Thermal Sensors

ICE 550: Robotics and Automation

Class 11: Robotic Sensors Thermal Sensors

Dr. S. MeenatchisundaramEmail: [email protected]

S.Meenatchisundaram, MIT, Manipal, Jan May 2015

Thermo-resistive sensors:


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.

Thermo-resistive sensors


Two basic types:

Resistive Temperature Detector (RTD)

Metal wire

Thin film

Silicon based

Thermistors (Thermal Resistor)

NTC (Negative Temperature Coefficient)

PTC (Positive Temperature Coefficient)



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.



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.



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.

Thermistors:


Thermistor: Thermal resistor

Became available: early 1960s

Based on oxides of semiconductors

High temperature coefficients

NTC

High resistances (typically)

Thermistors:

Thermistors:


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

Thermocouples:


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.

Thermocouples:


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.

Thermocouples:


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.

Thermocouples:


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.

Thermal Sensor Characteristics:


Problems:


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.

Thermal Sensors

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