Electronic Instrumentation and control systems

Electronic Instrumentation

1. Electronic Instrumentation and Control By: Dr. Ankita Malhotra Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

2. Unit 1: PRINCIPAL OF MEASUREMENT, TESTING AND MEASURING INSRUMENTS Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

3. Measurement The measurement of a given quantity is essentially an act or the result of comparison between the quantity (whose magnitude is unknown) and a predefined standard. Since two quantities are compared, the result is expressed in numerical values. In order that the results of the measurement are meaningful, there are two basic requirements : ( i ) The standard used for comparison purposes must be accurately defined and should be commonly accepted, · And (ii) The apparatus used and the method adopted must be provable Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

4. Significance of measurement- To validate and understand the new hypothesis, phenomenon and relations between them To pave the way for new discoveries backed by earlier measurements. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

5. Methods of measuremen t Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

6. Generalized Measurement System * Data/signal conditioning includes : A/D converter, amplifiers, filters, modulators, attenuators and processes like addition, subtraction, integration, differentiation etc. *Data presentation element includes : analog/digital display, recorders, storage devices, processors, printers etc. Fig: Block diagram of generalized measurement system Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

7. Performance Characteristics of Measuring Systems/Instruments Measurement involves using an instrument as a physical means of determining a quantity or variable. In simple cases, an instrument consists of a single unit which gives an output signal according to the unknown variable applied to it. The knowledge of performance characteristics of an instrument/measuring device is essential for minimizing the errors and to select the most suitable instrument for specific measurements. It can be divided into two distinct categories Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

8. Static Characteristics of Instruments Accuracy : It is the closeness with which an instrument reading approaches the true value of the quantity being measured. Point accuracy, Accuracy as percentage of scale range, Accuracy as percentage of true value. 2. Precision : It is a measure of the· reproducibility of the measurement, that is given a fixed value of variable. Precision is a measure of the degree to which successive measurements differ from each other. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

9. 3. Linearity : The characteristic of the instrument which defines that the output is linearly proportional to the input. This is the most desirable characteristic of an instrument because the conversion from a linear scale reading to the corresponding measured value of input quantity is most convenient if one merely has to multiply by a fixed constant rather than consult a non-linear calibration curve or compute from non-linear calibration equations. Also when the instrument is part of a large data or complex system, linear behavior of the part often simplifies the design and analysis of the whole system. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

10. 4.Sensitivity : Sensitivity of an instrument or an instrumentation system is the ratio of the magnitude of the output signal/response to the magnitude of input. signal /quantity being measured. It is the ratio of change in output of an instrument to the change in input. Sensitivity= Inverse sensitivity (deflection factor)= Fig : sensitivity of an instrument Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

11. 5- Hysteresis : Hysteresis is a phenomenon which depicts different output effects while loading and unloading. Hysteresis takes place due to the fact, that all the energy put into the stressed parts when loading is not recoverable while unloading. When the input of an instrument is varied from zero to its full scale and then if the input is decreased from its full scale value to zero, the output varies. Fig: Hysteresis effects Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

12. Threshold : Threshold is the smallest measurable input, below which no output change can be identified. Dead Time : It is the time required for the instrument to respond to the change in output. For a certain range of inputs, the output value does not change. Dead time is the time before which the instrument starts to respond after the output has been changed. Dead Zone : It is the largest change of input quantity for which there is no output. Resolution: It is the smallest change in the measured value to which the instrument will respond. It is smallest increment in the input value that can be detected by the instrument. Thus, resolution defines the smallest measurable input change while threshold defined the smallest measurable input. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

13. Dynamic Characteristics Many measurements are concerned with rapidly varying quantities and therefore, for such cases we must examine the dynamic relations which exist between the output and input. This is normally done with the help of differential equations. Performance criteria based upon dynamic relations constitute the dynamic characteristics. D ynamic Response Transient and Steady-state response Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

14. Dynamic response cntd . When an input is applied to an instrument or a measurement system, the instrument or the system cannot take up its final steady state position immediately. On the other hand, the system go through a transient state before it finally settles to its final 'steady state’ position. Some measurements are made under conditions that sufficient time is available for the instrument or the measurement system to settle to its final steady state conditions. Under such conditions the study of behavior of the system under transient state, called 'transient response' is not of much of importance ; only steady state response of the system need be considered. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

15. Invariably measurement systems are subjected to inputs which are not static but dynamic in nature. i.e. the inputs vary with time Since the input varies from instant to instant, so does the output. The behavior of the system under such conditions is described by the dynamic response of the system. The variations in the input, that are used practically to achieve dynamic behavior are : i )Step input, ii) Ramp input ,iii) Parabolic input iv)Sinusoidal input: *Note: The systems exhibit a characteristic sluggishness on account of presence of energy storage elements like capacitance, inductance etc. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

16. Dynamic Characteristics The dynamic characteristics of any measurement system are : ( i ) Speed of response (ii) Lag (iii) Fidelity (iv) Dynamic error Speed of Response. It is the rapidity with which an instrument responds to changes in the measured quantity. Response Time is defined as the time required by instrument or system to settle to its final steady position after the application of the input. Lag: Measuring lag is defined as the delay in the response of an instrument to a change in the measured quantity. The lags are of two types : 1. Retardation lag : As soon as there is a change in the measured quantity, the measurement system begins to respond. 2. Time delay : The response of the measurement system starts after a dead time, once the input is applied. They cause dynamic error. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

17. Fidelity: Fidelity of a system is defined as the ability of the system to reproduce the output in the same form as the input. Dynamic Error. It is the difference between , the true value of the quantity changing with time and the value indicated by the instrument if no static error is assumed. However, the total dynamic error of the instrument is the combination of its fidelity and the time lag or phase difference between input and output of the system. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

18. Calibration To ensure proper functioning of closed loop feedback systems, calibration is one of the essential requirements. Calibration is a condition and also a process. Calibration is the condition of an electronic instrument when its output or response is as close as possible to the specified value. Calibration is a process of examination. To calibrate an instrument means to determine how close to the true value its indication or output really is. This process also includes the correction and/or documentation of degree of error. The signals produced by various instruments may be altered and create undesirable results due to wearing or aging of components. These instruments are designed so that adjustments can be made to compensate for the deterioration of their internal components. The process of making these adjustments is called 'calibration'. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

19. Need of calibration The value of internal· electronic components of the instruments changes as well as its mechanical parts wear. This results in malfunctioning of the instruments. Calibration is required to overcome this problem . Calibration procedures are performed for other reasons, such as : Before new installation After repair After extended shut down After product repair Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

20. Types of calibration Three-point calibration : The measurements are made at three points and compared with the defined standards. The three points chosen are: Starting point, end point and centre point of the measurement scale. Disadvantage: cannot accurately calibrate the instrument over the entire scale, as it cannot ensure proper linear response of the system. Five-point calibration: Measurements are made at Test point values of 10,30, 50, 70 and 90 percent of the instruments input ranges. It is more accurate. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

21. Errors The error of an instrument is the algebraic difference between the observed value and the true value of the quantity being measured. Error may be expressed either as absolute or as percentage of error. TYPES OF ERRORS: Gross errors : human errors Systematic errors: Fixed errors due to components Schematic errors : Static and Dynamic Static errors due to limitations of instrument Dynamic errors due to delay in instrument’s response Random errors: Due to unknown causes. These errors differ in values when repeated measurements of the same variable is made. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

22. Limiting Errors The accuracy of an instrument, we know is specified by the manufacturer within a certain percentage of the full scale reading. The components for e.g. resistor, capacitor, inductor are guaranteed to be within some percentage of their rated value. This percentage shows the deviation of that quantity from its specified value. This deviation from the specified value is called as limiting error. These errors are also called as guarantee errors . For e.g. : if the value of a capacitor specified by the manufacture is 4.7 μ F with a tolerance of ± 5% then the actual value of capacitance is guaranteed to be within these limits. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

23. Measurement of resistance Low resistance: All resistance, of the order of 1 ohm and below are classified as low resistance. In practice such resistance, are· present in armature and series windings of large machines, ammeter shunts, cable lengths, contacts etc. Medium resistance :This class includes resistances from about 1 ohm upwards to about 1,00,000 ohms. The majority of electrical apparatus have resistance in this range. High resistance: The resistances of the order of 1,00,000 ohm and above are classified as high resistance. The following methods are explained in this section for measurement of low, medium and high resistances. a) Ohmmeter b) Wheatstone's bridge c)Kelvin's double bridge d) Megger Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

24. Kelvin’s Double bridge This method is one of the best available method for the precise measurement of low resistance. It is known as double bridge as it incorporates two sets of ratio arms. One set of ratio arms is composed R1,R2 and the other set is composed of Ra, Rb . Rx is the resistor to be measured· and R is adjustable resistor having very small tolerance. The ratio R1/R2 is kept same Ra/ Rb Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

25. For zero deflection of galvanometer Ry is resistance of connecting lead between points m and n. Substituting value of E: Similarly: But k l p o m n E I k I 1 I 2 Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

26. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

27. The known resistance R3 is adjusted such that galvanometer indicates zero and then using above equation Rx can be calculated. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

28. Advantages of Kelvin’s bridge In a typical Kelvin's double bridge the range of resistance covered is l Ω to 10 μΩ with an accuracy of± 0.05% to± 0.2%. The resistance of the connecting lead R, has no effect on measurement Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

29. Wheatstone’s Bridge This is the best and commonest method of measuring medium resistance. The galvanometer is a sensitive micro ammeter with a zero centre scale. When there is no current through the meter, the galvanometer pointer rests at 0, i.e. mid scale. Current in one direction causes the pointer to deflect on one side and current in the opposite direction deflects it to the other side. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

30. The resistors R1 R2 and R3 are known and Rx is unknown resistor to be measured. The bridge is balanced when there is no current through galvanometer or when the potential at points C and D are equal i.e. the potential across galvanometer is zero. When bridge is balanced: Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

31. For measurement of unknown resistor Rx, R3 is adjusted such that bridge is balanced and galvanometer current is zero. Then using above equation, the unknown resistor Rx is calculated. R3 is precision resistor having very small tolerance. When the bridge is in unbalanced condition, current flows through the galvanometer, causing deflection of its pointer. To determine current flowing through galvanometer when bridge is unbalanced, Thevenin ' s theorem can be used. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

32. Thevenin’s equivalent to find current flowing through Galvanometer Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

33. Thevenin's equivalent circuit for unbalanced Wheatstone's bridge If Rg is the internal resistance of galvanometer, then Ig is given by: Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

34. Mega ohm Bridge This method is used for measuring high resistance above 50M Ω It is called as mega ohmmeter Megger is a portable ohmmeter with built-in high voltage source. Working Principle of Ohmmeter The instrument is connected with a battery, a series adjustable resistor and an instrument which gives the reading. The resistance to be measured is connected at terminal ab. When the circuit is completed by connecting output resistance, the circuit current flows and so the deflection is measured. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

35. D’arsonval Movement This type of instruments consists of a permanent magnet and a coil which carries current and is placed in between them. Due to high intensity magnetic fields the deflecting torque produced is of large value due to which sensitivity of the meter is also increased. If the direction of current is reversed in these types of instruments, then torque direction will also be reversed so these types of instruments are applicable in DC measurements only. The deflecting torque is directly proportional to the deflection angle hence these types of instruments have the linear scale. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

36. There are many advantages due to which we use D’Arsonval type instrument. They are- They have uniform scale. Effective eddy current damping. Low power consumption. No hysteresis loss. They are not affected by stray fields. However they suffer from drawbacks such as- It cannot be used with AC signal. Costlier compared to MI instruments. There may be error due to ageing of springs by which we may not get accurate result. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

37. Basic types of ohmmeter Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

38. Mega ohmmeter ( Megger ) Megger is essentially a portable ohmmeter with built-in high voltage source. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

39. It mainly has two elements, a magnet type dc generator to supply current for making measurement and an ohmmeter which measures the resistance value. The meter used differs slightly from the standard D' Arsonoval movement. It has two windings. Coil A is the current coil with one terminal connected to negative output and other connected in series with R1 to the test lead P2• It is wound in such a way that it moves the pointer towards the zero end of the scale when current flows through it from generator. Coil B is the voltage coil and is connected across the generator output through resistance R. This coil is wound such that the pointer moves towards the high resistance end of the scale when current flows through it from generator. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

40. The two coils are mounted on the same shaft but at right angles to each other. The unknown resistance Rx is connected between test leads P1 and P2. If the test leads are left open, no current flows in coil A and coil B alone moves the pointer as current flows through it. Coil B moves a pointer such that it indicates infinity or open. When the unknown resistor Rx is connected across P 1 and. P 2, current flows from the generator· though coil A, resistors R1 and. Rx· The corresponding torque developed moves the pointer away from the infinity position, into a field of gradually increasing strength, until the torque fields between coils A and B are equal. The value of R1 is so chosen that even if the line terminals are a short circuited, coil A does not get damaged. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

41. Measurement of inductance Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

42. Maxwell’s Inductance Bridge This bridge circuit measures an inductance by comparison with a variable standard self inductance. The condition for balance of bridge require that there should be no current through the detector. This requires, that the potential difference between points b and d should be zero i.e. E1 = E3. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

43. In the circuit: Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

44. Comparing real and imaginary part: Note – To measure unknown Lx; R 2 and L 2 are adjusted to keep the bridge balanced Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

45. Maxwell's Inductance-Capacitance Bridge It measures unknown inductance in terms of known capacitance Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

46. When bridge is balanced Equating real and imaginary parts: Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

47. Thus the measurement of unknown inductance is independent of the excitation frequency. The scale of the resistance can be calibrated to read inductance directly. Quality factor (Q) for inductance is, For high values of Q, R 1 becomes excessively large and it is impractical to obtain a satisfactory variable standard resistance in the range. Therefore Maxwell's bridge is suitable for measurement of inductance with low Q values. For measurement of inductance,. unknown inductor is connected in one of the arms and resistor R1 and capacitor C1 are adjuste d to balance the bridge. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

48. Advantages of Maxwell’s bridge The equation for Rx and Lx are independent. The measurement is independent of frequency. The bridge yields simple expression for inductance Lx and resistance Rx· It is very useful for measurement of a wide range . of inductance at power and audio frequencies. It can measure inductance from 1 H to 1000 H with-± 2 % error. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

49. Disadvantages of Maxwell’s bridge This bridge requires a variable standard capacitor which may be expensive if calibrated to a high degree of accuracy. The bridge is limited to measurement of low Q vaIues {1 < Q. < 10). The measurement of high Q coils demand a large value for resistance R1. The Maxwell's bride is also unsuited for coils with very low Q values i.e. Q < 1. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

50. Hay's Bridge This method of measurement is particularly suited for the measurement of inductance having high Q Values Hay's bridge differs from Maxwell's bridge by having a resistance R1 in series with a capacitor C1 instead of being parallel Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

51. When bridge is balanced, Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

52. Equating real and imaginary parts: Putting the value of Lx from equation 2) in equation 1), we get: 1) 2) 3) Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

53. Putting the value of Rx from equation 3) to equation 2) As ω appears in the expression for Lx, this bridge is frequency sensitive. Quality factor (Q) for capacitor is Substituting in equation 4): 4) Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

54. For Q>10,1/Q2 can be neglected, thus: ( Similar to Maxwell's bridge) For Q less than 10, the 1/Q2 term can not be neglected. Hence this bridge is not suited for measurement of inductors having Q less than 10. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

55. Advantages : (1) It can measure inductances with high Q i.e. Q > 10. (2) A commercial bridge measures inductance from 1 J1H to 100 H with± 2% error. (3) It can be used for measuring incremental inductance. Disadvantages·: (1)The unknown value of inductance depends on loss of inductor(Q) and also on operating frequency. (2)It can not measure inductance with low Q i.e. Q < 10 Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

56. Measurement of Capacitance Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

57. Schering's Bridge It measures capacitances and their insulating properties precisely. This bridge is widely used for testing small capacitors at low voltage with very high precision. The standard capacitor c3 is a high quality mica capacitor with low loss for general measurements or an air capacitor having a very stable value and a very small electric field for insulation-measurement. V For measuring unknown capacitor, it is connected to one of the arms of the bridge and C1 and R2 are adjusted to obtain bridge balance. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

58. When bridge is balance, Thus Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

59. Comparing real and imaginary parts: Thus, once bridge is balanced, unknown capacitance can be calculated using above equation The dissipation factor D, which is inverse of quality factor of a series RC circuit is given by Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

60. Advantages Commercial bridge measure from 100 pf -1 μF with± 2% accuracy. The direct reading for Cx can be obtained by graduating C3 accordingly, if resistance ratio is maintained at a fixed value. It can measure small capacitors at low voltage precisely. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

61. UNIT -2 SENSORS and TRANSDUCERS Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

62. Transducers and their characteristics Transducer: A device which converts one form of energy into the other form. Characteristics of Transducers: Ruggedness : It is the· ability of a transducer to withstand overloads. A good transducer must have a high degree of ruggedness. Linearity : It is the ability of transducer to reproduce input-output characteristics symmetrically and linearly. Overall linearity is the main factor considered. Repeatability : It is. the· ability of a transducer to reproduce the output signal exactly when the same input is applied repeatedly under the same environmental conditions. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

63. Accuracy : It is defined as closeness of the actual output produced by a transducer to the ideal or true value of the quantity being measured. It should be high for any transducer. High stability and reliability :There should be a minimum· amount of error in measurement and it should be unaffected by temperature, vibrations and other environmental variations. Speed of resp onse : It shows how quickly a transducer responds to the changes in the quantity being measured. The speed of response should be as high as possible. Sensitivit y : The sensitivity of a transducer is defined as the output produced per unit change in the input quantity being measured. For example the _sensitivity of a thermocouple is expressed in m V /°C. The sensitivity should be as high as possible. Size : A transducer must have small size, proper shape and minimum volume so that it can be placed at any location for measurements . Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

64. Classification of transducers Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

65. Mechanical and electrical transducers Output is either mechanical or electrical in nature E.g. of mechanical transducer: springs, pendulum etc. E.g. of electrical transducer: potentiometer, electromagnet etc. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

66. Active Transducers These transducers do not need any external source of power for their operation. Therefore they are also called as self generating type transducers. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

67. Passive Transducers These transducers need external power supply for their operation. So they are not self-generating type" transducers. A DC power supply or an ,audio frequency generator is used as an external power source. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

68. Classification Based on the Quantity to be Measured Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

69. Analog and Digital Transducers The output is either analog or digital in nature. E.g. of analog transducer: thermocouple, strain gauge etc. E.g. of digital transducer: signal encoders Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

70. Primary and secondary transducers Primary transducers: Which convert the primary input, which is mostly the physical quantity into other form. Secondary transducers: which convert secondary input, processed by primary transducer into the desired form Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

71. Transducer and Inverse Transducer Transducers convert non-electrical quantity to electrical quantity while inverse transducers convert electrical quantity to a non electrical quantity; Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

72. According-to Transduction Principle 1 . Capacitive transduction: In capacitive transduction transducers, the measurand is converted to a change in the capacitance. 2. Electromagnetic transduction: In electromagnetic transduction, the measurand is converted to voltage induced 1n conductor by change in the magnetic flux, in absense of excitation. The electromagnetic transducers are self generating active transducers. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

73. 3 . Inductive induction :In inductive transduction, the measured is converted into a change in the self inductance of a single coil. It is achieved by displacing the core of the coil that is attached to a mechanical sensing element 4. Piezo electric induction: In piezoelectric induction the measurand is converted into a change· in· electrostatic charge q or voltage V generated by crystals when mechanically· it is stressed Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

74. 5. Photovoltaic transduction:In photovoltaic transduction ·the measurand is converted to voltage generated when the junction between dissimilar materials is illuminated 6.Photo conductive transduction:In photoconductive transduction the measurand is converted to change in resistance of semiconductor material by the change in light incident on the material . Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

75. Transducer Selection Factor 1. Nature of measurement : The selection of a transducer depends on the nature of quantity to be measured e.g. for measuring capacitance the capacitance sensors need to be used . 2. Cost and availability 3. Measuring system compatibility : The transducer selected must be compatible with the electrical system used. The output impedance of the transducer and the measuring system must not affect each other. 4. Loading effect- : If the transducer affects the value to be measured, errors may be introduced in the measured. Hence, in order to ·minimize. the errors the loading effect should be minimum. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

76. 5. Environmental considerations : output can be affected by electromagnetic interference, shock , temperature etc. 6. Operating range : The transducer must be selected such that it provides good resolution and range. 7. Sensitivity : It should be as high as possible. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

77. Sensors Sensors are the devices used to sense changes in a physical quantity in the surrounding environment in which they are kept. Sensors detect the presence of energy, changes in or the transfer of energy. Sensors detect by receiving a signal from a device such as a transducer, then responding to that signal by converting it into an output that can easily be read and understood. Typically sensors convert a recognized signal into an electrical – analog or digital – output that is readable. In other words, a transducer converts one form of energy into another while the sensor that the transducer is part of converts the output of the transducer to a readable format. Transducers convert one form of energy to another, but they do not quantify the conversions. The light bulb converts electrical energy into light and heat; however, it does not quantify how much light or heat . A battery converts chemical energy into electrical energy but it does not quantify exactly how much electrical energy is being converted. If the purpose of a device is to quantify an energy level, it is a sensor. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

78. Examples of sensors Thermal sensors : Thermometer, Thermocouple gauge etc. Mechanical Sensors : Pressure sensor ,Barometer ,Altimeter , Gas flow sensor , Accelerometer etc. Electrical Sensors: Ohmmeter, Voltmeter, galvanometer etc. Optical sensors: photo detectors, photo resistors, infra-red detectors etc. Others: motion sensors, chemical sensors etc. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

79. Inductive Sensor Inductive sensors use currents induced by magnetic fields to detect nearby metal objects. The inductive sensor uses a  coil (an inductor)  to generate a high frequency  magnetic field as shown below. If there is a metal object near the changing magnetic field, induced current (eddy current) will flow in the object. This resulting eddy current flow sets up a new magnetic field that opposes the original magnetic field. The net effect is that it changes the inductance of the coil in the inductive sensor. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

80. Single-ended eddycurren ~ type inductive sensor Single-ended eddy current type inductive sensor Push-pull eddy current type inductive sensor Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

81. Inductive sensor Inductive sensors are used in the industry to avoid collisions/detect collision or to detect part position. Their range is limited by the magnetic field of the sensor. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

82. Capacitive sensors Capacitive sensors are primary sensors for measurement of displacements. They are also known as proximity sensors in the sense that they measure the nearness of an object without any mechanical coupling between them. They can detect displacement of metallic and non-metallic objects both. They are commonly used in touch switches. It is non-loading, noncontact and non-invasive type sensor for displacement measurements . Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

83. Displacements conveyed to the movable plate may be so arranged as to either vary A which is the area common to the plates or alter the extent of penetration of a dielectric material inside the plates. The capacitance of such sensors is as low as 1 PF and with the help of suitable electrical circuitry, it is possible to detect very small variations in capacitance and measure displacements of the order of 25 nm. Capacitive sensors with cylindrical electrodes are popular for measurements of pressure of fluids and level of fluids and granular materials. For composition measurement, variation effect of dielectric .90nstant due to variation in composition, absorption of moisture and other effects is used. The dielectric constant of certain insulators and semiconductors varies with temperature. This effect is used for measurement of temperature. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

84. Capacitive Pressure Sensors It consists of a fixed plate and a movable plate. The movable plate is either a metallic diaphragm or the membrane. Capacitive pressure sensor using diaphragm is shown in diagram. Due to deformation of the clamped diaphragm for the deflection y at any radius r from the centre of the diaphragm, the capacitance changes. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

85. Consider an annular element of width dr at a distance r from the centre. Its capacitance can be given by For small deflections with y/d << l, d = initial spacing between the electrodes without any pressure difference Δ p y = displacement of the annular element from its initial position Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

86. The total capacitance due to the deflected diaphragm is given by For a diaphragm, the deflection y at any radius r from the centre is given by: The fractional change in capacitance can be given by Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

87. Cylindrical capacitive sensor for pressure measurements The capacitance Co of the sensor is given by Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

88. The dielectric constants of gases and liquids vary with pressure. To measure pressure, of such fluids, the cylindrical electrode arrangement shown in figure is used so that the pressure is continuously measured. under flowing conditions. The walls of the metallic pipe are used as the electrode and a· solid cylindrical rod running along the pip serve as the inner electrode . Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

89. Capacitive Displacement sensor Single ended capacitive displacement sensor The capacitive displacement sensor is fundamentally a proximity sensor If the sensor has ·a solid insulating material of dielectric constant ε , the capacitance is given by Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

90. If the air gap is decreased by Δ x, the capacitance increases by Δ C which is given by, Fractional change in capacitance: Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

91. Push-pull type capacitive displacement The range of linearity can be extended to a large extent by using push-pull sensor. Unity-ratio arm Wheatstone bridge circuit may be used for measuring ΔC/Co. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

92. Potentiometers A resistive potentiometer (Pot) consists of a resistance element with a sliding contact, called a wiper. The motion of sliding contact may be translatory or rotational or a combination of both. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

93. Under no load condition: Where Vo=Output voltage under no load condition Ei = Input voltage Rx =Resistance after displacement of contactor Rp = Full resistance of potentiometer For the same position of contactor, output voltage will get reduced if voltmeter has finite input resistance. In that case, fraction value x' is given by: Voltmeter with resistance RL Voʹ = voltage under loading Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

94. Let, Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

95. The error is given by: Percentage error = Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

96. Advantages: 1.They are inexpensive. 2. They are simple to operate and are very useful for applications where the requirements are not particularly severe. 3. They are useful for the measurement of large amplitudes of displacement. 4. Electrical efficiency is very high and they provide sufficient output to allow control operations. Disadvantages: 1. When using a linear potentiometer, a large force is required to move the sliding contacts. 2. The sliding contacts can wear out, become misaligned and generate noise. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

97. Linear Variable Differential Transformers (LVDT) for Measurement of Pressure and Displacement It is a variable inductance displacement transducer. The LVDT consists of a primary winding and two identical secondary windings. These windings are axially spaced. A rod shaped magnetic core is positioned centrally inside the coil assembly. This rod provides a low reluctance path for the magnetic flux linking the coils (windings). The moving object, displacement of which is to be measured is coupled to this movable rod. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

98. Equivalent circuit The output voltage is given by: Transfer characteristics of LVDT Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

99. Operation of LVDT : The primary winding is connected to the ac source. Assume that the core is exactly at the centre of the coil assembly. Then the flux linked to both the secondary windings will be equal. Due to equal flux linkage, secondary induced voltages are equal but they have opposite polarities. e o = 0 Now if the core is displaced from its null position towards secondary-1 then the flux linked to secondary-1 increases and flux linked to secondary-2 decreases. e o 1 > e o 2 When core is at centre Null position e o = positive Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

100. If the core is displaced towards the secondary-2 then flux liked to secondary-2 will be more than secondary-1 e o 2 > e o 1 Thus the magnitude of output signal Is made to vary "linearly'' with the mechanical displacement. Hence the word "Linear," is used in LVDT. The output is obtained "differentially" between the two secondary windings. Hence the word "differential" is used in LVDT. e o = negative Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

101. Operation of LVDT Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

102. Performance characteristics of LVDT i ) Null voltage : Ideally, the output of L VDT should be zero when its core is at null position. But practically a small residual voltage (called as null voltage) is observed at the null position of the core. This is due to the presence of harmonics in the output of excitation source and stray capacitance between the primary and secondary windings. ii) Resolution : If we assume that the L VDT is frictionless then it can respond to very minute movement of the core to produce a proportional output voltage. This makes the resolution of the LVDT ideally infinite. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

103. iii) Linearity : As is clear from transfer characteristics of, the LVDT response is quite linear. iv) Sensitivity :it is typically 1 to 2 m V /0.01 mm for the LVDT. v) Excitation voltage and excitation frequency : Maximum excitation voltage is limited by the maximum primary current which can be allowed to flow. The excitation frequency should be precisely adjusted to a frequency where the best possible Sensitivity of detection can be obtained. This frequency is between 1 kHz to 10 kHz. vi ) Dynamic response: This indicates how fast the LVDT responds to the displacement of the core. Dynamic response is 'dependent on the excitation frequency and the core weight. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

104. Advantages of LVDT 1. Very fine resolution 2. High accuracy 3. Very good stability 4. Linearity of transfer characteristic(better than 0.25 %) . 5. Ease of fabrication and installation 6. Ability to operate at high temperature_ 7. High sensitivity (2mVNolt/10 microns at 4kHz excitation). Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

105. Disadvantages: 1. LVDT is sensitive to the external magnetic fields. To minimize this effect magnetic shielding is necessary. 2. Complicated circuitry is needed. 3. Due to mass of the core, LVDT is not suitable for dynamic measurement (fast displacements). 4. Larger displacements are needed to get appreciable differential output. Applications: In addition to displacement measurement the L VDT is used in measurement of pressure, load, acceleration, force, weight etc. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

106. Rotary Variable Differential Transformer (RVDT) : It is used to sense angular displacement. It is similar to L VDT except the shape of core and may be rotated between its windings by means of a shaft. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

107. At the null position of its core, the output voltage of secondary windings S1 and S2 are equal and in opposition. Therefore the net output is zero. Any angular displacement from the null position will result in a differential voltage output. Hence the response of transducer is linear. Clockwise rotation produces an increasing voltage of a secondary winding of one phase while anticlockwise rotation produces an increasing voltage of opposite phase. Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering

108. Strain Gauges Dr. Ankita Malhotra SVKM's D J Sanghvi College of Engineering