2. Transducer
A device which converts one form of energy to another.
Typical energy domains are mechanical, electrical, chemical, magnetic,
optical and thermal.
When input is a physical quantity and output electrical → Sensor
When input is electrical and output a physical quantity → Actuator
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Sensors
Physical
parameter
4. TEMPERATURE SENSORS
Temperature is a scalar quantity that determines the direction of
heat flow between two bodies.
Sensing methods: contact and non-contact
Contact Sensor: is in direct physical contact with the object to
be sensed to monitor solids, liquids, gases over wide range
Non-contact : Interprets the radiant energy of a heat source to
energy in electromagnetic spectrum
Monitor non-reflective solids and liquids
Temperature sensors generate output signals in one of two ways:
1. through a change in output voltage
2. through a change in resistance of the sensor‘s electrical circuit
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5. TEMPERATURE SENSORS CLASSIFCATION:
Contact Sensing
Thermocouple (Thermoelectric)
Thermistor (Thermal Resistors)
- Negative temperature coefficient device (NTC)
- Positive temperature coefficient device (PTC)
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6. THERMOCOUPLES
Most common temperature sensing device.
Accurate temperature measurements can be made with thermocouples
sensors at low cost with shop-built probes and ordinary low-level
voltmeters.
Thermocouples can measure temperature at a point in a range of -250C to
+3500C.
Typical Industrial Thermocouple General Thermocouple
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7. Thermocouple Concept
• Principle of operation is based on the Seebeck effect, discovered
by Thomas Seebeck in 1822,
- Electrons flow from one wire to other, due to different energy
potentials of alloys
- As temperature changes, current flows
- Voltage is measured between the two alloys (Small voltage, less
than 10 mV)
• Seebeck effect: when any conductor subjected to thermal
gradient, generates a voltage.
• A temperature gradient along a conductor creates an EMF.
8. Thermocouple Concept……
• If two conductors of different materials are joined at one
point, an EMF is created between the open ends which is
dependent upon the temperature of the junction.
• As T1 increases, so does Voltage (V).
• EMF also depends on the temperature of the open ends T2.
• The junction is placed in the process, the other end is in
iced water at 0C. This is called the reference junction.
9. Thermistors: THERMAL RESISTORS
• Thermistor is a combination of the words thermal and resistor.
• Thermistor was invented by Samuel Ruben in 1930
• A thermistor is a type of resistor used to measure temperature
changes, relying on the change in its resistance with changing
temperature.
– Typically have a negative temperature coefficient (NTC),
– Resistance decreases with increasing temperature
• Thermistor can measures across the range of -40~150 ±0.35 °C
Leads, coated Glass encased Surface mount
10. Relationship b/w resistance & temperature
• Assume a simple linear relationship between resistance
and temperature for the following discussion:
ΔR = k ΔT
Where
• ΔR = change in resistance
• ΔT = change in temperature
• k = first-order temperature coefficient of resistance
15. Modulation
• “Chop” LED on and off at many kHz rate
• Bandpass filter after photodiode at the same
frequency as chopping
• Threshold circuit after BPF generates on/off
output
16. Magnetic Position Sensors
• Reed switches (sense permanent magnet)
• Inductive proximity sensors (eddy current)
• Hall Sensors (sense permanent magnet)
18. Solar cells
Solar cells converts light energy into electrical energy. They’re mainly used
for generating solar energy and are made of single-crystal silicon PN
junctions, similar photodiodes but with a broader response curve.
• Unlike photodiodes connected in a reverse-bias configuration, solar cells
are connected in a forward-bias configuration much like typical diodes.
These cells are designed to be sensitive to sunlight instead of a narrow
range of the electromagnetic spectrum. When exposed to solar
radiation, a cell generates a potential difference of 0.58V.
• Typically, several solar cells are connected in series in a panel to output
a greater voltage. This DC voltage can drive a resistive load or be
converted to AC for transmission.
19. Capacitive Transducers
capacitive transducers can be
made to be self-contained units.
Some transducers work by
making one of the capacitor
plates movable, either in such a
way as to vary the overlapping
area or the distance between the
plates.
Other transducers work by
moving a dielectric material in
and out between two fixed
plates to detect and transmit the
physical position of mechanical
parts via electrical signals
)
( farads
d
kA
C o
where,
k = dielectric constant.
A = Area of the plate
o = 8.854x10-12, in farad per meter.
d= the plate spacing in meters.
20. Differential Capacitive Transducers
have 3 wire connections: one wire for each of the "end" plates
and one for the "common" plate.
implementation in a bridge circuit
21. Differential Capacitive Transducers
This bridge circuit is similar in function with strain gauges: it is
not intended to be in a "balanced" condition all the time, but
rather the degree of imbalance represents the magnitude of the
quantity being measured.
