A Turn for the Better -- Gripper Extends Robot's Work Envelope

When packaging stamped parts, a KUKA robot reaches out beyond its "natural" work envelope - an advantage which makes investment in a larger robot unnecessary. This cost-effective solution is based on an eccentric gripper by means of which the carrier being used can be filled all the way to the edge. Of decisive importance here is the rotating end-effector, with which the robot can put the material down in such a way that bending of the intermediate layers is prevented.

Styner + Bienz AG, a specialist in non-cutting stamping and forming technology, grew out of a workshop for production of small tools and repair of machines founded in Bern, Switzerland in 1924. Today the company, which manufactures not only tools but also complete systems for mass production of parts, has its headquarters in the town of Niederwangen in Bern Canton. In 1974 Styner + Bienz acquired AWM Werkzeugbau AG, thus gaining access to expertise in plastic injection molding, a field with high potential for the future. Since 1997, the companies have done business as a group under the name adval tech Holding AG. This way even the company name emphasizes the added value gained through the use of their products.

With 500 employees, Styner + Bienz has annual sales of about 100 million Swiss francs. Certified in accordance with EN ISO 9001, this global player concentrates its marketing efforts on Switzerland, Germany, Scandinavia, the US and Asia. Primary customers are the automotive and telecommunications industries and manufacturers of large household appliances. "We consider ourselves to be not just tool and die makers and mechanical engineers, but also as service providers who develop complete solutions for our customers", emphasizes Rudolf Lüthi, Director of Research and Development at Styner + Bienz AG.

Demand for process reliability
As a part of planning work for a new product, a special retaining bracket, company decision-makers decided to have a robotic cell installed. Today, a KUKA KR 15 robot takes from the conveyor system the stampings required by the automotive industry, i.e. components for fastening steering systems,  and places them on europallets fitted with box-type frames. The robot completes ten cycles per minute. The most important requirement here, besides a short cycle time and high acceleration values, is high process reliability. This is because a malfunction would require the simultaneous use of two human operators in order to keep up with the quantities produced by the preceding press.

The robotic cell was developed by Insys Industriesysteme AG from the Swiss town of Worblaufen. This systems partner of KUKA Roboter GmbH, Augsburg, Germany was awarded the contract in October 1998. Assembly began in February 1999 and commissioning took place as scheduled in March of the same year. Previous to this, Insys had tested the most important system components during trial operation at its own plant. In addition to the robot and its controller, the cell consists of the chain conveyor which feeds the cell, the roller conveyor leading away from the cell, and sensors which monitor the end positions and the presence of pallets. Also provided with the cell were the safety devices, including light barriers at the roller conveyor entrance and exit, and a transfer station for the cardboard which is used for intermediate layers in packing.

"We decided in favor of Insys and KUKA Roboter in part because both companies can point to impressive references from the automotive industry. In addition, the geographic proximity of the systems partner was important to us. This ensures prompt customer support and thus the high availability we require", says Rudolf Lüthi about the selection process. "We also rate very highly the fact that Insys was always flexible in responding to our wishes while we were working together on the concept."

Eccentric gripper
The retaining brackets are transported from the stamping press to the robot by a chain conveyor. Above the cell, the conveyor separates two of the parts and holds them in a defined position for pickup by the KR 15. The robot picks up the two retaining brackets using a double gripper specially developed for this application. The gripper is about 400 mm long, and of eccentric construction. The connecting piece attached to the rear of the end-effector and leading to the robot's "wrist" is placed outside of the center point of the gripper. This offset expands the work envelope of the robot by 100 mm. The KR 15 gains 50 mm with reference to its reach, plus another 50 mm, since by rotating the end-effector 180° it can place the retaining brackets closer to the center of its axis 1. Finally, the user profits from the ease with which the robot can reach the edges and corners of the 800 mm x 1200 mm carrier while it is sitting on the roller conveyor. Were it not for this economical solution, Styner + Bienz would have had to employ a larger robot, which would have meant higher investment costs.

The KR 15 always lays down eight retaining brackets parallel to the long side of the carrier, and five crosswise. The parts are stacked with their leg piece uppermost in three layers, and with intermediate layers of cardboard in between. These are supplemented by a cardboard base layer and top layer added by the transfer station. The robot can use its gripper to handle three different models of the retaining bracket, all of which are practically the same size.

Rotation of the stampings makes their leg pieces into a stable supporting structure, which prevents the intermediate layers from bending. If, on the other hand, the box-type frame bulges inwards so that the retaining brackets in the outer row would strike the edge of the frame while being placed in the carrier, then the robot uses a metal roller attached to the gripper to press the frame outwards. This procedure, too is only possible with a rotating end-effector.

Once the carriers are completely loaded, they are moved on the roller conveyor out of the safeguarded zone. The goods are then picked up by forklift trucks, which also supply the system with empty pallets and intermediate layers.

The sequences are coordinated by the PC-based KR C1 KUKA robot controller, which communicates with the lower-level PLC of the peripheral equipment via a CAN bus. If the model of the retaining bracket being produced is changed, the operator simply calls up the corresponding program in the controller. This can be done quickly and easily using the familiar Windows interface of the KUKA Control Panel.

Flexible use
Styner + Bienz calculates that the entire robotic cell will have paid for itself in about three years. Its service life is much longer, however - a significant economic factor. This is even more true when one considers that the flexible KR 15 can, if required, be expanded to handle other retaining brackets, or even entirely different components.

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