'Growing' the A-class in a robot park

DaimlerChrysler's Mercedes A-class body shop relies on a variety of joining processes. The entire spectrum of welding, bonding and bolt fastening tasks is covered by 330 KUKA robots of a single type. Compactly integrated in the production flow, the robots represent a 'robot park' in which the car body grows step by step into its final form.

DaimlerChrysler AG's car plant in Rastatt was constructed from 1990 to 1992 for a total investment of DM 600 million. The plant's approximately 5,000 employees have been producing the company's A-class car there since 1997. This model series is produced with a cycle time of 1.2 minutes, and is shipped from Baden-Württemberg to all parts of the world - in Japan it was voted import car of the year. The plant stands on a site of 147 hectares, including a 24,800 m2 industrial park, where ten suppliers are directly tied into the production. Some DM 500 million of additional investment went into the A-class.

An optimal approach to planning for the new class of cars was provided by combining their corporate philosophy with an orientation to benchmark partners in the sphere of international Best Practice companies. DaimlerChrysler itself has set very high quality standards; the A-class is expected to assume a leading position in its market segment worldwide. To achieve this goal, the Rastatt Production System (RPS) has been developed. The RPS defines standards for the plant's most important processes and methods, uses regular audits to identify opportunities for optimization and improvement, and visualizes the current status with reference to defined specifications.

Reduced number of variants
The body shop, which has more than 550 employees and 58,000 m2 of floor space, helps achieve quality and cost targets in part by reducing the number of variants. Thus, here in the body shop the A-class has only five variants, which are limited, moreover, to the roof and doors. Thanks to this strategy, DaimlerChrysler can react flexibly to changing priorities in the orders on hand without interrupting the production process. Though the plant is otherwise fully-equipped, so far no press lines have been installed, and most of the sheet metal panels come from sister plants in Bremen and Sindelfingen.

The nearly 100% automated body shop, in which robots make about 3,900 spot welds on 290 different sheet metal parts, has a very clear layout which uses the 'fishbone principle'. This term stands for a production philosophy in which the manufacture of subassemblies, and thus most of the added value, is shifted to manufacturing cells connected with the main assembly line in a fishbone pattern. The cells are separated from the main line by short buffer segments. The advantages of this concept are that materials flow takes place over short, transparent paths, and that parts can move along the main line at a constant rate, independent of any fluctuations in the time required for assembly in the individual cells.

The body shop (RB) at the Rastatt plant is divided into three independent areas. While RB 1 is responsible for finishing the underbody, in RB 2 the inner side panels and the roof bow are attached, and in a second step the outer side panels and the roof are fitted. In RB 3 the front door and rear side-doors, the hood and the front end are mounted.

The user, after intensive evaluation and weighting of other automatic and manual processes, decided to employ almost exclusively robots to execute the entire spectrum of tasks. With regard to handling, for example, an investigation was made as to whether conventional conveyor systems or robots would be the more cost-effective alternative. The result: handling of subassemblies is carried out by robots; only the transfer on the main line was left to a skid conveyor system.

One robot type for all joining processes
Here, DaimlerChrysler relies entirely on KUKA IR 360/125 and IR 360/150 robots. These two robot types are of modular construction and are mechanically identical; they differ only in their respective load-bearing capacity, which the manufacturer raises from 125 to 150 kg simply by means of an additional counterweight. The KUKA robots - unlike parallelogram robots - require no other counterweights; this function is carried out by a hydraulic counterbalancing unit, which also makes for an extremely small interference contour when the robot rotates about axis 1. A total of 380 KUKA robots are in use at the Rastatt plant: 330 of them in the body shop, and the rest primarily in assembly, but also in finishing, where they bond window glass, seal seams and mount fenders.

Limiting the robots to one payload class makes maintenance, servicing and employee training simpler, and allows the number of spare parts kept on hand to be reduced - advantages which ultimately improve cost-effectiveness.

The primary joining process is resistance spot welding, in which the robots make up to 28 spot welds within 60 seconds. This is supplemented by clinching, stud welding, adhesive bonding and bolt fastening. MIG and MAG welding were not considered, since DaimlerChrysler's decision-makers wanted to avoid the rework and low process reliability associated with seam welding processes.

The tool used for welding is a water-hydraulic spot weld gun. The reason for using this gun was that high gun forces were required; a hydraulic gun cylinder generates more force, and is also faster. Moreover, the medium can also be used for cooling.

Another process used in Rastatt is 'spot-weld bonding'. This method combines spot welding with a structural adhesive which is applied before welding, and which cures after the weld is made. This structural adhesive also has a sealing function, and it greatly increases the torsional stiffness of the body. Structural bonding, which likewise improves the visual appearance of the body, is used on the doors in place of welding. In connecting the side panel to the underbody, spot-weld bonding is once again used. Robots achieve excellent standards with regard to quality and continuous path velocity during adhesive and sealant application - standards which cannot be obtained manually. As in spot welding, this is largely due to the robot's superior repeatability.

Flexible load-bearing capacity
In deciding in favor of KUKA Roboter GmbH, Augsburg, Germany, DaimlerChrysler expects to benefit from continuity, and to foster a partnership which has proved successful at other DaimlerChrysler plants. One feature which was decisive in awarding the contract was the robots' flexibility with regard to their carrying capacity, since this allows DaimlerChrysler to react quickly and economically to changes which require heavier tools and/or less favorable mass centers of the workpieces. Other important points were KUKA's efficient after-sales service and optional customer-specific solutions.

Additional factors in favor of KUKA robots were their continuous-path accuracy and DaimlerChrysler's experience with them, which has confirmed the long service life and high availability of the IR 360. On top of that were the ease of maintenance when removing motors and cable bundles, and the easy programming by means of teaching. All of these advantages add up to an attractive price/performance ratio and cost-effectiveness over the robot's entire service life.

The contract for the robots and their controllers was awarded in May 1995, and installation of systems for the body shop and finishing of the A-class began in the spring of 1996. At that time, the assembly shop was still being used to produce the US model of the new E-class. The systems were operated for the first time in November of that same year; the first automatic operation was in February of 1997. DaimlerChrysler has been producing A-class cars for sale to customers since May 1997, and since September 1998 at maximum capacity.

Short cycle times through high acceleration
KUKA Roboter found itself confronted with a wide range of different requirements; at the top of the priority list were short cycle times, which required high acceleration capability on the part of the robots. Simultaneous engineering was being carried out while the systems were being designed. This resulted in a larger number of spot welds than originally intended, which meant that the available margin with regard to cycle times was reduced virtually to zero. Subsequently, cells which had been planned for two robots now had to contain four.

One important factor was the soft electrode impact and soft pressure buildup provided by the hydraulic spot weld gun - this lengthened electrode service life. Another benefit was reduced indentation depth and thus higher weld strength, as well as improved appearance, which is also dependent on the length of pressure application.

An additional relevant criterion named by DaimlerChrysler was keeping floor space requirements to a minimum. A prime example of this is provided by a cell in which two robots are installed, each with a working range of 360°. In an extremely confined space, the robots have to make bolted connections and remove aluminum cross-members from two carriers. Moreover, in this station they also attach the MDS (magnetic data storage) for production control of the A-class. The MDS is fastened to the towing lug on the front end, and stays with the vehicle all the way through to final assembly.

Individual parameters for each weld spot
The KUKA RC 30/51 C controller which is used allows user programs and interfaces to be generated in conformity with DaimlerChrysler internal standards. The welding controllers, which like the other application controllers maintain communication with the corresponding robot controllers, independently compensate for electrode wear. For this purpose, all robotic guns are equipped with steppers, and the guns on the main line have tip dressers. The steppers increase the power input when necessary; the dressers reshape the electrode tips so that they can remain in use longer. Since individual parameters for each spot weld are stored in the controller, the steppers can also be used to make precision adjustments to individual welds.

KUKA Roboter additionally developed a fully-functional interface with the component and vehicle recognition system, and also provided robot training for the user's employees, thus further enhancing the robots' already almost 100% availability.

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