CA1184253A - Control system and method for dc pulse modulated arc welding - Google Patents

Abstract

Control System and Method for DC Pulse Modulated Arc Welding Abstract A method of pulsed direct current (DC), arc welding with a feedback control system is disclosed wherein the duty cycle of current pulses supplied by a power supply is controlled in response to resistance sensed at an arc gap to maintain a constant time-averaged power flow to work pieces which are welded. The feedlock control system includes a voltage sensor, a high-low regulator, and a duty cycle signal generator. This pulsed DC arc welding is especially useful with a type of pulsed DC arc welding wherein the ratio of peak current to maintenance current is maintained at a selected high value and the current is cycled in a time duration whereby oxides on the surface of the work pieces are dissipated when making the weld. Also, a control system is disclosed for automatically adjusting pulse width of current pulses supplied at an arc gap by a pulsed DC arc welding system power supply, to control power flow to work pieces at the arc gap.

Description

Control System and Method for DC Pulse Modulated Arc Weldin~

This invention relates to control systems for and me~hod~ of arc welding and, more par~icularly, relates to control sys~ems and methods of pul~ed direct current (DC) arc welding.

There are many situations in which it i~ desirable to arc weld together ~wo pieces of metal. For e~ample, a heat exchanger for an air conditioning system may be made from sections of thin wall alu~inu~ tubing which are joined to pro~ide a continuous circuit or the flow of a refrigerant. The ~ections must be joined 60 thst there are no leaks. One method fo~ accomplishing this is by arc welding.

One problem encountered in arc welding is the pre~ence of foreign materials on the surfaces of the work pieces which are being welded together. These foreign materials can degrade the quality of the weld if they are not removed. Metals such as alu~inu~, ma~nesium, and beryllium copper, pose an especially difficul~ surface contaminant problem since o~ides instantaneou~ly form on the surfaces of these metals ~hen they are expo~ed to air. O~ides ~ay be removed by using a noD~etal chlorine or fluorine base fl~x during the welding p~ocess but this flux is corrosive and i8 not co~patible with the enYironment. Therefo~e, it is desirable to arc weld, especially to arc weld metals such as sluminum, ~agnesium~
and beryllium copper without using a flu~.

Fluxless welding is possible by usi~g certain alternating currPnt (AC~ arc welding techniques. United States Patents 3,894,210 to Smith, et al and 3,818,177 to Needham, et al disclo~e such AC ar~
~elding techni~ues. These techniques are especially useful for ~elding cer~ain materials, ~uch as alumin~, magnesiu~, and beryllium copper, since a weld can be ~ade even if ~ides are p~esent on the surf3ces of the wo~ piece6. However, there are ,~

-2-many ~ituations when it i8 desirable to use direct current (DC) arc welding. For example, it is difficult to weld thin wall sections of al~minum tubing used in making heat exchangers for air conditioning systems by using an AC arc welding technique. This is because AC arc welding requires a significant power flow to the work piecPs to make a weld and dissipate oxides without using a flux. This po~er flow heats the work pieces to an undesirable temperature because the thin wall tubing does not pro~ide a sufficient heat sin~ fo~ conducting away heat energy. Thus, si~nificant ~agging in the weld area can occur and there is a possibility that the work pieces will be burned through. This distortion of the weld area can be reduced if DC arc welding is used. Also, electrode life can be increased if DC arc welding is used rather than AC arc welding. Furthermore, power flow to the work pieces may be more precisely controlled when using DC arc welding. These are just some of the advantages inherent in DC arc welding when welding certain materials such as the thin wall sections of aluminum tubing used in making heat exchangers for air conditioning systems. Therefore, it is preferable to weld these materials by usi~g DC arc welding rather than by using other techniques such as fluxless AC arc welding.

One disadvantage of conventional DC arc welding is that this type of arc welding is not generally capable of fluxless welding of certain materials, such as aluminum, magne~ium, a~d beryllium copper, which form difficult to reduce oxides on their surfaces.
However, there is a novel method of pulsed DC arc welding for welding these ~aterials ~ithout using a flu~. According to this novel method, special pulses of positive direct current are applied at an arc gap to arc weld work pieces a~ the arc gap. The sp~cial pulses have a for~ which is similar to conYentional DC pulses except that the ~atio of the magnitude of the peak current to the ~agntiude of the ~aintenance current at the leading edge of each currcnt pulse is selected to have a ~pecial feature. E~entially, ~his ratio is ~a~i~i2ed and the increase from the maintenance

-3 current level to the pea~ current value is adjusted to occur in a time interval whereby a thenmal shock effect is created. A related kind of thermal shock effect is well known in the field of vacuu~
bra~ing as part of a multi-step hea~ treatmen~ process in ~hich materials are joined together by brazing. Basically, this ther~al shock efEect results from rapidly heating work pieces having surface oxides with a coefficient of thermal expansion which is substantially less than the coefficient of thermal expansion of the underlying pure material. The rapid heating causes an uneven rate of expansion which fractures and splits apart the oxides on the surfaces of the work pieces.

The split apart oxides are pushed away from the weld area due to the melting and joining of the underlying pure materials during the novel arc welding process disclosed above. Other physical phenomena also may be responsible for the exemplary welds formed when using this novel arc welding method but the th~nmal shock effect is believed to be the primary mechanism by which the oxides are dissipated. Regardless of the exact physical phenomena underlying the oxide dissipation, the feature of maximi~ing the ratio of peak current to maintenance current at the leading edge of each current pulse i~ an esse~tial elemen~ of this novel method of DC arc welding. This feature is best explained when it is assumed that the thermal shock effect is the primary mechanism by which the oxideæ are dissipated.

The optimal values for the maintenance current, peak current and time duration in which the increase from the maintenance current level to peak current value oscurs, when src welding according to 3C the novel arc welding method described above, are selected through a trial and error process. These optimal values depend on the kind of material being welded, the thic~ness of the work pieces being welded, and other such factors~

.. _ . _ . , .. .. __._ .... _ . , .. _, _ ._ . _ .. . . .

Also, power flow from the welding electrode to ~he wor~ pieces is an important factor in determining weld quality. Good quality welds cannot always be ~ade because of ch~nges in this power flow as a function of time. It is especially difficult to continually make good quality welds on certain materials, such as thin wall alu~in~ tubing, when mass producing products, such as heat exchangers for air conditioninp systems, because of this variation in power flow. This problem is prèsen~ even if the novel method of fluxless pulsed DC arc welding described above is used in the ~anufacturing process.

These changes in power flow usually are caused by variations in the resistance between the welding electrode and the work pieces due to inhomogeneities in the ionized gas, variations in work piece dimensions resulting in a changing arc gap separation, naturally occurring fluctuations in power supply output voltage and other such pheno~ena. This variation in resistance between the welding electrode and the work pieces directly affects the a~ount of power which reaches the work pieces from the welding electrode. It is desirable to maintain this power flow at a constant optimal value since it is this power flow which primarily detenmines weld quality.

Conve~tional arc welding systems of the pulsed DC t~pe do not specifically address the proble~ of con~rolling power flow to the work pieces. Typically; these systemæ regulate current flow by adjusting the voltage applied across the arc gap in response to variations in arc gap resistance to ~aintain tbe curren~ flow at cons~ant preset levels. Therefore, the ~onmal operation of a current regulated pulsed DC system resul~s in variations iD the power flow ~o the wor~ pieces.

A ~ethod of controlling this power flow, when using a pulsed DC
power supply, is by changing the pulse width of the current pulses supplied to the arc gap. If a periodic ~eries of curre~t pulses is . . .

being ~sed ~hi~ amounts to changing the duty cycle Gf the current pulses. Thus, this method can be called pulse width ~odulation or duty cycle modulation. This method of cont{olling power flow is especially useful whell the form o~ the DC pulses must be maintained as required when arc welding according to the novel pulsed DC arc welding method described above. Therefore, it is desirable to have a method and to provide a control system for an arc welding system pulsed DC po~er supply which is capable of pr~cisely adjus~ing power flow to work pieces by modulating the pulse width of current pulses supplied by the power supply to the work pieces.
Preferably, this pulse width modulation is done without otherwise altering the general form of the current pulses. Furthermore, it is desirable to provide a method and a control system for an arc welding system pulsed DC power supply which is capable of adjusting power flow by pulse width modulation to co~pensate for variations in resistance between the weld-ing electrode and the work pieces.
In addition, it is desirable to have a control system for a pulsed DC arc welding system power supply which automatically contrGls the pulse width of current pul8es supplied by the power supply to the arc gap. Preferably9 this control should be capable of use with a conventional DC power supply for controlling the pulse width of any type of pulse generated by the power supply.

According to the present invention tbe fore~oing features are attained by a method of adjusting the pulse ~idth of currenk pulses, which are supplied to work pieces at an arc gap dnring arc welding, with a feedback control circuit comprifiing a voltage sensor, high-low regulator, and pulse width control signal generator. The feedback control circuit controls a pulsed DC arc welding power supply. The voltage sensor detects the voltage drop across the arc gap as a function of ti~e. This voltage is directly proportional to the resistance across the arc gap. ~hus, the voltage sensor direc~ly indicates varia~ions in parameters affecting pow~r flow to work pieces at the arc gap such a~ a change in the separstion distance between the welding electrode and the work pieces. The detected vol~age is input~ed to the high-low regulator which processes this voltage signal ~o determine whether the voltage has increased above a selected high limit or ha~
decreased below a selected low limit. If either of these conditions has occurred, the high low regulator generates an output signal which is supplied to the pulse width signal generator.

The pulse width signal genera~or c~ntinuously provides a voltage control signal to the pulsed DC arc welding power supply which controls the duty cycle of the current pulses supplied by the power supply to the arc gap. An operator initially selects a particular duty cycle for the current pulses which gives optimal weld characteristics. This optimal duty cycle varies dependin~ OQ the kind of material being welded, the thickness of the work pieces and other such factors. This optimal duty cycle is selected through a trial and error process. The high-low regulator provides a supplemental voltage signal to the pulse width si~nal generator which alters the control signal from the generator. The generator control signal is altered to decrease or increase the duty cycle of the current pulses supplied by the DC power supply to the arc gap in response to changes in the s~pplemental voltage signal. Thus ~he duty cycle of the current pulses is changed in response to changes in the voltage drop across the arc gap. This change in the duty cycle maintains the time-averaged power flow to the work pieces at the arc gap at the power flvw level associated with the opti~al duty cycle initially selected. None of the other characteristics of the current pul~es, such as frequency an~ peak current, are affected. Thus, the power flow is automatically maintained at the optimal level without changing the o~erall fon~
of the current pulses. Therefore, this optimal power flow is ~aintained even if there are changes in parameters which affect power flow to the work pieces such as change in the eparation distance between the welding electrode and the wo~k pie~es.

s~

Instead of the feedback control circuit described above, according to another aspect of the present invention 3 a programmable pulse width control device, in the control circuitry for an arc welding system power supply, is used to adjust the pulse width of the current pulses supplied at the arc gap from the power supply. This adjustment compensates for changes in parameters effecting power flow to the work pieces at the arc gap. If the arc welding power supply is a conventional pulsed DC supply, the progra~mable pulse width control device may be simply a normally closed time delay switch connected in parallel with a variable resistance device.
This programmable device operates to intexpose the variable resistance device in a conventional impulsar control circuit for the arc welding system power supply when the normally closed switch is opened after a preselected time delay. The device is electrically connected between an impuslar pulse width adjustor and impulsar output signal generator to automatically alter the pulse width of the voltage control signal outputted by the impulsar.
This causes the power supply to automatically alter the duty cycle of the current pulses supplied at the arc gap since the i~pulsar directly controls the operation of ~he power supply of a conventional arc ~elding system. If a more complicated current program is desired then the inser~ion of more resistance devices with time delay switches can be used or another such circuit arrangement devised.

This invention will now be described by way of example, with reference to the accompanyi~g drawing in which:

Figure 1 shows a block diagram of an arc welding ~ystem includin~ a feedback control circuit 2 for adjusting the pulse width of curren~
pulses supplied by a power supply to an arc gap.

Figure 2 is a schematic graph of tbe amplitude of current pulses which are appli~d to work pieces to ~ain~ain ~he time-averaged power flow to the work pieces constant when the voltage drop sensed at $hQ arc gap is increasing.

Figure 3 shows specific circuit components for the feedback control circuit shown in Figure 1.

Figure 4 shows a block diagra~ of an arc welding system including a controller for automatically adjusting the pulse width of current pulses supplied by a power supply to an arc gap.
Figure 5 shows specific circuit components for the automatic controller shown in Figure 4.

Figures 6 and 7 show how the impul~ar output system shown in Figure 5 operates to generate output voltage control signals of two different duty cycles in response to input voltage control signals Gf two different magnitudes when the impulsar output system includes a comparator.

Referring now to Figure 1, a block diagram of an arc welding system is shown includi~g a feedback control circui~ 2 for ~odulating the pulse width of direct current (DC) pulses applied at an arc gap 3.
The pulses are ~odulated in pulse width while maintaini~g their peak ~agnitude constant to provide a constant time-averaged power flow across the arc gap 3. Current flo~ across the arc gap 3 from the electrode 12 to the work pieces 1 is deter~ined by the operation of power ~upply 4. The power supply 4 can be o~e of a variety of power supplies which are available commercially. If the novel method of pulsed DC arc ~elding described previously is to be used it may be necessary to have a power supply 4 ~ith a relatively high peak current capability, depending on the type of work pieces being welded, to provide ~he required ratio of pea~ current value ~o maintenance current level at the leading edge of each surrent pulse as required by thifi novel ~ethod. Conventio~al power ..... .. , . . . ~

5~ 3 g supplies may be ~odified by those ~f ordinary skill in the art to provide a power supply with such a high peak current capability.

Impul~ar 10 in connection with current regulator 11 controls the operation oI the power supply 4. This is a conventional type of control for a power supply 4. Also, a hi8h ~oltage, high frequency arc starter 5 co~trols the initial flow of current across the arc gap 3. This too is a conventional feature of arc welding systems.
The arc starter 5 provides a high voltage to initiate curre~t flow across the arc gap 3 by ionizing inert gas supplied to the arc gap 3 from the gas supply means 6 through passageways 14 in the electrode holder 15. After the initiation of current flow the arc starter 5 discontinues operation. Subsequently, the inert gas is ionized by the operation of the power supply 4 to sustain current flow across the arc gap 3 throughout the ~rc welding process. The continuous supply of inert gas prevents impurities from reaching the weld and prevents formation of surface films 9 such as oxides, on the work pieces 1 during the arc welding process. However, it is not necessary to supply i~ert ~as during the welding process if other steps are taken, such as providing a vacuu~ at the arc gap 2, to prevent oxide fonmation and impurities from reaching the weld.

The electrode holder 15 can be one of a variety of co~structions.
For e~ample, the holder 15 can be a ~oving head type whereiD the work pieces 1 and the holder 15 are rotAted relative to each other to effect welding at selected positions o~ the work pieces 1. The holder 15 can be operated to make a continu~us weld on the ~ork pieces I or a series of spot welds.

A voltage sensor 7 senses the voltage drop ~cross the arc ~ap 3 through the electrical leads 19 and 20. This voltage is directly portional to the resistance across the arc gap 3. The ~oltage se~sor 7 supplies a~ electrical signal ~hich indicates the resi~tanse se~sed at the arc gap 3 to a high-low re~ulator 8. The high low regulator B provides duty cycle sig~al ge~erator 9 with a control signal indicating whether the pulse width of the current pulses needs to be increased or decre~sed ~o maintain a constant time-averaged power flow to the work pieces 1 at the arc gap 3.
The high-low regulator 8 is designed so that a control signal is S æupplied to the duty cycle signal generator 9 only when the voltage sensed at the arc gap 3 by the voltage sensox 7 exceeds a preselected high value or is below a preselected low value. The duty cycle generator 9 supplies a continuous control signal to the impulsar 10 to result in a preselected baseline pulsed DC flow across the arc gap 3. However, when ~he duty cycle signal generator 9 receives a signal from the high-low regulator 8 it responds to al~er the operation of impulsar 10. The duty cycle signal generator 9 supplie6 a signal to the impulsar 10 to increase the pulse width of the current pulses or decrease the pulse width of the current pulses depending on the control signal received from the high-low regulator 8.

It should be noted tha~ the voltage sensor 7 is not ~he only type of sensor which ~ay be used to sense power flow related conditions ~ at the arc gap 3. ~or example, a thin film resistance temperature detector (RTD~ may be attached to the work pieces 1 to generate aD
electrical signal which is a function of $he temperature of the work pieces 1. Change in the temperature of the work pieces 1 are a reliable indicator of variations in the power flow to the work pieces 1. The electrical signal o~ the RTD device can be used to supply the high~low regulator 8 with a voltage signal representing power flow which can be processed by the high-low xegulator $ in the same manner as the electrical signal from the voltage sensor 7 is processed.
Referring now to Figure 2, a schematic graph is shown o~ curre~
pulses varyi~g in pulse width as a function of ti~e but having a constant period To between pulses. Tke positive DC pulses are preferably of the special ~ovel type d~scribed previously ~herein the leading edge of each current pulse i~ chosen to have a ratio of peak curr~nt to ~aintenance current which is ma~i~ized to provid~ a thenmal shock effect to dissipa~ oxides which ~ay have formed on the surfaces of the work pieces l. ~he difference in the thermal coefficient of expansions of an o~ide layer and the underlying pure metal results in the thermal shock effect which dissipates the oxides. The present method of pul~e width modulation is especially designed for this type of pulsed DC arc welding.

The main principle of the present invention is the variation in pulse width to maintain power flow to the work pieces l constant for varying resistances at the arc g8p 3. ~he constant power flow to the work pieces improves the quality of the weld. Sagging may occur if the weld is made by supplying an excessive a~ount of power to the work pieces l. Also, there is a possibility of burning through the work pieces if too much power is supplied to the work pieces 1. If too little power is supplied to the work pieces 1 there may not be sufficient power to fully penetrate the work pieces l. A weaker and less durable weld results compared to when optimal power flow to the work piece is maintained. The present invention reduces the possibility of poor weld quality by always maintaining optimal power flow to the work pieces.

For purposes of explanation, assume that a constant ti~e-averaged power flow Pc gives tbe optimal ~eld for a par~icular pulsed DC arc welding process. The time-averaged power equation is:
P=~ V(t)I(t)dt, where V(t) i8 the voltage drop acros~ the arc gap 3 as a function of time, I(t) is the current flow acroæs the arc gap as a function of ti~e, and T is the period of the current pulses.
Assume a constant period T~ corresponding to a fixed frequency Fo9 a constant voltage drop VO~ and a periodically varying current flow given by the following function repeating itself during each successive p~riod To:

.. . .. . .

~ Ip o5 t< x I(t) =
I XC tC T
m -- o wbere I is a constant peak current value, where I is a constant ~aintenance current Yalue, which for purposes of this discussion can be assumed to be zero, and where X is the duty cycle of the current pulses. Assuming that I is zero, then integrating and sol~Jin~ the power equation for X gives:

X = PC/(VoIp) This is the duty cycle necessary to su~tain an optimal power flow Pc to the work pieces I at the arc gap 3, while main~aining a peak pulsed current flow of Ip, when the voltage drop across the arc gap 3 is a constant VO.

If the voltage drop across the arc gap 3 increase~ to 2VO then solving for X gives:

X = PC/ (2VoIp) Thus, the duty cycle must change to one-half th~ original duty cycle to sustain the optimal power flow Pc to the work pieces 1.
If the voltage drop increases to 4Vo then X ~ PC/(4VoIp) and the duty cycle must change to one-quarter the original duty cycle to sustain this constant optimal power flow P .

The ahove-described ~ariatioD in duty cycle ~ is illustrated in Figure 2 where initially it has been assumed that a 50% duty cycle is required to sustain an optimal po~er flow Pc to the wvrk pieces 1 at a constant voltage drop VO across th~ arc gap 3. A 50% duty cyclP corresponds to current flowing across the arc gap 3 fo~ 5~
of the operating time. During the other 50% of the op~rating ti~e only the main~enance current I flow~ across the arc gap 3. The second two pulses shown in Figure 2 represen~ the pulses g~Qerated when the voltage sensor 7 senses an increased voltage drop at the arc gap 3 equal to 2V . An increased voltage drop indicates an increase in resistance across the arc gap 3 which means that the pulse width must be decreased to sustain the sa~e power flow Pc to the work pieces 1 at the arc gap 3. Thus, as shown by the second two current pulses the duty cycle is decreased to 25~,. The third group of two pulses illustrates the variaton in pulse width as the voltage drop acrsss the arc gap increases further to 4VO indicating a further increase in resistance across the arc gap 3. The pulse width is decreased to give a 12 1/2~ duty cycle. Thus, although the instantaneous power delivered to the work piece varies the time-averaged power delivered to the work pieces is constant.
Also, it should be noted that the peak a~plitude of the current pulses is maintained constant at a value I .
p Figure 3 shows specific electrical components for the feedback control circuit 2 comprising voltage sensor 7, high-low regulator 8, and duty cycle sip,nal ge~erator 9 of the arc welding syste~
shown in Figure 1. The voltage sensor 7 comprises variable resistance device 21, resistor 22, and light emitting diode (LED) 23. Electrical leads 19 and 20 are connected across the arc gap 3 as shown in Figure 1. The variable resistance device 21 acts BS a voltage divider to control the flow of current through resistor 22 and light emitting diode 23.

High-low regul3tor 8 comprises a variety of co~ponents includi~g phototra~sistor 29, op a~p 30, op Bmp 32 and variable resistance devices 34 and 35. Phototransistor 29 and capacitor 38, which are electrically con~ected in parallel to voltage supply 31, provide a voltage signal, representing the varying arc vsltage which i8 se~sed by the voltage sensor 7 and transferred ~o the phototxansistor 29 from L~D 23, to the i~verting i~puts o the op amps 30, 32. This representative voltage signal is provided to the inverting input of op a~p 30 thrsugh isolation resistor 39 ~nd to the inverting input of op amp 32 through isolation resistor 40.
This representative voltage signal is the input signal which is processed by the high-low regulator 8 to ~odulate the operation of the duty cycle generator 9. The phototransistor and capacitor 38 are connected to ground through resistor 25. It should be noted that the term "ground", when used in describing the high-lo~
regulator 8 and the duty cycle ~enerator 9, is equivalent to circuit common.
A voltage supply 33 supplies a reference voltage, which is adjusted by variable resistance device 34, to the inverting input of op amp 30. Similarly, variable resistance device 35 adjusts the reference voltage supplied by voltage supply 33 to provide an adjusted reference signal to the inverting input of Qp amp 32. The signal from the variable resistance device 34 is provided to the op a~p 30 through isolation resistor 36 and the signal from the variable resistance device 35 is provided to op amp 32 through isolation resistor 37. These adjusted reference voltage signals are summed with the representative voltage signal from the phototransistor 29 at the inverting inputs of the op amps 30 and 32.

The non-inverting inputs of the op amps 30 and 32 are connected to ground through resistors 41 and 42, respectively. Variable resistance device 43 and capacitor 44 are connected in parallel to op amp 30 ~o control the gain of the op amp 30. Also 9 ~ime delay switch 45 is connected in parallel to the op amp 3~ to provide a shunting capability across the op amp 30. ~imilarly for op amp 32, variable resistanc~ device 46, time delay means 47 and capacitor 48 are connected in parallel to the op a~p 32 for the same pu~poses.
Diode 49, resistor 50 and variable resistance device 52 are con~ected in series at the ouput of op amp 30. Diode 49 bloc~s the transmission of negati~e output voltage signals fr~ the Dp amp 30.
Similarly, op amp 32 has diode 53, resistor 54 and variable resistance device 55 cGnnected in series at the ol-tput of op a~p 3~. Diode 53 blocks positive output voltage sig~als from the op amp 32. It should be noted that the variable resis~ance devi~es 52 nnd 55 control the magnitude of the voltage signals outputted from the op amps 30 and 42, respectively. The voltage signals from the op amps 30 and 32 are sum~ed at point 56 and supplied to the duty cycle signal generator 9 shown in Figure 1.

The duty cycle signal generator 9 comprises a voltage divider including variable resistance device 57 with resistor 60 and variable resistance device 48 with resistor 61. Also, the generator 9 includes op amp 59. A voltage supply 62 supplies vol~age to the variable resistance devices 57 and 58. A time delay switch 63 is connected in parallel to the variable resistance device 58 to provide a means for shunting variable resistance device 58. The outpnt from the voltage divider is supplied through resistor 64 to the inverting input of op amp 59. This voltage signal is summed with the voltage signal from the high-low regulator 9. The non-inverting input of op amp 59 is connected to ground through resistor 65. The gain of op amp 59 is controlled by variable resistance device 66 and capacitor 62. The output from op amp 59 is supplied to impulsar 10, as shown in Figure 1, through lead 69. The voltage divider insures that a reference signal is always generated by op amp 59 for supply to impulsar 10. lf a signal is present at point 5S it is summed with the output from the voltage divider to provide an adjusted input signal to the op amp 59 which adjusts the output signal from op a~p S9.

In operation, voltage is sensed at the arc gap 3 through leads 19 and 20 of voltage sensor 7. This voltage sig~al is adjusted by the variable resistance device 21 and supplied thro~gh the resistor 22 to the light emitting diode (LED) 23. The light emitting diode 23 emits light having in intensity which varies in direct proportion to the voltage sensed at the arc gap 3. Thus, a higher sensed ~oltage causes the L~D ~3 ~o emit light of a greater intensity.

The phototransistor 29 detects the intensity of the light from the LED 23. This interaction of the photo~ransistor 2g and the LED 23 occurs within an area 24 of the feedback control circuit 2, as shown in Figure 3. The phototransistor 29 is utilized to electrically isolate the high-low regulator B fro~ the voltage sensor 7 thereby isolating the arc welding power supply 4 from the high-low regulator 8. The relatively slow response speed of the phototransistor 29 eliminates undesirable spurious electrical interference from being pic~ed up by the high-low reglllator 8.
Capacitor 38 a~d phototransistor 29 function to create a representative voltage signal from the variable voltage signal which is sensed at the arc gap 3 by the voltage sensor 7 and which is transferred to the phototransistor 29 from the LED 23. The phototransistor 29 is powered by a voltage supply 31.
The representative voltage signal fro~ the phototransistor 29 is sl~ed at the i~ver~ing inputs of the op amps 30 snd 32 with an adjusted reference voltage from voltage supply 33. The reference voltage supplied to op amp 30 is adjusted by variable resistance device 34 and the reference voltage supplied to op amp 32 is adjusted by variable resistance device 35. These reference voltage signals are adjusted to cancel particular representative voltage signals from the phototransi~tor 29 which are generated when particular preselected voltages are sensed at the arc gap 3. The selected voltage~ are a hi8h voltage and a low voltage corresponding to a high power flow level to the work pieces 1 and a low power flow level to the work pieces 1, respec~ively. The hi8h power flow level is a power flow level above She optimal time-averaged power flow level and ~he low power flow level is a power flow level below this optimal level. The high a~d low power flow levels are limits beyond which it is ~ndesirable to have the time-averaged power flow deviate if optimal welding is ~o be achieved. The high and low voltages ~ay be selected to equal each other if no deviation from optimal time-aversged power flow is to 3~ be tolera~ed. ~owever 9 u~ually some deviation is allowed to preven~ the high~low regulator 8 from constantly modulating the duty cycle of the current pulses supplied across the arc gap 3 to the work pieces 1.

For example, if a representative voltage signal rom phototransistor 29 to the inverting input of op amp 30 of minus 2.5 volts occurs at the selected high voltage signal corresponding to a high power flow level which it is desired not ~o exceed, then variable resistance device 34 is set so that a plus 2.5 volts is supplied from the voltage supply 33 to this inverting i.nput of op amp 30. A positive output voltage signal, which is allowed to pass to the duty cycle generator ~ by diode 49, appears from the op amp 30 only when the phototransistor 29 provides a representative voltage signal to the inverting input of op amp 30 of less than minus 2.5 volts. The amount by which this representative voltage signal is less than the ~inus 2.5 volts corresponds to the a~ount by which the duty cycle of the cu~rent pulses at the arc gap 3 ~ust be decreased ~o maintain the optimal time-averaged power flow to the work pieces 1. Alternatively, if a minus 1.5 volts representative signal occurs at the selected low voltage signal corresponding ~o a low power flow level which it is desired not to fall below, then variable resistance device 35 is set so that a plus 1.5 volts is supplied from the voltage supply 33 to th~
inverting input of op amp 32. A negative outpu~ ~oltage signal~
which is allowed to pass to the duty cycle generator 9 by diode 53, appears fro~ the op amp 32 only when the phototransistor 29 provides a representatiYe voltage signal to the inverting input of op amp 32 o greater than minus 1.5 ~olts. The amo~n~ by which this representative voltage signal is greater than ~he minus 1.5 volts corresponds to the amount by which the duty ~ycle of the current pulses at the arc gap 3 ~st be increa6ed to ~aintain the optimal ti~e-averaged po~er flow the work pieces 1.

Thu~" the op a~ps 30, 32 and diodes 49, 53 operate to p~ovide an output voltage signal only when the voltage sensed at the arc gap 7 and transmitted to the phototransistor 29 exceed certain limi~s which are set by the variable resistance devices 34 and 35. If the variation in the voltage at the arc gap 3 does not exceed one of these preset limits then no voltage signal is outputted from ~he Qp amps 30 and 32 to point 56. However, i the voltage sensed should e~ceed either of the preselected limits then a voltage proportional to the amount by which the voltage ~ce~ds the limit is outputted from ei~her op amp 30 or op amp 32. The magnitude of this output voltage signal is adjusted by ~he resistance devices 50, 52 for the op amp 30 and by the resistance devices 54, 55 for the op amp 32.
This ajusted ~oltage signal is supplied to the inYerting input of the op amp 59.

The voltage divider provides a continuous signal to the inverting input of op amp 59. Thus, if no voltage signal is supplied from op amps 30 and 32 to the op amp 59 the op amp 59 will still have an output corresponding to the signal supplied at its inverting input from the voltage divider. If there is a signal present at the point 56, this signal is summed with the signal from the voltage divider to provide an altered signal at the inver~ing input of op amp 59O The voltage signal at poiDt 56 varies depending on the voltage sensed at the arc gap 3 by ~he voltage sensor 7. The signal from op amp 59 is transmitted tbrough the lead 69 to the impulsar 10 of the arc welding system.

Time delay switches 45, 47 and 63 are used to prevent the feedback control circuit 2 from improperly operating during the start-up period for the arc welding syste~. Initially, switches 45 and 47 are closed and switch 63 is open9 as show~ in ~igure 3. When ~witche~ 45 and 47 are cl~sed op amps 30 and 3~, respectively, are shunted and thereby prevented from operating. If the op a~p 30 was ~llowed to operate when the arc starter 5 is ionizin~ the inert ~as at the arc gap 3 and initiatin8 current flo~ ~cross the axc gap 3, ~hen it would defeat the operation of the arc ~tarter 5~ After the arc starter 5 has co~pleted its function and after a irst pres~lected time delay the normally open switch 63 closes ~nd the normally closed switch 47 opens. During this first preselected time delay the cuxrent pulses supplied at the arc gap 3 have a larger duty cycle (pulse width) than desired for steady-state operation of the welding system. This larger duty cycle, during this first time delay, insures that proper heat transfer, fusion, and penetration is occurring at the ~ork pieces 1 during the start-up period. When nonmally open switch 63 closes the variable resistance device 58 is shunted thereby lowering the voltage signal which is provided to op amp 59 from the voltage divider circuit.
This alters the voltage control signal outputted from op a~p 59 to step-down the duty cycle of the current pulses supplied to the arc ~ap 3. The duty cycle of the current pulses is decreased to the duty cycle desired for steady-state operation, that i~, to that duty cycle which has previously been determined to result in optimal power flow to the work pieces 1. Normally closed switch 47 opens at the same time tha~ non~ally open switch 63 closes thereby enabling op amp 32 to pro~ide low limit regulation of the current pulses. Thus, if the voltage drop across the arc gap 3 is not sufficient to achieve optimal power flow at the d~creased duty cycle then the op amp 32 operates to increase the duty cycle of the current pulses to compensate for this deficiency.

After a second preselected time delay, after step-down, the switch 45 opens to enable op amp 30 to provide high limi~ regulation of the current pulses. Op amy 30 is not enabled ustil a~ter step~down to prevent the op amp 30 from interfering with arc stabilization during the start-up period. The opening of switch 45 ends the start~up period. Also, thi~ allows the hi~h low regulator 8~
through the operation of op a~Rs 30 a~d 32, to modulate the duty cycle control signal outputted by op amp 59 of the duty cycle signal generator 9~ as described previously.

The i~pulsar 10 of a conventional ars welding system com~o~ly includes a circuit having a co~parator for g~n~rating a control ~20-signal for the current regulator ll. Thus, the level of the voltage signal supplied by the duty cycle signal generator 9 to the impulsar 10 determines the duty cycle of the current pul~es supplied by the power supply 4 to the arc gap 3. As the magnitude of the voltage signal from the op amp 59 of the duty cycle generator 9 increases the duty cycle of the pulses supplied a~ the arc gap 3 increases. This is accomplished by supplying the voltage signal from the duty cycle signal generator 9 to the comparator of the impulsar lO and then properly processing the output voltage signal from this co~parstor. An e~a~ple of the operation of such a comparator is explained in the following discussion of Figures 6 and 7. It should be noted that there are many techniques of utilizing the volta~e signal supplied from the du~y cycle signal generator 9 to adjust the pulse width of the current pulfies supplied at the arc gap 3. Al~o, it should be noted that the selected technique depends on the construction of the particular impulsar 10 which is being used. The foregoing is only one such technique for an impulsar which includes a particular comparator circuit.

Referrin8 now to ~igure 4, a block diagram is shown for an arc welding system having a time delay programmable pulse width controller 80 used as part of the control sy~te~ for the power supply of the arc welding system. As show~ in Figure b, wor~
pieces 70 and an electrode 71 fonm an arc gap 73 across which a voltsge is supplied by power supply 74. High frequenoy high voltage arc starter 75 is also connected across the 8rc gap 73 ~o provide an initial hi8h voltage for ionizing the inert gas ~upplied at the arc gap 73 from the ga~ supply meanG 76 at the be~i~nin~ of a welding cycle and for initiating current flow across the arc gap 73. Ater this initial ionization and after the initial ~urrent flow begins the arc starter 75 di3continues operation. The power supply 74 is a commercially available pulsed positi~e ~C power ~upply. A conventional current regulat3r 77 controlled by a 5~3 conventional impulsar is used to control the opera~ion of ~h~ power ~upply 74.

As shown in Figure 4, the impulsar is depicted as divided into two parts. One part is designated an iEpulsar pulse width adju~tor 79 and the other is designated an impulsar output system 78. The impulsar pulse width adjustor 79 is that part of the conventional impulsar which generates internal control sig als for the impulsar, typically voltage signals, for controlling operation of electrical devices of the impulsar. Typically, the magnitude of an internal voltage control signal 101 supplied through an electrical lead 81 from the pulse width adjustor 79 the impulsar output system 79 determines what pulse width control signal 104 will be outputted from the impulsar, as depicted in Figures 6 and 7. Other internal control signals may flow from the impulsar pulse width adjustor 79 to the i~pulsar output system 78 via electrical connector 82.

The impulsar output system 7B is that part of the conventional impulsar which generates an output control signal 104 for ~he current regulator 77 in response to the iDternal voltage control signal 101 from the impulsar pulse width adjustor 79. Typically, the impulsar output system 78 includes a co~parator which co~pares a reference voltage si~nal 102, such a~ a voltage ramp function~ to the internal volta~e control ~ignal 101 from the pulse width 2S adjustor 79. Figures 6 and 7 depict how the co~para~or operates to generate outpu~ voltage signals 103 of different pulse widths in response to internal voltage control ~ignals 101 of differ~nt magnitudes. Basically, the comparator generates an output voltage ~ignal 103 only when the referencP voltage signal 102 equals or exceeds the internal voltage control ~ignal 101. Thus7 the internal voltage control signal 101 shown in Figure 6 re~ults in a Rmaller pulse width for the comparator output Yoltage ~ignal 103 compared to when the reduced internal Yoltage co~trol ~ignal 101 ~hown in Figure 7 is ~tilized. Other conventional circuit ele~ents of the impul~ar output ~ystem 78 re~pond to the co~parator output _ ~ . . ~ . .

voltage signal 103 to generate an impulsar output control signal 104 for the current regulator 77. This output control si~nal 104 is keyed to the o~f-times of the comparator ou~put voltage signal 103 so that the duty cycle of the current pulses supplied by the S power supply 74 to the arc gap 73 is keyed to the off-times of the comparator output volta~e signal 103. Thus, an increaae in the internal voltage control signal, 101 9 which causes a decrease in the pulse width of the comparator output voltage signal 103, results in a corresponding increase in the duty cycle of the current pulses supplied at the arc gap 73.

The impulsar pulse width adjustor 79 of the arc welding system dir~ctly controls the operation of the i~pulsar output system 78.
However, according to the principles of the present invention, a time delay programmable pulse width controller 80 is interposed between the conventional pulse width adjustor 79 of the impulsar and the impulsar cutput system 78. This programmable pulse width controller 80 operates to automatically adjust the impulsar output system 78 to control the current regulator 77, and thus power supply 74, to provide current pulses of varying pulse width according to a predetermined programmed sequence.

The programmable pulse width controller 80 can be of a variety oE
constructions. The controller 80 can be most simply constructed by providing a variable resistance device 89 and a time delay switch 88 connected in parallel to each other and in series bPt~een the pulse width adjustor 79 and the impulsar output system 76. Such a oontroller 80 is shown in Figure 5. It should be noted that the circuit shown in Figure 5 is a simple e~a~plP of a progra~able pulse width controller 80. Other cii^cuits could ~e devised by one of ordinary skill in the art to provide other more comple~ current programs.

In operation, positive DC puls2s are ~upplied at the ~rc gap 73 by the power supply 74 as controlled by the current regulator 77 i~

response to the input from the impulsar output system 78.
Initially a signal is supplied through the normally closed contacts of the time delay switch 88. ~owever, time delay switch 88 operates after a preselected time delay to open the nor~ally closed contacts. This interposes the variable resistance device 89 between the impulsar pulse width adjustor 7g and the impulsar output system 78. This results in a different signal being provided to the current regulator 77 and in ~urn to the power supply 74. This different signal adjusts the duty cycle of the pulses supplied at the arc gap 73. Typically, the duty cycle of the current pulses is decreased after the time delay. Nonmally, a dec~ease is required since there is a heat build-up a~ ~he work pieces 70 during the staxt-up period of operation of the arc welding system. Thus, it is necessary to reduce the power flow to the work pieces 70 after a period of time to maintain the optimal power flow which will consistently achieve good quality welds. The particular time delay and amount of reduction in duty cycle to achieve optimal welding depends on the particular work pieces 70 being welded. These parameters are best selected through a trial and error process.

Finally, it should be noted that9 although the pulse width modulation of DC pulses according to the principles of the present invention is particularly suited for welding materials, such as aluminum, when using ~he special ~ovel type of current pulse described previously, the present invention is not li~ited to use with this type of pulse. Pulse width modulation according to the principles of the prese~t invention provides precise control of power flow to work pieces at an arc gap when arc welding practically any kind of material with DC pulses. For exa~ple, conventional DC pulses used in welding together stainless steel wor~ pieces, especially thin wall pieces of stainless steel9 can be ~odulated according to the principles of the p~esent invention to proside precise control of the power flow to the ~ork pieces to make high quality welds. Therefore, ~hile the present invention 5~

has been described in connected with particular embodiments, it is to be u~derstood that various other embodiments and modifications may be made without departing from the scope of the invention heretofore described and claimed in the appended claims.

Claims (10)

The embodiments of the invention on which an exclusive property or privilege is claimed are defined as follows:

1. A feedback control device for a pulsed direct current (DC) arc welding system for arc welding work pieces comprising a welding electrode for supplying electrical power to the work pieces at an arc gap and a power supply means for supplying a voltage to the welding electrode to generate a periodic series of direct current pulses which are supplied across the arc gap to weld the work pieces; a monitoring means for sensing the resistance across the arc gap as the work pieces are welded; and pulse width adjustment means for modulating the duration of the current pulses in response to the resistance sensed at the arc gap, while maintaining the peak magnitude of the current pulses constant, to provide a constant time-averaged power flow to the work pieces.

2. The feedback control device as recited in claim 1 wherein the monitoring means comprises a variable resistance device electrically connected in series between the work pieces and the electrode to form a first electrical circuit; and a light emitting diode electrically connected in parallel to the variable resistance device and in series between the electrode and the work pieces to form a second electrical circuit whereby said diode emits light which varies in intensity in direct proportion to the resistance across the arc gap and whereby said diode emits light having a background intensity which is determined by the resistance setting of the variable resistance device and the magnitude of the voltage supplied to the welding electrode.

3. The feedback control device as recited in claim 1 wherein the pulse width adjustment means comprises a high low regulator means for responding to the resistance sensed by the monitoring means to provide a first output signal only when the sensed resistance at the arc gap exceeds a high limit and to provide a different second output signal when the sensed resistance at the arc gap is below a low limit; and a duty cycle control means for decreasing the duty cycle of the current pulses generated by the power supply means when the high-low regulator means provides the first output signal indicating that the resistance at the arc gap is above the high limit and for increasing the duty cycle of the current pulses generated by the power supply means when the high-low regulator means provides the second output signal indicating that the resistance at the arc gap is below the low limit whereby the current pulses applied across the arc gap have a constant peak current value and are varied in pulse width to provide a constant time-averaged power flow to the work pieces.

4. The feedback control device as recited in claim 3 wherein the high-low regulator means comprises a first operational amplifier means for providing a positive output voltage signal to the duty cycle control means when the monitoring means senses a resistance across the arc gap below the low limit; a second operational amplifier means for providing a negative output voltage signal to the duty cycle control means when the monitoring means senses a resistance across the arc gap which is above the high limit; a reference power supply means for supplying summing voltages to the inputs of the first and second operational amplifier means; and a phototransistor for detecting the intensity of the light emitted from the light emitting diode and for generating a voltage, indicating the value of the resistance sensed at the arc gap, which is summed with the summing voltages of the reference power supply means at the inputs of the operational amplifier means to produce an effective input signal for the first operational amplifier means only when the sensed resistance exceeds the high limit and to produce an effective input signal for the second operational amplifier means only when the sensed resistance is less than the low limit.

5. The feedback control device as recited in claim 3 wherein the duty cycle control means comprises a voltage divider including first and second variable resistance device means which are electrically connected in series between a voltage supply and ground, and a summing means including an operational amplifier having its inverting input electrically connected to the voltage divider at a point between the first and second variable resistance devices and to the output of the high-low regulator means, said operational amplifier generating an output voltage signal which is used to control the power supply means in response to the summed input voltages from the voltage divider and high-low regulator means.

6. A method of arc welding of work pieces which includes positioning an electrode and the work pieces relative to each other to form an arc gap; providing a maintenance current flow across the arc gap, said maintenance current providing a power flow which is insufficient to increase the temperature of the work pieces to the melting temperature of the work pieces; increasing the magnitude of the current flowing across the arc gap to a peak value which can provide sufficient power flow to melt the work pieces and which is of sufficient magnitude that a power flow is provided which is capable of dissipating oxides on the surfaces of the work pieces during the time interval in which the increase in current flow occurs; holding the current flow across the arc gap at substantially the increased value for a duration of time sufficient to provide enough energy to heat the work pieces to their melting temperature; decreasing the magnitude of the current flowing across the arc gap to substantially the maintenance current value to allow the temperature of the work pieces to decrease to a temperature below their melting temperature whereby a weld is made on the work pieces; periodically cycling the current flow across the arc gap by repeating the steps of increasing, holding and decreasing the current flow to vary the magnitude of the current flowing across the arc gap between the maintenance current value and the peak current value to form a periodic series of current pulses which are applied to the work pieces; adjusting the duration of time at which the current flow across the arc gap is held at substantially the increased value to modulate the duration of the current pulses while maintaining the peak current value constant to provide a selected power flow to the work pieces; and changing the relative position of the electrode and the work pieces to direct each current pulse to a selected portion of the work pieces.

7. The method as recited in claim 6 which further comprises providing inert gas continuously at the arc gap after positioning the electrode and the work pieces relative to each other to form the arc gap; and then applying a voltage across the arc gap sufficient to ionize the inert gas and to initiate current flow across the arc gap prior to providing the maintenance current flow across the arc gap.

8. The method as recited in claims 6 and 7 which further comprises continuously sensing the resistance at the arc gap; and wherein the step of adjusting includes varying the duration of time at which the current flow across the arc gap is held at substantially the increased value in response to the resistance sensed at the arc gap to provide a constant time-averaged power flow to the work pieces.

9. A control device for the power supply of a pulsed DC arc welding system used to weld work pieces at an arc gap comprising a welding electrode for supplying electrical power to the work pieces; power supply means for supplying a voltage to the welding electrode to create a periodic series of direct current pulses which are supplied across the arc gap to weld the work pieces, said control device characterized by an impulsar means for providing a control signal to the power supply means said control signal controlling the pulse width of the current pulses applied at the arc gap; and a programmable pulse width control means for automatically adjusting the impulsar control signal to provide a preselected adjustment to the pulse width of the current pulses without otherwise substantially affecting the form of the current pulses.

10. The control device as recited in claim 9 wherein the impulsar means comprises a comparator means having an input terminal and an output terminal, said output terminal providing the control signal to the power supply means for controlling the pulse width of the current pulses supplied cross the arc gap, and said input terminal receiving an electrical signal from the programmable pulse width control means which controls the signal generated at the output terminal of the comparator; and wherein the programmable pulse width control means is characterized by a time delay normally closed switch means for providing a first electrical signal to the input terminal of the comparator means which causes the impulsar means to generate a first pulse width control signal; and a variable resistance device electrically connected in parallel with the switch means for providing a different second electrical signal to the input terminal of the comparator means which causes the impulsar means to generate a different second pulse width control signal, said second signal effective only when the normally closed switch means opens to allow the variable resistance device to control the operation of the comparator means after a fixed time period during which the normally closed switch means is controlling the operation of the comparator means.