Strong but Gentle Robots Used for Handling Bananas

When working with fruit, gentle handling is essential. Proof that robots have the necessary sensitive touch is provided by a banana ripening facility, where the strong but gentle machines tirelessly handle crates of bananas. These KUKA Long Range palletizing robots are confronted with a special challenge: they also have to be able to pick up boxes which have been crushed against each other or became tilted during sea transport.

The banana ripening facility belongs to Coop Schweiz, and was built in 1995 in Kaiseraugst, Aargau Canton, Switzerland on a 10,500 m2 site. It provides comprehensive supply coverage to all 15 distribution points of the regional co-operatives. With 30 employees, the company handles 23,000 tons of bananas per year. Up to twelve railroad cars are needed daily to transport the fruit to Kaiseraugst.

Bananas have to be convincingly fresh at their point of sale (POS). For this reason, it is important to monitor the logistical chain all the way from the harvest through to the sales venue, and to maintain a constant temperature and air humidity of about 90%. Thus, while on board ship the bananas, which are harvested in Costa Rica, Columbia, Ecuador and Ghana, are kept at 14°C in a low-oxygen environment. The railroad cars used for transport to Switzerland from the ports of Antwerp, Bremerhaven, Hamburg and Zeebrugge maintain the same temperature. In Kaiseraugst, the fruit is then placed in 31 ripening cells, each of which can hold up to 1,200 boxes. During this period, the temperature is regulated according to the desired length of the ripening process, which varies between four and eight days; generally this temperature is from 13° to 18°C. The temperature in the other operating areas of the ripening facility is from 14° to 16°C.

Long reach
Coop restacks the crates of bananas before shipping them. This gives the company an opportunity to inspect the quality of the fruit, and also to ensure that the goods are shipped exclusively on europallets. Previously these stacking operations were carried out manually; since the beginning of January 2000 the tasks have been performed by a robotic cell. The reason for this automation was the active self-service which has now become standard in the Coop stores. Since customers now weigh and attach prices to the products themselves at the POS, this operation, which can only be carried out manually, no longer has to be done at the ripening facility.

The concept for the robotic application came from KUKA systems partner Trapo AG, based in Gescher-Hochmoor near Münster, Germany. The cell's central elements are three KR 160 PA Long Range palletizing robots manufactured by KUKA Roboter GmbH, Augsburg, Germany. These Long Range robots are, as their name implies, designed with a very long reach; their radius of 3.5 meters means they can cover an area which holds up to six europallets. The KR 160 PAs can lift loads with a weight of up to 160 kg and supplementary loads of up to 50 kg. They also demonstrate great flexibility - they can be gentle or robust, depending on the task.

With their specially-developed gripper systems, the robots are precisely adapted for their tasks at Coop. For example, the gripper is able to compensate for differences within a layer which might result from slippage of individual crates of bananas.

Sensitive handling prevents bruising
"We are confident that with automatic handling, the crates with the delicate bananas will be picked up and put down gently, and will not be tilted or tipped over", emphasizes Peter Werren, Director of Coop Schweiz's banana ripening facility. "Otherwise, bruises would occur which would cause the fruit to turn brown."

Alois Gneithing, responsible for Trapo's sales in Baden-Württemberg and Switzerland, adds the following: "Because the robots also have to handle banana crates which have been crushed against each other or have become tilted during sea transport, only jointed-arm robots could come into consideration; conventional technology was eliminated from the outset. We devised this solution using a centering frame and a float-mounted chain conveyor. The combination of these elements can compensate for displaced stacking patterns."

The process
Forklifts with retractable masts retrieve the pallets, which contain a total of 50 banana crates, from the ripening chambers, transport them to the robotic cells and place them on one of the four chain conveyors located there. Each of the two unstacking robots is fed by two conveyors. After being set down, the pallet advances up to a light barrier, where it stops. Then the system positions the top layer centrally relative to the robot gripper, thus aligning it for programmed robotic handling. To achieve this, the centering frame, which is lowered from above, and the chain conveyor both move in the X and the Y axis. This process is repeated for each of the ten layers.

The Long Range palletizer alternately grips two banana crates lengthwise and three crosswise; this depends on the packing pattern of the pallets, whose dimensions are generally 1000 mm x 1200 mm. To do this, the robot rotates the gripper and adjusts the integrated jaws to match the load. The jaws pneumatically clamp the sides of the boxes; sensors report when the correct pressure has been generated for the boxes to be picked up. If the KUKA robot has to grip three banana crates, they are first separated from each other by a vacuum suction system. The distance between the boxes which is thus created is necessary for a secure grip to be achieved. The robot maintains this distance by means of spacers which it advances from its gripper.

After picking up the banana crates, the KR 160 PA sets them down on a roller conveyor. Once three boxes have been gripped by the robot, they are first placed on a spur line, which then passes them on to the roller conveyor. Empty pallets travel automatically on a traversing carriage to a magazine, where they are kept until a forklift truck comes to remove them. The boxes, on the other hand, travel along separate roller conveyors from the two robotic cells to the stacking robot. Quality stations for visual inspection are set up along this path. Crates containing bananas which require additional processing are transferred by the system to an autonomous sorting circuit. Order picking and packaging are also carried out on this circuit, for example when the customer wants goods by the piece.

The stacking robot, which is also a KR 160 Long Range palletizer, simultaneously takes two banana boxes from each of the two roller conveyors, and places them in seven or eight layers on a europallet. To ensure that the robot stacks the boxes exactly on top of each other, thus providing the necessary stability, a centering blade is installed on the frame which surrounds the carrier. After each layer, the blade advances in the middle of the pallet one level higher. Just before setting the boxes down, the robot uses its gripper to push the two pairs of boxes against the blade from both sides, thus guaranteeing a tight stacking pattern.

Once completely loaded, the pallets travel on the outgoing roller conveyor until they reach an automatic binding machine, and after that a transfer station for pickup by forklift truck or pallet truck. Finally, the goods are placed in a temperature-controlled shipping area.

Significant and uniform improvement in quality
"Thanks to our investment in the three robots and their peripheral systems, today we achieve uniformly and significantly higher quality", explains Peter Werren. "In addition, the robot's extraordinary acceleration capacity guarantees short cycle times, and therefore high throughput. We benefit from this on the weekends, too, when the robots have to deal with very large quantities. Another decisive factor for us was improving working conditions for our employees. After all, the crates of bananas weigh about 19 kg, and they add up to 80 to 120 tons a day."

Since the robots are equipped to handle up to 1,000 banana crates per hour, the system still offers reserve capacity. The control technology used can also be adapted to new requirements, since it allows the modification of all motions in the variable area. The system also demonstrates flexibility with regard to the carriers used, since it can handle pallets with dimensions up to 1200 mm x 1200 mm.

Because of the large throughput required, high availability is also important. For this reason, Coop Schweiz is considering the use of the remote servicing offered by KUKA Roboter to further enhance operational readiness, which is already approaching 100% in any case.

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