Abstract: A device for positioning a part comprises a signal generator which, upon movement of said part within the vicinity of a required position of the part, produces a signal value which passes through zero at a point when said part is in said required position, and a servo system responsive to said signal value to transmit movement to said part until said signal value reaches zero. The signal generator may operate to define a predetermined stop position into which a part, for example a machine part or a workpiece, is to be moved, or, in cooperation with an appropriately movable member, it may be used to trace a predetermined pattern of movement so that the part to be moved follows said pattern.