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• 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.
Non-electrical physical quantity: temperature, sound or light
Electrical signal
Temperature transducers Thermocouples Resistance-Temperature Detectors (RTD) Thermistors
Resistive position transducers Displacement transducers Strain gauge
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
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
• 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.
• 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.
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
• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors
• Resistive position transducers• Displacement transducers• Strain gauge
• 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.
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.
wherec and k = constants of the thermocouple materialsT1 = the temperature of the ‘hot’ junction
T2 = the temperature of the ‘cold’ or ‘reference’ junction
)()( 22
2121 TTkTTcE
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.
(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
(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.
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)
• 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.
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.
A straight line has been drawn between the points of the curve that represent temperature, T1 and T2, and T0 represent the midpoint temperature.
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
Straight line equation
R2 = resistance at T2
R1 = resistance at T1
)()(
1
12
12
0 TT
RR
TRo
More accurate representation of R-T curve over some span of temperatures.
R(T) = quadratic approximation of resistance at temperature T
R(T0) = resistance at temperature T0
α1 = linear fractional change in resistance
with temperatureα2 = quadratic fractional change in
resistance with temperature ΔT = T - T0
212
21 ])(1)[()( TTTTTTRTR o
Solution
Solution
Platinum
Copper
TungstenNickel
Platinum: very repeatable, sensitive, expensive
Nickel: not quite repeatable, more sensitive, less expensive
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
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.
Wire is in a coil to achieve small size and improve thermal conductivity to decrease response time.
Protect from the environment
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
• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors
• Resistive position transducers• Displacement transducers• Strain gauge
Distance
Electrical signal
To VRR
RV
21
2
2R
1R
• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors
• Resistive position transducers• Displacement transducers• Strain gauge
Capacitive transducer Inductive transducer Variable inductance transducer
• The capacitance of a parallel-plate capacitor is given by
ε = dielectric constantεo = 8.854 x 1o-12, in farad per meterA = the area of the plate, in square meterd = the plate spacing in meters
d
AC o
Principle: if there is a relative motion between a conductor and magnetic field, a voltage is induced in the conductor.
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.
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.
Core at the centerV1 = V2
Vo = 0
Core moves towards S1
V1 > V2
Vo increase
Core moves towards S2
V2 > V1
Vo decrease
• Temperature transducers▫ Thermocouples▫ Resistance-Temperature Detectors (RTD)▫ Thermistors
• Resistive position transducers• Displacement transducers• Strain gauge
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.
In general, stress is expressed as
is the average stress, also called engineering or nominal stress
and is the force acting over the area .
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.
• The strain is defined as the fractional change in length
• Strain is thus a unitless quantity
l
lstrain
• The strain is defined as the fractional change in length
• Strain is thus a unitless quantity
l
lstrain
From the equation of resistance,
R = resistanceρ = specific resistance of the conductor
materialL = the length of the conductor in metersA = the area of the conductor in square
meters
A
LR
To measurepressure
When a strain produced by a force is applied on the wires, L increase and A decrease.
L – increaseA – decrease
From the equation of resistance,
R – increase
A
LR
LL
RRK
/
/
K = the gauge factorR = the initial resistance in ohms (without
strain)ΔR = the change of initial resistance in
ohmsL = the initial length in meters (without
strain)ΔL = the change of initial length in meters
LL
RRK
/
/
K = the gauge factorR = the initial resistance in ohms (without
strain)ΔR = the change of initial resistance in
ohmsL = the initial length in meters (without
strain)ΔL = the change of initial length in meters
G
RRK
/
