Double document detection system having dectector calibration

Abstract

An electro-optical system for double document detection wherein an improved calibration is employed. A microcontroller generates a series of increasingly wider pulses, which are transformed into a series of respective increasingly higher ramp pulses to drive a light emitter at gradually increased power until a predetermined value of an output signal of a light detector is achieved. Then the pulse width corresponding to the predetermined value of the output signal is fixed. During feeding of documents, the microcontroller monitors the output signal to determine a double-feed document condition if the output signal exceeds a predetermined value. To detect a 'true' double-feed document condition, a software timer counts a prolongation of the alteration of the output signal. Also, in order not to miss a 'true' double-feed document condition when the area of overlapping is small, the distance between a front edge of the first overlapped document and a rear edge of the last overlapped document is measured. The system also declares a double-feed document condition if a space between two fed documents is no greater than a predetermined value. The system also provides a safety circuit to protect the light emitter from excessive power dissipation. If the situation of power down in the system occurs, a recalibration is not needed, since the system is provided with a non-volatile memory to store the fixed width. Each stage of the system is indicated by LEDs.