thermal sensors
DESCRIPTION
Thermal Sensors for RoboticsTRANSCRIPT
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ICE 550: Robotics and Automation
Class 11: Robotic Sensors Thermal Sensors
Dr. S. MeenatchisundaramEmail: [email protected]
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
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Thermo-resistive sensors:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
These work on the principle that the electricalresistance of a conductor change with temperature.
If a constant voltage is applied to the conductor thenthe current flowing through it will change withtemperature.
The resistivity of the conductor change withtemperature. This usually means the resistance getsbigger as the conductor gets hotter.
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Thermo-resistive sensors
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
Two basic types:
Resistive Temperature Detector (RTD)
Metal wire
Thin film
Silicon based
Thermistors (Thermal Resistor)
NTC (Negative Temperature Coefficient)
PTC (Positive Temperature Coefficient)
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Thermo-resistive sensors
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
Resistance of a length of wire
Conductivity is:
Resistance as a function of temperature:
- Temperature Coefficient of Resistance (TCR) [C1]
0 - Conductivity of the conductor at reference temperature T0T - temperature [C ]
T0 - Usually given at 20C but may be given at othertemperatures as necessary.
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Thermo-resistive sensors
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
The earlier equation can be rewritten which relates theresistance and temperature as
is the temperature coefficient of resistance. Ro is the resistanceat 0oC. Sometimes the equation is given as
A basic temperature sensor is made by winding a thin resistancewire into a small sensor head.
The resistance of the wire then represents the temperature.
This has an advantage over a thermocouple in that it isunaffected by the temperature of the gauge end. The main typeof wire used is PLATINUM.
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Thermo-resistive sensors
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
The sensors are usually manufactured to have aresistance of 100 at 0o C and the value of is0.00385 to 0.00390.
A typical operating range is 200 to 400oC.
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Thermistors:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
Thermistor: Thermal resistor
Became available: early 1960s
Based on oxides of semiconductors
High temperature coefficients
NTC
High resistances (typically)
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Thermistors:
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Thermistors:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
Transfer function:
[] and [K] are constants R(T): resistance of the device
T: temperature in K
Relation is nonlinear but: Only mildly nonlinear (b is small)
Approximate transfer function
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Thermocouples:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
When two wires with dissimilar electrical properties arejoined at both ends and one junction is made hot andthe other cold, a small electric current is producedproportional to the difference in the temperature.
Seebeck discovered this effect.
It is true no matter how the ends are joined so the coldend may be joined at a sensitive millivolt meter. Thehot junction forms the sensor end.
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Thermocouples:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
Peltier showed that heat is absorbed at the hot end andrejected at the cold end.
Thompson showed that part of the e.m.f. is due tothe temperature gradient in the wire as well as thetemperature difference between the junctions.
Most thermocouple metals produce a relationshipbetween the two temperatures and the e.m.f asfollows.
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Thermocouples:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
and are constants for the type of thermocouple.The relationship is nearly linear over the operatingrange.
The actual characteristic and suitable operatingtemperatures depends upon the metals used in thewires.
The various types are designated in international andnational standards. Typical linear operating ranges areshown for standard types.
It is important that thermocouples are standard sothat the same e.m.f will always represent the sametemperature.
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Thermocouples:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
Thermocouples come in several forms. They may be wiresinsulated from each other with plastic or glass fibre materials.
For high temperature work, the wire pairs are put inside a tubewith mineral insulation.
For industrial uses the sensor comes in a metal enclosure such asstainless steel.
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Thermal Sensor Characteristics:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
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Problems:
S.Meenatchisundaram, MIT, Manipal, Jan May 2015
1. A Platinum resistance thermometer has a resistance of 100 at0oC and the value of is 0.00385. In operation the resistance is101 . Calculate the temperature.
2. A thermocouple produces an e.m.f. in mV according to thetemperature difference between the sensor tip 1 and the gaugehead 2. = 3.5 x 10
-2 and = 8.2 x 10-6. The gauge head is at20oC. The mV output is 10 mV. Calculate the temperature atthe sensor.
3. A thermocouple produces an e.m.f. in mV according to thetemperature difference between the sensor tip 1 and the gaugehead 2. = 3.5 x 10
-2 and = 8.2 x 10-6. The gauge head is at20oC. Determine the mV output when the tip is at 220oC.