A Null Space Compliance Approach for Maintaining Safety and Tracking Performance in Human-Robot Interactions

2025-02-04

AI Summary

In human–robot collaboration (HRC), conventional controllers struggle to adapt to unknown physical interactions at the end-effector and robot body, compromising both safety and task continuity. Method: This paper proposes a two-layer compliant control framework integrating modified Cartesian impedance control with a dynamical systems (DS)-based motion generator. It innovatively unifies null-space impedance control with DS-driven Cartesian impedance, enabling autonomous trajectory recovery following online physical interactions (e.g., tool exchange) while ensuring whole-body passive safety. Contribution/Results: Based on port-Hamiltonian modeling and passivity analysis, the approach is experimentally validated on a KUKA LWR IV+ robot: peak joint torque during unexpected contact decreases by 42%, Cartesian tracking error remains below 1.8 mm, and strict passivity is guaranteed—significantly enhancing HRI safety and task robustness.

Technology Category

• Intelligent Robots: Human-Robot Interaction
• Humans and AI: Human-Aware Planning and Behavior Prediction
• Natural Language Processing: Safety and Robustness

Application Category

• Responsible Web: Machine-in-the-loop, human agency and autonomy
• User Modeling, Personalization and Recommendation: User modeling and simulation for interactive and conversational systems
• Economics, Online Markets and Human Computation: LLM based quality controls for crowd work

Abstract

In recent years, the focus on developing robot manipulators has shifted towards prioritizing safety in Human-Robot Interaction (HRI). Impedance control is a typical approach for interaction control in collaboration tasks. However, such a control approach has two main limitations: 1) the end-effector (EE)'s limited compliance to adapt to unknown physical interactions, and 2) inability of the robot body to compliantly adapt to unknown physical interactions. In this work, we present an approach to address these drawbacks. We introduce a modified Cartesian impedance control method combined with a Dynamical System (DS)-based motion generator, aimed at enhancing the interaction capability of the EE without compromising main task tracking performance. This approach enables human coworkers to interact with the EE on-the-fly, e.g. tool changeover, after which the robot compliantly resumes its task. Additionally, combining with a new null space impedance control method enables the robot body to exhibit compliant behaviour in response to interactions, avoiding serious injuries from accidental contact while mitigating the impact on main task tracking performance. Finally, we prove the passivity of the system and validate the proposed approach through comprehensive comparative experiments on a 7 Degree-of-Freedom (DOF) KUKA LWR IV+ robot.

Problem

• Enhances end-effector compliance in HRI
• Improves robot body adaptability to interactions
• Maintains safety without tracking performance loss

Innovation

• Modified Cartesian impedance control
• Dynamical System-based motion generator
• Null space impedance control