Resistive & Capacitive Transducer

8/10/2019 Resistive & Capacitive Transducer

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Resistive

&CapacitiveTransducers


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Presented To:

Presented By:


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Overviews Transducers

Types of transducers Active and Passive)

Resistive Transducers

Definition Working Principle Types Advantages and disadvantages Applications Capacitive transducers

Definition

Working principle Types Advantages and disadvantages Applications


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Transducers

Transducer is an electronic device that converts a signal,in the form of energy, into another form of energy. Itconverts energy to a readable form e.g. thermometer that

converts heat energy into height of mercury column.ORTransducer covert physical quantity into electricalsignal. So without transducer, it would be very difficult

to measure the physical quantity e.g. light intensity,speed, temperature, radiation, electric flux etc. Thequantities are first converted into electrical signal ,thenthey are monitored by dedicated equipments.


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EXAMPLES:

Microphones ,loudspeakers, thermometers, pressuresensors, LEDs and even common bulb are considered as

transducers.


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Active and Passive transducers:

ACTIVE transducers generates an electrical signaldirectly in response to the physical parameter and does

not require an external power source for its operation.(e.g. piezo electrical sensors and photo voltaic cells)

PASSIVE transducers needs external source of powerfor their operation so they are not self generating type oftransducers.(e.g. resistive , capacitive and inductive transducers)


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Resistive Transducer

Resistive transducers are those in which the resistancechange due to the change in some physicalphenomenon . They can be used for measuring variousphysical quantities like temperature, pressure,

displacement, force, vibrations etc.


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Working Principle of

Resistive transducers:The variable resistance transducer elements workon the principle that the resistance of theconductor is directly proportional to the length of

the conductor and inversely proportional to thearea of the conductor.

R = L/AWhere is constant of resistivity of the materialand is measured in ohm-m Resistance can be changed if any of these

value p ,L or A is changed


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Working Principle of Resistive

transducers: The resistance of some materials also changes

with the change in their temperature.

For example:

Resistance of metal is increased byincreasing the temperature and resistance of semi

conductor decreased by increasing thetemperature. This principle is primarily used forthe measurement of temperature.


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Types of Resistive Transducers:

Strain GaugeWire Strain Gauge

Foil Strain Gauge

Semiconductor Strain Gauges Potentiometer

Thermistors

Resistance Thermometers


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Strain Gauge:

`

The strain gauge is a passive, resistive transducer

which measures the mechanical elongation andcompression with the help of resistance change.

This change in resistance takes place due to

variation in length and cross sectional area of thegauge wire, when an external force acts on it.


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Working Principle of Strain Gauge:Each metal has its specific resistance. An external

force increases/decreases the resistance byelongating/contracting it. Suppose the originalresistance is R and a strain initiated change inresistance is R. Then, the following relation isconcluded:

where, Ks is a gauge factor, this coefficientexpressing strain gauge sensitivity.


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Types of Strain Gauge:

1. WIRE STRAIN GAUGE :a) Unbonded form

b) Bonded form2. FOIL STRAIN GAUGE

3. SEMICONDUCTOR STRAIN GAUGE


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WIRE STRAIN GAUGE:In this type of strain gauge the strain is sensed

with the help of wire.UNBONDED STRAIN GAUGE:

1. Unbonded strain gauge consist of a wire

stretched between two points in an insulatingmedium i.e. air.

2. The diameter of wire used is about 25um.

3. The wires are kept under tension so that thereis no free vibration.

4. Unbonded strain gauge are usually connectedin a bridge circuit.


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Working principle of

Unbonded Strain Gauge:When an external load is applied the resistanceof strain gauge changes, causing an unbalancedof the bridge circuit resulting in an output

voltage. This voltage is proportional to strain.


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USES OF UNBONDED STRAIN

GAUGE:1. Unbonded strain gauge is used in places wherethe gauge is to be detached and used again and

again.2. Unbonded strain gauges are used in force,pressure and acceleration measurement.

NOTE: This gauge has a very high accuracy. But it

occupies more space.


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BONDED STRAIN GAUGE:A metallic bonded strain gauge is shown in fig:

A fine wire element about 25um or less indiameter is looped back and forth on a carrier

or mounting plate, which is usually cemented tothe member undergoing stress.NOTE: They are reasonably inexpensive, smallin size, low mass, can measure static & dynamicstrain.


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WORKING PRINCIPLE OF

BONDED STRAIN GAUGE:A tensile stress tends to elongate the wire andthereby increase its length and decrease its

cross sectional area. The combined effect is anincrease in resistance, as seen from theformulae:

R =

L/AL= length

A= cross sectional area

= specific resistance of material (ohm m)


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Foil Strain Gauge:

In this type of strain gauge the strain is sensedwith the help of metal foil. The metals and alloys used for the foil and

wire are nichrome, constantan (Ni+Cu),isoelastic (Ni+Cr+Mo), nickel and platinum.


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SEMICONDUCTOR STRAIN

GAUGE: Semiconductor gauge are used in applicationwhere a high gauge factoris desired. A high

gauge factor means relatively higher change

in resistance that can be measured with goodaccuracy. The resistance of the semiconductor gauge

change as strain is applied to it. The

semiconductor gauge depends for theiraction upon the piezo-resistive effect i.e.change in value of resistance due to change

in resistivity.


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Silicon and germanium are used as resistive material forsemiconductor gauges.

Semiconductor strain gauge

SEMICONDUCTOR STRAIN GAUGE:


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ADVANTAGES OFSEMICONDUCTOR STRAIN

GAUGE:1. They have a high gauge factor of about +130

this allow measurement of very small strain

about 0.01 micro strain.2. Semiconductor strain gauges can be very

small in size, ranges in length from

0.7-7.0mm.BUT

As they are very sensitive to temperature so

they are expensive.


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POTENTIOMETER:

A potentiometeris an instrument for measuringthe potential (voltage) in a circuit.

Potentiometer is one of most commonly useddevice for measurement of the displacement of

the body. The potentiometer is of resistive type because it

works on the principle of change of resistance

of the wire with its length. A potentiometric transducer converts the

measured quantities i.e. displacements,geometric dimensions, or angles of rotation into

changes in the electrical resistance.


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WORKING PRINCIPLE:The potentiometer is an electric circuit in which

the resistance can be changed manually by thesliding contacts. Vs is applied voltage across thetwo points of the wire A and B. C is the variablecontact point between A and B. The resistance of

the conductor AC changes as the length of thewire AC changes.


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POTENTIOMETER AS ATRANSDUCER

To measure the displacement of the body, which ismoving, is connected to the sliding element of thepotentiometer.

As the body moves, the position of the sliderlocated on the potentiometer also changes so theresistance between the fixed point and the sliderchanges. Due to this the voltage Vo across thesepoints also changes. The change in voltage or theresistance is proportional to the change in thedisplacement of the body. Thus the voltage change

indicates the displacement of the body.


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The potentiometer can be used for the

measurement of translational displacement:

The potentiometer can be used for the

measurement of rotational displacement:


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ADVANTAGES OFPOTENTIOMETERS:

1. They are inexpensive.

2. They are simple to operate.

3. Electrical efficiency is very high.

4. More precise.

5. No loss of power during measurement.

BUT

When using a linear potentiometer, a large forceis required to move sliding contact. Slidingcontacts can wear out and generate noise.


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THERMISTOR A thermistor is a temperature-sensingelement composed of semiconductor material(typically a mix of metal oxides) that exhibits a

large change in resistance in response to asmall change in temperature . Thermistor have a negative temperature

coefficient (NTC) i.e. resistance decreases as

temperature rises. High sensitivity to temperature changes

makes the thermistor extremely useful forprecision temperature measurements.


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ContinuedThe thermistor may be in the form of beads, discs

and rods.BEAD THERMISTOR:

Smallest thermistors are made in the form of

beads. Some are as small as 0.15mm in diameter.Its probes are made up of glass and are used formeasuring the temperature of liquid.


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ADVANTAGES OF

THERMISTORS:1. It is small in size and low cost.2. Fast response over narrow temperature

range.

3. Good sensitivity in the NTC region.

BUT

It is unsuitable for wide temperature rangeand are only suitable for up to about 180C.It has very low excitation curent to avoid selfheating.


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Resistance Thermometer:

1. Resistance thermometers are also known asresistance temperature devices (RTD).

2. The resistance of a conductor changes when its

temperature changes, this property is utilizedin resistance thermometers to measuretemperature.

3. Resistance thermometers are useful insituations where a high degree of accuracy isrequired.

iff b Th i d


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Difference between Thermistor andResistance Thermometer:

THERMISTOR RESISTANCE

THERMOMETER

1. Made up of semi- Made up of metals.

conductors.2. Have negative temp Have positive temperature

coefficient. Coefficient.

3. Less temperature Greater temperaturerange i.e. 180C range i.e. 660C

4. Faster response slow response as compare

to thermistor


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APPLICATION OF RESISTIVE

TRANSDUCER

Strain gauges are used in CAT scan machine

C ti d


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Continued

A thermistor can be used to control temperatureinside a green house.

C ti d


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Continued

Strain gauges are used in Mammography

machines for measurement of force applied.

C ti d


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Continued

Behind the dimmer switch (vary brightness oflight)stands the potentiometer (resistivetransducer).

C ti d


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Continued

Thermistors are used for Engine Oil Temperature

Monitoring


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Capacitive Transducer:

Capacitive transducer are those in which thecapacitance change due to the change in somephysical phenomenon into electrical signal.


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Working Principal of

Capacitive Transducer Capacitive Transducers measure thedisplacement by measuring the change in

capacitance. Where capacitance is given by,

C = capacitance

A= aread= distance between plates

K= o & r= dielectric constant

d

AC

r0

m

F120

1085.8


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Working Principle of Capacitive

Transducer

Means change in capacitance occurs due to: Change in overlapped area of plates

Change in distance between the two plates

Change in dielectric medium.

.


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Capacitive Transducer(ForThickness Measurement)

Capacitive transducer can be used to measurethe thickness of the material.

Material whose thickness is to be measured isplaced in between the two metal plates forming

a parallel plate capacitor.

C = A/D
http://www.lionprecision.com/tech-library/appnotes/cap-0030-thickness-measurement.html


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Dielectric Gauge(For Liquid LevelMeasurement)

Capacitive transducers can be used in liquidsand free flowing solids for continuous levelmeasurement.


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WORKING PRINCIPLE

The principle of capacitive level measurement isbased on change of capacitance. An insulatedelectrode acts as one plate of capacitor and thetank wall (or reference electrode in a non-

metallic vessel) acts as the other plate. Thecapacitance depends on the fluid level. An emptytank has a lower capacitance while a filled tank

has a higher capacitance.

Ad t Of C iti


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Advantages Of CapacitiveTransducer

Construction is very simple. Cost of the transducer is low.

Has very high sensitivity.

BUT High impedance output.

High temperature sensitivity.

Need for complex electronics

APPLICATIONS OF CAPACITIVE


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APPLICATIONS OF CAPACITIVETRANSDUCERS:

Capacitive transducers are used in touch screenmobiles.

Continued


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Continued

Capacitive transducers are used in Smart beds to detectpostures that are potentially harmful to the spinal

column.

Continue


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Continue

Capacitive transducers are used in water level detection.


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Date post:	02-Jun-2018
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Resistive & Capacitive Transducer

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