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.
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3. Transducers (Briefly)
Transducer Anything
Any measureable (conversion)
quantity in out
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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4. Transducers
Definition: 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
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5. Types and applications
Some 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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6. Quantities and units
Parameter Unit – NB : Shaded boxes indicate a base SI unit.
Name Symbol
Mass Kilogram kg
Length / Displacement Metre m
Velocity Metres per second m/s
Temperature (SI) Kelvin K
Temperature (Alt) Celsius C
Acceleration Metres per second squared m/s2 (m/s/s)
Light Intensity Candela cd
Illuminance Lux lx
Force Newtons N
Pressure Pascal (Newton per square Pa
metre)
Sound level (relative) Decibel dB
Magnetic Flux Density Tesla (Webers per metre T (Wb/m2)
squared)
Time Second s
Current Ampere A
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7. 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.
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8. 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.
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9. Hysteresis
A transducer should
produce the same output
Ideal –
whether the value has Negligible
been reached due to a Hysteresis Hysteresis
continually increasing
input or a continually
Output
decreasing input.
Input
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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
% 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.
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11. Precision (Reproducibility, Repeatability)
The ability of the transducer to produce the same
output each time the same input is applied.
Poor Accuracy Poor Accuracy Good Accuracy
Poor Precision Good Precision Good Precision
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12. Sensitivity
Sensitivity is the ability of the transducer to generate
an output for a given change in input.
Change in output
Sensitivity =
Change in input
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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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
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14. Span
•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
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15. Linearity
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.
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19. Thermocouples
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.
20. 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
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21. 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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22. 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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24. Wheatstone bridge
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.
+
R1 R3
mV
R2 R4
_
Transducers (e.g.
Thermistors or RTDs)
can replace the resistors.
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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.
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26. 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.
27. 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.
29. Light dependant resistors LDRS
• The LDR is a
semiconductor
device.
• Its resistance is
dependant on the
light intensity that
falls on the device.
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30. 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)
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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.
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32. 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.
33. 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
34. 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.
35. Optocouplers
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
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
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38. 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
39. 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.
40. 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
41. • 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.
42. Microphone
Voice Coil Diaphragm
Dust Cover
Permanent Magnet S
Sound in
N
Signal to S
amplifier
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43. 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.
44. 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.
45. Hall Effect Transducers
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.
47. Hall effect devices
Hall 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
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48. 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.
49. 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
50. 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.
51. 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.
52. 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
