Feature Maintenance management
Electric actuation has superior robot welding force control
cations for over 40 years. With a range from full robot cells to spare electrode tips, the company’s welding guns have used hydraulic or pneumatic actuation to apply the welding force. Motor driven ball screw actuators
P
had also been used but the company was not happy with their durability. Therefore, when the company won an order from a battery manufacturer in Newport, South Wales, it approached Olsen Engineering for a better solution. Ian Brent-Smith, MD of PW
Resistance Welding Products, explains, “Ball screw actuators can work, but we were after long life on this particular battery welding application, and the life of a roller screw actuator is 10 times that of a ball screw equivalent. “With spot welding, the problem is
that the balls in a ball screw are working in one place all the time, whereas in a more typical full-travel application like opening a gate for example, a ball screw has equal force over the whole of its travel.
Force holding “In the case of welding, we take the actuator to a stop position and expect it to hold a force, so the balls are concen- trated in one small area all the time and wear out. With a roller screw you have the same position holding, but over a massively greater surface area so the issue of wear practically disappears.” Olsen supplied 12 Exlar IM40
actuators, using roller screw drive and internal anti-rotation mechanisms, fitted to servo drive motors powering CNC-controlled Fanuc welding robots. The reason the battery manufacturer
opted for robotic cell fabrication was the sheer weight of the batteries to be manufactured for use in power station cold start applications or in aerospace and defence. They were too heavy for a person to lift and the robotic cells have now been running for 24 hours a day for more than four years. Brent-Smith says: “We had a call
from Fanuc Robotics, which was involved in an automation project with the manufacturer to automate the welding of the battery terminals. The batteries are made up of several indi- vidual electrochemical cells and between each cell there is a pair of
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W Resistance Welding Products has been designing and making high quality spot welders for automotive and aerospace appli-
High compression force is crucial for battery welding applications and such a force is best applied in a linear motion rather than rotary motion for very large batteries. Here Piers Olsen of Olsen Engineering proves the theory with reference to the work of PW Resistance Welding Products
lead tags that are welded. Although robotic welding was already being used, the manufacturer was not happy with the result, so I drew a sketch of an improved welding machine. One of the problems was that the welding machine the manufacturer was using had pivot- ing arms, so there is only one point where the tips of the electrodes would be meeting in parallel. If those electrode tips wear, then they are no longer paral- lel. What was needed was linear movement of the electrodes, so we designed and developed a welding machine with linear movement.”
Seventh robot axis Brent-Smith explains that, because of the high force required for a reliable weld, which could be upwards of 1.1 tonnes, PW Resistance Welding Products wanted something that would last a long time, which favoured an electromechanically actu- ated roller screw mechanism. Because the Exlar IM40 actuator can be fitted to a wide range of motors, in this case Fanuc, it meant it could be treated as the seventh axis on the robot. “In this way, Fanuc could program it
directly from the robot pendant. They can put an extra drive in the robot controller and it effectively becomes part of another axis of the robot, so programming is integrated. “We recommended using a medium
frequency controller working at 1kHz, which allows the battery manufacturer to change the weld time by thousandths of a second, making it more accurate. “It also uses direct current (DC) so
the heating effect is faster, and the whole system is much more control- lable. The weld results from the first machine we built for the manufacturer were superb, so they then bought five more of our machines for the rest of their factories around the world.”
Force-speed characteristics As well as the Exlar actuator, Olsen also provides a mounting and matching belt drive for any type of motor, in this case Fanuc. The ratio chosen depends on the rotational speed of the motor, and relat-
Top left: Ian Brent-Smith of PW Resistance Welding Products
ing it to the pitch of a roller screw to get the right force-speed characteristics. “Speed, another key feature on spot
welding applications which need rapid switching between weld positions and force control for the weld, was also not an issue here,” says Brent-Smith. “Welding thick chunks of battery
Welders are designed in CAD
lead takes a long time,” he explains. “Electrodes are separated by a 1mm- thick plastic sheet with a hole in it, the shape of which, under a compression force of over one tonne, indents itself almost like extrusion into the lead on both sides to allow a weld current to pass through. The older scissors type could not provide as good a force con- trol as that provided by linear motion. High compression force is crucial.” Fanuc’s expertise in automotive
welding applications means that the robot has dedicated software for spot welding guns. The tricky bit was on account of the welding gun itself being a bespoke design, according to Fanuc project manager Julian Cashman. “We were not using a conventional
A weld gun application using linear motion
off-the-shelf welding gun so we could not apply the sufficient force that we needed to apply because of the surface area of the weld. That is when we approached Olsen Engineering to see if we could couple our servo motor to an electric actuator for our Fanuc R2000iB 170CF short-arm robot.”
Energy saving characteristics It has been long well known in the automotive industry that electric actua- tors save energy compared with pneu- matic cylinders, especially on weld gun, riveting, crimping and sealing applications, since electric cylinders do not use energy while they are at rest.
Oslen Engineering T: 01686 689812
consultolsen.com Enter 323
Summary Although electric actuation is more expensive when directly comparing cylinder-to-cylinder cost, electric cylin- ders consume less than a 10th of the energy and do not require a compressor or maintenance. They produce more repeatable welding performance and force can be adjusted on the fly if needed, with long life with no air leaks.
SEPTEMBER 2013 Factory Equipment
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