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Transducers
Revision 01 2
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 3
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...
Revision 01 4
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.
Revision 01 5
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 6
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
Revision 01 7
• 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
Revision 01 8
• 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
Revision 01 9
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
Revision 01 10
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.
Revision 01 11
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.
Revision 01 12
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
Revision 01 13
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
Revision 01 14
•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
Revision 01 15
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
Revision 01 16
Units we need to know.
Revision 01 17
Revision 01 18
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
Revision 01 20
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
Revision 01 21
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 22
Thermistors (Cont)
Like RTDs, thermistors are often enclosed in a housing suitable for either contact or non-contact applications in industry.
Revision 01 23
Transducers (Briefly)
R1 = 250ΩUnknown(initially 250Ω)
R1 = 250ΩR1 = 250Ω
Vout
0V
+10VBridge circuits
Use:WeighersConveyors (Tonnes/Hr)PressureRTD Temperature measurement
Revision 01 24
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.
Revision 01 25
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
Revision 01 29
• The LDR is a semiconductor device.
• Its resistance is dependant on the light intensity that falls on the device.
Light dependant resistors LDRS
Revision 01 30
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)
Revision 01 31
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
Revision 01 36
Opto-coupler devices
www.qsl.net
I solation circuits
Revision 01 37
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.
Revision 01 42
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
Revision 01 46
Hall effectMagnetic reed
Revision 01 47
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
Revision 01 53
Tachogenerator
Permeant magnet tacho- generator
Shaft mounted tacho
Revision 01 54
Tachogenerator