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An absolute encoder maintains position information when power is removed from the encoder. [5] The position of the encoder is available immediately on applying power. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration point to maintain position accuracy.
The encoder can be either incremental or absolute. In an incremental system, position is determined by motion over time; in contrast, in an absolute system, motion is determined by position over time. Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current.
Many incremental encoders have an additional output signal, typically designated index [2] or Z, [3] which indicates the encoder is located at a particular reference position. Also, some encoders provide a status output (typically designated alarm) [4] that indicates internal fault conditions such as a bearing failure or sensor malfunction.
An encoder is a sensor which turns a position into an electronic signal. There are two forms: Absolute encoders give an absolute position value. Incremental encoders count movement rather than position. With detection of a datum position and the use of a counter, an absolute position may be derived.
Some types of resolvers include both types, with the 2-pole windings used for absolute position and the multipole windings for accurate position. Two-pole resolvers can usually reach angular accuracy up to about ±5 ′, whereas a multipole resolver can provide better accuracy, up to 10″ for 16-pole resolvers, to even 1″ for 128-pole resolvers.
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Synchronous Serial Interface (SSI) is a widely used serial interface standard for industrial applications between a master (e.g. controller) and a slave (e.g. sensor). SSI is based on RS-422 [1] standards and has a high protocol efficiency in addition to its implementation over various hardware platforms, making it very popular among sensor manufacturers.
Absolute encoders can determine their position at power-on but are more complicated and expensive. Incremental encoders are simpler, cheaper, and work at faster speeds. Incremental systems, like stepper motors, often combine their inherent ability to measure intervals of rotation with a simple zero-position sensor to set their position at start-up.