Transducers

Transducers• Definition: Technically…• A device that converts one energy form to another (eg, mechanical to electrical).• Any device or component that converts an input signal of one form to an output signal of another form• An element or device which receives information in the form of one quantity and converts it to information in the same or an other quantity or form.• A device for translating the magnitude of one quantity into another quantity. Revision 01 2

Transducers (Briefly) Transducer Anything Any measureable (conversion) quantity in outeg. any measurable quantity:• energy: sound, electrical, mechanical, light, chemical,• pressure, level, density, temp, pH, flow, temperature• position, distance, mass, time• etc, etc. This allows for a VERY broad interpretation... Revision 01 3

TransducersDefinition: Practical and realistic…• A sensor that converts one energy form to another (eg. mechanical to electrical). eg. Things that AREN’T generally referred • Microphone to as transducers: • Thermocouples • Valves • Thermistors • Motors • Tacho-generators • Solenoids • a diode can be used • Alarms to measure temperature. • Contactor • pH probe • Heater • Ultrasonic level detector • Power transformer • etc, etc. • Hydraulic cylinder Revision 01 4

Types and applicationsSome common transducers and common uses• Thermistor/thermocouple temperature eg;motors• LDRs/LEDs flame or smoke• Opto-coupler data transfer• Speaker/microphone acoustic/sound• Magnetic pickup stylus/vibration• Strain guage tension• Hall effect magnetism• Peltier effect device temperature• Piezzo stress/pressure Revision 01 5

Quantities and units Parameter Unit – NB : Shaded boxes indicate a base SI unit. Name SymbolMass Kilogram kgLength / Displacement Metre mVelocity Metres per second m/sTemperature (SI) Kelvin KTemperature (Alt) Celsius CAcceleration Metres per second squared m/s2 (m/s/s)Light Intensity Candela cdIlluminance Lux lxForce Newtons NPressure Pascal (Newton per square Pa metre)Sound level (relative) Decibel dBMagnetic Flux Density Tesla (Webers per metre T (Wb/m2) squared)Time Second sCurrent Ampere A Revision 01 6

Classification of transducers• There are many ways to classify transducers: – By what they are measuring • General classification. • Specific classification. – By the output signal type. – By whether or not they produce their own supply. (Active or Passive) – Input to output. – Contact type or not – Direct or indirect. – Method used to sense input. Revision 01 7

Transducer parameters• Transducer operating characteristics are usually defined by a number of parameters.• Some of the main parameters to be considered are: – Sensitivity – Range – Span – Linearity – Hysteresis – Accuracy – Precision (Reproducibility, Repeatability) – And others. Revision 01 8

HysteresisA transducer shouldproduce the same output Ideal –whether the value has Negligiblebeen reached due to a Hysteresis Hysteresiscontinually increasinginput or a continually Outputdecreasing input. Input Revision 01 9

Accuracy Accuracy can be expressed as a comparison of the static error of the transducer compared to the actual value (at full scale) expressed as a percentage of full scale. (Accuracy may also be expressed in other ways.) (Measured value – Actual value) x 100% Accuracy = Actual value E.g. A temperature transducer that reads 102 C at full scale, when the temperature is 100 C, has an accuracy equal to 2% of full scale. Revision 01 10

Precision (Reproducibility, Repeatability)The ability of the transducer to produce the sameoutput each time the same input is applied. Poor Accuracy Poor Accuracy Good Accuracy Poor Precision Good Precision Good Precision Revision 01 11

SensitivitySensitivity is the ability of the transducer to generatean output for a given change in input. Change in outputSensitivity = Change in inputE.g. A thermocouple that increases outputvoltage by 3mV per degree Celsius temperaturechange has a sensitivity of 3mV/ C Revision 01 12

RangeThe highest and lowest values that thetransducer is designed to measure.E.g. A Temperature transducer may have a rangeof –50 C to +50 C Revision 01 13

Span•The difference between the upper and lowervalues the transducer is designed to measure.•E.g. A Temperature transducer that has a rangeof –50 C to +50 C has a span of 100 C Revision 01 14

LinearityLinearity refers to the change in outputcompared to the change in input. If the changein output is proportional to the change ininput, the transducer is said to be linear. Revision 01 15

Units we need to know. Revision 01 16

Revision 01 17

Measuring temperature Thermocouple Thermistor Revision 01 18

ThermocouplesAs the junction temperature increases a small voltage is created in the loop.The voltage produced at the junction of the dissimilar metals is due to aphenomenon called the “Seebeck Effect”.• The higher the temperature at the junction, the greater the voltageproduced by that junction.• The relationship between voltage and temperature is constant andtherefore will graph as a linear line.

Thermistors• Thermistors are made from semi-conductor materials.• Semi-conductor thermistors Resistance have a Negative Temperature Coefficient (NTC). i.e. as temperature increases, the resistance decreases. Temperature Revision 01 20

Thermistor construction• Thermistors come in a variety of sizes and shapes.• Beads, disks, rods and probes are some of the more common styles. Revision 01 21

Thermistors (Cont) Like RTDs, thermistors are often enclosed in a housing suitable for either contact or non- contact applications in industry. Revision 01 22

Transducers (Briefly) Bridge circuits +10V Unknown R1 = 250Ω (initially 250Ω)Use: Weighers Conveyors (Tonnes/Hr) Pressure Vout RTD Temperature measurement R1 = 250Ω R1 = 250Ω 0V Revision 01 23

Wheatstone bridgeA circuit invented by Sir Charles Wheatstone inthe mid-1800s. It is essentially two matchedvoltage dividers with a galvanometer across thenetwork to sense any difference in potential. + R1 R3 mV R2 R4 _ Transducers (e.g. Thermistors or RTDs) can replace the resistors. Revision 01 24

Optical devices• Many measurement and control systems utilise light and light-intensity as a way of detecting other physical properties.• Using direct or reflected light can provide an ideal non-contact sensing mechanism. Revision 01 25

Photoelectric TransducersPhotoelectric transducers are devices that produce anelectrical variation in response to a change in lightintensity, or produce a light intensity variation due to achange in applied electrical energy. Photoelectrictransducers operate in three classifications, they are: • Photoconductive, • Photovoltaic, • Photoemissive.

PhotoconductiveThe photoconductive device is a semiconductor cell whichproduces a change in it’s resistance in response to a change inlight intensity.The three most common photoconductive transducers are the • Light Dependant Resistor (LDR), • Phototransistor • Photodiode.

Light Dependant Resistor

Light dependant resistors LDRS • The LDR is a semiconductor device. • Its resistance is dependant on the light intensity that falls on the device. Revision 01 29

Light dependant resistors• As the light intensity increases, the resistance of the LDR decreases. Resistance• The LDR is a non-linear device with resistance ranging from about 10 MΩ in complete darkness to 100Ω in full sunlight. Light Intensity (cd) Revision 01 30

Phototransistor• The phototransistor is a three-layer semiconductor device with a light-sensitive collector-base p-n junction.• The current flowing through the collector emitter circuit will be controlled by the amount of light falling on the collector-base junction. Revision 01 31

As light intensity increases, the base-collector junction resistance of the phototransistor decreases. This decrease inresistance increases the base current that in turn increases the flow of collector current. The relationship between light intensity and current flow is generally constant and therefore will graph as a linear line. These linear transfer characteristics are shown below.

Solar cell• As the light (protons) intensity increases, an imbalance of electrons and holes are created, which gives an increase to the open circuit potential voltage difference and therefore a current flow within a circuit. The relationship between light intensity and open circuit voltage is not constant and therefore will not graph as a linear line

Light Emitting Diode • This LED is a semi conductive P-N junction enclosed in a coloured case to enhance the colour of the light output. Silicon is not used as it produces mainly heat rather than light. • The semi conductive materials used in the manufacture of LED’s determines the colour of the emitted light. By using different materials, such colours as red, yellow, green, and even invisible light spectrums such as infra-red can be produced.

OptocouplersOptocouplers belong to a family of devices used to electricallyisolate circuits.This isolation may be required to protect circuits from surgevoltages and to filter certain noise.Photoelectric transducers are effective in producing highquality fast responding Optocouplers which can be used inmany varying applications.The basic Optocoupler consists of a photo emissivedevice, LED, and a photoconductivedevice, phototransistor, contained in a single package

Opto-coupler deviceswww.qsl.netIsolation circuits Revision 01 36

Piezo devicesThe principle of piezoelectric action has been known for quite sometime. Materials such as quartz and man made products such asBarium Titanate and Lead Zirconate demonstrate a characteristic inthat when pressure is applied over one axis, there tends to be apolarization of electric charge over the adjacent axis. This isdemonstrated below Revision 01 37

Piezo DevicesWhether they are Piezoelectric or Piezoceramic devices, theapplication is very wide, almost wherever we wish to measurepressure you will find these devices being used. Although notexhaustive, some examples include; • Pressure switches • Piezoelectric pressure gauges • Djfferential pressure measuring transducers, and • Sonar transducers • Vibration detectors etc • Ignition devices

Resistive Strain Gauge A Resistive strain gauge is a device that converts a change in applied force into a change in produced resistance. A strain gauge consists of a length of resistive wire that is bonded to the surface of an object that receives an applied force.

Acoustic TransducersAcoustic transducers are devices that convert a variation inelectrical energy into a change in mechanical energy, (physicalvibrations or oscillations, ie. sound waves). Orconversely, convert a variation in sound wave energy intoelectrical energy.Common examples of acoustic transducers are the:• Acoustic speakers,• Acoustic microphone,• Piezoceramic transducers, and• Magnetostrictive transducers

• The magnetic field produced in the voice coil, when current is applied, is at right angles to the magnetic field produced by the permanent magnet.• Therefore the two fields attract or repel each other depending on the polarity of the signal current. This attraction and repulsion causes an inward or outward movement of the voice coil and cone which results in sound waves being produced.• The volume and frequency of the sound produced is dependant upon the amplitude and frequency of the input signal current.

Microphone Voice Coil Diaphragm Dust CoverPermanent Magnet S Sound in N Signal to S amplifier Revision 01 42

Piezoelectric Buzzer• Piezo electric buzzers and speakers are used in a wide variety of applications from simple low fidelity applications such as a warning buzzer to high fidelity, high frequency audio speaker applications. Regardless of the application, the principle of piezoelectric operation remains a constant.

Displacement Position And Proximity TransducersFloat transducers are used in tank level monitoring applications. These devices use a sender that is either a switch or some form of resistive device. A combination of these devices can be seen in an automotive application where the switch is used to indicate tank low level and the potentiometer sender provides a proportional indication of actual tank level.

Hall Effect TransducersThe Hall Effect describes a condition if current flow in a conductor being affectedby the presence of a magnetic field If an electric current flows through aconductor in a magnetic field, the magnetic field exerts a transverse force on themoving charge carriers which tends to push them to one side of the conductor.This is most evident in a thin flat conductor. A build up of charge at the sides ofthe conductors will balance this magnetic influence, producing a measurablevoltage between the two sides of the conductor. The presence of thismeasurable transverse voltage is called the Hall effect after E. H. Hall whodiscovered it in 1879.

Hall effectMagnetic reed Revision 01 46

Hall effect devicesHall effect devices can be used to:• Measure the velocity of charged particles in a magnetic field (flow meter)• Measure the proximity of magnetic materials (Linear displacement)• Detect pulses of magnetism e.g. as in a tachometer Revision 01 47

Capacitive Transducers• Capacitive transducers use a changing capacitive reactance within the transducer to produce a proportional output. The typical capacitive transducer. is used as a proximity device with one electrode charged and the other affected as it approaches in close proximity. The surrounding air is used as a dielectric to produce a reactance that is proportional to the distance between the to electrodes of the capacitor.

Reed Switches• The reed switch is an encapsulated inductive influenced switch that can be activated by the presence of a magnetic source. These devices are common in float sensor Tank Level Indicators which can be found in the liquid Level State management system in a modem warship. The item at Figure 1below is a typical reed switch that may be found in a range of these types of equipment

Inductive Proximity Sensors• Inductive proximity sensors rely on the effect of a magnet approaching a high turns ratio coil that produces a voltage proportional to the relative distance of that magnetic source from that coil. Another variat ion is to have the inductive source coupled via the proximity of the magnetic field. The sensor generates a magnetic field and as the magnetic conductive material approaches the magnetic field, it provides a decreasingly reluctant path to magnetism. This effect is proportional to the distance of the object from the sensor and produces an increasing output, the closer the object gets to the sensor.

Position and displacement measurement Potentiometers• Measurement of displacement with a potentiometer relies on the fact that the resistance between the sliding contact and the reference end of the resistance element is proportional to the distance between the two points.

Linear Variable Differential Transformer (LVDT)• Using AC instead of DC, we are able to avoid sliding contact between parts if we use a variable transformer instead of a potentiometer. Devices made for this purpose are called LVDT’s, which stands for Linear Variable Differential Transformer. The design of an LVDT looks similar to the layout in the diagram at Figure below

Tachogenerator Shaft mounted tachoPermeant magnettacho- generator Revision 01 53

Tachogenerator Revision 01 54

Transducers

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