Equipped with new control technology to handle higher volumes and faster wire speeds, aluminum welding technology is ideal for automotive manufacturing applications.
Push-pull wire feed systems can achieve higher reliability on aluminum MIG welding with speeds of 5 meters/min in lap welds and 2 m/min for fillet-lap welds.
A brushless DC servo-motor drive yields stable feeding and high speed response.
A horizontal to vertical weld. In high speed welding, operators need the ability to manage the waveform for optimum fluidity and bead shape.
An in-line weaving application. Arc transfer in high speed applications can employ robotic movements that are varied with quick response.
Extensive corporate research has helped Panasonic Factory Automation, Elgin, IL, to develop their MIG Force push-pull wire feed system to achieve the highest possible reliability for aluminum MIG welding. Speeds achieved are 5 meters/min in lap welds and 2 m/min for fillet-lap welds.
Overall Approach of MIG Force
The design goals were to control the wire feed rate (speed), pulse waveform, and robot movement --within a single tightly integrated system. Arc stability and wire feed rate are the key interacting variables to be controlled. Arc stability during high speed welding affects bead appearance, which, in turn, affects more than 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 wire feed occurs at the start and end of every weld. Therefore, two precise steps must occur:
- Exact control of wire feed speed at the start and end of welds is necessary.
- 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. The shielding gas should be at a stable flow rate at all times.
The new aluminum wire push-pull system enables the 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 percent 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 other process issues:
- A planetary roller wire feed provides a reliable feeding system without galling the wire. It automatically straightens the wire to yield correct targeting of the arc.
- The brushless DC servo-motor drive yields stable feeding and high speed response.
- The compact, light weight design permits use with robots as small as 6 kg.
- The power source provides sophisticated control of arc transfer, including the programming required to manage high-speed welding
The Robot Rules
The system uses Panasonic's G-series robot as its centerpiece. Precise control of wire feed speed and waveform modification programs for the power supply is achieved. The effect is to micromanage, thus coordinating all welding conditions. This technology contrasts with some other conventional arc welding robot technologies, 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 parameters of the arc process as comprising the following nine sets of variables:
- Peak current.
- Base current.
- Rise time.
- Falling time.
- Wire feed speed.
- Arc voltage.
- Waveform response time.
- Pulse mode.
- Pulse frequency.
Arc Management 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 many power supplies. Such machines actually provide lower degrees of "waveform control." Many systems modify pulse frequency and peak level to maintain constant power and heat input, which 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 high speed welding of aluminum, there is always significant extraction of heat from the weld to the parent metal. Consider what happens in a component (for example, while welding a lap joint of 2 mm sheet to 2 mm sheet) if a change in weld configuration occurs so that one needs to weld a similar joint, but this time 2 mm sheet to 4 mm sheet. 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 an advanced system, the waveform will be changed to account for the added withdrawal of heat in order 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 Amps of base current to increase overall heat input instantaneously. Extensive observations document that in high speed welding, the operator must have the ability to manage the waveform, such that optimum fluidity and bead shape result.
Constant Wire Feed
Successful feeding of aluminum wire depends on 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 be adjusted such that the motor sees only the feeding load.
MIG Force introduces a proprietary planetary roller system, a simple compact device, to achieve accurate feeding and torch/arc location. Two rollers confront one another at 45 degrees and are arranged in the housing such that a fixed angle is maintained relative to the centerline of the wire being fed, regardless 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. 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, accurate wire feed speeds are achieved.
In comparison, conventional grooved rollers pressure the wire that rides in center of the groove. To increase feedability, 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 traditional problems encountered in welding with small diameter aluminum wire is that of achieving high productivity despite the tendency of the wire to buckle in some feeding systems. In the MIG Force system, a buckling detection circuit monitors wire feeding resistance, preventing buckling and associated downtime by assuring arc starting reliability and freedom from "bird nesting."
Automotive and similar users demand efficient, high speed, stop-and-start "stitch" welding capability for 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 wire buckling 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 in which a tight, direction-ally controlled arc is needed.
- Hybrid — for high speed applications in which robot movement may be varied and a quick arc response is needed.
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. The machines are also able to control pulse frequency width, checking 20 times per pulse to verify that the correct waveform is operating. This yields better one-drop/pulse performance. Such a rapid response is necessary 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. The consequent arc voltage changes the bead characteristics in undesirable ways, leading to increased spatter and undercut.
Edited from information supplied by Panasonic Factory Automation, Elgin, IL Ph.:(888) PAN-WELD; Fax: (847) 288-4564; www.panasonicfa.com
Panasonic and Welding
From a modest beginning in 1947 as a manufacturer, Panasonic grew rapidly and began manufacturing welding robots in 1981. Specializing in arc welding (as opposed to resistance welding), Panasonic soon became a major private-label supplier to many branded robot products. Having entered the U.S. market in 1983 though OEM suppliers, 1987 marked the first U.S.-based Panasonic robotic engineering and sales office. A year later, the first Panasonic-brand robots were sold to a number of end users through the U.S. Company, Panasonic Factory Automation.
Major penetration of the automotive markets was heralded in 1999 with PFA's receipt of an order from Budd-Canada for over 200 arc welding robots, associated power sources, and its online process monitoring system, PanaPRO. By year-end 2000, Panasonic had sold over 50,000 arc welding robots worldwide.