Reinforcement learning for robotic assembly of fuel cell turbocharger parts with tight tolerances
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Reinforcement learning for robotic assembly of fuel cell turbocharger parts with tight tolerances Franziska Aschersleben1 · Rudolf Griemert1 · Felix Gabriel1 · Klaus Dröder1 Received: 31 March 2020 / Accepted: 9 June 2020 © The Author(s) 2020
Abstract The efficiency of a fuel cell is not only dependent on the stack, but also to a large extent on the turbocharger, which is responsible for providing the required airflow. Since the individual components, especially those of the rotor, are subject to high demands on manufacturing accuracy, it is crucial to ensure a precise and robust assembly. In order to achieve a scalable assembly process, this paper presents a method for a robot-based assembly of the rotationally symmetric components of the rotor. The assembly task has been reduced to the two essential problems: search and insertion. On this basis, a system was developed, which is able to learn the joining process independently and compensate for positioning inaccuracies with the help of reinforcement learning in combination with a position-controlled robot. The applied reinforcement learning strategy is based on the measurement data of a 6-axis force/torque sensor, with which the current contact state can be evaluated and a decision for the next step can be made. The experimental verification shows that an automation of the assembly process is possible with the proposed strategy. The robot is able to perform the search operation successfully, whereas limitations to the achievable accuracies of the insertion process could be found. Keywords Fuel cell · Assembly · Peg-in-hole · Reinforcement learning · Industrial robot
1 Introduction The fuel cell technology has a great importance regarding the reduction of emission in the automotive sector. While approximately one-fifth of global C O2 emissions are caused by vehicles with combustion engines [1], fuel cells only emit water as reaction product. The energy originates in an electrochemical cell by converting hydrogen and oxygen [2]. A turbocharger provides the required oxygen in the form of air with a specific volume and pressure and has a major impact on the efficiency of the system [3]. Therefore, accurate manufacturing and assembly are necessary. In comparison to a turbocharger used in cars with conventional engines, it consists of a compressor as well as a turbine and an electric engine in between. The rotor of the examined turbocharger at hand consists of six parts that are rotationally symmetric and manufactured with tight tolerances regarding the diameter, * Franziska Aschersleben f.aschersleben@tu‑braunschweig.de 1
Institute of Machine Tools and Production Technology, Technische Universität Braunschweig, Langer Kamp 19 b, 38106 Braunschweig, Germany
coaxiality and radial run-out to meet the dynamic requirements even at high rotational speed. The connection of these parts plays an important role and involves the application of a shrink fit, which requires an exact positioning of joining partners. The effective joining diameters varies betwee
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