Download - What is Transducer?

TRANSDUCERCHAPTER 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, 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.

What is transducer?Non-electrical physical quantity: temperature, sound or light

Electrical signal

TRANSDUCER Temperature transducers Thermocouples Resistance-Temperature Detectors (RTD) Thermistors

Resistive position transducers Displacement transducers Strain gauge

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

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

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.

Signal conditioning Types of devices that use signal conditioning include signal filters, instrument amplifiers, sampleand-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-tovoltage converters, current loop converters, and charge converters.

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



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.

Thermocouple - Principle

Twisting or welding of 2 wires

In normal operation, cold junction is placed in an ice bath

In normal operation, cold junction is placed in an ice bath

Thermocouples Type K Type J Type E Type N Type T : Chromel-Alumel : Iron-Constantan : Chromel-Constantan : Nicros-Nisil : Copper-Constantan

It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature.

Magnitude of thermal EMF E = c(T1 T2 ) + k (T T )2 1 2 2

wherec and k = constants of the thermocouple materials T1 = the temperature of the hot junction T2 = the temperature of the cold or reference junction

ProblemA thermocouple was found to have linear calibration between 0C and 400C with emf at maximum temperature (reference junction temperature 0C) equal to 20.68 mV. a) Determine the correction which must be made to the indicated emf if the cold junction temperature is 25C. b) If the indicated emf is 8.82 mV in the thermocouple circuit, determine the temperature of the hot junction.

Solution(a) Sensitivity of the thermocouple = 20.68/(400-0) = 0.0517 mV/C Since the thermocouple is calibrated at the reference junction of 0C and is being used at 25C, then the correction which must be made, Ecorr between 0C and 25C Ecorr = 0.0517 x 25 Ecorr = 1.293 mV

Solution(b) Indicated emf between the hot junction and reference junction at 25C = 8.92 mV Difference of temperature between hot and cold junctions = 8.92/0.0517 = 172.53C Since the reference junction temperature is 25C, hot junction temperature = 172.53 + 25 = 197.53C.

Thermocouple - applications Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. Example: Type K Type J Type E (-100C

: Chromel-Alumel (-190C to 1260C) : Iron-Constantan (-190C to 760C) : Chromel-Constantan to 1260C)

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 0100 C with 0.1 C accuracy. For such applications, thermistors and RTDs are more suitable.

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.

Resistance vs Temperature Approximations

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.

Resistance vs Temperature ApproximationsStraight line equation

R (T ) = R(To )[1 + o T ] T1 < T < T2R(T) = approximation of resistance at temperature T R(T0) = resistance at temperature T0 o = fractional change in resistance per degree of temperature at T0 T = T - T0

Resistance vs Temperature Linear ApproximationsStraight line equation

1 R2 R1 o = ( ) R (T0 ) T2 T1R2 R1 = resistance at T2 = resistance at T1

Example

Example

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

RTD quadratic approximationR (T ) = R (To )[1 + 1T + 2 (T ) ] T1 < T < T22

R(T) = quadratic approximation of resistance at temperature T R(T0) = resistance at temperature T0 1 = linear fractional change in resistance with temperature 2 = quadratic fractional change in resistance with temperature T = T - T0

Example

ExampleSolution

ExampleSolution

Nickel

Tungsten Copper

Platinum

Platinum: very repeatable, sensitive, expensive Nickel: not quite repeatable, more sensitive, less expensive

RTD - sensitivity Sensitivity is shown by the value o Platinum 0.004/ C Nickel 0.005/ C

Thus, for a 100 platinum RTD, a change of only 0.4 would be expected if the temperature is changed by 1C

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.

Construction of a platinum resistance thermometer


Wire is in a coil to achieve small size and improve thermal conductivity to decrease response time.


Protect from the environment

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 Symbol semiconductor resistance

Thermistor: resistance vs temperature

Thermistor

Scan example 6.3 module page 109



Resistive position transducersDistance

Electrical signal

Resistive position transducers

Resistive position transducers

Resistive position transducersR1 R2

R2 Vo = VT R1 + R2



Displacement transducers Capacitive transducer Inductive transducer Variable inductance transducer

Capacitive transducers The capacitance of a parallel-plate capacitor is given by

= dielectric constant o = 8.854 x 1o-12 , in farad per meter A = the area of the plate, in square meter d = the plate spacing in meters

oA C= d

Capacitive transducers physical design

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

Inductive transducers tachometer with a permanent magnet stator

Inductive transducer tachometer with a permanent magnet rotor

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.

Variable Inductance Transducers construction

Variable Inductance Transducers schematic diagram

Variable Inductance Transducers operationWhen 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.

Variable Inductance Transducers operationCore at the center V1 = V2 Vo = 0

Variable Inductance Transducers operationCore moves towards S1 V1 > V2 Vo increase

Variable Inductance Transducers operationCore moves towards S2 V2 > V1 Vo decrease

Variable Inductance Transducers with absolute magnitude



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.

StressIn general, stress is expressed as

is the average stress, also called engineering or nominal stress and is the force acting over the area .

StrainStrain 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.

Strain The strain is defined as the fractional change in length

l strain = l Strain is thus a unitless quantity

Strain The strain is defined as the fractional change in length

l strain = l Strain is thus a unitless quantity

Stress-strain curve

Strain gaugeFrom the equation of resistance,

L R= A

R = resistance = specific resistance of the conductor material L = the length of the conductor in meters A = the area of the conductor in square meters

Strain gaugeTo measure pressure

When a strain produced by a force is applied on the wires, L increase and A decrease.

Strain gaugeL increase A decrease From the equation of resistance, R increase

L R= A

Strain gauge the gauge factor

R / R K= L / LK = the gauge factor R = the initial resistance in ohms (without strain) R = the change of initial resistance in ohms L = the initial length in meters (without strain)


R / R K= L / LK = the gauge factor R = the initial resistance in ohms (without strain) R = the change of initial resistance in ohms L = the initial length in meters (without strain)


R / R K= G