Double document detection system having dectector calibration

A double document detection system comprising:

• a microcontroller,
• a microcontroller-controlled means for calibrating the system in a calibration cycle,
• microcontroller-controlled means for generation of a series of identical pulses of the width predetermined in the calibration cycle,
• microcontroller-controlled means for storing said predetermined width of the pulse,
• means for transforming pulses of the predetermined width into respective ramp pulses, each ramp pulse having a peak time,
• means for driving the light emitter by drive pulses corresponding to said ramp pulses, wherein the drive pulses are generated at the peak time of the respective ramp pulse,
• microcontroller-controlled means for continually comparing the output signal with a required value, and
• microcontroller-controlled means for detection of a double-feed document condition when the output signal exceeds the required value.

In an apparatus, wherein a light emitter generates light in response to drive pulses generated by a transistor, the transistor having a base, wherein the transistor generates drive pulses corresponding to pulses supplied to the base, and wherein excessive power dissipation may cause a damage of said transistor and said light emitter, a safety circuit for prolonging the light emitter life and for protecting both the transistor and the light emitter the safety circuit having a circuit ground, the safety circuit comprising:

• a NOR gate including inputs and an output;
• a microcontroller including a plurality of outputs such that one of the outputs of the microcontroller is connected to one of the inputs of the NOR gate;
• a resistor and a capacitor, the resistor having first and second terminals, the capacitor having first and second terminals, wherein the second terminal of the resistor and the first terminal of the capacitor are connected to each other and to the first input of the NOR gate, and wherein the second terminal of the capacitor is connected to the circuit ground;
• wherein the output of the NOR gate is connected to the base of the transistor, such that the NOR gate shuts down the transistor if the power of the supplied pulses exceeds a permissible value.

A method for double-feed document detection in a document handling machine, wherein documents are successively advanced along a document feed path between a light emitter and a corresponding light detector, wherein the light emitter generates light of an energy level penetrating at least one document in the document feed path and causing an output signal of the light detector, wherein the output signal changes its value in respect to the opacity of at least one document in the document feed path, wherein the value of the output signal is analyzed to detect if there is more than one document in the document feed path, the method including a calibration cycle and a subsequent operation cycle, wherein the light emitter is protected from damage by excessive power dissipation, wherein the calibration cycle comprises the steps of:

• providing a single calibration document in the document feed path,
• generating a series of increasingly wider pulses,
• transforming said series of increasingly wider pulses into a series of respective ramp pulses of increasing height,
• generating a series of drive pulses corresponding to the increasingly higher ramp pulses for driving the light emitter,
• monitoring the respective output signal's value until it reaches a predetermined value,
• fixing the width of one of the increasingly wider pulses corresponding to said predetermined value,
• storing the pulse of the fixed width for restoring the fixed width of the pulse in the situation of power outage immediately after the power up, and
• removing the calibration document from the document feed path;
• and
• wherein the operation cycle comprising the steps of:
• generating pulses of the fixed width determined in the calibration cycle,
• transforming the pulses of determined width in the calibration cycle into ramp pulses, each ramp pulse having a peak time,
• generating drive pulses corresponding to said ramp pulses, wherein each ramp pulse starts from the same voltage level, wherein said drive pulses are not generated until the peak time of the respective ramp pulse;
• and
• monitoring the output signal corresponding to at least one document in the document feed path,
• comparing the output signal with a required value,
• indicating a double-feed document condition if the monitored output signal exceeds the required value,
• counting a prolongation of the detected double-feed document condition,
• detecting a "true" double if the prolongation of the double-feed document condition exceeds the predetermined value, double-feed document condition including a first and a second overlapped documents,
• measuring a distance between a front edge of the first overlapped document and a rear edge of the second overlapped document, and
• detecting a double-feed document condition if said distance exceeds a predetermined distance, wherein the predetermined distance may be set up for various document sizes and speeds of the documents motion.