Transducer & Sensor

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Contents…contd

• Resistive Position Transducers

• Capacitive Transducers

• Inductive Transducers

• Strain Gauge

• Photoelectric Transducer

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Objectives

• Ability to understanding the definition, functions & categories of transducers.

• List the classes and types and examples of transducers.

• Operations and applications for each transducers

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Sensor and Transducer

• Sensor is a device that detects a change in a physical stimulusand turns it into a signal which can be measured or recorded.

E.g. : Thermistor

• Transducer is a device that transforms energy from one form toother'.

E.g. Thermistor with it associate circuit convert heat toelectricity.

As a comparison……

‘Sensor' for the sensing element itself and 'transducer' for thesensing element plus any associated circuitry. All transducerswould thus contain a sensor and most (not all) sensors wouldalso be transducers.

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Sensing Process

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Definition of a Transducer

Transducer is any device that converts energy in one form

to another energy. The majority either convert electrical

energy to mechanical displacement or convert some

non-electrical physical quantity, such as temperature,

sound or light to an electrical signal.

Type of Transducers

Electrical Transducers• Converts the input measurand into an electrical voltage/current

Mechanical Transducers• Converts the input measurand into a mechanical energy

Transducer Electrical

signalNon electrical

quantity

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Functions of Transducer

1. To sense the presence, magnitude, change in, and

frequency of some measurand.

2. To provide an electrical output that, when appropriately

processed and applied to readout device, gives accurate

quantitative data about the measurand

Transducer Electrical

output

Measurand

Excitation

Measurand – refers to the quantity, property or condition which the

transducer translates to an electrical signal.

Transducer can be classified according to

their application, based primarily on the physical

quantity, property, or condition that is measured.

A transducer can be classified

(i) On the basis of transduction form used,(ii) As primary and secondary transducers(iii) As passive and active transducers(iv) As analog and digital transducers and(v) As transducers and inverse transducers

Classification of Transducers

1. Classification based upon principle of transduction

The transducers can be classified on the basis of principle of

transduction as resistive, inductive, capacitive etc. depending upon how

they convert the input quantity into resistance, inductance or capacitance

respectively.

2. Primary and Secondary Transducers

Primary transducers are transducers which convert a physical quantity

into another form. But the secondary transducers convert the output

signal from the primary transducer into a usable output (i.e. an electrical

signal).

For eg., the Bourdon tube converts the input pressure into a displacement, as primary transducer and theLVDT converts this displacement into an analogous voltage, as secondary transducer.

Fig. Measurement of pressure using Bourdon tube and LVDT

3. Passive and Active Transducers

(i) Active transducer

Generates an electrical signal directly in response to the

physical parameter and does not require an external

power source for its operation. Thus active transducers

are self-generating devices.

Typical examples of active transducers are piezo electric

sensors (for generation of charge corresponding to

pressure) and photo voltaic cells (for generation of voltage

in response to illumination).

(ii) Passive transducers

Derive the power required for transduction from an

auxiliary power source. Thus passive transducers require

an external electrical source to convert the physical

parameters into an electrical signal. They are also known

as externally powered transducers. They depend upon the

change in an electrical parameter (R, L and C).

Typical examples are strain gauges (for resistance change

in response to pressure) and thermistors (for resistance

change corresponding to temperature variations).

Active Transducers - Do not requires external power produce an analog voltage

Passive Transducers -Require external power source to operate

Active Transducermeasurand electrical output

Passive Transducermeasurand electrical output

external power

Contents

• Objectives

• Sensor and Transducer

• Sensing Process

• Definition of Transducer

• Function of Transducer

• Classification of Transducer

• Selecting of Transducer

• Temperature Transducer

Selecting a Transducers

1. Operating range

=> The transducer should maintain range

requirements and good resolution.

2. Sensitivity

=> the transducer must be sensitive enough to allow

sufficient output.

3. Environmental compatibility

=> ability to make applicable and interactions

5. Minimum sensitivity measurand. => The transducer must

be minimally sensitivity

6. Accuracy => Subject to repeatability and calibration error

7. Physical condition => Depend on its usage

8. Electrical parameters => Length and type of cable

required, signal to noise ratio (SNR) when combined with

amplifiers and frequency response limitations.

Transducers

• Resistive Transducer

=> Potentiometer

• Resistive Position Transducer

• Capacitive Transducer

• Inductive Transducer

• Temperature transducers

• Strain Gauge

• Photoelectric

Potentiometer

• Electromechanical device containing a resistance element that is contacted by a movable slider

• The motion of the movable slider may be translatory or rotational.

Translatory type

Rotational Type

Helipots Type

Resistive Position Transducer

APPLICATION

• Potentiometer senses displacement by means of sensing

shaft, which is mechanically connected to the point or objects

whose displacement, is to be measured.

• Example:

Petrol-tank level indicator.

In this case, potentiometer is used to indicate/sense the petrol

level in a tank as shown in Figure below. The output signal

(voltage) is proportional to the petrol level.

The principle of the resistance transducer is that the physical variable under measurement causes a resistance change in the sensing element.

A common requirement in industrial measurement and control work is to be able to sense the position of an object or distanceit has moved.

.

Resistive Position Transducer

A

LR

Potentiometer

R: resistance change

: density

L: Length

A: area

Cont’d…

Figure shows the construction of a displacement transducer uses a resistance element with a sliding contact or wiper linked to the object being monitored.

The resistance between the slider and one end of the resistance element depends on the position of the object. The output voltage depends on the wiper position and therefore is a function of the shaft position.

FIG 1 (a) FIG 1 (b)

Consider Fig 1 (b), if the circuit is unloaded, the output voltage V0 is a

certain fraction of VT, depending on the position of the wiper:

21

20

RR

R

V

V

T

This equation shows that the output voltage is directly

proportional to the position of the wiper (R2), if the resistance of

the transducer is distributed uniformly along the length of travel of

the wiper.

EXAMPLE 1

A displacement transducer with a shaft stroke of 4 in. is used

in the circuit of figure 1 (b). R1 +R2 is 1000 Ω and VT = 4 V.

The wiper is 1.5 in from B. Find V0?

Capacitive Transducer

• Capacitive transducers are nothing but the capacitors

with the variable capacitance. These are mainly used

for the measurement of displacement, pressure etc.

• A capacitor consists of two parallel plates separated

by an air space or by a dieletric (insulating material).

• The capacitance of the pair of plates is a measure of

the amount of charge that can be transferred before a

certain voltage is reached.

• If the capacitance is large, more charge is needed to

establish a given voltage difference.

Capacitive Transducer

The capacitance of a parallel plate capacitor is given by

wherek = dielectric constantA = the area of the plate, in m2

εo = 8.854 x 10-12 F/md = the plate placing in m

)(0 Faradsd

kAC

Cont’d

Forms of Capacitance Transducers

Rotary plate capacitor

Rectilinear Capacitance Transducer

Thin diaphragm

Cont’d

Rotary plate capacitor:

The capacitance of this unit proportional to theamount of the fixed plate that is covered, thatshaded by moving plate. This type of transducerwill give sign proportional to curvilineardisplacement or angular velocity.

Cont’d

Rectilinear capacitance transducer:

It consists of a fixed cylinder and a moving cylinder. These pieces are configured so the moving piece fits inside the fixed piece but insulated from it.

Cont’d

Thin diaphragm:

A transducer that varies the spacing between surfaces. The dielectric is either air or vacuum.

Often used as Capacitance microphones.

Advantages:

1. Has excellent frequency response

2. Can measure both static and dynamic phenomena.

Disadvantages:

1. Sensitivity to temperature variations

2. the possibility of erratic or distortion signals owing to

long lead length

Applications:

1. As frequency modulator in RF oscillator

2. In capacitance microphone

3. Use the capacitance transducer in an ac bridge circuit

Cont’d

Example :

A capacitive transducer is used for the measurement oflinear displacement, X, as shown in below. The parallel platehas a dimension of 5.0cm X 5.0cm and is separated by adistance of 1.0cm. The space between the plates is filledwith a dielectric material of 1.0cm thick, which has adielectric constant of 4.0. If the dielectric constant for air is1.0cm, determine the value of the capacitance when x isequal to:

(i) 0.0cm

(ii) 2.0cm

Inductive Transducer

Inductive transducers may be either of the self

generating or passive type. The self generating type

utilises the basic electrical generator principle, i.e, a

motion between a conductor and magnetic field

induces a voltage in the conductor (generator action).

This relative motion between the field and the

conductor is supplied by changes in the measurand.

An inductive electromechanical transducer is a

device that converts physical motion (position

change) into a change in inductance. Transducers of

variable inductance type work upon one of the

following principles:

1. Variation of self inductance

2. Variation of mutual inductance

Cont..

Inductive transducers are mainly used for the

measurement of displacement. The displacement to be

measured is arranged to cause variation in any of three

variables:

1. Number of turns

2. Geometric configuration

3. Permeability of the magnetic material or magnetic

circuits

LVDT• LVDT works under the principle of mutual induction, and the

displacement which is a non-electrical energy is converted into an electrical energy. And the way how the energy is getting converted is described in working of LVDT in a detailed manner.

• LVDT consist of a cylinder former where it is surrounded by one primary winding in the centre of the former and the two secondary windings at the sides. The number of turns in both the secondary windings are equal, but they are opposite to each other, i.e., if the left secondary windings is in the clockwise direction, the right secondary windings will be in the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the two secondary coil. The two secondary coil is represented as S1 and S2. Esteem iron core is placed in the centre of the cylindrical former which can move in to and fro motion as shown in the figure. The AC excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.

S1

S2

Working of LVDT

• Working of LVDT can be understand by splitting the cases into 3 based on the iron core position inside the insulated former

• Case 1

On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero

i.e., V1-V2=0

• Case 2:

When an external force is applied and if the steel iron core tends to move in the left hand side direction then the emf voltage induced in the secondary coil is greater when compared to the emf induced in the secondary coil 2. Therefore the net output will be V1- V2

• Case 3:

When an external force is applied and if the steel iron core moves in the right hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. therefore the net output voltage will be V2-V1

Advantages of LVDT:

• * Infinite resolution is present in LVDT* High output* LVDT gives High sensitivity* Very good linearity* Ruggedness* LVDT Provides Less friction* Low hysteresis* LVDT gives Low power consumption

Disadvantages of LVDT

• * Very high displacement is required for generating high voltages.* Shielding is required since it is sensitive to magnetic field.* The performance of the transducer gets affected by vibrations* Its is greatly affected by temperature changes.

Applications of LVDT:

• LVDT is used to measure displacement ranging from fraction millimetre to centimetre.

• Acting as a secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc..

Strain GaugeThe strain gauge is an example of a passive transducer that

uses electric resistance variation in wires to sense the strain

produced by a force on wires. It is a very versatile detector

and transducer for measuring weight, pressure, mechanical

force, or displacement.

The construction of a bonded strain

gauge (see figure) shows a fine wire

element looped back and forth on a

mounting plate, which is usually

cemented to the member undergoing

stress. A tensile stress tends to

elongate the wire and thereby

increase its length and decrease its

cross-sectional area.

Strain Gauge

• It is resistive type of transducers which when undergoes deformation/strain , experiences change in its resistance. The change in resistance is directly proportional to change in length or strain.

• The change in resistance can be measured and thus strain can be found out.

• can be used for measurement of very small strain/change in length which otherwise cant be measured accurately by any other devices.

• It is passive type of transducer.

• The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern.

strain Gauge………

(a) (b)

Strain Gauge Applications

• Measurement of pressure

• Measurement of force

• Measurement of small displacement

• Measurement of Torque

• Measurement of Load etc.

Principle

• If a metal conductor is stretched or compressed, its resistance changes on account of the fact that both length and diameter of the conductor changes.

Equation for a strain gauge under stress can be written as ∆𝑅

𝑅=

∆𝐿

∆𝐿+ 2𝜈

∆𝐿

∆𝐿+

Δ𝜌

𝜌………….(1)

dividing Equation 1 by “ ∆𝐿

∆𝐿“ ,

∆𝑅

𝑅/

∆𝐿

∆𝐿= 1+2𝜈

∆𝐿

∆𝐿+

Δ𝜌

𝜌, (

∆𝑅

𝑅/

∆𝐿

∆𝐿= 𝐺𝑓)

𝐺𝑓 =1+2𝜈 +Δ𝜌

𝜌/∆𝐿

∆𝐿

Here 𝐺𝑓 is called as gauge factor of a strain gauge which can be defined as

“Gauge factor 𝐺𝑓 is defined as the ratio of per unit

change in resistance to per unit change in length”

Since ; 𝐺𝑓 =∆𝑅

𝑅/

∆𝐿

∆𝐿

or;∆𝑅

𝑅= 𝐺𝑓

∆𝐿

∆𝐿= 𝐺𝑓𝜖

𝐺𝑓 = 1+2𝜈 +Δ𝜌

𝜌/𝜖

Δ𝜌

𝜌/𝜖 is very small

There for;

𝐺𝑓 = 1+2𝜈

Types of strain gauge

•Unbounded Strain Gauge

•Bonded wire Strain Gauge

•Bonded Metal Foil Strain gauge

•Vacuum Deposit Strain Gauge

•Semiconductor strain gauge

•Diffused metal strain gauge

Metallic strain gauge – formed from thin resistance

wire or etched from thin sheets of metal foil.

Wire gauge (small) – to minimum leakage – for high

T applications

Semiconductor strain gauge – high output

transducers as load cells

Strain gauge is generally used as one arm of

bridge

Temperature Transducers

Temperature transducers can be divided into four main categories:

1. Resistance Temperature Detectors (RTD)2. Thermocouples3. Thermistor

Detectors of wire resistance temperature common employ platinum,

nickel or resistance wire elements, whose resistance variation with

temperature has high intrinsic accuracy. They are available in many

configurations and size and as shielded or open units for both

immersion and surface applications.

The relationship between temperature and resistance of conductors

can be calculated from the equation:

where

R = the resistance of the conductor at temperature t (0C)

R0 = the resistance at the reference temperature, usually

200C

α = the temperature coefficient of resistance

ΔT = the difference between the operating and the

reference temperature

1) Resistance Temperature Detector (RTD)

)1(0 TRR

RTD (resistance temperature detector) => A temperature sensor that operates on the

measurement principle that a material’s electrical

resistance changes with temperature.

Exercise:

2) Thermocouple

It consists of two wires of different metals are joined

together at one end, a temperature difference between this

end and the other end of wires produces a voltage between

the wires. The magnitude of this voltage depends on the

materials used for the wires and the amount of temperature

difference between the joined ends and the other ends.

How does a thermocouple

look like ?

please note the two wires (of two

different metals) joined in the junction.

How does a thermocouple work ?

In normal operation, cold junction is

placed in an ice bath

Cont’d

The emf of the thermocouple :

E = c(T1 – T2) + k(T12 – T2

2)

Where

c and k = constant of the thermocouple

materials

T1 = The temperature of the “hot”

junction

T2 = The temperature of the “cold” or

“reference” junction

Cont’d

Advantages of thermocouples:

1. It has rugged construction

2. Temperature range -2700C to 27000C

3. Using extension leads and compensating cables,

long transmission distances for temperature

measurement are possible

4. Bridge circuits are not needed for temperature

measurement

5. Comparatively cheaper in cost

6. Calibration checks can be easily performed

7. Thermocouples offer good reproducibility

8. Speed of response is high compared to the filled

system thermometer

9. Measurement accuracy is quite good

Cont’d

Thermocouples and thermocouple assemblies: (a) Uninsulated

thermocouple (b) insulated thermocouple (c) probe assembly

(d) thermocouple well

(i) Thermistor is a type of resistor used to measure

temperature changes, relying on the change in its

resistance with changing temperature.

(ii)Thermistor (THERmally sensitive resISTOR)

=> non-metallic resistors (semiconductor material),

made by sintering mixtures of metallic oxides such as

manganese, nickel, cobalt, copper and uranium.’

3) Thermistor …cont

Termistors have negative temperature coefficient (NTC).

That is, their resistance decreases as their temperature

rises.

Types of thermistor Resistance

Disc 1 to 1MΩ

Washer 1 to 50kΩ

Rod high resistance

Advantages of Thermistor

• Small size and low cost

• Fast response over narrow temperature range

• Good sensitivity in Negative Temperature Coefficient

(NTC) region

• Cold junction compensation not required due to

dependence of resistance on absolute temperature.

• Contact and lead resistance problems not encountered

due to large resistance

Limitations of Thermistor

• Non linearity in resistance vs temperature

characteristics

• Unsuitable for wide temperature range

• Very low excitation current to avoids self heating

• Need of shielded power lines, filters, etc due to high

resistance

Electrical symbol of a thermistor

Cont’d

Figure 3: Various configurations of thermistor