A linear potentiometer transducer consists of a potentiometer, which is short circuited by a slider. The other end of the slider is connected to a slider arm. The force summing device on the slider arm causes linear displacement of the slider causing the short circuit of a certain portion of the resistance in the potentiometer. Let the whole resistance positions on the potentiometer be ABC. Let the resistance position caused by the slider movement be BC. As the movement of the slider moves further to the right, the amount of resistance increases. This increase in resistance value can be noted according to the corresponding change in the linear displacement of the slider. The change in resistance can be calculated with the help of a Wheatstone bridge.
Another easy method than calculating the resistance with the help of a bridge connection is to connect a constant current source in series with the potentiometer. Thus a voltage will be developed. This voltage can be measured and hence the resistance, R = V/I.
Some of the most commonly used potentiometers for this purpose and their basic working is explained below.
1. Wire-Wound Potentiometer – The most commonly used resistance elements in this potentiometer are nickel, chromium or nickel copper. As these materials have a very low temperature coefficient of resistance, they can be used to handle large currents and also can be used up to 5 Hertz. They are also very cost effective. The winding of the resistance wire will depend on the different types of resistance changes due to the slider motion like linear, arithmetic, logarithmic and so on.
2. Cermet Potentiometer – This potentiometer is made from a material called Cermet which is made by mixing a paste of precious metal particles and a ceramic. Some of the most common mixtures used are palladium silver glass and palladium oxide glass. This device is used mostly for ac purposes as it has a low temperature coefficient of resistance and huge power ratings at high temperatures. Out of the lot, this device is mostly used as it is cost-effective.
3. Hot-Moulded Carbon Potentiometers – As the name implies, it is made by depositing a thin film of carbon and a thermosetting plastic binder. This device is mostly used for alternating current (ac) purposes.
4. Carbon Film Potentiometers – This potentiometer is made by coating a thin layer of carbon film on a non-conductive base. The temperature coefficient of resistance of this device is 1000 x 10-6 ohms/degree Celsius.
5. Thin Metal Film Potentiometer – This device is in the form of a thin vapour deposited layer of metal on glass or ceramic base. This is also used for ac applications.
- Simple design and simple working
- Can be used for measuring even large displacements.
- The device produces a large output and hence can be used for control purposes without further amplification steps. Thus the whole operation is bounded to a single device.
- Can produce a high electrical efficiency.
- All devices other than wire-wound potentiometer can be used for a large frequency range.
- Except wire wound, all other potentiometers can provide excellent resolutions.
- A huge force may be required for the slider movement.
- Can produce unwanted noise due to alignment problems, wear and tear of the sliding contact. This may also affect the total life of the device.