Process for the detection of superimposed sheets, apparatus for performing the process and application to bank notes

DETAILED DESCRIPTION This object is achieved by the fact that the process for the detection of superimposed sheets by the evaluation of their surface mass comprises the stages of passing the sheets between a particle source and a particle detector, counting the number of particles received by the detector by measuring the frequencies of the pulses of the detector in order to determine the surface mass and eliminating the notes with a high level of superimposition by comparing the count frequency with a limit value.
In summarizing, the count frequency drop due to the presence of an increased thickness is determined.
The first aforementioned object must be achieved even when the sheets are travelling at high speed, such as one or two meters per second.
To this end and according to another feature, the comparison with a limiting frequency value of the pulses of the particle detector is obtained by discrimination between the frequency of the pulses of the detector and the frequency of the pulses of a given frequency oscillator FO.
Another object of the invention is to propose a process making it possible to eliminate double sheets, whose superimposition level is small.
This second object is achieved by the fact that measurement takes place of the width of the sheets by means of a coding wheel associated with the transportation system, the coding wheel supplying a number of pulses proportional to the rotation during the sheet passage time, these pulses decrementing a preloaded subtract counter to a mean value representing a normal width of a single sheet, the elimination order corresponding to the passage through a threshold of the subtract counter content.
A third object is to propose a process making it possible to eliminate excessively short sheets (e.
g.
a sheet folded back on itself).
This third object is achieved in that a logic circuit connected to the outputs of the subtract counter emits an elimination order if, at the time when the signal from the sheet presence detection circuit disappears, the content of said subtract counter has still not reached a given minimum value