Source: {"pile_set_name": "USPTO Backgrounds"}

It is known to provide such belt with a marker element that travels along with the belt and to arrange a sensor in a fixed position next to the belt for sensing the marker and to produce a control signal from the sensed marker. For producing a proper control signal, the run of the belt must be monitored with regard to any lateral excursions and also with regard to any slippage between the belt and its driving drum. The slippage is easily determined when a marker on the belt is sensed by a sensor and if the rated belt speed is known. Sensors for this purpose may be, for example, of the inductive pick-up type. Such sensors provide an impulse when the marking element passes the sensor.
It is also known to ascertain lateral belt excursions away from a centered position, by means of end switches which have sensor elements that are contacted by a belt that moves away from its central position. These switches with their sensors are arranged alongside the belt edges in stationary positions. However, the use of these end switches with their sensors requires a substantial structural investment and expense. Further end switches have the disadvantage that they make the monitoring system more troubleprone. Thus, end switches require frequent maintenance work.
German Patent Publication (DE-OS) 2,936,344 discloses a method and apparatus for controlling the linearity of the movement of a conveyor belt relative to a centered position. In this known system the effective drum radius or diameter of a guide drum or of a driving drum is increased so as to urge the belt back to its centered position. In other words, the diameter increase is made on that side toward which the belt is deviating from its centered course. Such a system has its merits. However, it is not readily adaptable for use in connection, for example, with belts in weighing scales also referred to as belt weighers.