Vibration Damping for Highly Compliant Robots

Kurzfassung

In order to guarantee save interaction of robots with the environment and to ensure mechanical robustness, one of the key technical properties of robotic systems is physical compliance of the actuation systems. This is mostly achieved by a highly elastic element decoupling the link from the motor. The flexible elements cause a partially undesired vibration dynamics in the robotic structure when excited with harmonic torque on the link sides. The aim of this thesis is to analyze and to reduce oscillations through optimally chosen settings of the controller. To achieve that, we focus on two control approaches, namely Elastic Structure Preserving Impedance (ESpi) and Visco-Elastic Structure Preserving Impedance (VESpi) controller developed by Keppler et al. in [Kep+18a] and [Kep+18b]. To analyze the closed loop system, we focus on a non-tracking case, one joint and linear spring characteristic. As the aim is to absorb the introduced energy as efficiently as possible, we investigate the effect on certain tuning par meters. The physically motivated design approach of ESpi and VESpi controllers enables us to represent the closed-loop system as a nonconservative multispring-damper two-mass oscillator. Taking the concept of a tuned mass damper (TMD) into account, we extend the existing rule of how to choose the impedance of the absorber presented in for use in the VESpi and ESpi system. This is achieved in two steps: Firstly, we derive an analytical model of the closed-loop system and find parameters for the minimax amplitude in the frequency response. Secondly, we run a Monte Carlo simulation using a visco-elastic two-mass-system controlled by ESpi and VESpi. We want to obtain cost values for vibration and power efficiency that represent vibration efficiency and control effort. The results provide a guideline to determine the parameters for either minimum amplitude at link-side or best power efficiency. One of the most interesting contributions is that the VESpi controller - in contrast to ESpi - features optimal damping characteristic for all excitation frequencies given the optimal setting. On the other hand, the control approach ESpi cannot be used as a TMD due to the placement of the damper. Nevertheless, an optimal setting for a wide range of frequencies can be found.