An auxiliary module has a gearbox, a motor, a torque detector and a controller. The system includes one or more data collection stations, each of which may include an X-ray scanner that scans the items under inspection to obtain data about the items. An image is formed on the first surface of the medium while proceeding the medium through the printing path. In one embodiment, there may be between two and four alignment fields, and between two and four SSPM mark pairs around each alignment field. The bias current is adjusted according to the differential input to the differential pair, the difference between the currents of the differential pair, or any one of the currents of the differential pair.