Stability and Robustness of Disturbance Observer based Motion Control Systems (1912.05046v1)

Abstract: This paper analyzes the robustness and stability of a disturbance observer (DOB) and a reaction torque observer (RTOB) based robust motion control systems. Conventionally, a DOB is analyzed by using an ideal velocity measurement that is obtained without using a low-pass-filter (LPF); however, it is impractical due to noise constraints. An LPF of velocity measurement changes the robustness of a DOB significantly and puts a new design constraint on the bandwidth of a DOB. An RTOB, which is used to estimate environmental impedance, is an application of a DOB. The stability of an RTOB based robust force control system has not been reported yet since its oversimplified model is derived by assuming that an RTOB has a feed-forward control structure. However, in reality, it has a feed-back control structure; therefore, not only the performance but also the stability is affected by the design parameters of a RTOB. A new practical stability analysis method is proposed for a RTOB based robust force control system. Besides that novel and practical design methods, which improve the robustness of a DOB and the stability and performance of an RTOB based robust force control system, are proposed by using the new analysis methods. The validity of the proposals is verified by simulation and experimental results.

• The paper introduces practical design criteria for DOB systems that integrate low-pass filtering effects to balance robustness and stability.
• The paper proposes a novel stability analysis method for RTOB-based force control, correcting traditional feed-forward assumptions in feedback structures.
• The paper validates design methodologies through simulations and experiments, offering numerical guidelines to optimize performance in industrial applications.

Analysis of Stability and Robustness in Disturbance Observer-Based Motion Control Systems

The paper "Stability and Robustness of Disturbance Observer-Based Motion Control Systems" by Emre Sariyildiz and Kouhei Ohnishi presents a comprehensive examination of disturbance observer (DOB) and reaction torque observer (RTOB)-based control systems. By critically analyzing these methods, the authors provide insights into the robustness and stability of motion control systems, which are integral to various applications such as robotics and industrial automation.

The challenges associated with conventional DOB analyses, typically performed with idealized velocity measurements devoid of low-pass filtering (LPF), are addressed. This is significant because, in practical applications, noise constraints necessitate the use of LPF, thereby imposing constraints on the DOB bandwidth and consequently affecting system robustness.

Key Contributions

1. DOB Analysis with Practical Constraints: The paper underscores the effect of LPF on DOB performance and introduces design criteria that consider the trade-off between robustness and stability. The introduction of LPF in velocity measurement alters the classical robustness analysis, necessitating a reconsideration of DOB bandwidth and inertia parameter selection.
2. Stability Analysis of RTOB-based Systems: A novel stability analysis method is proposed for RTOB-based force control systems. Traditionally modeled with a feed-forward control assumption, the authors correct this to accurately reflect the feedback control structure in practice. This correction impacts both performance and stability considerations.
3. Design Methodologies: The work presents new design methodologies aimed at enhancing the robustness of DOB systems and the stability of RTOB-based force control systems. Specific design proposals include the use of phase lead-lag compensators by adjusting observer bandwidths.

Results and Implications

Simulation and experimental validation show that the design approaches proposed effectively balance the trade-off between stability and robustness. The fidelity of these systems in estimating disturbances and system uncertainties is confirmed across a variety of scenarios.

• Numerical Constraints: The paper sets forth specific numerical constraints and guidelines—such as the relationship between DOB and RTOB bandwidths—that ensure improved system stability while maintaining adequate performance.
• Impact on Practical Design: By integrating practical constraints into DOB designs, this research provides a pathway for more resilient industrial motion control systems. This advancement is crucial as these systems are vital in high-precision applications where disturbances can significantly impair functionality.

Future Developments

This research opens the door for further exploration into adaptive DOB and RTOB designs, particularly in environments laden with uncertainty and non-linear disturbances. The proposed adaptive methods, including parameter tuning via real-time feedback, could further bolster the efficacy of DOB and RTOB systems in dynamic settings.

Overall, this paper enhances the understanding of disturbance observer-based control systems, providing a robust framework for improving the reliability and accuracy of motion control systems across various applications. Future explorations can build on this foundation to fine-tune and adapt these systems for even greater precision and stability in complex operational environments.