U.S. patent number 3,896,287 [Application Number 05/378,265] was granted by the patent office on 1975-07-22 for direct current arc power supply.
This patent grant is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to George E. Cook.
United States Patent |
3,896,287 |
Cook |
July 22, 1975 |
Direct current arc power supply
Abstract
An arc power supply system having a three phase transformer and
a full wave rectifier which includes silicon controlled rectifiers.
A feedback control system includes first and second cascaded
operational amplifiers. The first amplifier sums (1) a reference
set point signal, (2) an adjustable voltage feedback signal and (3)
an adjustable current feedback signal. The first amplifier provides
low pass filtering to smooth the otherwise high ripple content of
the two feedback signals. The second amplifier provides both
integral and proportional control and drives a firing circuit. By
the use of a single slope adjustment control, the current and
voltage feedback signals may be adjusted to provide infinite and
continuous control of the voltampere characteristic of the power
supply between a constant current characteristic and a constant
potential characteristic. An adjustable start circuit provides
control of the average output voltage, and hence the initial arc
energy, at the start of operation to more easily establish the
arc.
Inventors: |
Cook; George E. (Brentwood,
TN) |
Assignee: |
Air Products and Chemicals,
Inc. (Allentown, PA)
|
Family
ID: |
23492408 |
Appl.
No.: |
05/378,265 |
Filed: |
July 11, 1973 |
Current U.S.
Class: |
219/130.33;
363/79 |
Current CPC
Class: |
H05H
1/36 (20130101); H02M 7/1557 (20130101); B23K
9/1056 (20130101); B23K 9/073 (20130101) |
Current International
Class: |
B23K
9/06 (20060101); B23K 9/10 (20060101); B23K
9/073 (20060101); H02M 7/12 (20060101); H02M
7/155 (20060101); H05H 1/36 (20060101); H05H
1/26 (20060101); B23k 009/10 () |
Field of
Search: |
;219/131F,131WR,131R,135
;321/19 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
A F. Manz, "The One Knob Welder," Welding Journal, Sept. 1968, pp.
720-725..
|
Primary Examiner: Truhe; J. V.
Assistant Examiner: Shaw; Clifford C.
Attorney, Agent or Firm: Simmons; James C. Moyerman;
Barry
Claims
I claim:
1. An arc power supply for controlling the volt-ampere
characteristic of the direct current applied by way of lead means
to a pair of electrodes which establishes and maintains an arc
between said electrodes, comprising:
power means operable for providing through said lead means direct
current to said electrodes,
reference setpoint means to produce a set reference signal related
to a desired value of said direct current,
adjustable means connected to said lead means for sensing said arc
current and providing an adjustable current feedback signal,
adjustable means connected to said lead means for sensing the
voltage across said electrodes and providing an adjustable voltage
feedback signal,
feedback control means coupled to said power means for summing said
set reference signal, said adjustable current feedback signal and
said adjustable voltage feedback signal for applying a control
signal to said power means to control said volt-ampere
characteristic, and
means for simultaneously adjusting both said adjustable current
sensing means and said adjustable voltage sensing means to provide
infinite and continuous control of said volt-ampere characteristic
of said direct current between and including a constant current
characteristic and a constant potential characteristic.
2. The arc power supply of claim 1 in which said simultaneously
adjusting means comprises means ganging said adjustable current
sensing means and said adjustable voltage sensing means to provide
a single slope adjustment control which when set (1) to one extreme
said current feedback signal is maximum and said voltage feedback
signal is zero thereby providing said constant current volt-ampere
characteristic and (2) to another extreme said current feedback
signal is zero and said voltage feedback signal is maximum thereby
providing said constant potential volt-ampere characteristic.
3. The arc power supply of claim 1 in which said adjustable current
sensing means comprises a current sensor coupled to said lead means
and voltage potentiometric slope adjustment means coupled to said
current sensor for providing said adjustable current feedback
signal proportional to the current flow through said lead means,
and in which said adjustable voltage sensing means comprises load
resistor means coupled to said lead means and voltage
potentiometric slope adjustment means coupled to said load resistor
means for providing said adjustable voltage feedback signal
proportional to said voltage across said electrodes.
4. The arc power supply of claim 3 in which said simultaneously
adjusting means comprises means for ganging said current and
voltage potentiometric means so that (1) at one extreme setting of
said ganging means the current feedback signal is maximum and the
voltage feedback signal is of zero value to produce a control
signal proportional to said set reference signal for establishing a
desired setpoint of current, and (2) at another extreme setting of
said ganging means and voltage feedback signal is maximum and the
current feedback signal is of zero value to provide a control
signal proportional to the set reference signal for establishing a
desired setpoint of voltage; with intermediate settings of said
ganging means producing proportional control signals related to the
slope characteristics established by the current and voltage
feedback signals.
5. The arc power supply of claim 1 in which said feedback control
means includes first amplifier means for summing said set
reference, said adjustable current and adjustable voltage feedback
signals and for providing a low pass filtering characteristic for
smoothing the high ripple content of said feedback signals.
6. The arc power supply of claim 5 in which said feedback control
means includes second amplifier means connected to the output of
said first amplifier means for providing integral plus proportional
control of the first amplifier output and for applying the
resultant signal as said control signal to said power means.
7. The arc power supply of claim 6 in which said power means
comprises rectifying means having silicon controlled rectifiers for
rectifying a source of alternating current for providing said
direct current and firing means adapted to provide properly timed
and spaced firing pulses to rectifiers of said rectifier means
under the control of said control signal.
8. The arc power supply of claim 5 in which said first amplifier
means comprises first, second, and third input circuits connected
respectively to said reference setpoint means, said adjustable
current sensing means and said adjustable voltage sensing means, a
first operational amplifier having an input connected to a common
junction coupling said first, second, and third filter input
circuits, a resistance-capacitance network providing feedback for
said first operational amplifier, and said first, second, and third
input citcuits and said feedback network forming a Butterworth
network.
9. The arc power supply of claim 8 in which said second input
circuit is a resistance-capacitance network which together with
said resistance-capacitance feedback circuit provide (1) adequate
filtering of said current feedback signal and (2) proper voltage
gain so that said current feedback signal is compatible with said
set reference signal and said voltage feedback signal.
10. The arc power supply of claim 9 in which said third input
circuit is a resistance-capacitance network which in conjunction
with said resistance-capacitance feedback circuit provides adequate
unity gain filtering of said voltage feedback signal.
11. The arc power supply of claim 10 in which said first input
circuit is a resistance-capacitance circuit having values selected
in conjunction with the values of said resistance-capacitance
feedback circuit provide a transfer function of the said set
reference signal which is compatible with the current feedback
signal and said voltage feedback signal.
12. The arc power supply of claim 6 in which said second amplifier
means includes a second operational amplifier having a feedback
integrator capacitor to integrate the applied signal to affect zero
error in the steady state.
13. The arc power supply of claim 1 in which there is provided
start means coupled to said feedback control means for varying said
control signal by a start level setting at the start of operation
to provide said direct current of value higher than said desired
value established by said set reference signal.
14. The arc power supply of claim 13 in which said start means
comprises start reference means for establishing a start reference
signal, means connected to said adjustable current sensing means
for comparing a nonadjustable current feedback signal with said
start reference signal for (1) applying at the start of operation
said start level setting and (2) returning said control signal to
its normal value when the arc current increases sufficiently to
increase the nonadjustable current feedback signal to a
predetermined value with respect to said start reference signal so
that said direct current decreases to its desired value established
by said set reference signal.
15. The arc power supply of claim 14 in which said comparing means
comprises a third operational amplifier and a relay connected to an
output of said third operational amplifier, said relay having
normally closed contacts connected in circuit with said feedback
control means for applying said start level setting when said
contacts are closed and for removing said start level setting when
said contacts are opened upon actuation of said relay by said third
operational amplifier.
16. A direct current arc power supply for establishing and
maintaining an arc between a pair of electrodes in which upon start
of operation a higher than normal valued startup direct current is
applied to said electrodes, comprising
power means operable for providing direct current to said
electrodes,
reference setpoint means to produce a set reference signal related
to a desired value of said direct current,
means for sensing said arc current and providing a current feedback
signal,
means for sensing the voltage across said electrodes and providing
a voltage feedback signal,
feedback control means coupled to said power means for summing said
set reference signal, said current feedback signal and said voltage
feedback signal for applying a control signal to said power means
to control the volt-ampere characteristic of said direct
current,
start reference means to produce a start reference signal, and
start means for comparing said current feedback signal with said
start reference signal for varying said control signal upon startup
to produce said direct current of value higher than said desired
value which value decreases to said desired value when said current
feedback signal increases to a predetermined value with respect to
said start reference signal.
17. The arc power supply of claim 16 in which said start means
includes an operational amplifier having one input coupled to said
current sensing means, relay means connected to an output of said
operational amplifier and having a normally closed relay contact,
start level setting means connected to said normally closed relay
contact and to said feedback control means for varying said control
signal by a start level setting until said current feedback signal
increases to said predetermined value at which time said relay is
actuated.
18. The arc power supply of claim 17 in which said feedback control
means includes an operational amplifier having an integrator
capacitor connected in a feedback loop therewith and providing at
an output said control signal, said start level setting means
coupled in another feedback loop whereby upon startup a start level
setting is applied to the input of said operational amplifier until
said current feedback signal increases to said predetermined
value.
19. A method of controlling the volt-ampere characteristic of
direct current applied to establish and maintain an arc between a
pair of electrodes which comprises
producing a set reference signal related to a desired value of said
direct current,
sensing the arc current and providing an adjustable current
feedback signal related to the value of the arc current,
sensing the voltage across the electrodes and providing an
adjustable voltage feedback signal related to the value of the
electrode voltage,
summing the set reference signal, the adjustable current feedback
signal and the adjustable voltage feedback signal and controlling
the volt-ampere characteristic as a function of these signals,
and
simultaneously varying both the adjustable current feedback signal
and the adjustable voltage feedback signal to provide an infinite
and continuous control of the volt-ampere slope characteristic of
the direct current between and including a constant current mode
and a constant potential mode.
20. The method of claim 19 in which the simultaneously varying step
includes simultaneously varying the signals so that (1) at one
extreme the adjustable current feedback signal is maximum and the
adjustable voltage feedback signal is zero thereby providing a
constant current volt-ampere characteristic and (2) at another
extreme the adjustable current feedback signal is zero and the
adjustable voltage feedback signal is maximum thereby providing a
constant potential volt-ampere characteristic.
21. The method of claim 19 in which the summing step includes
providing a low pass filtering characteristic for smoothing the
high ripple content of said adjustable current and voltage feedback
signals and then providing integral plus proportional control.
22. The method of claim 19 in which the summing step includes
varying the controlling of the volt-ampere characteristic by a
start level setting at the start of operation to provide the direct
current of value higher than said desired value established by said
set reference signal.
Description
BACKGROUND OF THE INVENTION
l. Field of the Invention 1.
This invention relates to the field of art of direct current arc
power supplies having feedback control systems.
1. Prior Art
Direct current power supplies are known in which an arc is
established and maintained between a pair of electrodes. The
incoming line voltage is reduced in potential by a transformer, the
output of which is full wave rectified for establishing a desired
direct current voltage and current output. Such an arc welding
supply is disclosed in U.S. Pat. No. 3,549,978 in which a polyphase
transformer-rectifier system includes silicon controlled rectifiers
and has both current and voltage feedback control. This feedback
control is effective to establish the desired phasing or firing of
the silicon controlled rectifiers. In this patent, the feedback
control system comprises a pair of summing amplifiers, the first of
which sums (1) a reference set voltage and (2) the current
feedback. The second amplifier sums (1) an output of the first
amplifier and (2) the voltage feedback. A separate slope adjustment
circuit is provided in the current feedback to establish
substantially zero slope even though the resistance in the output
line tends to establish a drooping characteristic. Such prior power
supplies have left much to be desired in that the range of slope
control in the constant potential mode is limited for stable
operation. As a result, such prior supplies have been limited
primarily to the short arc or spray transfer mode of welding where
either a constant potential or a slightly drooping volt-ampere
characteristic is desired.
SUMMARY OF THE INVENTION
In order to overcome the limitations of prior art devices, there is
disclosed herein a direct current arc power supply for controlling
the volt-ampere characteristic of direct current applied to
establish and maintain an arc between a pair of electrodes. A set
reference signal is produced related to a desired value of direct
current. The arc current is sensed to provide an adjustable current
feedback signal. In addition, the voltage across the electrodes is
sensed to provide an adjustable voltage feedback signal. A feedback
control system sums the set reference signal, the adjustable
current feedback signal and the adjustable voltage feedback signal
and applies a resultant control signal to a power circuit to
control the volt-ampere slope characteristic. There is
simultaneously adjusted both the adjustable current sensing means
and the adjustable voltage sensing means to provide infinite and
continuous control of the volt-ampere characteristic of the power
supply between a constant current characteristic and a constant
potential characteristic.
Further in accordance with the invention, the simultaneous
adjustment is provided by a single slope adjustment control in the
form of a mechanical gang between a current slope potentiometer and
a voltage slope potentiometer. These two potentiometers are
interconnected so that with the single slope adjustment control set
to one extreme, the current feedback signal is maximum and the
voltage feedback signal is zero thereby providing a true constant
current volt-ampere characteristic. With the single slope
adjustment control set to the other extreme, the current feedback
signal is zero and the voltage feedback signal is maximum, thereby
providing a true constant potential volt-ampere characteristic.
This single slope adjustment control may be adjusted continuously
between the two extremes to yield an infinitely adjustable
volt-ampere slope characteristic.
In this manner, the arc power supply may be used in applications
without the adverse stability characteristics inherent in the prior
art supplies. Thus, the invention may be used not only in the short
arc or spray transfer modes of welding (Gas Metal Arc Welding) but
may be used equally well for automatic or manual Gas Tungsten Arc
Welding and Shielded Metal Arc Welding. The arc power supply may
use solid state construction with high gain feedback operational
amplifiers to provide a highly reliable and long life system in
addition to compensating for ambient temperature conditions in the
circuitry of the feedback control system. As semiconductor devices
increase in temperature, they tend to decrease in resistance in
direct opposition to the increase in resistance associated with
increase in temperature of resistors in the circuit. Thus in
accordance with the invention, there is provided highly desirable
continuously adjustable slope and setpoint control with a stable
output which is essentially independent of load, line and
temperature conditions.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a direct current arc power supply in
accordance with the invention;
FIGS. 2A-B are volt-ampere characteristics helpful in explaining
the operation of the arc power supply of FIG. 1; and
FIG. 3 is a schematic circuit diagram showing details of the block
diagram of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown an arc power supply system
10 in which a three phase power source 11 supplies a polyphase
welding transformer 12. Transformer 12 provides an alternating
current input to a full wave polyphase rectifier assembly 14.
Rectifier 14 has a pair of output leads 15 and 16 with output lead
or load line 16 indicated as the negative lead and output lead or
load line 15 indicated as the positive lead.
In the illustrated embodiment of the supply system of FIG. 1,
output negative lead 16 is connected to an electrode 20 while
output positive lead 15 is connected by way of a shunt 22 to a work
member 23. When supply system 10 is energized, an arc 26 is
established and maintained between electrode 20 and work member
23.
The foregoing connections of leads 15 and 16 are useful for both
the manual and automatic modes of the Gas Tungsten Arc Welding
("GTAW" or "TIG") process for certain modes of the Shielded Metal
Arc Welding process (dependent on the particular type of shielded
electrode 20 used). For the remaining modes of the Shielded Metal
Arc Welding process and for both the short arc and spray transfer
modes of the Gas Metal Arc Welding process, lead 15 is connected to
electrode 20 while lead 16 is connected to work member 23.
For proper arc welding in the manual mode of the Gas Tungsten Arc
Welding process, the volt-ampere characteristic of system 10, as
shown in FIG. 2A, is preset so that a desired straight line
characteristic 30 is substantially tangent to an ideal constant
power characteristic (K) 31. Voltage V is the voltage across arc 26
taken between leads 15 and 16 while current I is the actual current
through arc 26. The tangency is provided at a desired nominal
operating point 33. Ideal constant power characteristic 31 is a
known characteristic in the Gas Tungsten Arc Welding process.
With the volt-ampere characteristic of system 10 set in this
manner, automatic compensation for variations in changes of
electrode 20 with respect to work member 23 may be achieved. For
example, if electrode 20 is moved away from member 23, the voltage
across arc 26 increases from its nominal value along characteristic
30 to a point 35, for example. Thus the voltage increases from
V.sub.1 to V.sub.2. This in turn results in a decrease in the arc
current I.sub.1 to I.sub.2, thereby tending to maintain the power
input to arc 26 approximately constant.
In the automatic mode of the Gas Tungsten Arc Welding process, a
true constant current volt-ampere characteristic is generally
desired. In the automatic mode, arc voltage is normally maintained
constant by means of a separate automatic arc voltage controller
which controls electrode 26, as described for example in U.S. Pat.
No. 2,516,777 For Control Apparatus for Automatic Welding
Heads.
In the spray transfer mode of the Gas Metal Arc Welding process,
the volt-ampere characteristic of system 10 is preset normally to
either a true constant potential characteristic or a slightly
drooping (negative slope) characteristic. In the short arc mode of
the Gas Metal Arc Welding process, the volt-ampere characteristic
of system 10 is preset normally to a higher magnitude negative
slope characteristic than that in the spray transfer mode. On the
other hand, in the Shielded Metal Arc Welding mode, the volt-ampere
characteristic is set the same as in the manual mode of the Gas
Tungsten Arc Welding process.
Referring again to supply system 10, a reference set point voltage
input 40 (set in a manner to be described) is applied to one input
42a of a summing filter amplifier 42. A second input 42c of
amplifier 42 is connected to a current feedback unit 45 while a
third input 42b to amplifier 42 is taken from a voltage feedback
unit 46. In this manner, amplifier 42 is effective to sum the set
reference signal, an adjustable current feedback signal and an
adjustable voltage feedback signal.
Current feedback unit 45 obtains its current input from a current
sensor or shunt 22 which provides a signal proportional to the
current through lead 15 (arc current I). Load resistors 17 are
connected between leads 15 and 16 and voltage feedback unit 46 is
connected across load resistors 17 to provide a signal proportional
to the voltage across arc 26. Units 45 and 46 comprise
potentiometers which are mechanically ganged by gang 47. The
potentiometers are ganged so that with a setting at one extreme (as
for example, clockwise) the voltage feedback signal at voltage
feedback conductor 44 is of zero value and the current feedback
signal at current feedback conductor 43 is of maximum magnitude.
Accordingly, at the output 50 of amplifier 42 there is produced a
voltage signal proportional to the setting of reference 40 which at
that time establishes the desired set point of current.
On the other hand, with gang 47 set at the other extreme (as for
example counterclockwise), the current feedback signal at conductor
43 is of zero magnitude while the voltage feedback signal at
conductor 44 is at a maximum value.
Accordingly, the output of amplifier 42 provides a voltage signal
proportional to the setting of reference 40 which establishes the
desired set point of voltage.
For intermediate settings of gang 47, output 50 produces modified
proportional set point signals related to the slope characteristic
established by the current and voltage feedback signals. One of the
slope characteristics is shown in FIG. 2A as characteristic 30
while another is shown as characteristic 37.
Amplifier 42 also provides low pass filtering characteristic which
is effective to smooth the otherwise high ripple content of the
current feedback applied to input 42c and the voltage feedback
applied to input 42b.
Output 50 of amplifier 42 is connected to an input of amplifier of
52 which provides both integral plus proportional control of the
feedback for system 10. The output of amplifier 52 is effective to
control the firing networks of firing unit 55. Firing unit 55 is
adapted to provide properly timed and spaced firing pulses to a
bank of triggered silicon controlled rectifiers in rectifier 14.
With this proper phasing of rectifier 14, there is established and
maintained the desired voltage and current characteristics at
output leads 15 and 16 and therefore to arc 26.
In order to assist in the start of supply system 10, a start
circuit 53 is effective to provide controlled arc energy at the
start of operation to more easily establish arc 26. When the weld
current increases sufficiently, a relay is energized thereby
removing the start weld setting so that the weld current assumes
its normal desired value as established by reference 40.
It will now be understood that current feedback unit 45 comprises a
separate potentiometric slope adjustment connected between shunt 22
and input 42c while voltage feedback unit 46 comprises a separate
potentiometric slope adjustment connected between load resistors 17
and input 42b. The current slope potentiometer is mechanically
ganged to the voltage slope potentiometer.
With the single slope adjustment control (gang 47) set to one
extreme, the current feedback signal is maximum and the voltage
feedback signal is zero, thereby providing a true constant current
volt-ampere characteristic. Specifically with gang 47 in its
extreme clockwise position, the current feedback signal at
conductor 43 is at a maximum and the current through arc 26 is
maintained substantially constant at a value determined by the
setting of reference. For example as shown in FIG. 2B depending
upon the setting of reference 40, the current I through arc 26 may
be 100 amps at one setting, 500 amps at another setting, etc. Each
constant current characteristic varies between a voltage value
approaching zero volts (as for example, 5 volts) to the maximum
voltage of supply system 10.
When the single slope adjustment control (gang 47) is set to the
other extreme, the current feedback signal is zero and the voltage
feedback signal is maximum, thereby providing a true constant
potential volt-ampere characteristic. Specifically, with gang 47
and its extreme counterclockwise position, the voltage feedback at
conductor 44 is maximum and the voltage across arc 26 is maintained
substantially constant at a voltage value determined by the setting
of reference 40. For example as shown in FIG. 2b, the voltage
across arc 26 may be 20 volts at one setting, 40 volts at another
setting, etc. with current varying from a minimum to a maximum
value.
The single slope adjustment control (gang 47) may be adjusted
continuously between the two extremes to yield an infinitely
adjustable volt-ampere slope characteristic.
Referring now to FIG. 3, there is shown a detailed drawing of
supply system 10 in which transformer 12 comprises a three phase
delta primary 60 and a six phase star secondary 62. The windings of
primary 60 and secondary 62 are suitably coupled on a common core
(not shown) to provide a constant potential transformer. Power
source 11 is coupled to primary 60. The common 64 or star point of
secondary 62 is connected by way of lead or load line 15 through
current sensing shunt 22 to work member 23. The outer end of each
phase winding of secondary 62 is connected through respective
silicon controlled rectifiers (SCR) 70a-b, 71a-b, 72a-b and then
through a smoothing and stabilizing reactor 67 and lead or load
line 16 to electrode 20. Reactor 67 is provided with taps so that
different values may be selected for different modes of
welding.
It will be understood that diametrically opposite phase windings of
the star connected secondary 62 are connected to simultaneously
conducting SCR's 70a-b, 71a-b, and 72a-b. Thus phase 62a is
connected to a pair of SCR's 70a-b and then through reactor 67 to
line 16. The adjacent phase 62b is connected to a similar pair of
SCR's 71a-b and the final phase 62c is connected to a final pair of
SCR's 72a-b. Each pair of SCR's 70a-b, 71a-b, and 72a-b is
connected to a firing unit 55 for simultaneous pulsing with the
three pairs being pulsed in proper sequence. In this manner, the
portion of the half-wave of each winding applied across leads 15-16
is controlled by the phased firing of the respective SCR's.
In the Gas Tungsten Arc Welding process, it is desirable to utilize
a background supply comprising auxiliary secondary windings 66a-c
in the main power transformer 60. Windings 66a-c are connected in
three phase delta with the output thereof coupled to a background
rectifier 69. Rectifier 69 may be connected as a three phase bridge
rectifier. The output of the rectifier 69 is coupled through a
switch 69a to leads 15 and 16. This circuit operates to help
establish a more stable arc by smoothing the ripple produced by the
arc.
Firing unit 55 may be any one of the firing units well known in the
art to provide the required phase control and firing of SCR's. For
example unit 55 may be a firing unit Part No. R613F372 manufactured
by Firing Circuit, Inc., Norwalk, Conn.
Firing unit 55 is actuated by an input conductor 74 which is
coupled by way of a gain control potentiometer 75 to output 77 of
amplifier 52. The signal at conductor 74 determines the particular
time in each half cycle at which a firing pulse is applied to a
particular SCR and thereby determine the particular time in the
phase that SCR conducts the output applied to it from secondary 62.
Since power source 11 is also applied to firing unit 55, this power
input is synchronized with the control voltage applied to an SCR.
The firing of the SCR is modified by the input signal at conductor
74 which reflects the current feedback produced by unit 45 through
summation amplifier 42 thereby to establish a desired voltage and
current slope characteristic.
Shunt 22 comprises a millivolt shunt connected in series with lead
15. The upper end 22b of shunt 22 is coupled by way of common lead
80 to the junction of fixed contacts of current and voltage
feedback potentiometers 82, 83 respectively. Common 80 is the
common for the entire electronic feedback circuit. The lower end
22a of shunt 22 is coupled by way of conductor 84 to the other
fixed contact of current potentiometer 82. In this manner,
potentiometer 82 is coupled across shunt 22 and the current
feedback signal is taken from moveable arm 82a of potentiometer 82
with respect to common 80. That current feedback signal is applied
to input 42c of summing filter amplifier 42.
Within amplifier 42, input 42c is coupled to an input circuit 85
comprising resistors 80a-c in series and capacitors 97a-b. One end
of the series resistance circuit is connected to input 42c and the
other end is connected to a summing junction 92 of an operational
amplifier 100 which comprises the amplifying device of summing
filter amplifier 42. The feedback network 102 of operational
amplifier 100 comprises resistors 103-106 and capacitors 108-111.
The values of these resistance and capacitance feedback components
are selected in conjunction with the values of resistors 90a-c and
capacitors 97a-b to provide adequate filtering of the current
feedback millivolt signal. These components are further selected to
provide proper voltage gain to make the current feedback millivolt
signal compatible with the reference set point voltage applied to
input 42a and the voltage feedback signal applied to input 42b. The
voltage levels applied to inputs 42a-b may for example each be
adjusted to approximately a 10 volt maximum level.
For sensing voltage feedback, a series circuit of a potentiometer
17b and a fixed resistor 17a (load resistors 17) is connected
between the other fixed contact of potentiometer 83 and load line
16. In this manner, the potential between load lines 15 and 16 is
developed across the series circuit combination of resistors 17a-b
and 83 with the voltage feedback signal taken from arm 83a of
potentiometer 83 with respect to common. By suitably moving arm
83a, the voltage feedback signal is adjusted to a level compatible
with the set point reference voltage 40. The voltage feedback
signal is applied by way of conductor 44 to input 42b. Within
amplifier 42, input 42b is connected to input circuit 86 comprising
resistors 93a- c in series circuit coupled to junction 92 and
capacitors 98a-b. The values of the components of this
resistance-capacitance network are selected in conjunction with the
feedback resistance-capacitance network 102 to provide adequate,
unity gain filtering of the voltage feedback signal.
For the reference set point voltage, reference circuit 40 comprises
a potentiometer 95 having its arm connected to input 42a. One fixed
contact of the potentiometer is connected to common while the other
fixed contact is connected through a potentiometer 96 to a positive
supply. Input 42a is coupled within amplifier 42 to an input
circuit 87 comprising resistors 94a-c in series circuit and
capacitors 99a-b. The values of these components are selected in
conjunction with the values of the components of the
resistance-capacitance feedback network 102 to provide a transfer
function for the reference input which is compatible with the
current feedback to input 42c and the voltage feedback to input
42b.
The ends of input circuits 85-87 remote from inputs 42c, 42b and
42a, respectively are summed at junction 92 which is coupled to the
negative input of operational amplifier 100. Input circuits 85-87
and feedback network 102 effectively define a Butterworth filter
network.
An additional input to junction 92 may be traced by way of a
resistor 116 and then to an arm of a potentiometer 115. One fixed
contact of potentiometer 115 is connected by way of a resistor 114
to a positive supply and the other fixed contact is connected to
common. Potentiometer 115 and resistor 114 form an adjustable
voltage divider network used to calibrate the low end of the
reference potentiometer readout in amperes when the slope is
adjusted to the constant current mode. Potentiometer 96, previously
described, is used to calibrate the high end of reference
potentiometer 95 readout in amperes when the slope is adjusted to
the constant current mode.
It will be understood that the arms 82a and 83a are ganged together
by gang 47 to provide a single slope adjustment control as
previously described. With gang 47 at its extreme right position
(corresponding to the extreme clockwise position) it will be seen
that arm 82a is at its furthest position from common while arm 83a
is at its closest position. Accordingly, the current feedback is at
a maximum and the voltage feedback is at a minimum. On the other
hand, with gang 47 in its extreme lefthand position (corresponding
to the extreme counterclockwise position) arm 82a is at its closest
position to common while arm 83a is at its furthest position.
Accordingly, the current feedback is zero and the voltage feedback
is maximum.
The output of operational amplifier 100 is applied by way of a
potentiometer 120 and a resistor 121 to the negative input of an
operational amplifier 125; the positive input of which is connected
by way of a resistor 123 to common. The output of amplifier 125 is
connected by way of an integrator capacitor 130 and a resistor 131
to the negative input. In this manner, operational amplifier 125
operates as an augmented integrator, or integral plus proportional
amplifier. In the steady state if a perturbation occurs in the
error signal at the output of operational amplifier 100, then
capacitor 130 charges in such a direction as to drive the output of
amplifier 125 and hence the firing circuit 55 to that level
necessary to reduce the error signal to zero. Potentiometer 120 may
be adjusted to set the closed loop gain of the feedback
circuit.
Start circuit 53 is provided to assist in the start of supply
system 10. It will be understood that in both manual and automatic
welding that it is desired to have higher weld currents at the
start of operation in order to more easily establish arc 26.
However, once the arc has been established, it is necessary that
the weld current be decreased to its normal desired value as
established by reference 40.
In order to provide this start level, current feedback by way of
the conductors 84 and 129 is applied through a resistor 132 to a
negative input 135a of an operational amplifier 135. The current
feedback produces an effective negative potential at input 135a.
Input 135a is also connected by way of a voltage divider network
136 to a positive supply. The positive input of amplifier 135 is
connected by way of a resistor 136 to common. When the value of the
negative potential produced by the current feedback reaches a level
greater than the positive potential developed by network 136, then
the output of amplifier 135 changes polarity thereby to turn on a
switching transistor 140 as shown. The collector of transistor 140
is coupled to a relay 142, the normally closed contacts 142a of
which are coupled between an arm of a potentiometer 144 and a diode
145. As shown, this circuit is connected between the input and
output of amplifier 125, thereby providing an output clamp on this
amplifier.
Accordingly, at start up, relay 142 is deenergized and the normally
closed contact 142a is effective to apply the start weld level
setting of potentiometer 144 to the input of amplifier 125. This
level setting is applied to firing unit 55 for the higher value
start up current. When the weld current I increases sufficiently to
produce a potential at input 135a of amplifier 135 greater than the
setting of voltage divider 136 than transistor 140 switches thereby
energizing relay 142 and opening the contact 142a. With contact
142a open, the start weld setting is removed and amplifier 52
operates normally.
Having thus described my invention what is desired to be secured by
Letters Patent of the United States is set forth in the appended
claims.
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