Robot welding force control

PW Resistance Welding Products has been designing and making high quality spot welders for automotive and aerospace applications 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.

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.

Motor driven ball screw actuators 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, managing director 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 ten 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.

“In the case of welding, we take the actuator to a stop position and expect it to hold a force, so the balls are concentrated 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 proven roller screw drive and internal anti-rotation mechanisms, which were 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 over four years.

“Our welders are designed in CAD by a design house in Bulgaria that we have had links with for a long time now,” reveals Brent-Smith. “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 individual electrochemical cells and between each cell there is a pair of lead tags that are welded.

“Although robotic welding was already being used, the manufacturer was not happy with the result, so I visited the site and drew a sketch of what an improved welding machine would look like. One of the problems was that the welding machine the manufacturer was using had pivoting 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 parallel. We deduced that what was needed was linear movement of the electrodes, so we designed and developed a welding machine with linear movement. We needed the movement to be controllable, and pneumatic actuation is not controllable, so we then talked to Olsen Engineering about the possibility of using an electric linear actuator.”

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 actuated roller screw mechanism. Because the Exlar IM40 actuator can be fitted to a wide range of motors, in this case Fanuc, it meant that it could be treated as the seventh axis on the robot.

Brent-Smith adds, “In this way, Fanuc could program it directly from the robot pendant without having to do anything else to it. They can put an extra drive in the robot controller and it effectively becomes part of another axis of the robot, so the programming is totally integrated.

“The old AC controller was working at 50Hz, which meant that the smallest welding time change was one fiftieth of a second and this can mean a big energy change. Therefore 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 much more accurate.

“It also uses direct current (DC) so the heating effect is much faster, and the whole system is much more controllable. 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.”

A key feature of the Exlar IM40, the anti-rotation feature, is important in welding applications where the actuator rod is unconstrained, but on this occasion it was not needed. The welders have massive linear slides so there is no way for the rod to rotate.

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 relating 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,” Brent-Smith continues.

“Welding thick chunks of battery 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 control 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 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.”

It has been long well known in the automotive industry that electric actuators save energy compared with pneumatic cylinders, especially on weld gun, riveting, crimping and sealing applications, since electric cylinders do not use energy while they are at rest.

Henrob riveting machines for example use roller screw electric actuation as standard for its machines. The energy saving was first scientifically proven in 2003 by the University of Pittsburgh on a continuous spot welding line showing savings of 90% when using all electric vs. pneumatic actuation.

Recent studies by the University of Frankfurt in 2012 also show such savings of less than a tenth of the energy required by pneumatic actuation. This does not include the additional on-going maintenance costs, plus expensive compressor capital costs, not to mention the air leakage costs which are usually significant.

Bentley introduced all electric weld gun technology in 2008 for a new 18-robot production line for improved welding control, variable force control for different thickness and different materials for optimised quality.

Jaguar Landrover has over 500 robots at its Halewood site in Liverpool and as part of the original design concept the sustainability of the new Landrover Evoque was considered. The 7,000-plus design engineers at Gaydon made sure that most of the materials used can be re-cycled and the cost of energy to produce the vehicles (one every 82 seconds) is optimised.

Recently during 2012 and 2013, Fiat Italy introduced new production lines and decided to utilise all-electric welding by installing ~ 2m Euros worth of Exlar GSX series electric cylinders. Comau robot weld guns globally use Exlar GSX Electric cylinders as standard.

Although electric actuation is more expensive when directly comparing cylinder-to-cylinder cost, electric cylinders consume less than one tenth 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.