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

TL;DR

The paper addresses safety and tracking challenges in human-robot interaction by integrating a Dynamical-System-based motion generator with a modified Cartesian impedance loop and a novel null-space impedance controller. The DS-based motion generator enables on-the-fly, interactable EE trajectories, while the null-space impedance strategy extends compliant behavior to the robot body without compromising main-task tracking, backed by a passivity analysis. Key contributions include (1) a unified control scheme that preserves tracking during unknown external interactions, (2) a null-space impedance law with joint friction compensation that dissipates energy in the null space, and (3) experimental validation on a 7-DOF KUKA LWR IV+ showing improved safety and interaction performance over classical impedance and force-estimation baselines. The results demonstrate practical impact for safe, interruptible HRI in manufacturing and other collaborative environments, enabling tool changes and safe handling without requiring force measurements or dynamic-model estimation.

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.

Figures (11)

• Figure 1: Interaction with robot while (I) holding the EE, (II) interacting with the robot body without affecting tracking, and (III) interacting using a handheld F/T sensor.
• Figure 2: Control Diagram: (I) Cartesian motion generation, (II) tracking control, (III) modified Cartesian impedance control, (IV) proposed joint friction compensation, and (V) proposed null space impedance control.
• Figure 3: Experimental setup: 7-DOF robot arm with a 3D-printed tool, a mounted and a handheld F/T sensor for measuring interactive forces.
• Figure 4: Non-interactive (\ref{['A']}): (a) Position, (b) velocity, and (c) orientation trackings for the proposed method.
• Figure 5: Interaction with the EE (\ref{['B']}): position and measured interaction force using our proposed method. Shaded areas indicate periods of interaction.