Real-Time Wireless Control Networks

Team

Faculty: Chenyang Lu, Yixin Chen

PhD Students: Abusayeed Saifullah, Chengjie Wu

Alumni: Octav Chipara, You Xu

---

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 theory and 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 proved that an optimal real-time transmission scheduling for feedback control loops in a WirelessHART network is NP-hard. 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 such as Least Laxity First (LLF) 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.

Fixed-priority transmission scheduling: We provided an analysis for finding the communication delay bounds of periodic real-time flows in a WirelessHART control network for fixed priority scheduling, a common class of scheduling policies adopted in real-time systems. The key insight in the analysis is a mapping of real-time multi-channel transmission scheduling to real-time multiprocessor scheduling that allows to exploit the rich literature of the latter upon incorporating the WirelessHART features. 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. Our analysis establishes safe and tight upper bounds on end-to-end delays, thereby enabling effective schedulability tests for WirelessHART networks. 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.

Real-time scheduling and control co-design for wireless control

In a wireless control system, the choice of sampling rates of the feedback control loops must balance between control performance and real-time communication. A low sampling rate usually degrades the control performance while a high one may cause excessive communication delays degrading 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 5 methods to solve it: 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.

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.