Encoder is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. There are two main types of encoders: absolute and incremental. The output of absolute encoders indicates the current position of the shaft, making them angle transducers. The output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance, and position.
Encoders can use different technologies and can be classified depending on these technologies. These encoder types can be listed as conductive, optical, on axis magnetic and off axis magnetic.
Conductive encoders: A series of circumferential copper tracks etched onto a PCB is used to encode the information. Contact brushes sense the conductive areas. This form of encoder is now rarely seen except as a user input in digital multimeters.
Optical encoders: This uses a light shining onto a photodiode through slits in a metal or glass disc. Reflective versions also exist. This is one of the most common technologies. Optical encoders are very sensitive to dust.
On axis magnetic encoders: This technology typically uses a specially magnetized 2 pole neodymium magnet the same size as the motor shaft that typically requires a custom motor shaft be used. The accuracy is very bad and does not allow many resolution options. This technology does not typically offer UVW or Z pulse outputs. Due to the 2 pole magnet there is lots of jitter on the output due to the internal interpolation.
Off axis magnetic encoders: This technology typically employs the use of rubber bonded ferrite magnets attached to a metal hub. This offers flexibility in design and low cost for custom applications. Due to the flexibility in many off axis encoder chips they can be programmed to accept any number of pole widths so the chip can be placed in any position required for the application. Magnetic encoders operate in harsh environments where optical encoders would fail to work.