Patent GB-840373-A (GB)
Title: Improvements relating to electrical indicating and alarm systems
Inventor: REDDING ROBERT JAMES
IPC: G01R17/08
Filed: 1956-02-23
Abstract: 840,373. Electric analogue computing; voltage measuring; potentiometers. EVER SHED & VIGNOLES Ltd. Feb. 19, 1957 [Feb. 23, 1956], No. 5667/56. Class 37. An electrical system indicating the change of a physical quantity during successive equal time intervals including two storage devices responsive to a current representing the physical quantity and to a comparison current thereof; the comparison current in one such device being held constant at a value corresponding to the magnitude of the quantity at the beginning of a time interval and the comparison current in the other being adjusted to vary continuously with the magnitude of the quantity during such time interval; an indicator continuously measuring the difference between the two currents, and a device for cyclically interchanging the functions of the two storage devices at the beginning of each successive time interval. A signal current representing e.g. liquid pressure energizes over lines 9, 10 (Fig. 1) the series coils 6a, 6b of storage devices 1, 2 comprising spring-controlled lever arms 4a, 4b respectively carrying such coils together with oppositely sensed balancing coils 7a, 7b in fields of pot magnets 3a, 3b. Contacts 15a, 15b on the arms vibrate between pairs of positive and negative contacts 16a, 17a; 16b, 17b, and for equality of currents in a corresponding pair of coils, the contact is floating, but at unbalance, one or other fixed contact is engaged with the moving contact in the operative storage device, selected by cam 35 and switch 19a, 19b. A large capacitance 23a or 23b is thus positively or negatively charged in series with a resistance between the grid and cathode of triode 20a or 20b and as shown capacitance 23a is charged to a level such that the anode current in 20a and coil 7a balances the instant value of the input current, after which cam 35 opens 19a and closes 19b for a fixed time interval (e.g. 30 seconds) during which the charge on 23a and anode current of 20a are held at the value corresponding to the input current just preceding change-over. Similarly capacitance 23b is charged to a level at which the anode current in 20b and coil 7b balances the instant value of input current, after which 19b opens and 19a closes so that the charge on 23b and anode current in 20b are held to values representing the alternate input current just before changeover. The respective anode currents also flow in alternate coils of a differential moving-coil instrument 25 whose pointer carries a contact movable between carrier contacts 28, 29 to operate an alarm 31 on closure to either. At each instant of changeover, the currents in the triodes and balance coils are equal, but over each elapsed time interval the deflection of pointer 26 represents the average rate of change of input over the elapsed time, and operates the alarm on exceeding a predetermined value. Contacts 28, 29 may operate separate alarms. The coil current of instrument 25 may flow (Fig. 2) in oppositely sensed coils 40, 41 respectively carried by spring-loaded arm 42 together with rebalancing coil 50 in the field of pot magnet 45; the arm carrying a contact 46 oscillatory between positive and negative contacts 47, 48 and connected over normally open switch 51 briefly closed by cam 35 immediately before the rebalancing of storage units 1, 2 to the grid of triode 54 connected to its cathode over capacitance 53 in series with resistance 52. Anode current energizes coil 50 to rebalance the deflection of arm 42 in response to the difference in energization of coils 40, 41 so that the balance current represents the rate of change of the input signal current and deflects indicator 56 and recorder 57. Cathode followers may be interposed between capacitances 23a, 23b and the triodes 20a, 20b to reduce the effective shunt resistance values and the Provisional Specification states that two co-operating pairs of storage units may be provided; each pair being rebalanced at intervals midway between rebalance of the other pair. Specifications 806,514 and 806,515 are referred to.