Device for detecting ferromagnetic materials

Claim 1

Apparatus for detecting small rapidly moving ferromagnetic objects comprising: (a) a source of alternating current; (b) an inductive sensing coil having a magnetic field, powered by said source of alternating current; (c) a balancing impedance in series connection with said inductive sensing coil, said balancing impedance being adjusted in a manner such that voltage amplitudes across said inductive sensing coil and said balancing impedance are approximately equal in the absence of ferromagnetic material in said coil's magnetic field; (d) first and second means for rectifying electrical signals, said first means half-wave rectifying a signal appearing across said inductive sensing coil, and said second means half-wave rectifying a signal appearing across said balancing impedance, said first and second rectifying means simultaneously producing half-wave rectified signals of oppposite polarity. (e) summing means joining the simultaneous outputs of said first and second rectifying means and producing an electrical signal proportional to the algebraic sum of the signals produced by said first and second rectifying means; (f) means for sensing a rapidly repetitive change in the electrical signal produced by said summing means in response exclusively to the rapid passage of ferromagnetic objects in succession through said coil's magnetic field; (g) a band-pass filter for blocking DC voltage interposed between the junction of said summing means and said means for sensing a change in the electrical signal produced by said summing means; and (h) means for indicating the sensed change in said electrical signal whereby the number of said objects may be counted.