Patent GB-714985-A (GB)
Title: Improvements in or relating to apparatus for the measurement of the effective valuesof secondary currents in electric resistance welding machines
IPC: G01R15/00
Filed: 1952-09-08
Abstract: 714,985. Current measuring. ELECTROMECANIQUE SOC. ANON. Sept. 8, 1952, [Sept. 12, 1951.] No. 22566/52. Class 37. Apparatus for measuring the integral value with respect to time of the square of the welding current in the secondary of a resistance welding machine comprises a welding current detector producing a voltage proportional to the welding current at any instant which is applied in succession to a squaring circuit and an integrating circuit to produce a voltage proportional to the instantaneous value of the integral of the squared welding current; such voltage being stored in a memorizing circuit over successive welds to which is connected an instrument for determining the resultant stored voltage. In Fig.1 arms 2, 3, support electrodes 4, 5, of an electric welding machine which embrace the work 7, 8. The arm 3 is surrounded by a complex magnetic current detecting and amplifying device 6, 9, comprising (Fig. 2) two saturable magnetic cores 16, 17 oppositely wound with coils 18, 19 connected in series with a sinusoidal A.C. source 20 of higher frequency than the welding current and the input of a bridge rectifier 2I. The output of the latter energizes a load resistance 23, in series with a tapped range adjusting rheostat 24 and a degenerative feedback winding 22 embracing both magnetic cores and generating a flux therein opposing that of the welding current. The voltage across resistance 23 is connected through a changeover switch 38 across the opposed grids of identical nonlinear triodes 29, 30 of a squaring circuit 10, whose grid resistances 27, 28 are returned to earth and whose anodes are connected to a stabilized common H.T. supply through resistance 37, while the cathodes are returned to an adjustable point of an H.T. potentiometer 35, 36, 45, 46 whereby the cathode voltage is adjustable for calibration when the switch 38 is reversed to apply a constant voltage 40 between the grids. The squared output from the common anodes is directly connected through resistance 48 to an integrator circuit I2 wherein the anodes of identical triodes 42, 43 are connected to a stabilized H.T. source through a common resistance 47, the cathodes are returned to the junction of resistances 44, 45 of the potentiometer 35, 36, 44, 45, 46, and the input is applied to the grid of triode 42 while condensers 49 to 52 are selectively connectible between the anode and grid of triode 43 by switch 53. The grid of triode 43 is.returned to the junction of potentiometer resistances 45, 46 through leak resistor 77, which is in parallel with a discharging switch 75 in series with resistance 76. From the anodes of triodes 42, 43 the integrated signal is directly coupled through resistance 58 to a memory circuit I3 comprising a cathode follower triode 56 whose cathode output is connected to charge a reservoir condenser 55 through a diode connected triode 57, and the accumulated condenser voltage is directly coupled through resistance 64 to a differential amplifier I4 comprising cathode follower triodes 6o, 6I, the input being applied to the grid of triode 6o and an adjustable balancing signal from an H.T. potentiometer 67, 68, 69 being applied to the grid of triode 6I. An internal measuring instrument or an external instrument connected to output terminals, shown at I5, are alternatively connectible between the cathodes of triodes 60,61 by switch 71, and indicate the required squared integrated value of effective welding current. A switch 66 in series with resistance 65 across the memory condenser 55 is ganged with switch 75, and the switches are closed after each measurement to discharge the integrating and memory condensers, and the differential amplifier 14 may then be re-adjusted for correct zero of the measuring instrument. Other forms of squaring, integrating, memory, and differential amplifier circuits are described and an alternative current detecting device is stated to comprise a manganin ring surrounding an element of the secondary welding circuit; the P.D. between opposite faces of the ring being connected to the inputs of the squaring circuit 10.