CA1246690A - Resistance welder - Google Patents
Resistance welderInfo
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- CA1246690A CA1246690A CA000477445A CA477445A CA1246690A CA 1246690 A CA1246690 A CA 1246690A CA 000477445 A CA000477445 A CA 000477445A CA 477445 A CA477445 A CA 477445A CA 1246690 A CA1246690 A CA 1246690A
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Abstract
Abstract of the Disclosure An electrical circuit for a resistance welder comprises 2 rectifier connected to an AC source, typically of 60 Hz. If the source is a three-phase source it is convenient to apply the source voltage to a full-wave bridge rectifier to produce an output voltage that has both a DC and an AC component. This output voltage is applied to a controlled thyristor inverter that transforms it into a wave that is substantially rectangular. An electronic circuit controls the thyristor inverter to determine the length of time it operates, the relative pulse width of the rectangular wave, and other desired features of the inverted voltage. The output of the inverter is applied to a step-down transformer which has a center-tapped secondary. The secondary is connected through a full-wave rectifier to welding contacts that supply welding current and the necessary force to a workpiece to accomplish resistance welding.
Description
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RESISTANCE WELDER
Background of the Invention This invention relates to resistance welding In particular t it relates to improvements in resistance 5 welding that are appropriate for use wi~h robot welders and automatic and press welders.
Resistance weldin~ is a well-known way to join together two electrical conductors. It comprises passing an elec~rical current through the conductors in an amount 10 sufficient to cause localized heating that melts the conductors, joining them together. This is normally accomplished by placing a pair of electrodes against the joined electrical conductors, applying pressure and an electrical voltage to the electrodes, and timing the I2R
I5 heating to an amount that is sufficient to weld the materials without creating excessive melting. This can be accomplished either by a DC or AC voltage in most applications. However, because of the typical junction F
; resistance between two elec~rical conductors that are to 20 be 30ined, it is normally desirable to reduce the voltage below the value of typical line voltages by a step-down transformer to apply voltages of the order of a few volts or tenths of a volt and currents of the order of thousands or tens of tho~sands of amperes. The simplest such 25 arrangement comprises a step-down transformer, a switch to control the application of an electrical voltage to the step-~own transformer, and a pair of electrodes connected electrically to the secondary winding of ~he step-down transformer~. When the electrod;es are placed on opposite 30 sides of the workpieces to be j~ined, closing the switch applies a voltage across ~he junction ~Qf the electrical conductors and the resultant electrical heating melts the spot under t~e electrodes to weld the electrical conductocs toget~e ,~
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., ~, Practical considerations of actual welding operations lead to ~he addition of Yarious refinements to the process described above. In order to minimize the cost of ~ransformers used to supply welding currents, it is desirable to insure that the peak voltage applied to the transformer from an electrical source places the c~re of the transformer in saturation, at or beyond the knee of the B-H curve of the core. Because of this fact, it is necessary to insure that the state of the core of the transformer is known whenever a voltage i5 applied to the transformer. If the last voltage that was applied to the transformer leaves the transformer magnetized in a particular direction and the next applied voltage causes current flow in the same direction, then the magnetizing current required by the transformer, together with the load current, may overload the transformer during the first cycle. This is an undesirable situation that is readily avoided by making certain that the control circuit for the welding transformer always applies full cycles of the input voltage to accomplish welding and that it always begins the application of voltage on that portion of the input voltage that ~oes in the same direction. This a~sures that the first cycle of applied voltage always encircles the oriyin ~f the hysteresis loop of the transfo`rmer core, avoiding a current that is far into : saturation, and also assuring that the peak voltage is constant during each welding cycle. It then remains only : to apply the welding voltage for ~ predete~mined number of cycles of the input voltage to accomplish a weld. The predetermined number of cycles is determined by experiPnce but is typically a number that is s~all enough tha~ it mus~ be con~rolled electronically bec~use the neces,~ary time peri~d of application is too s,hor~ to be controlled reliaSly by an operator.
The basic resistance welding system described above '~L24~i~i9~ -has serious disadvantages when it is applied to resistance welding in produc~ion lines. A production weld between two pieces of sheet steel, whether or not they are galvanized, typically reauires a current of the order of 10,000 to 30,0~0 amperes. A
transformer such as in EP-Al-64 750, EP-Al-l9 747 or BE-A-759 605, this is wound to supply such currents ln the secondary will typically weigh of the order of 200 to 600 pounds and will need to ~e cooled by water or other externa] cooling means. Electrical leads to carry such curren~ts are substantial in size. The usual means to handle such problems as these in production lines in the automotive and other industries is to suspend transformers from an overhead support, to run insulated conductors to a welding head that includes watercooled electrodes, and to have an operator place the electrodes at the spot to be welded and apply the external Eorce to hold the e]ectrodes in place while the weld is made. The system described above presents a number of problems when it is converted for use with robot or automatic welders.
Robots are generally limited in the amount of weight that they can handle, and their operation is normally improved by reducing the amount o~ that weight. Automatic welders are limited in the closeness with which they can make adjacent welds, by the size of their transformers. Robot welders are also hampered qreatly in operation by being connected to large electrical cables that are designed~to handle welding currents of thousands of amperes, and the mobility of robots shortens the useful life of such large cables.
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Accordin~ to one aspect oF the present invention, there is provided an electrical circuit for a resistance welder that reduces the weight in the vicinity of the weld.
According to a further aspect of the present invention, there is provided an electrical circuit for a resistance welder that is adaptable for use with a robot welding system.
According to a further aspect of the present invention, there is provided a small, lightweight weldinq transformer which, because of its size and weight, can be mounted close to the weld zone, allowing the use of lightweight primary leads and short secondary leadsO
According to a stil] further aspect o-E the present Invention, there is provided an~electrical circuit Eor a resistance welder that is adaptable Eor use with an automatic welding system.
According to yet another aspect of the present invention, there ~is provided a method of welding an electrically c~nducting workpiece by resistance weldinq comprising the steps of rectifylng an AC voltage~at a line frequency to produce a rectified voltage, invertlng~the rectified voltage at~an operating frequency that is higher than the line frequency to produce an AC voltage at the operating frequency, transforming with a transformer, the AC
voltage at the operating freguency to a lower voltaqe to produce a step-d~own voltage, characteri2ed by applying the step-down voltage from~a~center~tap of the transformer through welding contacts to the~workpiece~.
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Accordinq to a still further aspect of the present invention, there is provided an apparatus for weldinq an electrically conducting workpiece by resistance welding comprising means -Eor rectiEyinq an AC voltage at a line frequency to produce a rectified voltage, means for invertinq the rectified voltage at an operatinq frequency that is higher than the line frequency to produce an AC voltage at the operating frequency, means for transEorminq the AC voltage at the operating frequency to a lower voltage to produce a step-down voltage characterized by means for applying the step-do~7n voltaqe from a center tap of the transformer means through welding contacts to the workpiece.
According to yet another aspect of the present invention, there is provided An elec~rical circuit ~or welding an electrically conductive workpiece by resistance weldin~, to be connected between an AC voltage source at a power Erequency and a pair oE welding electrodes, the ciruit comprising a rectifier connected to the AC voltage source to produce a rectified voltage, means connected to the rectiEier for invertinq the rectified voltage of the rectifier at an operating frequency that is higher :than the -Erequency of the AC voltaqe source to produce an AC
voltage at the operating frequency~ a step-down transformer connected to the means for invertin~ the rectified voltaqe to produce~at a pair of secondary term1nals an output voltage at the operat~ing Er~quency that is lower in amplitude than the voltage produced~by the~means for inv~ertlhg the rectified voltage, ea~ch of the s~econdary terminals of the step-down transformer also - : :
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connected to one of the pair of weld;na electrodes characterized in that said step-down transEormer has a center tap connected to the other of the pair of welding electrodes.
According to a still further aspect oE the present inven-tion, there is provided an electrical circuit for welding an electrically conductive workpiece by resistance welding, be connected between an AC voltage source at a power frequency and a pair of weldina electrodes, the circuit comprising a rectiEier connected to the AC voltage source to produce a rectified voltage, means connected to the rectifier for inverting the rectified voltage of the rectifier at an operating frequency that is higher than the frequency of the AC voltage source to produce an AC
voltage at the operating frequency, a step-down transformer connected to the means for inverting the rectiEied voltage to produce at a pair of secondary terminals an output voltage at the operating fre~uency that is lower in amplitude than the voltage produced~by the means for inverting the rectified voltage, each of the secondary terminals of the step-down transformer also connected to one of the pair of welding electrodes, characterized by the step-down transformer having a center tap, and a full-wave ~rectifier being connected to the secondàry terminals of the ~step-down transformer, to the center tap, and to the welding electrodes to rectify the output voltage of the step-down transformer and apply a rectified output voltage to the welding electrodes.
Brief Description of the Drawings Fiqure 1 1s~a;~block~diagram ~of~a clrcu~it for the practice -of~the ~present invention. ~ ~ ~
Fiqure 2 1s a~more detalled hlock diagram~of th~e circuit for~the~practlce of the present invent~ion.
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Fig. 3 is a circuit diagram that provides more detail about the operation of inverting uni~s 44 and 46 of Fig. 2.
Fig. 4 is a detailed circuit diagram of the circuit for timer 48 of Fig. 2.
~ig. 5 is a detailed circuit diagram of drive circuit 52 of Fig. 2.
Fig. 6 is a cutaway perspective view of a step-down transformer that has been built and used for the practice of the present invention.
Fig. 7 is a set of time plots of vol~acle wave forms in the circuit of Fig. 4.
Detailed Description of the Invention Fig. 1 is a block diagram of a circuit for the practice of the~present invention. In Fig. 1 a source I0 o AC voltage is connected to a rectifier 12. The connection is shown here as being made~with three leads which is most likely when source 10 i5 a source of~
three-phase AC voltage, However, electrical energy of any number of phases co~ld be used. ~ectifier 12 is connected to produce as an output a rectified voltage which is appropriate~ly~described as DC voltage wlth an AC
component. ~The output of rectifier 12 is taken~through cont`ro~l circui~t 14,~ which~is a~controlled~`inverter, to transormer~ 16. Control circ~uit 14 is used~to convert the output~o~;~rect;lfier 12 lnto an~AC vo~ltage at a~frequency tha~t is~higher~than the inpu~t frequency and with an RMS~
~Yalue that is controlla~le.~ ~ ~
Trans~forme~r: 16: is~shown dotted~here because lt may be useEul~to~change the~voltage that i~ ~upplied to leads 18 and:~ 19~as~an input tQ st~p-down transormer 20 However, it shoul~ be understood that under svlme circumstances it might be desirable to connect lea~s 18 : ~: : :
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and 19 directly to control circuit 14. This is a ma~ter o~ design choice.
St~p-down transformer 20 has a secondary that is centertapped. The secondary leads of step-down transformer 20 are connected to rectifiers 22 and 24 to form a full-wave rectifier. A common connection from rectifiers 22 and 24 is taken to welding electrode 26.
Center tap 2~ of step-down transformer 20 is connected to welding electrode 30. When welding electrodes 26 and 30 are placed on opposite sides of a workpiece to be welded and control circuit 14 is operated to supply current through welding electrodes 26 and 30, a resistance weld may be effected between the pieces thus joined. When the frequency of the output voltage of control circuit 14 is higher than the frequency of source 10, then step-down transformer 20 can be made smaller and lighter than it could otherwise be. This facilitates its use by human operators and it also makes possible the placing of the transformer in the arm of a robot welder, thus freeing the arm for a wider range of motions. In addition, the smaller transformer makes, it possible to make welds closer together in automatic welding machines.
Fig. 2 is a more detailed block diagram of the circuit for ~he practice of the present invention. In Fi~. 2 t rectifier 12 is connected to source lO to supply olta~e between bus leads 4Q and 42. Inverting uni~s 44 and 46 are~connected between bus leads 40 and 42 and their midpoints are~connected to leads 18 and 19 thence to step-down trans~ormer 20. ~n the block diagram and circuit of Fig~ 2~ transformer 16 of Fiy. l has been omitted. As st~ted, this is a matter of design choice.
The b~lance of the circui~ in Fig. 2 includes rectifiers ;~ 22 and 24 that are connected to the secondary willding of step-down transformer 2Q to produce a full-wave-lrectified output through ~elding electrodes 26 and 30, through a ~2~
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workpiece that is not shown, and back to cen~er tap 28.
Control of the circuit of Fig. 2 is initiated in timer 48, which both sets the frequency of operation of in~erting units 44 and 46 and also controls relative timing to control the amplitude of the current flow through welding electrodes 26 and 30. A c~rrent-sensing element 50 is connected to supply an input to timer 48 to cut off operation of inverting units 44 and 46 if current in bus lead 40 e~ceeds a predetermined value.
Timer 48 prod~ces two outputs that are taken to drive circuit 52, which is connected to drive power transistors or other switching elements in inverting units 44 and 46 so that current flows during a first half cycle through the upper portion of inverting units 44, through lead 18 and the primary of step-down transformer 20, through lead 19 to the lower half of invertin~ unit 46, thence to bus lead 42. The second half cycle sees current flow thr~ugh the upper half of inverting unit 46, through lead 19 to the primary of step-down transformer ~l but flo~ing in the opposite direction. Current continues to flow through lead 19 to inverting unit 44 where it ~lows through the bottom half of inverting unit 44 to bus lead 42. Details of this control will ~ecome apparent ~rom examining more detailed circuit diagrams. Acceptable ; 25 s~itching elements for inverting unit 44 incIude thyristors, SCRs, gate-t~rnof~ devices, and the like.
Fig. 3 is a circuit diagram that provides more detail about the operotion of invertin~ units ~4 and 46 of Fig~ 2. In Fig. 3, AC power from source 10 is takeR
~ 30 through three fuses 60, three Iimiting resistors 62 and :~ :three co~acts 64 to rectifier 12. Source 10 is typically a three-phase source at a freq~ency of 60 Hz. ~nd a convenient power voltage ~uch as 430 volts or the like.
Contacts 64 are here shown as being energized by t~ontactor : 35 63 under the control of pushbutton 65. ~his supposes that 91:3 ~o pushbutton 65 is operated as a part of the openiny and closing of welding electrodes 26 and 30 before weld current is applied and after a weld is completed. In the alternative, contactor 63 may be controlled by timer 14 o~
Fig. 2.
In Fig. 3, rectifier 12 comprises an appropriate number of diodes 66 connected to form a full-wave bridge rectifier. Six diodes 66 are shown here, but it should be evident that that number may be changed according to the desired current to be handled, voltage to be applied to the diodes, and also to a different number of phases than three. These are mat~ers of desisn choice. The output of rectifier 12 is a full-wave-rectified voltage that is positive at bus 40 with respect to bus 42. Inver~ing lS units 44 and 46 are connected between bus leads 40 and 42. Inverting units 44 and 46 are identical, so only one inverting unit 44 will be described. In inverting unit 44, a power transistor 68 is connected in series with another power transistor 70 which in turn is connected to 2G a negative bus lead 42. Power transistor 68 is driven by a Darlington transistor array 72 which in turn is driven by drive circuit 52. Power transistor 70 is similarly d~iven hy a Darlington transistor array 74 which is also driven by driver 52. Inverting unit 46 comprises power transistors 76 and 78 that are similarly driven by Darlington transistor arrays that are not shown here~
Power ~transistor 68 is bypassed by a diode 80 a~d also by the series combination of resistor B2 ~nd capacitor 84, which suppresses a rapid rate of rise of volta~e across 30~ po~er tran~istor 6~. This may ~e unnecess~ry with some choices of power transistor 68. ~ower tran~istors 70, 76 and 78 ~re similarly bypassed. The common poi~ ~6 between~power transistors 6~ and 7C is connec~ed through lead la to one end of the primary windi~g ~f step-~o~n trans~ormer 20, and the common point 88 of power ~2~
transi~tors 76 and 78 is connected through lead 19 to ~he other end of the primary winding of step-down transformer 20. A stabilizing cap~citor 90 is c~nnected through a resistor 92 between positive bus lead 40 and negative bus lead 42.
Current-sensing element 50 comprises a current transformer 94 that senses current flow in positive bus lead 40 and also in capacitor 90. This combined connection prevents false trips when capacitor g0 is charging, when the circuit is first energi2ed. Current transformer 94 is connected to current sensor 96 which generates a signal that is proportîonal to the current measured. This cignal is taken to comparator 9B where it is compared with a predeter~ined voltage. When current flow generates a signal that exceeds the predetermined level, the signal is taken to timer 48 of Fig. 2 to control operation of timer 4~.
When the circuit of Fig. 3 was built and tested, the input voltage from source 10 was at a frequency of 60 Hz., and the timing circuit of timer 48 of Fig. 2 was operated so as to generate an input to step-down transformer 20 at 1200 Hz. Under the~e conditions it is appropriate to ignore the change from cycle to cycle of the voltage between bus leads 4~ and 42, even though, as the conventlonal output of a full-wave three-phase bridge rectifier, it is known to have components of AC voltage at 360 Hz. and multiples of that frequency. Operation of the c~lrcuit is well approximated by assuming that a DC voltage is applied between bus leads 40 and 42. An AC voltage is 30 applied to step-down ~ransormer 2D by first causing power transistors 6B and ~8 to conduct, while power transistors 70 and 76 are not conducting. This gene~ates one half-cy~le of AC voltage to be applied to step-down ~ transformer 20~ Conditions are then chanqed 50 that power transistvrs 76 and 70 are caused to conduct, while power I
transistors 68 and 78 are switched off. This applies a volt3ge ~o step-down transformer 20 in the opposite direction, suppl~ing the other half-cycle of AC voltage to step-down transformer 20. The voltage applied to the pri~ary of step-down transformer 20 is essentially a square wave at 1200 ffz. The secondary of step-down transformer 20 responds to the square wave at 12Q0 Hzo to produce what is substantially a square wave at 1200 ~Z.
with a slight ripple that is full-wave rectified to be applied at welding electrodes 26 and 30.
Fig. 4 is a detailed circuit diagram of the circuit for timer 48 of Fig. 2. In Fig. 4, a single-shot 110 generates a single rectangular pulse that is of the order of 1.6 milliseconds in duration. Thls pulse is taken to a weld memory circuit 112. A pulse generator 114 develops pulses at a predetermined frequency and of a width that is variable. These pulses are also taken as an input to weld memory circuit 112. A weld timer circuît 116 generates a rectangular pulse of variable duration ~hat is connected to weld memory circuit 112 to enable a weld for a predetermined time. Weld memory circuit 112 is also disabled by a signal from current sensing elements 50 of Fig. 2 and Fig. 3 indicating the presence of an overcurrent.
An output signal from weld memory circuit 112 is taken to delayed firing circuit 118, a flipflop that delays its signal. The output of delayed firing circui~
118, the output of pulse gen~erator 114, and the output of weld memory circui~ 112`are taken through a NO~ gate 120 ~;~ 30 to flip~flop 122. Flipflop 122 generates two outputs that are rectangular waves of oppos~i~e signs. One of these is taken to drive transistor 124 and the other is taken to ~rive transistor 126. Output5 of drive transistors 124 and 126 rep~esent the output ~f timer 48 which is taken as 35 two inputs to drive circuit 52 of Fig. 2.
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Considering the circuit of Fig. 4 in more detail, single-shot 110 includes a switch 128 which changes the state sf the inputs to a pair of NAND gates 130 and 132.
These are connected to form a flipflop that produces an output that is taken to single-shot 134. This is an anti-bounce circuit. Switch 128 is here indicated as a push bu~ton because that is the form in which it was used in the circuit that was built. It wo~ld also be possible to initiate the triggering of single-shot 110 ~ith an electrical signal from another portion of the circuit or from a microprocessor used in a system of welding control. This is a matter of design choice and convenience, Pulse generator 114 of Fig. 4 comprises a re~riggerable and resettable monostable circuit that will be described for convenience as pulse generator 136.
; Capacitor 138 and a network ~hat includes resistors 140 and 142, potentiometer 144 and diodes 146 and 148 is ; connected through resistor I50 to the positive voltage ~20 supply. The common point of diodes 146 and 148 is connected tQ pulse genera~or 136 so ~hat one or the other of diodes 146 or 148 is switched into conduction according to the sign of the voltage applied at their co~on point.
When diode 148 is switched into conduction, a resistance 25; that is equal to the sum of the resistance of resistors 142 and the right-hand half of potentiometer I44 is connec~ed in series with capacitor }38 to determine one pulse time. When~diode 146 is ~witched into conduction, he resistance that determines the period of the opposite half of the pulse is that of the sum of~resistQr 140 and the remaining portion of potentiometer 144. The resuIt is that a change in the setting of potentiometer 144 chanses the relati~e len~th of the pulses without changing the value of their sum. This prod~es at output terminal 152 a square wave at constan~ frequency which here ls 1200 Hz.
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and having periods of conduction of each sign that can be varied by varying the setting of potentiometer 144.
Weld-time timer 116 uses a single-shot 154.
Capacitor 156 is connected in series witl- resistor 158 and they both are connected to single-shot 154. The sum of the res~stance of resistor 158 and resistor 160 and variable resistor 162 combines with the value of capacitor 156 to determine the period of weld time. The resistance that is selected by adjusting the setting of variable resistor 162 thus adj~sts the length of time that the welder stays on. It should be evident that this function may be controlled on an analog basis by adjusting resistance and corresponding RC times in a single-shot as shown here. In the alternative a microprocessor could be used to control weld time either according to a predetermined schedule of times or in response to vari~us otheL items of information s~ch as measurements of weld yuali~y or the like. These are matters of design choice that will vary according to the use that is contemplated for the circuit, In the weld-time timer 116 as shown, that resuIt is a reotangular pulse at output terminal 164 that is eq~al in duration to the length of the desir2d weld, typically seconds or fractions of a second.
:: Signals from single-shot 110, pulse generator 114 and weld-timer 116 are all taken to weld memory circuit : :
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112~ The output of single-shot 110 is applied as one : : input to NOR gate 166 and the signal from ou~put terminal 1~4 of weld-time timer 116 is applied as the other input : to NOR gate 166. The output of NOR gate 166 is taken as an input to the D ter~inal of flipflop 168, which is clocked by the si~nal from terminal 152. ~lipflop 168 can be reset by a signal from curr2nt sensing element 50 of Fig. 2 in case of an overload. The NQT-Q output of flipflop 168 is ~aken as a reset signal to single shot 154 3S and also as an input to delayed firing circuit 118, where - ~ -it is applied to a single shot 170. The output of single shot 170 is taken to NOR gate 120 where it is applied as ~ne input. Other inp~ts to NOR gate 1~0 are taken from te~minal 152, the output of pulse generator 114, and the Q
output of flipflop 168. The output of NOR gate 120 is taken as an input to flipflop 122 where it is applied to inverter 172 and as one input to each of NAND gates 174 and 176. The output of inverter 172 is taken as a clocking input to flipflop 178 which provides opposite-going outputs that are talcen respectively as inputs to NAND gates 174 and 176. The result i5 to produce two equal and opposite rec~angular wave forms that are taken as inputs to drive transistors 124 and 1260 Referring again tO the three inputs to NOR gate 120, the inp~t from delayed-firing circuit 118 causes the first pulse in any welding interval to be of a shorter pulse width than the succeeding pulses. This prevents saturation of the core of step-down transformer 20 of Fig.
at the start of a weld. The input to NOR gate 1 0 from weld memory unit 112 determines the total time that rectangular pulses are allowed to appear at the output of ~OR gate 120. This is the length that is determined for a : single weld. The input to N~R gate 120 from terminal 152 causes all but the first pulse of any one weld cycle to be ; 25 rectangular pulses at a fixed frequency and o a length that is determined by the setting of potentiometer 144.
Fig. 5 is a detailed CifCUit diagram:of drive circuit~:52 of Fig. 2. :In Fig. 5, a timer 184 recei~es as : ~an input a signal frvm drive transistor 124 of Fig~. 4. An identical timer 186 receives~an equivalen~ si~nal from ~ drive transistor 126. Since the circui~s in whi~h :timers :~ 184 and 186 are used are identical, only:~hat as~svcia~ed : with timer 184 will be described in detail.
~ Ti~e~ ~84 produces a pulse that is taken to power 35 amplifiec 18~ of Fig. 5 wt)ere it i5 used to d~ e :
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negative-going amplifier 190. I~he input pulse from drive transistor 124 is also taken to a positive-g3i~ng am~lif ier 192. Negative-goi~g amplifier l90 and positive-going amplifier 1~2 are both connected to p~imary 1~4 of transformer 196 with one portion of a cycle of currel-t being supplied b~ each of these amplifiers. Transformer 196 has a secondary winding 198 and a secondary winding 200. Secondary winding 198 is connected to a shaping circuit 202 and secondary winding 200 is connected to a shaping circuit 204. St~aping circuit 202 is connected to the circuit of Fig. 3 to trigger conduction of power transistor 6B. Shaping circuit 204 is connecteci to the circuit of Fig. 3 to trigger conduction of power transistor 78. The corresponding shaping circuits of the id~ntical portion of Fig. 5 are similarly connected as indicated, one to trigger the conduction of power transistor 76 in Fig. 3 and the other, to trigger the conouction of power transistor 70 o Fig. 3.
The circuit of Fig. 4 that supplied inputs to timers 20 184 and l86 were described as being opposite in sense. It therefore follows that the voltages to transformer 1~6 and its symmetrical equivalent in Fig. 5 will be opposite in phase. Referring to the outputs of shapiny circuits 20~
a~d 20~, in connection with the circuit of Fig. 3~ it can 25 be seen that when pulse shaping circ~its 202 and 204 ; produce currents that will cause transistors 68 and 78 to conduct. Conduction through step-down transformer 20 of Fig. 3 will be rom left to right. The opposite is true when the input polarity is reversed so that the inputs to 30 Fig. 3 will turn on power transistors 70 and 76, causing current flow ~r~m right to ~eft tl)rough step-down transformer 20 of Fig. 3~ The result of ~his ~peration will be the application to step-down ~ransformer 20 of Pig. 3 of a square wave of current at a frequency deter~ined by pulse generatvr 114 o~ ~ig. 4. The RMS
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value of the cureent in step-down transformer 20 of Fig. 3 will be determined by the setting of potentiometer 144 of Fig. 3. The number of such pulses, representing the length of a weld, will be determined by the setting of variable resistor 162 of Fig. 4.
~ig. 6 is a c~taway perspective ~iew of step-down transformer 20 that has been built and used for the practice of the present invention. In Fig. 6, ferromagnetic core 210 is enclosed by a primary winding 212 and another primary winding 214 that are connected together. A secondary winding 216 begins from a terminal 218. Secondary ~inding 216 is a sin~le thickness of a water-cooled electrical conductor, placed to enclose primary winding 212 and an associated core 21û. Secondary winding 216 continues to cer~ter terminal 22û which will compri~e a center tap of the secondary winding 216. In order to keep the winding sellse of the secondary in ~he proper direction, secondary ~inding 216 is next taken around primary winding 214 in a direction to corner 22Z, then to corner 224, through window 226 to terminal 228, completing secondary winding 216.
Resistance welding oE sheet metal of gauges in common use in the automotive industry typically takes currents of the order of 10,000 or 20,000 amperes. While 25 ~the circuit in Fig. 3 shows two rectifiers 22 and 24, the realization of that circuit that was built using the ~ :transformer of Fig. 6 shows four, the num~er necessary to :~ ~ : carry the desired current. In Fig. 6, diode 230 was used in pa~raIlel with diode 232 to carry the necessary amount 30: of:~current. These two diodes in parallel form the equivalent of diode 22 o~ Fig. 3. Similarly, in Fig. 6, di~de 234 is placed in parallel with diode 236 to effect the e~uivalent of rectifie~ 24 o~ Fig. 3. A c~mon connestion a~on~ divdes 23~, 232, 234, and 236 is not shown in Fig. 6 but will be made by clamping an electrical 6~
conductor in the space 238 that now separates them. A
plurality of inlets 240 and outlets 242 carry cooling water that is passed internally through ducts 244 in secondary winding 216.
The transformer of Fig. 6 is one that has been built and ~ested for use in the circuit of Fig. 3. It is shown here for certain of its features rather than as a necessary way to build a transformer~ Those features include a secondary winding that has two turns with a center tap that is available for a connection. It has means or placing rectifying semiconductors in a water-cooled terminal attached to the transformer that allows them to be clamped readily to the common terminal.
One feature h~wever that represents a particular feature of the present invention is the ~act that the use of a frequency above the line frequency to be applied to the primary of the transformer Or Fig. 6 allows the ùse of less iron in core 210 that will be necessary at a lower frequency. The smaller amount of iron, and hence the smaller amount of copper required, reduces the weight of the transformer of Fig. 6,and makes it easier to locate the transformer of Fig. 6 in a robot arm or in an automatic welder.
Fig. 7 is a set of time plots of voltage wave forms in the cir~uit of Fig. 4. Each o~ the wave forms is identifie~ at an appropriate place in the abscissa by the element number of ~he item of eg~ipment in ~ig. 4 of which the wave form represents the output. Referring to Fig. 7, the wave form marked "114" is a rectangular wave ~hat is generated by the free-running pulse genera~or 114. That wave ~orm be~ins its rise at a ~ime marked Tl and repeats with a period of 417 microseconds. The time of fall of this ~ectangular h~ave form i5 indicated b~y arrows as being ~ariahle, since that ~ime can be set ~y adjusting potentiometer 144 of Fig. 4. A second wave form that is shown in Fig. 7 is that of single-shot 110 marke~ as '~110,", which is a rectangular wave of 1.6 milliseconds in duration. That rectangular pulse is shown as starting at time To in Fig. 7, a time that is determined by operating switch 128 of Fig. 4 and that can egually as well be determined by other signals or by programming, as has been described.
After time To~ time Tl is determined as the first occurence of a rise in the rectangular wave form of pulse generator 114. ~his sets the time of the rectangular p~lse marked "112," which is the output of weld memory circuit 112. This is a single rectangular pulse that begins ~t time Tl and continues to the end of the weld, a time measured typically in tenths of a second or seconds. Time Tl is also the starting time of the wave form marked "118." This is a single rectangular pulse tha~ begins at Tl and ends after 208 microseconds. This is the output of delayed firing circuit li8 which causes or may cause the first pulse in a weld cycle to be of shorter duration than the rest of the pulses.
Consider now the wave form in Fig. 7, marked "120,"
which is the output of NOR gate 120. This is the negation o~ the logical union of wave forms "112," i'll4," and "118"
of Fig. 7. Time T2 is seen as the fall time of the rectangular wave representing the output of pulse generator 114, while time T3 is defined as the time of fall of the rectangular pulse that is the output of delayed firing circuit 118. If time T2 occurs before ~0 time T3 as shown herer then the wave form marked "120"
begins at time T3 and thereaf teY is the negation of wave form "114.`' If time T2 is selected to be later than T3 then the wave form "120" will be the negation of wave form "~14O n Wave form ~120~ is ~hen the source of the wave forms marked ~174" and nl76" which are respectively ~S6 ,~
the outputs of NAND gates 174 and 176 of Fig. 4. As can be seen, wave form "174" comprises alternate p,ulses selected from "120," and wave form "176" comprises the remaining pulses of wave form "120." These switcll inverting units 44 and 46 alternately to produce the output square wave as desired.
We claim:
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RESISTANCE WELDER
Background of the Invention This invention relates to resistance welding In particular t it relates to improvements in resistance 5 welding that are appropriate for use wi~h robot welders and automatic and press welders.
Resistance weldin~ is a well-known way to join together two electrical conductors. It comprises passing an elec~rical current through the conductors in an amount 10 sufficient to cause localized heating that melts the conductors, joining them together. This is normally accomplished by placing a pair of electrodes against the joined electrical conductors, applying pressure and an electrical voltage to the electrodes, and timing the I2R
I5 heating to an amount that is sufficient to weld the materials without creating excessive melting. This can be accomplished either by a DC or AC voltage in most applications. However, because of the typical junction F
; resistance between two elec~rical conductors that are to 20 be 30ined, it is normally desirable to reduce the voltage below the value of typical line voltages by a step-down transformer to apply voltages of the order of a few volts or tenths of a volt and currents of the order of thousands or tens of tho~sands of amperes. The simplest such 25 arrangement comprises a step-down transformer, a switch to control the application of an electrical voltage to the step-~own transformer, and a pair of electrodes connected electrically to the secondary winding of ~he step-down transformer~. When the electrod;es are placed on opposite 30 sides of the workpieces to be j~ined, closing the switch applies a voltage across ~he junction ~Qf the electrical conductors and the resultant electrical heating melts the spot under t~e electrodes to weld the electrical conductocs toget~e ,~
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., ~, Practical considerations of actual welding operations lead to ~he addition of Yarious refinements to the process described above. In order to minimize the cost of ~ransformers used to supply welding currents, it is desirable to insure that the peak voltage applied to the transformer from an electrical source places the c~re of the transformer in saturation, at or beyond the knee of the B-H curve of the core. Because of this fact, it is necessary to insure that the state of the core of the transformer is known whenever a voltage i5 applied to the transformer. If the last voltage that was applied to the transformer leaves the transformer magnetized in a particular direction and the next applied voltage causes current flow in the same direction, then the magnetizing current required by the transformer, together with the load current, may overload the transformer during the first cycle. This is an undesirable situation that is readily avoided by making certain that the control circuit for the welding transformer always applies full cycles of the input voltage to accomplish welding and that it always begins the application of voltage on that portion of the input voltage that ~oes in the same direction. This a~sures that the first cycle of applied voltage always encircles the oriyin ~f the hysteresis loop of the transfo`rmer core, avoiding a current that is far into : saturation, and also assuring that the peak voltage is constant during each welding cycle. It then remains only : to apply the welding voltage for ~ predete~mined number of cycles of the input voltage to accomplish a weld. The predetermined number of cycles is determined by experiPnce but is typically a number that is s~all enough tha~ it mus~ be con~rolled electronically bec~use the neces,~ary time peri~d of application is too s,hor~ to be controlled reliaSly by an operator.
The basic resistance welding system described above '~L24~i~i9~ -has serious disadvantages when it is applied to resistance welding in produc~ion lines. A production weld between two pieces of sheet steel, whether or not they are galvanized, typically reauires a current of the order of 10,000 to 30,0~0 amperes. A
transformer such as in EP-Al-64 750, EP-Al-l9 747 or BE-A-759 605, this is wound to supply such currents ln the secondary will typically weigh of the order of 200 to 600 pounds and will need to ~e cooled by water or other externa] cooling means. Electrical leads to carry such curren~ts are substantial in size. The usual means to handle such problems as these in production lines in the automotive and other industries is to suspend transformers from an overhead support, to run insulated conductors to a welding head that includes watercooled electrodes, and to have an operator place the electrodes at the spot to be welded and apply the external Eorce to hold the e]ectrodes in place while the weld is made. The system described above presents a number of problems when it is converted for use with robot or automatic welders.
Robots are generally limited in the amount of weight that they can handle, and their operation is normally improved by reducing the amount o~ that weight. Automatic welders are limited in the closeness with which they can make adjacent welds, by the size of their transformers. Robot welders are also hampered qreatly in operation by being connected to large electrical cables that are designed~to handle welding currents of thousands of amperes, and the mobility of robots shortens the useful life of such large cables.
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Accordin~ to one aspect oF the present invention, there is provided an electrical circuit for a resistance welder that reduces the weight in the vicinity of the weld.
According to a further aspect of the present invention, there is provided an electrical circuit for a resistance welder that is adaptable for use with a robot welding system.
According to a further aspect of the present invention, there is provided a small, lightweight weldinq transformer which, because of its size and weight, can be mounted close to the weld zone, allowing the use of lightweight primary leads and short secondary leadsO
According to a stil] further aspect o-E the present Invention, there is provided an~electrical circuit Eor a resistance welder that is adaptable Eor use with an automatic welding system.
According to yet another aspect of the present invention, there ~is provided a method of welding an electrically c~nducting workpiece by resistance weldinq comprising the steps of rectifylng an AC voltage~at a line frequency to produce a rectified voltage, invertlng~the rectified voltage at~an operating frequency that is higher than the line frequency to produce an AC voltage at the operating frequency, transforming with a transformer, the AC
voltage at the operating freguency to a lower voltaqe to produce a step-d~own voltage, characteri2ed by applying the step-down voltage from~a~center~tap of the transformer through welding contacts to the~workpiece~.
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Accordinq to a still further aspect of the present invention, there is provided an apparatus for weldinq an electrically conducting workpiece by resistance welding comprising means -Eor rectiEyinq an AC voltage at a line frequency to produce a rectified voltage, means for invertinq the rectified voltage at an operatinq frequency that is higher than the line frequency to produce an AC voltage at the operating frequency, means for transEorminq the AC voltage at the operating frequency to a lower voltage to produce a step-down voltage characterized by means for applying the step-do~7n voltaqe from a center tap of the transformer means through welding contacts to the workpiece.
According to yet another aspect of the present invention, there is provided An elec~rical circuit ~or welding an electrically conductive workpiece by resistance weldin~, to be connected between an AC voltage source at a power Erequency and a pair oE welding electrodes, the ciruit comprising a rectifier connected to the AC voltage source to produce a rectified voltage, means connected to the rectiEier for invertinq the rectified voltage of the rectifier at an operating frequency that is higher :than the -Erequency of the AC voltaqe source to produce an AC
voltage at the operating frequency~ a step-down transformer connected to the means for invertin~ the rectified voltaqe to produce~at a pair of secondary term1nals an output voltage at the operat~ing Er~quency that is lower in amplitude than the voltage produced~by the~means for inv~ertlhg the rectified voltage, ea~ch of the s~econdary terminals of the step-down transformer also - : :
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connected to one of the pair of weld;na electrodes characterized in that said step-down transEormer has a center tap connected to the other of the pair of welding electrodes.
According to a still further aspect oE the present inven-tion, there is provided an electrical circuit for welding an electrically conductive workpiece by resistance welding, be connected between an AC voltage source at a power frequency and a pair of weldina electrodes, the circuit comprising a rectiEier connected to the AC voltage source to produce a rectified voltage, means connected to the rectifier for inverting the rectified voltage of the rectifier at an operating frequency that is higher than the frequency of the AC voltage source to produce an AC
voltage at the operating frequency, a step-down transformer connected to the means for inverting the rectiEied voltage to produce at a pair of secondary terminals an output voltage at the operating fre~uency that is lower in amplitude than the voltage produced~by the means for inverting the rectified voltage, each of the secondary terminals of the step-down transformer also connected to one of the pair of welding electrodes, characterized by the step-down transformer having a center tap, and a full-wave ~rectifier being connected to the secondàry terminals of the ~step-down transformer, to the center tap, and to the welding electrodes to rectify the output voltage of the step-down transformer and apply a rectified output voltage to the welding electrodes.
Brief Description of the Drawings Fiqure 1 1s~a;~block~diagram ~of~a clrcu~it for the practice -of~the ~present invention. ~ ~ ~
Fiqure 2 1s a~more detalled hlock diagram~of th~e circuit for~the~practlce of the present invent~ion.
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Fig. 3 is a circuit diagram that provides more detail about the operation of inverting uni~s 44 and 46 of Fig. 2.
Fig. 4 is a detailed circuit diagram of the circuit for timer 48 of Fig. 2.
~ig. 5 is a detailed circuit diagram of drive circuit 52 of Fig. 2.
Fig. 6 is a cutaway perspective view of a step-down transformer that has been built and used for the practice of the present invention.
Fig. 7 is a set of time plots of vol~acle wave forms in the circuit of Fig. 4.
Detailed Description of the Invention Fig. 1 is a block diagram of a circuit for the practice of the~present invention. In Fig. 1 a source I0 o AC voltage is connected to a rectifier 12. The connection is shown here as being made~with three leads which is most likely when source 10 i5 a source of~
three-phase AC voltage, However, electrical energy of any number of phases co~ld be used. ~ectifier 12 is connected to produce as an output a rectified voltage which is appropriate~ly~described as DC voltage wlth an AC
component. ~The output of rectifier 12 is taken~through cont`ro~l circui~t 14,~ which~is a~controlled~`inverter, to transormer~ 16. Control circ~uit 14 is used~to convert the output~o~;~rect;lfier 12 lnto an~AC vo~ltage at a~frequency tha~t is~higher~than the inpu~t frequency and with an RMS~
~Yalue that is controlla~le.~ ~ ~
Trans~forme~r: 16: is~shown dotted~here because lt may be useEul~to~change the~voltage that i~ ~upplied to leads 18 and:~ 19~as~an input tQ st~p-down transormer 20 However, it shoul~ be understood that under svlme circumstances it might be desirable to connect lea~s 18 : ~: : :
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and 19 directly to control circuit 14. This is a ma~ter o~ design choice.
St~p-down transformer 20 has a secondary that is centertapped. The secondary leads of step-down transformer 20 are connected to rectifiers 22 and 24 to form a full-wave rectifier. A common connection from rectifiers 22 and 24 is taken to welding electrode 26.
Center tap 2~ of step-down transformer 20 is connected to welding electrode 30. When welding electrodes 26 and 30 are placed on opposite sides of a workpiece to be welded and control circuit 14 is operated to supply current through welding electrodes 26 and 30, a resistance weld may be effected between the pieces thus joined. When the frequency of the output voltage of control circuit 14 is higher than the frequency of source 10, then step-down transformer 20 can be made smaller and lighter than it could otherwise be. This facilitates its use by human operators and it also makes possible the placing of the transformer in the arm of a robot welder, thus freeing the arm for a wider range of motions. In addition, the smaller transformer makes, it possible to make welds closer together in automatic welding machines.
Fig. 2 is a more detailed block diagram of the circuit for ~he practice of the present invention. In Fi~. 2 t rectifier 12 is connected to source lO to supply olta~e between bus leads 4Q and 42. Inverting uni~s 44 and 46 are~connected between bus leads 40 and 42 and their midpoints are~connected to leads 18 and 19 thence to step-down trans~ormer 20. ~n the block diagram and circuit of Fig~ 2~ transformer 16 of Fiy. l has been omitted. As st~ted, this is a matter of design choice.
The b~lance of the circui~ in Fig. 2 includes rectifiers ;~ 22 and 24 that are connected to the secondary willding of step-down transformer 2Q to produce a full-wave-lrectified output through ~elding electrodes 26 and 30, through a ~2~
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workpiece that is not shown, and back to cen~er tap 28.
Control of the circuit of Fig. 2 is initiated in timer 48, which both sets the frequency of operation of in~erting units 44 and 46 and also controls relative timing to control the amplitude of the current flow through welding electrodes 26 and 30. A c~rrent-sensing element 50 is connected to supply an input to timer 48 to cut off operation of inverting units 44 and 46 if current in bus lead 40 e~ceeds a predetermined value.
Timer 48 prod~ces two outputs that are taken to drive circuit 52, which is connected to drive power transistors or other switching elements in inverting units 44 and 46 so that current flows during a first half cycle through the upper portion of inverting units 44, through lead 18 and the primary of step-down transformer 20, through lead 19 to the lower half of invertin~ unit 46, thence to bus lead 42. The second half cycle sees current flow thr~ugh the upper half of inverting unit 46, through lead 19 to the primary of step-down transformer ~l but flo~ing in the opposite direction. Current continues to flow through lead 19 to inverting unit 44 where it ~lows through the bottom half of inverting unit 44 to bus lead 42. Details of this control will ~ecome apparent ~rom examining more detailed circuit diagrams. Acceptable ; 25 s~itching elements for inverting unit 44 incIude thyristors, SCRs, gate-t~rnof~ devices, and the like.
Fig. 3 is a circuit diagram that provides more detail about the operotion of invertin~ units ~4 and 46 of Fig~ 2. In Fig. 3, AC power from source 10 is takeR
~ 30 through three fuses 60, three Iimiting resistors 62 and :~ :three co~acts 64 to rectifier 12. Source 10 is typically a three-phase source at a freq~ency of 60 Hz. ~nd a convenient power voltage ~uch as 430 volts or the like.
Contacts 64 are here shown as being energized by t~ontactor : 35 63 under the control of pushbutton 65. ~his supposes that 91:3 ~o pushbutton 65 is operated as a part of the openiny and closing of welding electrodes 26 and 30 before weld current is applied and after a weld is completed. In the alternative, contactor 63 may be controlled by timer 14 o~
Fig. 2.
In Fig. 3, rectifier 12 comprises an appropriate number of diodes 66 connected to form a full-wave bridge rectifier. Six diodes 66 are shown here, but it should be evident that that number may be changed according to the desired current to be handled, voltage to be applied to the diodes, and also to a different number of phases than three. These are mat~ers of desisn choice. The output of rectifier 12 is a full-wave-rectified voltage that is positive at bus 40 with respect to bus 42. Inver~ing lS units 44 and 46 are connected between bus leads 40 and 42. Inverting units 44 and 46 are identical, so only one inverting unit 44 will be described. In inverting unit 44, a power transistor 68 is connected in series with another power transistor 70 which in turn is connected to 2G a negative bus lead 42. Power transistor 68 is driven by a Darlington transistor array 72 which in turn is driven by drive circuit 52. Power transistor 70 is similarly d~iven hy a Darlington transistor array 74 which is also driven by driver 52. Inverting unit 46 comprises power transistors 76 and 78 that are similarly driven by Darlington transistor arrays that are not shown here~
Power ~transistor 68 is bypassed by a diode 80 a~d also by the series combination of resistor B2 ~nd capacitor 84, which suppresses a rapid rate of rise of volta~e across 30~ po~er tran~istor 6~. This may ~e unnecess~ry with some choices of power transistor 68. ~ower tran~istors 70, 76 and 78 ~re similarly bypassed. The common poi~ ~6 between~power transistors 6~ and 7C is connec~ed through lead la to one end of the primary windi~g ~f step-~o~n trans~ormer 20, and the common point 88 of power ~2~
transi~tors 76 and 78 is connected through lead 19 to ~he other end of the primary winding of step-down transformer 20. A stabilizing cap~citor 90 is c~nnected through a resistor 92 between positive bus lead 40 and negative bus lead 42.
Current-sensing element 50 comprises a current transformer 94 that senses current flow in positive bus lead 40 and also in capacitor 90. This combined connection prevents false trips when capacitor g0 is charging, when the circuit is first energi2ed. Current transformer 94 is connected to current sensor 96 which generates a signal that is proportîonal to the current measured. This cignal is taken to comparator 9B where it is compared with a predeter~ined voltage. When current flow generates a signal that exceeds the predetermined level, the signal is taken to timer 48 of Fig. 2 to control operation of timer 4~.
When the circuit of Fig. 3 was built and tested, the input voltage from source 10 was at a frequency of 60 Hz., and the timing circuit of timer 48 of Fig. 2 was operated so as to generate an input to step-down transformer 20 at 1200 Hz. Under the~e conditions it is appropriate to ignore the change from cycle to cycle of the voltage between bus leads 4~ and 42, even though, as the conventlonal output of a full-wave three-phase bridge rectifier, it is known to have components of AC voltage at 360 Hz. and multiples of that frequency. Operation of the c~lrcuit is well approximated by assuming that a DC voltage is applied between bus leads 40 and 42. An AC voltage is 30 applied to step-down ~ransormer 2D by first causing power transistors 6B and ~8 to conduct, while power transistors 70 and 76 are not conducting. This gene~ates one half-cy~le of AC voltage to be applied to step-down ~ transformer 20~ Conditions are then chanqed 50 that power transistvrs 76 and 70 are caused to conduct, while power I
transistors 68 and 78 are switched off. This applies a volt3ge ~o step-down transformer 20 in the opposite direction, suppl~ing the other half-cycle of AC voltage to step-down transformer 20. The voltage applied to the pri~ary of step-down transformer 20 is essentially a square wave at 1200 ffz. The secondary of step-down transformer 20 responds to the square wave at 12Q0 Hzo to produce what is substantially a square wave at 1200 ~Z.
with a slight ripple that is full-wave rectified to be applied at welding electrodes 26 and 30.
Fig. 4 is a detailed circuit diagram of the circuit for timer 48 of Fig. 2. In Fig. 4, a single-shot 110 generates a single rectangular pulse that is of the order of 1.6 milliseconds in duration. Thls pulse is taken to a weld memory circuit 112. A pulse generator 114 develops pulses at a predetermined frequency and of a width that is variable. These pulses are also taken as an input to weld memory circuit 112. A weld timer circuît 116 generates a rectangular pulse of variable duration ~hat is connected to weld memory circuit 112 to enable a weld for a predetermined time. Weld memory circuit 112 is also disabled by a signal from current sensing elements 50 of Fig. 2 and Fig. 3 indicating the presence of an overcurrent.
An output signal from weld memory circuit 112 is taken to delayed firing circuit 118, a flipflop that delays its signal. The output of delayed firing circui~
118, the output of pulse gen~erator 114, and the output of weld memory circui~ 112`are taken through a NO~ gate 120 ~;~ 30 to flip~flop 122. Flipflop 122 generates two outputs that are rectangular waves of oppos~i~e signs. One of these is taken to drive transistor 124 and the other is taken to ~rive transistor 126. Output5 of drive transistors 124 and 126 rep~esent the output ~f timer 48 which is taken as 35 two inputs to drive circuit 52 of Fig. 2.
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Considering the circuit of Fig. 4 in more detail, single-shot 110 includes a switch 128 which changes the state sf the inputs to a pair of NAND gates 130 and 132.
These are connected to form a flipflop that produces an output that is taken to single-shot 134. This is an anti-bounce circuit. Switch 128 is here indicated as a push bu~ton because that is the form in which it was used in the circuit that was built. It wo~ld also be possible to initiate the triggering of single-shot 110 ~ith an electrical signal from another portion of the circuit or from a microprocessor used in a system of welding control. This is a matter of design choice and convenience, Pulse generator 114 of Fig. 4 comprises a re~riggerable and resettable monostable circuit that will be described for convenience as pulse generator 136.
; Capacitor 138 and a network ~hat includes resistors 140 and 142, potentiometer 144 and diodes 146 and 148 is ; connected through resistor I50 to the positive voltage ~20 supply. The common point of diodes 146 and 148 is connected tQ pulse genera~or 136 so ~hat one or the other of diodes 146 or 148 is switched into conduction according to the sign of the voltage applied at their co~on point.
When diode 148 is switched into conduction, a resistance 25; that is equal to the sum of the resistance of resistors 142 and the right-hand half of potentiometer I44 is connec~ed in series with capacitor }38 to determine one pulse time. When~diode 146 is ~witched into conduction, he resistance that determines the period of the opposite half of the pulse is that of the sum of~resistQr 140 and the remaining portion of potentiometer 144. The resuIt is that a change in the setting of potentiometer 144 chanses the relati~e len~th of the pulses without changing the value of their sum. This prod~es at output terminal 152 a square wave at constan~ frequency which here ls 1200 Hz.
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and having periods of conduction of each sign that can be varied by varying the setting of potentiometer 144.
Weld-time timer 116 uses a single-shot 154.
Capacitor 156 is connected in series witl- resistor 158 and they both are connected to single-shot 154. The sum of the res~stance of resistor 158 and resistor 160 and variable resistor 162 combines with the value of capacitor 156 to determine the period of weld time. The resistance that is selected by adjusting the setting of variable resistor 162 thus adj~sts the length of time that the welder stays on. It should be evident that this function may be controlled on an analog basis by adjusting resistance and corresponding RC times in a single-shot as shown here. In the alternative a microprocessor could be used to control weld time either according to a predetermined schedule of times or in response to vari~us otheL items of information s~ch as measurements of weld yuali~y or the like. These are matters of design choice that will vary according to the use that is contemplated for the circuit, In the weld-time timer 116 as shown, that resuIt is a reotangular pulse at output terminal 164 that is eq~al in duration to the length of the desir2d weld, typically seconds or fractions of a second.
:: Signals from single-shot 110, pulse generator 114 and weld-timer 116 are all taken to weld memory circuit : :
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112~ The output of single-shot 110 is applied as one : : input to NOR gate 166 and the signal from ou~put terminal 1~4 of weld-time timer 116 is applied as the other input : to NOR gate 166. The output of NOR gate 166 is taken as an input to the D ter~inal of flipflop 168, which is clocked by the si~nal from terminal 152. ~lipflop 168 can be reset by a signal from curr2nt sensing element 50 of Fig. 2 in case of an overload. The NQT-Q output of flipflop 168 is ~aken as a reset signal to single shot 154 3S and also as an input to delayed firing circuit 118, where - ~ -it is applied to a single shot 170. The output of single shot 170 is taken to NOR gate 120 where it is applied as ~ne input. Other inp~ts to NOR gate 1~0 are taken from te~minal 152, the output of pulse generator 114, and the Q
output of flipflop 168. The output of NOR gate 120 is taken as an input to flipflop 122 where it is applied to inverter 172 and as one input to each of NAND gates 174 and 176. The output of inverter 172 is taken as a clocking input to flipflop 178 which provides opposite-going outputs that are talcen respectively as inputs to NAND gates 174 and 176. The result i5 to produce two equal and opposite rec~angular wave forms that are taken as inputs to drive transistors 124 and 1260 Referring again tO the three inputs to NOR gate 120, the inp~t from delayed-firing circuit 118 causes the first pulse in any welding interval to be of a shorter pulse width than the succeeding pulses. This prevents saturation of the core of step-down transformer 20 of Fig.
at the start of a weld. The input to NOR gate 1 0 from weld memory unit 112 determines the total time that rectangular pulses are allowed to appear at the output of ~OR gate 120. This is the length that is determined for a : single weld. The input to N~R gate 120 from terminal 152 causes all but the first pulse of any one weld cycle to be ; 25 rectangular pulses at a fixed frequency and o a length that is determined by the setting of potentiometer 144.
Fig. 5 is a detailed CifCUit diagram:of drive circuit~:52 of Fig. 2. :In Fig. 5, a timer 184 recei~es as : ~an input a signal frvm drive transistor 124 of Fig~. 4. An identical timer 186 receives~an equivalen~ si~nal from ~ drive transistor 126. Since the circui~s in whi~h :timers :~ 184 and 186 are used are identical, only:~hat as~svcia~ed : with timer 184 will be described in detail.
~ Ti~e~ ~84 produces a pulse that is taken to power 35 amplifiec 18~ of Fig. 5 wt)ere it i5 used to d~ e :
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negative-going amplifier 190. I~he input pulse from drive transistor 124 is also taken to a positive-g3i~ng am~lif ier 192. Negative-goi~g amplifier l90 and positive-going amplifier 1~2 are both connected to p~imary 1~4 of transformer 196 with one portion of a cycle of currel-t being supplied b~ each of these amplifiers. Transformer 196 has a secondary winding 198 and a secondary winding 200. Secondary winding 198 is connected to a shaping circuit 202 and secondary winding 200 is connected to a shaping circuit 204. St~aping circuit 202 is connected to the circuit of Fig. 3 to trigger conduction of power transistor 6B. Shaping circuit 204 is connecteci to the circuit of Fig. 3 to trigger conduction of power transistor 78. The corresponding shaping circuits of the id~ntical portion of Fig. 5 are similarly connected as indicated, one to trigger the conduction of power transistor 76 in Fig. 3 and the other, to trigger the conouction of power transistor 70 o Fig. 3.
The circuit of Fig. 4 that supplied inputs to timers 20 184 and l86 were described as being opposite in sense. It therefore follows that the voltages to transformer 1~6 and its symmetrical equivalent in Fig. 5 will be opposite in phase. Referring to the outputs of shapiny circuits 20~
a~d 20~, in connection with the circuit of Fig. 3~ it can 25 be seen that when pulse shaping circ~its 202 and 204 ; produce currents that will cause transistors 68 and 78 to conduct. Conduction through step-down transformer 20 of Fig. 3 will be rom left to right. The opposite is true when the input polarity is reversed so that the inputs to 30 Fig. 3 will turn on power transistors 70 and 76, causing current flow ~r~m right to ~eft tl)rough step-down transformer 20 of Fig. 3~ The result of ~his ~peration will be the application to step-down ~ransformer 20 of Pig. 3 of a square wave of current at a frequency deter~ined by pulse generatvr 114 o~ ~ig. 4. The RMS
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value of the cureent in step-down transformer 20 of Fig. 3 will be determined by the setting of potentiometer 144 of Fig. 3. The number of such pulses, representing the length of a weld, will be determined by the setting of variable resistor 162 of Fig. 4.
~ig. 6 is a c~taway perspective ~iew of step-down transformer 20 that has been built and used for the practice of the present invention. In Fig. 6, ferromagnetic core 210 is enclosed by a primary winding 212 and another primary winding 214 that are connected together. A secondary winding 216 begins from a terminal 218. Secondary ~inding 216 is a sin~le thickness of a water-cooled electrical conductor, placed to enclose primary winding 212 and an associated core 21û. Secondary winding 216 continues to cer~ter terminal 22û which will compri~e a center tap of the secondary winding 216. In order to keep the winding sellse of the secondary in ~he proper direction, secondary ~inding 216 is next taken around primary winding 214 in a direction to corner 22Z, then to corner 224, through window 226 to terminal 228, completing secondary winding 216.
Resistance welding oE sheet metal of gauges in common use in the automotive industry typically takes currents of the order of 10,000 or 20,000 amperes. While 25 ~the circuit in Fig. 3 shows two rectifiers 22 and 24, the realization of that circuit that was built using the ~ :transformer of Fig. 6 shows four, the num~er necessary to :~ ~ : carry the desired current. In Fig. 6, diode 230 was used in pa~raIlel with diode 232 to carry the necessary amount 30: of:~current. These two diodes in parallel form the equivalent of diode 22 o~ Fig. 3. Similarly, in Fig. 6, di~de 234 is placed in parallel with diode 236 to effect the e~uivalent of rectifie~ 24 o~ Fig. 3. A c~mon connestion a~on~ divdes 23~, 232, 234, and 236 is not shown in Fig. 6 but will be made by clamping an electrical 6~
conductor in the space 238 that now separates them. A
plurality of inlets 240 and outlets 242 carry cooling water that is passed internally through ducts 244 in secondary winding 216.
The transformer of Fig. 6 is one that has been built and ~ested for use in the circuit of Fig. 3. It is shown here for certain of its features rather than as a necessary way to build a transformer~ Those features include a secondary winding that has two turns with a center tap that is available for a connection. It has means or placing rectifying semiconductors in a water-cooled terminal attached to the transformer that allows them to be clamped readily to the common terminal.
One feature h~wever that represents a particular feature of the present invention is the ~act that the use of a frequency above the line frequency to be applied to the primary of the transformer Or Fig. 6 allows the ùse of less iron in core 210 that will be necessary at a lower frequency. The smaller amount of iron, and hence the smaller amount of copper required, reduces the weight of the transformer of Fig. 6,and makes it easier to locate the transformer of Fig. 6 in a robot arm or in an automatic welder.
Fig. 7 is a set of time plots of voltage wave forms in the cir~uit of Fig. 4. Each o~ the wave forms is identifie~ at an appropriate place in the abscissa by the element number of ~he item of eg~ipment in ~ig. 4 of which the wave form represents the output. Referring to Fig. 7, the wave form marked "114" is a rectangular wave ~hat is generated by the free-running pulse genera~or 114. That wave ~orm be~ins its rise at a ~ime marked Tl and repeats with a period of 417 microseconds. The time of fall of this ~ectangular h~ave form i5 indicated b~y arrows as being ~ariahle, since that ~ime can be set ~y adjusting potentiometer 144 of Fig. 4. A second wave form that is shown in Fig. 7 is that of single-shot 110 marke~ as '~110,", which is a rectangular wave of 1.6 milliseconds in duration. That rectangular pulse is shown as starting at time To in Fig. 7, a time that is determined by operating switch 128 of Fig. 4 and that can egually as well be determined by other signals or by programming, as has been described.
After time To~ time Tl is determined as the first occurence of a rise in the rectangular wave form of pulse generator 114. ~his sets the time of the rectangular p~lse marked "112," which is the output of weld memory circuit 112. This is a single rectangular pulse that begins ~t time Tl and continues to the end of the weld, a time measured typically in tenths of a second or seconds. Time Tl is also the starting time of the wave form marked "118." This is a single rectangular pulse tha~ begins at Tl and ends after 208 microseconds. This is the output of delayed firing circuit li8 which causes or may cause the first pulse in a weld cycle to be of shorter duration than the rest of the pulses.
Consider now the wave form in Fig. 7, marked "120,"
which is the output of NOR gate 120. This is the negation o~ the logical union of wave forms "112," i'll4," and "118"
of Fig. 7. Time T2 is seen as the fall time of the rectangular wave representing the output of pulse generator 114, while time T3 is defined as the time of fall of the rectangular pulse that is the output of delayed firing circuit 118. If time T2 occurs before ~0 time T3 as shown herer then the wave form marked "120"
begins at time T3 and thereaf teY is the negation of wave form "114.`' If time T2 is selected to be later than T3 then the wave form "120" will be the negation of wave form "~14O n Wave form ~120~ is ~hen the source of the wave forms marked ~174" and nl76" which are respectively ~S6 ,~
the outputs of NAND gates 174 and 176 of Fig. 4. As can be seen, wave form "174" comprises alternate p,ulses selected from "120," and wave form "176" comprises the remaining pulses of wave form "120." These switcll inverting units 44 and 46 alternately to produce the output square wave as desired.
We claim:
: ~
:
Claims (9)
1. A method of welding an electrically conducting workpiece by resistance welding comprising the steps of:
a) rectifying an AC voltage at a line frequency to produce a rectified voltage;
b) inverting the rectified voltage at an operating frequency that is higher than the line frequency to produce an AC
voltage at the operating frequency;
c) transforming with a transformer, the AC voltage at the operating frequency to a lower voltage to produce a step-down voltage; characterized by applying the step-down voltage from a center tap of the transformer through welding contacts to the workpiece.
a) rectifying an AC voltage at a line frequency to produce a rectified voltage;
b) inverting the rectified voltage at an operating frequency that is higher than the line frequency to produce an AC
voltage at the operating frequency;
c) transforming with a transformer, the AC voltage at the operating frequency to a lower voltage to produce a step-down voltage; characterized by applying the step-down voltage from a center tap of the transformer through welding contacts to the workpiece.
2. The method of claim 1, further characterized by the steps of:
a) rectifying the AC voltage at the operating frequency to produce a multiphase DC voltage; and b) applying the multiphase DC voltage through welding contacts to the workpiece.
a) rectifying the AC voltage at the operating frequency to produce a multiphase DC voltage; and b) applying the multiphase DC voltage through welding contacts to the workpiece.
3. An apparatus for welding an electrically conducting workpiece by resistance welding comprising;
a) means for rectifying an AC voltage at a line frequency to produce a rectified voltage;
b) means for inverting the rectified voltage at an operating frequency that is higher than the line frequency to produce an AC voltage at the operating frequency;
c) means for transforming the AC voltage at the operating frequency to a lower voltage to produce a step-down voltage, characterized by means for applying the step-down voltage from a center tap of the transformer means through welding contacts to the workpiece.
a) means for rectifying an AC voltage at a line frequency to produce a rectified voltage;
b) means for inverting the rectified voltage at an operating frequency that is higher than the line frequency to produce an AC voltage at the operating frequency;
c) means for transforming the AC voltage at the operating frequency to a lower voltage to produce a step-down voltage, characterized by means for applying the step-down voltage from a center tap of the transformer means through welding contacts to the workpiece.
4. The apparatus of claim 3 further characterized by:
a) means for rectifying the AV voltage at the operating frequency to produce a multiphase DC voltage; and b) means for applying the DC multiphase voltage through welding contacts to the workpiece.
a) means for rectifying the AV voltage at the operating frequency to produce a multiphase DC voltage; and b) means for applying the DC multiphase voltage through welding contacts to the workpiece.
5. An electrical circuit for welding an electrically conductive workpiece by resistance welding, to be connected between an AC voltage source at a power frequency and a pair of welding electrodes, the circuit comprising:
a) a rectifier connected to the AC voltage source to produce a rectified voltage;
b) means connected to the rectifier for inverting the rectified voltage of the rectifier at an operating frequency that is higher than the frequency of the AC voltage source to produce an AC
voltage at the operating frequency;
c) a step-down transformer connected to the means for inverting the rectified voltage to produce at a pair of secondary terminals an output voltage at the operating frequency that is lower in amplitude than the voltage produced by the means for inverting the rectified voltage, each of the secondary terminals of the step-down transformer also connected to one of the pair of welding electrodes characterized in that said step-down transformer has a center tap connected to the other of the pair of welding electrodes.
a) a rectifier connected to the AC voltage source to produce a rectified voltage;
b) means connected to the rectifier for inverting the rectified voltage of the rectifier at an operating frequency that is higher than the frequency of the AC voltage source to produce an AC
voltage at the operating frequency;
c) a step-down transformer connected to the means for inverting the rectified voltage to produce at a pair of secondary terminals an output voltage at the operating frequency that is lower in amplitude than the voltage produced by the means for inverting the rectified voltage, each of the secondary terminals of the step-down transformer also connected to one of the pair of welding electrodes characterized in that said step-down transformer has a center tap connected to the other of the pair of welding electrodes.
6. An electrical circuit for welding an electrically conductive workpiece by resistance welding, be connected between an AC voltage source at a power frequency and a pair of welding electrodes the circuit comprising:
a) a rectifier connected to the AC voltage source to produce a rectified voltage;
b) means connected to the rectifier for inverting the rectified voltage of the rectifier at an operating frequency that is higher than the frequency of the AC voltage source to produce an AC
voltage at the operating frequency;
c) a step-down transformer connected to the means for inverting the rectified voltage to produce at a pair of secondary terminals an output voltage at the operating frequency that is lower in amplitude than the voltage produced by the means for inverting the rectified voltage, each of the secondary terminals of the step-down transformer also connected to one of the pair of welding electrodes, characterized by the step-down transformer having a center tap, and a full-wave rectifier being connected to the secondary terminals of the step-down transformer, to the center tap, and to the welding electrodes to rectify the output voltage of the step-down transformer and apply a rectified output voltage to the welding electrodes.
a) a rectifier connected to the AC voltage source to produce a rectified voltage;
b) means connected to the rectifier for inverting the rectified voltage of the rectifier at an operating frequency that is higher than the frequency of the AC voltage source to produce an AC
voltage at the operating frequency;
c) a step-down transformer connected to the means for inverting the rectified voltage to produce at a pair of secondary terminals an output voltage at the operating frequency that is lower in amplitude than the voltage produced by the means for inverting the rectified voltage, each of the secondary terminals of the step-down transformer also connected to one of the pair of welding electrodes, characterized by the step-down transformer having a center tap, and a full-wave rectifier being connected to the secondary terminals of the step-down transformer, to the center tap, and to the welding electrodes to rectify the output voltage of the step-down transformer and apply a rectified output voltage to the welding electrodes.
7. The circuit of claim 6, characterized in that the means for inverting comprises:
a) a timer circuit monitoring the primary circuit and producing the rectangular pulses of a controllable width at the operating frequency; and b) a plurality of semiconducting devices connected to the timer circuit and to the step-down transformer to switch current through the step-down transformer in alternating direction at the operating frequency.
a) a timer circuit monitoring the primary circuit and producing the rectangular pulses of a controllable width at the operating frequency; and b) a plurality of semiconducting devices connected to the timer circuit and to the step-down transformer to switch current through the step-down transformer in alternating direction at the operating frequency.
8. The circuit of claim 6, characterized in that the step-down transformer comprises:
a) a ferromagnetic core;
b) a primary winding enclosing the ferromagnetic core and c) a secondary winding formed of a single water-cooled electrical conductor wound in two turns in the same direction about the ferromagnetic core.
a) a ferromagnetic core;
b) a primary winding enclosing the ferromagnetic core and c) a secondary winding formed of a single water-cooled electrical conductor wound in two turns in the same direction about the ferromagnetic core.
9. The circuit of claim 8, characterized in that the center tap of the step-down transformer is at a junction of the two turns.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000477445A CA1246690A (en) | 1985-03-25 | 1985-03-25 | Resistance welder |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000477445A CA1246690A (en) | 1985-03-25 | 1985-03-25 | Resistance welder |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1246690A true CA1246690A (en) | 1988-12-13 |
Family
ID=4130119
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000477445A Expired CA1246690A (en) | 1985-03-25 | 1985-03-25 | Resistance welder |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1246690A (en) |
-
1985
- 1985-03-25 CA CA000477445A patent/CA1246690A/en not_active Expired
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