U.S. patent application number 11/341992 was filed with the patent office on 2006-07-27 for automatic gas control for a plasma arc torch.
This patent application is currently assigned to Hypertherm, Inc.. Invention is credited to Guy T. Best, Aaron D. Brandt, Zheng Duan, Girish R. Kamath, Stephen M. Liebold, Jon W. Lindsay, Christopher S. Passage, Shane M. Selmer.
Application Number | 20060163216 11/341992 |
Document ID | / |
Family ID | 36337439 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060163216 |
Kind Code |
A1 |
Brandt; Aaron D. ; et
al. |
July 27, 2006 |
Automatic gas control for a plasma arc torch
Abstract
A method and apparatus for controlling a gas supply to a plasma
arc torch uses a proportional control solenoid valve positioned
adjacent the torch to manipulate the gas flow to the torch, thereby
extending electrode life during arc transfer and shutdown. Swirl
ring design can be simplified and gas supply and distribution
systems become less complicated. The invention also allows
manipulation of shield gas flow to reduce divot formation when
making interior cuts. The system can be controlled with a digital
signal processor utilizing a feedback loop from a sensor.
Inventors: |
Brandt; Aaron D.; (West
Lebanon, NH) ; Passage; Christopher S.; (Canaan,
NH) ; Selmer; Shane M.; (Ascutney, VT) ;
Kamath; Girish R.; (Lebanon, NH) ; Best; Guy T.;
(Bethel, VT) ; Liebold; Stephen M.; (Grantham,
NH) ; Lindsay; Jon W.; (West Lebanon, NH) ;
Duan; Zheng; (Hanover, NH) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE 14TH FL
BOSTON
MA
02110
US
|
Assignee: |
Hypertherm, Inc.
Hanover
NH
|
Family ID: |
36337439 |
Appl. No.: |
11/341992 |
Filed: |
January 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11045012 |
Jan 27, 2005 |
|
|
|
11341992 |
Jan 27, 2006 |
|
|
|
Current U.S.
Class: |
219/121.39 |
Current CPC
Class: |
B23K 10/00 20130101;
H05H 1/34 20130101; H05H 1/36 20130101; B23K 10/02 20130101 |
Class at
Publication: |
219/121.39 |
International
Class: |
B23K 9/00 20060101
B23K009/00; B23K 9/02 20060101 B23K009/02 |
Claims
1. A method for piercing a workpiece with a plasma arc torch
comprising the steps of: establishing a plasma gas flow to a plasma
chamber in the plasma arc torch, the plasma chamber defined at
least in part by an electrode and a nozzle; initiating a plasma arc
in the plasma chamber between the electrode and the nozzle;
transferring the plasma arc to the workpiece; and adjusting a flow
of the shield gas based on cut characteristics of the workpiece,
without significantly adjusting the plasma gas flow.
2. The method of claim 1 further comprising the step of increasing
a current flow to the torch to pierce the workpiece.
3. The method of claim 1 wherein the adjusting step includes
increasing the shield gas flow during piercing to compensate for an
increased workpiece thickness.
4. The method of claim 1 wherein the cut characteristics include at
least one of a workpiece thickness, a type of processes being used,
a current level of the torch, a material type of the workpiece, or
the cut shape and geometry.
5. The method of claim 1 wherein adjustment of the shield gas flow
is accomplished using a programmable control valve located adjacent
the torch.
6. The method of claim 1 wherein the programmable control valve is
directly coupled to a body of the plasma arc torch.
7. The method of claim 5 wherein the programmable control valve is
a proportional solenoid control valve.
8. A plasma arc torch for cutting a workpiece, the plasma torch
having a plasma gas source to supply a plasma chamber and a shield
gas source to supply a shield gas to pass through a space between a
nozzle and a shield, such that an electrical current passing
between an electrode and a nozzle produces a plasma arc that exits
the torch through a nozzle exit orifice, the plasma torch
comprising: a means for sensing a parameter in a first fluid line
that supplies at least one of a plasma gas from the plasma gas
source or a shield gas from the shield gas source; and a means for
controlling a flow of the first gas based on the sensed parameter
using a programmable control valve disposed in the first fluid line
adjacent the plasma torch.
9. The plasma arc torch of claim 8 wherein the programmable control
valve is directly coupled to the plasma arc torch.
10. The plasma arc torch of claim 8 further comprising a plasma gas
vent valve that includes at least one of a programmable control
valve or a plurality of on-off solenoid valves.
11. A method for control of a gas flow to a plasma arc torch
including a plasma chamber disposed within a torch body comprising:
providing a first fluid line for supplying a first gas to the
torch; positioning a programmable control valve in the first fluid
line adjacent the torch to control a flow of the first gas; and
manipulating the programmable control valve thereby a) controlling
the flow of the first gas to the torch during operation of the
torch; and b) compensating for a volume in the first fluid line
between the programmable control valve and the torch.
12. The method of claim 11 wherein the programmable control valve
is directly coupled to a torch body of the plasma arc torch.
13. The method of claim 11 further comprising the step of adjusting
a ramp down time of the flowing first gas to prolong a life of a
plasma arc torch electrode based on a cut duration or a current
level of the electrode.
14. The method of claim 13 wherein the adjusting step includes
increasing the ramp down time based on an increased cut duration or
an increased current level of the electrode.
15. The method of claim 13 wherein the adjusting step includes
decreasing the ramp down time based on a reduced cut duration of
the electrode.
16. The method of claim 11 wherein the programmable control valve
controls the flow of the first gas a plurality of times during a
ramp cycle of the first gas.
17. The method of claim 11 wherein the first gas is a plasma gas
and at least a portion of the plasma gas is vented to the
atmosphere through a vent valve.
18. The method of claim 17 wherein the vent valve includes two or
more on-off solenoid valves mounted in parallel.
19. The method of claim 17 wherein the vent valve includes a
programmable control valve.
20. The method of claim 11 wherein the first gas comprises at least
one of oxygen, nitrogen, hydrogen, methane, helium, or argon.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Ser. No.
11/045,012, filed on Jan. 27, 2005, the contents of which are
incorporated herein by reference. This application claims benefit
of and priority to U.S. Ser. No. 11/045,012.
FIELD OF THE INVENTION
[0002] The invention relates generally to plasma arc cutting
torches, and more particularly to control systems and methods for
controlling gas supplies to such torches to enhance their
operation.
BACKGROUND OF THE INVENTION
[0003] Plasma arc cutting torches have a wide variety of uses, such
as the cutting of aluminum sheet metal, thick plates of steel or
stainless steel, or thin sheets of galvanized metal. As illustrated
in FIG. 1, a plasma torch 10 includes a torch body 12, an electrode
14 and a nozzle 16 mounted within the body. The nozzle has a
central exit orifice 18 and can be surrounded by a shield 22. An
exit port 24 of the shield is generally aligned with the exit
orifice 18 of the nozzle. A power supply (not shown) is used to
create an arc between the electrode 14 and the nozzle 16 ionizing a
plasma gas that is supplied from a plasma gas source 30. The
ionized plasma gas exits the torch 10 through the exit orifice 18
of the nozzle and the exit port 24 of the shield, and is used to
cut a workpiece (not shown). Once the plasma arc has been
initiated, the current flow can be transferred from the nozzle to
the workpiece.
[0004] The shield 22 is mounted to a retaining cap 26 on the torch
body 12. Shield gas from a shield gas source 40 can be introduced
to a space between the nozzle 16 and the shield 22. At least a
portion of the shield gas exits the torch with the plasma arc
(i.e., the ionized plasma gas) through the exit port 24 of the
shield. The shield gas cools the shield and helps protect the
shield from workpiece splatter during a cutting or piercing
operation of the torch. The torch can include a swirl ring (not
shown) in the flow path of the plasma gas and/or the shield gas to
impart a swirling motion to the gas for improving torch
performance.
[0005] During operation of the torch, certain consumable parts
become worn and have to be replaced. These consumables include
torch electrodes, nozzles, and shields. Previous patents assigned
to Hypertherm, Inc. of Hanover, N.H. teach techniques for
prolonging the life of these consumables. For example, U.S. Pat.
No. 5,070,227, the contents of which are incorporated herein by
reference, teaches that the life of an electrode can be extended by
controlled reduction of the plasma gas flow a short time before
commencement of the torch current flow reduction, as the cut cycle
is ended. U.S. Pat. No. 5,166,494, the contents of which are
incorporated herein by reference, describes altering the flow of
plasma gas in conjunction with the transfer of the current flow
from the nozzle to the workpiece, and U.S. Pat. No. 5,170,033, the
contents of which are incorporated herein by reference, explains
that a chamber in the swirl ring can be created and sized to
favorably affect the dynamic flow characteristics of the flowing
gas when torch operating conditions are changed.
[0006] FIG. 1 illustrates a known gas distribution feed system for
a plasma arc torch. Gas from a gas supply system or a gas cylinder
(e.g., plasma gas supply 30 or shield gas supply 40) is regulated
to a desired operating pressure using a pressure regulator 31, 41.
The reduced-pressure gas passes through an on-off solenoid valve
33, 43 and optionally through a manually operated needle valve 35,
45. On-off solenoid valves (e.g., 33, 43) generally produce an
exponentially decreasing pressure decay curve upon closure of the
valve. After exiting the needle valve 35, 45, the gas flows to a
plasma torch 10. The plasma gas is channeled to the plasma chamber,
to a space located between the electrode and the nozzle. The shield
gas flows to a space between the shield and the nozzle. A more
complex gas distribution feed system is described in U.S. Pat. No.
5,396,043, assigned to Hypertherm, Inc., the contents of which are
incorporated herein by reference. The '043 patent describes a
plurality of plasma and/or shield gas flow channels and valves of
different sizes and configurations that can be used to provide a
pre-flow, an operating flow, and a quick charge for use in
different operating modes of the torch, such as during piercing
operation of the torch, or cutting operation.
[0007] Unfortunately, there are drawbacks associated with these
different approaches. For example, although the gas flow reduction
scheme of the '227 patent extends the lifetime of the electrode, to
fully optimize the technology it is necessary to customize the
length of hose between the on-off solenoid valve 33 and the torch
10 to achieve a proper volume and resulting gas ramp-down
characteristics for a particular torch and consumable set (e.g.,
electrode and nozzle). This hose volume customization needs to be
matched, e.g., to the specific closing characteristics of the
on-off solenoid valve 33, such that a precise gas flow profile is
achieved about the electrode 14 as the cut cycle is ended. More
specifically, it was previously necessary to position on-off
solenoid valves 33, 43 at a specified distance from the torch such
as 12 inches, 4 feet, or 6 feet, depending upon the system being
configured. Empirical determination of the pressure decay curve
along with other mechanical adjustments and compensations were then
performed to obtain a prolonged life of different consumable sets
(e.g., electrodes and nozzles). Such tedious empirical
determinations were performed for different torches, systems,
consumable sets, and cutting conditions. In such systems,
relocating the on-off solenoid valve by one foot, for example, from
4 feet away from the torch to 5 feet away from the torch without
recalibrating the current ramp down rates resulted in a dramatic
reduction in electrode life (on the order of 30%).
[0008] These control difficulties are exacerbated by the rapid
system dynamics, which can all take place within a time span of
about 300 milliseconds or less. The determination of the proper
hose length and current ramp down characteristics to achieve an
acceptable termination gas flow profile is empirically acquired and
is extremely time consuming. Similar developmental problems are
encountered when customizing the gas flow characteristics required
for optimal use of the '494 patent, e.g., while transferring the
current from the nozzle to the workpiece.
[0009] The chambered swirl ring of the '033 patent, while achieving
an increase in electrode life, requires fabrication of a complex
swirl ring design. Moreover, the inlet and outlet port diameters of
such a swirl ring must be carefully fabricated to precise
tolerances to achieve the desired gas flow characteristics.
Although proper sizing of the swirl ring chamber volume and inlet
and outlet port diameters can achieve the desired gas flow results,
a given swirl ring is generally useful for only a certain torch
type or consumable set necessitating the storage and availability
of different swirl rings with varying design criteria. Performance
of such swirl rings can also be adversely affected, e.g., by
varying gas supply pressures and other gas flow parameters.
[0010] Finally, considering the gas distribution feed system of the
'043 patent, the multiple flow channels for each gas stream are
complex and require many component parts. What is needed is a less
complicated, less expensive system to accomplish desired gas flow
objectives.
[0011] What is also needed is a method and apparatus that can
reliably accomplish all of these objectives using fewer component
parts and at a reduced manufacturing cost.
SUMMARY OF THE INVENTION
[0012] The present invention achieves these objectives by
positioning a programmable control valve in the gas line adjacent
the torch and manipulating it to control the gas flow.
[0013] One aspect of the invention features a method for extending
the life of a torch consumable such as an electrode, nozzle, or
shield that includes providing a first fluid line such as a plasma
gas line for supplying a plasma gas to the torch, and positioning a
programmable control valve in the first fluid line adjacent the
torch to control a flow of the plasma gas. Manipulation of
programmable control valve controls the flow of the plasma gas to
the torch during operation of the torch and compensates for a
volume in the first fluid line between the proportional solenoid
control valve and the torch. Embodiments include locating the
programmable control valve near the torch and adjusting an opening
of the programmable control valve, such as a flow orifice, or a
valve and seat assembly, to change or adjust the flow of the plasma
gas a plurality of times during a ramp cycle of the plasma gas.
Examples include plasma gas ramp up, such as during torch start up,
and plasma gas ramp down near the end of a torch cutting cycle. A
control output from a digital signal processor can be used to
adjust the programmable control valve to perform at least one of
the gas flow controlling or the volume compensating steps. In some
embodiments the programmable control valve is a proportional
solenoid control valve, such as a Burkert proportional solenoid
control valve, although other types of control valves that have
suitable control characteristics and response times can be
used.
[0014] The method for extending the life of the torch consumable
can include a sensor disposed between the torch and the
programmable control valve, such that the digital signal processor
uses a signal from the sensor to adjust the control output to the
programmable control valve. The sensor can be a pressure sensor, a
flow sensor (such as a mass flow meter), an electrical sensor such
as a current measurement, a temperature sensor such as an IR
(infrared measurement), and more. The method can also include
positioning the sensor in the first fluid line between the
programmable control valve and the torch, sensing a parameter (such
as one of those mentioned above) in the first fluid line, and using
the sensed parameter to adjust the programmable control valve
during the controlling step.
[0015] The method can also include the step of extending the life
of a consumable (such as an electrode or a nozzle) by controlling a
ramp-up flow of the plasma gas during start-up of the torch using
the programmable control valve, especially when the current flow is
transferred from the nozzle to the workpiece. It can also include
the step of controlling a ramp-down flow of the plasma gas during
shut-down of the torch using the programmable control valve, which
can also extend the life of the consumable. In some embodiments,
both the ramp-up flow of the plasma gas and the ramp-down flow of
the plasma gas are controlled using the programmable control valve.
This can be used to reduce the cycle time of workpiece cuts by the
torch, thereby increasing production line throughput and capacity.
In some embodiments, the torch consumable either is or includes an
electrode and operation of the torch includes controlling at least
one of a ramp-up or a ramp-down of a flow of the plasma gas based
on a type of the electrode installed in the torch, i.e., some
electrodes perform better and/or last longer when start-up and/or
shutdown is accompanied by a customized plasma gas flow curve.
[0016] Another aspect of the invention features a method for
control of a gas flow to a plasma arc torch that includes a plasma
chamber disposed within a body of the plasma arc torch. The method
includes providing a first fluid line for supplying a first gas to
the torch, positioning a programmable control valve in the first
fluid line adjacent the torch to control a flow of the first gas,
and manipulating the programmable control valve (such as a
proportional solenoid control valve). Embodiments include locating
the programmable control valve near the torch and adjusting an
opening of the programmable control valve, e.g., by using a valve
with a rising stem-type plunger, to change or adjust the flow of
the first gas a plurality of times during a ramp cycle of the first
gas. Examples include first gas ramp up, such as during torch start
up, and first gas ramp down near the end of a torch cutting cycle.
Manipulation of the programmable control valve is used to control
the flow of the first gas to the torch during operation of the
torch and to compensate for a volume in the first fluid line
between the programmable control valve and the torch. The
programmable control valve can be a proportional solenoid control
valve. In some embodiments the first gas is a plasma gas that
supplies the plasma chamber. In others the first gas can be a
shield gas that flows to space between a nozzle and a shield of the
torch.
[0017] The programmable control valve can be directly coupled to
the torch body of the torch, e.g., such that there is no hose
length between the valve and the torch. In some embodiments, at
least a portion of the programmable control valve is disposed
within the torch. The method can also the step of increasing a ramp
down time of the flowing first gas to prolong a life of the
electrode. This ramp down time can be increased based on, e.g., an
increased cut duration of the torch or a higher operating current
level of the torch. In some embodiments the ramp down time interval
can be decreased to increase the electrode life, e.g., when the
torch is being used to make short duration cuts.
[0018] The method can also include a vent valve for venting the
plasma gas (e.g., to atmosphere), which can be used to decrease the
amount of time required to vent the plasma gas, e.g., at the end of
a cutting cycle. The vent valve can be a programmable control
valve, such as a proportional solenoid control valve. In some
embodiments the vent valve can include two or more on-off solenoid
valves that are mounted in parallel.
[0019] This method can also include positioning a second
programmable control valve in a second fluid line, which can be
used to supply a second gas to the torch. The first gas can be a
plasma gas and the second gas can be a shield gas, and the flow of
the second gas can cool and protect from splatter a shield that
surrounds a nozzle and is mounted on the torch body. The method can
also include diverting at least a portion of the flow of the second
gas (e.g., a shield gas) through a third fluid line to join with
the flow of the plasma gas to the torch. A third programmable
control valve can be positioned in the third fluid line adjacent
the torch, to control the diverted shield gas flow. The third
programmable control valve can be manipulated to control the flow
of the diverted portion of the second gas and to compensate for a
volume in the third fluid line between the third programmable
control valve and the torch. In another embodiment, the method can
include diverting at least a portion of the flow of the plasma gas
through a third fluid line to join with the flow of the second gas
(e.g., the shield gas) to the torch. The third programmable control
valve can be positioned in the third fluid line adjacent the torch
to control the diverted plasma gas flow. The third programmable
control valve can be manipulated to control the flow of the
diverted portion of the plasma gas and to compensate for a volume
in the third fluid line between the third programmable control
valve and the torch.
[0020] In some embodiments the second and/or the third programmable
control valve is a proportional solenoid control valve. A control
output from a digital signal processor can be used to adjust any or
all of the programmable control valves to perform at least one of
their respective controlling and the compensating steps. A sensor,
such as a pressure, flow, temperature, or mass flow sensor can be
located between the torch and the programmable control valve, such
that the digital signal processor uses a signal from the sensor to
adjust the control output to the programmable control valve.
Embodiments include the first gas comprising at least one of
oxygen, nitrogen, hydrogen, methane, helium, or argon. In some
embodiments, the plasma chamber is defined by an electrode and a
nozzle, and the first gas is a shield gas that cools and protects
the shield from splatter, where the shield surrounds the nozzle and
is mounted on the torch body, e.g., via a retaining cap.
[0021] The method can also include the step of adjusting the flow
of the first gas (e.g., a shield gas) to reduce formation of a
divot in an interior cut of a workpiece, to control slag formation,
and/or to improve the quality of a corner cut within a workpiece.
Embodiments include controlling the flow of the shield gas to be a
piercing flow during a piercing operation of the torch (e.g., when
initially penetrating a workpiece), or to be a cutting flow during
a cutting operation of the torch (such as during a prolonged
cutting operation).
[0022] Another aspect of the invention features a method for
control of a shield gas flow to a shield surrounding a nozzle and
mounted on a torch body of a plasma arc torch that includes
providing a first fluid line for supplying a shield gas to the
torch and positioning a programmable control valve in the first
fluid line adjacent the torch, to control the flow of the shield
gas. The programmable control valve is manipulated to control the
flow of the shield gas to the torch during operation of the torch
and to compensate for a volume in the first fluid line between the
proportional solenoid control valve and the torch. The shield gas
flow is adjusted to reduce formation of a divot in an interior cut
of a workpiece. Embodiments include locating the programmable
control valve near the torch and adjusting an opening of the
programmable control valve to change or adjust the flow of the
shield gas a plurality of times during a ramp cycle of the shield
gas. Examples include shield gas ramp up, such as during torch
start up, and shield gas ramp down near the end of a torch cutting
cycle. In some embodiments the programmable control valve is a
proportional solenoid control valve, although other types of valves
with suitable flow characteristics and adequate response dynamics
can also be used.
[0023] Yet another aspect of the invention features a plasma arc
torch for cutting a workpiece, wherein the plasma torch has a
plasma gas source to supply a plasma chamber. An electrical current
passing between an electrode and a nozzle of the torch produces a
plasma arc that exits the torch through a nozzle exit orifice. The
plasma torch includes a means for sensing a parameter in a first
fluid line that supplies a plasma gas from the plasma gas source
and a means for controlling a flow of the plasma gas to the plasma
chamber based on the sensed parameter using a programmable control
valve disposed in the first fluid line adjacent the plasma torch.
In some embodiments the programmable control valve is a
proportional solenoid control valve. The sensed parameter can be a
pressure or a flow of the plasma gas. The torch can also include a
means for controlling a flow of a shield gas from a shield gas
source to the torch through a second fluid line, the means
comprising a second programmable control valve disposed in the
second fluid line adjacent the plasma torch. The second
programmable control valve can be a proportional solenoid control
valve.
[0024] The torch can also include a swirl ring that imparts a
swirling motion to at least one of the plasma gas or the shield
gas, and a control output from a digital signal processor can be
used to manipulate one or both of the programmable control valve
and the second programmable control valves. The output of the
digital signal processor can be adjusted based on the type or
thickness of the workpiece to be cut, such that the plasma and/or
shield gas flows are thereby adjusted to compensate for these
variables. In some embodiments the plasma gas includes oxygen,
nitrogen, hydrogen, methane, argon, or mixtures thereof. The nozzle
of the plasma torch can be surrounded by a shield mounted to a
retaining cap and having an exit port that aligns with the exit
orifice of the nozzle.
[0025] Another aspect of the invention features a plasma cutting
system that includes a power supply and a plasma arc torch for
cutting a workpiece. The plasma torch includes a plasma gas source
to supply a plasma chamber such that an electrical current passing
between an electrode and a nozzle produces a plasma arc that exits
the torch through an exit orifice in the nozzle. The plasma torch
includes a means for sensing a parameter in a first fluid line,
such as pressure, temperature, or flow, which line supplies a
plasma gas from the plasma gas source to the torch. The torch also
includes a means for controlling a flow of the plasma gas to the
plasma chamber based on the sensed parameter using a programmable
control valve disposed in the first fluid line adjacent the plasma
torch. The programmable control valve can be a proportional
solenoid control valve. Other types of valves with suitable control
parameters and response dynamics can also be used.
[0026] Another aspect of the invention features a plasma arc torch
for cutting a workpiece that has a plasma gas source to supply a
plasma chamber, and a shield gas source to supply a shield gas. The
shield gas passes through a space between a nozzle and a shield,
such that an electrical current passing between an electrode and a
nozzle produces a plasma arc that exits the torch through a nozzle
exit orifice. The plasma torch includes a means for sensing a
parameter in a first fluid line that supplies at least one of a
plasma gas from the plasma gas source or a shield gas from the
shield gas source, and a means for controlling a flow of the first
gas based on the sensed parameter using a programmable control
valve disposed in the first fluid line adjacent the plasma torch.
The programmable control valve can be directly coupled to the
plasma arc torch.
[0027] Some embodiments include a plasma gas vent valve that
includes at least one of a programmable control valve or a
plurality of on-off solenoid valves. The vent valve can vent the
plasma gas to the atmosphere.
[0028] Yet another aspect of the invention features a method for
piercing a workpiece with a plasma arc torch. The method includes
the step of establishing a plasma gas flow to a plasma chamber in
the plasma arc torch, such that the plasma chamber is defined at
least in part by an electrode and a nozzle. A plasma arc is
initiated between the electrode and the nozzle, and transferred to
the workpiece. The flow of the shield gas is adjusted based on cut
characteristics of the workpiece, but without significantly
adjusting the plasma gas flow. The cut characteristics can include
workpiece thickness, the type of material of which the workpiece is
made, the type of cut or cuts to be made, and/or the types of
process gas or gases being used. The method includes the step of
increasing a current flow to the torch to pierce the workpiece,
e.g., from a pilot current level to a cutting current level.
[0029] The shield gas flow can be adjusted by increasing the shield
gas flow during piercing to compensate for an increased workpiece
thickness. This can occur when the torch is operating a cutting
current level, a pilot current level, or at a piercing current
level. Not all embodiments of the invention include using all three
of these current levels. Cut characteristics can include at least
one of a workpiece thickness, a type of processes being used, a
current level of the torch (e.g., a pilot, cutting, or piercing
current level), a material type of the workpiece, or the cut shape
and geometry.
[0030] Adjustment of the shield gas flow can be accomplished using
a programmable control valve located adjacent the torch. In some
embodiments, the programmable control valve is directly coupled to
a body of the plasma arc torch. The programmable control valve can
be a proportional solenoid control valve.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The foregoing discussion will be understood more readily
from the following detailed description of the invention, when
taken in conjunction with the accompanying drawings, in which:
[0032] FIG. 1 illustrates a known gas supply system for a plasma
arc torch;
[0033] FIG. 2 is a schematic representation of a plasma gas supply
system to a plasma arc torch incorporating features of the
invention;
[0034] FIG. 3 is a chart depicting plasma gas flow control test
data during startup of a plasma arc torch, in accordance with an
embodiment of the invention
[0035] FIG. 4 is a schematic representation of a shield gas supply
system to a plasma arc torch incorporating features of the
invention;
[0036] FIG. 5 illustrates the control of divot formation achievable
with an embodiment of the invention;
[0037] FIG. 6 is a schematic representation of a combined plasma
gas and shield gas supply system to a plasma arc torch; and
[0038] FIG. 7 is a more detailed representation of a combined
plasma gas and shield gas supply system to a plasma arc torch.
DETAILED DESCRIPTION
[0039] FIG. 2 is a schematic representation of a plasma gas supply
system to a plasma arc torch incorporating features of the
invention. A plasma gas, e.g., including oxygen, nitrogen, and/or
argon flows from a plasma gas supply 30 to a plasma gas supply line
201. This first gas supply line channels plasma gas to the plasma
torch 10 through a programmable control valve 205 such as a
proportional solenoid control valve. The plasma gas flows to a
plasma chamber 207 of the torch, and can pass through a sensor 210,
such as a flow sensor or a pressure sensor. In some embodiments a
signal 212 from the plasma gas sensor 210 can pass to a digital
signal processor (DSP) 215 (e.g., a microprocessor, computer,
computerized numeric controller (CNC), or PLC), which in turn can
manipulate an output 218 to the programmable control valve 205
based on the signal 212. However, the sensor 210 is optional and
embodiments include a plasma gas supply line 201 with no sensor
210, and manipulation of the output 218 of the DSP 215 can be based
upon other, e.g., predetermined, operating parameters.
[0040] Valves suitable for use as a programmable control valve 205
with the invention include actuated valves such as ball, plunger,
needle, and varying orifice valves. Although valves that control
flow using a flow orifice or opening of variable size are
preferred, other valves having suitable flow characteristics and
response times can also be used. Servo valves of the same valve
styles can also be used. Another suitable programmable control
valve is a proportional solenoid control valve such as a Type 6022
or Type 6023 proportional solenoid control valve available from
Burkert Fluid Control Systems (http://www.burkert-usa.com) of
Irvine, Calif. For example, Burkert valves with a valve coefficient
(C.sub.v) of 0.12 suitable for use with most aspects of the
invention. The input to the valve (e.g., the output 218 from the
DSP 215) can be of many different forms, such as an 800 Hz PWM
(pulse width modulation) signal, a 0-10 volt DC signal, a 4-20 ma
current loop signal (e.g., 24 volt DC ), or others, such as are
known to those of skill in the art. This listing of valves is not
intended to be comprehensive and other valves and types of valves
meeting the required performance characteristics can also be used
to achieve the objectives of the invention.
[0041] The programmable control valve 205 differs from previous
valves used in gas supplies to plasma arc torches, such as those
described above. Applicants have discovered that use of a
programmable control valve 205 such as a proportional solenoid
control valve having an analog-type control range, suitable flow
characteristics, and fast response times, when positioned adjacent
the torch allows for very precise dynamic (real-time) manipulation
of the gas flow with a control precision that has a dramatic effect
on torch operation and consumable life. An example of the precise
gas flow control achieved by the invention is illustrated in FIG.
3. In order to achieve results such as these, it is necessary to
position the programmable control valve 205 adjacent the torch,
i.e., within a reasonable distance. Preliminary testing indicates
the plasma gas programmable control valve 205 can be located up to
10 feet away from the torch if certain parameters, such as inside
line size diameter are met. For example, for a plasma gas flowing
to a plasma arc torch through a plasma gas line having an inside
diameter of 1/8 of an inch at between 70 and 90 psig, adequate gas
control characteristics can be achieved if the programmable control
valve is located not more than 10 feet from torch. Embodiments also
include locating the programmable control valve only 6 feet from
the torch, and closer. In some embodiments, the programmable
control valve functionality is directly coupled, closely coupled
to, or is located within the torch. "Directly coupled" means
attached (e.g., bolted) to the torch body, or having components of
the programmable control valve at least partially installed or
mounted within the plasma torch.
[0042] As the proximity of the programmable control valve to the
torch is increased, i.e., as the programmable control valve is
located closer to the torch, the control dynamics of the system
improve. Positioning the programmable control valve adjacent the
torch permits an effective dynamic response time of the control
system to be achieved, thereby reducing a time constant of the
system response and allowing a precise and meaningful system
response to be achieved. As can be seen from FIG. 3, the required
speed of response for this example is significantly faster than 300
ms. The mechanical response of the programmable control valve 205
to system changes, e.g., as indicated by sensor 210, occurs at
least a plurality of times within a gas ramp (i.e., a ramp up or a
ramp down) cycle (e.g., about 300 ms). This rapid mechanical valve
response rate is advantageous for both increasing gas ramp cycles
(ramp up) and for decreasing gas ramp cycles (ramp down). Moreover,
such rapid valve response allows non-constant setpoints (e.g., flow
or pressure) to be used, further enhancing the versatility and
usefulness of the system. An optimized, predetermined setpoint
curve can be used for a given set of cutting conditions, and the
predetermined setpoint curve values can be controlled and
maintained throughout the gas ramp cycle (e.g., over 300 ms) by the
programmable control valve 205. As explained above, achievement of
these benefits requires that the programmable control valve 205 be
positioned adjacent the torch to provide a suitably fast system
response time to changes in valve position, or that this
functionality be located within the torch.
[0043] Such positioning of the programmable control valve 205, 405
allows a single hose arrangement to be used for many different
system configurations and consumable set combinations. This is
especially advantageous for mechanized (i.e., robotic) systems and
automated cutting tables. For example, the programmable control
valve 205 can be manipulated to compensate for the limited volume
in the hose between the programmable control valve 205 and the
torch. However, due to the compressible nature of the flowing
fluid, it is necessary that the programmable control valve 205 be
positioned adjacent the torch 10 as described above.
[0044] FIG. 3 is a chart depicting plasma gas flow control test
data during startup of a plasma arc torch, in accordance with an
embodiment of the invention. In this example, current flow (the
curve labeled as "C") is steadily increased from approximately 35
amps to approximately 130 amps from time zero to approximately 200
ms. Previous plasma gas flow results using on on-off solenoid valve
33 are labeled on this chart as curve P. Two different control
schemes of the programmable control valve 205 are depicted. The
curve labeled N shows a pressure controlled response to the torch
using a PID controller and a pressure control setpoint of 85 psig.
The curve labeled S illustrates a pressure controlled response
using a similar PID controller, but including a step change in the
pressure control setpoint. For this example, the initial pressure
setpoint is 75 psig through about 350 ms, whereupon the pressure
setpoint is changed to 85 psig. Other suitable control schemes will
become apparent to the skilled artisan. For example, a pressure
setpoint of 75 psig could be used until 250 ms followed by a step
change in the setpoint value to 80 psig. At 500 ms a pressure
control setpoint step change from 80 to 85 psig could be
implemented. Many other setpoint control schemes are also possible,
including many more step changes at varying times, or a
continuously changing setpoint value over time that follows a
smooth contiguous curve.
[0045] As is apparent from FIG. 3, to maintain sufficiently precise
control of the gas flow (or pressure) over the course of the gas
ramp up cycle, a plurality of adjustments by the programmable
control valve 205 to the gas flow are necessary. Preferably, these
programmable control valve output adjustments are based on a
feedback control loop using measurement signals from sensor 210.
For effective control, a plurality of programmable control valve
output gas flow adjustments are made to cause the gas flow profile
to match a desired, predetermined, or previously-used setpoint
curve. Each programmable control valve 205 output adjustment
results in a repositioning of the mechanical valve, e.g., of a
valve plunger with respect to a valve seat within the valve trim
section of the valve. When positioned adjacent or at least
partially within the torch, not only can the programmable control
valve 205 be used to match a setpoint curve of the form of curve P,
but setpoint step changes and curves having a continuously changing
setpoint value over time (such as those described above) can also
be achieved. The programmable control valve is positioned adjacent
the torch (as described above) to reduce the time constant of the
system response, thereby enabling rapid system dynamics to achieve
precise gas control. In this manner, a plurality of programmable
control valve outputs to the gas flow, via a corresponding
transmission of a plurality of adjustments via the mechanical valve
mechanism, can be effectively achieved during a gas flow ramp cycle
(e.g., within a few hundred milliseconds), resulting in the
consumable life extensions and other benefits described herein.
Embodiments also include only three to five programmable control
valve output adjustments during a gas ramp cycle, while other
embodiments include dozens or even continuous adjustments.
[0046] In contrast, if the programmable control valve 205 is not
located adjacent the torch, i.e., is too far from the torch, then
the time constant of the system is too slow and rapid, fine valve
trim adjustments of the programmable control valve 205 do not
result in the fine, predictable, controllable flow required to
accomplish the objectives of the invention.
[0047] Moreover, the plurality of programmable control valve output
adjustments described above can be used in conjunction with sensor
210. A feedback control loop using a signal from the sensor 210,
e.g., via DSP 215, can be used to efficiently control the gas flow.
Using this technique in combination with programmable control valve
205 located adjacent the torch allows realization of unprecedented
plasma torch control benefits to be achieved when the feedback loop
cycles a plurality of times during a ramp cycle of the plasma (or
shield) gas.
[0048] In addition to the PID controller described above, other
control schemes can also be used, such as a gap action controller,
a hysteresis controller, and other control methods and techniques
known to the skilled artisan. Moreover, sensor control parameters
other than pressure can be used. For example, embodiments include
using a flow sensor with a sufficiently rapid response time in
place of a pressure sensor, a temperature signal from a strategic
point within or about the torch, a mass flow sensor, or visual or
electrical measurements (such as current flow). Although feedback
control techniques are often preferred, open loop control systems
can also be employed in connection with control of the plasma gas
flow. Such open loop control methods can be based on, e.g.,
empirical test results or calculated values.
[0049] The invention can also be used to realize additional
advantages. Now that precise flow or pressure control of the plasma
gas is no longer linked to the specific hardware configuration
employed, plasma gas ramp-up flow controls (e.g., during arc
transfer from the nozzle to the workpiece) and/or plasma gas
ramp-down flow controls (e.g., during torch shutdown) can be
customized to more fully optimize the lifetime of torch electrodes,
nozzles, and the like. Compensation for various nozzle orifice and
shield exit port sizes, varying volumes into the plasma chamber and
between the electrode and nozzle, and for trapped volumes between
the torch 10 and the programmable control valve 205 can now be
readily achieved. Moreover, consumable lifetime optimization can
now be performed by precisely synchronizing and optimizing plasma
gas flow with torch current increases and/or decreases. Such
synchronization can take many forms. For example, the ramp rate of
gas flow can be the same as or greater than the rate of change of
the current during one portion of the cycle, and less than the rate
of change of the current during another portion. Many combinations
are now possible. Previous gas flow optimization results have been
rudimentary in comparison with those achievable by the present
invention, having been severely limited by the gas flow profiles
previously obtainable. Achievable gas flow profiles have now been
decoupled from the mechanical constraints of previously-used
control hardware and system arrangements.
[0050] The properly positioned plasma gas programmable control
valve 205 can also be manipulated to achieve other objectives. For
example, the function of the chambered swirl ring described in U.S.
Pat. No. 5,170,033 (described above) can now be achieved using the
programmable valve 205. Whereas previously it was necessary to
physically size the chamber inlet and/or outlet ports to achieve
the desired swirl ring gas flow characteristics, the present
invention allows similar results to be achieved using a swirl ring
having no chamber. Moreover, the swirl ring ports can be oversized,
and the requisite flow control can be achieved using the
programmable control valve 205 and, optionally, sensor 210. The
resulting system is thus simpler and less costly to manufacture.
The system can also be used to respond to plasma gas supply
pressure fluctuations. This feature is particularly useful for shop
operations having torches supplied from a header system that is
prone to such fluctuations.
[0051] FIG. 4 is a schematic representation of a shield gas supply
system to a plasma arc torch incorporating features of the
invention. A shield gas, e.g., including oxygen, nitrogen,
hydrogen, methane, argon, helium, air, and/or mixtures of these
gases, flows from a shield gas supply 40 to a shield gas supply
line 401. This gas supply line channels shield gas to a space 407
between the nozzle 16 and the shield 22 through a programmable
control valve 405 such as a proportional solenoid control valve.
The types of valves described above in connection with the
programmable control valve 205 for the plasma gas can also be used
as a programmable control valve 405 for the shield gas. As
described above, the programmable control valve 405 must be located
adjacent the torch, e.g., within 2 feet.
[0052] Preliminary testing indicates that for shield gas flow the
programmable control valve 205 can be located up to 10 feet away
from the torch if certain parameters, such as inside line size
diameter are met. For example, for a shield gas flowing to a plasma
arc torch through a gas line having an inside diameter of 3/16 of
an inch at between 30 and 50 psig, adequate shield gas control
characteristics can be achieved if the programmable control valve
is located not more than 10 feet from torch. Embodiments also
include locating the programmable control valve only 6 feet from
the torch, and closer. As discussed above in connection with
programmable control valve 205, the proximity of the programmable
control valve to the torch is increased, i.e., as the programmable
control valve is located closer to the torch, the control dynamics
of the system improve. The mechanical response of the programmable
control valve 405 to system changes, e.g., as indicated by sensor
410, must occur at least a plurality of times within a gas ramp
cycle. Gas ramp cycle times for both plasma and shield gases can be
about 300 ms, although longer ramp cycles, and considerably shorter
ramp cycles, are sometimes used and are within the scope of the
invention. As with plasma gas control, shield gas setpoints can
also be variable over time having, e.g., the form of a curve, a
step function, or a linear ramp through the gas ramp cycle. The
programmable control valve 405 is located adjacent the torch to
achieve benefits of this embodiment of the invention by providing a
time constant of the shield gas response system that is
sufficiently small to allow precise process control of the gas flow
to be achieved. The operational requirements of the shield gas
programmable control valve 405 are similar to those of the plasma
gas programmable control valve 205 described above, in that a
plurality of mechanical output adjustments by the programmable
control valve 405 to the gas flow during the gas ramp cycle are
required to achieve the objectives of the invention.
[0053] After passing through the programmable control valve 405 the
shield gas can be routed through a sensor 410. An output 418 from a
DSP 215 can be used to manipulate the programmable control valve
405 based on a signal 412 from the sensor 410. Sensor types and
control strategies described above for use with the plasma gas
supply system can also be used with the shield gas system. Although
feedback control techniques are often preferred, open loop control
systems can also be employed for the shield gas. Such open loop
control methods can be based on, e.g., empirical test results or
calculated values. For example, in the absence of real time
feedback information, the results of previous testing can be used
to determine a preferred output curve. This empirical test
information can also be used in actual feedback control situations
as a control setpoint from which the feedback control loop
operates. Valve output amounts can also be at least partially based
on factors such as the calculated volume through which the gas flow
will pass before reaching the torch, the gas supply pressure, and
the like, and these calculated values can be used to determine
valve output amounts or to adjust valve output dynamics (e.g.,
control loop setpoint adjustment or tuning). Similar techniques can
be used for plasma gas control.
[0054] Additional control objectives can be achieved using the
shield gas control system. The shield gas control techniques
described herein can be implemented either in combination with, or
independently of the enhanced plasma gas control described above.
For example, torch performance is improved by appropriate shield
gas control during piercing and cutting operations as taught in
U.S. Pat. No. 5,396,043, described above. Such objectives can be
readily obtained using the present invention, but using fewer
components and gas lines than previously required. For example, the
single shield gas supply line 401 of the present invention can be
used to provide the pre-flow, operating flow, and quick charge
flows described in the '043 patent. Moreover, positioning the
programmable control valve 405 near the torch 10 allows it to be
used to manipulate the shield gas flow to provide both the piercing
gas flow and the cutting gas flow at different times, as described
in the '043 patent. For example, during workpiece piercing
operation (e.g., at the commencement of a cut) rapid and strategic
increases in shield gas flow can be used to reduce slag formation
and slag blowback to the torch shield 22.
[0055] Improved control of divot formation can also be obtained.
FIG. 5 illustrates divot formation results achievable with an
embodiment of the invention. When creating an interior cut within a
workpiece divots and dross are commonly formed, especially at the
end of the cut. A divot is formed when the end of the cut perimeter
is returned, e.g., to the starting point. FIG. 5 illustrates the
perimeter of an interior circular cut 505 within a workpiece 510.
In some situations an interior divot A can be formed when the start
and end point of the cut is at A. In other situations an exterior
divot B can be formed, when the start and end point of the cut is
at B. Preferably, in this example, a circular interior cut 505
would be achieved resulting a circular hole without the formation
of any divots (i.e., without A or B).
[0056] Different techniques can be used to achieve a divot-free
cut. For example, withdrawing the torch from the workpiece surface
while still in full operating mode will reduce divot and dross
formation, but results in premature damage to the torch electrode.
Alternatively, the current flow can be slammed off (immediately
truncated) at the end of the cut, but this also results in
premature electrode failure. Another technique involves using a
controlled ramp down of the shield gas flow, along with torch
current ramp down flow management, to achieve a divot-free cut. The
invention allows unprecedented benefits of this technique to be
maximized. Embodiments of the invention also provide for cut
optimization when performing interior corner cuts (not shown).
Controlled reduction of shield gas and current flow as the torch
makes an interior corner cut results in improved corner cut
quality.
[0057] Additional advantages can be realized utilizing the
invention. For example, torch operation can be optimized to
accommodate different end cut requirements, such as for small holes
and other specialized workpiece cut designs and features. Moreover,
the ability to precisely and independently control shield gas flow
and torch current allows customization to be performed to better
accommodate different workpiece materials and thicknesses.
[0058] Additional illustrative embodiments of the invention include
improved shield gas flow for workpiece piercing operations. After
arc ignition and transfer of the arc current to the workpiece, some
cutting applications establish a piercing flow, e.g., to initiate a
cut in the center of a workpiece. The programmable control valve
405 for the shield gas can be used to increase the shield gas flow
(or pressure) during piercing operations, thereby increasing the
amount of shield gas force and momentum at the cutting location on
the workpiece, independently of the plasma gas control flow or
amount. After the workpiece has been pierced, the shield gas flow
can be reduced to normal flow rates, e.g., for a subsequent cutting
operation. This subsequent reduction in flow reduces shield gas
consumption and can be set to improve or control the cut angle and
reduce dross formation.
[0059] The extra shield gas force provided during piercing assists
the pierce operation, helping to clear away molten workpiece
material as the initial hole is pierced through the workpiece.
Extra plasma gas momentum can also be provided using the
programmable control valve 205 for the plasma gas, but high plasma
gas flow/pressure during piercing operation can reduce the life
expectancy of the electrode. For this reason, it is generally
preferred to provide the additional gas momentum using the shield
gas flow, and to provide little or no additional momentum from the
plasma gas. The ability to utilize increased shield gas flow during
piercing and a lower flow during steady-state cutting allows
piercing capabilities to be enhanced without sacrificing electrode
life.
[0060] Consideration of additional process factors can also result
in enhanced performance. For example, different types of workpiece
piercings benefit from different operating characteristics.
Piercing types are affected by factors such as the process gases
(shield and plasma gases) being used, the actual and nominal
current levels, the material being cut, the thickness of the
workpiece, and the cut shape and geometry. Customization of the
shield and/or plasma gas flows allows cut performance enhancements
to be achieved by compensating for variables such as these. For
example, increasing shield gas flows as torch piercing current
levels are increased results in improved piercing operations.
Similarly, benefits are also achieved by increasing shield piercing
gas flow as the thickness of the workpiece to be pierced increases.
After the initial workpiece piercing is accomplished the shield gas
can be reduced to improve the cut angle (i.e., make it more
vertical) and reduce dross formation.
[0061] However, the increase in shield gas piercing flow described
above can be unhelpful or undesirable for thin workpieces or
workpieces that have fine (intricate) cut features. Piercing (and
cutting) of such features may actually benefit from reduction of
the shield gas flow. Additionally, benefits can be achieved and
operation of the system streamlined by selecting pierce plasma and
shield flow rates, e.g., automatically, from DSP 215 (e.g., a CNC)
based on the pierce-type characteristics described above. These
parameters can be stored, e.g., in electronic form, in a "cut
table" for quick reference and utilization by the cut program. The
cut table can store cutting shapes and parameters for ready
reference by the cut program or, e.g., for selection and use by the
equipment operator.
[0062] FIG. 6 is a schematic representation of a combined plasma
gas and shield gas supply system to a plasma arc torch. A plasma
gas programmable control valve 205, such as a proportional solenoid
control valve, is positioned adjacent the torch 10 in the plasma
gas supply line 201. A shield gas programmable control valve 405,
such as a proportional solenoid control valve, is positioned
adjacent the torch 10 in the shield gas supply line 401.
Optionally, a sensor 210 is present in the plasma gas line 201 and
provides a control signal 212 to a DSP 215. Also optionally, a
sensor 410 is present in the shield gas supply line 401 and
provides a control signal 412 to DSP 215. The sensors 210, 410 can
measure different types of physical parameters, such as flow,
pressure, and others, such as those described above.
[0063] In addition to advantages described above, this embodiment
of the invention allows additional advantages to be realized. For
example, precise and dynamic real-time control of three independent
variables is now possible--torch current, plasma gas flow, and
shield gas flow. Manipulation of these variables allows
unprecedented optimization of cut cycle times since faster ramp-up
and ramp-down times can now be achieved without sacrificing
consumable life. In many operations, and especially in automated
mechanized operations, workpiece cut cycle times can be further
optimized in view of, e.g., electrode and nozzle life. This
embodiment of the invention allows processing time to be reduced
using plasma gas ramp-up and ramp-down controls in conjunction with
precise current controls, along with the time saving benefits
achieved by faster and better shield gas piercing and cutting
controls. A more precise optimization between cut cycle time and
consumables life expectancy can now be achieved, resulting in
greater productivity, more efficient utilization of manufacturing
equipment and resources, and increased cost savings or throughput.
Of course, the workpiece piercing, divot, cut angle, and dross
minimization benefits described above can also be achieved.
[0064] Embodiments of the invention include enhanced gas ramp up
and ramp down flow controls for the plasma gas and/or the shield
gas based, e.g., on the shape, thickness, material, cut type, and
cut duration for the workpiece being processed. It is known that
plasma gas ramping techniques are especially useful for prolonging
electrode life when the steady state cutting process is
transitioned to torch shutdown (i.e., extinguishing the arc), and
when the initiated plasma arc is transferred to the workpiece. More
precise control of the shield and/or plasma gas flows is achievable
to better compensate for variables such as the shape to be cut, the
material and thickness of the workpiece, the type of cut to be made
(e.g., a piercing cut, a lengthy contour cut, a straight cut, or a
corner or an intricate feature), divot reduction, and the cut
duration. These setpoint adjustments can be effectively controlled
during one or more stages of a cutting cycle (e.g., plasma arc
initiation; transfer of the plasma arc to the workpiece; piercing;
steady-state cutting; shape cutting; and arc extinguishment),
resulting in benefits that were not previously achievable.
[0065] For example, embodiments include adjusting the plasma gas
ramp down rate as the cutting process is being shut down such that
the ramp down rate is slower for longer duration cuts. As described
herein, a slower ramp down rate means a gas ramp down period that
has a longer time interval. A ramp down cycle can commence when
reduction of a cutting current begins, and this ramp down cycle can
finish when the current flow is terminated. In this example, a 100
millisecond ramp down time period would be considered faster than a
500 millisecond time period for the same ramp down cycle.
Conversely, a gas ramp up cycle can commence when a pilot current
begins to increase to a cutting level, and the ramp up cycle can
finish when the cutting current level is achieved. A faster ramp up
cycle will take less time (have a shorter duration) than a shorter
ramp up cycle.
[0066] Applicants have learned that electrode life is prolonged
when the plasma gas ramp down time is increased after the electrode
has been in operation for a longer period of time. For example,
when performing a series of 60 second cuts with a torch, increasing
the plasma gas ramp down time (e.g., from 100 milliseconds to 400
milliseconds) dramatically increases the life of the torch
electrode. Moreover, Applicants have also learned that an increased
plasma gas ramp down time extends electrode life expectancy when a
torch is operated at a higher current (e.g., 260 amps or 400 amps)
as compared with a torch that is operated at a lower current level
(e.g., 130 amps or 70 amps). It is believed that the additional
ramp down time facilitates non-turbulent cooling of the emissive
material (e.g., hafnium) at the electrode tip.
[0067] As an example, Applicants have learned that a slower ramp
down rate (i.e., more ramp down time) also enhances electrode life
for repetitive workpiece cuts of equivalent time duration when
applied to a torch/electrode that is operated at a high current
level. For example, applying an increased (i.e., slower) ramp down
time to a 260 amp electrode that is performing 4 second cuts
extends the life of such an electrode as compared with the amount
of electrode life achieved by applying the same ramp down time
increase to a 130 amp electrode operating under similar conditions.
In this example, the electrode life enhancement of the lower
current electrode (the 130 amp electrode) is not as pronounced as
the electrode life enhancement of the higher current (the 260 amp)
electrode. For these examples, the electrode life is measured by
the number of torch starts that can be accomplished with the
electrode before the electrode fails. Similarly, to prolong
electrode life a slower plasma gas ramp down rate (i.e., a longer
ramp down time) should be used when reducing a cutting current from
400 to 75 amps, as compared with from 200 to 75 amps. For example,
where this ramp down time was previously 100 milliseconds, the
programmable control valve 205 can be used to lengthen this ramp
down time period to 500 milliseconds. Applicants use these
parameters to achieve an optimized balance between electrode life
and cycle time ("duty cycle"). For example, faster cut to cut cycle
times for short cut durations on thinner workpieces, such as fine
features and holes allows these pieces to be produced more rapidly
without sacrificing electrode life. But longer cycle times
employing customized and longer gas ramp times (e.g., for the
plasma gas) are warranted for thicker workpieces that require
longer cuts and that generate and accumulate additional internal
electrode heat.
[0068] Applicants have also learned that when performing a series
of short cuts with a plasma torch (e.g., a series of 4 second torch
as contrasted with a series of 60 second torch cuts), shortening
the plasma gas ramp down time from about 400 milliseconds to about
100 milliseconds doubles the electrode life. Thus, the invention
allows a given torch or cutting system to utilize customized plasma
gas flow control profiles for different cutting situations, thereby
extending and prolonging electrode life.
[0069] These types of benefits were previously unachievable for a
number of reasons. Previous gas control systems tended to have
inflexible gas control capabilities. On-off solenoid valves
(sometimes in combinations) were used, valve timing was adjusted to
empirically compensate for gas hose length effects, and swirl rings
were ported to achieve specific objectives. Prior art systems thus
lacked versatility when different cutting parameters were required.
Either significant setup efforts were required to reconfigure them
for a new cutting operation, or they were setup to achieve overall
"average" acceptable performance. That is, they could do several
things acceptably well, but did not achieve exceptional results for
any given cutting situation. Previous gas control technologies in
the art also lacked sufficient precision to identify and obtain the
results of the invention. Applicants understand the performance
impact of fine, precise control of the plasma and/or shield gas
flows and can achieve additional performance enhancements. Gas ramp
up times can be varied based on pierce type (e.g., whether internal
features or external features are being cut), the shape to be cut,
material thickness, material type (e.g., aluminum, stainless steel,
carbon steel), cut type (e.g., a fine cut with intricate features,
an interior cut, a straight cut, or a gentle contour), and actual
or predicted cut duration. Actual cut duration information can be
determined directly based on power supply output characteristics,
such as output current level and history. Pierce type
characteristics can be used to limit the size of the divot during
pierce, especially when cutting internal features. Gas ramp up
and/or ramp down curves can be slowed to produce improved cut
characteristics when the cut geometry includes cutting fine,
intricate shapes or when the workpiece is especially thin.
[0070] Although feedback control techniques are generally
preferred, open loop control systems can also be employed. Such
open loop control methods can be based on, e.g., empirical test
results or calculated values. The power supply output information
can also be used for open loop control, e.g., using the techniques
similar to those described above for gas control.
[0071] Such ramp up and ramp down techniques for the plasma and/or
shield gases can be conveniently implemented, e.g., automatically,
using DSP 215 (e.g., a CNC). In some embodiments, cut tables
including specific cut information based on variables such as those
identified above can be used. These parameters can be stored, e.g.,
in electronic form, in a "cut table" for quick reference and
utilization by the cut program. When a workpiece requiring a longer
cut length and cut duration is to be cut, the cut table information
has a longer plasma gas ramp down time. Thus, the cut table can
store cutting shapes and parameters for ready reference by the cut
program or, e.g., for selection and use by the equipment operator.
In such embodiments, a manual mode can allow the operator to
include having no plasma gas ramp down (i.e., an instantaneous ramp
down time) at the end of a cut. For example, automated ramp up
sequences can be used in combination with abrupt or manual gas
termination techniques.
[0072] Using such a system, automated control of torch current
levels and gas flow rates can be conveniently and consistently
utilized for consistent and optimized cutting operations. Such a
system can be employed to consistently take full advantage of
specific cutting information that has been previously optimized in
view of the many inter-related variables that affect cutting
operations,
[0073] FIG. 7 is a more detailed representation of a combined
plasma gas and shield gas supply system to a plasma arc torch that
provides enhanced operating flexibility. This figure illustrates
how seven different supply gases can be efficiently incorporated
into a system providing two cut gases 701, 702, each cut gas being
available to provide plasma gas and/or shield gas.
[0074] For example, different plasma gas compositions are preferred
when cutting different workpiece materials or thicknesses. FIG. 7
illustrates a system that can supply H5 (5% hydrogen content, 95%
argon) via on-off solenoid valve 710, H35 (35% hydrogen content,
65% argon) via on-off solenoid valve 711, F5 (5% hydrogen content,
95% nitrogen) via on-off solenoid valve 712, and/or methane (CH4)
via on-off solenoid valve 713 as a first cut gas 701, such as a
plasma gas. Nitrogen (N2) as a first cut gas can also be supplied
via on-off solenoid valve 714 and air via on-off solenoid valve
715. A check valve 703 can be included in the first cut gas line.
This first cut gas 701 can be channeled through a plasma gas
programmable control valve 205 positioned adjacent the plasma
torch, and through sensor 210 as described above.
[0075] Nitrogen can also be supplied as a second cut gas 702 via
on-off solenoid valve 720, air via on-off solenoid valve 721,
helium via on-off solenoid valve 723, and O2 (oxygen) via on-off
solenoid valve 722. A check valve 704 can be included in the second
cut gas line. As illustrated, the second cut gas can be used as a
shield gas, which can pass through a shield gas programmable
control valve 405 positioned adjacent the plasma arc torch, and
through sensor 410 as described above. Although not shown, a DSP
215 can be used to manipulate, e.g., the programmable control
valves 205, 205A, 405, and 405A, and/or some or all of the on-off
solenoid valves 710, 711, 712, 713, 714, 715, 720, 721, 722, and
730.
[0076] This embodiment also features crossover lines 730 and 732.
Crossover line 730 allows the first cut gas 701 to be used as a
shield gas; crossover line 732 allows the second cut gas 702 to be
used as a plasma gas, as illustrated. Each of the crossover lines
includes a programmable control valve 205A, 405A, such as a
proportional solenoid control valve, which is used to precisely
control the amount of crossover gas flow. For embodiments
incorporating this crossover flow functionality, it is important
that any crossover programmable control valve 205A, 405A be
positioned adjacent the plasma arc torch. Crossover gas flow can be
used to augment or replace other gases already flowing in the
system.
[0077] Although the previous discussion has focused primarily on
programmable control valves (e.g., 205, 405) that are located
adjacent the torch (e.g., within 2 feet, 6 feet, or 10 feet of the
torch), embodiments also include torches into which the
programmable control valve functionality has been integrated. For
example, the plasma torch and a programmable control valve can be a
unitary assembly, i.e., with no connecting hose between the two.
Such closely-coupled embodiments include a programmable control
valve that is directly attached, i.e., directly coupled (e.g.,
bolted) to the housing of a plasma torch. In some embodiments, the
programmable control valve apparatus is actually disposed within
the plasma torch body. Locating programmable control valve
functionality (e.g., for the plasma gas or the shield gas) within
the torch reduces the number of external components present in a
plasma torch system.
[0078] Also illustrated in FIG. 7 is an optional vent valve 730
that can be used to vent plasma gas to the atmosphere (ATM). Vent
valve 730 can be an on-off solenoid valve. In some embodiments (not
shown) vent valve 730 can be a programmable control valve. This
embodiment is particularly useful when cutting conditions are
encountered in which it is desirable to decrease the flow rate of
plasma gas flowing to the plasma chamber 207 more rapidly than is
possible with only the plasma gas programmable control valve(s)
205, 205A. In other embodiments (not shown), vent valve 730 can be
two or more on-off solenoid valves mounted in parallel, providing
for increased venting capacity and control. The two or more vent
valves of these embodiments can be operated independently or
simultaneously and can have different sizes (i.e., different valve
coefficients (C.sub.v's)).
[0079] Of course, other combinations are possible. Gas supplies
other than those illustrated can be used, and various techniques
known to those of skill in the art can be used, e.g., to mix
different gas sources. For example, a plasma gas mixture can be
formed by mixing methane and H35 using techniques and
instrumentation (not shown) known to the skilled artisan. In
addition to the mixing techniques described above using crossover
lines 730 and 732, additional gas supplies can be added adjacent
the torch and these additional gas supplies can be controlled using
programmable control valves. An embodiment includes adding a
reducing gas stream (such as methane) to the shield gas between
programmable control valve 405 and sensor 410, such that the flow
of the reducing gas is controlled with a programmable control
valve. More than one additional gas (such as the reducing gas
stream) can be added in this manner. Similar techniques can be used
for the plasma gas. Many other arrangements and combinations are
also possible.
[0080] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *
References