Method of testing the weight per unit area of thin material

Abstract

A method and a device for determining the weight per unit area and/or the thickness of thin material in sheets, for example banknotes, by aid of ultrasonic waves. The device comprises a single- or multi-channel transmitter-receiver arrangement in which the banknote passing through is exposed to a pulsating sound field. The sound is transmitted to the receiver via the banknote which is set vibrating by the sound. For the evaluation of the receiver signal a time interval dependent on the sound propagation time between the transmitter and the receiver is defined which begins when the primary sound arrives at the receiver and ends before reflected portions of the transmitted sound or portions of sound from adjacent transmitters arrive at the receiver. During the time interval the receiver signal which is free from noise signals is integrated. The integration value is a measurement of the density per unit area of the test material. A special integration method allows for the detection of two or more overlapping banknotes.

Dependencies are inferred from the claim text.