This invention concerns a method and apparatus for sensing the passage of sheets through a nip between a pair of guide rollers by sensing the relative deflection of the guide rollers during the passage of a sheet through the nip.
One example of such a method and apparatus is shown in Published International (PCT) Patent Application WO-A-82/01698. In this example, a sheet passes under a roller which is connected to a linear variable differential transformer. A signal representing the change of signal from the sensor in response to the passage of an expected thickness of a sheet is preset and to this is added a signal representing the datum value obtained from the sensor when the roller rests on a guide surface. When a note passes through the nip, the output signal is fed to a comparator, which also receives the expected thickness signal and the datum value, the comparator determining whether or not a sheet is present. Between the passage of successive sheets through the nip, the datum value is rechecked and a suitable correction is made if necessary.
Such an arrangement works well if the rollers and the shafts on which they are mounted are made with high mechanical precision but it is expensive to achieve such precision. The problem which arises when the roller and shaft system is not of high mechanical precision is that the "noise" level of the sensor output is comparable to the signal level corresponding to the passage of a single note, particularly for the thinner currencies. The object of the present invention is to achieve good results in sheet sensing with a roller and shaft system which is built to a lower precision and is consequently less costly.