Patent GB-613905-A (GB)
Title: Improvements in and relating to electrical timing apparatus
IPC: B23K11/24
Filed: 1943-11-04
Abstract: 613,905. Control systems for consumption apparatus. BRITISH THOMSON-HOUSTON CO., Ltd. (General Electric Co.). Nov. 4, 1943, No. 18291. [Class 38 (iv)] To reduce the number of timing units in an electrical system for timing a number of sequential operations, as, for instance, occur in resistance welding, one or more of the timing units control a plurality of operations with different time settings. As shown, a welding machine 10 comprises electrodes 11 moved into and out of engagement with the work 15 by a fluidoperated device 16 controlled by a solenoid-operated valve 17, the electrodes being energized through a transformer 13. The machine is controlled by timing units TD1- TR3 and associated control relays CR1-CR3 so that it performs the sequence of timed operations indicated at the foot of Fig. 2, and continues to repeat the sequence as long as the push-button switch S1 is closed. The " squeeze " time is that allowed for the electrodes to be moved into contact with the work, the "hold " time determines the period that the work is under pressure from the electrodes after the welding current has ceased to flow, and the " off " period is that allowed between consecutive sequencies when performing repeated operations. The "weld " time itself comprises three heating periods alternating with two cooling periods. Timing unit TD1 controls the " off " and " heating times, as indicated by the hatched areas associated with the line pertaining to this unit in Fig. 2, unit TD2 determines the " squeeze," " cool " and " hold " periods, whilst the " weld " time is under the control of unit TD3. The periods of energization of timing units and control relays are indicated in Fig. 2 by a rise in level of their identification lines. The three timing units are supplied from A.C. rails 24, 25 and comprise, in the case of TD3, a relay 37 arranged in the anode circuit of a grid-controlled space discharge device 31, means (e.g. contacts 47 of relay CR2) being provided for the opening or closing of the cathode connection to supply rail 25, and a timing condenser 39 shunted by a discharge resistor 48 being connected to the control grid 87 and to an adjustable tapping 39 on a potentiometer 40-42 across the supply rails. A switch 49 may short-circuit a portion of resistor 48 to modify the discharge time of condenser 38. With contacts 47 open condenser 38 charges up negatively through resistors 41, 40, 43 and 44 and by control grid-cathode rectification, to a voltage dependent on the setting of 39. This voltage is applied as bias to the control grid. If now the contacts 47 are closed the anodecathode circuit of device 31 is completed, but the flow of anode current is delayed until such time as condenser 38 has discharged through 48 to a voltage sufficiently low to allow the device to conduct, since high resistor 42 now replaces 41 in the charging circuit. Relay 37 then picks up. Relay 35 of unit TD1 is similarly controlled by a space discharge device 29 but control relay CR3 has contacts 50, 51 for changing over the connections of timing condenser 52 from resistor 55 to resistor 58, whereby two timing periods, each adjustable, are provided. Unit TD2 operates similarly, but has charging resistors 71, 75 and 77, selectively brought into circuit by contacts 65, 66 of CR2 and contacts 67, 68 of relay 37, to provide three adjustable timing periods. The sequence of timing operations is effected as follows : Closure of line switches 27 energize supply rails 24, 25 and relay 35 of TD1 immediately picks up as timing condenser 52 is uncharged and the anode-cathode circuit of discharge device 29 is completed over contacts 103 of CR3. Contacts 97 of relay 35 thus close and a circuit for energizing CR3 is prepared which is completed by closure of push-button S1 when the welding operation is to be initiated. Closure of contacts 97 leaves the anode circuit of discharge .device 30 of TD2 open only at the pushbutton, so that when the button is pressed CR3 picks up and TD2 starts to time the " squeeze period, CR3 energizing the valveoperating coil 18 of the machine over contacts 99. Pick up of CR3 interrupts, at contacts 103, the anode circuit of TD1 and changes over its changing resistors at contacts 50, 51. TD1 thus falls out but the energizing circuit of CR3 is not interrupted by the opening of contacts 97 as contacts 100 of CR3 in parallel therewith are now closed. At the end of the " squeeze " period discharge device 30 of TD2 conducts, relay 36 picks up and, over its contacts 104, establishes and energizing circuit for CR1 (including conductors 111, 110, 109, 117, 116, 115 and contacts S1, 100, 106, 104, 102 and 95) which then picks up, closing the primary welding circuit at 92, energizing CR2 over contacts 93 and disconnecting TD2 at 94. Closure of contacts 104 again brings TD1 into operation to time the first heating period, whilst CR2 on picking up establishes its own maintaining circuit over contacts 107 and starts TD3 timing by the closure of contacts 47. After TD1 has timed the first welding operation its device 29 conducts, relay 35 picks up to open its contacts 95 which deenergize CR1, thereby stopping the flow of welding current. TD2 consequently starts timing the cooling period, but with a delay different from that of the heating period as the contacts 65, 66 have been moved from their normal positions by energization of CR2. When device 30 of TD2 conducts CR1 again picks up to establish another heating period timed by TD1, which becomes energized upon pick up of relay 36 and consequent closure of contacts 104. TD1 and TD2 continue to operate successively to provide alternate heating and cooling periods until their action is arrested by discharge device 31 of TD3 conducting and thereby opening contacts 106 of its relay 37, which, concurrent with the opening of contacts 105 of TD2, de-energize CR2 and CR3 and causes CR1 finally to drop out. Relay 37 connects, at contacts 68 a further timing circuit to TD2 which, through the closing of 94, times the " hold " period at the end of which relay 36 picks up to energize TD1 for timing the " off period. The sequence of operations will be repeated if S1 is maintained closed; closure of a switch 22, however, permits continuous operation independent of S1  after the latter has initially been depressed to operate CR3. A switch 23 is also provided to shunt the contacts 97 of TD1, whereby the " squeeze " period may be eliminated.