incremental encoder and rotary encoder！

Encoders come in incremental and absolute styles. Like linear and rotary encoders,
incremental and absolute encoders have similarities, but they differ in wiring and
movement identification.
An incremental encoder only reads pulses to provide information about the relative
motion of the shaft. It has no information about location when powered up; it can
only show how far the shaft has moved since the encoder was powered up. It reports back these position changes with electrical “pulses”. These pulse streams can either be single channel (one output wire from the encoder) or dual channel (two wires – see also the “Quadrature” section below). Think of an incremental encoder as a tape measure with no numbers on it, only tick marks: you can tell how far you’ve moved, but you don’t know exactly where you are unless you measure from a known spot

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