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Transducer main

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Transducer main

  1. 1. SENSOR AND INSTRUMENTATION EE -305 SHAILENDRA GAUTAM Department of Electrical & Electronics Engineering A.B.E.S. Engineering College
  2. 2. Measurement • Measurement is an act or the result of a quantitative comparison between an unknown magnitude and the predefined standard. • The result is expressed in numerical values. • Internationally accepted standard : • Mass : Kg • distance : km
  3. 3. Which quantities do we need to measure? • temperature • wind speed and direction • pressure • humidity • visibility • cloud distribution • cloud type • type and amount of precipitation
  4. 4. INSTRUMENT ? • An instrument is a device that measures a physical quantity such as flow, temperature, level, distance, angle, or pressure. • For example : Instruments may be as simple as direct reading thermometers or may be complex multi- variable process analyzers. Instruments are often part of a control system in refineries, factories, and vehicles. • Instruments can be classified as : mechanical, electrical, electronics instruments.
  5. 5. Instrumentation system
  6. 6. What is transducer? • A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts "one type of energy or physical attribute to another for various purposes including measurement or information transfer". For example : Light Level Light Dependant Resistor (LDR) Photodiode LED's & Displays Temperature : Thermocouple
  7. 7. What is transducer? Non-electrical physical quantity: temperature, sound or light Electrical signal
  8. 8. sensor : • A sensor (also called detector) is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. • A thermocouple converts temperature to an output voltage which can be read by a voltmeter. • The audio loudspeaker, which converts electrical voltage variations representing music or speech, to mechanical cone vibration and hence vibrates air molecules creating acoustical energy. • traffic lights etc.
  9. 9. Difference between transducer and sensor. • Transducers and sensors are physical devices that are used in electrical, electronic and many other types of gadgets and appliances. • Transducers are used to convert one energy type into another while sensors measure energy levels and convert them into electrical signals that can be measured digitally.
  10. 10. Characteristics and choice of Transducers: 1. Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive ,optoelectronic, piezo electric etc. 2. Sensitivity: The transducer must be sensitive enough to produce detectable output. 3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range. 4. Accuracy: High accuracy is assured. 5. Cross sensitivity: It has to be taken into account when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan. 6. Errors: The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.
  11. 11. 7 Transient and frequency response : The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error. 8. Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects. 9. Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down. 10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals. 11. Usage and Ruggedness : ruggedness both electrical as well as mechanical intensities of a transducer must be considered. 12. Electrical aspects : length and type of cable, signal to noise ratio, frequency response. 13. Stability and reliability : high degree of stability to be operated during its operation and storage life. 14. Static characteristics : low static errors, low non-linearity, low hystersis, high resolution and a high degree of repeatability.
  12. 12. Classification of transducer Transducers can be classified : 1. on the basis of transduction form used 2. as primary and secondary transducers. 3. as passive and active transducers 4. as analog and digital tranducers 5. as transducers and inverse transducers.
  13. 13. Displacement sensors: converting displacement into electrical signals. Types of displacement sensors: 1. Potentiometric 2. LVDT (Linear Variable Differential Transformer) 3. Optical encoder
  14. 14. POTENTIOMETRIC DISPLACEMENT SENSOR RESISTIVE POTENTIOMETER ( POT) :
  15. 15. Stress • Stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces. It was introduced into the theory of elasticity by Cauchy around 1822. Stress is a concept that is based on the concept of continuum.
  16. 16. Stress In general, stress is expressed as is the average stress, also called engineering or nominal stress and is the force acting over the area .
  17. 17. Strain Strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape.
  18. 18. Strain • The strain is defined as the fractional change in length • Strain is thus a unitless quantity l l strain ∆ =
  19. 19. Strain gauge L – increase A – decrease From the equation of resistance, R – increase A L R ρ =
  20. 20. Strain gauge – the gauge factor LL RR FG / / .. ∆ ∆ = K = the gauge factor R = the initial resistance in ohms (without strain) ΔR = the change of initial resistance in ohms L = the initial length in meters (without strain) ΔL = the change of initial length in meters ε RR FG / .. ∆ =
  21. 21. Classification of strain gauge 1. wire strain gauge (a) bonded wire strain gauge (b) Unbonded metal strain gauge (c) bonded metal foil strain gauge 2. semiconductor strain gauge.
  22. 22. BONDED WIRE STRAIN GAUGE A resistance wire strain gauge consist of a grid of fine resistance wire. The grid is cemented to carrier which may be a thin sheet of paper bakelite or teflon. The wire is covered on top with a thin sheet of material so as to prevent it from any mechanical demage. The carrier is bonded with an adhesive material to the specimen which permit a good transfer of strain from carrier to grid of wires.
  23. 23. UNBONDED METAL STRAIN GAUGE The unbonded meter wire gauges employ preloaded resistance wire connected in Wheatstone bridge as shown in fig. At initial preload the strain and resistance of the four arms are nominally equal with the result the output voltage of the bridge is equal to zero. Application of pressure produces a small displacement , the displacement increases a tension in two wire and decreases it in the other two thereby increase the resistance of two wire which are in tension and decreasing the resistance of the remaining two wire . This causes an unbalance of the bridge producing an output voltage which is proportional to the input displacement and hence to the
  24. 24. Bonded metal foil strain gauge This class of strain gauge is only an extension of the bonded metal wire strain gauges. Base (carrier) Materials: several types of base material are used to support the wires. Adhesive: The adhesive acts as bonding materials. successful strain gauge bonding depends upon careful surface preparation and use of the correct bonding agent.  strain be faithfully transferred on to the strain gauge, the bond has to be formed between the surface to be strained and the plastic backing material on which the gauge is mounted . .Leads: The leads should be of materials which have low and stable resistivity and also a low resistance temperature coefficent.  grid pattern is formed with a thin foil.  larger surface area, therefore higher heat dissipation capability and better bonding property.
  25. 25. Semiconductor strain gauges Semiconductor gauge are used in application where a high gauge factor is desired. A high gauge factor means relatively higher change in resistance that can be measured with good accuracy. The resistance of the semiconductor gauge change as strain is applied to it. The semiconductor gauge depends for their action upon the piezo-resistive effect i.e. change in value of resistance due to change in resistivity. Silicon and germanium are used as resistive material for semiconductor gauges
  26. 26. Pressure sensors (transducers) pressure = force area pressure may be considered as stress. pressure sensors - measurement of pressure. Types of pressure transducers :  LVDT based Diaphragm  Piezoelectric
  27. 27. Diaphragm (a) flat diaphragm; (b) corrugated diaphragm A diaphragm usually is designed so that the deflection-versus-pressure characteristics are as linear as possible over a specified pressure range, and with a minimum of hysteresis and minimum shift in the zero point.
  28. 28. Diaphragm Uses the elastic deformation of a flexible membrane that separates two different pressures.  The deformation of the diaphragm is dependent on the difference in pressure between the two faces.  The diaphragm expands when very small pressures are applied.
  29. 29. LVDT based diaphragm
  30. 30. Electromagnetic Flowmeters • Magnetic flowmeters have been widely used in industry for many years. • Unlike many other types of flowmeters, they offer true noninvasive measurements. • They are easy to install and use to the extent that existing pipes in a process can be turned into meters simply by adding external electrodes and suitable magnets. • They can measure reverse flows and are insensitive to viscosity, density, and flow disturbances. • Electromagnetic flowmeters can rapidly respond to flow changes and they are linear devices for a wide range of measurements. • As in the case of many electric devices, the underlying principle of the electromagnetic flowmeter is Faraday’s law of electromagnetic induction. • The induced voltages in an electromagnetic flowmeter are linearly proportional to the mean velocity of liquids or to the volumetric flow rates.
  31. 31. • As is the case in many applications, if the pipe walls are made from nonconducting elements, then the induced voltage is independent of the properties of the fluid. • The accuracy of these meters can be as low as 0.25% and, in most applications, an accuracy of 1% is used. • At worst, 5% accuracy is obtained in some difficult applications where impurities of liquids and the contact resistances of the electrodes are inferior as in the case of low-purity sodium liquid solutions. • Faraday’s Law of Induction • This law states that if a conductor of length l (m) is moving with a velocity v (m/s–1 ), perpendicular to a magnetic field of flux density B (Tesla), then the induced voltage e across the ends of conductor can be expressed by: Blve =
  32. 32. ,BDve = , 4 2 vDAvQ π == D BQ e π 4 =
  33. 33. Performance Considerations Reynolds number constraints Entrained gas or particles for doppler Clean liquids for time of flight Installed without process shut down Straight upstream piping requirements V ADVANTAGES No Moving Parts Unobstructed Flow Passage Wide Rangeability DISADVANTAGES For Liquids Only (limited gas) Flow Profile Dependent Errors Due To Deposits
  34. 34. Signal conditioning • In electronics, signal conditioning means manipulating an analogue signal in such a way that it meets the requirements of the next stage for further processing. For example, the output of an electronic temperature sensor, which is probably in the millivolts range is probably too low for an Analog-to-digital converter (ADC) to process directly. In this case the signal conditioning is the amplification necessary to bring the voltage level up to that required by the ADC.
  35. 35. Signal conditioning • Types of devices that use signal conditioning include signal filters, instrument amplifiers, sample- and-hold amplifiers, isolation amplifiers, signal isolators, multiplexers, bridge conditioners, analog-to-digital converters, digital-to-analog converters, frequency converters or translators, voltage converters or inverters, frequency-to-voltage converters, voltage-to-frequency converters, current-to- voltage converters, current loop converters, and charge converters.
  36. 36. Signal conditioning • Signal inputs accepted by signal conditioners include DC voltage and current, AC voltage and current, frequency and electric charge • Outputs for signal conditioning equipment can be voltage, current, frequency, timer or counter, relay, resistance or potentiometer, and other specialized outputs
  37. 37. TRANSDUCER • Temperature transducers ▫ Thermocouples ▫ Resistance-Temperature Detectors (RTD) ▫ Thermistors • Resistive position transducers • Displacement transducers • Strain gauge
  38. 38. Thermocouple • In 1821, T.J. Seebeck discovered that an electric potential occurs when 2 different metals are joined into a loop and the two junctions are held at different temperatures. • Seebeck emf – a voltage difference between the two ends of the conductor that depends on the temperature difference of the ends and a material property. • If the ends of the wire have the same temperature, no emf occurs, even if the middle of the wire is hotter or colder.
  39. 39. Thermocouple - Principle Twisting or welding of 2 wires
  40. 40. In normal operation, cold junction is placed in an ice bath
  41. 41. In normal operation, cold junction is placed in an ice bath
  42. 42. Thermocouples • Type K : Chromel-Alumel • Type J : Iron-Constantan • Type E : Chromel-Constantan • Type N : Nicros-Nisil • Type T : Copper-Constantan • It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature.
  43. 43. Magnitude of thermal EMF where c and k = constants of the thermocouple materials T1 = the temperature of the ‘hot’ junction T2 = the temperature of the ‘cold’ or ‘reference’ junction )()( 2 2 2 121 TTkTTcE −+−=
  44. 44. Problem A thermocouple was found to have linear calibration between 0⁰C and 400⁰C with emf at maximum temperature (reference junction temperature 0⁰C) equal to 20.68 mV. a) Determine the correction which must be made to the indicated emf if the cold junction temperature is 25⁰C. b) If the indicated emf is 8.82 mV in the thermocouple circuit, determine the temperature of the hot junction.
  45. 45. Solution (a) Sensitivity of the thermocouple = 20.68/(400-0) = 0.0517 mV/⁰C Since the thermocouple is calibrated at the reference junction of 0⁰C and is being used at 25⁰C, then the correction which must be made, Ecorr between 0⁰C and 25⁰C Ecorr = 0.0517 x 25 Ecorr = 1.293 mV
  46. 46. Solution (b) Indicated emf between the hot junction and reference junction at 25⁰C = 8.92 mV Difference of temperature between hot and cold junctions = 8.92/0.0517 = 172.53⁰C Since the reference junction temperature is 25⁰C, hot junction temperature = 172.53 + 25 = 197.53⁰C.
  47. 47. Thermocouple - applications • Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. Example: Type K : Chromel-Alumel (-190⁰C to 1260⁰C) Type J : Iron-Constantan (-190⁰C to 760⁰C) Type E : Chromel-Constantan (-100⁰C to 1260⁰C)
  48. 48. Thermocouple - applications • Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. • They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications, thermistors and RTDs are more suitable.
  49. 49. Resistance temperature detector (RTD) Resistance temperature detectors (RTDs), also called resistance thermometers, are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. Temperature Metal Resistance The resistance ideally varies linearly with temperature.
  50. 50. Resistance vs Temperature Approximations
  51. 51. Resistance vs Temperature Approximations • A straight line has been drawn between the points of the curve that represent temperature, T1 and T2, and T0 represent the midpoint temperature.
  52. 52. Resistance vs Temperature Approximations Straight line equation R(T) = approximation of resistance at temperature T R(T0) = resistance at temperature T0 αo = fractional change in resistance per degree of temperature at T0 ΔT = T - T0 21]1)[()( TTTTTRTR oo <<∆+= α
  53. 53. Resistance vs Temperature Linear Approximations Straight line equation R2 = resistance at T2 R1 = resistance at T1 )( )( 1 12 12 0 TT RR TR o − − =α
  54. 54. Example
  55. 55. RTD – quadratic approximation • More accurate representation of R-T curve over some span of temperatures.
  56. 56. RTD – quadratic approximation R(T) = quadratic approximation of resistance at temperature T R(T0) = resistance at temperature T0 α1 = linear fractional change in resistance with temperature α2 = quadratic fractional change in resistance with temperature ΔT = T - T0 21 2 21 ])(1)[()( TTTTTTRTR o <<∆+∆+= αα
  57. 57. Example
  58. 58. Example Solution
  59. 59. Example Solution
  60. 60. Platinum Copper Tungsten Nickel Platinum: very repeatable, sensitive, expensive Nickel: not quite repeatable, more sensitive, less expensive
  61. 61. RTD - sensitivity • Sensitivity is shown by the value αo ▫ Platinum – 0.004/ °C ▫ Nickel – 0.005/ °C • Thus, for a 100Ω platinum RTD, a change of only 0.4 Ω would be expected if the temperature is changed by 1°C
  62. 62. RTD – response time • Generally 0.5 to 5 seconds or more • The slowness of response is due principally to the slowness of thermal conductivity in bringing the device into thermal equilibrium with its environment.
  63. 63. Construction of a platinum resistance thermometer
  64. 64. Construction of a platinum resistance thermometer Wire is in a coil to achieve small size and improve thermal conductivity to decrease response time.
  65. 65. Construction of a platinum resistance thermometer Protect from the environment
  66. 66. Thermistor • Semiconductor resistance sensors • Unlike metals, thermistors respond negatively to temperature and their coefficient of resistance is of the order of 10 times higher than that of platinum or copper. • Temperature semiconductor resistance • Symbol
  67. 67. Thermistor: resistance vs temperature
  68. 68. Thermistor
  69. 69. • Scan example 6.3 module page 109
  70. 70. TRANSDUCER • Temperature transducers ▫ Thermocouples ▫ Resistance-Temperature Detectors (RTD) ▫ Thermistors • Resistive position transducers • Displacement transducers • Strain gauge
  71. 71. Resistive position transducers Distance Electrical signal
  72. 72. Resistive position transducers
  73. 73. Resistive position transducers
  74. 74. Resistive position transducers To V RR R V 21 2 + =2R 1R
  75. 75. TRANSDUCER • Temperature transducers ▫ Thermocouples ▫ Resistance-Temperature Detectors (RTD) ▫ Thermistors • Resistive position transducers • Displacement transducers • Strain gauge
  76. 76. Displacement transducers • Capacitive transducer • Inductive transducer • Variable inductance transducer
  77. 77. Capacitive transducers • The capacitance of a parallel-plate capacitor is given by ε = dielectric constant εo = 8.854 x 1o-12 , in farad per meter A = the area of the plate, in square meter d = the plate spacing in meters d A C oεε =
  78. 78. Capacitive transducers – physical design
  79. 79. Inductive transducers • Principle: if there is a relative motion between a conductor and magnetic field, a voltage is induced in the conductor.
  80. 80. Inductive transducers – tachometer with a permanent magnet stator
  81. 81. Inductive transducer – tachometer with a permanent magnet rotor
  82. 82. Variable Inductance Transducers • Principle: modulation of the excitation signal. • Consist of a primary winding and two secondary windings, wound over a hollow tube and positioned so that the primary is between two secondary.
  83. 83. Variable Inductance Transducers - construction
  84. 84. Variable Inductance Transducers – schematic diagram
  85. 85. Variable Inductance Transducers – operation When the core is in the center, the voltage induced in the two secondaries is equal. When the core is moved in one direction from the center, the voltage induced in one winding is increased and that in the others is decreased. Movement in the opposite direction reverse the effect.
  86. 86. Variable Inductance Transducers – operation Core at the center V1 = V2 Vo = 0
  87. 87. Variable Inductance Transducers – operation Core moves towards S1 V1 > V2 Vo increase
  88. 88. Variable Inductance Transducers – operation Core moves towards S2 V2 > V1 Vo decrease
  89. 89. Variable Inductance Transducers – with absolute magnitude
  90. 90. TRANSDUCER • Temperature transducers ▫ Thermocouples ▫ Resistance-Temperature Detectors (RTD) ▫ Thermistors • Resistive position transducers • Displacement transducers • Strain gauge
  91. 91. Strain • The strain is defined as the fractional change in length • Strain is thus a unitless quantity l l strain ∆ =
  92. 92. Strain gauge From the equation of resistance, R = resistance ρ = specific resistance of the conductor material L = the length of the conductor in meters A = the area of the conductor in square meters A L R ρ =
  93. 93. Strain gauge – the gauge factor LL RR K / / ∆ ∆ = K = the gauge factor R = the initial resistance in ohms (without strain) ΔR = the change of initial resistance in ohms L = the initial length in meters (without strain) ΔL = the change of initial length in meters
  94. 94. Strain gauge – the gauge factor G RR K /∆ =

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