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Disturbance Compensation for Safe Kinematic Control of Robotic Systems with Closed Architecture

Fan Zhang, Jinfeng Chen, Joseph J. B. Mvogo Ahanda, Hanz Richter, Ge Lv, Bin Hu, Qin Lin

TL;DR

The paper tackles the challenge of achieving safe, high-precision kinematic control for robots with closed inner-loop architectures and uncertain dynamics. It introduces ESOR-QP, an outer-loop add-on that combines extended-state-observer-based disturbance rejection with a robust control barrier function to guarantee tracking and safety using only kinematic commands. The authors provide stability and safety proofs and validate the approach experimentally on a PUMA 500, demonstrating robust performance under payload changes and disturbances at 1 kHz. The framework offers a practical, implementation-friendly solution for upgrading legacy robotic systems without modifying the inner-loop controller, with clear implications for industrial automation and safety-critical robotics.

Abstract

In commercial robotic systems, it is common to encounter a closed inner-loop torque controller that is not user-modifiable. However, the outer-loop controller, which sends kinematic commands such as position or velocity for the inner-loop controller to track, is typically exposed to users. In this work, we focus on the development of an easily integrated add-on at the outer-loop layer by combining disturbance rejection control and robust control barrier function for high-performance tracking and safe control of the whole dynamic system of an industrial manipulator. This is particularly beneficial when 1) the inner-loop controller is imperfect, unmodifiable, and uncertain; and 2) the dynamic model exhibits significant uncertainty. Stability analysis, formal safety guarantee proof, and hardware experiments with a PUMA robotic manipulator are presented. Our solution demonstrates superior performance in terms of simplicity of implementation, robustness, tracking precision, and safety compared to the state of the art. Video: https://youtu.be/zw1tanvrV8Q

Disturbance Compensation for Safe Kinematic Control of Robotic Systems with Closed Architecture

TL;DR

The paper tackles the challenge of achieving safe, high-precision kinematic control for robots with closed inner-loop architectures and uncertain dynamics. It introduces ESOR-QP, an outer-loop add-on that combines extended-state-observer-based disturbance rejection with a robust control barrier function to guarantee tracking and safety using only kinematic commands. The authors provide stability and safety proofs and validate the approach experimentally on a PUMA 500, demonstrating robust performance under payload changes and disturbances at 1 kHz. The framework offers a practical, implementation-friendly solution for upgrading legacy robotic systems without modifying the inner-loop controller, with clear implications for industrial automation and safety-critical robotics.

Abstract

In commercial robotic systems, it is common to encounter a closed inner-loop torque controller that is not user-modifiable. However, the outer-loop controller, which sends kinematic commands such as position or velocity for the inner-loop controller to track, is typically exposed to users. In this work, we focus on the development of an easily integrated add-on at the outer-loop layer by combining disturbance rejection control and robust control barrier function for high-performance tracking and safe control of the whole dynamic system of an industrial manipulator. This is particularly beneficial when 1) the inner-loop controller is imperfect, unmodifiable, and uncertain; and 2) the dynamic model exhibits significant uncertainty. Stability analysis, formal safety guarantee proof, and hardware experiments with a PUMA robotic manipulator are presented. Our solution demonstrates superior performance in terms of simplicity of implementation, robustness, tracking precision, and safety compared to the state of the art. Video: https://youtu.be/zw1tanvrV8Q

Paper Structure

This paper contains 30 sections, 3 theorems, 66 equations, 14 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Related Work
  3. Related Work
  4. Adaptive Control
  5. Reduced-order model-based CBF
  6. Robust CBF
  7. Preliminary and Problem Formulation
  8. System Dynamics
  9. Robot Kinematic Control with Known Model
  10. Control Barrier Functions
  11. Proposed Framework
  12. Robot Kinematic Control with Nominal Model
  13. Extended State Observer Design
  14. Controller Design
  15. Controller Stability Analysis
  16. ...and 15 more sections

Key Result

Theorem 1

Given the robotic system eq:SECOND with kinematic control input, the ESO eq: ESO with appropriate observer bandwidth $\omega_{o_i}$ for each joint, and the nominal control law eq:overall control law with appropriate tuned $k_p$ and $k_d$, we have the following properties:

Figures (14)

  • Figure 1: Trajectory tracking of the PUMA 500 for an infinity-shaped reference. The trajectory using a poor inner-loop controller is shown in red, while the improved tracking performance (ours) is shown in blue.
  • Figure 2: The proposed framework in this paper, is marked as the Outer-loop Controller. The total estimated disturbance $\hat{f}$ is used for both disturbance compensation and safe control using ESOR-QP.
  • Figure 3: PUMA 500 robot used as a testbed in our research. The figure is from our previous work khalaf2019trajectory.
  • Figure 4: The comparison of control bounds projected into each state with the true model (blue), nominal model (red), and nominal model with ESO (yellow). ESO can narrow the gap between the nominal model and the true model.
  • Figure 5: Poor inner-loop PD controller's performance for $10$ s. (a) Trajectory tracking performance in the joint space. (b) Trajectory tracking performance in the Cartesian space. The inner-loop controller can not track the reference trajectory.
  • ...and 9 more figures

Theorems & Definitions (11)

  • Definition 1
  • Definition 2
  • Remark 1
  • Theorem 1
  • Proof 1
  • Theorem 2
  • Proof 2
  • Theorem 3
  • Proof 3
  • Remark 2
  • ...and 1 more