Detecting the passing of magnetic articles using a transducer-signal detector having a switchable dual-mode threshold

A proximity-detection method for detecting the passing of magnetic articles comprising:

• a) employing a magnetic-field-to-voltage transducer near the passing magnetic articles, sensing an ambient magnetic field and applying the output voltage of said transducer V.sub.H to an amplifier, and generating from the output from said amplifier a voltage Vsig that is directly related to the proximity of the articles that may be passing;
• b) generating a binary proximity-detector output voltage Vout by generating a track-and-hold signal voltage, V.sub.t&h, that tracks each slope portion of Vsig until a peak excursion is reached and holds the peak excursion voltage until at a time t.sub.x the difference voltage, V.sub.dif, between Vsig and V.sub.t&h reaches the threshold voltage of a switchable dual-threshold signal detector, and at successive times t.sub.x producing from said signal detector the successive binary transitions in Vout of one and the other polarities corresponding respectively to a time t.sub.x following a positive excursion in Vsig and a time t.sub.x following a negative excursion in Vsig;
• c) defining an initial interval in which only the first positive and first negative excursions in Vsig transpire after start-up of the detecting;
• d) setting said dual-threshold signal detector at start-up of the proximity detecting to the larger (V.sub.thB) of the two threshold voltages;
• and
• e) when during the initial interval, the excursion of one polarity in Vsig becomes greater than the predetermined reference voltage (V.sub.HI), switching said dual-threshold signal detector to the smaller (V.sub.thA) of the two threshold voltages.