Electromagnetic detector for metallic materials having an improved phase

Abstract

An electromagnetic detector device embodying a linear voltage differential transformer. A pair of coils of the differential transformer serve as pick-up coils connected in a detector circuit which is phase sensitive and capable of discriminating between signals representing a target metallic material, such as carbon steel particles trapped in a filter element, and signals induced by other metallic material, as well as stray signals including electrical noise. Input signals representing a target metallic material are fed to an operational amplifier circuit including a first non-inverting amplifier and a second inverting amplifier which produce a non-inverted signal and an inverted signal which are fed to an analog switch. The switch is operated as a chopper by a driver signal having a predetermined phase and the same frequency relative to the signal driving an input coil to the transformer. The phase of the driver signal is adjustable to provide phase discrimination. The coils of the transformer are wound on a core providing a sensor in which a filter element may be placed for test.