Driven shielding capacitive proximity sensor

Abstract

A capacitive proximity sensing element, backed by a reflector driven at the same voltage as and in phase with the sensor, is used to reflect the field lines away from a grounded robot arm towards an intruding object, thus dramatically increasing the sensor's range and sensitivity.

We claim: 
     
       1. A capacitive type proximity sensor having improved range and sensitivity between a machine and an intruding object in the immediate vicinity of the machine, comprising: an outer electrical conductor on said machine forming one electrode of a sensor capacitor, .[.the.]. .Iadd.an .Iaddend.other electrode comprising said .Iadd.intruding .Iaddend.object, said outer .Iadd.electrical .Iaddend.conductor comprising a first thin sheet of conductive material;   an intermediate electrical conductor located between said outer .Iadd.electrical .Iaddend.conductor and said machine and being of a size substantially larger than said outer .Iadd.electrical .Iaddend.conductor to act as a shield for reducing the parasitic capacitance between said outer .Iadd.electrical .Iaddend.conductor and said machine, said intermediate .Iadd.electrical .Iaddend.conductor comprising a second thin sheet of conductive material substantially wider than said .[.first.]. .Iadd.outer electrical .Iaddend.conductor;   said outer and intermediate conductors attached to a surface on said machine .[.with no gap between the conductors.]. .Iadd.wherein said conductors are spaced apart in substantially overlapping parallel fashion, .Iaddend.and no gap between said surface and said intermediate conductor and .[.said outer and intermediate conductors coplanar with each other.]. and said surface of said machine, said surface of said machine acting as a ground plane.Iadd., with said intermediate electrical conductor and said surface of said machine spaced apart in substantially overlapping parallel fashion.Iaddend.;   first circuit means for coupling in phase the instantaneous voltage at said outer electrical conductor to said intermediate electrical conductor;   second circuit means coupled to said outer .Iadd.electrical .Iaddend.conductor and being responsive to the capacitance of said sensor capacitor for generating a control signal to said machine.   
     
     
       2. The sensor of claim 1 wherein said first circuit means comprises a voltage follower circuit comprised of an operational amplifier. 
     
     
       3. The sensor of claim 2 wherein said second circuit means comprises a signal generator having an output signal indicative of the capacitance of said sensor capacitor. 
     
     
       4. The sensor of claim 3 wherein said signal generator comprises a variable frequency oscillator. 
     
     
       5. The sensor of claim 1 wherein said machine comprises a robotic apparatus. 
     
     
       6. The sensor of claim 5 wherein said robotic apparatus comprises a movable member of said robotic apparatus. 
     
     
       7. The sensor of claim 6 wherein said movable member comprises an articulated member. 
     
     
       8. The sensor of claim 7 wherein said articulated member comprises a robotic arm.