Sep 30 2013
A rotary encoder is a sensor designed to generate an electrical signal in response to a rotational movement to determine or control the speed or position of a mechanical device. The device is mounted on a cylindrical shaft, and it is used in combination with mechanical conversion devices such as rack and pinions and linear slides to measure linear movement.
There are two main types of a rotary encoder: An incremental encoder designed to generate a series of pulses, as it rotates, and an absolute encoder which generates an output signal that is in digital bits, with each bit corresponding to a known position.
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Image credit: Photos.com.
Encoder Technologies
Rotary encoders are usually implemented in various technologies:
Magnetic technology - Strips of magnetized material are kept on a rotating disk and sensed by a magnetoresistive or Hall-effect sensor. Hall effect sensors are used to sense gear teeth without the need for a separate encoder disc.
Optical technology – This is one of the most common technologies that works based on the light shining on a photodiode via slits in a glass disk or metal.
Conductive tracks – This technology employs a series of copper pads etched onto a printed circuit board to encode the information.
Working Principle of Rotary Encoder
Absolute Rotary Encoder
The absolute rotary encoder is of two basic types: mechanical and optical
The mechanical absolute encoder consists of a metal disk having a set of concentric rings of openings. The metal disk is fixed to an insulating disk that is rigidly fixed to a shaft.
A series of sliding contacts is fixed to a stationary object such that each contact connected to a separate electrical sensor slides against the metal disk at a different distance from the shaft. When the metal disk is supplied with an electric current, a metal pattern is formed so that each possible position of the axle creates a unique binary code.
The optical encoder utilizes a rotating glass or plastic disk containing concentric rings, which have patterns etched onto them. An LED is used to shoot light through a grid that projects a pattern onto the rotating disk. A photodiode is used on the other side of the rotating disk to read the pattern of light being passed through. The pattern on the grid, combined with the pattern on the rotating disk generates a code that can be read by a controlling device to determine the angle of the shaft.
Incremental Rotary Encoder
An incremental rotary encoder provides cyclical outputs when the encoder is rotated. The encoder can be either mechanical or optical. The mechanical type encoder requires de-bouncing, and is typically used as digital potentiometers on equipment including consumer devices.
The optical type encoder is used when higher speeds are encountered or a higher degree of precision is required.
Applications
Some of the major applications of rotary encoders include:
- Industrial controls
- Special purpose photographic lenses
- Controlled stress rheometers
- Rotating RADAR platforms
- Computer input devices such as trackballs and optomechanical mice.
References