The device that converts the angular motion or position of an axle or shaft to a digital or analog code is called a rotary encoder. There are two main types of rotary encoders: incremental and absolute. The output of absolute encoders shows the current position of the shaft, which makes them angle transducers. The output of the incremental encoders on the other hand, provides information about the movement of the shaft, which is usually further processed elsewhere into information such as position, distance and speed.
Rotary encoders are used in many applications which require very precise shaft unlimited rotation – including robotics, industrial controls, computer input devices, controlled stress rheometers, special purpose photographic lenses and rotating radar platforms. There are 4 types of encoder technologies: optical, conductive, on-axis magnetic and off-axis magnetic.
Optical Encoder Technology
The optical encoder technology uses the light which shines into a photodiode through slits in a glass or metal disc. Reflective versions are also available on the market. The optical encoder technology is one of the more commonly used encoder technologies, and they’re very sensitive to dust.
Conductive Encoder Technology
This technology enforces a series of circumferential copper tracks which are etched onto a PCB to encode information. Contact brushes sense these conductive areas, which makes it the go-to option for user input in digital multimeters.
This is a technology that typically uses specially magnetised 2 pole neodymium magnets. These magnets are the same size as the motor shaft, which typically requires a custom motor shaft to be used. The accuracy isn’t all that good and doesn’t allow many resolution options. It doesn’t typically offer Z or UVW pulse outputs due, and there’s a lot of jitter on the output due to the 2 pole magnets which cause internal interpolation.
The off-axis magnetic technology employs the use of rubber bonded ferrite magnets which are attached to a metal hub, which offers flexibility in low cost and design for custom applications. Thanks to this flexibility, many off-axis encoder chips can be programmed to accept any number of pole widths so the chip can be put in any position required for the application. This technology can operate in harsh environments where the optical technology fails to work.
To summarize, these technologies are used to track the position of the motor shaft on permanent magnet brushless motors that are very commonly used in robots, CNC machines and other industrial equipment. Incremental encoders are used on the induction type servomotors and absolute encoders are used on permanent magnet brushless motors.