Note: Descriptions are shown in the official language in which they were submitted.
CA 02489137 2004-12-03
Method and Apparatus For Feeding Wire to a Welding Arc
FIELD OF THE INVENTION
The present invention relates generally to the art
of welding. More specifically, it relates to welding using
a short circuit or pulse process.
BACKGROUND OF THE INVENTION
There are many different arc welding processes
used for numerous welding applications. While different
processes share some characteristics, such as using an
electric arc and/or current flow to provide the heat for the
weld, different processes have characteristics that render
them desirable for particular applications.
MIG welding is a widely used process that gives
high heat input into the wire electrode and the workpiece,
and thus can give high deposition rates. However, the
process can be unstable and control of the arc length can be
difficult. Also, for some application MIG can be too hot
(cause too much heating of the workpiece). The MIG process
is often performed as a short circuit or pulse welding.
Another known welding process is called controlled
short circuit welding, or short circuit welding.. Short
circuit welding is often performed as a MIG process.
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Generally, short circuit welding includes a.short circuit
state, wherein the welding wire is touching the weld pool
thus creating a short circuit, and an arc state, wherein an
arc is formed between the welding wire and the weld pool.
During the arc state the wire melts, and during the short
circuit state the molten metal is transferred from the end
of the wire to the weld puddle.
Disadvantages of short circuit welding relate to
the transitions between states, and instability of the
process. Transition from the short'circuit state to the arc
state was typically caused by providing sufficient current
to "pinch" off a droplet. The pinching off at high current
can result in a violent disintegration of the molten metal
bridge producing excessive weld spatter. Instability also
results from the weld pool being pushed away.
Many attempts in the prior art were made to create
a stable short circuit or pulse welding power supply, such
as those shown in US Patents 4717,807, 4835360, 4866247,
4897523, 4954691, 4972064, 500132.6, 5003154, 5148001,
5742029, 5961863, 6051810 and 6160241, which may be referred
to for further details. These patents generally disclose
complicated control schemes that fail to control the process
to provide a stable and effective weld. They include control
schemes that try to control the deposition of material and/or
predict or cause a transition to the subsequent state-based
on the'total energy put into the weld, the length of the
stick out, total watts, time'of the preceding state, etc.
These schemes share a common failure: they attempt
to control both the energy of the weld and the transition
between states using output current or power. This
necessarily entails a sacrificing of one control goal
(either energy.. to the weld or state transition) for the sake
of the other. The net result is that the control schemes do
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not perform well at either controlling the energy into the
weld or controlling the transition.
Another short circuit welding control system is
disclosed in US Patent No. 6326591. This system adequately
controls the energy into the weld, but it does not provide
independent control of the transitions between states.
The present inventors have published descriptions
of a controlled short circuit welding process where
mechanical movement of the wire (advancing and stopping,
slowing or retracting) is used to control the transition
between welding states. The short circuit state is entered
by advancing the wire until the wire touches the weld pool.
The arc state is entered by retracting the wire until the
wire does not touch the weld pool, and an arc forms. This
system allows a typical output control to be used to control
the energy delivered to the weld. By separating control of
the transitions from control of energy, the system allows
for better control of each.
A controlled short circuit or pulse welding system
requires the capability of advancing and stopping, slowing
or retracting the wire. The inventors have disclosed in the
literature the use of a stepper motor to control the wire
movement. A stepper motor adequately provides for short
term advancing and retracting of the wire.
However, a stepper motor does not necessarily
provide adequate feeding of the wire over the long term.
Accordingly, a system that provides for advancing and
retracting of the wire, and long term feeding of the wire,
is desirable.
One problem with controlled short circuit welding
arises when the wire is retracted. The wire from the source
is feeding toward the weld, and has momentum in that
direction. The retracting motor moves the wire in the
opposite direction. With nothing to compensate for the
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opposing forces, the wire might not feed in a smooth and
efficient manner. Accordingly, a controlled short circuit
or pulse welder that compensates for the reversal of the
wire is desirable.
Another problem with controlled short circuit or
pulse welding is that the prior art has not fully taken
advantage of the process control made possible by the
mechanical control of the state transitions. Thus, a
controlled short circuit or pulse welder that provides for
electrical control of the arc for the purpose of controlling
heat into the weld, and not for causing transitions from one
state to another, is desirable.
The prior art has not adequately addressed the
needs of short circuit or pulse welding at lower currents
with thicker wires. The difficult to implement control
schemes, in particular, make it difficult to weld.with
thicker wire, such as 2.4 mm diameter wire, e.g., at low
currents, such as less than 100 amps. Accordingly, a
controlled short circuit or pulse welding process that may
be used at low currents relative to the wire diameter is
desirable.
Pulse welding generally consists of the output
current alternating between a background current and a
higher peak current. Most of the transfer (of the wire to
the weld) occurs during the peak state. Pulse MIG welding
systems are also well known. They have variety of power
topologies and control schemes that provides the pulse
power. Many pulse processes desire a short arc length.
However, short arc lengths can result in inadvertent
shorting of the wire to the weld pool. Accordingly, a
system and method that allows for shorter arc lengths
without resulting in an unsatisfactory number of inadvertent
shorts.
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Spray transfer is another known process. As in
all welding processes, spray transfer is best done with
controls that optimize the process. Difficulties with
spray processes include controlling the arc length and
starting the process. Accordingly, spray transfer with a
controlled arc, such as mechanical control, is desired.
SUMMARY OF THE PRESENT INVENTION
A wire feeder includes a motor that advances,
adjusts movement of the wire including slowing, stopping or
reversing movement of the wire. It may be used within a
short circuit, pulse or spray process. The wire feeder may
be part of a welding system. The wire feeder may have a
motor near the torch, reel, or both.
Other principal features and advantages of the
invention will become apparent to those skilled in the art
upon review of the following drawings, the detailed
description and the appended claims.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagram of a welding system, in
accordance with the present invention;
Figure 2 is a torch with a buffer and reversible
motors in accordance with the present invention;
Figure 3 is a cross-sectional view of the torch of
Figure 2;
Figure 4 is a detailed' cross-sectional view of a
buffer in accordance with the present invention;
Figure 5 is a cross-sectional view of a weld cable
used as part of a buffer in accordance with the present
invention;
Figure 6 is one wave form of a process cycle in
accordance with the preferred embodiment;
Figure 7 is one current wave form of a process
cycle in accordance with another embodiment; and
Figure 8 is graph of the wire feed speed of a
process cycle in accordance with one embodiment of the
invention.
Before explaining at least one embodiment of the
invention in detail it is to be understood that the
invention is not limited in its application to the details
of construction and the arrangement of the components set
forth in the following description or illustrated in the
drawings. The invention'is capable of other embodiments or
of being practiced or carried out in various ways. Also, it
is to be understood that the phraseology and terminology
employed herein is for the purpose of description and should
not be regarded as limiting. Like reference numerals are
used to indicate like components.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the present invention will be illustrated
with reference to a particular welding system using
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particular components, it should be understood at the outset
that the invention may also be implemented with other
systems,. components, and modules, and be used in other
environments.
Generally, the present invention is a method and
apparatus for controlled short circuit or pulse welding that
includes mechanical control of transitions between the arc
and short circuit states. In. various embodiments the
process includes a pulse mode or transfer. Control of
energy to the weld is effected using the output current or
voltage magnitude, wave *shape, time, etc. Thus, the
transitions are caused to occur by controlling the wire
movement, and current can be coordinated with, the
transitions to reduce spatter, instability, or other
undesirable features, by, for example, changing the current
as the transition occurs, or in anticipation of. the
transition. Alternatives include using the mechanical
control described herein with a spray transfer process.
The mechanical control of the process allows the
process to better have a desired arc length. Desired arc
length is an arc length, constant or varying, for part or
all of the process that helps the process perform better,
and may be user set, process set, or controlled. Often,
shorter arc lengths will be cooler, and thus may be
advantageous for some applications. For example,
applications such as welding thinner gauge materials, (auto
body, furniture etc.) or pipe welding may be performed with
pulse or spray using mechanical control.
Mec .anical control of the states is performed by
advancing and slowing, stopping or retracting, or
combination thereof, the wire at the arc. Reversing,
slowing or stopping the wire causes an arc to form.
Advancing the wire causes a short to form. Slowing or
stopping the wire causes the arc to form=because the wire
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doesn't advance fast enough to maintain the short while the
ball forms. Also, the forward momentum of the ball can
cause it to separate from the wire.
An advance followed by a slowing, stopping or
retracting defines one process cycle. (Process cycle, as
used herein, includes one cycle of the states of the process
such as an arc state followed by a short circuit state, or
an arc state, followed by a ghort circuit state, followed by
a pulse state, or it may be defined by current levels --
peak, background, peak, background...etc.) One process cycle
may include multiple speed changes for each current cycle,
or multiple current cycles for each speed cycle.
The advancing and slowing, stopping or retracting
(each of which can be called retarding the advancement) are,
in the preferred embodiment, accomplished using a pair of
motors disposed on either side of the wire, opposite one
another and near (or mounted on) the torch. The motors are,
in various embodiments stepper motors, servo motors,
planetary drive motors, zero backlash motors, dc motors, dc
brushless motors, dc direct drive motors. gearless motors, or
replaced with ca linear actuator. The pair is disposed one
after the other in one embodiment. A single motor is used
in another embodiment.
Stepper motors are used in the preferred
embodiment, and the number, and angle or size of the step is
controlled to control the length of wire advanced or
retracted.
Another embodiment provides for a dc direct drive
motor (such as a solenoid that moves the drive to and form
the wire to directly drive the wire) to reverse, slow, stall
or stop (retard) the wire. Other mechanisms, such as
clamps, magnetics, induction, linear actuators, etc, are
used to reverse, slow, stall or stop the wire in other
embodiments. The motor may be a single motor, or two
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motors, mounted at or near the torch, and may be used with
or without a motor at the wire reel. When no motor is used
at the reel, a buffer is not always used. Another
embodiment provides for a motor (such as one named above) at
the reel, and no motor at the torch. Yet another embodiment
alters the wire path length in or near the torch, thus
"taking up" the wire being fed from the reel. For example,
a solenoid in the torch is uped to deflect the wire,
effectively slowing, stopping or reversing the advancement
of the wire to the weld. The motor at the reel can be any
motor.
The preferred embodiment includes a wire feed
motor mounted near the source of wire, such as a reel of
wire, that drives the wire to the torch (although other
embodiments omit this motor). As the reversible motors
retract the wire (and the wire feed motor continues to feed
the wire) a buffer is provided to account for the increase
in wire between the wire feed motor and the reversible
motors. Similarly, when the reversible motors advance the
wire, wire is withdrawn from the buffer. Controllable
motors are used to slow or stop the wire in other
embodiments. The reversible or controllable motors move the
end of the wire in addition to the movement from the wire
feed motor, or they superimpose motion onto motion imposed
by the wire feed motor. The speed of the wire feed motor is
slaved to the average speed of the reversible or
controllable motors, so that,' on'average, they both drive
the same length of wire, in the preferred embodiment.
The buffer may be anything that stores and returns
the extra wire, or provides an increased wire path length
between the source and the torch. The buffer of the
preferred embodiment includes a wire liner about the wire
for at least a portion of the distance from the source to
the torch. The liner is disposed in a tube that is wider,
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and the liner can bend and flex within the tube, thus
increasing the length of wire in a given length of tube.
The tube is mounted to a hol.Low shaft, and the wire passes
through the shaft. The shaft is fixed in one position.
S Thus, as the wire. is retracted, the wire moves relative to
the tube and shaft (or the tube and shaft may be said to
move relative to the wire). The shaft could be.mounted to
slide along the axis of the Wire, and thus move relative to
the tip of the torch, thereby increasing the length of the
wire path between the tip (arc-end) of the torch and the
wire source end of the torch.
Alternatively, the liner may be mounted to the
shaft, and the wire moves relative to the liner. The liner
is compressible, such as a coil spring, so that as the wire
retracts, the spring compresses, in the preferred
embodiment. Sensors may be provided that sense. the amount
of wire in the buffer, or the tension of the wire, and the
process controlled (average wire feed speed e.g.) may be
controlled in response thereto.
A controller is provided that causes the motors to
retard the movement of the wire (reverse, slow or stop) at
least once per process cycle in the preferred embodiment,
and controls the current output based on mean arc current
(average current during the arc state only, or a function
25' thereof), power,'energy, voltage, or other welding output
-parameters. Feedback may include one or more of short
detection, buffer feedback, tension feedback, pool
oscillation, in addition to traditional welding parameters.
Alternatives include reversing less frequently than once per
cycle. One alternative provides for repeated reversals,
slowings or stopping during the weld (i.e., not merely at
the conclusion of the weld), but not once per cycle. When a
pulse process is used to implement the invention each pulse
is.considered a process cycle.
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For example, braking at the end of the arc cycle
can feed forces between wire and droplet, which may disrupt
the liquid bridge without retracting action. This is
particularly present with lower wire diameters and higher
short circuiting frequencies, but may apply in other
circumstances. The droplet has the speed of the wire before
braking. This kinetic energy can be enough for disrupting
the liquid path. In this cage, no retracting is needed, and
slowing or stopping is used.
The control may include controlling heat,
penetration and/or bead formation by controlling the
advancement of the wire into the weld pool. The relative
time in arc state and.short state (arc balance) may be set
by the user (as may be the time in the pulse state if it is
used). Control of parameters such as polarity (balance),
gas mixtures etc. may be done in coordination with the
relative arc/short times (or other parameters).
Referring now to Figure 1, a welding system 100
includes, in accordance with'the preferred embodiment, a
power supply 102, a wire feeder 104, a controller 106 and a
torch 108, and a supply line 112 which feeds welding
current, gas, water, control, and current for motors to
torch 108, that cooperate to provide welding current on weld
cables 105 and 107 to a workpiece 110. Power supply 102,
wire feeder 104 and controller 106 may be commercially
available welding system components, such as a Miller
Invision 456 power supply, and a modified Miller XR wire
feeder. Power supply, as used herein, includes any device
capable of supplying welding, plasma cutting, and/or
induction heating power including resonant power supplies,
quasi-resonant power supplies, etc., as well as control
circuitry and other ancillary circuitry associated
therewith. Power source, or source of power, as used
herein, includes the power circuitry such as rectifiers,
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switches, transformers, SCRs, etc. that process and provide
the output power. Wire feeder, as used herein, includes the
motor or mechanism that drives the wire, the mounting for
the wire, and controls related thereto, and associated
hardware and software. It can include a motor near the
source of wire that pushes the wire to the weld, and/or
motor(s) near the torch that pulls the wire into the line
and to the contact tip,.or slows, stops or pulls the wire
back from the contact tip. Wire path as used herein,
includes the path the wire takes from the wire source to the
torch or power supply, and may include through a liner, a
buffer, etc.
Controller 106 is part of wire feeder 104 and
power supply 102 in this embodiment. Controller 106 also
includes control modules adapted for the present invention,
such as a reversible wire feeder control module. to control
the reversible motors, a mean arc current module, and the
control module for the mechanical control of the arc states.
Controller, as used herein, includes digital and analog
circuitry, discrete or integrated circuitry,
microprocessors, DSPs, etc., and software, hardware and
firmware, located on one or more boards, used to control a
device such as a power supply and/or wire feeder. Control
module, as used herein, may be digital or analog, and
includes hardware or software, that performs a specified
control function. For example, a mean arc current control
module controls the output to provide a desired mean arc
current.
Figure 2 shows torch 108 in more detail. Torch
108 includes, in addition to the features of prior art
torches, a pair of motor housing 203 and 205 have motors
disposed within to drive the wire to or from the weld, and a
buffer 201 to take up wire 209 when it is retracted, and
provide wire 209 when it is advanced. Buffer, as used
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herein, includes components used to take up the wire when
the wire direction is reversed and provide wire when the
wire is advanced. The end of the wire at the arc is shown
as 207. The motor housings and buffer are adjacent to the
torch in the preferred embodiment, and near the torch in
other embodiments. Adjacent the torch, as used herein,
includes abutting, touching or part of the torch, directly
or through a housing. Near the torch, as used herein,
includes much closer to the torch than the source of wire,
such as more than 75% of the way from the source to the
torch. One embodiment provides that a handheld torch
includes a small spool of wire mounted on the torch.
Figure.3 is a cross-sectional view of the torch of
Figure 2, taken. along lines A-A. A pair of motors 301 and
302 are preferably stepper motors (although they may be
other motors) and drive the wire and are disposed adjacent
to the wire, and directly opposite one another, on opposite
sides of the wire, thereby substantially equalizing forces
on the wire. In alternative embodiments they are disposed
one following the other, or on the same side of the wire.
Directly opposite one another, as used herein, includes at
substantially the same position along a wire path. Disposed
adjacent the wire, as used herein, includes being close
enough to the wire to push or pull the wire. Drive the
wire, as used herein, includes one or both of moving the
wire toward the torch and moving the wire away from the
torch.
Buffer 201 may also be seen on Figure 3, and is
shown in more detail on Figure 4, and includes a shaft 401
mounted on a support 403. Shaft 401 has a hollow axis,
through which wire 209 passes. Weld cable 105 (Figures 1
and 5) is comprised of an outer tube 501 and a-liner 503,
with wire 209 disposed therein. The outer diameter of line
503 is substantially smaller than the inner diameter of tube
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501, to allow for wire length to be taken up or stored by
liner 503 flexing within tube 501. Liner 503 is preferably
a coil spring that allows for compression and expansion to
further buffer the wire. Storing a length of wire, as used
herein, includes taking up wire when the wire direction is
reversed. Substantially more than an, outer diameter of the
liner, as used herein includes enough room to move and flex.
Wire liner, as used herein, Lncludes a tube in which the
wire can easily move. Tube 501 is mounted to shaft 401 so
that wire 209 moves with respect to shaft 401.
A sensor can be included that senses the amount of
wire taken up by buffer 201. Examples of such sensors
include a wheel with an encoder that is turned as the wire
moves past it, or a linear transformer, with the liner being
comprised of a ferrite or magnetic material. The controller
includes a buffer feedback input that receives the feedback,
and provides a wire feed motor output that is responsive to
the buffer feedback. Tension in the wire can also be sensed
and used to control the process.
Control of the process from an electrical
standpoint is easier since process control is performed
using mechanical control of the wire position. Therefore,
the welding current becomes an independent process
parameter, totally, opposite to the conventional MIG process.
One desirable control scheme uses mean arc current
(average current during the arc state, or a function
thereof) as the control variable. This allows better
control of the melting and heat to the weld, and reduces
spatter and instability, compared to prior art control
schemes. It is possible to use mean arc current to control
the heat, since arc current is not used to cause the
transition from arc to short (or the opposite). The control
of the states can be coordinated with the current control.
For example, if a state transition is to occur at a time T1,
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the current transition can occur shortly before that, so as
to avoid disrupting the weld pool. Another control feature
is to allow the user to set relative arc and short time, or
balance between EP and EN.
One desirable arc waveform is shown in Figure 6,
and includes an arc current waveform with three segments -
an initial high current segment, an intermediate current
segment, and a low current segment. The low current segment
is entered into prior to the short forming, thereby
enhancing a smooth transition to the short circuit state.
'Another arc waveform is shown in Figure 7, and is
similar to prior art waveforms. The current is increased
during the short, and then reduced before the short is
cleared and an arc forms. Then, during the arc,. a higher
current is provided, followed by a gradual current
reduction. The current and/or energy during the arc phase,
or portions thereof, may be totalized.
The waveform of Figure 7 and the prior art such as
US Patents 4717807, 4835360, 4866247, 4897523, 4954691,
4972064, 5001326, 5003154, 5148001, 5742029, 5961863,
6051810, 6160241, and 6326591 is combined with the present
invention in one embodiment. The prior art teaches to
control the process by current control. This embodiment of
the present invention replaces the prior process control
with mechanical control (slowing, stopping, reversing), but
retains the wave form.
Another embodiment uses the prior art process
control, but uses mechanical control (slowing, stopping or
reversing the wire) to clear the short (create the arc) if
the short fails to clear when expected according to the
prior art or if it has not cleared after a period of time.
The failure to establish the arc can be determined by
monitoring. output voltage. This embodiment is particularly
helpful to stabilize some of the relatively unstable prior
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art processes, by providing a "failsafe" transition to the
arc state.
Because the welding current becomes an independent
process parameter, the current can be set to the value,
which directs the process into the wanted situation by
physical determined behavior. For allow spatter material
transfer, the forces onto the liquid have to be low, when
the cross section of the electrical conductor is low.
Therefore, in one embodiment, the currents have to be low
during those phases. During the middle part of the short
circuit state, where larger cross section of the electrical
conductor is present, high forces can be used to move
liquids. Also, high currents during the middle part of the
short circuit state are possible. During the arc phase, the
current can be used for movement of the liquid and
determining the melting rate.
The present invention may be used with known
control schemes, but implement them in a more desirable
fashion by eliminating the need for current levels to cause
transitions. For example, schemes using either arc length
or stick-out as a control variable can be implemented easily
because the stepper motors allow stick-out to be measured
precisely. Because the transitions are caused mechanically,
the arc length may be redetermined each process cycle.
Turning now to Figure 8, the wire feed speed of a
process cycle of the preferred embodiment is shown. Upon
detection of a short, as indicated by the voltage dropping
below a threshold, the wire feed speed is commanded to be a
constant reverse speed. It takes some time for the motor to
effect the commanded change. When the reverse wire feed
speed is reached, it is held constant. Eventually,=the
reversing wire forms an arc. The controller continues to
command the constant reverse wire feed speed for a length of
time that will provide a desired arc length. The desired
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arc length divided by the wire feed speed will result in the
time necessary to continue the constant reverse speed.
Thereafter, the wire feed speed is commanded to be the
forward wire feed speed. Again, it takes some time for the
wire feed speed to reach the commanded forward wire feed
speed. The forward speed is held constant until a short is
formed.
In various alternatives, the controller commands a
faster or slower wire feed speed in reverse when the open
circuit is detected. By commanding a faster reverse speed
the desired arc length will be obtained more quickly. Other
modifications, such as, other delays, other than constant
speeds, changing the commanded speed to forward prior to the
desired arc length being obtained to accommodate for the
length of time it takes for the motor to bring the wire feed
speed back to the commanded forward speed, may be used.
The present invention may be implemented with a
variety of processes, including but not limited to electrode
positive, electrode negative, alternating polarity, ac mig,
mig brazing, hard facing, and welding with thick wire at low
currents. For example, welding on a 2.4 mm wire may be
performed at 100 amps, or even 35 or fewer amps with the
present invention. Prior art systems required more current
on thick wire to cause the short to clear and to enter the
arc state. The present invention doesn't rely on current to
clear the short, so thick wire and low current may be used.
The control preferably ties the speed of the wire
feed motor to the average speed of the stepper motors, so
that the wire feed speed follows the process speed.
Averaging speed over 20-30 process cycles (about 500 msec.)
provides for effectivecontrol.
Pool oscillation frequency can be found by
monitoring the distance the wire travels until a short is
created, or an arc is created. One control scheme provides
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that the state transitions are timed to coincide with the
natural frequency of pool oscillation. The controller
includes a frequency module and a pool oscillation feedback
circuit that effect this control scheme. A short detection
feedback circuit may be used as part of the control loop.
Another embodiment included implementing
mechanical control of the wire in a pulse process. The
mechanical control can be used to control arc length and/or
help avoid inadvertent shorts. One embodiment provides that
the wire be slowed, stopped or reversed during one of the
phases of the process, such as during the background current
phase, or during the peak current phase. If the wire is
slowed or stopped it is less likely to short, and the
process can thus be made more stable. Preferably, the
mechanical control is linked with the electrical control, so
that the stopping occurs on a regular basis. In one
embodiment the arc voltage (or other output parameter) is
monitored to determine when a short occurs. At that time,
the wire is slowed or stopped or reversed. Thus, the short
20, can be prevented, or more quickly cleared, and the process
becomes more stable.
Yet another embodiment includes implementing
mechanical control of the wire in a spray process. The
mechanical control can be used to control arc length and/or
help avoid inadvertent shorts. If the wire is slowed,or
stopped it is less likely to short, and the process can thus
.be made more stable. Preferably, the mechanical control is
linked with the electrical control, so that the stopping
occurs on a regular basis. in one embodiment the arc
voltage (or other output parameter) is monitored to
determine arc length. The wire is advanced, slowed, stopped
or reversed to maintain a desired arc length. Thus, the
process can be cooled, and/or, more stable. Other processes
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such as pulse or short circuit welding can be used with arc
length control.
One application of a combined mechanical-pulse
process is used to weld titanium. The combined process runs
cooler, and can have a shorter arc, and produce more
desirable welds by countering molten surface tension with
mechanical control. The combined process can be as
described above, wherein mechanical control is added to a
pulse process, or it can be performed.using distinct
mechanically controlled short-arc processes, followed by,
preceding or alternated with MIG processes.
Turning now to Figure 8, the wire feed speed of a
process cycle of the preferred embodiment is shown. Upon
detection of a short, as indicated by the voltage dropping
below a threshold, the wire feed speed is commanded to be a
constant reverse speed. It takes some time for, the motor to
effect the commanded change. When the reverse wire feed
speed is reached, it is held constant. Eventually, the
reversing wire forms an arc. The controller continues to
command the 'reverse wire feed speed for a length of time
that will provide a desired arc length. The desired arc
length divided by the wire feed speed will result in the
time necessary to continue the constant reverse speed.
Thereafter, the wire feed speed is commanded to be the
forward wire feed speed. Again, it takes some time for the
wire feed speed to reach the commanded forward wire feed
speed. The forward speed is-held constant until a short is
formed.
In some alternatives, the controller commands a
faster or slower wire feed speed in reverse when the open
circuit is detected. By commanding a faster reverse speed
the desired arc length will be obtained more quickly. Other
modifications, such as, other delays, other than constant
speeds, changing the commanded speed to forward prior to the
CA 02489137 2004-12-03
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desired arc length being obtained to accommodate for the
length of time it takes for the motor to bring the wire feed
speed back to the. commanded forward speed.
In various alternatives, the controller commands a
faster or slower wire feed speed in reverse when the open
circuit is detected. By commanding 4. faster reverse speed
the desired arc length will be obtained more quickly. Other
modifications, such as, other delays, other than constant
speeds, changing the commanded speed to forward prior to the
desired arc length being obtained to accommodate for the
length of time it takes for the motor to bring the wire feed
speed-back to the commanded forward speed, may be used.
Numerous modifications may be made to the present
invention which still fall within the intended scope hereof.
Thus, it should be apparent that there has been provided in
accordance with the present invention a method and apparatus
for controlled short circuit and/or MIG/pulse welding that
fully satisfies the objectives and advantages set forth
above. Although the invention has been described in
conjunction with specific embodiments thereof, it is evident
that many alternatives, modifications and variations will be
apparent to those skilled in the art. Accordingly, it is
intended to embrace all such alternatives, modifications and
variations that fall within the spirit and broad scope of
the claims.
