Safe Collision and Clamping Reaction for Parallel Robots During Human-Robot Collaboration

Parallel robots (PRs) offer the potential for safe human-robot collaboration because of their low moving masses. Due to the in-parallel kinematic chains, the risk of contact in the form of collisions and clamping at a chain increases. Ensuring safety is investigated in this work through various contact reactions on a real planar PR. External forces are estimated based on proprioceptive information and a dynamics model, which allows contact detection. Retraction along the direction of the estimated line of action provides an instantaneous response to limit the occurring contact forces within the experiment to 70 N at a maximum velocity of 0.4 m/s. A reduction in the stiffness of a Cartesian impedance control is investigated as a further strategy. For clamping, a feedforward neural network (FNN) is trained and tested in different joint angle configurations to classify whether a collision or clamping occurs with an accuracy of 80%. A second FNN classifies the clamping kinematic chain to enable a subsequent kinematic projection of the clamping joint angle onto the rotational platform coordinates. In this way, a structure opening is performed in addition to the softer retraction movement. The reaction strategies are compared in real-world experiments at different velocities and controller stiffnesses to demonstrate their effectiveness. The results show that in all collision and clamping experiments the PR terminates the contact in less than 130 ms.

I. Introduction

Safety in physical human-robot collaboration (HRC) is ensured by limiting the kinetic energy resulting from the contact partners' effective mass and relative speed [1]. Accordingly, lightweight serial robots reduce the collision energy in the event of contact. Another approach is using a parallel robot (PR) to reduce moving masses and maintain the same energy limits at significantly higher speeds or to decrease the demanded energy for a fixed trajectory. PRs are characterized by typically base-mounted drives with kinematic chains connected to a mobile platform [2]. An example of a PR is shown in Fig. 1(a).