Feedback Control Goes Wireless: Guaranteed Stability over Low-power Multi-hop Networks (1804.08986v4)

Abstract: Closing feedback loops fast and over long distances is key to emerging applications; for example, robot motion control and swarm coordination require update intervals of tens of milliseconds. Low-power wireless technology is preferred for its low cost, small form factor, and flexibility, especially if the devices support multi-hop communication. So far, however, feedback control over wireless multi-hop networks has only been shown for update intervals on the order of seconds. This paper presents a wireless embedded system that tames imperfections impairing control performance (e.g., jitter and message loss), and a control design that exploits the essential properties of this system to provably guarantee closed-loop stability for physical processes with linear time-invariant dynamics. Using experiments on a cyber-physical testbed with 20 wireless nodes and multiple cart-pole systems, we are the first to demonstrate and evaluate feedback control and coordination over wireless multi-hop networks for update intervals of 20 to 50 milliseconds.

• The paper presents a co-designed wireless embedded system that integrates communication and control to overcome network impairments like jitter and message loss.
• The paper develops a state-feedback control framework that guarantees mean-square stability for LTI dynamics in the presence of delayed and lossy feedback.
• The paper validates its approach on a 20-node testbed, achieving over 99.9% reliability and robust performance even with up to 75% message loss.

Overview of "Feedback Control Goes Wireless: Guaranteed Stability over Low-power Multi-hop Networks"

The paper "Feedback Control Goes Wireless: Guaranteed Stability over Low-power Multi-hop Networks" addresses a pivotal challenge in the area of cyber-physical systems (CPS): the rapid and stable closure of feedback loops over wireless networks with low power and multi-hop communication capabilities. This research is particularly significant for applications requiring fast update intervals, such as robotic systems and swarm coordination.

Technological Contributions

The paper delivers three primary contributions:

1. System Design: The authors present a wireless embedded system explicitly tailored for supporting feedback control over multi-hop networks. This system is designed to overcome typical wireless network impairments, such as jitter and message loss. The co-design approach tightly integrates the wireless communication protocol and the control system to enhance stability and performance.
2. Control Framework: The control design exploits the characteristics of the wireless embedded system to provide stability guarantees for physical processes with linear time-invariant (LTI) dynamics. The system handles delayed feedback due to wireless network constraints by predicting system states and implementing static state-feedback control modified for non-ideal network conditions.
3. Empirical Validation: Using a testbed comprised of 20 wireless nodes and multiple cart-pole systems, the authors demonstrate the system's ability to stabilize and synchronize physical systems over multi-hop networks with update intervals as short as 20 to 50 milliseconds. A series of experiments showcase the robustness of the system against message loss and its scalability for multi-agent coordination tasks.

Experimental Insights and Practical Implications

The experimental results underline the feasibility of deploying CPS applications with fast and reliable feedback control over wireless multi-hop networks. For instance:

• Performance Metrics: The system maintains pole angles and cart positions within stable regimes despite the high variability typical of wireless networks. It achieves a reliability of over 99.9\% in message delivery, even in networks with significant environmental interference.
• Efficiency and Scalability: The system demonstrates a trade-off between update interval length and energy efficiency, with longer intervals reducing the radio duty cycle. This property is crucial for energy-constrained CPS applications.
• Robustness: The system's ability to handle message losses up to 75\% and tolerate bursts of continuously lost messages ensures reliability in adverse conditions.

Theoretical Implications

From a theoretical standpoint, the research validates the application of LTI control theory in multi-hop wireless networks under non-ideal conditions. The formal proof of mean-square stability provided in the paper offers a robust framework for further exploration and application in various CPS domains.

Future Directions and Implications

This work sets the foundation for applying these methods to larger, more complex systems that demand real-time feedback over distributed networks. The implications span across many fields, from autonomous vehicle platooning systems to large-scale environmental monitoring. Furthermore, the robust design principles demonstrated here could inspire developments in resource-constrained environments where traditional network infrastructure might be impractical or inefficient.

Conclusion

The paper represents an integration of network theory and control systems, advancing the capabilities of CPS through innovative design and empirical validation. By ensuring robust stability over wireless network imperfections, it opens new pathways for the deployment of reliable and efficient feedback-driven applications in complex, dynamic environments.