Note: Descriptions are shown in the official language in which they were submitted.
~.~23q~?3S
--1--
CONSTANT POTENTIAL/CONSTANT CURRENT AUTOMATIC CONTROL
SYSTEM FOR AIR-CARBON ARC ~ETAL REMOVA~
This application is a divisional of co-pending
Application Serial Number 451,844, filed April 12, 1984.
This invention relates to the automatic
control of the current or voltage of an arc drawn
between an electrode and a workpiece for purposes of
air-carbon arc cutting and gouging.
In air-carbon arc cutting and gouging of
metal, it is highly desirable to be able to accurately
control the spaclng of an electrode with respect to a
workpiece so as to eEfect an accurate cut. AS described
in U.S. Patent 3,659,071, it has previously been
possible to control the spacing of an electrode with
respect to the workpiece to a close tolerance by sensing
any variation in arc voltage from a steeply drooping
voltage source. The electrode then would be
automatically moved in a direction to cancel the
variation.
Prior air-carbon arc metal removal systems
have been limited in that they have not been compatible
with both constant potential and constant current
sources nor have they provided certain control and auto-
mation features. U. S. Patent 4,320,282 shows control
of arc sustaining parameters to mitigate arc quenchingimpulse noise.
~.
-
3~3S
--2
One embodiment of the presen-t invention is a
system for automatically controlling the feed of a
carbon electrode in an air-carbon arc metal removal
system having arc power produced by ei-ther a constant
potential source or a constant current source.
A preerred embodiment of the invention
automatically turns the arc source off if the arc
voltage drops below or rises above a predetermined
threshold for a predetermined amount of time.
In accordance with one embodiment of the
present invention, there is provided a control sys-tem
which senses arc current and arc voltage and controls
the feed of a carbon electrode in an air-carbon arc
metal removal system having arc power produced by either
a constant potential power source or a constant current
power source comprising:
means for sensing the magnitude of the arc
current and providing an arc current command siynal;
means for sensing the magnitude of the arc
voltage and providing an arc voltage command signal;
feedback control means for controlling the
feed of the carbon electrode toward and away from a
workpiece in accordance with an applied input signal;
and
switching means for alternately applying as
the input signal to the feedback means ~1) the arc
current command signal when the constant potential
source provides weld power thereby to maintain the arc
~.~3~3~;
--3--
current at a predetermined value, or (2) the arc voltage
command signal when the constant current source
provides weld power thereby to maintain the arc voltage
at a predetermined value.
In greater detail of the above, and in a
preferred embodiment, in air-carbon arc metal removal a
control system in provided for automatically controlling
the arc between a carbon electrode and a conductive
workpiece when powered from either a constant potential
or constant current power source~ The electrode is
controlled to alternately feed and retract from the
conductive workpiece to maintain either a specific arc
voltage or arc current level as set by an operator. For
a constant potential source, the arc current is
maintained at a predetermined value by sensing the
magnitude of the arc current and thereby
3q3~3S
controlling the electrode. For a constant current
source, the arc voltage is maintained at a predetermined
value by sensing the magnitude of the arc voltage and
thereby controlling the electrode. The control system
senses the presence of an arc current above a threshold
for a predetermined interval and provides relative mo-
tion between the workpiece carriage and the electrode
thus allowing the electrode arc to cut to a controlled
depth groove in the ~orkpiece, In the event that the
electrode arc current drops below the predeter~ined
threshold, a low current sensing circuit is activated.
If the electrode arc current remains below the thres-
hold for a predetermined time interval as a result,
typically, of reaching the ed~e of the workpiece, the
workpiece carriage travel is terminated. If the arc
current remains below the threshold for longer than a
predetermined amount of time, the electrode power is
turned off. This may also be accomplished if the arc
voltage rises above a predetermined threshold. The con-
trol system also senses if the arc voltage drops and
remains below a threshold for a predetermined time
period. This latter capability permits automatic detec-
tion and shutdown in the presence of electrode-to-work-
piece shortouts.
~ Z3~35
s--
Brief Description of the Drawings
Fig. 1 is a block diagram showing the
interconnection of the automatic arc control system
invention with a welding power source and a carbon
electrode;
.
Fig. 2 ls a functional block diagram of the
preferred embodiment of the automatic arc control
system;
Fig. 3 is a schematic of the functional block
diagram of Fig. 2.
~.Z3~3S
--6--
Detailed Description of the Invention
Referring to the block diagrams of Figs. 1 and
2, an automatic arc control system ln i5 connected to an
electrical welding power source 20, electrode feed
retract motor 30, carbon electrode 58, and workpiece
travel motor ~0. One signal monitored by automatic
control system 10 is the weld feedback "F~" volts on
cable 21. This signal is used to monitor the
electrode-to-workpiece voltage across arc 70. Si~ilar-
ly, the weld P~ current derived from arc current shunt
60 is monitored by automatic control system 10 via cable
61. The weld ~B current is proportional to the current
passing through arc 70. Automatic outputs of control
system 10 include the operation of contactor relay
contacts 184b, feed/retract signals by way of conductors
11,12 to electrode feed/retract motor 30, and the travel
enable signal by way of conductor 13 to the workpiece
travel motor 4QO
The operator begins operation of system 10 by
adjusting the mode select switch 90 to either a constant
current or constant potential setting depending upon the
kind (constant potential ~CP" or constant current "CCn)
of welding power source 20 being used. He then adjusts
the arc level command ~0 to a prescribed arc current or
voltage level consistent with the mode switch 90
setting: e.g., if a constant potential power source 20
is used, a specific arc current level is set. If a
constant current source ~0 is used, an arc voltage level
~.~3G~935
i5 set. The operator adjusts the travel delay 100 to a
satisfactory value and initiates the arc cutting process
by way of a conventional process start, stop control 110.
Assuming that the mode select switch 90 is in
the constant potential scurce mode when the electrode
arc 70 is first struck, the weld FB current will ini-
tially exceed the value set into arc level command 80.
The weld FB current, appropriately filtered and convert-
ed to a voltage analog for computational purposes by
signal conditioner 23Q, is ~ed to feedback command arc
control system 150 and feedback command arc control
system 140. These two arc control systems alternately
energize the feed and retract modes of electrode feed
retract motor 30 to cause carbon electrode 58 to alter-
nately feed and retract from the conductive workpiece 60
thereby to maintain arc 70 at a specific feedback
current level. This is in response to a prescribed arc
level com~and 80 as set into the automatic arc control
system 10 by the operator.
If the mode select switch 90 was in the
constant current source mode 92 the magnitude of the
weld FB voltage 21 is then sensed. The weld F~ voltage
is filtered by signal conditioner 220 and applied to
feedback command arc control systems 140 and 150.
These two arc control systems alternately energize feed
retract motor 30 so that electrode 5~ alternately feeds
and retracts from workpiece 121 thereby to maintain arc
70 at a specific voltage level.
....
~ 23a?~3s
--8--
~ he output of signal conditioner 230 is also fed
to low current threshold 240. If the output voltage of
signal conditioner 230 represents a weld current greater
than threshold, (threshold typically being 200 amperes)
then workpiece travel system 160 initiates a travel
timing interval. If the weld current remains above
threshold during the travel timing interval, the work-
piece carriage 120 moves rearward with respect to the
stationary carbon electrode 58, thus cutting a groove of
prescribed depth in workpiece 60.
The electrode feed retract process previously
described is sufficiently accurate so as to allow an
electrode 58 to workpiece 121 gap 70 as small, typical-
ly, as one hundredth of an inch. At such spacing, the
electrode may occasionally weld to the workpiece causing
a shortout. A sustained shortout can damage the work-
piece 121 as well as the power source 20. The weld FB
voltage, suitably filtered and adjusted in amplitude by
signal conditioner 220 is fed to low arc voltage monitor
190. Low arc voltage monitor 190 contains an arc volt-
age threshold detector ~typically set at 27 volts when
maximum weld voltage is 100 volts). If the weld F~
voltage analog at the output of signal conditioner 220
falls below the arc voltage threshold for a predeter-
mined length of time (as would happen during a shortout)
low arc voltage monitor 190 provides a process stop
signal to process stop system 180. The contactor reiay
in the process stop system 180 is thereby disabled,
~ ~3~93~ii
g
opening contactor relay contacts 184b and thus turning
the arc cutting process of~ and avoiding damage due to a
sustained shortout.
Absence oE an arc current (because the
contactor is off) is now sensed by arc off retract
system 200. Relay contacts in relay switching system
210 are closed by the arc off retract system 200. The
electrode 5~ is thereby caused to retract from workpiece
121. Electrode retraction is timed by a retraction
interval timer in the arc off retract system 200. The
retraction interval is long enough to ensure that
electrode 58 safely exits from workpiece 121. Thus arc
control system 10 automatically retracts the electrode
58 during process shutdown.
An alternate process leading to shutdown results
if the weld FB current analog at the output of signal
conditioner 230 drops below the low current threshold, as
measured by low current threshold detector 240, for a
predetermined time as measured by low arc current monitor
170. This condition, caused by the electrode 58
reaching the end of the workpiece 121, causes the
process stop system 180 to disable the contactor
relay 184, shutting down the process. The foregoing
may also be accomplished in a further embodiment by
sensing a high arc voltage threshold instead o~ a low arc
current threshold. Specifically, a switch assembly (not
shown) would connect the input of monitor 170 to
~I.Z3~35
--10--
the arc FB voltage analog at the output of signal condi-
; tioner 220 instead of the output oE detector 24a.
Accordingly, monitor 170 would then be effective to
operate as a high voltage threshold detector. When the
output of conditioner 220 rises above a high voltage
threshold for a predetermined time as measured by monitor
170, process stop system 180 would disable relay 184
thereby shutting down the process.
A more detailed understanding of the
preferred embodiment of the automatic arc control system
10 may be had by a review of the functional blocks and
logic elements of Fig. 3.
Mode Switch 90
Switch 90 is ganged by way of arm 101 to a
three pole double throw switch 99 in relay switching
block 210. In combination these switches select either
the constant current or constant potential modes of
operation. When mode switch 90 is in the CC position,
switch contacts 91,92 connect a voltage feedback signal
at the output of signal conditioner 220 to the inputs of
the feedback command system 150 and feedback command
system 140. In this position, switch 99 contacts 94,95
connect relay contacts 143a to electrode retract line 12
and contacts 96,97 connect relay contact 154a to elec-
trode feed line 11.
~.~3~
-11
When switch 90 is in the CP position, contacts
91,93 connect a current feedback signal at the output of
signal conditioner 230 to the inputs of systems 140,150.
In this CP position, switch 99 contacts 95,98a connect
relay contacts 143a to electrode feed line 11 and switch
contacts 97,98 connect relay contacts 154a to line 12.
Thus, the action of swtiches 90 and 99 is: (1)
to select either a voltage or current feedback signal,
and (2) to directly apply or reverse the connection of
relay contacts 143a,154a to electrode feed line 11 and
electrode retract line 12.
Feedback Command System 140
The feedback command system 140 comprises
command feedback comparator 141, relay driver 142, and
lS solid state relay 143. The principal function of the
feedback command system 140 is the activation of the
electrode feed retract motor 30 via the solid state
relay 143 when the weld voltage or current feedback
signal (which feedback signal is present depends on mode
switch 90 position) is less than the arc level command
set 80 as entered by the operator.
~.2~3~i
In the constant current mode of operation, the
arc level command set ~0 i5 a voltage analog of the
desired arc voltage. ~hen the weld voltage feedback
signal drops below the arc level command set ~0, then
feedback command system 140 causes electrode 58 to
retract from the workpiece 121 raising the voltage
feedback signal~ In the constant potential mode of
operation, the arc level command se~ 80 is a voltage
analog of a desired arc current. ~hen the weld current
feedback signal drops below the arc level co~mand set
80, then feedback command system 140 causes the
electrode 58 to feed into the workpiece 121 raising the
weld current feedback signal.
Solid state relay 143 will control either the
electrode feed or retract mode of the electrode feed
retract motor 30 depending upon the mode switch 90 posi-
tion. If the mode switch 90 were in the constant cur-
rent position for reading voltage feedback, then acti-
vating relay 143 enables the electrode retract mode of
the electrode feed retract motor 30. If switch 90 were
in its CP position for reading current feedback, then
relay 143 enables the feed mode.
Feedback Command System 1~0
The feedback command system 150 comprises
command feedback comparator 151, a unity gain signal
-~ 23~35;
inverter 152, relay driver 153, ancl solid state relaty
154. The principal function of the feedback command
system 150 is the actuation of the electrode feed re-
tract motor 30 via the solid state relay 154 when the
weld voltage or current feedback signal is greater than
the level of the operator-set command 80. If switch 90
were in its CC ~osition for reading voltage feedback,
then relay 154 enables the feed ~ode. Alternatively, if
switch 90 were in its CP position for reading voltage
feedback, then relay 154 enables the retract mode. The
feedback command logic 150 and the feedback command
logic 140 previously discussed work in opposition: if
one causes retraction of the electrode 58 under its co~-
parator threshold conditions the other causes electrode
58 to feed under its comparator threshold conditions.
In this manner, the null adjust 130 causes an offset in
the threshold of comparator 141 which results in a delay
or hysteresis effect minimizing oscillation between the
retract and feed modes.
Signal Conditioner 230
The signal conditioner 230 includes an opera-
tional amplifier configured as a conventional ~utter-
worth filter. Additionally the gain of the operational
amplifier is set to convert the arc current shunt 60
full scale output of 5Q millivolts to 10 volts. The
~andwidth limi~ing of the ~utterworth filter is
..
~ 23~?35i
-14-
necessary to re~ove high frequency components fro~ the
arc current derived signal.
Low Current Threshold Detector 240
The low current threshold detectcr 241 is ad-
justed to change state (typically from +15v to -15v)
when the weld FB current signal analog at the output of
the signal conditioner 230 is greater than a predeter
mined threshold (typically 2 volts when the full scale
is 10 volts).
Workpiece Travel System 160
The workpiece travel system 160 comprises an
operational amplifier configured as a positive~going
integrator 163 and having an input potentiometer 100,
relay driver 164 and travel relay 165. System 160 en-
; 15 ables the workpiece carriage travel ~otor 40 via the
travel relay 165 after the weld current exceeds the low
current threshold 240 setting for a prescribed travel
ti~ing interval.
The travel timing interval measure~ent is accom-
plished by setting integrator 163 output initially at
zero volts and linearly increasing the output with re-
spect to time until relay driver 164 enables travel re-
lay 165. The workpiece travel system 60 disables the
workpiece travel motor 40 if the weld current drops
~.23 (:~3~
-15-
below the low current threshold longer than a prescribed
interval. As shown in Fig. 1, workpiece 60 travels
beneath a stationary electrode 58. In a further embodi-
ment the workpiece may be stationary while the electrode
carriage may travel, with control by the workpiece
travel syste~ 160.
Low Arc Current Monitor
The low arc current monitor 170 comprises a sig-
nal inverting unity gain operational amplifier 171
and an operational amplifier configured as a positive-
going integrator 172. Monitor 170 disables the
contactor relay 182 if the weld current drops below the
low current threshold detector 240 setting for a
prescribed "low arc current" length of time (typically
.6 seconds~. The measurement of the low arc current
ti~e interval is achieved by causing the output of
integrator 172 to linearly drop from a maximum positive
signal towards zero volts. Relay driver 182 (in the
process stop system 180~ disables relay 182 when
integrator 172 output approaches zero voltsO
As shown in Fig. 3~ the input of integrator 172
is connected through resistor 17S and potentiometer 176.
In addition, integrator 172 may be shunted so that it
does not provide low current shutoff by means of a
switch 1770 These components are provided since conven-
tional power supplies 20 have differing characteristics
~2~
-16-
upon start up. Some power supplies may have a violent
flow of current which has the effect of blowing the tip
off the electrode causing shutdown. Other power
supplies have more gentle start-up. Potentiometer wiper
176a may be adjusted to match many of these characteris-
tics. For example, moving wiper to the left may match
so-called nor~al characteristics of power supplies with
less violent start-up. As wiper 176a is moved to the
right, it would match special characteristics of more
violent start-up by increasing the input resistance to
ti~ing integrator 172. When the operator does not want
low current shutoff to occur at all, he may control this
manual].y by closing switch 177 to clamp off the inte-
grator.
Signal Conditioner 220
The signal conditioner 220 is similar to signal
conditioner 230 previously described. A Butterworth
filter is used to eliminate high frequency co~ponents
contained in the weld voltage which is actually the
voltage across arc 70. The gain of signal conditioner
220 is set to provide a full scale output of 10 volts
when the weld voltage lon cable 21) is at about 100
volts.
Low Arc Voltage Monitor 190
The low arc voltage monitor 190 co~.prises low
voltage threshold detector 192, and an operational
~.23~3~35i
-17-
amplifier configured as a positive-going integrator 193.
Monitor 190 provides a signal to process stop system
180 to disable the contactor relay 182 if the weld FB
voltage drops below a prescribed "low arc voltage"
threshold (typically 27 volts when the maximum weld
voltage is 100 volts) for a pres~ribed "low arc voltage"
length of time (typically .6 seconds). Dropping below
the prescribed 'low arc voltageK threshold for an ex-
tended period is an indication of electrode 58 shorting
to workpiece 121. The shortout is a situation requiring
that the arc cutting process be shut down relatively
quickly if damage to the workpiece 121 and power supply
20 is to be avoided. Timing the shortout is necessary
to avoid shutting down on a momentary electrode-to-
workpiece contact. Normally ti-e., when the weld F~
voltage analog exceeds the low voltage threshold setting
of threshold detector 192) the output of ti~ing
integrator 193 is positive causing relay driver 182 to
enable contactor relay 182. When the weld FB voltage
; 20 drops below the threshold detector 192 setting, the
output of threshold detector-192 changes from a negative
maximum to a positive maximum (typically from -15 volts
to +15 volts). The out~ut of integrator 193 begins to
linearly drop with respect to time. The integration
time constant of integratOr 193 is set to allow the
integrator to drop to the voltage where relay driver 182
changes state in a time period of about .6 second from
the start of low weld ~ voltage.
335
-18-
Should the weld FB voltage rise above the low
voltage threshold before relay driver 182 changes state,
the process remains unaffected. The integrator 193
merely integrates back to a positive maximum.
Arc Off Retract System 200
The arc off retract system 200 comprises an
operational amplifier configured as a positive-going
integrator 201, a voltage comparator 202, relay driver
203, and retract override relay 206. Retract system 200
initiates the electrode retract mode of electrode feed
retract motor 30 via the retract override relay 206 when
the arc current drops below the prescribed low current
threshold in low c~rrent threshold detector 240.
Retract timing integrator 2Ql causes arc off comparator
202 and relay driver 203 to deenergize retract override
relay 206 after sufficient time has passed to ensure
: that electrode feed retract motor 30 has retracted elec-
trode 58 safely from ~orkpiece 121. Deenergization of
retract override relay 206 terminates the electrode re-
tract mode of electrode feed retract motor 30.
Process Stop System 180
The process stop system 180 comprises a logic
"or" r~lay driver 182 and contactor relay 18~. Process
stop 180 stops the electrode arc generation process if
either of the inputs to low arc voltage monitor lgO or
~.Z~3~
-19
low arc current monitor 170 drop below threshcld. Like-
wise, if both the low arc voltage monitor 190 and lo~J
arc current monitor 170 outputs are above threshold, the
contactor relay 1~4 can be disabled by the start stop
control 110.
Relay Switching 210
Relay switching 210 comprises contacts of relays
184, 165, 206, 154, 143. The function of the relay
switching 210 is the initiation of the electrode feed,
electrode retract, weld volts, and workpiece travel
functions in response to enabling signals. Normally
open contacts 1~4a-b and 165a are actuated by relays 184
and 165 respectively. ~lormally open contacts 206a are
activated by retract override relay 206. Further
normally open contacts 154a, 143a are actuated by relays
154 and 143 respectively.
Relays 143,154 are solid state devices to
ensure rapid actuation (less than 5 ~illiseconds) of the
feed and retract actions of the electrode motor 30.
-~ 23~3~
-20-
In automatic control system 10, the following
components have been used for the function and operation
described and shown:
Ref NumeFal Component ~y~
141,151,152,163, Quad. Operational MC4741
171,172,1921193, Amplifier
201,202,221
142,153,164, Relay ~river EXAR
182,203 XP~2201
:~ 10 231,241 Dual Operational XR1458
A~plifier
.
.
