UNIT – II SENSORS AND TRANSDUCERS 1.0 INTRODUCTION: v Sensor is used to produce a varying signal according to the quantity being measured. v Sensor is an element in a mechatronic system which acquires a physical parameter and changes it into signal that can be processed by the system. v The active element of a sensor is known as transducer. v Transducer converts the measured quantity, property (or) condition into a usable electrical output. v The mechatronic system requires sensors to measure physical quantities such as position, distance, force, strain, temperature, vibration and acceleration. Simply sensors are also called transducers. 2.0 PERFORMANCE TERMINOLOGY: v 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. v The sensor should be capable of faithfully and accurately detecting any changes that occur in the measured parameter. v 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: v The range of a transducer defines the limits between which the input can vary. v The difference between the limits (maximum value - minimum value) is known as span. v 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 load cell is 80 N (i.e., 100-20) 2. Error: v If the transducer is ideally designed and made from appropriate materials with ideal workmanship, then output will indicate the true value. But in actual practice the output of the transducer will deviate from the true value. v The algebraic difference between the indicated value and the true value of the measured parameter is termed as the error of the device. v Error = Indicated value —true value v 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 actual temperature is 3 1° C, then the error is —1°C. 3. Accuracy: v Accuracy is the extent to which the value indicated by the measurement system would be wrong. v Accuracy is the summation of all possible errors that are likely to occur. v For example, a thermocouple has an accuracy of ± 1° C. This means that reading given by the thermocouple can be expected to lie within + 1°C (or) — 1°C of the true value. v Accuracy is also expressed as a percentage of the full range output (or) fullscale deflection. v 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 to 200°C, then the reading given can be expected to be within + 8°C (or) —8° C of the true reading. 4. Sensitivity: v The sensitivity is the relationship showing how much output we can get per unit input. v ie sensitivity = Output / Input 5. Hysteresis error: v When a device is used to measure any parameter plot the graph of output Vs value of measured quantity. v First for increasing values of the measured quantity and then for decreasing values of the measured quantity. v The two output readings obtained usually differ from each other.
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Fig.1.1 Hysteresis error v This is because of a certain amount of internal (or) external friction in the response of the sensing element. v The maximum difference in between any part of output readings so obtained is known as hysteresis error. v The hysteresis error can be reduced by proper design and selection of the mechanical components, introducing greater flexibility and providing suitable heat treatment to the materials. 6. Non-linearIty error: v A linear relationship is assumed between the input and output and hence, a straight line is drawn in the graph as shown here.
Fig.1.2 v Some transducers, do not have linear relationship and errors occur as a result of the assumption of linearity. v The error is defined as the maximum difference from the straight line. v There are three methods to find the the numerical error. They are namely, (i) End range value (ii) Best straight line for all values (iii) Best straight line, through zero point. v In the first method, (fig 1.2), the straight line is drawn by joining the output values at the end points of the range.
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v In the next method, the straight line is drawn by using the method of least squares to determine the best fit line by considering all data values are in error. Refer fig (1.3). v In the last method, the straight line is drawn by using the method of least squares to determine the best fit line which passes through the zero point.
Fig.1.3
Fig.1.4
7. Repeatability/Reproducibility: v The repeatability and reproducibility of a transducer are its ability to give the same output for repeated applications of the same input value. v Repeatability is also defined as the measure of the deviation of test results mean value. 8. Reliability: v The reliability of a system is defined as the possibility that it will perform its assigned functions for a specific period of time under given conditions. v The reliability of a device (or) system is affected not only by the choice of individual parts in system but also by manufacturing methods, quality of maintenance and the type of user. 9. Stability: v The stability of a transducer is its ability to give the same output when used to measure a constant input over a period of time. v The term drift is the change in output that occurs over time. v The drift can be expressed as a percentage of the full range. v Zero drift means if there is change in output when there is zero input. 10. Dead band / time: v There will be no output for certain range of input values. This is known as dead band. There will be no output until the input has reached a particular value. v The length of time from the application of an input until the output begins to respond and change is known as Dead time.
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11. Resolution: v Resolution is defined as the smallest increment in the measured value that can be detected. v The resolution is the smallest change in the input value which will produce an observable change in the input. v Resolution is also known as the degree of fineness with which measurements can be made. v For example, if a micrometer with a minimum graduation of 1mm is. used to measure to the nearest 0.5 mm, then by interpolation, the resolution is estimated as 0.5 mm. 12. Backlash: v Backlash is defined as the maximum distance (or) angle through which any part of a mechanical system can be moved in one direction without causing any motion of the attached part. v Backlash is an undesirable phenomenon and is important in the precision design of gear trains. 13. Output Impedance: v Before defining impedance, we should know about Ohm’ s law. v Ohm’ s law is used to define the relationship between voltage V, Current I and Resistance R. (i.e.,) V=IR v Ohm’ s law can be extended to the AC circuit analysis of resistor, capacitor and inductor elements as v=ZI where Z is called impedance of the elements. So impedance is similar to resistance. v The sensors produce electrical output. v When these sensors are interfaced with an electronic circuit, it is necessary to know the output impedance. v This impedance is connected in either series (or) parallel with that circuit and the inclusion of the sensor will modi1 the behaviour of the system 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. 5
Position Sensors: v The determination of the position of some object with reference to some reference point. 1. Photo electric sensors, 2. H sensors. Proximity Sensors: v Used to determine when an object has moved to within some particular critical 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 required 2. The resolution required 3. The size of the displacement 4. Displacement type (linear or angular) 5. The cost and material made 1. Contact Sensors: v The measured object is mechanical contact with the sensor. v In the contact sensors there is a sensing shaft which is direct contact with the object being monitored. v The movement of the shaft may be used to make changes in electrical voltage, capacitance, resistance.
2. Non-contact sensors: v The measured object is no physical contact between the measured object and the sensor. v In the non-contact sensors the measured object causing a change in the air pressure in the sensor, or a change in inductance or capacitance. 3.1 DISPLACEMENT SENSORS: v A potentiometer can be used to convert rotary or linear displacement to a voltage. v The potentiometers can be classified into three types.
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1. Potentiometer Sensor v Potentiometers consists of a resistance element with a sliding contact and the sliding contact can be moved over the length of the element. This sliding contact is called Wiper. v The motion of the sliding contact may be linear or rotational. v The Fig.1.5 shows the linear potentiometer and the Fig.1.6 shows the rotary potentiometer. v The rotary potentiometer consists of a circular wire-wound track over which a rotatable sliding contact can be rotated. v The wire-wound track may be single turn or helical turn. Displacement and Position Sensor Types: The displacement and position sensors are grouped into: 1. Contact sensors 2. Non-contact sensors 1. Rotary 2. Linear 3. 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 of displacement 2. Strain Gauged Element: v The change in length divided by original length is called strain. v The strain gauge consists of metal wire, metal foil strip. When subject to strain, the resistance ‘ R’ changes, and the change in resistance L is proportional to strain E.
where G is a constant (gauge factor). v In the Fig.1.9 the strain gauge is attached to flexible elements in the form of cantilevers, rings, U shapes. v When the flexible element is bent, as a result of this the electrical resistance will change due to force applied by a contact point. v The change in resistance is the measure of displacement. v The Fig.1.8 and 1.9 shows the strain gauges and strain gauged elements. v The major types of strain gauges are I. Metal wire strain gauges, 2. Metal foil strain gauges,
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3. Semiconductors strain gauges.
Fig.1.8
Fig.1.9
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v In Metal Wire Strain Gauges a wire stretched between two points in an insulating medium such as air. v The wires may be made of various copper nickel, chrome nickel or nickel iron alloys. They are about 0.003 mm in diameter and gauge factor of 2. The length of wire is 25 mm or less. v 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, usually polyimide. The foil is terminated at both ends with large metallic pads. v The size of the entire gauge is very small and has a length of 5 mm to 15 mm length. v In Semiconductor strain gauges the p type and n type silicon semiconductors are used. v 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 and effective length range from 1.25 to 12 mm. 4. Linear Variable Differential Transformer (LVDT)
Fig.1.10
Fig.1.11
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v It consists of three coils symmetrically spaced along an insulated tube. v The central coil is primary and other two are secondary. v A magnetic core is moved through the central tube, so that the displacement being monitored. v When voltage is supplied to the primary coil, alternating e.m.f.s are induced in the secondary coils. v Suppose the magnetic core is in central, the e.rn.f. induced in each coil is same because of magnetic material in each coil is same and oppose to each other. So there is no output. v If the core is displaced from the central position there is a greater amount of magnetic core in one coil than the other. This will create a higher e.m.f. in one coil and lesser e.m.f. in the other coil. This will make a net difference in two e.m.f.s and the displacement being monitored. v 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 and secondary coils.
Advantages of L VDT: 1. High range 2. Friction and electrical isolation 3. Low hysteresis 4. Power consumption is less. 5.Push Pull Displacement Sensor: v It has three plates with the upper pair forming one capacitor and the lower pair forming another capacitor. v There is a non-linear relationship form between the change in capacitance AC and the displacement X. v The displacement moves the central plate between the two other plates. v The result of this, the central plate moving downwards and to increase the plate separation of the upper capacitor and decrease the separation of the lower capacitor. v 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 v position sensors report the position of an object with respect to a reference part. v The information can be an angle as in many degree a dish antenna has turned. v The following are the position sensors. 1. Photoelectric Sensors v It is used to detect the object by breaking a beam of light (Refer Fig.1.12(a)) or radiation falling on a device or by detecting the light reflected back by the object (Refer Fig.1.12(b)).
Fig.1.12 2. Hall effect Sensors v Hall effect: Hall effect is defined as when a beam of charged particles passes through magnetic field, the beam is deflected from its straight line path due to the forces acting on the particles. v A current flowing in a conductor like a beam is deflected by a magnetic field.
Fig.1.13 v The working principle of a Hall effect sensor is that if a strip of conducting material carries a current in the presence of a transverse ngne1i shown in Fig.1.13.
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v The difference of potential is produced between the opposite edges of the conductor. The magnitude of the voltage depends upon the current and magnetic field. v In the Fig. the current is passed through leads 1 and 2 of the strip. The output leads connected with Hall strip. v When a transverse magnetic field passes through the strip the voltage difference occur in the output leads. v 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 of current. 4. It is used for measurement of power.
Digital Optical Encoder: v A digital optical encoder is a device that converts motion into a sequence of digital pulses. v By counting or decoding these bits and the pulses can be converted into relative or absolute position measurements. v Encoders are in Rotary, linear configurations. v The Rotary encoders are in two forms. 1. Absolute encoder 2. Incremental encoder. 1. Absolute Encoder: v The absolute encoder is designed to produce a digital word that distinguishes ‘ N’ distinct positions of the shaft.
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Fig. 1.15. Components of an optical encoder v The Fig.1.15 shows the basic form of an absolute encoder. v The rotating disc has four concentric circles of slots and four sensors to detect the light pulses. v The slots are arranged in such a way that the output is made in the binary code. v The number of bits in the binary number will be equal to the number of tracks. v The most common types of numerical encoding used in the absolute encoder are gray and natural binary codes. v 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 track patterns 2. Incremental Encoder: v Working: A beam of light passes through the slots in a disc and it is detected by a suitable light sensor. v 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. Incremental encoder v So the angular position of the disc is determined by the number of pulses produced. In the above Fig. three tracks and three sensors are used. v The inner track has just one hole and other two tracks have a series of equally spaced holes. v The angle is determined by the number slots on the disc. 3.3 PROXIMITY SENSOR v A proximity simply tells the contra! system whether a moving part is at a certain place. v Proximity sensors come under the non contact type sensors. v The following are the some of the proximity sensors. 1. Pneumatic proximity sensor: v Working: Low-pressure air is allowed and to escape through a port which is placed in the front position of the sensor. This escaping air reduces the pressure in the nearby sensor output port, when there is no close by object.
Fig. 1.18. Pneumatic proximity sensor
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v If there is a close by object means the air will not escape readily, so the pressure increases in the sensor output port. This output from the sensor depends on the proximity of objects. 2. Eddy current proximity sensors:
Fig. 1.19. Eddy current sensor v Working: When alternating current is supplied to the coil means the alternating magnetic field is produced. If there is a metal object in close proximity to this alternating magnetic field the eddy current is induced in it. This eddy current will produce a magnetic field themselves and the impedance of the coil changes the amplitude of the alternating current. v The above Fig. shows the basic form of such sensor and it is used for the detection of non-magnetic conductive materials. 3. Inductive proximity switch: v It is used for the detection of metal objects and it consists of a coil wound around a core. v 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: v 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 belt depressing the belt by a spring loaded platform nearer to the sensor the presence of item in the conveyor is determined. v The closeness of switch is done by movement of this spring loaded platform.
Fig. 1.20. Microswitch 5. Reed switch: v It is a non-contact proximity switch. It is used for checking the closure of doors. v It consists of two magnetic switch contacts sealed in a glass tube.
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Fig. 1.21. Reed switch v When a magnet is brought close to the switch, the magnetic reeds are attracted 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 v Velocity sensors or tachogenerators are devices that give an output proportional to angular velocity. v These sensors find wide application in motor speed control systems. v The following are the various velocity sensors. 1. Electro Magnetic Transducer, v The most commonly used transducer for measurement of linear velocities is electromagnetic transducer. v The electromagnetic transducers are classified into two categories. 1. Moving Magnet Type: 2. Moving coil type. v In moving magnet type the sensing element is a rod that is rigidly coupled to the device whose velocity is being measured. v This rod is a permanent magnet. This permanent magnet is surrounded by a coil. v The motion of the magnet induces a voltage in the coil and the amplitude of the voltage is directly proportional to the velocity.
Fig. 2.22. Moving magnet type transducer 17
2. Moving coil type velocity transducer: v It is operated through the action of a coil moving in a magnetic field. v A voltage generated in the coil is proportional to the velocity of the coil. v This is a more satisfactory arrangement due to it forms a closed magnetic circuit with a constant air gap and the device is contained an antimagnetic case which reduces the effects of stray magnetic field.
Fig. 2.23. Moving coil type velocity transducer 3. Tachogenerators: v A sensor that converts speed of rotation directly into electrical signal is called a tachogenerator. v It is used to convert angular speed into a directly dependent voltage signal. (a) Toothed Rotor Variable Reluctance Tachogenerator: v It is used to measure angular velocity. v This tachogenerator consists of a metallic toothed rotor mounted on the shaft whose speed is to be measured. v A magnetic pick up is placed near the toothed rotor and this magnetic pick up consists of a housing, and the housing containing a small permanent magnet with a coil wound around it. v 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 in the pickup coil. Finally the output is in the form of pulses and wave shapes. v The pulses induced depend upon the number of teeth in the rotor and the rotational speed. When the speed is known, the rotational speed is calculated by measuring the frequency pulses.
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v Fig. 2.24. Toothed rotor tachometer generator v Suppose the rotor has ‘ n’ teeth and the speed of rotation is ‘ N’ r.p.s. and number of pulses per second is ‘ p’ . v The number of pulses per revolution = ‘ n’ = n
The advantage of toothed rotor variable reluctance tachogenerator is the information from this device can be easily transmitted and easy to calibrate. 4. A. C. Generator Form of Tachogenerator: v It consists of rotor, which rotates with the rotating shaft and a coil. v When the coil rotates in the magnetic field the e.m.f. is induced. v The magnet may be in the form of stationary permanent magnet or electromagnet. v The frequency of this alternating e.m.f. is used to measure the angular velocity. v The output voltage is rectified and it is measured with a permanent magnet moving coil (PMCC) voltmeter.
Fig. 2.25. A.C Tachometer generator
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4.2 MOTION SENSORS 1. Stroboscope: v Stroboscope is a simple portable manually operated device for periodic or rotary motions measurement. v It is a variable frequency flashing light instrument and the flashing is set by the operator. v If a strong light is caused to flash on a moving object at the time each flash occurs. The stroboscope occupies a given position, and the object will appear to be stationary. v The flashing light whose frequency can be varied and controlled, and this source is called strobotron. 2 Pyroelectric Sensors: v It consists of a polarised pyroelectric crystal with thin metal film electrodes on opposite faces. (Pyro electric materials, e.g., lithium tantalate are crystalline materials which generate charge in response to heat flow. When such materials heated to about 610°C in an electric field, the electric dipoles within the material line up and it becomes polarised as shown in Fig.). v Due to the crystal is polarised with charged surfaces, the ions are drawn from the surrounding air and electrons from any measurement circuit is connected to the sensor to balance the surface charge as shown in Fig. v For measurement of a human or heat source motion, the sensing element has to differentiate between general background heat radiation and a moving heat source. For that a single pyroelectric sensor is not capable to use and dual pyroelectric sensors are used as shown in Fig. v In this dual pyroelectric sensors the sensing element has the one front electrode and two back electrodes. When two sensors being connected means both sensors are receive the same heat signal and their outputs are cancelled.
Fig. 2.26. Pyroelectric sensors
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v Suppose a heat source moves from its position means the heat radiation moves from one of the sensing elements to the other, then the current is alternates in one direction first and then reversed to the other direction second. v A moving human gives an alternating current of 1O A. When the infrared radiation is incident on the dual pyroelectric sensor material and changes its temperature, the polarisation in the crystal is reduced. A focusing device is needed to direct the infrared radiation onto the sensor. 5.0 FLUID PRESSURE SENSORS v The devices which are used to monitor fluid pressure in industrial processes is diaphragms, bellows, capsules and tubes. v The types of pressure measurements required are (1) Absolute pressure measurement, (2) Differential pressure measurements. v In absolute pressure measurements the measurement is related to vacuum pressure (zero pressure) and in differential pressure measurement the difference in pressure is measured. The types of pressure measurement devices are discussed below. 1. Diaphragms v In this the pressure to be measured is applied to the diaphragm, causing it to deflect, and the deflection being proportional to the applied pressure. This movement can be monitored by some form of displacement sensor. (Example for displacement sensor is strain gauge) 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 pressure transducer v A specially designed strain gauge is also used for measuring pressure and it consisting of four strain gauges with, two measuring the strain in a circumferential direction while remaining two measure strain in a radial direction. The four strain gauges are connected to form the arms of a wheatstone bridge a shown in Fig.2.28. v The deflection at any point is shown in terms of +ve and —ye sign. The stress distribution on the diaphragm surface is almost ideal for practical purposes, since both compressive and tensile stresses exit. So this will allow the use of a four arm wheatstone bridge where all the gauges are active and consequently there is a large output. v The strain gauges I and 4 are placed at close to the centre and oriented to read tangential strain and its value is +ve maximum at this point. v The gauges 2 and 3 are oriented to read radial strain and it is placed close to the edge as possible. 2. Bellows v A metallic bellows is a series of circular parts as shown in Fig.2.29 and the parts are formed or joined in such a manner that they are expanded or contracted axially by change in pressure.
Fig. 2.29. Bellows 22
v The Fig.2.30 shows the bellows can be combined with a LVDT to give a pressure sensor with an electrical output. v The bellows are made up of materials like stainless steel, phosphor bronze, nickel, rubber and nylon. v The output pressure is calibrated through the LVDT.
Fig. 2.30. L VDT with bellows 3. Capsule
Fig. 2.31. Capsule v Capsules are one of the pressure measuring device and it can be considered to be just two corrugated diaphragms combined and give even greater sensitivity. v The capsules are more sensitive in measuring pressure. 4. Tube Pressure Sensors
Fig. 2.32. Tube pressure sensors v In tube pressure measurement the increase in pressure in a tube is cause the tube in circular cross-section. It is shown in the above Fig. The tubes having greater sensitivity while the pressure increases. v The tubes are made up of stainless steel and phosphor bronze. 5. Tactile Sensor v It is one form of pressure sensor and it is used to determine the pressure in Robotics in such a form fingertips of robotics contact with the object.
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v These type of sensors also used in ‘ touch display screens’ where physical contacts to be sensed. v The above Fig.2.33 shows the one form of tactile sensor. v It uses piezo electric polyvinylidene fluoride (PVDF) film. v There are two layers of such film is used and it is separated by a soft film which transmits vibrations.
Fig. 2.33. PVDF tactile sensor v The alternating voltage is supplied in the lower PVDF film and this results in mechanical oscillations of the film. v The intermediate film transmits these vibrations to the upper PVDF film. v Due to the piezoelectric effect the vibrations formed are cause an alternating voltage to be produced across the upper film. v So the pressure is applied to the upper PVDF film and its vibrations are affected the output voltage. 6. Piezoelectric sensor
Fig. 2.34. Sensor equivalent circuit v The electrical circuit for a piezo electric sensor is a charge generator in parallel with capacitance Cs and in parallel with Resistance Rs. v The effective circuit is as shown by the Fig. when the sensor is connected via a cable of capacitance C and resistance RA. v The sensor is charged subject to pressure change and the capacitor will discharge with time. The discharge time depends on the time constant of the circuit. LIQUID FLOW SENSORS v There are many devices used to measure the liquid flow. v The basic principle in measuring flow is the fluid flowing through the pipe per second is proportional to square root of pressure difference. v The following flow measuring devices are used to measure the liquid flow.
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1. Turbine Flowmeter v 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. v The rotor rotation depends upon the fluid flow and the angular velocity is proportional to the flow rate. v The rotor rotation is determined y the magnetic pick-up, which is connected to the coil. v The revolution of the rotor is determined by counting the number of pulses produced in the magnetic pick up. The accuracy of this instrument is ± 3%.
Fig. 2.35. 2. Orifice Plate v It is a simple disc with a central hole and it is placed in the tube through which the fluid flow.
Fig. 2.36. Orifice plate v From the above Fig.2.36 the pressure difference measured between a point equal to the diameter of the tube upstream and half the diameter of down stream. v The accuracy of this instrument is ±1.5%. LIQUID LEVEL MEASUREMENT The liquid level measurement is done by using 1. Differential pressure sensor and 2. Float system.
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1. Differential Pressure Sensor v In this the differential pressure cell determines the pressure difference between base of the liquid and atmospheric pressure. v The differential pressure sensor can be used in either form of open or closed vessel system.
Fig. 2.37. 2. Float System v In this method the level of liquid is measured by movement of a float. v The movement of float rotates the arm and slider will move across a potentiometer. v The output result is related to the height of the liquid.
Fig. 2.38. 6.0 TEMPERATURE SENSORS v Temperature measurements are amongst the most common and the most important measurements made in controlling industrial processes. v Changes that are commonly used to monitor temperature are, the expansion or contraction of solids, liquids or gases, the change in electrical resistance of conductors, semiconductors and thermoelectric e.m.f.s. The control system which are used to measure the temperature is as follows 1 Thermocouples v The most common electrical method of temperature measurement uses the thermocouples. v The basic principle of this is, if two different metals are joined together, a potentiometer difference occurs across the junction. v The potential difference depends on the metals used and the temperature of the junction. v When both junctions are at the same temperature, there is no net e.ni.f. But if there is a difference in temperature between the junction the e.m.f. will be produced. v This e.m.f. will depend upon the two metals and the temperature between the junctions. One junction is held at 0°C and the equation which is used to find out the e.m.f. is
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v
Fig. 2.39. Thermocouple v There are three e.m.f.s present in a thermoelectric circuit. In this the Seebeck e.m.f. is caused by the junction of dissimilar metals and the Pettier e.m.f. is caused by a current flow in the circuit, and the Thomson e.m.f. which results from a temperature gradient in the materials. v It is observed that all thermocouple circuits must involve at least two junctions. In that one of the junctions senses the desired or unknown temperature. v This junction is called the hot or measuring junction. The other junction is usually maintained at a known fixed temperature and this junction is called the cold or reference junction. v If the temperature of the reference or cold junction is known, the temperature of the hot or the measuring junction can be calculated by using the thermoelectric properties of the materials. v If thermocouple circuit can have other metals in the circuit and they will have no effect on the thermoelectric e.m.f. v A thermocouple can be used with the reference junction at a temperature other than v 0°C. v 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 intermediate temperature
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v Here to maintain the 0°C at one junction a compensation circuit is Used to provide an e.m.f. which varies with the temperature of the cold junction. v When it is added to the thermocouple e.m.f. it will generate a combined e.m.f. This is shown in Fig.2.42.
Fig. 2.42. Compensation thermocouple v In the above Fig.2.42, the wires from the measuring junction are screwed directly to an isothermal block terminal strip. v The temperature of the block is ambient temperature. v This reference temperature is measured by semiconductor sensor and compensation circuitry develops a voltage Ecomp which is combined with measuring junction and the net voltage across the voltmeter = T (Temperature being measured). v The isothermal block can accept many thermocouple pairs in multichannel instruments with microprocessor computing power since the T (reference junction sensor now sends its temperature data to the computer which computes the needed voltage correction for each thermocouple. v The thermocouples like E, J, K and T are relatively cheap and it has accuracies of about ± ito 3%. v The noble metal thermocouples are very high cost compared with this and it has accuracies of about ±1% better than the base metal thermocouples. v Thermocouples are used in applications ranging from measurement of room air temperature to that of a liquid metal bath. The problems which may be encountered are 1. Faulty reference junction, 2. Installation faults, 3. Junctions formed by users may involve excessive temperatures or faulty soldering techniques, 4. Gross errors can result due to wrong installation of thermocouple. 2. Resistance Temperature Detectors (RTDs) v Resistance temperature detectors (RTDs) or resistance thermometers are basic instruments for measurement of resistance. v The materials used for RTDs are Nickel, Iron, Platinum, Copper, Lead, Tungsten, Mercury, Manganin, Silver, etc. v The resistance of most metals increases over a limited temperature range and the relationship between Resistance and Temperature is shown below.
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v Fig. 2.43. Resistance temperature detector v The Resistance temperature detectors are simple, and resistive elements in the form of coils of wire and it is shown in the above Fig.2.44. v The equation which is used to find the linear relationship in RTD is
Fig. 2.44. RTD element Constructional Details ofRTDs: v The platinum, nickel and copper in the form wire are the most commonly used materials in the RTDs. v Thin film platinum elements are often made by depositing the metal on a suitable substrate wire- wound elements involving a platinum wire held by a high temperature glass adhesive inside a ceramic tube. v This is shown in Fig.2.45.
Fig. 2.45.
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Salient Features ofRTDs: 1. High degree of accuracy. 2. Resistance thermometer is interchangeable in a process without compensation or recalibration. 3. It is normally designed for fast response as well as accuracy to provide close control of processes. 3. Thermistors v Thermistor is a semiconductor device that has a negative temperature coefficient of resistance in contrast to positive coefficient displayed by most metals. v Thermistors are small pieces of material made from mixtures of metal oxides, such as Iron, cobalt, chromium, Nickel, and Manganese. v The shape of the materials is in terms of discs, beads and rods. v The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature. v The resistance of conventional metal-oxide thermistors decreases in a very non-linear manner with an increase in temperature is shown in the Fig.2.46 below. v The change in resistance per degree change in temperature is considerably larger than that which occurs with metals.
Fig. 2.46. Thermistors v The simple series circuit for measurement of temperature using a thermistor and the variation of resistance with temperature for a typical thermistor is shown in the below Fig.2.47.
Fig. 2.47. Thermistor v The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature. v Thermistors have many advantages when compared with other temperature sensors. v The main disadvantage is highly non-linear behaviour.
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4. Thermodiodes and Transistors (a) Thermodiodes: v Thermodiode is widely used method for measuring temperature. When the temperature of doped semiconductors changes, the mobility of their charge carriers changes and this affects the rate at which electrons and holes can diffuse across ap-n junction. 1. Measurement of temperature, 2. Control of temperature, 3. Temperature compensation, 4. Measurement of thermal conductivity, 5. Measurement of power at high frequencies, 6. Measurement of composition of gases, 7. Providing time delay, 8. Vacuum measurements. v The difference in voltage and current through the junction is a function of the temperature. The equation which is used to find the I is
v From the above equation the voltage ‘ V’ is proportional to the temperature on Kelvin scale and the potential difference measurement across a diode at constant current is used to measure the temperature. (b) Transistor: v In Thermo transistor the voltage across the junction between the base and the emitter depends on the temperature. v A common method is use of two transistors with different collector current and finding the difference in the base-emitter voltages between them, and this difference is the measure of temperature.
Fig. 2.48. Transistor v The thermotransistors can be combined with circuit components on a single chip to give a temperature sensor. v This is shown in the above Fig.2.48.
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5. Bimetallic strips
Fig. 2.49. Bimetallic thermostat v A Bimetallic thermostat consists of two different metal strips bounded together and they cannot move relative to each other. v These metals have different coefficients of expansion and when the temperature changes the composite strips bends into a curved strip, with the higher coefficient metal on the outside of the curve. v The basic principle in this is all metals try to change their physical dimensions at different rates when subjected to same change in temperature. v This deformation may be used as a temperature- controlled switch, as in the simple thermostat. v The Fig.2.49 shows the Bimetallic thermostat which was commonly used with domestic heating systems. 7.0 LIGHT SENSORS 1. Photodiodes v Diodes like photodiodes and semiconductor diodes are connected into a circuit in reverse bias giving a very high resistance. v When light falls on the junction the resistance of the diode will drop and the current in the circuit will rise.
Fig. 2.50. v The Fig.2.51 shows the diode characteristics. 32
v If the diode is sufficiently reverse biased, it will breakdown. v The current passing through the diode when forward biased only. v If an A.C. voltage is applied across a diode, it can be regarded as only switching on when forward bias it and being off in the reverse direction. v The photodiodes have a very fast response to light and it can be used as a variable resistance device controlled by the light incident on it. 2. Photo Transistors The transistors are come in two forms 1. npn, 2. pnp.
Fig. 2.51. v The main current flows in at the collector and out at the emitter in npn transistor. v The main current flowing in at the emitter and out at the collector in pnp transistor. v The phototransistors have a light sensitive collector-base p-n junction. v There is a very small collector to emitter current when there is no incident light. Suppose the light is incident a base current is produced and it is proportional to the light intensity. v So this will produce a collector current and it is used for measure of the light intensity. v The example for photo transistors is photo Darlington arrangement.
Fig. 2.52. Photo Darlington arrangement 3. The Photo Resistor v Its resistance depends on the intensity of light falling on it, and the resistance will decrease linearly as the intensity increases. v The photoresistor like cadmium sulphide has most responsive to the light having wavelengths of about 520 mm to 700 mm. 4. Array of Light Sensors v This will be used in small space like rooms to determine the variations of light 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? 34
24. What is a comparator? 25. Name an application of a Schmitt trigger. 26. Why integrators are preferred over differentiators in analog computers? 27. What is a voltage follower? 28. What is the advantage of CMOS Schmitt trigger? PART-B 1. Explain the terminologies used in transducers. 2. What are all the displacement sensors? Explain each one briefly. 3. Explain the position sensors with neat figure. 4. Define proximity and explain the proximity sensors. 5. What are all the velocity and Motion sensors? 6. How the pressure is measured? Explain the pressure sensors neatly. 7. Explain the temperature measurement sensors. 8. Explain the light sensors with neat figure. 9. What are all the points to be considered while selecting the sensors? 10. Explain the signal processing. 11. Explain some applications of operational amplifier. 12. Explain the operation of successive approximation ADC. 13. How do a dual slope ADC and single slope ADC differ? 14. What is Flash ADC ? Discuss. 15. Explain the construction of R-DR ladder DAC. 16. Discuss the various terms associated with ADC.
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