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Capacitive proximity switch for evaluating minor changes in capacitance and method therefor

  • US 6,590,401 B1
  • Filed: 04/02/2001
  • Issued: 07/08/2003
  • Est. Priority Date: 08/03/1998
  • Status: Expired due to Fees
First Claim
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1. A capacitive proximity switch for the evaluation of capacitance changes with an electrical alternating current measurement bridge of four bridge branches, which reach every two of four switching knots of the bridge, wherein an alternating current is positioned as a bridge feed voltage (ubr) near two opposite, not neighboring switching knots, and respectively the two bridge branches between the two feed in switching knots build each of the two halves of the bridge, and there is an evaluated bridge diagonal voltage on the two remaining switching knots over diagonal voltage path (DSp1, DSp2), wherein the bridge diagonal voltage (Ud) is removable, for detecting an approaching or moving away face, in particular an electrically bad conductive face or metallic face, which face is part of variable capacitor (C1) in one of the bridge branches of the bridge circuit, wherein at least one capacitor(C1,C2,C3,C4)(capacitive bridge) or at least one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10) (capacitive ohmic bridge) is disposed in the remaining bridge branches as further reactances, wherein rectifiers (N1,N2,N3,N4,S1,S2,S3,S4) are arranged to the rectification of two bridge branch voltages (uc1,uc3) separately according to respective bridge halves in the diagonal voltage paths (DSp1, DSp2) and the diagonal bridge voltage (Ud) is evaluated only after the rectification of the two bridge branch voltages (uc1,uc3) as changing direct current corresponding to the capacitance change of the variable capacitor, characterized in that,a) the proximity switch comprises a flat multilayer printed circuit board (10) of at least two electrically insulating layers (13,14), wherein an electrically conducting intermediate layer (11) as a first face of a capacitor in one of the two bridge branches of one of the bridge halves is disposed between the at least two electrically insulating layers (13,14) b) at one of the layers (13,14) at the opposite side of the intermediate layer (11) a flat electrically conducting covering is placed onto one of the two layers (13,14) as a sensor (12), wherein the electrically conducting covering forms the second face of the capacitor c) the face (15) is mobile relative to the sensor (12) and the face (15) forms together with the sensor (12) a flat variable capacitor of the bridge circuit.

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