Method and apparatus for plural document detection

Apparatus for detecting the presence of overlapped sheets on a transport, the transport defining a substantially planar path along which the sheets are linearly conveyed, said detecting apparatus comprising:

• an ultrasound beam generator, said beam generator radiating bursts of ultrasonic energy at a preselected frequency, the radiated energy comprising wavefronts which move generally in the direction of the beam axis, the beam generator being supported on a first side of the transport path such that the beam axis intersects the plane of the transport path at an acute angle, said preselected frequency being commensurate with a wavelength which is greater than the largest spacing between facing surfaces of a pair of overlapping sheets being conveyed by the transport and said frequency and angle being selected such that overlapped sheets in said air gap will function as an interference filter and the wavefronts of the radiated ultrasound energy will destructively combine;
• a receiver transducer, said receiver transducer generating electrical output signals commensurate with ultrasonic energy incident thereon, said receiver transducer being supported on the second side of the transport path in alignment with said beam axis, an air gap through which the sheets being transported will pass being defined between said beam generator and receiver transducer;
• an attenuation detector responsive to output signals provided by said receiver transducer and at least a first reference signal, said first reference signal being commensurate with an output signal produced by said receiver in the absence of multiple sheets of paper in said air gap, said attenuation detector providing a control signal indicative of results of a comparison of said receiver transducer output signal and said first reference signal;
• and
• an alarm signal generator responsive to the output of said attenuation detector, said alarm signal generator providing an output signal indicative of the presence of multiple overlapped sheets of paper in said air gap.

A method for the detection of overlapped sheets, the sheets being conveyed along a transport path and being supported in a plane during movement, said method comprising the steps of:

• positioning an ultrasound beam generator on a first side of the transport path, the beam generator being oriented such that the axis of the beam of ultrasound energy produced thereby will intersect the plane in which the sheets are supported at an acute angle;
• locating an ultrasound receiver on the second side of the transport path and in axial alignment with the beam generator, an air gap through which the sheets are transported thus being present between the beam generator and beam receiver, the ultrasound receiver providing electrical signals commensurate with ultrasonic energy incident thereon;
• energizing the ultrasound beam generator to cause the production of bursts of energy at a preselected frequency;
• determining from the signals provided by the receiver whether the degree of attenuation of ultrasound energy which traverses the air gap between the beam generator and ultrasound receiver is commensurate with the presence of closely spaced plural sheets in the air gap;
• and
• providing an alarm when the degree of attenuation of the ultrasound energy is indicative of plural sheets being present in the path of the beam of ultrasound energy.

Apparatus for detecting the presence of overlapped sheets on a transport, the transport defining a substantially planar path along which the sheets are linearly conveyed, said detecting apparatus comprising:

• an ultrasound beam generator, said beam generator radiating bursts of ultrasonic energy at a preselected frequency, the radiated energy comprising wavefronts which move generally in the direction of the beam axis, the beam generator being supported on a first side of the transport path such that the beam axis intersects the plane of the transport path at an acute angle;
• a receiver transducer, said receiver transducer generating electrical output signals commensurate with ultrasonic energy incident thereon, said receiver transducer being supported on the second side of the transport path in alignment with said beam axis, an air gap through which the sheets being transported will pass being defined between said beam generator and receiver transducer;
• an attenuation detector responsive to output signals provided by said receiver transducer and at least a first reference signal, said first reference signal being commensurate with an output signal produced by said receiver in the absence of multiple sheets of paper in said air gap, said attenuation detector providing a control signal indicative of results of a comparison of said receiver transducer output signal and said first reference signal;
• and
• an alarm signal generator responsive to the output of said attenuation detector, said alarm signal generator providing an output signal indicative of the presence of multiple overlapped sheets of paper in said air gap.