Mechatronics - UNIT1-SH

8/14/2019 Mechatronics - UNIT1-SH

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1.0 INTRODUCTION:

SENSORS AND TRANSDUCERS

+ Sensor is used to produce a varying signal according to thequantity being measured.

+ Sensor is an element in a mechatronic system which acquires aphysical parameter and changes it into signal that can be processed bythe system.

+ The active element of a sensor is known as transducer.

+ Transducer converts the measured quantity, property (or) conditioninto a usable electrical output.

+ The mechatronic system requires sensors to measure physicalquantities such as position, distance, force, strain, temperature,vibration and acceleration. Simply sensors are also called transducers.

2.0 PERFORMANCE TERMINOLOGY:+ The function of the sensor (or) transducer is to sense (or) detect a

parameter such as pressure, temperature flow, motion, resistance,voltage, current and power.

+ The sensor should be capable of faithfully and accuratelydetecting any changes that occur in the measured parameter.

+ The performance of transducers can be defined by using the following terms:

1. Range and span

2. Error

3. Accuracy

4. Sensitivity

5. Hysteresis error

6. Non linearity error

7. Repeatability/Reproducibility

8. Reliability

9. Stability

10. Dead band/time

11. Resolution

12. Backlash

13. Output impedance


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1. Range and Span:

+ The range of a transducer defines the limits between which the input can vary.

+ The difference between the limits (maximum value - minimum value) isknown as span.

+ For example a load cell is used to measure force. An input force can vary from

20 to 100 N. Then the range of load cell is 20 to 100 N. And the span of loadcell is 80 N (i.e., 100-20)

2. Error:

+ If the transducer is ideally designed and made from appropriate materialswith ideal workmanship, then output will indicate the true value. But inactual practice the output of the transducer will deviate from the true value.

+ The algebraic difference between the indicated value and the true value ofthe measured parameter is termed as the error of the device.

+ Error = Indicated value true value

+ For example, if the transducer gives a temperature reading of 30 C when

the actual temperature is 29 C, then the error is + 1 C. If the actualtemperature is

3 1 C, then the error is 1C.

3. Accuracy:

+ Accuracy is the extent to which the value indicated by themeasurement system would be wrong.

+ Accuracy is the summation of all possible errors that are likely to occur.

+ For example, a thermocouple has an accuracy of 1 C. This meansthat reading given by the thermocouple can be expected to lie within + 1 C(o r)

1 C of the true value.

+ Accuracy is also expressed as a percentage of the full range output (or)full- scale deflection.

+ For example, a thermocouple can be specified as having an accuracy of 4% of full range output. Hence if the range of the thermocouple is 0 to200C, then the reading given can be expected to be within + 8 C (or) 8 C of the true reading.

4. Sensitivity:

+ The sensitivity is the relationship showing how much output we can getperunit input.

+ ie sensitivity = Output / Input

5. Hysteresis error:+ When a device is used to measure any parameter plot the graph of output

Vs value of measured quantity.+ First for increasing values of the measured quantity and then fordecreasing

values of the measured quantity.+ The two output readings obtained usually differ from each other.


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11. Resolution:

+ Resolution is defined as the smallest increment in the measured valuethat can be detected.

+ The resolution is the smallest change in the input value which willproduce an observable change in the input.

+ Resolution is also known as the degree of fineness with whichmeasurements can be made.

+ For example, if a micrometer with a minimum graduation of 1mm is.used to measure to the nearest 0.5 mm, then by interpolation, theresolution is estimated as 0.5 mm.

12. Backlash:

+ Backlash is defined as the maximum distance (or) angle throughwhich any part of a mechanical system can be moved in onedirection without causing any motion of the attached part.

+ Backlash is an undesirable phenomenon and is important in the

precision design of geartrains.

13. Output Impedance:

+ Before defining impedance, we should know about Ohm s law.

+ Ohm s law is used to define the relationship between voltage V, Current Iand

ResistanceR.

(i.e.,) V=IR

+ Ohm s law can be extended to the AC circuit analysis of resistor,capacitorand inductor elements as

v=ZI

where Z is called impedance of the elements. So impedance issimilar to resistance.

+ The sensors produce electrical output.

+ When these sensors are interfaced with an electronic circuit, it isnecessary to know the output impedance.

+ This impedance is connected in either series (or) parallel with thatcircuit and the inclusion of the sensor will modi1 the behaviour of thesystem to which it is connected.

3.0 DISPLACEMENT, POSITION AND PROXIMITY

Displacement Sensors:

The measurement of the amount by which some object has been moved.1. Potentiometer,2. Resistance strain gauge,

3. LVDT,4. Push pull displacement sensor.


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Position Sensors:+ The determination of the position of some object with reference to

some reference point.1. Photo electric sensors,

2. Hsensors.

Proximity Sensors:

+ Used to determine when an object has moved to within some particularcritical distance.1. Pneumatic proximity sensor,2. Eddy current proximity sensor,

3. Inductive proximity switch,4. Micro switch,

5. Reed switch.

Factors to be considered while selecting displacement, Position and

Proximity sensors:1. The accuracy required2. The resolution required

3. The size of the displacement4. Displacement type (linear orangular)5. The cost and material made

1. Contact Sensors:

+ The measured object is mechanical contact with the sensor.+ In the contact sensors there is a sensing shaft which is direct contact

with the object being monitored.+ The movement of the shaft may be used to make changes in electrical

voltage, capacitance, resistance.

2. Non-contact sensors:

+ The measured object is no physical contact between the measuredobject and the sensor.

+ In the non-contact sensors the measured object causing a change in theair

pressure in the sensor, or a change in inductance orcapacitance.

3.1 DISPLACEMENT SENSORS:

+ A potentiometer can be used to convert rotary or lineardisplacement to a voltage.

+ The potentiometers can be classified into three types.


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1. Potentiometer Sensor

+ Potentiometers consists of a resistance element with a sliding contactand the sliding contact can be moved over the length of theelement. This sliding contact is called Wiper.

+ The motion of the sliding contact may be linear orrotational.

+ The Fig.1.5 shows the linear potentiometer and the Fig.1.6 shows therotary potentiometer.

+ The rotary potentiometer consists of a circular wire-wound track overwhich a rotatable sliding contact can be rotated.

+ The wire-wound track may be single turn or helical turn.

Displacement and Position SensorTypes:

The displacement and position sensors are grouped into:1. Contact sensors

2. Non-contact sensors1. Rotary

2. Linear3. Helical potentiometers

Fig.1.5

Fig.1.6


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Fig.1.7

Advantages of Resistance Potentiometers:

1. They are simple and in expensive,2. Electrical efficiency is high,

3. Simple in operation.4. Useful for measurement of large amplitudes ofdisplacement

2. Strain Gauged Element:

+ The change in length divided by original length is called strain.+ The strain gauge consists of metal wire, metal foil strip. When

subject to strain, the resistance R changes, and the changein resistance L is proportional to strain E.

where G is a constant (gauge factor).+ In the Fig.1.9 the strain gauge is attached to flexible elements in the

form ofcantilevers, rings, U shapes.+ When the flexible element is bent, as a result of this the electrical

resistance will change due to force applied by a contact point.+ The change in resistance is the measure ofdisplacement.+ The Fig.1.8 and 1.9 shows the strain gauges and strain gauged elements.+ The major types of strain gauges are

I. Metal wire strain gauges,2. Metal foil strain gauges,


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3. Semiconductors straingauges.

Fig.1.8

Fig.1.9


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+ In Metal Wire Strain Gauges a wire stretched between two pointsin an insulating medium such as air.

+ The wires may be made of various copper nickel, chrome nickel ornickel iron alloys. They are about 0.003 mm in diameter and gaugefactor of 2. The length of wire is 25 mm orless.

+ In Metal foil strain gauge the foil is usually made up of constantan, and it

is etched in a grid pattern onto a thin plastic backing material, usuallypolyimide. The foil is terminated at both ends with large metallic pads.

+ The size of the entire gauge is very small and has a length of 5 mm to15 mm length.

+ In Semiconductor strain gauges the p type and n type siliconsemiconductors are used.

+ The semiconductor strain gauges have the gauge factors of about +100 or

100. In p-type gauges resistance increases with tensile strain. While in n-type,

resistance decreases. Typical thickness is about 0.25 mm andeffective length range from 1.25 to 12 mm.

4. Linear Variable Differential Transformer (LVDT)

Fig.1.10

Fig.1.11


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+ It consists of three coils symmetrically spaced along an insulated tube.+ The central coil is primary and other two are secondary.+ A magnetic core is moved through the central tube, so that the

displacement being monitored.+ When voltage is supplied to the primary coil, alternating e.m.f.s are

induced in the secondary coils.

+ Suppose the magnetic core is in central, the e.rn.f. induced in each coilis same because of magnetic material in each coil is same and opposeto each other. So there is no output.

+ If the core is displaced from the central position there is a greateramount ofmagnetic core in one coil than the other. This will create ahigher e.m.f. in one coil and lesser e.m.f. in the other coil. This willmake a net difference in two e.m.f.s and the displacement beingmonitored.

+ The formulas which are used in LVDT are:

1. The e.m.f.s induced in the two secondary coils 1 and 2 are:

where K1, K2 are degree of coupling between the primary andsecondary coils.

Advantages of L VDT:1. High range2. Friction and electrical isolation3. Low hysteresis4. Power consumption is less.

5.Push Pull Displacement Sensor:

+ It has three plates with the upper pair forming one capacitor and thelowerpairforming anothercapacitor.

+ There is a non-linear relationship form between the change in capacitanceAC

and the displacement X.

+ The displacement moves the central plate between the two otherplates.+ The result of this, the central plate moving downwards and to

increase the plate separation of the upper capacitor and decrease theseparation of the lower capacitor.

+ Therefore, the capacitance of a parallel plate capacitor is given by

where C1 is in one arm of an a.c. bridge,


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C2 is in an other arm of an a.c. bridge.

= Relative permittivity of the dielectric between the

= Permittivity of free space

constant, x = Displacement,A = The area of overlap between the two plates,

d = The plate separation.

3.2 POSITION SENSORS+ position sensors report the position of an object with respect to a

reference part.+ The information can be an angle as in many degree a dish antenna hasturned.+ The following are the position sensors.

1. Photoelectric Sensors

+ It is used to detect the object by breaking a beam of light (ReferFig.1.12(a)) or radiation falling on a device or by detecting the light

reflected back by the object (ReferFig.1.12(b)).

Fig.1.12

2. Hall effect Sensors

+ Hall effect: Hall effect is defined as when a beam of charged particlespasses through magnetic field, the beam is deflected from its straightline path due to the forces acting on the particles.

+ A current flowing in a conductor like a beam is deflected by a magnetic field.

Fig.1.13

+ The working principle of a Hall effect sensor is that if a strip ofconducting material carries a current in the presence of atransverse ngne1i shown in Fig.1.13.


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+ The difference of potential is produced between the opposite edgesof the conductor. The magnitude of the voltage depends upon thecurrent and magnetic field.

+ In the Fig. the current is passed through leads 1 and 2 of the strip. Theoutput leads connected with Hall strip.

+ When a transverse magnetic field passes through the strip the

voltage difference occur in the output leads.+ The hail effect sensor have the advantages of being able to operate as

switches and it operate upto 100 KHz.

Fig.1.14

Applications of Hall Effect Sensors:

1. It is used as a Magnetic to electric transducer.2. It is used for the measurement of the position or displacement of a

structural element.

3. It is used for measurement ofcurrent.4. It is used for measurement ofpower.

Digital Optical Encoder:

+ A digital optical encoder is a device that converts motion into asequence ofdigital pulses.

+ By counting or decoding these bits and the pulses can be convertedinto

relative or absolute position measurements.+ Encoders are in Rotary, linearconfigurations.+

The Rotary encoders are in two forms.1. Absolute encoder2. Incremental encoder.

1. Absolute Encoder:

+ The absolute encoder is designed to produce a digital word thatdistinguishes

N distinct positions of the shaft.


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Fig. 1.15. Components of an opticalencoder

+ The Fig.1.15 shows the basic form of an absolute encoder.+ The rotating disc has four concentric circles of slots and four sensors

to detect the light pulses.

+ The slots are arranged in such a way that the output is made in thebinary

code.+ The number of bits in the binary number will be equal to the number oftracks.+ The most common types of numerical encoding used in the absolute

encoderare gray and natural binary codes.+ To illustrate the action of an absolute encoder, the gray code and

natural binary code disk track patterns for a simple 4 track (4-bit)encoder is shown in Fig.1.16

Fig. 1.16 4-bit gray code absolute encoder disk trackpatterns

2. Incremental Encoder:+ Working: A beam of light passes through the slots in a disc and it is

detected by a suitable light sensor.+ When the disc is rotated, the output is shown in terms of pulses

and these pulses being proportional to the angle of disc rotation.


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Fig. 1.17. Incrementalencoder

+ So the angular position of the disc is determined by the number ofpulses produced. In the above Fig. three tracks and three sensors areused.

+ The inner track has just one hole and other two tracks have a series ofequally

spacedholes.

+ The angle is determined by the number slots on the disc.

3.3 PROXIMITY SENSOR

+ A proximity simply tells the contra! system whether a moving partis at a certain place.

+ Proximity sensors come under the non contact type sensors.

+

The following are the some of the proximity sensors.

1. Pneumatic proximity sensor:+ Working: Low-pressure air is allowed and to escape through a port

which is placed in the front position of the sensor. This escapingair reduces the pressure in the nearby sensor output port, when thereis no close by object.

Fig. 1.18. Pneumatic proximity


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sensor


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+ If there is a close by object means the air will not escape readily,so the pressure increases in the sensor output port. This outputfrom the sensordepends on the proximity ofobjects.

2. Eddy current proximity sensors:

Fig. 1.19. Eddy currentsensor

+ Working: When alternating current is supplied to the coilmeans the alternating magnetic field is produced. If there is a metal

object in close proximity to this alternating magnetic field the eddycurrent is induced in it. This eddy current will produce a magnetic fieldthemselves and the impedance of the coil changes the amplitude of thealternating current.

+ The above Fig. shows the basic form of such sensor and it is usedfor the detection of non-magnetic conductive materials.

3. Inductive proximity switch:

+ It is used for the detection of metal objects and it consists of a coilwound around a core.

+ The metal object is close to the coil means it will produce a inductance

change in the coil. This inductive change is being monitored.

4. Microswitch:+ It is used for determining the presence of an item on a conveyor belt

and this might be actuated by the weight of the item on the beltdepressing the belt by a spring loaded platform nearer to the sensorthe presence of item in the conveyor is determined.

+ The closeness of switch is done by movement of this spring loadedplatform.

Fig. 1.20.Microswitch

5. Reed switch:+ It is a non-contact proximity switch. It is used for checking the

closure of doors.


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+ It consists of two magnetic switch contacts sealed in a glass tube.

Fig. 1.21. Reedswitch

+ When a magnet is brought close to the switch, the magnetic reeds areattracted each other and close the switch contacts.

4.0 VELOCITY AND MOTION:

To detect and monitor the velocity and motion the following sensors are used.

4.1 VELOCITY MEASUREMENT+ Velocity sensors or tachogenerators are devices that give an

output proportional to angularvelocity.+ These sensors find wide application in motor speed control systems.+ The following are the various velocity sensors.

1. Electro Magnetic Transducer,+ The most commonly used transducer for measurement of linear

velocities is electromagnetic transducer.+ The electromagnetic transducers are classified into two categories.

1. Moving Magnet Type:2. Moving coil

type.

+ In moving magnet type the sensing element is a rod that is rigidlycoupled to the device whose velocity is being measured.

+ This rod is a permanent magnet. This permanent magnet issurrounded by a coil.

+ The motion of the magnet induces a voltage in the coil and theamplitude ofthe voltage is directly proportional to the velocity.


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Fig. 2.22. Moving magnet typetransducer

2. Moving coil type velocity transducer:

+ It is operated through the action of a coil moving in a magnetic field.+ A voltage generated in the coil is proportional to the velocity of the coil.+ This is a more satisfactory arrangement due to it forms a closed

magnetic circuit with a constant air gap and the device is contained anantimagnetic case which reduces the effects of stray magnetic field.

Fig. 2.23. Moving coil type velocitytransducer

3. Tachogenerators:

+ A sensor that converts speed of rotation directly into electrical signal iscalled a tachogenerator.

+ It is used to convert angular speed into a directly dependent voltage signal.(a) Toothed Rotor Variable Reluctance Tachogenerator:

+ It is used to measure angularvelocity.+ This tachogenerator consists of a metallic toothed rotor mounted on the

shaft whose speed is to be measured.+ A magnetic pick up is placed near the toothed rotor and this magnetic pickup

consists of a housing, and the housing containing a small permanent


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magnet with a coil wound around it.+ When the rotor rotates, the reluctance of the air gap between pickup

and the toothed rotor changes and the rise in e.m.f. is induced inthe pickup coil. Finally the output is in the form of pulses and waveshapes.

+ The pulses induced depend upon the number of teeth in the rotor

and the rotational speed. When the speed is known, the rotationalspeed is calculated by measuring the frequency pulses.

+

Fig. 2.24. Toothed rotor tachometergenerator

+ Suppose the rotor has n teeth and the speed of rotation is Nr.p.s. and number of pulses per second is p.

+ The number of pulses per revolution = n = n

The advantage of toothed rotor variable reluctance tachogenerator is theinformation from this device can be easily transmitted and easy to calibrate.

4. A. C. Generator Form ofTachogenerator:+ It consists of rotor, which rotates with the rotating shaft and a coil.+ When the coil rotates in the magnetic field the e.m.f. is induced.+ The magnet may be in the form of stationary permanent

magnet or electromagnet.+ The frequency of this alternating e.m.f. is used to measure the

angular velocity.+ The output voltage is rectified and it is measured with a permanentmagnet

moving coil (PMCC)voltmeter.


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Fig. 2.25. A.C Tachometergenerator


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4.2 MOTION SENSORS1. Stroboscope:+ Stroboscope is a simple portable manually operated device for

periodic orrotary motions measurement.+ It is a variable frequency flashing light instrument and the flashing is

set by the operator.

+ If a strong light is caused to flash on a moving object at the timeeach flash occurs. The stroboscope occupies a given position, and theobject will appearto be stationary.

+ The flashing light whose frequency can be varied and controlled,and this source is called strobotron.

2 Pyroelectric Sensors:+ It consists of a polarised pyroelectric crystal with thin metal film

electrodes on opposite faces. (Pyro electric materials, e.g., lithiumtantalate are crystalline materials which generate charge in response toheat flow. When such materials heated to about 610 C in an electricfield, the electric dipoles within the material line up and it becomes

polarised as shown in Fig.).+ Due to the crystal is polarised with charged surfaces, the ions are

drawn from the surrounding air and electrons from any measurementcircuit is connected to the sensor to balance the surface charge asshown in Fig.

+ For measurement of a human or heat source motion, the sensingelement has to differentiate between general background heat radiationand a moving heat source. For that a single pyroelectric sensor isnot capable to use and dual pyroelectric sensors are used as shownin Fig.

+ In this dual pyroelectric sensors the sensing element has the onefront electrode and two back electrodes. When two sensors being

connected means both sensors are receive the same heat signal andtheir outputs are cancelled.


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Fig. 2.26. Pyroelectric

sensors


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+ Suppose a heat source moves from its position means the heatradiation moves from one of the sensing elements to the other, thenthe current is alternates in one direction first and then reversed to theother direction second.

+ A moving human gives an alternating current of 1O A. When theinfrared radiation is incident on the dual pyroelectric sensor material

and changes its temperature, the polarisation in the crystal isreduced. A focusing device is needed to direct the infrared radiationonto the sensor.

5.0 FLUID PRESSURE SENSORS+ The devices which are used to monitor fluid pressure in industrial

processes is diaphragms, bellows, capsules and tubes.+ The types of pressure measurements required are

(1) Absolute pressure measurement,(2) Differential pressure measurements.

+ In absolute pressure measurements the measurement is related tovacuum pressure (zero pressure) and in differential pressure

measurement the difference in pressure is measured. The types ofpressure measurement devices are discussed below.

1. Diaphragms+ In this the pressure to be measured is applied to the diaphragm, causing

it to deflect, and the deflection being proportional to the appliedpressure. This movement can be monitored by some form ofdisplacement sensor. (Example for displacement sensor is straingauge) and it is shown in Fig.2.27.

Fig. 2.27. Diaphragm pressure gauge


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Fig. 2.28. Diaphragm type strain gauge pressuretransducer

+ A specially designed strain gauge is also used for measuring pressureand it consisting of four strain gauges with, two measuring the strain in acircumferential direction while remaining two measure strain in aradial direction. The four strain gauges are connected to form thearms of a wheatstone bridge a shown in Fig.2.28.

+ The deflection at any point is shown in terms of +ve and ye sign. Thestress distribution on the diaphragm surface is almost ideal forpractical purposes, since both compressive and tensile stresses exit.So this will allow the use ofa four arm wheatstone bridge where all thegauges are active and consequently there is a large output.

+ The strain gauges I and 4 are placed at close to the centre andoriented to read tangential strain and its value is +ve maximum at thispoint.

+ The gauges 2 and 3 are oriented to read radial strain and it is placedclose to the edge as possible.

2. Bellows+ A metallic bellows is a series of circular parts as shown in Fig.2.29and the parts are formed or joined in such a manner that theyare expanded orcontracted axially by change in pressure.


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Fig. 2.29.Bellows


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+ The Fig.2.30 shows the bellows can be combined with a LVDT togive a pressure sensor with an electrical output.

+ The bellows are made up of materials like stainless steel, phosphorbronze, nickel, rubber and nylon.

+ The output pressure is calibrated through the LVDT.

Fig. 2.30. L VDT withbellows

3. Capsule

Fig. 2.31.Capsule

+Capsules are one of the pressure measuring device and it can beconsidered to be just two corrugated diaphragms combined and giveeven greatersensitivity.

+ The capsules are more sensitive in measuring pressure.

4. Tube Pressure Sensors

Fig. 2.32. Tube pressuresensors

+ In tube pressure measurement the increase in pressure in a tube iscause the tube in circular cross-section. It is shown in the above Fig.The tubes having greater sensitivity while the pressure increases.

+ The tubes are made up of stainless steel and phosphor bronze.

5. Tactile Sensor


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+ It is one form of pressure sensor and it is used to determine thepressure in

Robotics in such a form fingertips of robotics contact with theobject.


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+ These type of sensors also used in touch display screens wherephysical contacts to be sensed.

+ The above Fig.2.33 shows the one form of tactile sensor.+ It uses piezo electric polyvinylidene fluoride (PVDF) film.+ There are two layers of such film is used and it is separated by a

soft film which transmits vibrations.

Fig. 2.33. PVDF tactilesensor

+ The alternating voltage is supplied in the lower PVDF film and thisresults in mechanical oscillations of the film.

+ The intermediate film transmits these vibrations to the upper PVDF film.+ Due to the piezoelectric effect the vibrations formed are cause an

alternating voltage to be produced across the upperfilm.+ So the pressure is applied to the upper PVDF film and its vibrationsare

affected the output voltage.

6. Piezoelectric sensor

Fig. 2.34. Sensor equivalentcircuit

+ The electrical circuit for a piezo electric sensor is a chargegenerator in parallel with capacitance Cs and in parallel withResistance Rs.

+ The effective circuit is as shown by the Fig. when the sensor is

connected via a cable of capacitance C and resistance RA.+ The sensor is charged subject to pressure change and the

capacitor will discharge with time. The discharge time depends on thetime constant ofthe circuit.

LIQUID FLOW SENSORS+ There are many devices used to measure the liquid flow.+ The basic principle in measuring flow is the fluid flowing through the

pipe persecond is proportional to square root of pressure difference.+ The following flow measuring devices are used to measure the liquid flow.


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1. Turbine Flowmeter+ The Fig.2.35 shows the turbine flowmeter and it consists of a multi-

bladed rotor which is supported in the pipe along with the flow occurs.+ The rotor rotation depends upon the fluid flow and the angular

velocity is proportional to the flow rate.+ The rotor rotation is determined y the magnetic pick-up, which is

connected to the coil.+ The revolution of the rotor is determined by counting the number of

pulses produced in the magnetic pick up. The accuracy of thisinstrument is 3%.

Fig.2.35.

2. Orifice Plate+ It is a simple disc with a central hole and it is placed in the tube through

which the fluid flow.

Fig. 2.36. Orificeplate

+ From the above Fig.2.36 the pressure difference measured between

a point equal to the diameter of the tube upstream and half thediameter of down stream.

+ The accuracy of this instrument is 1.5%.

LIQUID LEVEL MEASUREMENTThe liquid level measurement is done by using

1. Differential pressure sensorand2. Float system.


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1. Differential Pressure Sensor+ In this the differential pressure cell determines the pressure difference

between base of the liquid and atmospheric pressure.+ The differential pressure sensor can be used in either form of open or

closed vessel system.

Fig.2.37.

2. Float System+ In this method the level of liquid is measured by movement of a float.+ The movement of float rotates the arm and slider will move

across a potentiometer.+ The output result is related to the height of the liquid.

Fig.

2.38.

6.0 TEMPERATURE SENSORS+ Temperature measurements are amongst the most common and

the most important measurements made in controlling industrialprocesses.

+ Changes that are commonly used to monitor temperature are, theexpansion or contraction of solids, liquids or gases, the change inelectrical resistance of conductors, semiconductors andthermoelectric e.m.f.s. The control system which are used to measurethe temperature is as follows

1 Thermocouples

+ The most common electrical method of temperature measurementuses the thermocouples.

+ The basic principle of this is, if two different metals are joinedtogether, a potentiometer difference occurs across thejunction.

+ The potential difference depends on the metals used and thetemperature ofthe junction.

+ When both junctions are at the same temperature, there is no net e.ni.f.But ifthere is a difference in temperature between the junction thee.m.f. will be produced.

+ This e.m.f. will depend upon the two metals and the temperature


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between the junctions. One junction is held at 0 C and the equationwhich is used to find out the e.m.f. is


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+

Fig. 2.39.Thermocouple

+ There are three e.m.f.s present in a thermoelectric circuit. In this theSeebeck e.m.f. is caused by the junction of dissimilar metals and thePettier e.m.f. is caused by a current flow in the circuit, and the Thomsone.m.f. which results from a temperature gradient in the materials.

+ It is observed that all thermocouple circuits must involve at

least two junctions. In that one of the junctions senses the desiredor unknown temperature.

+ This junction is called the hot or measuring junction. The otherjunction is usually maintained at a known fixed temperature and thisjunction is called the cold or referencejunction.

+ If the temperature of the reference or cold junction is known, thetemperature of the hot or the measuring junction can becalculated by using the thermoelectric properties of the materials.

+ If thermocouple circuit can have other metals in the circuit and they willhave

no effect on the thermoelectric e.m.f.+ A thermocouple can be used with the reference junction at a

temperature other than+ 0C.+ For that we assume a 0 C junction and the correction has to be

applied using the law of intermediate temperatures.

The equation used in this is

Fig. 2.41. Las of intermediatetemperature


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+ Here to maintain the 0 C at one junction a compensation circuit isUsed to provide an e.m.f. which varies with the temperature of the coldjunction.

+ When it is added to the thermocouple e.m.f. it will generate a combinede.m.f.

This is shown in Fig.2.42.

Fig. 2.42. Compensation thermocouple

+ In the above Fig.2.42, the wires from the measuring junction arescrewed directly to an isothermal block terminal strip.

+ The temperature of the block is ambient temperature.

+ This reference temperature is measured by semiconductorsensor and compensation circuitry develops a voltage Ecompwhich is combined with measuring junction and the net voltage acrossthe voltmeter = T (Temperature being measured).

+ The isothermal block can accept many thermocouple pairs inmultichannel instruments with microprocessor computing power

since the T (reference junction sensor now sends its temperaturedata to the computer which computes the needed voltage correctionfor each thermocouple.

+ The thermocouples like E, J, K and T are relatively cheap and it hasaccuracies

of about ito 3%.+ The noble metal thermocouples are very high cost compared with this

and it has accuracies of about 1% better than the base metalthermocouples.

+ Thermocouples are used in applications ranging from measurementof room air temperature to that of a liquid metal bath. The problemswhich may be encountered are

1. Faulty reference junction,2. Installation faults,3. Junctions formed by users may involve excessive temperatures

orfaulty soldering techniques,4. Gross errors can result due to wrong installation of thermocouple.

2. Resistance Temperature Detectors (RTDs)

+ Resistance temperature detectors (RTDs) or resistance thermometersare basic instruments for measurement ofresistance.

+ The materials used for RTDs are Nickel, Iron, Platinum, Copper,


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+

Fig. 2.43. Resistance temperaturedetector

+ The Resistance temperature detectors are simple, and resistiveelements in the form of coils of wire and it is shown in the aboveFig.2.44.

+ The equation which is used to find the linear relationship in RTD is

Fig. 2.44. RTDelement

Constructional Details ofRTDs:+ The platinum, nickel and copper in the form wire are the most

commonly used materials in the RTDs.+ Thin film platinum elements are often made by depositing the metal ona

suitable substrate wire- wound elements involving a platinum wireheld by a high temperature glass adhesive inside a ceramic tube.

+ This is shown in Fig.2.45.

Fig.


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2.45.


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Salient Features ofRTDs:1. High degree ofaccuracy.2. Resistance thermometer is interchangeable in a process withoutcompensation or recalibration.3. It is normally designed for fast response as well as accuracy toprovide close control ofprocesses.

3. Thermistors+ Thermistor is a semiconductor device that has a negative

temperature coefficient of resistance in contrast to positive coefficientdisplayed by most metals.

+ Thermistors are small pieces of material made from mixtures of metaloxides,

such as Iron, cobalt, chromium, Nickel, and Manganese.+ The shape of the materials is in terms of discs, beads and rods.+ The thermistor is an extremely sensitive device because its resistance

changes rapidly with temperature.

+ The resistance of conventional metal-oxide thermistors decreases in

a very non-linear manner with an increase in temperature is shownin the Fig.2.46 below.

+ The change in resistance per degree change in temperature isconsiderably

larger than that which occurs with metals.

Fig. 2.46.Thermistors

+ The simple series circuit for measurement of temperature using athermistorand the variation of resistance with temperature for atypical thermistor is shown in the below Fig.2.47.

Fig. 2.47.Thermistor

+ The thermistor is an extremely sensitive device because its resistancechanges rapidly with temperature.

+ Thermistors have many advantages when compared with othertemperature sensors.

+ The main disadvantage is highly non-linearbehaviour.


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4. Thermodiodes and Transistors

(a) Thermodiodes:+ Thermodiode is widely used method for measuring temperature.

When the temperature of doped semiconductors changes, themobility of their charge carriers changes and this affects the rate at

which electrons and holes can diffuse across ap-njunction.1. Measurement oftemperature,2. Control oftemperature,3. Temperature compensation,

4. Measurement of thermal conductivity,5. Measurement of power at high frequencies,

6. Measurement of composition ofgases,7. Providing time delay,

8. Vacuum measurements.

+ The difference in voltage and current through the junction is a functionofthe temperature. The equation which is used to find the I is

+ From the above equation the voltage V is proportional to thetemperature on Kelvin scale and the potential differencemeasurement across a diode at constant current is used to measurethe temperature.

(b) Transistor:+ In Thermo transistor the voltage across the junction between the base

and the emitter depends on the temperature.+ A common method is use of two transistors with different collector

currentand finding the difference in the base-emitter voltages between them,and this difference is the measure oftemperature.

Fig. 2.48.Transistor

+ The thermotransistors can be combined with circuit components on asingle chip to give a temperature sensor.

+ This is shown in the above Fig.2.48.


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5. Bimetallicstrips

Fig. 2.49. Bimetallic thermostat

+ A Bimetallic thermostat consists of two different metal stripsbounded together and they cannot move relative to each other.

+ These metals have different coefficients of expansion andwhen the temperature changes the composite strips bends into acurved strip, with the higher coefficient metal on the outside of thecurve.

+ The basic principle in this is all metals try to change their physicaldimensions at different rates when subjected to same change intemperature.

+ This deformation may be used as a temperature- controlled switch, asin the simple thermostat.

+ The Fig.2.49 shows the Bimetallic thermostat which was commonlyused with domestic heating systems.

7.0 LIGHT SENSORS1. Photodiodes+ Diodes like photodiodes and semiconductor diodes are connected into

a circuit in reverse bias giving a very high resistance.+ When light falls on the junction the resistance of the diode will drop and

the current in the circuit will rise.

Fig.2.50.


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+ The Fig.2.51 shows the diode characteristics.


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the light having wavelengths of about 520 mm to 700 mm.4. Array of Light Sensors+ This will be used in small space like rooms to determine the variations

oflight intensity across that space.

e.g., Automatic camera


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8.0 SELECTION OF SENSORS

The factors to be considered while selecting sensors are

1. The nature of output required from the sensor.2. The nature of measurement required.

3. The accuracy of the sensor.4. The cost of the sensor.5. The power requirement of the sensor.6. The speed response of the sensor.

7. The linearity of the sensor.8. The Reliability and Maintainability of the sensor.

9. Environmental conditions under which the measurement is to be made.10. Signal conditioning requirements.

11. The nominal and range of values of the sensor.12. Suitable output signals from the measurement.

PART- A

1. What is the use of sensors and transducers?

2. Differentiate between Range and Span.3. Give the formula for finding the repeatability of a transducer.

4. What is hysteresis error?

5. What is the difference between Accuracy and Precision ?6. What is threshold?

7. What is Dead time and Dead zone?

8. What is resolution?

9. What is Rise time and Settling time?

10. What is meant by Hall effect?

11. What are the velocity and motion sensors?

12. What is non-linearity error?

13. Give the example for measuring force.

14. What are the fluid pressure sensors?

15. What are the liquid flow measuring devices?

16. What are the two types diaphragms?

17. What are the Temperature measuring devices?

18. Give the example for light sensors.

19. What is the basic principle in thermocouples?

20. Give some materials used in thermocouples

21. What is offset voltage of an operational amplifier

22. What is the equation for V of an integrator?

23. What is a precision diode?


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Mechatronics - UNIT1-SH