Patent GB-750438-A (GB)
Title: Improvements in or relating to measuring and/or control installation for electric welding machines
IPC: B23K11/24
Filed: 1953-08-26
Abstract: 750,438. Control systems for resistance weld- Ing. SOC. ANON. ELECTROMECANIQUE. Aug. 26, 1953 [Sept. 2, 1952], No. 23638/53. Class 38 (4). [Also in Group XXXVI] A resistance welding machine comprises two electrode carrying arms having located in the field of the welding current I and laterally of one of such arms a saturable closed magnetic core wound so as to produce substantially zero electromagnetic field with two windings excited by an electric oscillator of frequency greater than that of the welding current and arranged so that the magnetic circuit of the induction flux of the welding current is substantially in air, a load resistance in series with the windings and the oscillator and a rectifier energized from the voltage appearing across the resistance; the values of the resistance and the oscillator output voltage and the characteristics of the wound magnetic core material being chosen so that the mean rectified voltage is a quadratic function of the welding current which is applicable (a) to an integrator with respect to time t and a memory circuit operating measuring apparatus calibrated in I<SP>2</SP>t and I#t, (b) to a trigger and memory circuit operating measuring apparatus calibrated in t and #t, (c) through a regulable resistance and cathode follower control circuit to regulate the saturation winding of a variable inductor controlling the phase shift of the instant of ignition of ignitrons controlling the welding feed circuit. In a resistance welding machine wherein the electrodes are applied to the work by opposed conducting arms (Fig. 13, not shown) the effective value of the welding current is controlled and measured in response to the time integral of the squared intensity of such current by apparatus shown schematically in Fig. 1 wherein an oscillator 2 operating at a frequency exceeding that of the welding current comprises two triodes 21, 22 (Fig. 4) superposed in series across a H.T. supply, the grid of triode 21 being coupled over a phaseshifting R.C. network 23 to the cathode circuit of an output cathode follower triode 24 directly excited from the anode of triode 22, whose grid receives an output stabilizing bias rectified from a portion of the cathode follower output, which is capacitance coupled to a load resistance 210 through a transductor 1 comprising opposed windings 211, 212 (Fig. 2) on a closed saturable sheet-metal magnetic core placed laterally of an electrode arm and in the magnetic field of the welding current; the working point of the transductor being in the saturation zone and the load resistance being chosen to give a quadratic response so that the mean voltage output of rectifier 3 comprising diode 31 and threshold adjusting resistance 32 is proportional to the square of the welding current. The transductor may be wound (Fig. 3, not shown) on two separate cores each adjacent to one electrode arm. This voltage is connected to an integrator 4 (Fig. 5) wherein triodes 41, 42 are superposed in series across the H.T. supply, the input to triode 41 being over a series grid resistance while the grid of triode 42 is held at a steady potentiometer voltage and the anode is directly coupled to the grid cathode follower 43 whose load resistor develops the output voltage which is also fed back to the grid of triode 41 over a switchable integrating capacitance 44. An alternative form of integrator is described (Fig. 6, not shown). The output proportional to f i2dt, where i is the welding current, is connected to a memory circuit 5 (Fig. 8) wherein capacitance 50 is charged over diode 51 to the input voltage, which it holds over a predetermined time for a reading or until discharged by closure of switch 53; the voltage being measured by a valve voltmeter 6 comprising cathode follower triodes 52, 62 with a milliammeter 60 indicating effective welding current and having scales graduated in i<SP>2</SP>t and i#t across the cathodes thereof. The memory circuit output is connected to the grid of one triode, that of the other being connected to the slider of a zero-set potentiometer 64 across the H.T. supply. For initial adjustment by means of threshold resistance 32 (Fig. 4) changeover switch 63 supplies a fixed voltage to the grid of triode 62 from the potentiometer, while changeover switch 65 disconnects the milliammeter from the cathode circuit of triode 52 and connects it over a series resistance direct to the integrator output. The output of the rectifier 3 is also amplified at 7 by series superposed triodes 70, 71 (Fig. 9), directly coupled to a trigger circuit 8 comprising triode 80 directly coupled to triode 81 whose cathode load output is positively fed back to the cathode of triode 80. Initially the circuit is in equilibrium with a low voltage at the output but on arrival of a rectified i2 signal at the beginning of a weld a square pulse is applied to the memory circuit 9 (Fig. 7) for the duration thereof, wherein the pulse charges capacitor 90 over a selected input resistance and diode 91, the capacitor being dischargeable by closure of switch 93 operated by relay 123 (Fig. 11) and its memory voltage being measurable by valve volt-meter 10 wherein triode 92 receiving the memory voltage on its grid has a cathode load resistance, and a measuring instrument 100 graduated in t and #t is connected between cathode and a zero adjustment potentiometer across the H.T. supply to give an indication proportional to the welding time. For automatically controlling the energy dissipated in each weld a trigger circuit 11 (Fig. 11) comprising triodes 110, 111, operating as described above, is fed from the integrator output to turn over release relay 120, in unit 12, and open contact 121 when the integrated input signal exceeds a predetermined threshold level. Relay 123 is operated over contact 124 which closes when the welding electrodes close upon the work to open a switch 93 (Fig. 7) normally shunting the integrator capacitance. Relay 130 in unit 13 is energizable over contacts 121, 124 to close the welding current switch contact 131. Initially on pressure of electrodes upon the work, switch 124 closes, contactor 131 closes, and welding commences. The integrator capacitance is simultaneously open-circuited by switch 93, and at the end of a time determined by the preset value of this capacitance the voltage representing f i<SP>2</SP>dt has reached a level sufficient to turn over the trigger circuit, release relay 120, open contact 121, and open welding contactor 131. Withdrawal of the electrodes opens switch 124, releases relay 123, and short-circuits the integrator capacitance over switch 93 to prepare for the next weld. The effective welding current is controlled by devices 14, 15 (Fig. 12) which operate by tapping off a fraction of the output of rectifier 3 over a series variable resistance 145 to the grid of triode 140 which is shunted to earth over capacitance 143, the anode and cathode being paralleled with corresponding electrodes of a similar triode 141 whose grid is earthed over a variable resistance 146 in shunt with a capacitance 144. The grid of triode 141 is connectible to the common anodes through a series resistance on closure of switch 147 by deenergization of relay 123 (Fig. 11) and the cathodes are earthed through the control winding 142 of a saturable inductance (not shown) controlling the phase shift of the instant of ignition of ignitrons regulating the fraction of each cycle during which the supply current is supplied to the welding electrodes. When the latter engage the work, relay 123 is energized, switch 147 opens, and capacitance 144 slowly discharges over resistance 146, whereby grid potential of triode 141 and current through winding 142 decreases to allow welding current to pass for a greater proportion of each supply cycle. At the end of the pre-heating period for the weld (corresponding to the values of 144, 146) the voltages across capacitors 144, 143 reach equality and there is no further change in the current through winding 142, so that the effective welding current is maintained at a constant level set by the input to resistance 145 for the remainder of the welding time.