Encoder is a sensor which turns a position into an electronic signal.
There are two forms:
Absolute encoders give an absolute position value.
Incremental encoders count movement rather than position.
With detection of a datum position and the use of a counter, an absolute position may be derived.
The position may be measured as either linear or angular position
Linear encoder, converts linear position to an electronic signal
Rotary encoder, converts rotary position to an electronic signal
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to analog or digital output signals.
A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller.
An absolute maintains position information when power is removed from the device. The position of the encoder is available immediately on applying power. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration point to maintain position accuracy.
An incremental is a linear or rotary electromechanical device that has two output signals, A and B, which issue pulses when the device is moved. Together, the A and B signals indicate both the occurrence of and direction of movement. Many incremental device have an additional output signal, typically designated index or Z, which indicates the device is located at a particular reference position. Also, some encoders provide a status output (typically designated alarm) that indicates internal fault conditions such as a bearing failure or sensor malfunction.