Tim Nacey, group manager, Industrial Group, Panasonic Factory Solutions Co. of America
Edited by Kimberley A. Gilles, associate editor

A brushless DC servomotor produces stable feeding and high-speed response.

Arc transfer in high-speed applications can use robotic movements that are varied and need quick arc response, such as in this in-line weaving application.

Panasonic's push-pull wire feed produces speeds of 196.9 in. on lap welds and 78.7 imp on fillet-lap welds.

Panasonic Factory Solutions Co. of America, Elgin, Ill., developed its MIG Force push-pull wire-feed system to help manufacturers meet the productivity and quality challenges of high-volume MIG welding of aluminum. The system tightly controls arc stability, robot movement, wire-feed speed, and pulse waveform.

The company developed a reliable push-pull wire-feed system to couple its G-series robot and 64-bit RISC controller. Arc stability during highspeed welding has an effect on bead appearance that is beyond aesthetics and weld quality. It also influences the mechanical and structural properties of the weldment.

Wire-feed speed control
During high-speed aluminum welding, rapid changes in wire-feed occur at the start and end of every weld. So the control of wire-feed speed must be exact at the start and end of welds, and the arc waveform also must be synchronized with wire-feed speed during the entire welding process.

Other vitally important aspects include: Precise control of the droplet detached from the wire must be maintained for continuous one-pulse/ onedrop transfer. The arc must respond rapidly to any changes in programmed stick out; and, the shielding gas should be kept at a stable flowrate.

Panasonic's push-pull system consistently feeds soft-aluminum MIG wires. The feeding system's push-assist motor delivers constant torque that can overcome nearly 100% of the natural friction and feeding resistance in the wire-delivery system. A magnetic, constant-torque system exerts only 50 grams or less of back pressure on the wire package, so the planetary gear "pull" system needs only a minimal force.

Additional engineering elements of the system address other process issues:

• A planetary roller wire-feed provides a reliable feeding system without galling the wire. It straightens the wire automatically for correct targeting of the arc.
• A brushless, DC servomotor drive ensures stable feeding and highspeed response.
• Robots as small as 6 kg can be used to manipulate the compact, lightweight-welding system.
• A power source controls arc transfer and the programming required to manage high-speed welding.

The robot rules
The system's centerpiece is Panasonic's G-series robot that precisely controls wire-feed speed and ultrahigh-speed, waveform modification programs for the power supply. The goal is to micromanage the variables so that all welding conditions are coordinated. This technology contrasts with conventional arc welding robot technologies in which the control of wire-feed resides in the power source and in which waveforms are fixed for the life of the power supply.

The robot's 64-bit RISC CPU ties the whole system together. It carries out the high-response tasks at 30 the speed of a twin 32-bit RISC chip set. Panasonic modeled the arc processor's parameters on nine variables: peak current, base current, rise time, falling time, wire-feed speed, arc voltage, waveform response time, pulse mode, and pulse frequency.

Arc management is key
The G2 robot controls arc waveform. This robot is the only part of the system that knows what type and at what time a weld must be made. Although some power supplies provide one, pre-programmed waveform, the G2 robot changes waveforms instantaneously to accommodate variables such as changes in material alignment and to ensure that high-quality welds are made. Typical machines actually provide lower degrees of "waveform control."

Some systems modify pulse frequency and peak levels to maintain constant power and heat input, but that may be counter-productive to achieving high-quality, high-speed welding. The waveform must be changed dynamically to match conditions in real production.

During high-speed aluminum welding, significant heat exchange from the weld to the parent metal always occurs.

The advanced waveform changes to account for the added heat withdrawal so the correct bead shape is produced. With 4-mm-thick (0.16-in.) sheet, the robot's program calls for the addition of 20-30 amps of base current to increase overall heat input instantaneously. In high-speed welding, the operator must have the capability to manage such waveform so that optimum fluidity and bead shape result.

Constant wire-feed
For successful feeding of aluminum wire, the force within the entire wirefeed path must be kept to an absolute minimum. To achieve the desired total load on the motor, the reflected load should be under 1 lb. This is accomplished by locating an adjustable, magnetically coupled push motor at some distance from the robot system and by adjusting the motor so it sees only the feeding load.

MIG Force has accurate feeding and torch/arc location because of its proprietary, compact planetary roller system. Two rollers are set in a housing at 45° to each other. The angle remains fixed relative to the centerline of the wire being fed. The wire is not "flattened" before entering the contact tip, so erratic contact and momentary disruption of feeding is avoided.

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 fed in its centerline direction. An added bonus is that the inherent cast and helix of the wire are removed, and the wire is straightened. Straight wire provides an arc in the correct location, minimizing welding defects. In addition, time and labor costs associated with changing drive rollers are eliminated because the planetary rollers do not need to be changed to handle different wire diameters. The precision timing belt and AC servomotor ensure high-precision roller specs so accurate wirefeedspeeds are achieved. In contrast, conventional grooved rollers put pressure on the wire as it rides in the groove's center. To boost feedability, more pressure must be applied to the drive rolls, and that increases the possibility of wire deformation. In addition, conventional grooved rollers cannot straighten the wire.

Buckling detection circuit
Traditionally, achieving high productivity while welding with small-diameter aluminum wire has been challenging because aluminum wire tends to buckle in feeding systems. To counter this, MIG Force uses a bucklingdetection circuit that monitors wire feeding resistance and prevents buckling. Associated downtime is reduced because the system ensures total arc starting reliability and freedom from "bird nesting" and burnbacks.

The circuit's servomotor detects any change in motor load at the instant it happens, and the robot software stops the drive motor before the wire buckles.

This feature especially is important during "stitch" welding, the stop-andstart welding technique used by automotive and other manufacturers on aluminum components.

MIG Force has produced up to 20,000 consecutive arc starts in a production environment without failure. It has also repeatedly produced quality welds at speeds over 200 ipm (5 meters/min) in certain applications.

Controlling the arc
MIG Force uses soft, hard, and hybrid arc-transfer modes for different application requirements. Soft transfer modes are used for wide, washed out beads or where fit-up may be a problem. Hard and hybrid transfer modes are used for high-speed applications. The hard mode is appropriate when a tight, directionally controlled arc is needed; and the hybrid mode is appropriate when robot movement may be varied and a quick arc response is needed.

Artificial intelligence
Inverter power sources for the MIG Force system have the required hybrid mode, a mix of both pulsewidth modulation and pulsefrequency modes used in high-speed robotic welding. Typical high-end power sources only modify pulse frequency for arc control, but this method is too slow to control the arc's micro-details. Leading edge machines also control pulse frequency width while checking 20 times/pulse to verify the correct waveform is operating. This yields better one-drop/pulse performance. Such fast reaction is needed since extremely consistent length is required and the "arc flare" — a longer-than-desired arc length — cannot be permitted. The resulting increase in arc voltage changes bead qualities, leading to increased spatter and undercut.