Panasonic Achieves Ultra High-Speed MIG Welding of Aluminum

Panasonic Factory Solutions Company Posted 04/05/2004

Elgin, IL.  Panasonic Factory Automation introduced pioneering high-speed aluminum welding technology for applications such as automotive manufacturing.  Extensive corporate research developed the MIG Force push-pull wire feed system to achieve highest-possible reliability for aluminum MIG welding.  Speeds achieved are 5 meters/min in lap welds and 2 m/min for fillet-lap welds.

The overall approach of MIG Force.
The design goals were to control wire feed speed, pulse waveform and robot movement – within a single tightly integrated system.  Arc stability and wire feed speed are the key interacting variables to be controlled.  Arc stability during high speed welding affects bead appearance, which in turn affects more than aesthetics; it also influences quality as well as mechanical and structural properties of the weldment. 

Wire feed speed control.
During high-speed aluminum welding, rapid wire feed speed occur at the start and end of every single weld.  Therefore, two precise steps must occur:  

1. Exact control of wire feed speed at the start and end of welds is necessary, and 
2. The arc waveform both at the start and end of welds and also while welding must be synchronized with wire feed speed.  Related aspects are also important.  The droplet detached from the wire should be precisely controlled to allow one-pulse:one drop transfer at all times.  If there are changes in programmed stick out the arc must also respond to them. Shielding gas should be at a stable flow rate at all times. 

The new aluminum wire push-pull system enables highly stable feeding of soft aluminum MIG wires.  One element of this feeding system is the push-assist motor, operating with constant torque that can be set to overcome virtually 100% of the natural friction and feeding resistance in the wire delivery system.   Only 50 grams or less of back pressure is exerted on the wire package by a magnetic, constant torque system, thus requiring minimal pulling force by the planetary gear ‘‘pull’‘ system.  There are many additional engineering elements employed to address many of the other issues: 

• An innovative planetary roller wire feed … provides a highly reliable feeding system without galling of the wire and automatically straightens the wire to yield correct targeting of the wire and thus, the arc
• Brushless DC servo motor drive … yields stable feeding and high speed response
• Compact, lightweight design …  permits use with robots as small as 6 kg.
• Power source … providing sophisticated control of arc transfer, including the programming required to manage high-speed welding

The robot rules.
Panasonic’s development approach uses its G-series robot as the centerpiece of the system.  Achieved is precise control of both wire feed speed and the waveform modification programs for power supply.  The effect is to micromanage, thus coordinating all welding conditions.  This technology contrasts to conventional arc welding robot technology, in which wire feed control resides in the power source and waveforms are fixed for the life of the power supply.

The 64-bit RISC CPU within the robot ties the whole system together.  It carries out the high-response tasks at 30X the speed of a twin 32-bit RISC chip set.  The technical approach modeled the details of the arc process as comprising the following nine unique sets of variables:

• Peak current
• Base current
• Rise time
• Falling time
• Wire feed speed
• Arc voltage
• Waveform response time
• Pulse mode
• Pulse frequency

Managing the arc is key.
In the G2 series of robots, the arc waveform is controlled by the robot, which alone in the system knows the weld to be made at any point in time.  It is not possible to plan that one pre-programmed waveform is sufficient, as is the case for most power supplies termed advanced.  Such machines actually provide lower degrees of ‘‘waveform control.’‘  Many of these systems modify pulse frequency and peak level to maintain constant power and heat input.  This may be exactly what is not needed in high speed welding.  As conditions change in real production welds, the waveform must be changed dynamically.  In simple colloquial terms, one size does not fit all when world-class quality is at stake.

During the high speed welding of aluminum, there is always significant extraction of heat from the weld to the parent metal.  Consider what happens in a real-world component if a change in weld configuration occurs from 2- to 2-mm (0.078- to 0.078-in.) lap joint to a situation if one part is increased to 4-mm, thus significantly increasing heat flow from the weld.  Without advanced waveform control, the only solution would be to slow down to allow time for the weld bead to flow to the joint. 

In leading-edge system, the waveform will be changed to account for the added withdrawal of heat, to achieve the correct bead shape.  In this example of encountering a 4-mm thickness, the robot’s program will call for an added 20-30 amperes base current to increase overall heat input instantaneously.  Extensive observations document that for high speed welding, the operator must have the ability to manage the waveform, such that optimum fluidity and bead shape result. 

Achieving rock-steady wire feed speed.
Successful feeding of aluminum wire depends upon achieving absolutely minimal force within the entire wire feed path of cabling, hoses and so on.  To achieve the desired total load on the motor, the reflected load should be less than 1-lb. To accomplish this, an adjustable magnetically coupled push motor can be located any distance from the robot system and adjusted such that the motors sees only the feeding load.

MIG Force introduces a newly designed, proprietary planetary roller system, a simple compact device to achieve highly accurate feeding as well as torch and arc location.  Two rollers confront one another at 45°and are arranged in the housing such that a fixed angle is maintained relative to the centerline of the wire being fed, irrespective of its diameter or hardness.  Wire is not ‘‘flattened’‘ prior to entering the contact tip, thus avoiding either erratic contact or momentary disruption to feeding.  Wire helix is also prevented, resulting in extremely straight wire and uniform arc location for automated applications.

When the housing is rotated, the two rollers are planetary-rotated touching the wire.  As the rollers turn on the circumference of the wire, it is thus fed in its centerline direction.  As an added, highly important bonus, the inherent cast and helix of the wire are removed and the wire is straightened to yield a major benefit. Straight wire yields an arc in the correct location, thus minimizing any welding defects.  With this contact mechanism, all wire diameters are fed with the same drive rolls to eliminate the lost production time and labor cost required to change drive rolls. A precision timing belt and an AC servomotor assure high-precision roller specs and thus, extremely accurate wire feed speeds are achieved. 

In comparison, conventional grooved rollers pressure the wire that rides in center of the groove.  To increase feed ability, increased pressure is applied to the drive rolls with the possibility of deformation of the wire. This kind of system cannot straighten the wire.  Among the traditional problems in welding with small dia. aluminum wire is to achieve high productivity given the pronounced tendency to buckling encountered with traditional feeding systems.  In the MIG Force system, a buckling detection circuit monitors wire feeding resistance, preventing buckling and associated downtime by assuring total arc starting reliability and freedom from ‘‘bird nesting. 

Automotive and similar users demand efficient high-speed, stop-and-start ‘‘stitch’‘ welding capability for increasingly utilized aluminum components.  This system has achieved up to 20,000 consecutive arc starts without failure in a production environment. Quality welds have been repeatedly demonstrated at speeds in excess of 5 meters/min (200 in/min. in certain applications).

Costly downtime due to burn-backs and bird nesting are prevented by a carefully engineered sequence.  A servomotor detects any instantaneous change in motor load. The robot software then stops the drive motor before buckling of wire occurs. All popular wire packages are readily handled, e.g. bulk drums or small spools.

Controlling the arc.
Three modes of arc transfer may be employed:

• Soft … for wide washed out beads or where fit up may be a problem
• Hard ... for high-speed applications where a tight directionally controlled arc is needed
• Hybrid …  for high speed applications where robot movement may be varied and quick arc response is needed

Power source is also special.
Artificial intelligence, inverter power sources have the required Hybrid mode, a mix of both pulse width modulation and pulse frequency modes of arc transfer for high speed robotic welding.  The typical high-end power source only modifies pulse frequency for arc control, but this is too slow to control the micro-details of the arc.  Leading edge machines are also able to control pulse frequency width, checking 20 times per pulse to verify if the correct waveform is operating.  This yields better one-drop/pulse performance.  Such fast reaction must occur since a very consistent arc length is required and the arc cannot be permitted to ‘‘flare.’‘  The pulse frequency control method will react to changes in robot stick out. Arc flare is, in reality, a longer-than-desired arc length, changing bead characteristics and leading to spatter and, and virtually eliminating undercut.

Background
Panasonic and welding.
From a modest beginning in 1947 as a manufacturer, Panasonic grew rapidly, and began manufacturing welding robots in 1981.  Focused to arc welding (as opposed to resistance welding), Panasonic soon became a major private-label supplier to many branded robot products.  Having entered the US market in 1983 though OEM suppliers, 1987 saw the first US-based Panasonic robotic applications engineering and sales office.  A year later, the first Panasonic-brand robots were sold to a number of end users through the US Company, Panasonic Factory Automation.  . 

Major penetration of the automotive markets was heralded in 1999 with its receipt of an order from Budd-Canada  for over 200 arc welding robots, associated power sources and its innovative online process monitoring system, PanaPRO®.  By year-end 2000, Panasonic has sold over 50,000 arc-welding robots worldwide.
 

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