Note: Descriptions are shown in the official language in which they were submitted.
53
--1--
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
53
-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~
-7-
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
-10-
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:
.. . .. . .
-12-
~ 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
-13-
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
-14-
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
-15-
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.
-16-
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
-17-
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
-18-
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
-19-
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
21-
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~
-23-
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~
-24-
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.
