What are Variable Resistance Transducers?
The variable resistance transducers are one of the most commonly used types of transducers. The variable resistance transducers are also called as resistive transducers or resistive sensors. They can be used for measuring various physical quantities like temperature, pressure, displacement, force, vibrations etc. These transducers are usually used as the secondary transducers, where the output from the primary mechanical transducer acts as the input for the variable resistance transducer. The output obtained from it is calibrated against the input quantity and it directly gives the value of the input.
Principle of Working of Variable Resistance Transducer
The variable resistance transducer elements work on the principle that the resistance of the conductor is directly proportional to the length of the conductor and inversely proportional to the area of the conductor. Thus if L is the length of the conductor (in m) and A is its area (in m square), its resistance (in ohms) is given by:
R = ρL/A
Where ρ is called as resistivity of the material and it is constant for the materials and is measured in ohm-m
The resistance of some materials also changes with the change in their temperature. This principle is primarily used for the measurement of temperature.
Some Examples of Variable Resistance Transducers
Here some of the popular variable resistance transducers that are being used for various applications:
1) Sliding contact devices: In the sliding contact type of variable resistance transducers there is a long conductor whose effective length is variable. One end of the conductor is fixed, while the position of the other end is decided by the slider or the brush that can move along the whole length of the conductor. The slider is connected to the body whose displacement is to be measured. When the body moves the slider also moves along the conductor so its effective length changes, due to which it resistance also changes. The effective resistance is measured as the resistance between the fixed position of the conductor and the position of the sliding contact. The value of the resistance is calibrated against the input quantity, whose value can be measured directly. One of most popular sliding contact type of variable resistance transducer is the potentiometer. These devices can be used to measured linear as well as angular displacement.
2) Wire resistance strain gauge: This is an interesting devise used for the measurement of force, stress and strain. When the tension is applied to the electrical conductor, its length increases while the cross section area decreases, due to which the resistance of the conductor changes. This change in resistance can be measured easily and is calibrated against the input quantity.
3) Thermistors: Thermistors work on the principle that resistance of some materials changes with the change in their temperature. When the temperature of the material changes, its resistance changes and it can be measured easily and calibrated against the input quantity. The commonly used thermistors are made up of the ceramic like semiconducting materials such as oxides of manganese, nickel and cobalt. Thermistors can be used for the measurement of temperature, as electric power sensing devices and also as the controls for various processes.
4) Thermocouple: The thermocouples work on the principle of Seeback effect, Peltier effect and Thomson effect. As per the Seeback effect, when two dissimilar elements are joined at their ends the electromotive force exists at their junction. As per Peltier effect, the amount of electromotive force generated depends on the temperature of the junction, while Thomson effect says that the amount of voltage generated depends on the temperature gradient along the conductors in the circuit. All these effects are used in the thermocouple. The voltage output from the thermocouple changes as its temperature changes or the temperature of the body in whose contact it is changes. The voltage output is calibrated against the temperature of the body that can be measured easily. Thermocouple is a very popular devise used for measurement of temperature.
Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck