Patent GB-648822-A (GB)
Title: Improvements in or relating to self-balancing electrical bridge circuits
IPC: G05D23/24
Filed: 1947-01-30
Abstract: 648,822. Automatic temperature - control systems. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. Jan. 30, 1947, No. 2918. Convention date, Feb. 2, 1946. [Class 38 (iv)] [Also in Groups XXXVI and XXXVIII] In a self-balancing bridge system for measuring unknown electrical quantities, an unbalance signal operates bridge rebalancing means which indicates or records the unknown, and deflects a contact over a current carrying impedance to derive a potential difference between the contact and one or more fixed but adjustable contacts on the impedance, which operates control means to regulate the value of the unknown quantity, operate an alarm signal, and unbalance an auxiliary bridge, whose output diagonal is connected in circuit with the control means and the fixed and moving impedance contacts, so that in the stable condition of the system the P.D. between the contacts is neutralized by the unbalance signal of the auxiliary bridge. Fig. 1 shows a known self-balancing Wheatstone measuring bridge comprising fixed resistances 2, 3, 4, a tapped potentiometer 5 and a resistance 1 controlled by the magnitude to be measured, for example, temperature. The bridge is A.C. energized and a known circuit in the diagonal between the potentiometer tap 6 and the join of resistances 1, 2 controls an electric motor 7 which traverses contact 6 to rebalance the bridge and operates an indicator or recorder (not shown). A further contact 10 is correspondingly traversed over a conductive endless belt which is electrically interrupted at diametrically opposite points 14, 15 and runs on conductive rollers 12, 13 connected to a battery, the centre point of which is returned to contact 10 through electrical control mechanism 17, responsive to current intensity and phase or sense, and which, when subjected to the reversal of current due to contact 10 passing interruption 14 when traversed in response to unbalance of the bridge by variations of unknown magnitude, readjusts its value and may operate an alarm. The relative positions of the interruptions may be varied by turning the rollers 12, 13. In a modification (Fig. 2) the circuit 12-15 is replaced by a potentiometer 18 fed with A.C. or D.C. and the control mechanism 17 is connected between traversing contact 10 and two adjustable interconnected contacts 20, 21 so that backlash is introduced into the control action. A single contact may be used. Fig. 3 shows a system according to the invention in which the automatic readjustment of the unknown magnitude is stabilized by a feed-back signal derived from an A.C. energized auxiliary bridge comprising resistances 22, 23, 24, 25 and a potentiometer 26 having a contact 27 movable in response to the control mechanism 17, which is connected in series with tap 27, adjustable contact 20, and contact 10 on potentiometer 18. Variation of the unknown quantity causes deviation of traversing contact 10 from the contact 20 as the main bridge is rebalanced, and develops a voltage therebetween which operates control mechanism 17 to traverse tap 27, unbalancing the auxiliary bridge to produce a secondary voltage which is combined in opposition with the primary voltage applied to control mechanism 17 regulating the unknownquantity. Contacts 6 and 10 return to normal in response to the regulation, followed by contact 27, until the voltage between contacts 10 and 20 is again balanced by the voltage derived from the secondary bridge unbalance. When over-running occurs, switch 28 is momentarily closed by mechanism 17 and short-circuits resistance 22 to develop a momentary unbalance voltage opposite in phase or sign to the operative voltage. The secondary bridge may be unbalanced by varying an arm in response to the mechanism 17. In a further modification (Fig. 4, not shown), sensitivity is increased by combining a voltage derived from the input of the main bridge rebalancing motor with the primary and secondary control signals so that the control mechanism operates immediately on the main unbalance.