Controlled Precision -- Robotic Welding with Touch and Arc Sensors

When the textile machine manufacturer Trützschler makes rotary drums for high-performance carding machines, extremely high precision is demanded. Although the drums are made of solid steel plates, once they have been turned and ground they must meet extremely tight tolerances at high rotational speeds. This level of precision is made possible by largely automated production methods such as the welding process, reliably controlled by a KUKA KR 6 using touch and arc sensors. High throughput is also required, this is ensured by the constant availability of the robotic cell. In addition, a second working position for the KR 6 eliminates robot "idle periods" during setting-up times.

The Trützschler GmbH & Co. KG textile machine factory was founded in 1888 in Crimmitschau, Saxony. In 1948 the factory was re-established in Odenkirchen, a district of Mönchengladbach in North Rhine-Westphalia. The company's headquarters have been there ever since. The business has been run by the same family for four generations now, and the company also has subsidiaries in the US, Brazil, India and Turkey, and a licensee in Japan.

Over the years, Trützschler has developed into a leading-edge manufacturer of spinning preparation machines, carding machines, draw frames, recycling systems and fiber preparation systems for non-woven lines, as well as their associated control systems. This technology from North Rhine-Westphalia sets standards for quality and cost-effectiveness, and is sold in about 100 countries; exports account for more than 90%. Of the over 2,000 employees worldwide, 1,100 work in Mönchengladbach and Viersen-Dülken.

Conversion to automation
Trützschler is one of the world's leading producers of carding machines, i.e. machines for opening and cleaning in the textile industry. The carding machine is the 'heart' of the spinning process, since the card sliver it produces has a lasting effect on the quality of all subsequent processing steps. Previously, the company welded the rotary drums used in its high-performance carding machines manually, which required considerable personnel resources. The welding torch was moved by a mechanical holder, and the operator made vertical and lateral corrections to the weld gun by means of a joystick. This meant that the strength-sapping manual lifting of the torch could be avoided, but the employee was still tied into the process from start to finish. At the beginning of 1999 Trützschler converted to automatic operation with a KUKA KR 6 robot, while retaining the pulsed MAG welding process. The employees who were no longer required in this area were without exception highly qualified, and now work in other departments.

The KR 6 is equipped with a welding torch and moves on a linear unit between two working stations with positioning systems, where it welds the rotary drums. Since the robots have six axes, and the linear unit and the two workpiece positioners each have one axis, the robot controller, which communicates via a CAN bus, has to coordinate the motions of nine axes in all. The scope of supply of the robotic cell encompassed, besides the equipment, including safety installations and welding fume extraction system, also the development of the complete solution with verification of functional capability. The supplier was the KUKA systems partner Paul von der Bank GmbH from Hilden near Düsseldorf, a specialist in welding processes. Trützschler gave von der Bank binding specifications for the cell's cycle time.

Sensors guarantee quality
To maintain continuously the high quality of welding demanded by the user, the KR 6 is equipped with both a touch sensor and an arc sensor. The touch sensor is used for seam finding; with the aid of this tactile sensor the robot detects before the welding process any geometrical variations on the component relative to the component used in the reference model for programming. During the welding process, on the other hand, the KR 6 tracks the seam with the help of the arc sensor. Any changes in the arc length indicate to the controller, on the basis of variations in the current strength from one side of the groove to the other, deviations of the welding torch from the center of the joint. In response to this, the robot compensates for the deviations by making appropriate changes to the axis positions.

"Apart from quality, for us a high availability was at the top of the priority list", says Dipl.-Ing. Andreas Ebenhöh, Director of Trützschler's branch plant in Dülken about the decision-making process. "In addition, this small robot is optimally designed for the weight of the welding torch we use, and it can move about easily when welding the insides of the drums. Other important factors were the robot's periphery and the first-class support provided by the systems partner. Additional criteria were the harmonious design concept, the geographic proximity of the von der Bank company and the price/performance ratio."

The contract was awarded in October 1998; installation and commissioning took place in February 1999. Implementation was subject to a very tight schedule, since the old system was urgently needed at the textile machine manufacturer's plant in Brazil. The KUKA systems partner helped gain employee acceptance of this modern technology through appropriate training. The familiar Windows interface of the KUKA Control Panel was a plus in this regard.

High flexibility
Daily capacity of the robotic cell in single-shift operation is up to eight sets of two drums. The various welding programs can be easily retrieved from the controller, guaranteeing high flexibility. The user thus has a free choice of welding either of the two sizes of roller in any desired sequence. Other advantages compared with submerged-arc welding, for example, are the high level of automation and high customer benefit - one of the stations can be loaded and unloaded while the robot works at the other station, without having to wait during setting-up times.

The large rotary drum has a diameter of 1200 mm and a plate thickness of 22 mm; the corresponding values for the small drum are 690 mm and 15 mm. During the cycle time, i.e. within 26 or 14 minutes, the robot welds a 1.1 m longitudinal seam and circumferential seams totalling 7.5 m on the large drum and 4.3 m on the small one. The welds on the rotary drums have to be very uniform, since in operation they will be turning side by side with a clearance of 25 hundredths of a millimeter, and must be perfectly balanced.

The process
The blank plates are cut using a CNC flame-cutting machine and provided with beveled edges on a continuously turning unit equipped with three torches. Transport to the subsequent CNC four-roll bending machine, which automatically shapes the tubular assembly, is carried out by a gantry crane. This crane, which uses a magnet to lift its load, links all of the areas of this production section.

After the bending process, employees tack the rotary assembly with four spot welds. Then run-on and run-off plates are welded to both ends of the seam. This way any weld defects which may occur at the beginning and end of the seam will be located outside of the drum.

After that, the crane puts the rotary assembly down on one of the workpiece positioners. To align the drum's longitudinal seam, the external robot axis is traversed manually in jog mode, while a laser beam provides a reference point for the seam center. After the robot has once more verified the position of the seam using the touch sensor, the longitudinal seams are welded inside and outside with the help of the arc sensor. Then the crane lifts the drum out of the cell. Employees remove the run-on and run-off plates and tack in two end plates. The assembly is returned to the cell, where the robot welds on the end plates using circumferential seams. The component is thus finished.

Much more cost-effective
"This robotic cell is a prime example of the economic benefits provided by modern production systems", says Andreas Ebenhöh in summary. "This investment enabled us to reduce manufacturing times by 35% compared with manual handling, which allowed us to organize the welding process much more efficiently." The required high availability (Trützschler achieves over 95%) is further enhanced by the geographic proximity and prompt service provided by von der Bank. Trützschler calculates that the payback period for the complete system will be three years.

Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld, Germany