Rotary Encoder or Shaft Encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. Rotary encoders are used in many applications that require precise shaft unlimited rotation—including industrial controls, robotics, special purpose photographic lenses, computer input devices (such as optomechanical mice and trackballs), and rotating radar platforms.

Types of Rotary Encoders

  • Absolute Rotary Encoder – The output of incremental encoders provide information about the motion of the shaft which is typically further processed elsewhere into information such as speed, distance, RPM and position. Digital absolute encoders produce a unique digital code for each distinct angle of the shaft. They come in two basic types: mechanical and optical rotary encoders.
  • Incremental (Relative) Rotary Encoder – The output of absolute encoders indicate the current position of the shaft, making them angle transducers. A traditional incremental encoder works differently by providing an A and a B pulse output that provide no usable count information in their own right. Rather, the counting is done in the external electronics. The point where the counting begins depends on the counter in the external electronics and not on the position of the encoder.

Rotary Encoder Technologies

  • Mechanical Rotary Encoders – A series of copper pads etched onto a PCB is used to encode the information. Contact brushes sense the conductive areas. The metal pattern is designed so that each possible position of the axle creates a unique binary code in which some of the contacts are connected to the current source (i.e. switched on) and others are not (i.e. switched off).
  • Optical Rotary Encoders – The optical encoder uses a light shining onto a photodiode through slits in a transparent plastic or glass disc. A (laser) light source and photo detector array reads the optical pattern that results from the disc’s position at any one time. This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft. This is one of the most common technologies.
  • Magnetic Rotary Encoders – Strips of magnetised material are placed on the rotating disc and are sensed by a Hall-effect sensor or magnetoresistive sensor. Hall effect sensors are also used to sense gear teeth directly, without the need for a separate encoder disc.

Rotary encoders are often used to track the position of the motor shaft on permanent magnet brushless motors, which are commonly used on CNC machines, robots, and other industrial equipment. Incremental (Quadrature) encoders are used on Induction Motor type servomotors, but absolute encoders are used in Permanent Magnet Brushless Motors, where applicable.

Analogue Rotary Encoder devices that perform a similar function of digital rotary encoders include the synchro, the resolver, the rotary variable differential transformer (RVDT) and the rotary potentiometer.