When dispensing media from a self-service terminal, such as bank notes from an ATM, it is very important that the correct amount of media (e.g. the correct amount of money) is dispensed. The media is stored in the SST in cassettes from which it is picked, one piece of media at a time, by a pick mechanism. Occasionally, the pick mechanism may accidentally remove more than one item of media from a cassette in a single pick operation and it is important that this is detected before it is dispensed as otherwise it would result in the user of the SST being dispensed more media than intended and than has been recorded. SSTs therefore include sensors which can detect double picks (i.e. where two pieces of media have been picked together in a single pick operation) and when a double pick is detected, the stack of two pieces of media is rejected (i.e. placed into a purge or reject bin) or recycled, if the SST has recycling capability.
A typical sensor which is used to detect double picks involves passing the media 100 between two rollers 101, 102 and detecting the resultant displacement of the axle 103 of one roller with respect to the axle 104 of the other roller, as shown in the schematic diagram of FIG. 1. The position of one of the rollers (e.g. roller 102) may be fixed. The axle displacement may be measured using a linear variable differential transformer (LVDT). An LVDT comprises three coils: a primary coil and two secondary coils, and the transfer of current between the primary and secondary coils is determined by the position of a magnetic core. At a centre position, the secondary voltages are equal, but if the magnetic core moves away from this centre position, one secondary voltage will increase and the other will decrease and therefore by monitoring these secondary voltages (or the difference between them), displacement can be detected. Instead of using a LVDT to detect the relative displacement of the rollers, the displacement may be detected using a strain gauge.