Compliance while resisting: a shear-thickening fluid controller for physical human-robot interaction

TL;DR

This work introduces a Shear-Thickening Fluid Controller (SFC) for physical human-robot interaction, addressing the need to be compliant during human traction while resisting external impacts. By mapping STF constitutive behavior to a nonlinear velocity-damping model, the authors develop a virtual-dynamics framework, analyze stability and passivity, and derive a describing-function based frequency-domain understanding. They derive discretization constraints, propose an automatic parameter-tuning algorithm, and validate SFC through simulations and real-world experiments on fixed and mobile manipulators, showing superior impact resistance and stable collaboration compared with linear and nonlinear admittance controllers. The results indicate that SFC offers safer, more comfortable, and robust pHRI with practical industrial potential and avenues for extensions to exoskeletons and rehabilitation robotics.

Abstract

Physical human-robot interaction (pHRI) is widely needed in many fields, such as industrial manipulation, home services, and medical rehabilitation, and puts higher demands on the safety of robots. Due to the uncertainty of the working environment, the pHRI may receive unexpected impact interference, which affects the safety and smoothness of the task execution. The commonly used linear admittance control (L-AC) can cope well with high-frequency small-amplitude noise, but for medium-frequency high-intensity impact, the effect is not as good. Inspired by the solid-liquid phase change nature of shear-thickening fluid, we propose a Shear-thickening Fluid Control (SFC) that can achieve both an easy human-robot collaboration and resistance to impact interference. The SFC's stability, passivity, and phase trajectory are analyzed in detail, the frequency and time domain properties are quantified, and parameter constraints in discrete control and coupled stability conditions are provided. We conducted simulations to compare the frequency and time domain characteristics of L-AC, nonlinear admittance controller (N-AC), and SFC, and validated their dynamic properties. In real-world experiments, we compared the performance of L-AC, N-AC, and SFC in both fixed and mobile manipulators. L-AC exhibits weak resistance to impact. N-AC can resist moderate impacts but not high-intensity ones, and may exhibit self-excited oscillations. In contrast, SFC demonstrated superior impact resistance and maintained stable collaboration, enhancing comfort in cooperative water delivery tasks. Additionally, a case study was conducted in a factory setting, further affirming the SFC's capability in facilitating human-robot collaborative manipulation and underscoring its potential in industrial applications.

Figures (28)

• Figure 1: Interactive robot motion modeling and control based on shear-thickening fluid dynamics: the robot can comply with human traction and resist external impact
• Figure 2: The structural relationship between the sections.
• Figure 3: Control framework for the pHRI underlying virtual robot dynamics. The interaction model's solution with input $\mathbf{f}_\text{ext}$ yields the value of $\dot {\bar{\textbf{x}}}$, which the velocity-controlled robot needs to adhere to. This is achieved by computing the desired joint velocities $\dot{\textbf{q}}_\text{d}$ via differential inverse kinematics and regulating the joint torque $\boldsymbol{\tau}$ to enable the joint velocities $\dot{\textbf{q}}$ to track $\dot{\textbf{q}}_\text{d}$.
• Figure 4: Schematic view of physical human-robot interaction. (a) An admittance-controlled robot featuring virtual damping (${\mu _\mathrm{a}}$) and virtual mass (${m_\mathrm{a}}$), maintains contact with a human possessing inertia (${m_\mathrm{h}}$), stiffness (${k_\mathrm{h}}$), and damping (${\mu_\mathrm{h}}$). (b) A robot that is controlled by SFC and equipped with STFs damping (${D(\dot x)}$) and virtual mass ($m$) is subjected to external environmental forces (${f_\mathrm{ext}}$).
• Figure 5: Sets of three kinds of interacting forces