GB847917A - A self balancing potentiometer system for use in association with weighing apparatus - Google Patents

Abstract

847,917. Weighing-apparatus. AVERYS Ltd., [formerly AVERY Ltd., W. & T.]. June 28, 1957 [July 4, 1956], No. 20661/56. Class 40(1). [Also in Group XXXVI] An automatically-balanced bridge for use in weighing-apparatus includes a plurality of balancing stages corresponding to different units of weight, each stage being controlled by a separate stepping switch energized when the previous stage has reached an over-balanced condition; the bridge output is amplified and operates a switch to energize the appropriate stepping switch and introduce biasing means to add one unit of that stage to produce the overbalanced condition. In Fig. 1 the spindle of the weight-indicator operates a potentiometer P1 in an A.C. bridge of which one arm comprises a 3-stage Kelvin-Varley potentiometer R1- R32. The bridge output is amplified at A1 and energizes a phase-sensitive relay RL1 which changes the negative D.C. supply from P to Q when the bridge changes from an under-balanced to an over-balanced condition, and vice-versa. Initially the bridge is under-balanced and on pressing button PB1 the stepping switch 84 is energized and moves to contact 2. Stepping switch S1 is now energized via C2 and shifts its contact along the " hundreds " resistors R1- R11 until the balance-point is passed. Relay RL1 now changes over to contact Q and drives switch S4 on to contact 3. Here resistors R35 and R36 are substituted for R34 to underbalance the bridge once more, and RL1 changes back to P so that when S4 advances to contact 4 the next stepping switch S2 is energized to bring in the " tens " resistors R12-R22. The same process is repeated for these and the " units " R23-R32 so that finally the bridge is balanced correct to the nearest unit. To ensure that the bridge, amplifier and relay RL1 are working correctly two further contacts 7 and 8 are provided to introduce one unit of unbalance in each direction successively. Only if the amplifier responds to this amount of unbalance and if the relay RL1 changes over its contacts P and Q accordingly will the switch S4 be returned to its original position to carry out a new weighing. In Fig. 2 (not shown) further contacts are included on the stepping switches to operate the solenoids of an electric type-writer. Instead of decade units the apparatus can be constructed to operate with other scales, e.g. binary or avoirdupois. Fig. 3 shows an antifraud circuit to ensure that the apparatus will not function unless the weighing mechanism is stationary. Any oscillation produces an A.C. signal from P1 which is amplified at A2, voltagedoubled at C2 and applied to a valve V1 to energize a relay RL2 and open-circuit the bridge. The latter can then be operated only when oscillations have ceased and C2 has discharged through R.