Easy Does ItEasy Does It
May 5, 2003
"The advantage of using a servo actuator is that the robot can bring the tips in fast and then go real slow right before hitting the metal. That way, you're not slamming the gun closed like you do with a pneumatic," says Paul Kirchheimer, a Ford Motor Co. welding engineer.
The ability of a new generation of electric servo-based weld gun actuators to provide infinite positioning is one of the oft-cited advantages of the technology-and the main reason automakers are doing away with their traditional pneumatic weld gun actuators. They rely on simple bang-bang motion to drive the tips closed at thrust forces ranging from 600 to 1,600 pounds, which too frequently results in poor weld quality.
At the component level, an in-line servo weld gun actuator can cost up to four times more than its pneumatic counterpart, says John Walker, national sales manager for Exlar Corp. (www.exlar.com), a servo actuator supplier. The difference shrinks, however, when other air-related component costs are factored in. And proponents contend that the operational savings of servo can typically offset the cost differential in under two years.
Automakers Climbing Onboard
After several years of testing the technology, North American automakers are buying into the idea. All are taking their first steps to replace pneumatic guns with servo-actuated weld guns in vehicle production.
The numbers are still small. In its first production commitment to the technology, Ford will employ servo actuators on about 10% of the estimated 1,000 to 1,200 robot spot welding guns used in the company's 2004 model year F-150 truck program, says Kirchheimer. But for upcoming Ford vehicle programs, about 25% of all new spot welding robots are now being ordered with servo-actuated guns, he adds, and the numbers are trending upward.
Chrysler is also buying about 25% servo guns for its new vehicle programs, says Bob Anderson, the controls, robotics, and welding manager. And though the numbers are smaller at General Motors, the company is also planning to move into the servo actuator technology.
"I clearly see this as the wave of the future," says Gadrian Zayas, a senior manufacturing engineer at the GM Technical Center, Warren, MI, who has spent the past two years evaluating servo actuators for robotic spot welding. Ford's Kirchheimer agrees. "I think you're going to see this servo weld gun actuator on 100% of welding robots within the next few years," he says.
The trend represents a big opportunity for suppliers of the servo actuator technology. By various estimates, the North American auto market alone totals 15,000 to 30,000 robot welding guns annually, and the car makers are looking for new servo-based designs that can better meet their needs.
Engineers initially deployed servo actuators in reverse parallel designs, in which a rotary brushless servo motor-typically supplied by the robot manufacturer-is mounted in parallel to a linear actuator, which the motor drives via cogged belt and gearing. Chrysler has been using 180 reverse parallel servo-driven robot weld guns in its Dodge Ram pick-up program since 2001, for example. Similar units are in production in other Chrysler programs. "The reverse parallels have been very reliable," notes Gerry Grzadzinski, Chrysler senior productivity executive.
But at weld gun maker Milco Manufacturing Co. (www.milcomfg.com)-which has supplied reverse parallel servo-actuated guns to Chrysler-General Manager John Pippin, Jr. says today's trend is toward smaller, lighter in-line linear servo actuators. In these units, a brushless servo motor is integral to the same shaft that supports a planetary roller screw or precision ball screw that works with a nut to drive the actuator stroke. A resolver, encoder, or other feedback device is attached to the back of the unit for motor control. The in-line motor/actuators "can be 30 pounds lighter" than the reverse parallel designs, Pippin says.
Several vendors are currently supplying in-line servo actuators for the North American market, including France-based Aro Products (www.aronet.com), a welding gun manufacturer that makes servo actuators for its own guns, as well as such motion control system vendors as Exlar Corp. and Tol-O-Matic Inc. (www.tolomatic.com).
Aro filed its first patents on servo welding technology some 20 years ago, and is now using servos in about half of the 4,000 welding guns it ships annually, according to Pierre Barthelemy, Aro Products vice president. Most of the servo-based guns are still sold in Europe, which leads North America in servo gun adoption. "But the U.S. market is now moving fast," Barthelemy says, with several hundred Aro servo-based guns delivered recently to U.S. car makers. Chrysler, for example, will use 200 Aro in-line servo-actuated guns at its Brampton, Ontario, plant for production of its 2004 model year LX luxury car, says Anderson.
Exlar, for its part, has been working since 1999 with welding gun maker and automotive tooling supplier ComauPico Inc. (www.comaupico.com), and has so far delivered 150 in-line servo actuators to the firm, for use with welding robots that build Thunderbird body sides for Ford, says Walker.
Details in the Design
Tol-O-Matic introduced its HT Series of in-line servo actuators in January. The actuators are designed specifically to meet requirements laid out by North American automotive OEMs, says Keith Hochhalter, vice president of engineering and director of new product development.
Among other things, the automakers wanted servo actuators that could match the high thrust capabilities of pneumatics, but fit the same footprint as the air cylinders they will replace, without the need for forced cooling, Hochhalter says. "We were told the actuator had to be less than 2 ft long and weigh less than 30 pounds," notes Jon Summers, Tol-O-Matic marketing manager. At 28 lbs and measuring 3.84 inch2 and 22.3 inches long, including a 6-inch stroke extension, Tol-O-Matic's HT23, the largest of three sizes offered, meets that requirement, while also providing 2,300 lbs of thrust.
One key to achieving a small size was the use of planetary roller screw/nut technology, which is more costly than a ball screw, but provides 4 to 10 times more dynamic thrust for a given size, says Hochhalter. The automakers also wanted a 6 million cycle life with no maintenance or lubrication requirements-a duty cycle unachievable with traditional ball screws of a size small enough to meet requirements, Summers adds.
Other important considerations included an actuator design that could withstand the axial impact loading common in automotive welding, caused by misalignment of tips when they are brought together under high pressure. This is a problem particularly in scissor-style guns, points out Paul Romslo, HT Series product manager. This side loading, if transferred to the screw/nut, causes excessive, life-shortening wear.
Some competitive actuators operate by turning the nut and passing the screw in and out, which makes side loading difficult to prevent, says Romslo. The HT Series instead rotates the screw while it is held in position, allowing the nut to move back and forth with the thrust tube attached. Four flat bearings around the nut coupling and a nose bearing at the front end of the thrust tube isolate the screw from axial loading, and also provide anti-rotation, another OEM requirement, Romslo says. Though they add about 2 inches to the actuator length, these flat and nose bearings are critical to screw/nut life, he notes.
Also adding to the size was the requirement for a design that does not need forced water cooling. "The price you pay to avoid forced cooling is a little bit larger motor, and therefore a little more weight, but what you gain is a lot less complexity because you don't have to plumb up the water supply," Romslo observes. "There's less risk also, because if you lose your water on a forced cooling design, you shut down your motor."
GM's Zayas confirms the tradeoff. GM-approved welding gun vendors include Aro, which uses a water-cooled motor in its inline actuator design, he notes, as well as Milco and Grossel Tool Co. (www.grosseltool.com), both of which use the HT Series actuator in their servo-actuated guns.
The Aro motor/actuators are "a lot smaller" than the HT Series design, Zayas says. "If you've got an application where you need the small size and weight in the actuator, then the water cooling is a small price to pay," says Zayas, because the gun already has water hoses that are needed to cool the welding tips. On the other hand, he observes, "the preferred method is air-cooled, and we've found that the HT Series without the forced cooling is still within the range of the robot payload."
GM plans to use 14 Grossel guns equipped with HT Series actuators for Chevrolet Cavalier production this year at its Lordstown, Ohio plant, Zayas says. And the automaker is also evaluating small numbers of Aro and additional Tol-O-Matic-equipped guns in other production applications.
|Servo Makes Inroads in Welding Apps|
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