Self-calibrating system for detecting media movement by using capacitors as sensors

In apparatus having a sheet feed path along which sheets are moved to a use location(s), an improved sheet movement detection system comprising:

• (a) first capacitor sensing means including a first pair of parallel plate members located in opposing relation on opposite sides of the feed path plane at an upstream position;
• (b) second capacitor sensing means including a second pair of parallel plate members which are correspondingly sized and spaced to said first plate members in opposing relation on opposite sides of the feed path plane at a downstream position, said second sensing means being located along said feed path so that a fed sheet can completely span the first and second pairs of plate members;
• (c) detection circuit means for applying potentials across the pairs of plate members in a circuit configuration such that output signals of the first and second capacitor sensing means are substracted from one another;
• and
• (d) position signal means for receiving the output of said detection circuit means during movement of a lead sheet edge through the second capacitor sensing means and providing a signal indicating the precise position of the sheet lead edge on said feed path.