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Chapter 3: Sensors and/or Transducers
1
Introduction to Different Types of
Sensors
Application of sensor
2
Application off sensor
Grey code encoder reflection sensor 3
Application of sensor
Infrared reflection sensor 4
Application of sensors
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 5
Goals of the Chapter
• Define classification of sensors and some terminologies
• Introduce various types of sensors for me...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
7Instrum. & Control Eng. for Energy...
Introduction
• Sensors
• Elements which generate variation of electrical quantities (EQ) in
response to variation of non-e...
Introduction …
• Advantages of using sensors include
1. Mechanical effects such as friction is reduced to the minimum
poss...
Introduction - Use of Sensors
1. Information gathering: Provide data for display purpose
• This gives an understanding of ...
Introduction – Sensor Requirements
• The main function of a sensor is to respond only for the
measurement under specified ...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
12Introduction to Instrumntaion Eng...
Classification of Sensors
• Sensors can be divided on the basis of
• Method of applications
• Method of energy conversion ...
Primary and Secondary Sensors
• Classification is based on the method of application
• Primary sensor
• The input NEQ is d...
Active and Passive Sensor
• Classification based on the basis of energy conversion
• Active sensor
• Generates voltage/cur...
Active and Passive ….
• Passive sensors
• Sensors that does not generate voltage or current, but produce
element variation...
Analog and Digital Sensors
• Classification based on the nature of the output signal
• Analog sensor
• Gives an output tha...
Sensor Classification
• Sensors can also be classified according to the application
• Example
• Measurement of displacemen...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Resistive sensors
• Potentiometers, temperature de...
Resistive Sensors - Potentiometer
• Examples: Displacement, liquid level (in petrol-tank level
indicator) using potentiome...
Resistive Sensors – Potentiometer …
• A linear or rotary movement of a moving contact on a slide
wire indicates the magnit...
Resistive Sensors – Temperature Dependent Resistors
• Two classes of thermal resistors are
• Metallic element
• Semiconduc...
Resistive Sensors – Temperature Dependent…
• Semiconductor based resistance thermometers elements
• The resistance of such...
Resistive Sensors – Temperature Dependent…
• The temperature coefficient of resistance is
• β is typically 4000 k and for ...
Resistive Sensors – Strain Gauges
• Is a secondary transducer that senses tensile or
compressive strain in a particular di...
Resistive Sensors – Strain Gauges …
• To find the change in resistance ∆R,
• Dividing both sides by R, we get the fraction...
Resistive Sensors – Strain Gauges …
• Then, Gauge Factor, G is defined as
• G is also known as Strain-Sensitivity factor; ...
Resistive Sensors – Strain Gauges …
• Sensitive measurements require very high Gauge factors in
the range of 100-300
• Suc...
Piezoresistive Pressure Sensor
• Piezoresistivity is a strain dependent resistivity in a single
crystal semiconductor
• Wh...
Resistive Sensors – Photoconductor
• Are light sensitive resistors with non-linear negative
temperature coefficient
• Are ...
Photoconductive Transducers
• A voltage is impressed on the semiconductor material
• When light strikes the semiconductor ...
Photovoltaic Cells
• When light strikes the barrier between the transparent metal layer
and the semiconductor material, a ...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Resistive sensors
• Capacitive sensors
• Inductive...
Capacitive Transducers
• The parallel plate capacitance is given by
• d= distance between plates
• A=overlapping area
• ε0...
Capacitive Transducers – Liquid Level Measurement
• A simple application of
such a transducer is for
liquid measurement
• ...
Capacitive Transducers – Pressure Sensor
• Use electrical property of a capacitor to measure the
displacement
• Diaphragm:...
Capacitive Transducers – Pressure Sensor
• Components: Fixed plate, diaphragm, displaying device,
dielectric material (air...
Capacitive Transducers - Linear Displacement
• Variable area capacitance displacement transducer
38Introduction to Instrum...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Resistive sensors
• Capacitive sensors
• Inductive...
Inductive Sensors
• For a coil of n turns, the inductance L is given by
• Where
• n: Number of turns of the coil
• l: Mean...
• Input voltage (alternating current): One primary coil
• There will be a magnetic coupling between the core and the coils...
LVDT - Output Characteristics
Output characteristics of an LVDT
42Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and ...
LVDT – Applications
• Measure linear mechanical displacement
• Provides resolution about 0.05mm, operating range from ± 0....
LVDT – Bourdon Tube Pressure Gauge
• LVDT can be combined
with a Bourdon tube
• LVDT converts
displacement into an
electri...
• When pressure is applied via the hole,
the bellow expands a distance d
• This displacement can be calibrated in
terms of...
LVDT and Bellow Combination
• Bellows produce small displacement
• Amplified by LVDT and potentiometer
46Introduction to I...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
• Thermoelectric transducers
• Phot...
Active Sensors - Thermocouple
• Thermoelectric transducers provide electrical signal in
response to change in temperature
...
Active Sensors – Thermocouple …
• Typical material combinations used as thermocouples
• To get higher output emf
• Connect...
Active Sensors – Thermocouple …
• Applications
• Temperature measurement
• Voltage measurement
• Rectifier based rms indic...
Active Sensors – Thermocouple Meter
• The measured a.c. voltage signal is
applied to a heater element
• A thermocouple sen...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
• Thermoelectric transducers
• Phot...
Photoelectric Transducers
• Versatile tools for detecting radiant energy or light
• Are extensively used in instrumentatio...
Photoelectric Transducers …
2. Photo diode
• A diode that is normally reverse-biased=> Current is very low
• When a photon...
Photo transistor
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 55
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
• Thermoelectric transducers
• Phot...
Piezoelelectric Transducers
• Convert mechanical energy into electrical energy
• If any crystal is subject to an external ...
Piezoelelectric Transducers
• A piezoelectric crystal is placed between two plate
electrodes
• Application of force on suc...
Piezoelelectric Transducers …
• Induced charge is proportional to the impressed force
Q = d F
• d= charge sensitivity (C/m...
Piezoelelectric Transducers ….
• Piezoelelectric sensors have good frequency response
• Example: Accelerometer
60
Piezoele...
Piezoelelectric Transducers …
• Example: Pressure Sensors
• Detect pressure changes by
the displacement of a thin
metal or...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
• Thermoelectric transducers
• Phot...
Hall-effect Transducers
• Hall voltage is produced when a material is
• Kept perpendicular to the magnetic field and
• A d...
Hall-effect Transducers …
• Like active sensors, it generates
voltage VH
• It also need an external control
current IC lik...
Hall-effect Transducers …
• Magnetic field forces
electrons to concentrate
on one side of the
conductor (mainly uses
semic...
Overview
• Introduction
• Classification of sensors
• Passive sensors
• Active sensors
• Thermoelectric transducers
• Phot...
Tachometric Generators
• Tachometer – any device used to measure shaft’s rotation
• Tachometric generator
• A machine, whe...
Tachometric Generators
• Voltage generated is proportional to rotation of the shaft
68
D.C. tachometric generator A.C. tac...
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Cn3 sensors and transducers-1

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introduction to instrument engineering

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Cn3 sensors and transducers-1

  1. 1. Chapter 3: Sensors and/or Transducers 1 Introduction to Different Types of Sensors
  2. 2. Application of sensor 2
  3. 3. Application off sensor Grey code encoder reflection sensor 3
  4. 4. Application of sensor Infrared reflection sensor 4
  5. 5. Application of sensors Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 5
  6. 6. Goals of the Chapter • Define classification of sensors and some terminologies • Introduce various types of sensors for measurement purpose and their applications • Example: Displacement, motion, level, pressure, temperature, … 6Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  7. 7. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors 7Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers
  8. 8. Introduction • Sensors • Elements which generate variation of electrical quantities (EQ) in response to variation of non-electrical quantities (NEQ) • Examples of EQ • Temperature, displacement, humidity, fluid flow, speed, pressure,… • Sensor are sometimes called transducers 8 Sensing element Signal conditioning element Signal conversion/ processing element Output presentation Non-electrical quantity Electrical signal Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  9. 9. Introduction … • Advantages of using sensors include 1. Mechanical effects such as friction is reduced to the minimum possibility 2. Very small power is required for controlling the electrical system 3. The electrical output can be amplified to any desired level 4. The electrical output can be detected and recorded remotely at a distance from the sensing medium and use modern digital computers 5. etc … 9Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  10. 10. Introduction - Use of Sensors 1. Information gathering: Provide data for display purpose • This gives an understanding of the current status of the system parameters • Example: Car speed sensor and speedometer, which records the speed of a car against time 1. System control: Signal from the sensor is an input to a controller 10 Controller System under control Output signalDesires signal Sensor Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  11. 11. Introduction – Sensor Requirements • The main function of a sensor is to respond only for the measurement under specified limits for which it is designed • Sensors should meet the following basic requirements 1. Ruggedness: Capable of withstanding overload • Some safety arrangements should be provided for overload protection 1. Linearity: Its input-output characteristics must be linear 2. Repeatability: It should reproduce the same output signal when the same input is applied again and again 3. High output signal quality 4. High reliability and stability 5. Good dynamic response 6. No hysteresis, … 11Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  12. 12. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors 12Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  13. 13. Classification of Sensors • Sensors can be divided on the basis of • Method of applications • Method of energy conversion used • Nature of output signals • Electrical principle • In general, the classification of sensors is given by • Primary and secondary sensors • Active and passive sensors • Analog and Digital sensors 13Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  14. 14. Primary and Secondary Sensors • Classification is based on the method of application • Primary sensor • The input NEQ is directly sensed by the sensor • The physical phenomenon is converted into another NEQ • Secondary sensor • The output of the primary sensor is fed to another (secondary) sensor that converts the NEQ to EQ 14 Load cell Strain- gauge NEQ Secondary sensor Weight (Force ∆F) Primary sensor NEQ EQ Displacement ∆d Resistance ∆R Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  15. 15. Active and Passive Sensor • Classification based on the basis of energy conversion • Active sensor • Generates voltage/current in response to NEQ variation • Are also called self-generating sensors • Normally, the output of active sensors is in µV or mV • Examples • Thermocouples: A change in temperature produces output voltage • Photovoltaic cell: Change solar energy into voltage • Hall-effect sensors, … 15 Active sensors NEQ Ex. Temperature EQ Voltage or current Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  16. 16. Active and Passive …. • Passive sensors • Sensors that does not generate voltage or current, but produce element variation in R, L, or C • Need an additional circuit to produce voltage or current variation • Examples • Thermistor: Change in temperature leads to change in resistance • Photo resistor: Change in light leads to change in resistance • Straingauge: Change in length or position into change in resistance) • LVDT, Mic 16 Passive sensors NEQ ∆R, ∆L, ∆C Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  17. 17. Analog and Digital Sensors • Classification based on the nature of the output signal • Analog sensor • Gives an output that varies continuously as the input changes • Output can have infinite number of values within the sensor’s range • Digital sensor • Has an output that varies in discrete steps or pulses or sampled form and so can have a finite number of values • E.g., Revolution counter: A cam, attached to a revolving body whose motion is being measured, opens and closes a switch • The switching operations are counted by an electronic counter 17Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  18. 18. Sensor Classification • Sensors can also be classified according to the application • Example • Measurement of displacement, motion, temperature, intensity, sensors 18Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  19. 19. Overview • Introduction • Classification of sensors • Passive sensors • Resistive sensors • Potentiometers, temperature dependent resistors, strain gauge, photoconductors (photoresistors), Piezoresistive • Capacitive sensors • Inductive sensors • Active sensors 19Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  20. 20. Resistive Sensors - Potentiometer • Examples: Displacement, liquid level (in petrol-tank level indicator) using potentiometer or rheostat • Convert s linear (translatory) or angular (rotary) displacement into a change of resistance in the resistive element provided with a movable contact 20 • Petrol-tank level indicator • Change in petrol level moves a potentiometer arm • Output signal is proportion to the external voltage source applied across the potentiometer • The energy in the output signal comes from the external power source Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  21. 21. Resistive Sensors – Potentiometer … • A linear or rotary movement of a moving contact on a slide wire indicates the magnitude of the variable as a change in resistance which can easily be converted by a proper electrical circuit into measurements of volt or current 21Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  22. 22. Resistive Sensors – Temperature Dependent Resistors • Two classes of thermal resistors are • Metallic element • Semiconductor • For most metals, the resistance increases with increase in temperature • Where α is the temperature coefficient of resistance and given as • Example: Platinum • Has a linear temperature-resistance characteristics • Reproducible over a wide range of temperature • Platinum Thermometers are used for temperature measurement 22 ]1[...]1[)(R 0 2 210 TRTTRT ααα +≈+++= 0 1 R R T ∆ ∆ =α Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  23. 23. Resistive Sensors – Temperature Dependent… • Semiconductor based resistance thermometers elements • The resistance of such elements decreases with increasing temperature • Example: Thermistor • The resistance-temperature relationship is non-linear and governed by • Where R0 is the resistance at absolute temp (in Kelvin) and β is material constant expressed in degree Kelvin • Most semiconductor materials used for thermometry possess high resistivity and high negative temperature coefficients 23 KTeRT TT 0 0 ) 11 ( 0 300;)(R 0 == −β Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  24. 24. Resistive Sensors – Temperature Dependent… • The temperature coefficient of resistance is • β is typically 4000 k and for T = 300k, 24 2 0 1 TR R T β α −= ∆ ∆ = 044.0 300 4000 22 −=−=−= T β α Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  25. 25. Resistive Sensors – Strain Gauges • Is a secondary transducer that senses tensile or compressive strain in a particular direction at a point on the surface of a body or structure • Used to measure force, pressure, displacement • Where e=∆l/l is the strain • The resistance of an unstrained conductor is given as • Under strained condition, resistance of conductor changes by ∆R because of ∆l, ∆A, and/or ∆ρ 25 )(R eR= A l ρ=R Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  26. 26. Resistive Sensors – Strain Gauges … • To find the change in resistance ∆R, • Dividing both sides by R, we get the fractional change as • Let us define eL = ∆l/l as the longitudinal stain and eT as the transversal strain • Also assume that eT = -νeL ,where ν is the Poisson’s Ratio 26 ρ ρρ ρ ρ ∆+∆−∆= ∆ ∂ ∂ +∆ ∂ ∂ +∆ ∂ ∂ =∆ A l A A l l A R A A R l l R 2 R ρ ρ∆ + ∆ − ∆ = ∆ A A l l R R Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  27. 27. Resistive Sensors – Strain Gauges … • Then, Gauge Factor, G is defined as • G is also known as Strain-Sensitivity factor; rearranging terms, we get • Where is the Piezoresistive term • For most metals, the Piezoresistive term is about 0.4 and 0.2 < ν < 0.5 • Thus, Gauge factor for metallic stain gauges is in the range 2.0–2.5 (not sensitive) 27 Le G ρρ υ / )21( ∆ ++= Le ρρ/∆ Lell G R/R / R/R ∆ = ∆ ∆ = Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  28. 28. Resistive Sensors – Strain Gauges … • Sensitive measurements require very high Gauge factors in the range of 100-300 • Such factor can be obtained from semiconductor strain gauges • Due to the significant contribution from the Piezoresistive term 28Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  29. 29. Piezoresistive Pressure Sensor • Piezoresistivity is a strain dependent resistivity in a single crystal semiconductor • When pressure is applied to the diaphragm, it causes a strain in the resistor • Resistance change is proportional to this strain, and hence change in pressure 29Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  30. 30. Resistive Sensors – Photoconductor • Are light sensitive resistors with non-linear negative temperature coefficient • Are resistive optical radiation transducers • Photoconductors have resistance variation that depends on illumination • The resistance illumination characteristics is given by • Where RD is Dark Resistance and E is illumination level in Lux • Photoconductors are used in • Cameras, light sensors in spectrophotometer • Counting systems where an object interrupts a light beam hitting the photoconductor, etc. 30 E DeR α− =R Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  31. 31. Photoconductive Transducers • A voltage is impressed on the semiconductor material • When light strikes the semiconductor material, there is a decrease in the resistance resulting in an increase in the current indicated by the meter • They enjoy a wide range of applications and are useful for measurement of radiation at all levels • The schematic diagram of this device is shown below 31Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  32. 32. Photovoltaic Cells • When light strikes the barrier between the transparent metal layer and the semiconductor material, a voltage is generated • The output of the device is strongly dependent on the load resistance R • The most widely used applications is the light exposure meter in photographic work Schematic of a photovoltaic cell. 32Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  33. 33. Overview • Introduction • Classification of sensors • Passive sensors • Resistive sensors • Capacitive sensors • Inductive sensors • Active sensors 33Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  34. 34. Capacitive Transducers • The parallel plate capacitance is given by • d= distance between plates • A=overlapping area • ε0 = 8.85x10-12 F/m is the absolute permittivity, εr =dielectric constant (εr =1 for air and εr =3 for plastics) d A C rεε0= • Displacement measurement can be achieved by varying d, overlapping area A and the dielectric constant Schematic of a capacitive transducer. 34Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  35. 35. Capacitive Transducers – Liquid Level Measurement • A simple application of such a transducer is for liquid measurement • The dielectric constant changes between the electrodes as long as there is a change in the level of the liquid Capacitive transducer for liquid level measurement. 35Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  36. 36. Capacitive Transducers – Pressure Sensor • Use electrical property of a capacitor to measure the displacement • Diaphragm: elastic pressure senor displaced in proportion to change in pressure • Acts as a plate of a capacitor 36Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  37. 37. Capacitive Transducers – Pressure Sensor • Components: Fixed plate, diaphragm, displaying device, dielectric material (air) • When the diaphragm deflects, there is change in the gap between the two plates which in turn deflects the meter d C 1 α 37 • Capacitance C of the capacitor is inversely proportional to distance d between the plates, i.e., Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  38. 38. Capacitive Transducers - Linear Displacement • Variable area capacitance displacement transducer 38Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  39. 39. Overview • Introduction • Classification of sensors • Passive sensors • Resistive sensors • Capacitive sensors • Inductive sensors • Active sensors 39Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  40. 40. Inductive Sensors • For a coil of n turns, the inductance L is given by • Where • n: Number of turns of the coil • l: Mean length of the magnetic path • A: Area of the magnetic path • µ: Permeability of the magnetic material • R: Magnetic reluctance of the circuit • Application of inductive sensors • Force, displacement, pressure, … • Inductance variation can be in the form of • Self inductance or • Mutual inductance: e.g., differential transformer 40 R n l n AL 22 == µ Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  41. 41. • Input voltage (alternating current): One primary coil • There will be a magnetic coupling between the core and the coils • Output voltage: Two secondary coils connected in series • Operates using the principle of variation of mutual inductance Linear Variable Differential Transformer (LVDT) Schematic diagram of a differential transformer • The output voltage is a function of the core’s displacement • Widely used for translating linear motion into an electrical signal 41Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  42. 42. LVDT - Output Characteristics Output characteristics of an LVDT 42Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  43. 43. LVDT – Applications • Measure linear mechanical displacement • Provides resolution about 0.05mm, operating range from ± 0.1mm to ± 300 mm, accuracy of ± 0.5% of full-scale reading • The input ac excitation of LVDT can range in frequency from 50 Hz to 20kHz • Used to measure position in control systems and precision manufacturing • Can also be used to measure force, pressure, acceleration, etc.. 43Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  44. 44. LVDT – Bourdon Tube Pressure Gauge • LVDT can be combined with a Bourdon tube • LVDT converts displacement into an electrical signal • The signal can be displayed on an electrical device calibrated in terms of pressure 44Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  45. 45. • When pressure is applied via the hole, the bellow expands a distance d • This displacement can be calibrated in terms of pressure • Where: • d: distance moved by the bellows in m, • A is cross sectional area of the bellow in m2 • λ is the stiffness of the below in N.m-1 LVDT – Bellows • The bellow is held inside a protective casing • Differential pressure sensor • Used to measure low pressure 45 λ A d Punknown = Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  46. 46. LVDT and Bellow Combination • Bellows produce small displacement • Amplified by LVDT and potentiometer 46Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  47. 47. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors • Thermoelectric transducers • Photoelectric transducers • Piezoelectric transducers • Hall-effect transudes • Tachometric generators 47Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  48. 48. Active Sensors - Thermocouple • Thermoelectric transducers provide electrical signal in response to change in temperature • Example: Thermocouple • Thermocouple: Converts thermal energy into electrical energy • Application: To measure temperature • Contains a pair of dissimilar metal wires joined together at one end (sensing or hot junction) and terminated at the other end (reference or cold junction) • When a temperature difference exists b/n the sensing junction and the reference, an emf is produced 48 )(....)()( 21 2 2 2 121 TTTTTTEemfInduced −≈+−+−== αβα Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  49. 49. Active Sensors – Thermocouple … • Typical material combinations used as thermocouples • To get higher output emf • Connect two or more Thermocouples in series • For measurement of average temperature • Connect Thermocouples in parallel 49 Type Materials Temp. Range Output voltage (mV) T Copper-Constantan -2000 C to 3500 C -5.6 to 17.82 J Iron-Constantan 0 to 7500 C 0 to 42.28 E Chromel-Constantan -200 to 9000 C -8.82 to 68.78 K Chromel-Alumel -200 to 12500 C -5.97 to 50.63 R Platinum = 13% Rhodium = 87% 0 to 14500 C 0 to 16.74 Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  50. 50. Active Sensors – Thermocouple … • Applications • Temperature measurement • Voltage measurement • Rectifier based rms indications are waveform dependent • They are normally designed for sinusoidal signals • Hence, error for non-sinusoidal signals • Use thermocouple based voltmeters • Here, temperature of a hot junction is proportional to the true rms value of the current 50Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  51. 51. Active Sensors – Thermocouple Meter • The measured a.c. voltage signal is applied to a heater element • A thermocouple senses the temperature of the heater due to heat generated ( ) • The d.c. voltage generated in the thermocouple is applied to a moving-coil meter • The thermocouple will be calibrated to read current (Irms) • AC with frequencies up to 100 MHz may be measured with thermocouple meters • One may also measure high frequency current by first rectifying the signal to DC and then measuring the DC Schematic of a thermocouple meter. 2 rmsI 51Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  52. 52. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors • Thermoelectric transducers • Photoelectric transducers • Piezoelectric transducers • Hall-effect transudes • Tachometric generators 52Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  53. 53. Photoelectric Transducers • Versatile tools for detecting radiant energy or light • Are extensively used in instrumentation • Most known photosensitive devices include 1.Photovoltaic cells • Semiconductor junction devices used to convert radiation energy into electrical energy 53 Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  54. 54. Photoelectric Transducers … 2. Photo diode • A diode that is normally reverse-biased=> Current is very low • When a photon is absorbed, electrons are freed so current starts to flow, i.e., the diode is forward biased • Has an opening in its case containing a lens which focuses incident light on the PN junction 3. Photo transistor • Also operate in reverse-biased • Responds to light intensity on its lens instead of base current 54Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  55. 55. Photo transistor Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 55
  56. 56. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors • Thermoelectric transducers • Photoelectric transducers • Piezoelectric transducers • Hall-effect transudes • Tachometric generators 56Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  57. 57. Piezoelelectric Transducers • Convert mechanical energy into electrical energy • If any crystal is subject to an external force F, there will be an atomic displacement, x • The displacement is related to the applied force in exactly the same way as elastic sensor such as spring • Asymmetric crystalline material such as Quartz, Rochelle Salt and Barium Tantalite produce an emf when they are placed under stress • An externally force, entering the sensor through its pressure port, applies pressure to the top of a crystal • This produces an emf across the crystal proportional to the magnitude of the applied pressure 57Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  58. 58. Piezoelelectric Transducers • A piezoelectric crystal is placed between two plate electrodes • Application of force on such a plate will develop a stress and a corresponding deformation The piezoelectric effect • With certain crystals, this deformation will produce a potential difference at the surface of the crystal • This effect is called piezoelectric effect 58Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  59. 59. Piezoelelectric Transducers … • Induced charge is proportional to the impressed force Q = d F • d= charge sensitivity (C/m2 )/(N/m2 ) = proportionality constant • Output voltage E= g t P • t= crystal thickness • P = impressed pressure • g=voltage sensitivity (V/m)/(N/m2 ) • Shear stress can also produce piezoelectric effect • Widely used as inexpensive pressure transducers for dynamic measurements 59Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  60. 60. Piezoelelectric Transducers …. • Piezoelelectric sensors have good frequency response • Example: Accelerometer 60 Piezoelectric accelerometer Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  61. 61. Piezoelelectric Transducers … • Example: Pressure Sensors • Detect pressure changes by the displacement of a thin metal or semiconductor diaphragm • A pressure applied on the diaphragm causes a strain on the piezoelectric crystal • The crystal generates voltage at the output • This voltage is proportional to the applied pressure 61Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  62. 62. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors • Thermoelectric transducers • Photoelectric transducers • Piezoelectric transducers • Hall-effect transudes • Tachometric generators 62Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  63. 63. Hall-effect Transducers • Hall voltage is produced when a material is • Kept perpendicular to the magnetic field and • A direct current is passed through it • The Hall-voltage is expressed as • Where • Ic: Control current flowing through the Hall-effect sensor, in Amps • β: Flux density of the magnetic field applied, in Wb/m2 • t: Thickness of the Hall-effect sensor, in meters • KH : Hall-effect coefficient • Hall-effect sensors are used to measure flux density • Can detect very week magnetic fields or small change in magnetic flux density 63 t I KV C HH β = Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  64. 64. Hall-effect Transducers … • Like active sensors, it generates voltage VH • It also need an external control current IC like passive sensors • The sensor can be used for measurement of • Magnetic quantities (B, φ) • Mobility of carriers • Very small amount of power 64Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  65. 65. Hall-effect Transducers … • Magnetic field forces electrons to concentrate on one side of the conductor (mainly uses semiconductor) • This accumulation creates emf, which is proportional to the magnetic field strength • Used in proximity sensors 65Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  66. 66. Overview • Introduction • Classification of sensors • Passive sensors • Active sensors • Thermoelectric transducers • Photoelectric transducers • Piezoelectric transducers • Hall-effect transudes • Tachometric generators 66Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  67. 67. Tachometric Generators • Tachometer – any device used to measure shaft’s rotation • Tachometric generator • A machine, when driven by a rotating mechanical force, produces an electric output proportional to the speed of rotation • Essentially a small generators • Tachometric generators connect to the rotating shaft, whose displacement is to be measured, by, e.g., • Direct coupling or • Means of belts or gears • They produce an output which primarily relates to speed • Displacement can be obtained by integrating speed • Types of Tachometric generators: Generally a.c. or d.c. 67Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
  68. 68. Tachometric Generators • Voltage generated is proportional to rotation of the shaft 68 D.C. tachometric generator A.C. tachometric generator

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