incremental shaft encoder into an absolute one. 2. Usually the two emitter/detector pairs are out of phase
Incremental shaft encoders have usually two pairs of photo emitters and photo detectors. Why is this?
Figure 1: Incremental Shaft Encoder
1. Using two pairs of emitters and detectors allows turning an incremental shaft
encoder into an absolute one.
2. Usually the two emitter/detector pairs are out of phase by an angle of 90 degrees,
so that the direction in which the the encoder disk is moving can be determined. If
there was only one such emitter/detector pair, the direction could not be determined.
3. The additional emitter/detector pair is only introduced, in order to increase the
redundancy of the shaft encoder. Hence, if one of them fails, the shaft encoder would
continue operating in the same way as beforehand.
4. The additional emitter/detector pair allows measuring additional information about
the shaft encoder motion. Whilst shaft encoders with a single emitter/detector pair
can strictly only measure the absolute angle of the shaft encoder, adding another pair
allows measuring differential speed and acceleration.
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