UNIT 2

Transducers

Transducer• Devices used to transform one kind of energy to

another.• Advantages of electrical transducers Easy amplification, small power requirements,

easy data transmission & recording

• Disadvantages Low reliability and high cost

• Requirement of a transducer

Linearity , Reproducibility , High reliability & sensitivity, Good dynamic response, high SNR, No hysteresis & ruggedness

TRANSDUCER• ACTIVE Self generating transducers or transducers

which develop their output in the form of voltage or current without any auxiliary source.eg: thermocouple, piezo electric transducers

• PASSIVE Transducers in which electrical parameters R,

L ,C changes with changes in input signal. It requires power supply for its operation .eg: resistive, capacitive, inductive trasducer

JOBY JOHN 3

TRANDUCER• ANALOGTransducer which converts input signal into an analog

output . Mainly divided into Electromechanical , opto electric Type

Eg: thermistor, LVDT, stain guages• DIGITAL Transducer which converts input signal into discrete

output /pulses . 2 types a) Frequency generating b) Digital encodersEg: digital encoders, optical encoders,

electromagnetic transducers

Electro Mechanical Transducers

Input(Physical Variable) is converted to Mechanical Displacement Or Strain, using Primary Sensors. Then , it is converted to Electrical Output with the help of electromechanical Transducers.

Electro Mechanical TransducerPotentiometric Transducers • Based on the linear dependence between total

impedance and conductor length.• Several technologies: wire bound, conductive

plastic, mixed,…• Both for linear and angular displacement

measurements• R(l) = (l/L) R0• Angular and rotary Sensor, Linear Translation

Sensor

Electro Mechanical Transducer• Potentiometers

Translational Type (passive) Rotational Type

• Can be used to measure position, displacement & level

• Advantages: Suitable for large amplitude of displacement ,Electrical efficiency is high, inexpensive

• Disadvantages: Large force is required to move sliding contacts and it also produce noise

Electro Mechanical TransducerInductive Transducers ( mainly for displacement)• Two Types : Self Generating Type (Electrical Generator

Principle) Or Passive Type ( Change in Inductance)• Active: Motion of conductor in a magnetic field

produces a voltage in the conductor• Passive :Variation in Self Inductance and Variation in

Mutual Inductance Implemented by 1.Number of lines 2. Geometric Configuration 3.Permeability of magnetic Material• L= e/(di/dt) = N2/R= N2µA/l

a) Number of Turns

• Angular Inductive

input output

• Linear Inductive• As N Changes , L and

output Changes

b) Change in Permeability• Iron core is surrounded by

the winding. When the iron core is inside the winding ,its permeability is increased ,hence the inductance. When the iron core is moved out of the winding, permeability decreases, resulting in reduction in self inductance of the coil

c)Variable Reluctance Type Transducer

• Coil is wound on a ferromagnetic core. Displacement to be measured is applied to ferromagnetic target ( No Physical contact with core, but separated by an air gap ).

• Reluctance of the magnetic path is determined by the Size of the air Gap.

• Self Inductance of the coilL=N2/Ri+Rg, where N - Number of turnsRi - Reluctance of iron partsRg - Reluctance of air gap

c)Variable Reluctance Type Transducer

• L= N2/Rg , since Ri is negligible• Rg = lg/µ0* Ag where lg – length of air gap, µ0 –

Permeability, Ag – Area of the flux path through air . Here µ0 and Ag are constants. Rg is proportional to lg

• When the target is near the core, length is small and hence self inductance is large , but when the target is away from the core, reluctance is large ,resulting in smaller value of inductance

Linear Variable Differential Transducers• Passive Inductive transducer

widely used for translating a linear motion into an electrical signal

Operation: • When core is in its null

position , equal voltages are induced in the two secondary windings Es1 = Es2 . Hence E0 = 0.(ie,E = Es1- Es2 ).

• If the core is moved to the left of the null position, E0= Es1- Es2, in phase with Es1.

(ie,E0 =+ve, ø = 00 )

• If the core is moved to the right of the null position, E0=Es2-Es1, in phase with Es2

( ie,E0 = -ve, ø =1800 )

Linear Variable Differential Transducers• Construction: Single

primary winding P1(connected to AC Source) and two secondary winding S1 & S2 (equal No. of turns and identically placed on either sides of the primary winding) wound on a hollow cylindrical former. A movable soft iron ( Nickel –Iron alloy, movement is attached to it) core slides within the former and affect the magnetic coupling between primary and two secondaries.

Linear Variable Differential TransducersVarious Core Positions and variation of output voltage & phase with displacement

Linear Variable Differential TransducersAdvantages Disadvantages

• Higher output volt for small changes in core position(50-300mv/mm)

• Output voltage is linear upto 5mm displacement

• Infinite resolution & High sensitivity

• Low Power consumption (less than 1 W) and low Hysteresis

• Less friction

• Sensitive to stray magnetic field.

• Temperature affect transducer operation.

• Demodulator network must be used if a dc output is required

Capacitive transducers• Changes in position is

converted to changes in the capacitance

• Capacitance C = KA/d K- dielectric constant A- Area of the capacitor Plates D- distance between the plates• Capacitance increases if 1.Effective area of the plate is

increased 2.If the material has high K value 3.If the distance between the

plates is reduced

A)Variable Distance Type (pressure - microphone)

• Here diaphragm is deflected with respect to the applied pressure leads to change in distance between the plates, hence the capacitance

Capacitive transducersb) Variable Area Type• Here one plate is kept

fixed called stator, and the movable plate is called rotor, to which movement is connected.

• Change in position of the rotor with respect to the stator causes change effective area , hence capacitance

Capacitive transducersC)Differential Capacitive Transducer• A movable plate is kept in midway

between the two fixed plates.Then C1= C2, and E1=E2 = E/2Differential output = 0.• Let the movable plate displaced x

in the upward direction, then C1= KA/(d-x) & C2=KA/(d+x)

Then E1= C2E/(C1+C2) =(d-x)E/2dand E2= C1E/(C1+C2) =(d+x)E/2d

Differential output = ± xE/d.

Capacitive transducersD)Variable Dielectric Type

Initially C= Kwl1/d+ KKrwl2/d

• Let the Dielectric is moved through a distance x, then change in capacitance

∆C= Kw x(Kr-1)/d

Uses of Capacitive Transducers• Can be used for the

measurement of presure, force, Humidity etc

Advantages• Good Frequency Response• High Input Impedance• Highly sensitive &

Operational with small force

• Good resolution Disadvantages• Nonlinear behaviour due

to edge effect• Temperature

variation ,dust particle in moisture affect the performance

Resistive TraducersResistance Temperature Detector(RTDs)• Based on the temperature dependence of resistivity of all

metals and alloys.• Although virtually all metals can be employed, platinum is

used almost exclusively: predictable response, long-term stability and durability ,ultimate in accuracy . Other RTDs are Nickel, Copper etc

• All RTD’s have positive temperature coefficientsAdvantages• Linearity over wide Temperature Range• Better Stability at higher temperaturesDisadvantages• Low sensitivity & high cost• Can be affected by contact resistance, shock, acceleration

RESISTIVE TRANDUCERS• Rt- Resistance of a conductor

at temperature t0C• R0- Resistance of a conductor

at temperature 00C• à- Temperature coefficient of

resistance• Then• Rt= R0(1+à∆t)

RESISTIVE TRANDUCERS• Thermistor• Thermo resistive sensors but fabricated with metal-oxide materials

that behave like semiconductors• They usually present a Negative temperature coefficient (NTC),

although PTC (Positive temperature coefficient) ,thermistors are also available.

• Low-accuracy and low stability sensors, but low cost, they present an exponential-like dependence on temperature.

• Thermistors are available in the form of beads, disc,washer and rod• Merits: Fast response, Good sensitivity in NTC region , Small

size ,low Cost.• Demerits: Non-linear behaviour, unsuitable for wide range, Self

heating

RESISTIVE TRANDUCERSResistance versus Temp . Graph

Thermistor• R ref - Resistance of a thermistor

at temperature T ref.

• R- Resistance of a thermistor at temperature T

• ß- Temperature coefficient of resistance

Then

RESISTIVE TRANDUCERS• Strain guages ( variable resistance transducers)• Passive transducer which is used to measure strain

produced by a Force by the changes in electrical resistance in wires.

• Piezoresistive effect : The impedance( due to the change in length or diameter) of a metal wire changes when the material is mechanically deformed. This is the origin for a widely used sensors known as strain gauges.

• When a stress is applied on wire , its length increases and diameter decreases . i.e , R = ÞL/A

• In these sensors, the unitary change in resistance is proportional to the elongation (strain) through a parameter known as gauge factor (K).

• ∆R/R = K (∆L/L). Measurement of sensitivity of a material to strain is called guage factor.

STRAIN GUAGESDifferent Types Strain Guages• Wire Strain Guage• Foil Stain Guage• Semiconductor Strain guage Resistance Wire Gauges have 2

Basic forms: Unbonded & Bonded• 1.UNBONDED

• It is connected in a bridge circuit• With no load applied bridge is

balanced. • When load is applied, resistance

change occurs resulting an output voltage in the bridge

• In unbonded Resistance strain guages, Wire streched Between two points in an Insulating Medium(D= 25µm) ,

STRAIN GUAGES2)BONDED• A wire is looped back and

forth on a mounting plate (carrier) which is usually cemented to a member undergoing stress

• R = ÞL/A

• Different Types of Wire Strain guages

• Grid Type• Rosette Type• Torque Type• Helical type

STRAIN GUAGES

• Grid Helical • Torque Rosette

STRAIN GUAGES• Grid Type Measuring axis of a strain gauge is along the longitudinal

axis, but in most applications , some degree of strain is present along the transverse axis of the gauge .i.e, transverse sensitivity can not be completely eliminated ,in high accuracy measurements. So compensation is required…

• Rosette typeCompensation for transverse sensitivity is to place several

gauges on members surface with a known angles between them(450)

• Characteristics of strain guages for better measurement 1.High value of K2.R should be as high as possible3.Low resistance temperature coefficient 4. No hysteresis

STRAIN GUAGES

Foil Strain Guage Strain is measured with the help of a metal foil . Ex: Nickel,

Platinum Nichrome, Constantan(Ni+cu), Isoelastic (Ni+Cr+Mo)Advantages• Greater dissipation capability due to large surface area. Hence in

can be operated in higher Temperature range• Better bonding & fabricated in any shape and etched on the

carrier.• Longitudinal sensitivity is 5% more than wire type with equal K

value• Transverse sensitivity & hysteresis is smaller to ½ to 1/3 of wire

Type • Resistance in between 50-1000 ohms & thickness of the film is

about 0.2mm

STRAIN GUAGESSemiconductor strain gauges• Here change in resistance due to resistivity change rather

than dimensional change in metallic gauge eg: Ge, SiAdvantages• High value of Gauge Factor (50 times more than wire type)

indicates higher change in resistance and hence good accuracy

• Hysteresis characteristics of Semiconductor strain gauges are excellent

• Better frequency response & small in sizeDisadvantages• Sensitive to temperature changes• Linearity is poor• Expensive

Piezo Electric Transducer• Active tranducer used for the measurement of

force or strain • Piezo electric effect: Crystalline materials such as

Quartz, barium titanate , produces an emf when they are placed under stress.(reversible)

• Structure: Here crystal is placed between a solid base and a force summing member. Force entering the transducer through its pressure port applies pressure vat the top of the crystal. This produces an emf across the crystal proportional to the magnitude of applied pressure

Piezo Electric Transducer• Basic expression for output voltage E =Q/Cp, where Q –

generated Charge, Cp - shunt capacitance. Let K- Coupling CoefficientK= Mech.E converted to Ele. E/Applied Mech. EK= Ele.E converted to Mech. E/Applied Ele. E Properties of Piezo electric crystal• Stability• High Output• Very good Frequency Response• Sensitive to temperature and humidity• Applications• Measurement of force, High frequency accelerometer, as

mass to frequency converter, Measurement of temperature

Load cell (Pressure cell)• Used to measure Heavy loads using strain guages

• As the stress/load is applied along Z direction, steel bar experience a compression along this direction, and an expansion along X & Y axis. As a result , gauge A experience a decrease in resistance while the other B undergoes increase in resistance. When these two gauges and gauges on the other sides of the steel are connected to form a bridge circuit, sensitivity is multiplied by four times. So it load cell is sensitive to small values of stress as well as heavy loads

Thermo CoupleActive transducer used for measurement of

temperature Principle: When a pair of wires made of disimilar

metals is joined together at one end, a temperature difference between the two ends of wires produces a voltage between the two wires (Seebeck effect)

• The magnitude of this voltage depends on the material used for the wire and temperature difference between the joined ends and other ends

• Temperature difference between the sensing junction and other ends is a critical factor, the latter are kept at constant reference temperature

Thermo Couple• A series of thermocouple connected together is

called a thermopile• Thermocouple is made of different alloys covering a

range of -2700 C to 27000 C

• J- Iron Constantan, T-Copper Constantan• E- Chromel-Constantan, • K- Chromel-Alumel, S- Platinum-Rhodium etc

Thermo Couple

Advantages

Limitations

• Rugged Construction• Wide range (-2700 C to 27000

C)

• Good Accuracy• No need of bridge circuit• Good reproducibility• Cheap

• Cold junction & other compensation are required for accurate measurement

• Nonlinear behavior • Signal amplification is

required

Ultrasonic Transducers• Ultrasonic sensors generate high frequency sound

waves(above 20khz) and evaluate the echo which is received back by the sensor. Sensors calculate the time interval between sending the signal and receiving the echo based on the the ultrasound frequency, and the sound velocity of the propagation medium etc to determine the distance.

• Piezo electric transducers can be used as transmitters and receivers

• When an alternating current is applied Piezo electric transducers, causes them to oscillate at very high frequencies, thus producing very high frequency sound waves. Also piezoelectric crystals generate a voltage when force is applied to them, So the same crystal can be used as an ultrasonic detector

Ultrasonic Transducers

• It can be used to detect level of a fluid in a tank, speed of an object through air or water etc

Ionization Transducer• Most widely used for low-pressure measurement• Operation: A regulated electron current (typically 10 mA) is

emitted from a heated filament. Electrons emitted from the filament move several times in back and forth movements around the grid before finally entering the grid (+150 volts). During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (Electron Ionization) and these ions pour into the collector (-30 volts) to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current .The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament . This current is amplified and displayed by a high-gain-differential amplifier .

Ionization Transducer• This ion current will differ for different gases at

the same pressure; that is, a hot filament ionization gauge is composition-dependent

Proximity Transducers

• Detect the presence of nearby objects without any physical contact

• Operation : A proximity sensor often emits an electromagnetic field or electromagnetic radiation and observes changes in the field or return signal.

• An Inductive proximity sensor always requires a metal target.

Proximity Transducers Target Signal Switching evaluator Amplifier

• Advantages: Due their non-contact mode of operation ,direct contact with the object is avoided (No moving parts or contacts, hence functions without wear).

• High degree of protection against vibration and shock, stress as well as dirt, dust and humidity ( can also be used in extreme conditions)

Proximity Transducers• Operation

Proximity Transducers• OPERATION : An inductive proximity sensor comprises an

LC oscillating circuit, a signal evaluator/trigger circuit and a switching amplifier/output circuit .

• The coil of this oscillating circuit generates a high-frequency alternating electromagnetic field. This field is emitted at the sensing face of the sensor.

• If attenuating material/ Target nears the sensing face, eddy currents are generated in the case of non ferrite metals . In the case of ferro magnetic metals, hysteresis and eddy current loss also occurs.

• These losses draw energy from the oscillating circuit and reduce oscillation.

• The signal evaluator detects this reduction and converts it into a switching signal.

Hall Effect sensors• Hall Effect :When a current carrying conductor is subjected

to magnetic field, a voltage is developed across the conductor transverse to the current flow and perpendicular to the magnetic field.

• The Hall coefficient is defined as the ratio of the induced electric field to the product of the current density and the applied magnetic field

• No magnetic field - Straight line current flow • With Perpendicular Mag. Field – Curved path and

accumulate on the surfaces ,produces an emfFor a simple metal strip( charge carriers: Electrons only) the Hall voltage VH= -IB/netI= current, B= Mag.Fluxt= thickness, n= charge carrier density

Hall Effect sensors• A Hall effect sensor : Analog/Digital transducer that varies

its output voltage in response to a magnetic field ( With a known magnetic field, its distance from the Hall plate can be determined).

• Hall sensor can be used to measure the current without interrupting the circuit.

• Hall effect sensors are used for proximity switching, displacement, positioning, speed detection, and current sensing applications

• Digital Type: Two magnets are placed on the disc .As the disc rotates ,LED tuns ON/OFF depends on the position of the coil & magnet

Hall Effect sensors

Photoconductive cell• Passive transducer used for the measurement of radiation

intensity.• Electrical resistance to the current flow varies as the light

intensity striking in it.• When the photocell has appropriate light is incident upon it, its

resistance is low and the current flow through the relay is high to operate the relay.

• When the light is interrupted shut off partially or completely, resistance of the photocell increases ,thereby reducing the current through the relay.

Photoconductive cell• Photo resistors/LDR are basically photocells (CdS, PbS,

InSb)• A photo resistor is made of a high

resistance semiconductor . If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron conduct electricity, thereby lowering resistance

Photovoltaic Cell• Active transducer which directly converts EM radiations/light into

electrical energy. • Generate a voltage proportional to light intensity (ex:

Si,Se,Ge,InAs,InSb)• AdvantagesActive TransducerTemperature range -100 to 1250CExtremely fast response

• Operation :Photons striking the cell pass through the thin upper layer and are absorbed by the electrons in the lower layer , causing the generation of conduction electron and holes. The depletion region separates conduction holes & electron causing a potential difference across the junction

Photo Diode• Symbol• When the device operates with a reverse voltage, it

functions as a photoconductive device. When operating without a reverse voltage, it functions as a photovoltaic device

• Response time of photodiode is very fast so that it may be used in applications where light fluctations occur at high frequency

• PIN photodiodeIntrinsic layer in betweenP and N region which increases depletion layerwidth thus reducing Capacitance. So faster response & low noise

Photo Transistor• Symbol

• More Sensitive than a photodiode (as much as100 times), by adding a junction resulting in NPN Device (but slower switching time)

• Illumination of the central region causes the release of electron hole pairs. This lower the potential barrier across both junctions, causing an increase in the flow of electrons from left region to central region and on to the right region.

Digital EncoderA digital optical encoder is a device that converts motion into a sequence of digital pulsesEncoders : linear and rotary (common type)Rotary encoders are manufactured in two basic forms:A) Absolute encoderHere, a unique digital word corresponds to each rotational position of the shaftB) Incremental encoder which produces digital pulses as the shaft rotates, allowing measurement of relative position of shaft

Digital Encoder (optical)• Most rotary encoders are composed of a glass or

plastic code disk with a photographically deposited radial pattern organized in tracks. As radial lines in each track interrupt the beam between a photo emitter-detector pair, digital pulses are produced. The optical disk of the absolute encoder is designed to produce a digital word that distinguishes N distinct positions of the shaft.

Absolute Encoder• For 8 tracks, the encoder is capable of producing

256 distinct positions or an angular resolution of 1.406 (360/256) degrees.

• The most common types of numerical encoding used in the absolute encoder are gray and binary codes.

• The gray code is designed so that only one track (one bit) will change state for each count transition, unlike the binary code where multiple tracks (bits) change at certain count transitions

Absolute Encoder• Gray code Absolute Encoder • Binary code Absolute Encoder

Incremental Encoder• The incremental encoder, sometimes called a relative

encoder• It consists of two tracks and two sensors whose

outputs are called channels A and B. As the shaft rotates, pulse trains occur on these channels at a frequency proportional to the shaft speed, and the phase relationship between the signals yields the direction of rotation

• By counting the number of pulses and knowing the resolution of the disk, the angular motion can be measured.

• The A and B channels are used to determine the direction of rotation by assessing which channels "leads" the other

Incremental Encoder• The signals from the two

channels are a 1/4 cycle out of phase with each other and are known as quadrature signals.

• Third output channel, called INDEX, yields one pulse per revolution, which is useful in counting full revolutions.

• It is also useful as a reference to define a home base or zero position

• Incremental encoder Pattern

Incremental Encoder• The direction of rotation(clockwise or counter-clockwise) is

determined by the level of one signal during an edge transition of the second signal

• In the 1X mode, A= with B =1 implies a clockwise pulse, and B=with A=1 implies a counter-clockwise pulse