Robotics/Feedback Sensors/Encoders

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Encoders are disks mounted on the axle of e.g. a motor in such a way that they turn along with said axle. There are single "channel" encoders which have a single circular pattern of slots. These are commonly used to measure the speed of a axle. There are also encoder with multiple channels, often with markings following a specific code, e.g. Grey code. These Encoder are used to detect a position rather than a speed.

[edit] Incremental Encoders

These Encoders need a single IR-LED and IR-phototransitor pair to measure speed. A second pair placed so that when the first one is in the middle of a marking, the second one falls on an edge of a marking, allows you to measure speed and direction. This methode is commonly used in computer mice.
Unless there is a extra marking and IR-LED / IR-phototransistor pair, these encoders have no start position.

[edit] Position Encoders

These encoder use multiple "channels" with markings following a specific code in order to encode the position of the axle they are mounted on. Each channel needs its own IR-LED and IR-phototransistor pair.