MR3491 SENSORS AND INSTRUMENTATION ( UNIT-I INTRODUCTION)

Prepared by A.R.Sivanesh AP/MECH, SRIET, Coimbatore

1. SENSORS AND INSTRUMENTATION UNIT I INTRODUCTION Prepared by A.R.SIVANESH Assistant Professor Department of Mechanical Engineering Sri Ranganathar Institute of Engineering and Technology, Coimbatore 1

2. Measurements Measurement: Comparison between a standard and what we want to measure (the measurand). Two quantities are compared the result is expressed in numerical values. 2

3. Basic requirements for a meaningful measurement The standard used for comparison purposes must be accurately defined and should be commonly accepted. The apparatus used and the method adopted must be provable ( verifiable) . 3

4. Two major functions of all branch of engineering Design of equipment and processes Proper Operation and maintenance of equipment and processes. 4

5. Methods of Measurement Direct Methods Indirect Methods 5

6. DIRECT METHODS : In these methods, the unknown quantity (called the measurand ) is directly compared against a standard. INDIRECT METHOD : Measurements by direct methods are not always possible, feasible and practicable. In engineering applications measurement systems are used which require need of indirect method for measurement purposes. 6

7. Instruments and Measurement Systems. instruments as a physical means M e as u r e m ent i n vo l v e t h e u se o f of determining quantities or variables. Because of modular nature of the elements within it, it is common to refer the measuring instrument as a MEASUREMENT SYSTEM. 7

8. Evolution of Instruments. Mechanical Electrical Electronic Instruments. MECHANICAL: These instruments are very reliable for static and stable conditions. But their disadvantage is that they are unable to respond rapidly to measurements of dynamic and transient conditions. 8

9. Co n td ELECTRICAL: It is faster than mechanical, indicating the output are rapid than mechanical methods. But it depends on the mechanical movement of the meters. The response is 0.5 to 24 seconds. ELECTRONIC: It is more reliable than other system. It uses semiconductor devices and weak signal can also be detected. 9

10. Classification Of Instruments Absolute Instruments. Secondary Instruments. ABSOLUTE: These instruments give the magnitude if the quantity under measurement terms of physical constants of the instrument. SECONDARY: These instruments are calibrated by the comparison with absolute instruments which have already been calibrated. 10

11. Further its classified as Deflection Type Instruments Null Type Instruments. 11

12. Functions of instrument and measuring system can be classified into three. They are: Indicating function. Recording function. Controlling function. Application of measurement systems are: Monitoring of process and operation. Control of processes and operation. Experimental engineering analysis. 12

13. Types Of Instrumentation System Intelligent Instrumentation (data has been refined for the purpose of presentation ) Dumb Instrumentation (data must be processed by the observer) 13

14. Elements of Generalized Measurement System Primary sensing element. Variable conversion element. Data presentation element. PRIMARY SENSING ELEMENT: The quantity under measurement makes its first contact with the primary sensing element of a measurement system. VARIABLE CONVERSION ELEMENT: It converts the output of the primary sensing element into suitable form to preserve the information content of the original signal. 14

15. Co n td . . DATA PRESENTATION ELEMENT: The information about the quantity under measurement has to be conveyed to the personnel handling the instrument or the system for monitoring, control or analysis purpose. 15

16. Functional Elements of an Instrumentation System PRIMARY SENSING ELE M E NT VARIABLE CONVER -SION ELE M E N T VARIABLE M ANI P U L A T I - O N ELE M E N T DATA TRANSMISSIO -N ELEMENT D A T A COND ITI ON I NG ELE M E NT INTERMEDIATE STAGE DETECTOR T RAN SD U C E R STAGE TE R M I N A T I NG STAGE QUANTITY TO BE M E A S URED DATA P R E S E N TA TION ELEMENT 16

17. SENSORS & TRANSDUCER 17

18. DEFINITION SENSOR It is defined as an element which produces signal relating to the quantity being measured sensor can be defined as “A device which provides a usable output in response to a specified measured.” TRANSDUCER 1. It is defined as an element when subjected t o s o m e physical exper i ences a chan g e re l a ted change or an element which converts a specified measured into a usable output by using a transduction principle. 2. It can also be defined as a device that converts a signal from one form of energy to another form. 18

19. TYPE OF SENSORS AND ITS APPLICATIONS IN MECHATRONICS SYSTEM WE NEED TO MEASURE THE FOLLOWING PHYSICAL QUANTITIES – SENSORS AND TRANSDUCERS ARE THE KEY ELEMENT USED FOR THE MEASUREMENT OF THE PHYSICAL QUANTITIES DISPLACEMENT TEMPERATURE PRESSURE STRESS POSITION AND PROXIMITY VELOCITY MOTION FORCE LIQUID FLOW LIQUID LEVEL LIGHT SENSORS 19

20. SENSORS ELEMENT IN A MEASUREMENT SYSTEM THAT ACQUIRES A PHYSICAL PARAMETER AND CHANGES INTO A SIGNAL(ALSO CAN BE DEFINED AS PART OF A TRANSDUCER WHICH SENSES OR RESPOND TO A PHYSICAL QUANTITY OR MEASURAND SENSOR NORMALLY SENSES THE FOLLOWING PHYSICAL QUANTITIES POSITION FORCES DISTANCE STRAIN VIBRATION TEMPERATURE ACCELERATION ETC. EXAMPLE OF SENSOR – A THERMOCOUPLE SENSES THE CHANGE IN TEMPERATURE 20

21. TRANSDUCER – CONVERTS ENERGY FROM ONE FORM TO ANOTHER TEMPERATURE, STRAIN ---------  ELECTRICAL ENERGY EXAMPLE- ACCELEROMETER GIVES OUTPUT VOLTAGE PROPORTIONAL TO THE MECHANICAL MOTION OF THE OBJECT 21

22. Active transducers generate electric current or voltage directly in response to environmental stimulation ( Active transducers are those which do not require any power source for their operation. They work on the energy conversion principle. They produce an electrical signal proportional to the input (physical quantity). For example, a thermocouple is an active transduce r .) Passive transducers produce a change in some passive electrical quantity, such as capacitance, resistance, or inductance, as a result of stimulation. These usually require additional electrical energy for excitation .( Transducers which require an external power source for their operation is called as a passive transducer . They produce an output signal in the form of some variation in resistance, capacitance or any other electrical parameter, which than has to be converted to an equivalent current or voltage signal.) 22

23. EXAMPLE OF ACTIVE TRANSDUCER 23

24. EXAMPLE OF PASSIVE TRANSDUCER 24

25. Static Characteristics Of Instruments And Measurement Systems ( Ref 4, Chapter 2) Application involved measurement of quantity that are either constant or varies slowly with time is known as static. A ccuracy D rift D ead Zone S tatic Error S ensitivity R eproducibility 25

26. Static Characteristics Static correction Scale range Scale span Noise Dead Time Hysteresis. Linearity 26

27. ACCURACY: It is the closeness with an instrument reading approaches the true value of the quantity being measured. TRUE VALUE: True value of quantity may be defined as the average of an infinite no. of measured value. SENSITIVITY is defined as the ratio of the magnitude of the output response to that of input response. 27

28. STATIC ERROR: It is defined as the difference between the measured value and true value of the quantity. A=Am-At W here Am =measured value of quantity At =true value of quantity. It is also called as the absolute static error. 28

29. SCALE RANGE: The scale range of an instrument is defined as the difference between the largest and the smallest reading of the instrument. Suppose highest point of calibration is X max units while the lowest is X min units, then the instrument range is between X min and X max . SCALE SPAN: Scale span or instrument span is given as Scale span= X max - X min It is the difference between highest and lowest point of calibration. 29

30. Reproducibility is specified in terms of scale readings over a given period of time. Drift is an undesirable quality in industrial instruments because it is rarely apparent and cannot be maintained. It is classified as Zero drift Span drift or sensitivity drift Zonal drift. 30

31. Dynamic Characteristics of Measurement System ( Ref 4, Chapter 4) Speed of response Measuring lag Fidelity Dynamic error 31

32. . SPEED OF RESPONSE :It is defined as the rapidity with which a measurement system responds to changes in measured quantity. It is one of the dynamic characteristics of a measurement system. FIDELITY: It is defined as the degree to which a measurement system indicates changes in the measured quantity without any dynamic error. 32

33. Dynamic Error It is the difference between the true value of the quantity changing with time and the value indicated by the measurement system if no static error is assumed. It is also called measurement error. It is one the dynamic characteristics. 33

34. Measuring Lag It is the retardation delay in the response of a measurement system to changes in the measured quantity. It is of 2 types: Retardation type: The response begins immediately after a change in measured quantity has occurred. Time delay: The response of the measurement system begins after a dead zone after the application of the input. 34

35. Errors in Measurement Limiting Errors (Guarantee Errors) Known Error Classification Gross Error Systematic Or Cumulative Error Random Or Residual Or Accidental Error Instrumental Environmental Observational 35

36. Gross Error Human Mistakes in reading , recording and calculating measurement results. The experimenter may grossly misread the scale. E.g.: Due to oversight instead of 21.5 o C, they may read as 31.5 o C They may transpose the reading while recording (like reading 25.8 o C and record as 28.5 o C) 36

37. Systematic Errors   INSTRUMENTAL ERROR: These errors arise due to 3 reasons- Due to inherent short comings in the instrument Due to misuse of the instrument Due to loading effects of the instrument ENVIRONMENTAL ERROR: These errors are due to conditions external to the measuring device. These may be effects of temperature, pressure, humidity, dust or of external electrostatic or magnetic field. OBSERVATIONAL ERROR: The error on account of parallax is the observational error. 37

38. Residual error This is also known as residual error. These er r o rs a r e d u e t o a m ultit u de o f s m all factors which change or fluctuate from one measurement to another. The happenings or d i s t u rba n ces ab o u t wh i ch we a r e un awa r e are lumped together and called“Random” o r “ Resi d ual”. H e n ce the e r rors ca u sed b y these are called random or residual errors. 38

39. Arithmetic Mean The most probable value of measured variable is the arithmetic mean of the number of readings taken. It is given by n n   x x  x 1  x 2  .. . . . x n Where x = arithmetic mean x1,x2,.. x3= readings of samples n= number of readings 39

40. Deviation Deviation is departure of the observed reading from the arithmetic mean of the group of readings. d 1  x 1  X d 2  x 2  X d 3  x 3  X d n  x n  X d 1  d 2  d 3  . . ...  d n  ie  ( x 1  X )  ( x 2  X )  ( x 3  X )  ..  ( x n  X )  ( x 1  x 2  x 3  . . .  x n )  n X  n X  n X  40

41. Standard Deviation The standard deviation of an infinite number of data is defined as the square root of the sum of the individual deviations squared divided by the number of readings. d n d 2 d 2 1 2 d 2 1 n  1   d 2  d 2  ...  d 2 2 3 4 n  1 S . D  s    20 observation    2 ob s e r v a t i o n    d 2  d 2  ...  d 2 2 3 4 n   S . D    41

42. Vari a nce n 42 2  2 2  2   20 o b s e r v a t i o n  n  1 V a r i a n c e   S . D  s    2 o b s e r v a t i o n   d 2  d 2 V a r i a n c e   S . D   

43. Pr o blem Question: The following 10 observation were recorded when measuring a voltage: 41.7,42.0,41.8,42.0,42.1, 41.9,42.0,41.9,42.5,41.8 volts. Mean Standard Deviation Probable Error Range. 43

44. Answer Mean=41.97 volt S.D=0.22 volt Probable error=0.15 volt Range=0.8 volt. 44

45. Calibration Calibration of all instruments is important since it affords the opportunity to check the instruments against a known standard and subsequently to find errors and accuracy. Calibration Procedure involve a comparison of the particular instrument with either a Primary standard a secondary standard with a higher accuracy than the instrument to be calibrated. an instrument of known accuracy. 45

46. Standards A standard is a physical representation of a unit of measurement. The term ‘standard’ is applied to a piece of equipment having a known measure of physical quantity. 46

47. Types of Standards –International Standards (defined based on international agreement ) –Primary Standards (maintained by national standards laboratories) –Secondary Standards ( used by industrial measurement laboratories) –Working Standards ( used in general laboratory) 47

48. Performance measures of sensors 1.Range and Span The range of a transducer defines the limits between which the input can vary on the working. The Span is the difference between the maximum value and the minimum value. For example, a load cell for the measurement of forces might have a range of 0 to 50kN and its span is 50kN (50 kN – 0 kN = 50kN). 48

49. 2.Error Error is the difference between the result of the measurement and the true value of the quantity being measured. Error = measured value – true value For example, measurement system gives a temperature reading of 50℃ , but the actual reading is 49 ℃, then the error is +1℃ (50℃ – 49℃). If the actual reading is 52 ℃ , then the error is -2℃ (50℃ – 52℃). The error can obtain in both positive and negative values. 49

50. 3.Accuracy It is the extent in which the value indicated by a measurement system might be wrong. It is the summation of all the possible errors that are likely to occur as well as the accuracy to which the transducer has been calibrated. For example, if the temperature of the system have a specified accuracy of  ± 5℃, this means that the reading given by the instrument to be lie within plus or minus 5 ℃ of the true value. Accuracy is mainly expressed in percentage of the full range. For example, a transducer having an accuracy of ± 10% of full range output of  0 to 500 ℃, then the reading can be expected from plus or minus 50 ℃ of the true reading i.e., from 450 ℃ to 550℃. 50

51. 4. Sensitivity It is normally termed as the relationship showing how much output there is per unit input, i.e., output/input. For example, a resistance thermometer has sensitivity of 1Ω/℃. This shows that the thermometer having sensitivity, where there is a deflection of 1Ω for every 1℃. This is also used to indicate the sensitivity to inputs other than being measured. 51

52. 5. Hysteresis Error Transducers can give different values of outputs to the same value of quantity being measured. So the output value will be obtained by continuously increasing or continuously decreasing change. This effect is called hysteresis. The difference between the decrease in change and increase in change on the system of measurement known as hysteresis error. 52

53. 6. Non-Linearity error In many Transducers linear relationship between the input and output is assumed over the working range. i.e., for the given input the obtained output will produce a graph of straight line. But some times this linearity will not be occurred due to certain possible errors. The error is defined as the maximum difference from the straight line. It is known as Non-Linearity error . Various methods are used for the numerical expressions of the non-linearity error. This error is generally defined by percentage of the full range output. We can identify the non-linearity error by observing the linear relationship of the input and output, plot them in a straight line on a graph. 53

54. Then the non-linearity function for the input and output also plot in the same graph. Surely, this non-linearity will not be in straight line. The difference between two graphs is called error (non-linearity error). Below image shows the graph of non-linearity error . 54

55. 7. Repeatability/Reproducibility Repeatability /reproducibility in transducer is defined as the ability to give the same output for the applications of the same input value. ADVERTISING The error occurring from the same output not given with repeated applications is usually expressed as a percentage of the full range output. Repeatability = (max value- min value)/ full range * 100 For example, the maximum resistance measured in system of 100 ℃ is  75Ω and the minimum resistance is 0.1 Ω  of  the range (0 to 75 Ω), then the repeatability is calculated by Repeatability = (75-0.1)/75  *100 Repeatability = 74.9/75 = 0.99 *100 Repeatability = 99 For the system the repeatability will be 99% for the same output value for the same input. 55

56. 8. Stability Stability of a transducer is the performance of a transducer which will give the same output when used to measure the same input for a period of time. Normally, stability is nothing but for the constant given input the output will be stable only for given period of time in the measurement system. 9. Drift The term drift is used to describe the change in output for a given period of time for the same input. The drift may be expressed as percentage of the full range of output. There is a term called  Zero Drift  which is used to describe the change in output on the system when there is no input or zero input. 56

57. 10. Dead Band The dead band of a transducer is the range of input values in the system for which there will be no output. For example , in a Load measurement system the change of resistance will define the amount of weight but if there will be no output for some range of input after that output will occur similarly. The space / time where there is no output for the input is known as Dead Band or Dead space. 11. Resolution When the input varies continuously over the range in the system, which may cause small change in output signals. Resolution is nothing but small change in input will cause the observable change in output also. For example, in wire wound potentiometer the slider moves from one turn to the next one which will change the output resistance reasonably. For a transducer giving a digital output will produce a smallest change in output signal is 1 bit. 57

58. SENSORS 58

59. Classification of sensors Detail classification of sensors in view of their applications in manufacturing is as follows. A. Displacement, position and proximity sensors Potentiometer • Strain-gauged element • Capacitive element • Differential transformers • Eddy current proximity sensors • Inductive proximity switch • Optical encoders • Pneumatic sensors • Proximity switches (magnetic) • Hall effect sensors B. Velocity and motion Incremental encoder • Tachogenerator • Pyroelectric sensors C. Force Strain gauge load cell D. Fluid pressure Diaphragm pressure gauge • Capsules, bellows, pressure tubes • Piezoelectric sensors • Tactile sensor • E. Liquid flow Orifice plate • Turbine meter F. Liquid level • Floats • Differential pressure G. Temperature Bimetallic strips • Resistance temperature detectors • Thermistors • Thermo-diodes and transistors • Thermocouples • Light sensors • Photo diodes • Photo resistors 59

60. Sensor Calibration Techniques 60

61. Sensor Calibration in simple terms can be defined as the comparison between the desired output and the measured output . These errors can be caused by various reasons. Some of the errors seen in sensors are errors due to improper zero-reference, errors due to shift's in sensor range, error due to mechanical damage, etc… Sensor Calibration Techniques 61

62. 62

63. Why do we need to calibrate sensors? 1.No sensor is perfect. Sample to sample manufacturing variations mean that even two sensors from the same manufacturer production run may yield slightly different readings. Differences in sensor design mean two different sensors may respond differently in similar conditions. This is especially true of ‘indirect’ sensors that calculate a measurement based on one or more actual measurements of some different, but related parameter. Sensors subject to heat, cold, shock, humidity etc. during storage, shipment and/or assembly may show a change in response. Some sensor technologies 'age' and their response will naturally change over time - requiring periodic re-calibration. 63

64. 2. The Sensor is only one component in the measurement system.  For example: With analog sensors, your ADC is part of the measurement system and subject to variability as well. Temperature measurements are subject to thermal gradients between the sensor and the measurement point. Light and color sensors can be affected by spectral distribution, ambient light, specular reflections and other optical phenomena. Inertial sensors almost always have some 'zero offset' error and are sensitive to alignment with the system being measured 64

65. What makes a good sensor? The two most important characteristic of a sensor are: Precision  - The ideal sensor will always produce the same output for the same input.   Resolution  - A good sensor will be able to reliably detect small changes in the measured parameter. 65

66. How Do We Calibrate? The first thing to decide is what your calibration reference will be.   Standard References If it is important to get accurate readings in some standard units, you will need a Standard Reference to calibrate against.  This can be: A calibrated sensor - If you have a sensor or instrument that is known to be accurate.  It can be used to make reference readings for comparison.  Most laboratories will have instruments that have been calibrated against NIST standards.  These will have documentation including the specfic reference against which they were calibrated, as well as any correction factors that need to be applied to the output. A standard physical reference - Reasonably accurate physical standards can be used as standard references for some types of sensors Rangefinders Rulers and Meter sticks Temperature Sensors Boiling Water - 100°C at sea-level Ice-water Bath - The "Triple Point" of water is 0.01°C at sea-level Accelerometers Gravity is a constant 1G on the surface of the earth. 66

67. Working Principle of Sensor Calibration Calibration of the sensors aids in enhancing their functionality and accuracy. Industries do sensor calibration using two well-known procedures. The first way involves businesses incorporating an internal calibration procedure within their production facility to undertake customized sensor calibration. In this case, the business incorporates the required hardware into its design for sensor output rectification. Through this procedure, the sensor calibration can be adjusted to meet the needs of a particular application. However, this procedure lengthens the time to market. As an alternative to this internal calibration process, several manufacturing firms offer sensor packages that include an excellent automotive-grade MEMS sensor in addition to full system-level calibration. Companies use onboard digital circuitry and software in this approach to assist designers in enhancing the performance and usability of the sensors. Digital circuitry, such as voltage regulation and analog signal filtering techniques, are used to shorten the product design cycle and reduce the number of components. Advanced sensor fusion methods are offered to the onboard processor to enhance overall performance and functionality. Some highly developed onboard signal processing algorithms also aid in shortening the production process, enabling quicker time to market. 67

68. Calibration Methods Three different types of calibration: One Point Calibration Two Point Calibration Multi-Point Curve Fitting 68

69. One Point Calibration One point calibration is the simplest type of calibration. If your sensor output is already scaled to useful measurement units, a one point calibration can be used to correct for sensor offset errors in the following cases: Only one measurement point is needed. If you have an application that only requires accurate measurement of a single level, there is no need to worry about the rest of the measurement range.  An example might be a temperature control system that needs to maintain the same temperature continuously. The sensor is known to be linear and have the correct slope over the desired measurement range. In this case, it is only necessary to calibrate one point in the measurement range and adjust the offset if necessary.  Many temperature sensors are good candidates for one-point calibration. 69

70. A one point calibration can also be used as a "drift check" to detect changes in response and/or deterioration in sensor performance.   For example, thermocouples used at very high temperatures exhibit an 'aging' effect.  This can be detected by performing periodic one point calibrations, and comparing the resulting offset with the previous calibration. 70

71. How to do it: To perform a one point calibration: Take a measurement with your sensor. Compare that measurement with your reference standard. Subtract the sensor reading from the reference reading to get the offet . In your code, add the offset to every sensor reading to obtain the calibrated value. 71

72. Two Point Calibration A Two Point Calibration is a little more complex.  But it can be applied to either raw or scaled sensor outputs. A Two Point calibration essentially re-scales the output and is capable of correcting both slope and offset errors.  Two point calibration can be used in cases where the sensor output is known to be reasonably linear over the measurement range.   72

73. How to do it: To perform a two point calibration: Take two measurements with your sensor:  One near the low end of the measurement range and one near the high end of the measurement range.  Record these readings as " RawLow " and " RawHigh " Repeat these measurements with your reference instrument.  Record these readings as " ReferenceLow " and " ReferenceHigh " Calculate " RawRange " as  RawHigh – RawLow . Calculate " ReferenceRange " as  ReferenceHigh – ReferenceLow In your code, calculate the " CorrectedValue " using the formula below: CorrectedValue  = ((( RawValue – RawLow ) * ReferenceRange ) / RawRange ) + ReferenceLow 73

74. Multi-Point Curve Fitting Sensors that are not linear over the measurement range require some curve-fitting to achieve accurate measurements over the measurement range. A common case requiring curve-fitting is thermocouples at extremely hot or cold temperatures.  While nearly linear over a fairly wide range, they do deviate significantly at extreme temperatures. 74

75. Sensor output signal types 75

76. sensor output signal types Digital vs. Analog First, we make a distinction between two types of outputs: an analog and a digital output. A sensor with a digital output signals a logical value. In other words: Yes or No, 0 or 1, True or False , Valid or Invalid . A digital output is very well-suited to indicate the presence of an object (at a certain distance) or detecting whether a set limit value has been reached. Does the sensor "see" the object or not? Is the value reached or not? During a detection or non-detection the logical value changes from a 0 to a 1, or vice versa! Examples of digital (switching) outputs are PNP/NPN, relay, solid state relay and PushPull . A sensor with an analog output is capable of giving a signal that is continuously partallel to the measured value. An analog signal is a signal that can register values without intervals. Think of a constantly fluctuating temperature in an outdoor location, such as the conveyor belts in the production of steel beams: the analog output changes parallel en mostly linear with the change of the measurement of the sensor. Another example is the change of a distance from 0 to 1.000 cm or a temperature drops from 200°C to 20°C. Examples of analog outputs are 0-10 Vdc , 4-20 mA, 0-5 Vdc or 0-20 mA. 76

77. Types of digital outputs: PNP or NPN Sensors with a PNP or NPN switching contact make use of a transistor output. The type of transistor output determines whether the sensor switches PNP or NPN. Sensors with a PNP or NPN switching output are equipped with at least three wires; A " + " (Pin 1 / brown wire), a " – " (Pin 3 / blue wire) and a switching wire (Pin 4 / black wire). 77

78. PNP switching output The load is switching between the switching wire (4) and the – (3) within a sensor with a PNP switching output. 78

79. NPN switching output The load is switching between the switching wire (4) and the + (1) within a sensor with a NPN switching output. 79

80. PushPull switching output A PushPull output means that the switching component of a sensor consists of two transistors. This is a type of output in which it is possible to alternately switch PNP as well as NPN. The circuit is designed in such a way that any voltage between a certain limit will make the sensor switch NPN, while a lower voltage provides a PNP output. Sensors with a PushPull output are versatile to use in applications that require a PNP and NPN output. The advantage is also that there is no need for developing the same sensor but with an NPN or PNP output. 80

81. Solid State Relay (SSR) output A solid state relay (SSR) is also known as an optocoupler relay or semiconductor relay. It is a type of relay without a mechanical switch, contrary to a more conventional relay. Conventional mechanical relays have the advantage of being able to switch higher power rates, but because of moving parts are susceptible to wear and tear. A solid state relay switches by use of a light-sensitive diode and is, because of this, free from wear and tear. In addition it is also capable of higher switching frequencies. 81

82. Analog Outputs The most widely used standards are current analog 4 to 20 mA and voltage of 0-10 Vdc . There are others like 1-5 VDC, 0-20 mA and 20mA -20mA. Serial Outputs The sensors with serial outputs are connected to networks for field devices and the information is serially transmitted (bit stream) through a cable network and sending data from the sensor to the controller or supervision by a communications port. Examples of field bus (field networks) mentioned below: Devicenet , Profibus dp , Foundation FieldBus and in recent years it has increased the functionality of EtherNet /IP as a network of field. 82