TRANSDUCER

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.

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

TRANSDUCER Temperature transducers Resistive position 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, 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 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

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

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 Hot junCtion cold junCtion voltmeter Twisting or welding of 2 wires

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

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

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.

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

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.

Solution Sensitivity of the thermocouple = 20.68/(400-0) = 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

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.

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)

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.

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 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

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

Example

Example

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

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

Example

Example Solution

Example Solution

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 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

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

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

Construction of a platinum resistance thermometer 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 semiconductor resistance Symbol

Thermistor: resistance vs temperature

Thermistor

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

Resistive position transducers Distance TRANSDUCER Electrical signal

Resistive position transducers

Resistive position transducers

Resistive position transducers

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

Displacement transducers Capacitive transducer Inductive transducer Variable inductance transducer

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

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 – 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.

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

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

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

Variable Inductance Transducers – with absolute magnitude

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

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.

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 .

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.

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

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

Stress-strain curve

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

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

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

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 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

Strain gauge – the gauge factor