An optical encoder is an electronic device that measures the extent to which a mechanical shaft has rotated. It can also measure the rate of rotation (angular speed).

An optical encoder consists of a pair of light-emitting diodes (LEDs), a photodetector, and a chopping wheel. The LEDs shine on the photodetector through the chopping wheel. The wheel has radial bands, alternately transparent and opaque (see illustration). The wheel is attached to the shaft. As the shaft turns, the light beam is interrupted. Each interruption actuates a counting circuit. The number of pulses is a direct function of the extent to which the shaft has rotated. The frequency of pulses is a direct function of the rotational speed. Two LEDs, placed in the correct positions, allow the encoder to indicate the direction (clockwise or counterclockwise) in which the shaft rotates.

Optical encoders are used in various robotic applications. In particular, they are used in manipulators to measure the extent of joint rotation.

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