PERFORMANCE CONSIDERATIONS OF ROTARY POTENTIOMETERS
The potentiometer is a resistively coupled transducer. The force required to move the slider arm comes from the motion source, and the resulting energy is dissipated through friction.
This energy conversion, unlike pure mechanical-to-electrical conversions, involves relatively high forces, and the energy is wasted rather than being converted into the output signal of the transducer. Furthermore, the electrical energy from the reference source is also dissipated through the resistor element (coil or film), resulting in an undesirable temperature rise and coil degradation. These are two obvious disadvantages of a potentiometer.
In coil-type pots there is another disadvantage, which is the finite resolution.
A coil, instead of a straight wire, is used to increase the resistance per unit travel of the slider arm. But the slider contact jumps from one turn to the next in this case.
Accordingly, the resolution of a coil-type potentiometer is determined by the number of turns in the coil. For a coil that has N turns, the resolution r, expressed as a percentage of the output range, is given by
Resolutions better (smaller) than 0.1% (i.e., 1000 turns) are available with coil potentiometers. Virtually infinitesimal (incorrectly termed infinite) resolutions are now possible with high-quality resistive film potentiometers, which use conductive plastics. In this case, the resolution is limited by other factors, such as mechanical limitations and signal-to-noise ratio. Nevertheless, resolutions on the order of 0.01 mm are possible with good rectilinear potentiometers.
Selection of a potentiometer involves many considerations.
A primary factor is the required resolution for the specific application. Power consumption, loading, and size are also important factors. The following design example highlights some of these considerations.
The sensitivity of a potentiometer represents the change (Duo) in the output signal associated with a given small change (Dq) in the measurand (the displacement).
This is usually nondimensionalize, using the actual value of the output signal (uo) and the actual value of the displacement (q). For a rotatory potentiometer in particular, the sensitivity S is given by
These relations may be nondimensionalize by multiplying by q/uo. An expression for S may be obtained by simply substituting
Some limitations and disadvantages of the potentiometer as a displacement measuring device are given below:
1. The force needed to move the slider (against friction and arm inertia) is provided by the displacement source. This mechanical loading distorts the measured signal itself.
2. High-frequency (or highly transient) measurements are not feasible because of such f actors as slider bounce, friction and inertia resistance, and induced voltages in the wiper arm and primary coil.
3. Variations in the supply voltage cause error.
4. Electrical loading error can be significant when the load resistance is low.
5. Resolution is limited by the number of turns in the coil and by the coil uniformity. This will limit smalldisplacement measurements.
6. Wear out and heating up (with associated oxidation) in the coil or film, and slider contact cause accelerated degradation.
There are several advantages associated with potentiometer devices, however, including the following:
1. They are relatively inexpensive.
2. Potentiometers provide high-voltage (low-impedance) output signals, requiring no amplification in most applications. Transducer impedance can be varied simply by changing the coil resistance and supply voltage.