1. TRANSDUCER

2. 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, pressure sensors).
The term transducer is commonly used in two senses; the sensor, used to detect a parameter in one form and report it in another (usually an electrical or digital signal), and the audio loudspeaker, which converts electrical voltage variations representing music or speech, to mechanical cone vibration and hence vibrates air molecules creating acoustical energy.

3. Non-electrical physical quantity: temperature, sound or light
What is transducer?
Non-electrical physical quantity: temperature, sound or light
TRANSDUCER
Electrical signal

4. TRANSDUCER Temperature transducers Resistive position transducers
Thermocouples
Resistance-Temperature Detectors (RTD)
Thermistors
Resistive position transducers
Displacement transducers
Strain gauge

5. Classification of transducer
Self generating type – do not require an external power, and produce an analog voltage or current when stimulated by some physical form of energy
Thermocouple
Photovoltaic cell
Moving coil generator

6. Classification of transducer
Passive transducers – require an external power, and the output is a measure of some variation (resistance or capacitance)
Slide-wire resistor
Resistance strain gauge
Differential transformer

7. 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.

8. 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.

9. 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

10. TRANSDUCER Temperature transducers Resistive position transducers
Thermocouples
Resistance-Temperature Detectors (RTD)
Thermistors
Resistive position transducers
Displacement transducers
Strain gauge

11. 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.

12. Thermocouple - Principle
Hot junCtion
cold junCtion
voltmeter
Twisting or welding of 2 wires

13. In normal operation, cold junction is placed in an ice bath
Hot junCtion
cold junCtion

14. In normal operation, cold junction is placed in an ice bath
UNKNOWN TEMPERATURE
KNOWN TEMPERATURE

15. 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.

17. 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

18. Problem
A thermocouple was found to have linear calibration between 0⁰C and 400⁰C with emf at maximum temperature (reference junction temperature 0⁰C) equal to 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.

19. Solution Sensitivity of the thermocouple = 20.68/(400-0)
= 20.68/(400-0)
= 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 = x 25
Ecorr = mV

20. 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/ = ⁰C Since the reference junction temperature is 25⁰C, hot junction temperature = = ⁰C.

21. 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)

22. 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.

23. 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.

24. Resistance vs Temperature Approximations

25. 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.

26. 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

27. Resistance vs Temperature Linear Approximations
Straight line equation R2 = resistance at T2 R1 = resistance at T1

28. Example

29. Example

30. RTD – quadratic approximation
More accurate representation of R-T curve over some span of temperatures.

31. 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

32. Example

33. Example
Solution

34. Example
Solution

35. Nickel
Tungsten
Copper
Platinum
Platinum: very repeatable, sensitive, expensive
Nickel: not quite repeatable, more sensitive, less expensive

36. 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

37. 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.

38. Construction of a platinum resistance thermometer

39. Construction of a platinum resistance thermometer
Wire is in a coil to achieve small size and improve thermal conductivity
to decrease response time.

40. Construction of a platinum resistance thermometer
Protect from the environment

41. 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

42. Thermistor: resistance vs temperature

43. Thermistor

45. TRANSDUCER Temperature transducers Resistive position transducers
Thermocouples
Resistance-Temperature Detectors (RTD)
Thermistors
Resistive position transducers
Displacement transducers
Strain gauge

46. Resistive position transducers
Distance
TRANSDUCER
Electrical signal

47. Resistive position transducers

48. Resistive position transducers

49. Resistive position transducers

51. TRANSDUCER Temperature transducers Resistive position transducers
Thermocouples
Resistance-Temperature Detectors (RTD)
Thermistors
Resistive position transducers
Displacement transducers
Strain gauge

52. Displacement transducers
Capacitive transducer
Inductive transducer
Variable inductance transducer

53. Capacitive transducers
The capacitance of a parallel-plate capacitor is given by
ε = dielectric constant
εo = x 1o-12, in farad per meter
A = the area of the plate, in square meter
d = the plate spacing in meters

54. Capacitive transducers – physical design

56. Inductive transducers
Principle: if there is a relative motion between a conductor and magnetic field, a voltage is induced in the conductor.

57. Inductive transducers – tachometer with a permanent magnet stator

58. Inductive transducer – tachometer with a permanent magnet rotor

59. 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.

60. Variable Inductance Transducers - construction

63. Variable Inductance Transducers – schematic diagram

65. 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.

66. Variable Inductance Transducers – operation
Core at the center V1 = V2 Vo = 0

67. Variable Inductance Transducers – operation
Core moves towards S1 V1 > V2 Vo increase

68. Variable Inductance Transducers – operation
Core moves towards S2 V2 > V1 Vo decrease
If the core is attached to a moving object, the lvdt output voltage can be a measure of the position of the object

69. Variable Inductance Transducers – with absolute magnitude

72. TRANSDUCER Temperature transducers Resistive position transducers
Thermocouples
Resistance-Temperature Detectors (RTD)
Thermistors
Resistive position transducers
Displacement transducers
Strain gauge

73. 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 Stress is a concept that is based on the concept of continuum.

74. 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

75. 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.

76. Strain The strain is defined as the fractional change in length
Strain is thus a unitless quantity

77. Strain The strain is defined as the fractional change in length
Strain is thus a unitless quantity

78. Stress-strain curve

79. 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

80. Strain gauge To measure pressure
When a strain produced by a force is applied on the wires, L increase and A decrease.

81. Strain gauge
L – increase A – decrease From the equation of resistance, R – increase

82. Strain gauge – the gauge factor
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

83. Strain gauge – the gauge factor
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
strain

84. Strain gauge – the gauge factor