Sim-to-Real Transfer of Bolting Tasks with Tight Tolerance

Dongwon Son, Hyunsoo Yang, Dongjun Lee

Abstract: In this paper, we propose a novel sim-to-real framework to solve bolting tasks with tight tolerance and complex contact geometry which are hard to be modeled. The sim-to-real has desirable features in terms of cost and safety, however, that of the assembly task is rare due to the lack of simulator, which can robustly render multi-contact assembly. We implement the sim-to-real transfer of nut tightening policy which is adaptive to uncertain bolt positions. This can be realized through developing a novel contact model, which is fast and robust to complex assembly geometry, and novel hierarchical controller with reinforcement learning (RL), which can perform the tasks with a narrow and complicated path. The fast and robust contact model is achieved by utilizing configuration space abstraction and passive midpoint integrator (PMI), which render the simulator robust even in a high stiffness contact condition. And we use sampling-based motion planning to construct a path library and design linear quadratic tracking controller as a low-level controller to be compliant and avoid local optima. Additionally, we use the RL agent as a high-level controller to make it possible to adapt to the bolt position uncertainty, thereby realizing sim-to-real. Experiments are performed to verify our proposed sim-to-real framework.