Real-Time Wireless Control Networks

Team

Faculty: Chenyang Lu, Yixin Chen

PhD Students: Abusayeed Saifullah, Chengjie Wu

Alumni: Octav Chipara, You Xu

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Wireless control networks represent a new generation of communication technology for industrial process monitoring and control. With the adoption of WirelessHART, an open wireless sensor-actuator network standard, recent years have seen successful real-world deployment of wireless control in process industries. Industrial control systems impose stringent real-time and reliability requirements on wireless control networks. We are developing a new real-time scheduling theory and network protocols for real-time wireless control networks. Our research addresses both practical problems in current WirelessHART networks and fundamental challenges faced by future wireless control networks.

Real-time scheduling theory for WirelessHART

We established a novel real-time scheduling framework for wireless control networks based on WirelessHART by bridging real-time scheduling theory and wireless networking.

Dynamic transmission scheduling: We devised both optimal and near optimal policies for dynamic priority scheduling of transmissions for real-time flows between sensors and actuators. We observed that transmission conflict (due to half-duplex radio of the nodes) plays a major role in communication delays, making the traditional real-time scheduling policies less effective for transmission scheduling in WirelessHART networks. Using this key observation, we designed an optimal algorithm based on branch and bound, and a heuristic called Conflict-aware Least Laxity First (C-LLF) for dynamic priority scheduling. C-LLF integrates the degree of conflicts associated with a transmission into LLF, and outperforms traditional real-time scheduling policies. This work was presented at [RTSS'10].

Fixed-priority transmission scheduling: For wireless control with firm requirements on network latency, a delay analysis is required to quickly assess the schedulability of the real-tme flows, specially for online admission control and workload adjustment in response to network dynamics. We provided an efficient analysis of the worst-case communication delays of periodic real-time flows that are scheduled based on fixed priorities in a WirelessHART network. The key insight in the analysis is to map real-time multi-channel transmission scheduling to real-time multiprocessor scheduling. This approach allows us to build on existing real-time scheduling theory while focusing on incorporating unqiue features of wireless communication in the analysis. Our analysis establishes safe upper bounds on end-to-end delays, thereby enabling effective schedulability tests for WirelessHART networks. Moreover, since priority assignment has a significant impact on the schedulability of real-time flows, we also proposed an optimal priority assignment policy based on local search and a near optimal policy based on heuristic search. A salient feature of the proposed search framework is that it leverages the lower and upper bounds of end-to-end delays provided by our schedulability analysis to significantly reduce the search space. The delay analysis and priority assignments were presented at [RTAS'11] and [ECRTS'11], respectively.

Cyber-physical co-design for wireless control systems

In a wireless control system, the choice of sampling rates of the feedback control loops must balance control performance and communication delays. A low sampling rate usually degrades the control performance while a high one may cause excessive communication delays also leading to degraded control performance. We addressed the scheduling-control co-design problem of sampling rate selection to optimize the overall control cost in a WirelessHART network. The resulting constrained optimization is challenging since it is non-differentiable, non-linear, and not in closed-form. We proposed and evaluated five optimization methods including greedy heuristic, subgradient method, simulated annealing based penalty method, and gradient descent method and interior point method upon a differentiable convex relaxation. Our result in this work has drawn some interesting conclusions towards co-design. In particular, we have shown the interior point method and the simulated annealing based adaptive penalty method as the two most effective approaches for rate selection. They represent the opposite ends of the tradeoff between control cost and execution time, while the interior method is likely the most effective approach in practice due to its run time efficiency. This work shows the promise of cyber-physical co-design, where tailoring real-time scheduling analysis allows for an elegant and efficient optimization approach for wireless control systems. This work was nominated for Best Paper Award at [RTAS'12].

Publications

• A. Saifullah, C. Wu, P. Tiwari, Y. Xu, Y. Fu, C. Lu and Y. Chen; Near Optimal Rate Selection for Wireless Control Systems, IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS '12), April 2012. (Best Paper Nominee) PDF

• A. Saifullah, Y. Xu, C. Lu and Y. Chen; Priority Assignment for Real-time Flows in WirelessHART Networks, Euromicro Conference on Real-Time Systems (ECRTS '11), July 2011. PDF

• O. Chipara, C. Wu, C. Lu and W.G. Griswold, Interference-Aware Real-Time Flow Scheduling for Wireless Sensor Networks, Euromicro Conference on Real-Time Systems (ECRTS '11), July 2011. PDF

• A. Saifullah, Y. Xu, C. Lu and Y. Chen; End-to-End Delay Analysis for Fixed Priority Scheduling in WirelessHART Networks, IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS '11), April 2011. PDF

• A. Saifullah, Y. Xu, C. Lu, and Y. Chen; Real-time Scheduling for WirelessHART Networks; IEEE Real-Time Systems Symposium (RTSS '10), December 2010. PDF

• O. Chipara, C. Lu and G.-C. Roman, Real-time Query Scheduling for Wireless Sensor Networks, IEEE Real-Time Systems Symposium (RTSS '07), December 2007. PDF

If you have any questions or comments, feel free to email Abusayeed Saifullah.

Talks

• Real-Time Wireless Control Networks: Challenges and Directions, NITRD National Workshop on the New Clockwork for Time-Critical Systems, October 2012. PDF

• Real-Time Wireless Sensor Networks, Royal Institute of Technology (KTH), Stockholm, Sweden, April 2010. PDF