Chengjie Wu

Real-Time Wireless Sensor-Actuator Networks for Industrial Cyber-Physical Systems

With recent adoption of wireless sensor-actuator networks (WSANs) in industrial automation, industrial wireless control systems have emerged as a frontier of cyber-physical systems. Despite their success in industrial monitoring applications, existing WSAN technologies face significant challenges in supporting control systems due to their lack of real-time performance and dynamic wireless conditions in industrial plants. This article reviews a series of recent advances in real-time WSANs for industrial control systems: 1) real-time scheduling algorithms and analyses for WSANs; 2) implementation and experimentation of industrial WSAN protocols; 3) cyber-physical codesign of wireless control systems that integrate wireless and control designs; and 4) a wireless cyber-physical simulator for codesign and evaluation of wireless control systems. This article concludes by highlighting research directions in industrial cyber-physical systems.

Near optimal rate selection for wireless control systems

With the advent of industrial standards such as Wireless Hart, process industries are now gravitating towards wireless control systems. Due to limited bandwidth in a wireless network shared by multiple control loops, it is critical to optimize the overall control performance. In this paper, we address the scheduling-control co-design problem of determining the optimal sampling rates of feedback control loops sharing a Wireless Hart network. The objective is to minimize the overall control cost while ensuring that all data flows meet their end-to-end deadlines. The resulting constrained optimization based on existing delay bounds for Wireless Hart networks is challenging since it is non-differentiable, non-linear, and not in closed-form. We propose four methods to solve this problem. First, we present a sub gradient method for rate selection. Second, we propose a greedy heuristic that usually achieves low control cost while significantly reducing the execution time. Third, we propose a global constrained optimization algorithm using a simulated annealing (SA) based penalty method. Finally, we formulate rate selection as a differentiable convex optimization problem that provides a closed-form solution through a gradient descent method. This is based on a new delay bound that is convex and differentiable, and hence simplifies the optimization problem. We evaluate all methods through simulations based on topologies of a 74-node wireless sensor network testbed. Surprisingly, the sub gradient method is disposed to incur the longest execution time as well as the highest control cost among all methods. SA and the greedy heuristic represent the opposite ends of the trade off between control cost and execution time, while the gradient descent method hits the balance between the two.

Interference-Aware Real-Time Flow Scheduling for Wireless Sensor Networks

With the emergence of wireless sensor networks, an enabling communication technology for distributed real-time systems, we face the critical challenge of meeting the end-to-end deadlines of real-time flows. This paper presents Real-time Flow Scheduling (RFS), a novel conflict-free real-time transmission scheduling approach for periodic real-time flows in wireless sensor networks. In contrast to existing transmission scheduling algorithms that ignore interference between transmissions or prevent spatial reuse within the same channel, RFS supports spatial reuse through a novel interference-aware transmission scheduling. While recent work on conflict-free transmission scheduling focused on specialized communication patterns such as queries and converge cast, RFS is designed for peer-to-peer real-time flows with arbitrary inter-flow interference. Moreover, RFS has three salient that make it particularly suitable for real-time systems: First, RFS includes a real-time schedulability analysis that accounts for interference between real-time flows. Second, RFS improves reliability by incorporating retransmissions in a flexible scheduling scheme. Finally, RFS enhances scalability by dividing the network into neighborhoods and provides real-time performance for flows crossing multiple neighborhoods through a novel application of the Release Guard protocol. RFS was evaluated through simulations based on the traces collected from an indoor wireless sensor network test bed. Compared to a traditional TDMA protocol, RFS reduces flow latencies by up to 2.5 times, while improving the real-time capacity by as much as 3.9 times.

Reliable Transport with Memory Consideration in Wireless Sensor Networks

Wireless sensor networks are often composed of resource-constrained sensor nodes with limited memory space, computational capacity and communication range. The links in WSN are often lossy and unreliable. In order to make fluent and reliable data transport on memory-constrained sensor nodes, we propose a new transport layer protocol reliable transport with memory consideration (RTMC), which provides both hop-by-hop retransmission and congestion control. We have implemented RTMC on MICA2 with only 4 K bytes RAM. Experiment, analysis and simulation results show that RTMC can use channel resource effectively and enable all of the segments to be received by the sink with low transport time and low memory cost.

Incorporating emergency alarms in reliable wireless process control

Recent years have witnessed adoption of wireless sensor-actuator networks (WSANs) in process control. Many real-world process control systems must handle various emergency alarms under stringent timing constraints in addition to regular control loops. However, despite considerable theoretical results on wireless control, the problem of incorporating emergency alarms in wireless control has received little attention. This paper presents, to the best of our knowledge, the first systematic approach to incorporate emergency alarms into wireless process control. The challenge in emergency communication lies in the fact that emergencies occur occasionally, but must be delivered within their deadlines when they occur. The contributions of this work are three-fold: (1) we propose efficient real-time emergency communication protocols based on slot stealing and event-based communication; (2) we build an open-source WirelessHART protocol stack in the Wireless Cyber-Physical Simulator (WCPS) for holistic simulations of wireless control systems; (3) we conduct systematic studies on a coupled water tank system controlled over a 6-hop 21-node WSAN. Our results demonstrate our real-time emergency communication approach enables timely emergency handling, while allowing regular feedback control loops to effectively share resources in WSANs during normal operations.

Near Optimal Rate Selection for Wireless Control Systems

With the advent of industrial standards such as Wireless Hart, process industries are now gravitating towards wireless control systems. Due to limited bandwidth in a wireless network shared by multiple control loops, it is critical to optimize the overall control performance. In this paper, we address the scheduling-control co-design problem of determining the optimal sampling rates of feedback control loops sharing a Wireless Hart network. The objective is to minimize the overall control cost while ensuring that all data flows meet their end-to-end deadlines. The resulting constrained optimization based on existing delay bounds for Wireless Hart networks is challenging since it is non-differentiable, non-linear, and not in closed-form. We propose four methods to solve this problem. First, we present a sub gradient method for rate selection. Second, we propose a greedy heuristic that usually achieves low control cost while significantly reducing the execution time. Third, we propose a global constrained optimization algorithm using a simulated annealing (SA) based penalty method. Finally, we formulate rate selection as a differentiable convex optimization problem that provides a closed-form solution through a gradient descent method. This is based on a new delay bound that is convex and differentiable, and hence simplifies the optimization problem. We evaluate all methods through simulations based on topologies of a 74-node wireless sensor network testbed. Surprisingly, the sub gradient method is disposed to incur the longest execution time as well as the highest control cost among all methods. SA and the greedy heuristic represent the opposite ends of the trade off between control cost and execution time, while the gradient descent method hits the balance between the two.

Analysis of EDF scheduling for Wireless Sensor-Actuator Networks

Industry is adopting Wireless Sensor-Actuator Networks (WSANs) as the communication infrastructure for process control applications. To meet the stringent real-time performance requirements of control systems, there is a critical need for fast end-to-end delay analysis for real-time flows that can be used for online admission control. This paper presents a new end-to-end delay analysis for periodic flows whose transmissions are scheduled based on the Earliest Deadline First (EDF) policy. Our analysis comprises novel techniques to bound the communication delays caused by channel contention and transmission conflicts in a WSAN. Furthermore, we propose a technique to reduce the pessimism in admission control by iteratively tightening the delay bounds for flows with short deadlines. Experiments on a WSAN testbed and simulations demonstrate the effectiveness of our analysis for online admission control of real-time flows.

Schedulability Analysis under Graph Routing in WirelessHART Networks

Wireless sensor-actuator networks are gaining ground as the communication infrastructure for process monitoring and control. Industrial applications demand a high degree of reliability and real-time guarantees in communication. Because wireless communication is susceptible to transmission failures in industrial environments, industrial wireless standards such as WirelessHART adopt reliable graph routing to handle transmission failures through retransmissions and route diversity. While these mechanisms are critical for reliable communication, they introduce substantial challenges in analyzing the schedulability of real-time flows. This paper presents the first worst-case end-to-end delay analysis for periodic real-time flows under reliable graph routing. The proposed analysis can be used to quickly assess the schedulability of real-time flows with stringent requirements on both reliability and latency. We have evaluated our schedulability analysis against experimental results on a wireless testbed of 69 nodes as well as simulations. Both experimental results and simulations show that our delay bounds are safe and enable effective schedulability tests under reliable graph routing.

Submodular game for distributed application allocation in shared sensor networks

Wireless sensor networks are evolving from single-application platforms towards an integrated infrastructure shared by multiple applications. Given the resource constraints of sensor nodes, it is important to optimize the allocation of applications to maximize the overall Quality of Monitoring (QoM). Recent solutions to this challenging application allocation problem are centralized in nature, limiting their scalability and robustness against network failures and dynamics. This paper presents a distributed game-theoretic approach to application allocation in shared sensor networks. We first transform the optimal application allocation problem to a submodular game and then develop a decentralized algorithm that only employs localized interactions among neighboring nodes. We prove that the network can converge to a pure strategy Nash equilibrium with an approximation bound of 1=2. Simulations based on three real-world datasets demonstrate that our algorithm is competitive against a state-of-the-art centralized algorithm in terms of QoM.

Implementation and Experimentation of Industrial Wireless Sensor-Actuator Network Protocols

Wireless sensor-actuator networks (WSANs) offer an appealing communication technology for process automation applications. However, such networks pose unique challenges due to their critical demands on reliability and real-time performance. While industrial WSANs have received attention in the research community, most published results to date focused on the theoretical aspects and were evaluated based on simulations. There is a critical need for experimental research on this important class of WSANs. We developed an experimental testbed by implementing several key network protocols of WirelessHART, an open standard for WSANs widely adopted in the process industries, including multi-channel TDMA with shared slots at the MAC layer and reliable graph routing supporting path redundancy. We then performed a comparative study of the two alternative routing approaches adopted by WirelessHART, namely source routing and graph routing. Our study shows that graph routing leads to significant improvement over source routing in term of worst-case reliability, at the cost of longer latency and higher energy consumption. It is therefore important to employ graph routing algorithms specifically designed to optimize latency and energy efficiency.