Transducers and Signal Conditioning

By:

Rajni

B.K.N. Government Polytechnic

Narnaul

What is TRANSDUCER ?? Converts one form of energy to another form. Transducers may be

electrical, magnetic, piezoelectric etc.

Electrical Transducers are devices which for the purpose of measurement

convert physical input quantities into an electrical output signal.

Why Conversion to Electrical Signal?

Easy to modify

Easy to process

Easy to Transmit

Easy to Display

No mass-inertia effects

Transducers are classified based

on:

(a) Based on element involved(transduction Principle):

Resistive, capacitive and inductive.

(b) Based on Energy consumption pattern:

Active Transducer:

Transducers which do not require any external source of energy are “active transducers”.

Examples:

Piezoelectric, Peizoresistive,Thermocouple, Moving coil generator.

Passive Transducer:

Transducers which require external source of energy for operation are “passive transducer”.

Examples:

Strain gauge, LVDT, etc.

(c)Based on Output base:

Analog and digital :

•Analog transducers give continuous output.

Examples: Strain gauge,thermocouple ,thermisters,L.V.D.T.

•Digital transducers give discrete output .

(d)Based on Stages involved:

Primary and Secondary :

•Transducers which do not involve any intermediate stage are “primary

transducers”.

•Transducers which involve intermediate stage are “secondary

transducers”.

•Examples: Measurement of pressure using Bourdon Tube and L.V.D.T.

(e) Transducers and Inverse transducers:

“Transducers” convert non electrical quantities to electrical.

“Inverse transducers” convert electrical to non-electrical.

Factors influencing choice of

transducers • Size

• Weight

• Shape

• Sensitivity to desired, modifying and interfering inputs.

• Accuracy

• Operating range

• Transient and Frequency response

• Static characteristics

Based on Transduction Principle:

1)RESISTIVE TRANSDUCERS: • The resistance of the metal conductor is given by:

R=resistance(Ω) ,L=length of conductor( m) , A=cross-sectional area of

conductor(m2) , ρ =Resistivity of conductor material (Ω-m)

• A resistive transducer can be designed by varying any one of the quantities

involved in the above equation .

• Translational and rotational potentiometers :

Change in the value of resistance with change in length of the conductor

can be used for the measurement of Translational or Rotatary displacements

Change in the

length of conductor

Change of resistance

Measurement of

displacement

R= ρ L/A

Major types of Resistive transducers

Resistive Potentiometers

Strain Gauges

Resistance temperature detector (RTD)

Thermistors

Light Dependent Resistor (LDR)

RESISTIVE POTENTIOMETERS • A potentiometer is a resistive transducer

• Converts linear or rotational displacement to voltage.

• It is a passive transducer.

• Principle : potential drop across a length is proportional to length.

• A resistance element is provided with a movable contact. The movable

contact motion can be:

translation

rotation

combination of the two such as helical

Consider a translational potentiometer , let ei and e0 are input and output voltages

respectively.

Under ideal condition , the output voltage varies linearly with displacement .

SENSITIVITY :

S= output =e0

input xi

e0

eex

Xi/xt

Advantages and disadvantages of

Resistive Potentiometer:

Advantages:

Measurement of large amplitudes of displacements.

Electrical efficiency is sufficiently high.

Provide sufficient output to allow control operation.

Inexpensive, simple to operate.

Useful for applications where requirements are not particularly severe.

Disadvantages:

When using a linear potentiometer, a large force is required to move the

sliding contacts.

Sliding contact may generate noise.

•Principle :Works on magnetic induction of material.

•induction of the magnetic material depends on-

(2) INDUCTIVE TRANSDUCERS

There are two common type inductive transducers :

1. Self inductance type

2. Two-coil mutual inductance type

•The “self inductance” of a coil refers to the flux linkage within the coil due

to current in the same coil.

•“Mutual inductance” refers to the flux linkages in a coil due to current in

adjacent coil.

number of turns of the coil on the material

size of the magnetic material,

the permeability of the flux path

The inductance and corresponding output is changed and the output is calibrated directly

against the value to be measured

The permeance of the flux path changes

mechanical element whose displacement to be measured is moved

Self Inductance Type Inductive Transducers

Self inductance type

transducer

Two-Coil Mutual Inductance Type Inductive Transducer: •working principle of Inductance transducer :

of mutual inductance of two coils

Where

K=co-efficient of coupling

L1 ,L2= self inductance of two coils

when two or more coils are magnetically

linked together by a common magnetic

flux,have the property of “Mutual

Inductance”.

Mutual inductance

between the two coils ,changed by

Positioning them on a

common soft iron core

Changing the number of

turns of either coil

Ltotal = L 1 + L 2 - 2M

(b) Magnetic field of the two coils are

differentially connected:

Total inductance of a circuit : •when one is stationary and other is movable :

Ltotal = L 1 + L 2 + 2M

(a) Magnetic field of the two coils are

cumulatively connected:

Linear Variable Differential Transformer

(LVDT) An L.V.D.T. is an “inductive transducer”:.

Principle :

• When a force is applied to the armature (or core), air gap changes .

• Ultimately, Inductance of the coil changes which is the measure of the

applied, force.

Construction and Working: Consists of a primary winding

and two secondary windings with

a magnetic armature or core in between.

A.C. signal is connected across the

primary.

A

The voltages induced in two secondary windings (by

transformer action) are V1 and a V2

•V0= V1- V2

•Practically, the coil is not at the centre position, thus

output of the transformer is equal to the difference of

V1 and V2.

•The differential output voltage of the transformer

(produced by the displacement of core) is linear over

a considerable range AB as shown. This explains the

name L.V.D.T.

• When the force (or pressure) to be measured is

applied to the L.V.D.T., the output voltage is the

measure of the resultant displacement of the core .

Moves OTHER SIDE , V0 will be out of phase with primary voltage . (V0 =-ve)

Moves ONE SIDE, V0 will be in phase with primary voltage . (V0 =+ve)

(c) Advantages of L.V.D.T:

• It is a frictionless device due to its unique feature of electrical

transformer.

• High range used for measurement of transformer from 1.25 to

250mm, so highly reliable.

• Immunity for external effects.

• High degree of sensitivity upto 40 V/mm.

•The power consumption is less than 1 W.

(d) Disadvantages of L.V.D.T.:

• Sensitivity to stray magnetic field ,so shielding is needed to

minimize the effect of stray magnetic fields.

• Affected by vibrations.

•Dynamic response is not good.

Uses of L.V.D.T.:

•High sensitivity is a major attraction.

•The L.V.D.T.s can be used, where displacement is from a fraction

of mm.

•They can also be used to measure force, pressure, weight, etc.

•LVDT is used for measurement control of thickness of metal

sheet rolled ,if thickness equals desired value, LVDT is balanced

•But as dynamic response poor, the L.V.D.T.s are not used for

dynamic (changing with time) measurements.

CAPACITIVE TRANSDUCERS:

The capacitive transducers are used to measure physical changes :force or

pressure .

The capacitance of a parallel plate capacitor is given by:

Where , A = Area of each plate , ϵr= relative permittivity

ϵ0 = permittivity of free space, d = Distance between the plates

•Any variation in distance ‘d’ between the plates will vary the capacitance of

the capacitor.

• Force F to be measured is applied at the diaphragm of the transducer.

•Measurement by changing

Distance b/w two plates

Overlapping area of plates

Dielectric Material

Construction : •Measure 1mm to several cm

displacement.

•One plate of capacitor is

movable and other is fixed.

•Capacitance changes linearly

with area of the plates but

non linearly with distance

changes b/w two plates.

When movable plate moves RIGHT, capacitance will decrease .

When movable plate moves LEFT, capacitance will increase .

PLATE DISTANCE CHANGE: PLATE AREA CHANGE:

dC

1 l-x) (w)C (*

Advantages of Capacitive Transducers: •They are extremely sensitive.

• They have a good frequency response upto 50 kHz.

•Their resolution is of the order of 2 mm.

•They need very small power to operate.

Disadvantages of Capacitive Transducers: • Sometimes they shown a non-linear behaviour.

• Their metallic parts are to be properly earthed.

•They are temperature sensitive.

Uses of Capacitive Transducers: • To measure linear as well as angular displacement.

•To measure force and pressure.

• Effectively measure humidity in gases , since change in dielectric constant of

gases and thereby change in capacitance.

•With mechanical modifiers ,used to measure volume weight and density.

• Active Transducers do not require

an auxiliary power source to produce

their output.

•They are known as“self -generating

type” as they develop their own

voltage or current output.

•Velocity,temperature ,light intensity

and force can be transduced with the

help of active transducers .

.

•Example :Thermocouples,

piezo-electric crystal

On the basis of Energy consumption:

(1) Active transducers (self-generating-no external power)

Types of Active Transducers

S.no Name of transducers

Application (The physical quantities measured)

1. Thermocouple Temperature

2. Moving coil generator

Velocity, vibrations

3. Piezoelectric transducers

Sound, velocity, acceleration, pressure vibration

(2) Passive transducers (externally powered transducers)

Types of Passive Transducer

Sr no Name of the transducers Application (The physical quantities

measured)

1.(a) Strain gauge Force, torque, displacement

(b) Resistance thermometer Temperature

(c) Thermistor Temperature

2.(a) Potentiometer(POT)

Displacement

•Derive the power required for: •Transduction from auxiliary power source.

•Conversion from the physical quantity under measurement .

Examples : Resistive ,Inductive and Capactive transducers .

A typical example of a passive transducers is a “POT”.

PIEZOELECTRIC TRANSDUCER ( Active transducer)

•On application of force on certain

surfaces of crystal

• Dimensions of crystal are changed by

application of mechanical force.

• Displacement of charges in crystal

produces potential .

•This is called as “piezoelectric effect”.

•

“Piezo electric Effect” is a reversible

act.

• As if varying potential is applied to the

crystal it will change the dimensions of

the crystal thereby , Deforming it.

•This effect is also known as “electro-

striction “.

Piezo-electric crystal used for

measurement of force

Piezo electric phenomenon was discovered by CURIE brothers in 1880 in

dielectric substance like:QUARTZ.

Electric potential appears across surfaces of a crystal

Displacement of charges

Dimensions of the crystal are changed

Apply mechanical force

Piezoelectric material Electrostriction

Deformed crystal

Dimension of the crystal are changed

Displacement of charges

Apply electric potential across surfaces of a crystal

TYPES OF PIEZOELECTRIC MATERIALS

Natural: Tourmaline, Quartz, Topaz, Rochelle salt (sodium potassium tartrate tetra

hydrate). Quartz is most stable : Output and sensitivity is quite small. Rochelle salt : The highest output among natural group but it can be

worked over a limited humidity range (rochelle salt are quite flexible and can be molded in different forms.) Synthetic:

Ammoniumdihydrogenphosphate(ADP),lithiumsulphate(LS),dipotassium tartarate(DKT)

Properties of piezo-electric materials High stability High output insensitivity to temperature and humidity. Ability to be formed into most desirable shape(Rochelle salt). .

Advantages of Piezo Electric transducers

•These are small, compact in size and light in weight.

•These are active transducers hence, do not need any external

source of power.

• They are also polarity sensitive.

• Natural piezoelectric crystals show good frequency response.

• Synthetic materials show higher voltage sensitivity.

• Materials are suitable for applications at temperarure upto 2000C.

Disadvantages of Piezo Electric transducers

•Due to small capacitance and large insulation resistance of the transducer

,the charge leaks away through its insulation resistance.

Thus, CHARGE AMPLIFIERS are used to measure the charge.

• The output voltage is effected by temperature variations.

• Uses of Piezoelectric Transducers

•For measurement and recording of dynamic changes in

mechanical variables such as pressure, force .

•Also used in Generation of ultra sonic waves.

(Transmitting and receiving ultra sonic systems are useful

for detection and location of underwater objects).

•Used to measure deformation.

• Measurement of acceleration.

Transducers and Inverse Transducers

“Transducers” are the devices, which convert a non-electrical quantity

into electrical output.

“Inverse transducers” are the devices which convert an electrical quantity

into non-electrical output.

The examples of “Inverse transducers” are:

• A piezo electrical transducer as, when a voltage applied to its surface, it

changes its dimensions i.e. it converts the voltage into a displacement.

•A current carrying coil moving in a magnetic field produces a rotational

torque displacement.

•An ammeter or voltmeter Converts current into mechanical

displacement.

• Data indicating/recording devices such as pen recorder, C.R.O. etc.

Working:

• The output signal (Usually in electrical form) is fed back .

•The feedback signal is converted by passing through an inverse transducer

into a non electrical form.

•Then, compared with the quantity under measurement (non electrical).

•The resulting error signal is passed through a transducer into the electrical

form and is applied to give output indication.

The most useful application of inverse transducers is in “Feedback

measurement system”.

Fig. shows a block diagram of a feed back measurement system using

inverse transducers.

The ‘firm’ lines show electrical devices/signals and dotted lines show

mechanical devices signals.

BASED ON THE TYPE OF OUTPUT

ANALOG TRANSDUCERS:

•

“The analog Transducers convert the

input quantity into an analog output

which is a continous function of time”.

An analog signal uses some property

of the medium to convey the signal's

information. such as voltage

•Example:1

• In sound recording, fluctuations in air

pressure strike the diaphragm of a

microphone

•This induces fluctuations in the current in

an electromagnetic microphone.

• The voltage or the current is said to be an

"analog" of the sound.

•A strain gauge , an LVDT ,a thermocouple

or a thermistor may be called as" Analog

Transducers ”

•Advantages:

•Analog signals are of

higher density

•Processing may be

achieved more simply than

with the digital equivalent

•Disadvantages

•limitations of accuracy of

the data.

•As the signal is copied and

re-copied, or transmitted

over long distances ,

random variations occur.

•system has noise – i.e.,

random unwanted variation

DIGITAL TRANSDUCER

•“These Transducers convert

the input quantity into an

electrical output which is in

the form of pulses” .

•The physical quantities measured

are transmitted as coded digital

signals.

• Digital Transducers are also

called “ Encoders”

• These Encoders are normally

available in form of “linear / rotary

displacement transducers”.

Classification of digital transducers

•Tachometer Transducer

•Incremental Transducer

•Absolute Transducer

•Tachometer Transducer has pulse for each increment of displacement.

•Motion are always in one direction.

•Digital counter accumulate these pulses to determine the displacement.

•Identical pulses are produced for motion in reverse direction: hence ,ERRORS.

APPLICATION :Used for SPEED measurements only

Tachometer Transducer:

Incremental Transducer:

• Consists of three signal generating Element.

• Two tracks are shifted by 1/4 cycle relative to each other.

• Up and down counter subtracts pulses when motion reverses.

• Third element produces one pulse per revolution.

Able to rotate through many revolutions.

False pulse occurs due to electric noise.

ADVANTAGE

DISADVANTAGE

ABSOLUTE TRANSDUCER: They use multiple tracks and outputs which are read out in

parallel to produce binary representation of angular shaft input position.

Advantage

•Electronic items tend to be cheaper than mechanical

• lower system noise

• lower system distortion

•Data from instrument can be recorded for future.

• Higher accuracy and reliability

•Ease of use

Disadvantage: •Indicate erratic values due to faulty or damaged electronic circuits

• high humidity and corrosive atmosphere damages internal parts of system and

result faulty values.

• sometimes, instrument show some readings even though there is no applied

measurable parameter.

Applications:

•Digital tachometer