Ferromagnetic article detector with dual Hall-sensors

Claim 1

A ferromagnetic article detector comprising: (a) a pair of essentially identical silicon-integrated-circuit-Hall-sensor chips each of the type capable of providing between a first and a second output thereof a differential output voltage that is a continuous function of the strength of a magnetic field perpendicular to a major face of the integrated circuit chip, each of said chips comprising a Hall element being co-planar with said major face of said each chip and a differential amplifier having an input connected to the output of said Hall element; and having an output connected to said first and second outputs (b) a permanent magnet in the shape of a bar having at least one substantially flat end and being magnetized in a direction perpendicular to said end, said pair of integrated circuit chips being mounted spaced apart at said magnet end with their major surfaces lying in a plane that is parallel to said bar end; (c) a differential comparator means having one and another inputs; and (d) a voltage summing means coupling to said one comparator means input a sum of the output voltages at the first differential output of one of said Hall-sensor chips and the second differential output of the other of said Hall sensor chips, and coupling to said another comparator means input a sum of the output voltages at the second differential output of said one chip and at the first differential output of said other chip, said differential comparator means being for producing a detector-output-signal that is proportioned to the difference between the output signals of said two amplifiers.