Patent GB-591093-A (GB)
Title: Improvements in and relating to electric measuring circuits
IPC: B23K11/24
Filed: 1945-05-03
Abstract: 591,093. Current and voltage measurements ; determining physical qualities of materials. BRITISH THOMSON-HOUSTON CO., Ltd. May 3, 1945, No. 11251. Convention date, May 4, 1944. [Class 37] Apparatus for use with a load circuit that is intermittently energized from a source of alternating current comprises means for obtaining a measurement of the last half-cycle value of an electrical condition that occurs in the load circuit during a period of energization thereof and means for maintaining the measurement during the immediately following period of deenergization of the load circuit. Such apparatus is particularly suited for indicating the quality of resistance spot welds formed between parts by a predetermined timed application of welding current flow through the agency of pressure exerting electrodes. In the case of ferrous metals, the voltage across the weld increases as the weld is formed and becomes substantially constant on the formation of a satisfactory weld, while in the case of non-ferrous welds the voltage across the weld decreases as the weld is formed and attains a substantially constant value when a weld of satisfactory quality has been formed. By measuring the last half-cycle value of voltage and comparing it with a predetermined value for like welds of desirable quality, the quality of the weld may be determined. In the apparatus shown in Fig. 1, closure of a switch 14 causes a welding transformer 3 to be energized one or more times from an A.C. source 5 depending on the control exerted on valves 7, 8 by a timer 13. Each period of energization lasts for a predetermined number of half-cycles ending with a half-cycle of the same polarity and the periods of energization will be separated from one another by predetermined periods of de-energization if more than one period of energization is occasioned by the timer 13. The magnitude of the current depends on the adjustment of a heat control 15. Across the welding electrodes 1 is connected the primary winding 17 of a control transformer 16, the voltage induced during the first positive halfcycle of current in the secondary 18 of which charges a capacitor 20 through a rectifier 23 and resistance 22. The charge on the capacitor 20 overcomes the negative bias voltage across a resistance 30 and renders a valve 28 conducting, thereby energizing a relay 24, which closes its contacts 25 and completes the charging circuit of the measuring capacitor 19. This is charged through a rectifier 21 by the next positive half-cycle of welding current to a voltage corresponding to the maximum voltage across the electrodes 1. At the end of the first negative half-cycle of welding current a positive voltage peak is produced in the secondary winding 53 of a transformer 54, of which the primary is formed by the electrodes 1. The secondary winding 53 is connected across a resistance 51 in the control circuit of a valve 45, and the positive voltage peak overcomes the negative bias voltage across a resistance 37 and capacitor 52 in the circuit of this valve which becomes conductive and discharges the measuring capacitor 19. A resistance 47 connected in series with a capacitor 48 across a resistance 46 in the anode circuit of the valve 45 causes the valve to return to its non-conducting condition after discharge of the capacitor 19. Thereafter the capacitor 19 is charged during positive half-cycles and discharged at the end of negative half-cycles of welding current and is left with a charge depending on the maximum voltage across the electrodes 1 at the end of a welding period, which ends with a half-cycle of positive polarity. Shortly after the welding period ends, the capacitor 20 has discharged through a resistance 34 sufficiently to reduce the current through the valve 28 so that the relay 24 opens its contacts 25. Any residual voltage of the welding circuit is therefore not impressed on the capacitor 19 when the electrodes 1 are separated from the work. The control grid 59 of a valve 58 is connected to be responsive to the charge on the capacitor 19 so that at the end of a welding period the current through this valve and a milliammeter 55 is a measure of the voltage of the capacitor 19 and is maintained during the de-energization period. The charge on the capacitor 19 also causes the terminal 69 of a cathode following circuit including the valve 58 to attain a corresponding value, which is applied to the control grid of a valve .64 and if greater than the cathode potential of this valve causes it to become conductive and thus connect a valve 61 across the secondary 65 of a transformer 38. The minimum voltage necessary to render the valve 64 conducting is determined by its cathode potential, which is adjustable by a potentiometer 36. This also determines the cathode potential of a valve 66, the grid of which is connected to the slider of a potentiometer 57 in the cathode following circuit. If the potential of the terminal 69 increases beyond a predetermined value for a given adjustment of the potentiometer 57, the valve 66 becomes conducting and shunts the valves 61, 64. The valve 61 therefore becomes illuminated when the voltage across the capacitor 19 is between predetermined limits and serves as a signal that a satisfactory weld has been made. In a modification, Fig. 2, the charge on the measuring capacitor 71, corresponding to the capacitor 19, Fig. 1, is applied through a resistance 72 to the control grid 73 of a valve 74, the circuit of which includes a milliammeter 75. The capacitor 71 is connected through a rectifier 86 to one of the secondary windings 87 of a control transformer 88 whose primary is connected across the welding electrodes 1. A loading circuit comprising a resistance 91 and a reversely connected rectifier 90 prevents D.C. saturation of the transformer 88. The discharge circuit of the capacitor 71 is formed by a resistance 94 in series with a gas-filled valve 93, the excitation of the control element 95 of which depends upon a component of voltage derived from another secondary 96 of the transformer 88 and a component derived from the secondary 97 of another control transformer 98. These control voltages are introduced into two parallel branches 101, 96 and 100, 97, 81 of the excitation circuit of the valve 93, one branch being polarized by a rectifier 100. The primary 99 of the transformer 98 is connected to a phaseshift network 102 through a variable resistance 103 and a series resonant circuit 104, 105 tuned to the frequency of the supply circuit 5. The output voltage of the transformer 98 is of peaked waveform and is adjusted in phase by means of the network 102 so that it occurs after current ceases to flow in the welding circuit as a result of a half-cycle of voltage corresponding in polarity to the last half-cycle of a welding period. In those circumstances, so long as the voltage from the secondary 96 is greater than the bias voltage across the resistance 80 and so long as the peak voltage wave derived from the secondary 97 is greater than the bias voltages across the resistances 80, 81, the valve 93 is rendered conductive and discharges the capacitor 71. Specification 591,052, [Group XXXV], and U.S.A. Specification 2,264,067 are referred to.