With shafts, gears and the like, the general method of determining the rotational speed is to use some form of tachometer or shaft encoder. These give out a pulse at regular angular intervals. It we have N pulses per rev then obviously we have a pulse every (360/N) degrees. Determining the speed is nominally very simple: just measure the time between successive pulses. If this period is T_k seconds and the angle travelled is (360/ N) degrees then the rotational speed is simply estimated by 360/(N*T_k) degrees/second or 60/(N*T_k) rpm.

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