Speed detector for use on a dc motor

Abstract

A speed detector, particularly useful on a permanent magnet DC motor, for providing a pulse indication from movement of the rotor teeth. During one revolution of the motor, a pulse is provided for each motor tooth. In a preferred embodiment, the pulse pickup probe is formed from a ferromagnetic stud which is anchored to the stator housing and extends through an opening formed in the center of the fixed permanent magnet to a position in close proximity to the rotor. The magnet through which the probe extends provides magnetic flux having a path through the stud and the rotor. The reluctance of the flux path through the rotor varies as a function of the alignment of a rotor tooth with respect to the ferromagnetic stud. The stud is formed to have a pickup end facing the rotor. The diameter of the pickup end is slightly greater than the rotor tooth width. A pickup coil is wound around a reduced diameter portion formed on the ferromagnetic stud, and generally lies within the opening through the permanent magnet. As a rotor tooth passes across the pickup end of the stud, a voltage pulse is induced in the pickup coil. The speed with which these voltage pulses are produced indicates motor speed. A small opening or openings extend from the head of the stud towards the reduced diameter portion for receiving leads which are connected to the pickup coil. A diode can be electrically connected to the pickup coil to produce a unidirectional pulse output.