On-off servomechanism controlled by a wheatstone bridge

Claim 1

A photocopying machine having a zoom lens housing (9) and supporting means (7, 8, 11, 13) therefor, wherein the improvement is in means for moving the zoom lens housing (9), comprising: a. a first DC source (1), b. a second DC source (2), c. a bridge circuit which includes at least two adjustableslider resistors (18, 20) with slider arms and which is connected to said second DC source (2) to receive current therefrom, d. two semiconductive switching elements (10, 12) each having a gate terminal and two main terminals, and each being normally an open circuit between its main terminals but changing to a closed circuit therebetween when turned ON, which elements (10, 12) are connected in back-to-back relation with each other, with the gate terminal of each element (10, 12) connected to a main terminal of the other element through a voltage dropping resistor (22, 24), and which elements (10, 12) are connected to said first DC source (1) and to said bridge circuit, both of said semiconductive switching elements (10, 12) being turned OFF when the bridge is balanced and one of said semiconductive switching elements (10, 12) being turned ON when the bridge is unbalanced, e. a motor (6), f. two relays (14, 16), each connected in series with a main terminal of one of said semiconductive switching elements (10, 12) to be turned on and off in response to the ON and OFF state respectively of the semiconductive switching element (10, 12) with which it is connected, said relays (14, 16) being connected to the motor (6) to drive the motor in one direction when one of said relays (14, 16) is turned on and to drive the motor (6) in the opposite direction when the other replay (14, 16) is tuRned on, g. means (3) for manually unbalancing the bridge, and h. a zoom lens housing (9) and means (7, 8, 11, 13) supporting the zoom lens housing, said motor being drivingly connected to the slider arm of one of said adjustable-slider resistors and to the supporting means (7, 8, 11, 13) of the zoom lens housing (9) to drive both the slider arm and the zoom lens housing (9) when the bridge circuit is unbalanced until said bridge circuit is balanced and to move thereby the zoom lens housing (9) toward a desired position.