High frequency oscillation type proximity switch

Abstract

A high frequency oscillation type proximity switch, comprising: an oscillation circuit having a first resonant frequency; a tank circuit connected in a positive feedback path of the oscillation circuit and including an LC resonant circuit having a second resonant frequency slightly different from the first resonant frequency; and a comparator circuit which discriminates the amplitude of an oscillation output of the oscillating circuit at a predetermined threshold level; the comparator producing a detection output signal when a nonmagnetic metallic object is brought to a certain proximity of a coil included in the LC resonant circuit and a level of the oscillation has thereby changed by a certain amount as a result of a change in the susceptance of the coil. Since the approach of a magnetic object increases the loss of the coil whereas the approach of a nonmagnetic metallic object increases the susceptance of the coil, it is possible to detect magnetic objects alone, nonmagnetic metallic objects alone, or both magnetic objects and nonmagnetic metallic objects alike by appropriately selecting the resonant frequencies of the oscillation circuit, and the tank circuit and the threshold level of the comparator circuit.