Rotary encoder
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A Gray code absolute rotary encoder with 13 tracks. At the top can be seen the housing, interrupter disk, and light source; at the bottom can be seen the sensing element and support components.
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 an analog or digital code. 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, RPM and position. The output of absolute encoders indicates the current position of the shaft, making them angle transducers. Rotary encoders are used in many applications that require precise shaft unlimited rotation—including industrial controls, robotics, special purpose photographic lenses,[1] computer input devices (such as optomechanical miceand trackballs), and rotating radar platforms. There are two main types: absolute and incremental (relative).
Contents
[hide]
• 1 Absolute rotary encoder
o 1.1 Construction
o 1.2 Mechanical absolute encoders
o 1.3 Optical absolute encoders
o 1.4 Standard binary encoding
o 1.5 Gray encoding
o 1.6 Single-track Gray encoding
o 1.7 Absolute encoder output formats
• 2 Incremental rotary encoder
• 3 Incremental versus absolute encoder terminology
o 3.1 Traditional absolute encoders
o 3.2 Traditional incremental encoders
o 3.3 Battery backed incremental encoders
• 4 Sine wave encoder
• 5 Use in industry
o 5.1 Encoders used on servomotors
• 6 Encoder technologies
• 7 See also
• 8 External links
• 9 References
[edit]Absolute rotary encoder
Absolute rotary encoder ROD 425
[edit]Construction
Digital absolute encoders produce a unique digital code for each distinct angle of the shaft. They come in two basic types: optical and mechanical.
[edit]Mechanical absolute encoders
A metal disc containing a set of concentric rings of openings is fixed to an insulating disc, which is rigidly fixed to the shaft. A row of sliding contacts is fixed to a stationary object so that each contact wipes against the metal disc at a different distance from the shaft. As the disc rotates with the shaft, some of the contacts touch metal, while others fall in the gaps where the metal has been cut out. The metal sheet is connected to a source of electric current, and each contact is connected to a separate electrical sensor. 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).
[edit]Optical absolute encoders
The optical encoder's disc is made of glass or plastic with transparent and opaque areas. A 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.
The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft (by using a special algorithm).
[edit]Standard binary encoding
Rotary encoder for angle-measuring devices marked in 3-bit binary. The inner ring corresponds to Contact 1 in the table. Black sectors are "on". Zero degrees is on the right-hand side, with angle increasing counterclockwise.
An example of a binary code, in an extremely simplified encoder with only three contacts, is shown below.