A Firm Grip -- Robots Transfer Vehicle Wheels to Chain Conveyor

The KUKA robots in use at Südrad, a manufacturer of steel disc wheels, not only have a "firm grip" on the wheels produced there; they have also mastered the art of transferring them to a circular conveyor moving at a variable velocity. The crucial requirements for implementation of this application were short cycle times, high acceleration capacity and adequate repeatability. Lasting gains in the system's efficiency are ensured by the double grippers used by each of the robots to hang up in a parallel position two steel wheels per cycle.

The Südrad company was founded in 1946 in Ebersbach/Fils, Germany. Today it comprises a holding company, Südrad GmbH, a production company, Südrad Autoräder GmbH & Co. KG, and a sales company, Südrad GmbH Radtechnik, all of which have their headquarters at Südrad's original location near Stuttgart. Their product is steel disc wheels for cars, trucks, tractors, trailers, construction equipment and other commercial vehicles.

More than four million wheels of more than 500 different types leave the plant every year. Primary customers are the entire automotive industry and the German consumer market. Südrad's sales and marketing area, however, extends to other countries in Europe and beyond; exports amount to 30% of all sales. With some 600 employees, the company has total annual sales of DM 130 to 150 million. This success is based on high quality; since its founding Südrad has defined quality as one of its most important company goals. Today, the effectiveness of the manufacturer's quality management systems is attested to by its certification according to both VDA 6, Part 1 and QS-9000, 3rd edition.

Handling "around the clock"
After the mechanical manufacturing processes, the steel disc wheels are placed on a circular conveyor, where they go through cathodic dip painting and the associated preparatory steps. Two KUKA KR 125 robots, working "around the clock", hang the wheels, whose diameter ranges from 13" to 17", on the conveyor. Previously, employees carried out this operation by hand, but today they work in other parts of the company. Although the wheels weigh a maximum of 15 kg, due to the weight of the double gripper the user decided in favor of a robot with a 125 kg carrying capacity.

While the robots themselves were supplied directly by KUKA Roboter GmbH, Augsburg, the end-effectors and periphery were made by Boll GmbH, a KUKA systems partner with headquarters in the town of Elsenfeld in Lower Franconia. Included in Boll's scope of supply were synchronization of the robots with the chain conveyor, locating the wheels before they are picked up, supplementary materials flow components, and the hook monitor for detecting and guiding the hangers of the circular conveyor. Cooperation between Südrad and Boll began in 1998, when Boll designed grippers for two other systems. KUKA was chosen as the robot manufacturer due to its geographic proximity, which guarantees prompt service, its attractive price/performance ratio, the high availability of its robots, and its PC-based controller.

Robots and conveyor synchronized
"Important criteria for the robotic cell were short cycle times, high robot acceleration and precise handling, since each robot has a maximum of twelve seconds per cycle", explains Rolf-Dieter Mansdörfer, Managing Director of Südrad GmbH Radtechnik. "The greatest challenge for Boll, however, was to synchronize the conveyor with the robots, which have to hand over the wheels with an accuracy of ± 2 mm."

In addition, the two KR 125s have to be flexible, since the diameter and thickness of the steel disc wheels vary. The robots handle from 15,000 to 18,000 parts per day; their production is coordinated with downstream processes. The cell, which began operation in August 1998, had to fit in a very tight space, which previously had also been used by the operators. Assembly of the robotic cell, including dismantling of old system components such as the wheel feed system and the circular conveyor track, was accomplished under great time pressure; only 14 days during Südrad's summer vacation were available.

Since the robotic cell plays such a decisive role in the complete system, it was absolutely essential that it be ready to begin work on time. For this reason, Boll assembled the complete cell and a circular conveyor at its own plant, and confirmed the system's functionality ahead of time by means of various test runs. This way the requirement for continuous high availability of nearly 100% was addressed from the very beginning. If a robot should nevertheless malfunction, the second robot can continue operation alone. In this case, the other half of the work would be carried out manually.

Controlled process
Two separate roller conveyors guide the steel disc wheels, segregated by type, to each of the two KR 125s. On the conveyor system, the parts are stopped and separated by parallel swivel arms. After that, a locating fixture simultaneously takes hold of two wheels at defined positions, allowing the robot to pick them up securely using its double gripper.

To ensure that transfer to the circular conveyor goes smoothly, the hangers are fixed in place by a holding device; thus clamped between guide belts, they remain in the same orientation until they leave the cell. In addition, Südrad uses light barriers and an inspection gage mounted in front of the first robotic cell to monitor the overhead conveyor hooks; this way the system determines whether the hooks are present and whether they are moving within the specified tolerance. If the controller determines that the tolerances are not being maintained, the robot is instructed via data transfer to let the hanger in question pass through the cell without being loaded.

Getting a grip on a wide variety of types
The KR 125s use their double grippers, which are adapted to the double hanger of the chain conveyor, to transfer the wheels, which are segregated by type and arranged in pairs one on top of the other. The end-effectors are in the form of three-finger grippers, and can handle the complete range of wheel types without retooling. All that is required to pick up a different wheel type is to call up the appropriate program in the robot controller using the KUKA Control Panel. One advantage here is that the Control Panel is equipped with the familiar Windows interface; this makes training and working with the controller much easier.

Safety functions are also integrated into the process; the controller verifies whether a wheel has been picked up by monitoring the positions "Gripper open" and "Gripper closed without wheel". Moreover, the gripper fingers are pivoted, and are equipped with a collision monitoring device. Since the chain conveyor has a stopping distance of up to 1000 mm when an Emergency Stop button is pressed, Boll has also built a predetermined breaking point into the end-effector. This functions as an overload protection device.

The robots generally take turns hanging wheels on the conveyor, i.e. each individual robot loads every second hanger. Other sequences are also stored in the controller, however, for example with priority for one of the two lines. "The robots manage the precise transfer to the moving circular conveyor through the use of a shaft encoder mounted on the conveyor", explains Dipl.-Ing. Ulrich Fischer, Director of Production Planning at Südrad Autoräder GmbH & Co. KG. "This way we can work with different velocities, which is necessary because the treatment times in the associated processes fluctuate, and we therefore also have to be able to adjust the cycle time of the cell flexibly."

A step in the right direction
The company calculates the dual robotic cell will have paid for itself in two years. This time period would have been shorter if fewer changes had been made in the periphery - changes which were necessary for synchronization of the circular conveyor.

On the basis of this experience, the company is already considering automating other areas; one possibility would be the removal of wheels from the conveyor after they are coated. At present this is still done manually, since the shipping carriers specified by the automotive industry vary considerably.

With regard to manufacturing, Südrad considers automation to be a step in the right direction in any case. Evidence for this is provided by the total of 14 KUKA robots which Südrad uses in various production areas, for example two systems for production of wheel discs for commercial vehicles. In each of these systems, three KR 125s load and unload presses and lathes. In addition, the robots carry out so-called orientation tasks, i.e. they detect the rotational positions of the parts. This application uses three robots to replace manual handling by twelve human workers; it makes use of another specially-developed double gripper, which provides long reach and short cycle times.

Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld