Case Studies Of Industrial Robots

Robots in action can be found everywhere. Robot Manufacturers give examples of robot applications in different industries.

Yaskawa, Germany - IFR Robot Supplier

Fully automatic welding of crane booms

Founded in 1949, Liebherr is a family run company and one of the world's largest manufacturers of construction machinery. Liebherr has also made a name for itself as a supplier of technically sophisticated, value adding products and services. Today, Liebherr is a global player with more than 38,000 employees in over 130 companies worldwide.

Liebherr does not operate the typical organisational structures. Instead, the decentralised group of companies is divided into manageable, independently operating company units characterised by customer focus and flexibility. One such unit is the Liebherr plant in Nenzing. Set in the idyllic surroundings of the Austrian Vorarlberg region, its workforce of around 1,700 employees is involved in the development and manufacture of cranes and handling equipment for maritime applications as well as in the extraction and transfer industries and in the fields of civil engineering and special heavy construction. The company produces and markets a wide range of different product lines. These include deck cranes, offshore cranes, mobile harbour cranes and reach stackers. On the international construction machinery market, the plant is represented by a broad spectrum of universal hydraulic cable excavators, crawler cranes and pile-driving/drilling equipment.

Liebherr cranes are characterised by their high quality and performance. In order to be able to maintain high levels of quality over the longer term in a dynamic market environment, innovative production processes are required. For the production of intermediate sections for crane booms, production planners had the ambition of switching to a fully automatic manufacturing process for some time. The two primary factors behind this investment in new manufacturing processes were increased productivity and improved working conditions. The manual joining and welding of the intermediate sections carried out until now were among the most arduous of tasks for employees, as they had to weld large numbers of tubes whist in positions detrimental to health.

Special welding system creates solutions

"We were not prepared to settle for a semi-automatic standard welding system, however. What we wanted was a complete solution: a fully automatic robotic welding system for crane booms that could accommodate around 50 different variants. We entrusted YASKAWA with the implementation of this project for a special system with high requirements," says Dipl.-Ing. (FH) SFI Tobias Stroehle, Head of Equipment Production at Liebherr.

It is common knowledge that YASKAWA Europe has a wealth of welding expertise thanks to the history of Motoman Robotec. "For this project we had to pull out all the stops in order to come up with a process-reliable solution in joint development with Liebherr. It is the complexity of the task, involving a number of really difficult work steps, that makes this project challenging," claims Sepp Hautzinger, Head of Sales Office Austria of YASKAWA Europe GmbH. 

The task of the system is the fully automatic production of various crane booms, varying from 3,000 to 12,500 mm in length, with a width of 900 to 3,000 mm. The robotic cell has to be extremely flexible, as the intermediate sections have to be produced in three component families in excess of 50 variants. The maximum work piece weight is around three tonnes. The crane booms consist of four longitudinal tubes - the so-called chord tubes - which, together with the horizontal posts and the bracing, form the basic frame. A large number of diagonal tubes have to be welded in between these chord tubes in order to reinforce the structure. The job of the robotic system is not only to carry out all the tack welding and other welding tasks, but also provision of the diagonal tubes and integrated quality assurance.

Convincing system layout

A glance at the system - or rather "systems", as Liebherr has ordered two completely identical robotic welding cells - is all it takes to discover how successfully the task has been implemented. The welding station and the provision of the diagonal tubes are physically separated from each other. While two six-axes robots of type MOTOMAN ES165 and MOTOMAN ES200RN perform the demanding handling and supply tasks in the cell, the two MOTOMAN EA1900 welding robots are responsible for the tack welding and subsequent final welding of the diagonal tubes. The ES200RN is mounted on a linear axis that runs along length of the system, linking the welding station and the work piece supply area, and thus serving as the interface between the two areas. The perfection with which the four robots work together in this very large system, with precision to within a tenth of a mm, is impressive. 

Production of the intermediate sections begins with certain preparatory tasks, such as the assembly of the four chord tubes between the face plates of the positioning table and loading the system with diagonal tubes using an electric pallet truck. The cell is also equipped with a carriage containing various small parts required for completion of the crane booms at the end of the manufacturing process. 

Fully automatic process sequence in the robotic welding cells

Once the preparatory work has been completed, the fully automatic manufacturing process is commenced by starting the system in automatic mode. In the first step, the MOTOMAN ES165 handling robot collects a diagonal tube from the magazine and loads it into a measuring station. Here, the diagonal tube is inspected for quality using cameras that precisely measure the contours of the cut ends of the tube. Meanwhile, exact positioning of the chord tubes on the system is carried out by means of corresponding supporting and locating fixtures. 

Following the quality inspection, the second MOTOMAN ES200RN handling robot fetches the diagonal tube from the measuring station and moves to the first loading position in the welding area. Here, the robot inserts the tube between the chord tubes - a complex step, as Sepp Hautzinger explains: "The six-axes robot has to thread the diagonal tube in between the chord tubes from underneath. For this, the robot not only has to be extremely agile, but also has to be able to work in a very precise and sensitive way. In order to achieve a tension free final position of the tube between the chord tubes, the six-axes robot opens its gripper on reaching the target coordinates and shakes the diagonal tube into the desired position - an operation for which a wealth of expertise was required."

Once the tube is positioned, the two welding robots move into action on their traversing axes and set to work tack welding the diagonal tube. At the same time, the two handling robots concentrate on getting the next diagonal tube ready as before. Once all the tubes in one plane (ladder) have been tack welded, the positioning table indexes the workpiece to the next position. The four robots continue in this way until the crane boom has been completed. 

Sophisticated robotic welding without component distortion

The two MOTOMAN EA1900 robots then perform the final MIG welding operation of the diagonal tubes, which are optimised for weight using high-strength fine-grained steel. This is a welding marathon with a sophisticated welding sequence that largely prevents distortion to the components and demands maximum performance and reliability from the robots. "An absolute prerequisite for this challenging job are top quality welds. Crane booms are safety-relevant components for which we have the strictest quality requirements. The MOTOMAN robots enable us to meet these requirements and produce exemplary weld seams,"says Dipl.-Ing. (FH) SFI Tobias Stroehle.

Once final welding has been completed, all that remains to be done is the tack welding and final welding of small component parts. For this, the handling robot collects parts from the carriage and positions them, then leaves the tack welding and final welding to the welding robots. In order to be able to handle all the small parts without the need for manual intervention, the handling robot can select the appropriate gripper from an automatic tool changing station. Once the small parts have been welded, the operator can inspect and unclamp the finished intermediate sections.

"The two innovative systems with the cooperating MOTOMAN robots demonstrate what is currently technically feasible in the production of crane booms. With this state of the art technology, we are setting new standards in terms of productivity and reproducible quality," enthuses Dipl.-Ing. (FH) SFI Tobias Stroehle from Liebherr.