Patent GB-714261-A (GB)
Title: Improvements relating to graph plotting apparatus
Inventor: HINDERWELL FRANCIS BESWICK
IPC: G08B5/22
Filed: 1951-08-01
Abstract: 714,261. Electric indicating systems. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. July 31, 1952 [Aug. 1, 1951], No. 18233/51. Addition to 688,549. Class 40 (1). Apparatus of the kind described in the parent Specification for plotting a graph relating two independent variables, comprises for each a continuously self balancing unit including an electric circuit for comparing an electrical quantity representing the respective variable with a reference electrical quantity end comprising an unbalance circuit responsive to the comparison difference for continuously restoring the balance by adjustments measuring the variable together with a chart table traversed in one direction by the balancing adjustments measuring one variable and a stylus traversed at right angles thereto by the adjustments measuring the other variable. Figs. 1, 2 show a graph plotting apparatus wherein a chart table T mounted on rollers R and rails L is moved along the X axis by cords W1 passing round a 'drum D1 driven through gearing by a servo-motor M1 also driving a variable circuit element Bl connected in a continuously self-balancing circuit wherein unbalance signals derived from comparison of a reference electrical quantity and an electrical quantity representing a first unknown are amplified, and drive the servo-motor to readjust element B for balance and deflect the chart proportionately to the value of the unknown. A recording device P, which may be a pen, a stylus co-operating with waxed paper, or (Provisional Specification) a pencil of light coacting with photosensitive paper, is moved over the chart along the Y axis by cords W2 passing round a drum D2 driven through gearing by a servo-motor M2 also driving a variable circuit element of a second continuously self-balancing circuit operating to control the motor and deflect the recording device in response to an electrical quantity representing a second unknown quantity as above, whereby a graph of the second quantity against the first is traced. A roller chart C rotated at constant speed by a. motor M3 under a recording device P<SP>1</SP> movable by cords W3 surrounding a drum D3 driven by motor M1 (or M2) traces the graph of either quantity against time. Fig. 3 shows a suitable self-balancing servomotor control circuit wherein a slidewire potentiometer B is energised through range determining resistances R3, R4, from a resistance Re across a battery E. An unknown voltage to be measured (e.g. from a thermocouple) is connected to the input terminals and compared with the voltage derived from the potentiometer slider S, the unbalance voltage being converted to A.C. and amplified at AMP 1 to energise one winding of a 2-phase reversible A.C. rebalancing motor M, rebalancing the potentiometer and deflecting the recording chart or recording stylus; the reference phase being derived from the A.C. supply to the converter and amplifier through a phase displacement condenser C. An A.C. motor M<SP>1</SP> drives fast and slow cam operated switches SW1, SW2 to energise the coil OC of an electromagnetic clutch at intervals to mechanically couple the. motor M to adjust the variable resistance Re; switch SW2 being simultaneously operated to disconnect the input voltage and connect a standard cell SC for comparison with the voltage at the junction of resistances R1, R2 across the potentiometer supply. Resistance Re is thus readjusted to restore this voltage to the value determined by the standard cell. Adjustment may be initiated at any time by closing switch SW4 across SW1, SW2. Resistances R1, R2 may be temperature sensitive to give cold junction temperature correction. Fig. 4 shows a modified self-balancing circuit wherein an A.C. energised Wheatstone bridge comprises a slidewire potentiometer B in series with resistances R3, R4, a fixed resistance R5, and a resistance R6 variable by an unknown quantity (e.g. a straingauge, resistance thermometer, or resistance variable by linear displacement.) The unbalance signal energises one winding of a 2-phase reversible motor M through amplifier AMP 2 as above, and the motor drives the potentiometer to restore balance, and deflects the chart or recording stylus according to the value of the unknown. The variable resistance R6 has a 3-wire connection to the bridge to reduce errors due to variations of lead resistance, and the bridge is energised through resistance R7 which controls its sensitivity. The bridge may be D.C. energised and the unbalance voltage converted to A.C. for application to the amplifier.