Rotary Position Sensor

Rotary position sensors are used to track the rotational position of a shaft. This can be either absolute or relative position.

Potentiometer

The inside of a potentiometer is a three terminal resistor. A resistive pad is connected between two terminals while a wiper connects between this pad and the last terminal. The wiper's position on the pad is controlled by the input shaft. As the wiper moves, the resistance measured from a pad terminal to the wiper terminal changes. In some applications, this resistance is directly used to control circuits. However, a potentiometer can be used as a controllable voltage divider by applying a voltage across the pad's terminals and measuring the voltage across a pad terminal to the wiper terminal. If the wiper terminal is connected to a load, the resultant output voltage can change, but for minimal loads this is typically negilible. There are limitations to the amount of power a potentiometer is capable of dissipating, so high power loads are typically not directly connected. Intermediate circuitry such as operational amplifiers or microcontrollers can take the output voltage and apply it to high power applications or convert to data for use elsewhere.

The ANSI electrical symbol is a resistor with an arrow pointing at the side, while the IEC electrical symbol is a box with an arrow pointing at the side. (TODO: Insert images of the symbols)

One issue with potentiometers is the output is not easily repeatable and can easily be off by 10%[citation needed]. However, in applications where this is not a concern potentiometers make for a cheap way of rotary position measurement. A second issue stems from the mechanical connection between the pad and wiper. Over time, the wiper and pad will wear down which could affect the measurement of the potentiometer.

Linear Output

To provide a consistent rate of change over the range of the potentiometer, a linearly resistive pad can be used. To achieve this, the pad is consistent throughout the range of the wiper, where the resistance across a one degree distance at any point on the pad is the same. Linear output potentiometers are commonly used for position measurement.

Logarithmic Output

In some specialized applications a logarithmic pad is used instead. Audio applications typically use logarithmic output potentiometers for volume control which matches how audio is heard by humans.

Encoder

Encoders typically output a digital signal.

Hall-effect

Uses a magnet to detect the rotation of a shaft.

Optical

Similar to Hall-effect encoders, but using optics instead.

Analog

Produces a signal similar to a resolver. Also known as a sin-cos encoder.

Resolver

Can have multiple poles. Analog signal signal output.

Linear Variable Differential Transformer

Very similar to a resolver where a core moves in a field applied by primary coil. This is then transferred to two output coils where the amplitude of the signal changes based on the core's position.

Linear Variable Inductance Transducer

Operates by measuring the change in inductance as an iron core moves in and out of a coil.