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Capacitive Document Sensing

IP.com Disclosure Number: IPCOM000090472D
Original Publication Date: 1969-Apr-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 56K

Publishing Venue

IBM

Related People

Meier, JH: AUTHOR

Abstract

The circuits are for detecting small changes in capacitance and are particularly useful for detecting the presence of documents in document handling apparatus.

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Capacitive Document Sensing

The circuits are for detecting small changes in capacitance and are particularly useful for detecting the presence of documents in document handling apparatus.

In A, transducer, CX comprising a pair of capacitor plates located on opposite sides of the document path, forms an unknown capacitance which varies depending upon whether or not a document is between the plates. The capacitance of CX is compared with the capacitance of a standard or reference transducer CR. The apparatus comprises a balanced comparison system in which, under normal conditions, the AC inputs supplied from generator G can be balanced at the inputs of operational amplifiers A1 and A2. The outputs of amplifiers A1 and A2 are connected to shields S1 and S2, which extend from the electronic circuitry to and around the active plates of the capacitors. By setting the gain of amplifiers A1 and A2 to unity, the shield voltage is kept exactly at the probe voltage and thus eliminates the effects of distributed capacitance between the active plates or probes of the two capacitors and ground. Emitter-followers could be used in the place of amplifiers A1 and A2. The output of amplifiers Al and A2 is capacity-coupled by capacitors C1 and C2 to a networks of resistors and diodes, which provide half-wave rectification of the output voltages and supply them to terminals 0T1 and 0T2. By connecting a threshold detector between terminals 0T1 and 0T2, the unbalance which is caused by...