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Head Stabilization for Wheeled Bipedal Robots via Force-Estimation-Based Admittance Control

Tianyu Wang, Chunxiang Yan, Xuanhong Liao, Tao Zhang, Ping Wang, Cong Wen, Dingchuan Liu, Haowen Yu, Ximin Lyu

TL;DR

This work addresses head-height stability for wheeled bipedal robots navigating unstructured terrain by integrating a proprioceptive ground contact force estimator with an admittance controller. The approach uses a 2-DoF planar leg model for real-time force estimation and a Wheeled Spring–Damper Inverted Pendulum (W-SDIP) for admittance-based height control, enabling terrain adaptability without extra force sensing hardware. The key contributions are the force-estimation module, the adaptive admittance control framework, and a height-control strategy that translates estimated ground forces into knee-torque commands, demonstrated in simulated terrain scenarios with significant head-stability improvements over a baseline. The findings suggest a low-cost, robust pathway for reliable sensor payload stabilization in fielded wheeled bipedal robots, with planned real-world validation and 3D-dynamic extension.

Abstract

Wheeled bipedal robots are emerging as flexible platforms for field exploration. However, head instability induced by uneven terrain can degrade the accuracy of onboard sensors or damage fragile payloads. Existing research primarily focuses on stabilizing the mobile platform but overlooks active stabilization of the head in the world frame, resulting in vertical oscillations that undermine overall stability. To address this challenge, we developed a model-based ground force estimation method for our 6-degree-of-freedom wheeled bipedal robot. Leveraging these force estimates, we implemented an admittance control algorithm to enhance terrain adaptability. Simulation experiments validated the real-time performance of the force estimator and the robot's robustness when traversing uneven terrain.

Head Stabilization for Wheeled Bipedal Robots via Force-Estimation-Based Admittance Control

TL;DR

This work addresses head-height stability for wheeled bipedal robots navigating unstructured terrain by integrating a proprioceptive ground contact force estimator with an admittance controller. The approach uses a 2-DoF planar leg model for real-time force estimation and a Wheeled Spring–Damper Inverted Pendulum (W-SDIP) for admittance-based height control, enabling terrain adaptability without extra force sensing hardware. The key contributions are the force-estimation module, the adaptive admittance control framework, and a height-control strategy that translates estimated ground forces into knee-torque commands, demonstrated in simulated terrain scenarios with significant head-stability improvements over a baseline. The findings suggest a low-cost, robust pathway for reliable sensor payload stabilization in fielded wheeled bipedal robots, with planned real-world validation and 3D-dynamic extension.

Abstract

Wheeled bipedal robots are emerging as flexible platforms for field exploration. However, head instability induced by uneven terrain can degrade the accuracy of onboard sensors or damage fragile payloads. Existing research primarily focuses on stabilizing the mobile platform but overlooks active stabilization of the head in the world frame, resulting in vertical oscillations that undermine overall stability. To address this challenge, we developed a model-based ground force estimation method for our 6-degree-of-freedom wheeled bipedal robot. Leveraging these force estimates, we implemented an admittance control algorithm to enhance terrain adaptability. Simulation experiments validated the real-time performance of the force estimator and the robot's robustness when traversing uneven terrain.

Paper Structure

This paper contains 15 sections, 16 equations, 6 figures, 4 tables.

Table of Contents

  1. INTRODUCTION
  2. MODELING
  3. 2-DoF Planar Leg Model
  4. Wheeled Spring–Damper Inverted Pendulum (W-SDIP) Model
  5. ADMITTANCE CONTROL WITH GROUND CONTACT FORCE ESTIMATION
  6. Force Estimator Module
  7. Ground contact detection
  8. The Computation of Contact Force
  9. Admittance Controller Module
  10. Height controller Module
  11. EXPERIMENTS
  12. Experiment 1: Single Slope Terrain
  13. Experiment 2: High-frequency Rugged Terrain
  14. Experiment 3: Continuous Undulating Terrain
  15. CONCLUSION AND FUTURE WORK

Figures (6)

  • Figure 1: Robot coordinate systems, generalized coordinates, actuated torques, and leg structure.
  • Figure 2: Admittance control Framework
  • Figure 3: Webots simulation environment (a) Experiment 1: Single Slope Terrain; (b) Experiment 2: High-frequency Rugged Terrain; (c) Experiment 3: Continuous Undulating Terrain;
  • Figure 4: Experiment 1: The vertical displacement and velocity of the robot's head along the z-axis, along with concurrent changes in contact forces
  • Figure 5: Experiment 2: The vertical displacement and velocity of the robot's head along the z-axis, along with concurrent changes in contact forces
  • ...and 1 more figures