A portable power tool having an electric motor with a rotatable armature shaft, and a tachometric generator

摘要

Several embodiments of tachometers generators for use in controlling the speed of electrical motor driven tools. In each embodiment a toothed rotor (28, 52) is fixed for rotation with the motor armature shaft (23, 53) and a sensor (55) is juxtaposed to the rotor. The sensor is comprised of a core (32. 56) having a first end portion (33, 57) juxtaposd at a first location relative to the rotor and a second end portion (36,58) that is juxtaposed to the rotor at a second, spaced location. A permanent magnet (37, 59) is interposed between the second end portion and the rotor with one of its poles aligned with a second end portion and the other of its poles aligned with the rotor. A coil (38, 61) is wound around an intermediate portion (34, 62) of the core so that a voltage will be generated in the coil by the alternating magnetic flux passing through the core during rotation of the rotor, which voltage is indicative of the speed of the armature shaft. The compact contruction facilitates installation in close places and the device provides a strong signal and exhibits a high signal to noise ratio.