POTENTIOMETER POSITION INPUT

A potentiometer position signal conditioner is very simple in terms of circuit design. The conditioner provides a way to adjust the output for the desired level for a given rotation angle of the potentiometer.

Potentiometers of almost any value can be accommodated, but certain parameters are easier and less expensive to accomplish if the potentiometer resistance is kept in certain limits. These parameters are discussed below.

EXCITATION VOLTAGE

The potentiometer is an adjustable voltage divider that gives an output voltage proportional to the position of the slide from the reference end of the potentiometer.

The magnitude of the voltage is set by the excitation voltage across the potentiometer. Typical values are 1 to 2 volts, but higher voltages can be used.

The lower the resistance of the potentiometer, the less influence wiring and noise pickup has on the system. However, excitation current increases as the value of the potentiometer decreases which can add cost to the excitation supply. Values between 1,000 and 10,000 ohms are the most common values in use.

The stability of the output is directly related to the stability of the excitation supply.

THE POTENTIOMETER

The potentiometer selected must be suitable for the task. Potentiometers that are rarely rotated and stay at rest when not being moved are relatively inexpensive and easily obtainable.

Cermet elements (a mixture of glass and metal particles fused to a ceramic substrate offer good temperature stability and infinite resolution of position.

Wire wound units offer better temperature stability, but have a resolution limit due to the wiper moving from turn to turn of the element.

In applications where the potentiometer is never still, such as a position servo loop, the constant movement of the wiper, due to noise jitter in the loop, wears out the element quickly. Special potentiometers are available for such applications.

Potentiometers are also available with ball bearings on the shaft, complex precision mounting features, and a host of other features.

RESPONSE TIME

Potentiometers rarely move fast enough to make response time much of a consideration. Consequently the signal conditioner can have a relatively slow response time which will help prevent noise picked up by the input wiring from passing through the conditioner.

Response times generally range from 1 second to 10 seconds, depending on application.

COST FACTORS FOR POTENTIOMETER INPUT SIGNAL CONDITIONER

1. Absolute accuracy of measurement.
2. Stability versus ambient temperature.
3. Environment (hazardous, humid, corrosive, etc.)
4. Span of measurement. (Span is the rotational angle used for full scale output divided by the maximum rotational angle of the potentiometer and is expressed as a percent; i.e. span = 25% rotation). The narrower the span the more costly it is to meet stability requirements.