U.S. patent application number 11/426063 was filed with the patent office on 2006-10-12 for method and apparatus for feeding wire to a welding arc.
This patent application is currently assigned to Illinois Tools Works Inc.. Invention is credited to Bruce P. Albrecht, Peter Heneoke, Gerd Huismann, Richard M. Hutchinson.
Application Number | 20060226137 11/426063 |
Document ID | / |
Family ID | 34890193 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060226137 |
Kind Code |
A1 |
Huismann; Gerd ; et
al. |
October 12, 2006 |
Method and Apparatus For Feeding Wire to a Welding Arc
Abstract
A method and apparatus for feeding wire in a welding system
include one or more motors disposed adjacent the wire to drive it.
A wire feed motor is also disposed along the wire path, and is
closer to the source of wire than the torch, and closer to the
source than the one or more motors. The motors may be a pair motors
disposed on opposite sides of the wire and move the wire to and
away from an arc end of a torch. They preferably reversing the
direction of the wire within one process cycle. The or more motors
may be a stepper motor, a servo motor, a zero backlash motor, a
gearless motor, a planetary drive motor, or a linear actuator (such
as a piston), in various embodiments.
Inventors: |
Huismann; Gerd; (Hamburg,
DE) ; Heneoke; Peter; (Am Silberg, DE) ;
Hutchinson; Richard M.; (New London, WI) ; Albrecht;
Bruce P.; (Grayslake, IL) |
Correspondence
Address: |
CORRIGAN LAW OFFICE
5 BRIARCLIFF CT
APPLETON
WI
54915
US
|
Assignee: |
Illinois Tools Works Inc.
Glenview
IL
|
Family ID: |
34890193 |
Appl. No.: |
11/426063 |
Filed: |
June 23, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10728629 |
Dec 5, 2003 |
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11426063 |
Jun 23, 2006 |
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10200884 |
Jul 23, 2002 |
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10728629 |
Dec 5, 2003 |
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Current U.S.
Class: |
219/137.71 |
Current CPC
Class: |
B23K 9/1336 20130101;
B23K 9/073 20130101; B23K 9/124 20130101 |
Class at
Publication: |
219/137.71 |
International
Class: |
B23K 9/10 20060101
B23K009/10 |
Claims
1. A wire feeder for feeding wire from a source of wire in a
welding system comprising: at least one stepper motor disposed
adjacent the wire and disposed to drive the wire; wire feed motor
disposed along a wire path from the source to a welding torch,
wherein the torch is closer to the at least one stepper motor than
the torch is to the wire feed motor, and wherein the wire feed
motor is disposed to contact the wire and move the wire from the
source to the torch; and the at least one stepper motor is disposed
to retard movement of the wire toward an arc end of the torch.
2. The wire feeder of claim 1, wherein the at least one stepper
motor is disposed to slow the movement of the wire.
3. The wire feeder of claim 1, wherein the at least one stepper
motor is disposed to stop the movement of the wire.
4. A wire feeder for feeding wire from a source of wire to a weld,
comprising a pair of motors disposed on opposite sides of the wire
and disposed to move the wire to an arc end of a torch, and to
retard movement of the wire to an arc end of the torch.
5. The wire feeder of claim 4, wherein the pair of motors is
disposed to slow the movement of the wire.
6. The wire feeder of claim 4, wherein the pair of motors is
disposed to stop the movement of the wire.
7. The wire feeder of claim 6, wherein the pair of motors are
disposed along a wire path from the source to the torch, adjacent
the torch.
8. The wire feeder of claim 4, wherein the pair of motors are
disposed along the wire path closer to the torch than to the
source.
9. The wire feeder of claim 4, further comprising a wire feed motor
disposed along the wire path, closer to the source than to the
torch, and disposed to contact the wire and move the wire from the
source to the torch.
10. The wire feeder of claim 4, wherein the source includes a reel
of wire mounted without a wire feed motor adjacent thereto.
11. The wire feeder of claim 4, wherein the pair of motors are
disposed directly opposite one another.
12. The wire feeder of claim 7, wherein the pair of motors are
stepper motors.
13. The wire feeder of claim 4, wherein the pair of motors are
disposed one after the other.
14-28. (canceled)
29. A method of providing wire from a source to a weld in a welding
system comprising driving the wire with a pair of motors disposed
on opposite sides of the wire and moving the wire to an arc end of
a torch, and retarding movement of the wire to the arc end of the
torch.
30. The wire feeder of claim 1, wherein the pair of motors is
disposed to slow the movement of the wire.
31. The wire feeder of claim 1, wherein the pair of motors is
disposed to stop the movement of the wire.
32-39. (canceled)
40. A method of providing wire from a source to a weld in a welding
system comprising driving the wire to, and retarding the movement
to an arc end of a torch within one process cycle.
41. The method of claim 29, wherein retarding includes slowing the
movement.
42. The method of claim 29, wherein retarding includes stopping the
movement.
43. A wire feeder for feeding wire from a source of wire in a
welding system comprising: means for feeding wire from the source
to a weld; and means for driving the wire to or retarding movement
to an arc end of a torch within one process cycle.
44-61. (canceled)
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] Another known welding process is called controlled short
circuit welding, or short circuit welding. Short circuit welding is
often performed as a MIG process. 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.
[0005] 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.
[0006] Many attempts in the prior art were made to create a stable
short circuit or pulse welding power supply, such as those shown in
U.S. Pat. Nos. 4,717,807, 4,835,360, 4,866,247, 4,897,523,
4,954,691, 4,972,064, 5,001,326, 5,003,154, 5,148,001, 5,742,029,
5,961,863, 6,051,810 and 6,160,241, hereby incorporated by
reference. 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.
[0007] 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 not perform well at either controlling the
energy into the weld or controlling the transition.
[0008] Another short circuit welding control system is disclosed in
U.S. Pat. No. 6,326,591. This system adequately controls the energy
into the weld, but it does not provide independent control of the
transitions between states.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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
[0017] A wire feeder includes a motor that advances, and slows,
stops or reverses the wire. It may be used with in 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.
[0018] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagram of a welding system, in accordance with
the present invention;
[0020] FIG. 2 is a torch with a buffer and reversible motors in
accordance with the present invention;
[0021] FIG. 3 is a cross-sectional view of the torch of FIG. 2;
[0022] FIG. 4 is a detailed cross-sectional view of a buffer in
accordance with the present invention;
[0023] FIG. 5 is a cross-sectional view of a weld cable used as
part of a buffer in accordance with the present invention;
[0024] FIG. 6 is one wave form of a process cycle in accordance
with the preferred embodiment;
[0025] FIG. 7 is one current wave form of a process cycle in
accordance with another embodiment; and
[0026] FIG. 8 is graph of the wire feed speed of a process cycle in
accordance with one embodiment of the invention.
[0027] 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
[0028] While the present invention will be illustrated with
reference to a particular welding system using 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.
[0029] 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.
[0030] 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.
[0031] Mechanical control of the states is performed by advancing
an 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 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.
[0032] 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
short 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.
[0033] 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 a linear actuator. The pair is
disposed one after the other in one embodiment. A single motor is
used in another embodiment.
[0034] 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.
[0035] 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
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 used 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.
[0036] 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.
[0037] 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, 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 hollow shaft, and the wire passes through the
shaft. The shaft is fixed in one position. 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.
[0038] 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.
[0039] 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 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.
[0040] 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 case, no retracting is needed,
and slowing or stopping is used.
[0041] 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).
[0042] Referring now to FIG. 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.RTM. power supply, and a modified Miller XR.RTM. 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, 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.
[0043] 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.
[0044] FIG. 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 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.
[0045] FIG. 3 is a cross-sectional view of the torch of FIG. 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 fro0Xthe torch.
[0046] Buffer 201 may also be seen on FIG. 3, and is shown in more
detail on FIG. 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 (FIGS. 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 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, includes 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.
[0047] 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.
[0048] 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.
[0049] 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, 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.
[0050] One desirable arc waveform is shown in FIG. 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.
[0051] Another arc waveform is shown in FIG. 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.
[0052] The waveform of FIG. 7 and the prior art such as U.S. Pat.
Nos. 4,717,807, 4,835,360, 4,866,247, 4,897,523, 4,954,691,
4,972,064, 5,001,326, 5,003,154, 5,148,001, 5,742,029, 5,961,863,
6,051,810, 6,160,241, and 6,326,591 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.
[0053] 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 art processes, by providing a "failsafe" transition to the
arc state.
[0054] 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 a low 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.
[0055] 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.
[0056] Turning now to FIG. 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
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.
[0057] 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.
[0058] 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.
[0059] 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 effective control.
[0060] 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 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.
[0061] Another embodiment includes 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 can be prevented, or more quickly cleared, and the
process becomes more stable.
[0062] 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 such as pulse or short circuit
welding can be used with arc length control.
[0063] 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.
[0064] Turning now to FIG. 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.
[0065] 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 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.
[0066] 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.
[0067] 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.
* * * * *