Learning-based Adaptive Admittance Controller for Efficient and Safe pHRI in Contact-rich Manufacturing Tasks

Abstract

This paper proposes an adaptive admittance controller for improving efficiency and safety in physical human-robot interaction (pHRI) tasks in small-batch manufacturing that involve contact with stiff environments, such as drilling, polishing, cutting, etc. We aim to minimize human effort and task completion time while maximizing precision and stability during the contact of the machine tool attached to the robot’s end-effector with the workpiece. To this end, a two-layered learning-based human intention recognition mechanism is proposed, utilizing only the kinematic and kinetic data from the robot and two force sensors. A "subtask detector" recognizes the human intent by estimating which phase of the task is being performed, e.g., Idle, Tool-Attachment, Driving, and Contact. Simultaneously, a "motion estimator" continuously quantifies intent more precisely during the Driving to predict when Contact will begin. The controller is adapted online according to the subtask while allowing early adaptation before the Contact to maximize precision and safety and prevent potential instabilities. Three sets of pHRI experiments were performed with multiple subjects under various conditions. Spring compression experiments were performed in virtual environments to train the data-driven models and validate the proposed adaptive system, and drilling experiments were performed in the physical world to test the proposed methods’ efficacy in real-life scenarios. Experimental results show subtask classification accuracy of 84% and motion estimation R<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> score of 0.96. Furthermore, 57% lower human effort was achieved during Driving as well as 53% lower oscillation amplitude at Contact as a result of the proposed system.