Intro to Transducers

8/12/2019 Intro to Transducers

http://slidepdf.com/reader/full/intro-to-transducers 1/31

1

Types of Transducers and Their Applications

Instrumentation and Product Testing



2

1.1 Distinction Between The Sensor And

Transducer

A sensible distinction is to use “sensor” forsensing element itself and “transducer” for the

sensing element plus any associated circuitry.



3

E +

Sensor

Transducer



4

1.2 Active and passive transducers

Passive transducer

A component whose output energy is supplied entirely

or almost entirely by its input signal is commonly called

a passive transducer.

Active transducer

An active transducer requires an auxiliary source of

power which supplies a major part of the output powerwhile the input signal supplies only an insignificant

portion. Normally, the output magnitude of the active

transducer is higher than that of the passive type.



5

We can further classify transducers according to their

function (displacement, temperature, force)

physical property (inductive, photo-voltaic,

piezo-electric)



6

2.1 Transducer of linear displacement

Force and displacement are closely linked. A

true/ideal displacement transducer is one which

requires a negligible force to make a large

displacement.





8

Features :

1. Simple and cheap2. Accuracy depends on quality and dimension of

the resistive wire used or the quality of the

resistive film.

3. The force required to operate, although small,depends on the size of the potentiometer, but

that required to start movement of the slider is

generally about twice that to keep it in motion.

4. The frequency response is limited by the mass of

the system but small transducer can have a flat

response up to 50 or 60 Hz.



9

Main demerit :

The output is very noisy when the slider moves.

Cause of the noise are many: dirt and corrosion on thewire; chatting and vibration of the slider contact;

variation of contact area as the slider moves;

resolution noise when the slider makes and breaks

contact with a turn of wire; etc.



10

Inductive type

An important displacement transducer used in industrial

and medical application is the linear variabledifferential transformer (LVDT).



11

MOTION

+ -

S1

+ -

~+ -

P

+ -

S2

E o®

E ® E

2

®

CF

F. Coil former;

C. Movable core;

P. Primary winding;

S1 and S2. Secondary

windings;

E 1. Induced voltage in S1;

E 2. Induced voltage in S2. E o. Output voltage E 1 - E 2



12

displacement

E 1

E 2

E 0= E 1- E 2 0

0Primary voltage

E 1

E 2

180

E 0= E 1- E 2

E 1

E 2

E 0= E 1- E 2

(a) Absolute magnitude output voltage; (b) phase-referenced output

voltage as a function of LVDT core position



13

Characteristics:

1. Due to no moving contact (non-contact) hence very low

noise level. Resolution is excellent.2. The frequency response is limited mechanically by the

mass of the core and electrically by the frequency of the

applied primary voltage (carrier), the frequency of this

carrier should be at last ten times that of the highestfrequency component to be measured.

Demerits:

1. Quite expensive.2. The operation can be severely affected by stray magnetic

A.C. fields or by the presence of large mass of metal near

by.



14

Capacitive methods

The capacitance of a capacitor can be changed by varying

its area, gap length or dielectric constant.

15



15

Differential capacitance method

x

d

d

P 1

P 2

E

C 2 E 2

C 1 E 1

- E E E x

d E 1 2

16



16

2.3 Digital methods

By means of coding (absolute linear encoder)

Identify the position of a movable test piece by a binary system of

notation.

5-digit scale for digital indication of linear position of a movable object

5-bit digital output for

linear displacement

5 photo sensors

Relative

motion

The resolution depends upon the number of bits comprising the

binary number.

The accuracy obviously depends upon the accuracy with which

the scale is drawn.

17



17

By pulse counting (incremental linear encoder)

Light source

Photo senser

motion

Counter

transparent

strip

opaque strip

Reset signal

Digital output for linear displacement

ck

18



18

All the above mentioned methods are impracticalwhen measuring long displacement (say > 1m) with

good accuracy (say, <0.01mm). Such measurement

may be required in many application areas, e.g. large

machine tools.

What should we do?

19



19

A grating measurement system can be though of as a

development of a well-known mechanical-opticalmodulating transducer.

Motion

Photo sensor

Index grating

Fixed scale

gratingLight source

to counter Photo sensor output:

20



20

Gratings are available fairly cheaply in widerange of size. The resolution of measurement is

essentially equal to the spacing between the

lines. Gratings up to 1000 lines per mm areavailable.

21



21

3.1 Transducers of Angular displacement

All the methods used in linear measurement can be

applied to angular measurement.

Analog Methods:Resistive, Inductive, and Capacitive

Digital Methods:

Absolute angular encoder, Use of maximal length,and Incremental angular encoder

22



22

4.1 Transducers of velocity

By electronic differentiator or integrator

As far as we know the linear displacement, finding of

velocity can be done by differentiator. If it is an analog

signal, op-amp differentiator may be used. If it is a digital

signal, then use numerical methods for the differentiation.

However, it is more usual to employ an acceleration

transducer and an integrator, because differentiation

accentuates the high frequency noise while integrating

reduces the high frequency noise.

23



23

Induction method

If a coil is moving inside a magnetic field is moved to

cut directly across the lines of flux, then a voltage is

induced in the coil :

e = B a n v

where B is the flux densitya is area of the coil

n is number of turn of the coil

v is relative velocity between the coil and the field

Therefore e is proportional to v, when other parameters

are constant. There are mainly two types --- moving coil

and moving magnet.

24



24

Soft-iron yoke

Coil andformer

S S

S

N

Pole-pieces

Magnet

Motion

NS

Motion

Moving coil transducer Moving magnet transducers

25



25

Doppler effect

It is most common means of measuring remote moving objects.

The police radar trap is a well known example of this technique.

The Doppler effect is a very effective and accurate means of

measuring velocity. If a narrow radio beam or ultrasonic beam

is aimed at an object the beam will be reflected back to thesource. However, if the object is moving the frequency of the

received signal differs from that of the transmitted signal.

The difference between the two frequencies being a measure of

the velocity of the moving object. The received frequency will

be higher than the transmitted frequency if the moving object is

travelling towards the receiver and lower if the object is

travelling away.

26



26

Digital methods

By counting the number of pulse per unit time from theincremental encoder linear encoder, we can know the

linear velocity. Its principle is the same as the digital

method in angular velocity, hence please refer to that

section for details.

27



27

4.2 Angular velocity

Analog method

Tachometer. There are two main types -

d.c. tacho and

a.c. tacho.

28



28

D.C. Tacho

N

S

Slip rings

The device is very convenientand widely used in control

systems where velocity

feedback is required, but is

unsatisfactory for precisionmeasurement because of

unavoidable ripple (due to the

finite number of poles) and

because of spikes due to the sliprings. The a.c. tacho is better in

this respect.

29



A.C. Tacho

The below shows two a.c. tachometer in which both the

magnitude of the generated e.m.f. and its frequency are proportional to the angular velocity.

N

S

N S

Coil

A.C. tacho with moving magnet

and stationary coils A.C. tacho with stationary

magnet and coils

Note: The direction of rotation is indicates by the sign of the DC voltage in DC

tachometer, while AC tachometer does not indicate the direction of rotation.

30



Digital method

By counting the number of pulses per unit time from a

digital angular displacement encoder.

The resolution depends on number of pulse perrevolution of the encoder and the length of the unit

time base.

31



Thank you

Documents

Intro to Transducers