss, which increases their reliability.

⦁ LVDT are of small size and are very lightweight, hence they can be
easily managed and aligned as per the requirements, and despite
their small size and lightweight nature, they can bear mechanical
shocks and vibrations.

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 ⦁ The coil and core of the LVDT are magnetically coupled with each
other, and there does not exist any direct connection, hence they
can be separated from each other. This can be done by inserting a
tube made of non-magnetic material between the core and the
former; here, the pressurized fluid is added to the inserted tube.
This assembly is usually utilized in hydraulics for various
measurements.

⦁ LVDT’s are built of quality materials and techniques that can easily
withstand corrosion, pressure and extreme temperatures. The null
point of the LVDT usually remains stable even at temperatures
above its operating temperature.

⦁ As there is no friction during the operation of LVDT, hence the

position of the core can be changed rapidly which results in the
dynamic responses of the LVDT. The only thing that is considered
to limit the dynamic responsive nature of the LVDT is the mass of
the core.

⦁ The LVDT provides the absolute value. It means that the LVDT
does not lose its position data in case of abrupt power failure. The
value of the output remains the same if the measurement is restarted
as it was measured before the power failure.

Disadvantages of LVDT

⦁ The major disadvantage of the LVDt is that an additional circuit is

required to deal with the stray magnetic field produces across the
electric circuit. The stray magnetic field arises due to the inductive

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 transducer mechanism of the LVDT.

⦁ The performance of the LVDT may lag due to the unwanted

vibrations or temperature changes in the device.

⦁ The output obtained by the LVDT is AC, hence, a demodulator is

required to get the DC output.

⦁ The fast dynamic responses of the LVDT may get limited due to the
mass of the movable core or due to the frequency of the applied
primary voltage.

Capacitive Transducers:

A capacitor consists of two conductors (plates) that are electrically

isolated from one another by a nonconductor (dielectric). When the two
conductors are at different potentials (voltages), the system is capable of
storing an electric charge. The storage capability of a capacitor is
measured in farads.The principle of operation of capacitive transducers is
based upon the equation for capacitance of a parallel plate capacitor as

Where, A = Overlapping area of plates; m2,

d = Distance between two plates; m,
E = Permittivity (dielectric constant); F/m.

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The capacitance is measured with a bridge circuits. The output
impedance Z of a capacitive

transducer is:

Z = 1/2πfC

Where: Z = Impedance, f = frequency, 50 Hz, C = capacitance

In general, the output impedance of a capacitive transducer is high. This

fact calls for a careful design of the output circuitry. The capacitive
transducers work on the principle of change in capacitance of the
capacitor. This change in capacitance could be caused by change in
overlapping area A of the plates, change in the distance d between the
plates and change in dielectric constant E.

In most of the cases the above changes are caused by the physical
variables, such as, displacement, force or pressure. Variation in
capacitance is also there when the dielectric medium between the plates
changes, as in the case of measurement of liquid or gas levels.

Therefore, the capacitive transducers are commonly used for

measurement of linear displacement, by employing the following effects

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as shown in Fig.

i) Change in capacitance due to change in overlapping area of plates.

ii) Change in capacitance due to change in distance between the two

plates.

iii) Change in capacitance due to change in dielectric between the two

plates

As may be seen in above Fig., all of the differential devices have three
wire connections rather than two: one wire for each of the end plates and
one for the common plate. As the capacitance between one of the end
plates and the common plate changes, the capacitance between the other
end plate and the common plate also changes in the opposite direction.
a) Transducers Using Change in Area of Plates
Examining the equation for capacitance, it is found that the capacitance
is directly proportional to the area, A of the plates. Thus, the capacitance
changes linearly with change in area of plates. Hence this type of
capacitive transducer is useful for measurement of moderate to large
displacements say from 1 mm to several cm. The area changes linearly

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with

displacement and also the capacitance.

For a parallel plate capacitor, the capacitance is:

The sensitivity is constant and therefore there is linear relationship

between capacitance and displacement. This type of a capacitive
transducer is suitable for measurement of linear displacement ranging
from 1 to 10 cm. The accuracy is as high as 0.005%.

b) Transducers Using Change in Distance between Plates

Fig. shows the basic form of a capacitive transducer employing change in

distance between the two plates to cause the change in capacitance. One
plate is fixed and the displacement to be measured is applied to the other
plate which is movable. Since, the capacitance, C, varies inversely as the
distance d, between the plates the response of this transducer is not
linear. Thus this transducer is useful only for measurement of extremely
small displacements.

Thus the sensitivity of this type of transducer is not constant but varies
over the range of the transducer. The relationship between variations of
capacitance with variation of distance between plates is hyperbolic and
is only approximately linear over a small range of displacement. The

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linearity can be closely approximated by use of a piece of dielectric
material like mica having a high dielectric constant, such as, a thin piece
of mica.

c) Transducers Using Change in dielectric constant between Plates

If the area (A) of and the distance (d) between the plates of a capacitor
remain constant, capacitance will vary only as a function of the
dielectric constant (E) of the substance filling the gap between the
plates. If the space between the plates of a capacitor is filled with an
insulator, the capacitance of the capacitor will change compared to the
situation in which there is vacuum between the plates. The change in the
capacitance is caused by a change in the electric field between the
plates.

The value of dielectric constant is initially set by design in the choice of

dielectric material used to make the capacitor. Many factors will cause
the e to change, and this change in ewill vary for different materials. The
major factors that will cause a change in e are moisture, voltage,
frequency, and temperature. The dielectric constant of a process material
can change due to variations in temperature, moisture, humidity,
material bulk density, and particle size etc. The e in the basic formula is
the effective dielectric constant of the total space between the electrodes.
This space may consist of the dielectric material, air, and even moisture,
if present. The figure shows that how in a capacitor the position of the
dielectric is varied to vary the capacitance. Physical variables, such as,
displacement, force or pressure can cause the movement of dielectric
material in the capacitor plates, resulting in changes in the effective

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dielectric constant, which in turn will change the capacitance.

The major advantages of capacitive transducers are that they require

extremely small forces to operate them and hence are very useful for use
in small systems. They are extremely sensitive and require small power
to operate them. Owing to their good frequency response they are very
useful for dynamic studies. The disadvantages of capacitive transducers
include their non-linear behaviour on account of edge effects and the
effects of stray capacitances especially when the transducers have a low
value of capacitance. Therefore guard rings must be used to eliminate
this effect. The metallic parts of the capacitive transducers must be
insulated from each other. In order to reduce the effects of stray
capacitances, the frames must be earthed. Capacitive transducers can be
used for measurement of both linear and angular displacements. The
capacitive transducers are highly sensitive and can be used for
measurement of extremely small displacements down to the order of
molecular dimensions, i.e., 0.1x10-6 mm. On the other hand, they can be
used for measurement of large displacements up to about 30 m as in
aeroplane altimeters. The change in area method is used for
measurement of displacements ranging from 10 to 100 mm. Capacitive
transducers can be used for the measurement of force and pressure. The
force and pressure to be measured are first converted to displacement
which causes a change of capacitance. Capacitive transducers can also

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be used directly as pressure transducers in all those cases where the
dielectric constant of a medium changes with pressure. They can be used
for measurement of humidity in gases and moisture content in soil / food
products etc.

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