Transducers

Transducers

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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.

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Transducers (Briefly)

Transducer(conversion)Any measureable

quantity in

Anythingout

eg. any measurable quantity:• energy: sound, electrical, mechanical,

light, chemical, • pressure, level, density, temp, pH, flow,

temperature• position, distance, mass, time• etc, etc.

eg. 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...

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Transducers

Definition: Practical and realistic…• A sensor that converts one energy form to

another (eg. mechanical to electrical). Things that AREN’T generally referred to as transducers:• Valves• Motors• Solenoids• Alarms• Contactor• Heater• Power transformer• Hydraulic cylinder

eg. • Microphone• Thermocouples• Thermistors• Tacho-generators • a diode can be used

to measure temperature.• pH probe• Ultrasonic level detector• etc, etc.

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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

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Quantities and units

AAmpereCurrent

sSecondTime

T (Wb/m2)Tesla (Webers per metre squared)

Magnetic Flux Density

dBDecibelSound level (relative)

PaPascal (Newton per square metre)

Pressure

NNewtonsForce

lxLuxIlluminance

cdCandelaLight Intensity

m/s2 (m/s/s)Metres per second squaredAcceleration

CCelsiusTemperature (Alt)

KKelvinTemperature (SI)

m/sMetres per secondVelocity

mMetreLength / Displacement

kgKilogramMass

SymbolName

Unit – NB : Shaded boxes indicate a base SI unit.Parameter

AAmpereCurrent

sSecondTime

T (Wb/m2)Tesla (Webers per metre squared)

Magnetic Flux Density

dBDecibelSound level (relative)

PaPascal (Newton per square metre)

Pressure

NNewtonsForce

lxLuxIlluminance

cdCandelaLight Intensity

m/s2 (m/s/s)Metres per second squaredAcceleration

CCelsiusTemperature (Alt)

KKelvinTemperature (SI)

m/sMetres per secondVelocity

mMetreLength / Displacement

kgKilogramMass

SymbolName

Unit – NB : Shaded boxes indicate a base SI unit.Parameter

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• 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.

Classification of transducers

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• 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.

Transducer parameters

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Hysteresis

A transducer should produce the same output whether the value has been reached due to a continually increasing input or a continually decreasing input.

Ou

tpu

t

Input

Hysteresis

Ideal –Negligible Hysteresis

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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

Actual value% Accuracy =

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.

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Precision (Reproducibility, Repeatability)

Poor AccuracyPoor Precision

Poor AccuracyGood Precision

Good AccuracyGood Precision

The ability of the transducer to produce the same output each time the same input is applied.

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Sensitivity

Sensitivity is the ability of the transducer to generate an output for a given change in input.

Change in output

Change in inputSensitivity =

E.g. A thermocouple that increases output voltage by 3mV per degree Celsius temperature change has a sensitivity of 3mV/ C

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Range

The highest and lowest values that the transducer is designed to measure.

E.g. A Temperature transducer may have a range of –50 C to +50 C

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•The difference between the upper and lower values the transducer is designed to measure.

•E.g. A Temperature transducer that has a range of –50 C to +50 C has a span of 100 C

Span

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Linearity refers to the change in output compared to the change in input. If the change in output is proportional to the change in input, the transducer is said to be linear.

Linearity

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Units we need to know.

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Measuring temperature

Thermocouple

Thermistor

As 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 a phenomenon called the “Seebeck Effect”.

• The higher the temperature at the junction, the greater the voltage produced by that junction.

• The relationship between voltage and temperature is constant and therefore will graph as a linear line.

Thermocouples

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Thermistors

• Thermistors are made from semi-conductor materials.

• Semi-conductor thermistors have a Negative Temperature Coefficient (NTC). i.e. as temperature increases, the resistance decreases.

Re

sist

an

ceTemperature

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Thermistor construction

• Thermistors come in a variety of sizes and shapes.

• Beads, disks, rods and probes are some of the more common styles.

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Thermistors (Cont)

Like RTDs, thermistors are often enclosed in a housing suitable for either contact or non-contact applications in industry.

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Transducers (Briefly)

R1 = 250ΩUnknown(initially 250Ω)

R1 = 250ΩR1 = 250Ω

Vout

0V

+10VBridge circuits

Use:WeighersConveyors (Tonnes/Hr)PressureRTD Temperature measurement

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Wheatstone bridge

mV

+

_

Transducers (e.g. Thermistors or RTDs) can replace the resistors.

R1

R2

R3

R4

A circuit invented by Sir Charles Wheatstone in the mid-1800s. It is essentially two matched voltage dividers with a galvanometer across the network to sense any difference in potential.

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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.

Photoelectric Transducers

Photoelectric transducers are devices that produce an electrical variation in response to a change in light intensity, or produce a light intensity variation due to a change in applied electrical energy. Photoelectric transducers operate in three classifications, they are:

• Photoconductive, • Photovoltaic,

• Photoemissive.

Photoconductive

The photoconductive device is a semiconductor cell which produces a change in it’s resistance in response to a change in light intensity.

The three most common photoconductive transducers are the

• Light Dependant Resistor (LDR),

• Phototransistor

• Photodiode.

Light Dependant Resistor

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• The LDR is a semiconductor device.

• Its resistance is dependant on the light intensity that falls on the device.

Light dependant resistors LDRS

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Light dependant resistors

• As the light intensity increases, the resistance of the LDR decreases.

• The LDR is a non-linear device with resistance ranging from about 10 MΩ in complete darkness to 100Ω in full sunlight.

Re

sist

an

ceLight Intensity (cd)

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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.

As light intensity increases, the base-collector junction resistance of the phototransistor decreases. This decrease in

resistance 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.

Optocouplers belong to a family of devices used to electrically isolate circuits.

This isolation may be required to protect circuits from surge voltages and to filter certain noise.

Photoelectric transducers are effective in producing high quality fast responding Optocouplers which can be used in many varying applications.

The basic Optocoupler consists of a photo emissive device, LED, and a photoconductive device, phototransistor, contained in a single package

Optocouplers

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Opto-coupler devices

www.qsl.net

I solation circuits

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Piezo devices

The principle of piezoelectric action has been known for quite some time. Materials such as quartz and man made products such as Barium Titanate and Lead Zirconate demonstrate a characteristic in that when pressure is applied over one axis, there tends to be a polarization of electric charge over the adjacent axis. This is demonstrated below

Piezo Devices

Whether they are Piezoelectric or Piezoceramic devices, the application is very wide, almost wherever we wish to measure pressure you will find these devices being used. Although not exhaustive, 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 Transducers Acoustic transducers are devices that convert a variation in electrical energy into a change in mechanical energy, (physical vibrations or oscillations, ie. sound waves). Or conversely, convert a variation in sound wave energy into electrical 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.

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S

S

N

Sou

nd in

Dust Cover

DiaphragmVoice Coil

Permanent Magnet

Signal toamplifier

Microphone

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 Transducers

Float 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.

The Hall Effect describes a condition if current flow in a conductor being affected by the presence of a magnetic field If an electric current flows through a conductor in a magnetic field, the magnetic field exerts a transverse force on the moving 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 of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall effect after E. H. Hall who discovered it in 1879.

Hall Effect Transducers

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Hall effectMagnetic reed

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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

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

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Tachogenerator

Permeant magnet tacho- generator

Shaft mounted tacho

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Tachogenerator