Automated cell assembly by a Cobot - A cooperation of HHN and ANSMANN AG

Automatische Zellmontage durch einen Roboter. Forschungsprojekt T1.

Automated cell assembly by a Cobot - A cooperation of HHN and ANSMANN AG

This research project has been carried out in a cooperation between the company ANSMANN AG and Heilbronn University. Within the scope of the cooperation, a concept for the autonomous assembly of lithium-ion battery packs with the help of a collaborative robot (cobot) was developed. The battery packs vary in size and shape and are produced by ANSMANN AG in small batches.

Despite this large product variety, the assembly process of a single lithium-ion cell is usually the same. Until now, this monotonous work was carried out manually by ANSMANN AG employees. Nevertheless, it requires a high level of concentration, as lithium-ion cells are sensitive hazardous goods: If only one battery pack was to be incorrectly assembled and then welded, short-circuited cells would outgas or even catch fire. A specially developed, intelligent automation system is designed to solve this dilemma. At its core is a cobot from Universal Robots. Using a standard gripper, it removes a battery cell from the magazine with either the positive or negative terminal pointing upwards - depending on the specifications of the defined assembly plan. It then moves to the assembly area where empty battery packs are fed. These have corresponding holes into which the cylindrical cells can be inserted.

Such a placement task is called a "peg-in-hole problem": A peg must be fed into a hole that is only slightly larger. Accordingly, in the cell assembly use case, the lithium-ion cells represent the pegs and the individual holes in the battery pack represent the holes. For us humans, this is literally a child's play: at a young age and even with their eyes closed, children can insert wooden shapes of a puzzle into their respective holes. The sense of touch of our fingers and the compliance of our movements make this possible. The latter means that movements do not have to be executed strictly as previously planned. Instead, they can be adapted to unpredicted circumstances, e.g. when collisions due to obstacles occur. This considerably reduces the intensity of the collisions.

The majority of ordinary industrial robots do not have a sense of touch, nor can they perform movements compliantly. They achieve their universal applicability through high travel speeds and accuracies that far exceed those of humans. However, they are only suitable to a limited extent for the assembly of lithium-ion cells, since the above potential danger also applies to mechanical damage to the cells. If the robot could not avoid unforeseen obstacles and could only perform movements in an rigid manner, collisions between the cell and the battery pack would lead to deformations and thus to a fire risk.

For this reason, a cobot was selected for this application. It has integrated force-torque sensors in each of its joints, which give its hand or end effector a sense of touch. This enables sensitive mounting of the cells in the corresponding holes of the battery pack. In addition, the specially developed assembly algorithm enables the robot to make compliant movements: It can thus avoid the forces that arise in collisions with protruding parts of the battery pack or the edges of the hole itself, and can therefore find the hole and insert the cell into it even with larger position tolerances. To do this, it uses a motion strategy modeled on humans: the gripped cell is not mounted vertically, but first tilted. This simplifies the hole search many times over. After the tilted cell is centered in the hole found, it can then be tilted back into the vertical orientation. Last but not least, it is pressed vertically into the hole.

Both the system and the algorithm are designed in such a way that not only battery packs of one type can be assembled, but a variety of different models. The developed process has a success rate of more than 99.5% - based on the assembly of more than 13,000 cells during a test by ANSMANN AG employees lasting several months. The cooperation between Heilbronn University and ANSMANN AG is being intensified in the form of further research aimed at increasing the efficiency of the system. For example, developments are currently underway on a new type of robotic gripper to which the lithium-ion cells can be fed directly by means of a pneumatic hose. This will eliminate the need for travel between the cell magazine and the assembly area. In addition, the cycle time of the actual cell assembly is to be reduced by reinforcement learning.

Heilbronn University thanks for the good cooperation with ANSMANN AG as well as for the support by the state of Baden-Württemberg.

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