Electrical sensing system for measuring ferrous particles within a fluid with temperature compensation circuit

Claim 1

A means for sensing ferrous contamination in a fluid comprising: (a) a generally cylindrical magnet; (b) an electrical coil located co-axially on one end of said magnet, said coil having a hole lengthwise through its center; (c) a pole piece of said magnet occupying the hole in said first coil and concentric with said magnet disposed at one end of the magnet, said pole piece of the magnet being available to accumulate ferrous contamination present in a fluid whereby any ferrous accumulation changes the induction of the coil; (d) An electronic circuit means for measuring inductance changes of said electrical coil including; a clock oscillator means providing two discrete frequencies, one high frequency and one low frequency; a frequency selection means to select, upon command, one of the two frequencies of the said clock oscillator means; a waveform generator; a waveform period counter which switches between one of two states corresponding to said high frequency and said low frequency of said clock oscillator; a sample and hold counter control means operatively connected to said waveform period counter; a first sample and hold amplifier; a second sample and hold amplifier; a constant current coil driver circuit driving a tuned circuit having a resonant frequency between said high frequency and said low frequency, said tuned circuit including said electrical coil; a first differential amplifier; and a second differential amplifier; wherein said clock oscillator is operatively connected to the frequency selection means, and said frequency selection means alternatively selects said high and low frequency from said oscillator means to introduce such frequency into the waveform generator, said waveform generator producing a frequency output proportional to the said selected high or low frequency and introducing said proportional output to the said coil driver constant current circuit, and the said waveform period counter; wherein further said constant current driver provides a constant current to said tuned circuit, the output of said tuned circuit being placed across the input of said first differential amplifier such that the output of said amplifier represents the voltage across said tuned circuit; the output of said amplifier being introduced to the input of said first and second sample and hold amplifiers, said first sample and hold amplifier driving an inverting input of said second differential amplifier and said second sample and hold amplifier driving a non-inverting input of said second differential amplifier said second differential amplifier providing an output proportional to inductance changes of said electrical coil; wherein further said first and second sample and hold amplifiers are commanded through said sample and hold counter control means to sample and hold by said waveform period counter in a manner corresponding to said high frequency and said low frequency of said clock oscillator means.