Rotary speed measuring apparatus

摘要

A coil sensor excited with alternating current and connected in a circuit for detecting changes in the coil impedance producing a signal exceeding a fixed threshold value in response to the revolution of a wheel with features providing periodic changes in the spacing between the coil and the wheel, has its periphery subdivided into zones where the surface material is alternately thereomagnetic and non-theromagnetic. With a suitable setting of the frequency of the exciting alternating current and a suitable choice of the materials with respect to their eddy current and magnetic induction effect on the inductance of the coil, it is possible to suppress the signal of once per revolution periodicity resulting from wheel eccentricity, bearing wobble or the like.