An electronic tachometer utilizes a transducer, typically an optical encoder disc, connected to a movable mechanical element. The transducer produces a pair of triangular waves of fixed relative phase, each having slopes proportional to the velocity of the mechanical element. The triangular waves are...http://www.google.co.uk/patents/US4228396?utm_source=gb-gplus-sharePatent US4228396 - Electronic tachometer and combined brushless motor commutation and tachometer system
