Welding Simulator Steps Up with Stepper Motors
Stepper motors retract electrode in welding trainer to simulate realistic performance.
Arc welding processes require the welder to use good workmanship in order to produce sound welds. Poor technique such as an overly long distance between the electrode and the work piece (arc length) can make the difference between a smooth, strong weld and one that’s full of defects and prone to failure. Learning the skill requires time and patience, along with a significant volume of supplies. In an effort to reduce the cost of training and train welders faster, the Lincoln Electric Co. (Cleveland, Ohio), along with software development partner VRSim, developed the VRTEX 360 welding simulator (see figure 1). The system features a welding helmet retrofit to display a virtual-reality environment, in combination with an ultra-realistic hand piece enabled by stepper motors from MICROMO (Clearwater, FL), to bring the shop floor welding experience to the classroom (see figure 2). Users not only see immediate results of their efforts, they get detailed quantitative feedback that helps novices and seasoned professionals alike perfect their technique.
In traditional arc welding, the welder must strike and maintain an electrical arc between an electrode and the workpiece, or coupon, completing an electrical circuit driven by a welding power supply. The arc heats an area of the coupon to thousands of degrees centigrade, hot enough to melt the coupon metal and a filler metal so that the two can combine to form a strong bond. Arc welding can be classed by whether the electrode itself melts to become the filler material (consumable-electrode arc welding) or whether the filler material comes from a separate source (non-consumable electrode arc welding).
When it comes to consumable-electrode systems, the most common arc welding process is shielded metal arc welding (SMAW), or stick welding. In stick welding, the hand piece holds a consumable electrode that melts and combines with a pool of molten metal on the work piece to form a weld bead.
The concept is straightforward, but the devil lies in the details. The results of stick welding are highly dependent on the length of the arc, as well as travel speed, electrode angle, and position of the electrode in the joint. The welder needs to move the electrode along the joint while maintaining a separation between the tip of the electrode and the coupon that is roughly equivalent to the electrode diameter ¾ typically, 1/16 in. to 1/8 in. If the arc length is too long, it will produce a flat bead with excess spatter; too short and the bead will be overly narrow and convex. The process is similarly sensitive to travel speed and electrode angles. The fact that the electrode is consumed during the process, requiring constant adjustments from the welder, makes maintaining proper technique even more difficult.
Learning to weld has traditionally been a matter of trial and error. It's an art as much as a skill, requiring students to go through coupon after coupon until they absorb the technique needed to consistently turn out good product. The VRTEXTM 360 provides a way for students to log hours of practice without consuming excessive amounts of materials. Even better, the system provides detailed, quantitative, real-time feedback on their technique.
When students put on the system’s virtual-reality welding helmet, they're transported to a virtual worksite (see figure 3). The virtual reality electrode holder behaves similarly to that of an electrode and electrode holder on a real stick welder. When the student strikes an arc with the simulator, they hear the sound of welding and they see the bead forming as they move the virtual reality electrode holder. As they continue to weld, the electrode retracts as though it was melting, forcing them to adjust to maintain proper arc length.
The simulator also presents the virtual weld on an adjacent monitor, along with a detailed analysis that both student and instructor can access for quantitative feedback. During the weld, the instructor can monitor real-time metrics like arc length, electrode angle, and travel speed, then review the data afterward with the student to help them understand what they did right and where they need to improve. The system can help new students master the technique more quickly and experienced welders to learn new skills.
Moving the Electrode
The true value of the VRTEX 360 lies in the realistic behavior of the electrode, which retracts into the handpiece by more than 7 in., simulating the way an electrode melts away during the course of a weld. The Lincoln Electric engineering team needed to design a mechanical assembly to move the electrode in response to feedback. Because burn rate varies depending on materials and process conditions, it wasn't enough to install a fixed-speed DC motor. The system needed a motor that provided variable-speed motion. At the same time, the motor needed to remain simple and reliable while providing the required torque in a small package. Lead engineer Antonius Aditjandra chose to go with a two-phase PreciStep micro-stepper motor from MICROMO. MICROMO also supplied the wiring harness, simplifying the assembly process for Lincoln Electric and eliminating the chance that a motor might be damaged in the wire-bonding process.
Although stepper motors are often run open loop, the motor in the VRTEXTM 360 required feedback for accurate simulation. A sensor monitors the distance between the end of the electrode and the coupon, as well as rod angle and speed. This allows it to monitor user performance as well as properly simulate the process.
On first blush, it would seem that a low-torque motor would be sufficient to handle such lightweight material. On the contrary, says Aditjandra. “Manufacturing plastic is always a challenge. Normally with plastic, you have 0.020-in. tolerance. In that situation, any out-of-tolerance part will create friction. I wanted to make sure that the stepper motor had enough torque to actually push through the worst-case scenario.”
The quality of the motors was essential. The housing had to be accurate to fit the manufacturing tolerances of the molded plastic parts. Performance consistency was even more important. The value of a simulator lies in delivering a measured, repeatable experience. “Consistency of torque is very important because when students are building their muscle memory, they need to be sure that they’re feeding the consumables at the proper burn rates,” Aditjandra says. The MICROMO stepper motors delivered. "Based on the testing that I've done running several stick devices at the same time to see how consistent their travel speed were, they’re very consistent.”
One of the benefits of working with MICROMO was the support they provided during the design phase. Courtesy of their Express Prototyping service, Aditjandra was able to try a variety of stepper motors to determine the right one. "I told them what I was looking for, and they sent some samples out almost immediately," he said. "One thing I liked, too, was that they always followed up with me. They asked how the test and development was going. They also provided a lot engineering support." When he passed along operating parameters and performance results, the MICROMO engineering team analyzed data, giving him feedback and helping him determine the optimum settings.
True to form, the stepper motors remain robust, predictable, and consistent. "I've not had any stepper motor failure so far," Aditjandra says. “The motors have been good."
So, the next time you buy a product that features a smooth, competent hand weld, you just might have Lincoln Electric—and MICROMO—to thank.