Dolvara Gunatilaka

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.

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.

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.

Maximizing Network Lifetime of WirelessHART Networks under Graph Routing

Industrial Wireless Sensor-Actuator Networks (WSANs) enable Internet of Things (IoT) to be incorporated in industrial plants. The dynamics of industrial environments and stringent reliability requirements necessitate high degrees of fault tolerance. WirelessHART is an important industrial standard for WSANs that have seen world-wide deployments. WirelessHART employs graph routing to enhance network reliability through multiple paths. Since many industrial devices operate on batteries in harsh environments where changing batteries is prohibitively labor-intensive, WirelessHART networks need to achieve a long network lifetime. To meet industrial demand for long-term reliable communication, this paper studies the problem of maximizing network lifetime for WirelessHART networks under graph routing. We first formulate the network lifetime maximization problem and prove it is NP-hard. Then, we propose an optimal algorithm based on integer programming, a linear programming relaxation algorithm and a greedy heuristic algorithm to prolong the network lifetime of WirelessHART networks. Experiments in a physical testbed and simulations show our algorithms can improve the network lifetime by up to 60% while preserving the reliability benefits of graph routing.

Conflict-Aware Real-Time Routing for Industrial Wireless Sensor-Actuator Networks

As process industries start to adopt wireless sensoractuator networks (WSANs) for control applications, it is crucial to achieve real-time communication in this emerging class of networks. Routing has significant impacts on end-to-end communication delays in WSANs. However, despite considerable research on real-time transmission scheduling and delay analysis for such networks, real-time routing remains an open question for WSANs. This paper presents a conflict-ware real-time routing approach for WSANs. This approach leverage a key observation that conflicts among transmissions sharing a common field device contribute significantly to communication delays in industrial WSANs such as WirelessHART networks. By incorporating conflict delays in the routing decisions, conflict-aware real-time routing algorithms allow a WSAN to accommodate more realtime flows while meeting their deadlines. Evaluation based on simulations and experiments on a real WSANs testbed show conflict-aware real-time routing can lead to up to three-fold improvement in real-time capacity of WSANs.

Empirical Study and Enhancements of Industrial Wireless Sensor–Actuator Network Protocols

Wireless sensor–actuator networks (WSANs) offer an appealing communication technology for process automation applications to incorporate the Internet of Things (IoT). In contrast to other IoT applications, process automation poses unique challenges for industrial WSAN due to its critical demands on reliable and real-time communication. While industrial WSANs have received increasing attention in the research community recently, most published results to date have 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 that has been widely adopted in the process industries based on the HART. We then performed a series of empirical studies showing that graph routing leads to significant improvement over source routing in terms of worst-case reliability, but 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. Our studies also suggest that channel hopping can mitigate the burstiness of transmission failures; a larger channel distance can reduce consecutive transmission failures over links sharing a common receiver. Based on these insights, we developed a novel channel hopping algorithm that utilizes far away channels for transmissions. Furthermore, it prevents links sharing the same destination from using channels with strong correlations. Our experimental results demonstrate that our algorithm can significantly improve network reliability and energy efficiency.

Maximizing Network Lifetime of Wireless Sensor-Actuator Networks under Graph Routing

Industrial Wireless Sensor-Actuator Networks (WSANs) enable Internet of Things (IoT) to be incorporated in industrial plants. The dynamics of industrial environments and stringent reliability requirements necessitate high degrees of fault tolerance. WirelessHART is an important industrial standard for WSANs that have seen world-wide deployments. WirelessHART employs graph routing to enhance network reliability through multiple paths. Since many industrial devices operate on batteries in harsh environments where changing batteries is prohibitively labor-intensive, WirelessHART networks need to achieve a long network lifetime. To meet industrial demand for long-term reliable communication, this paper studies the problem of maximizing network lifetime for WirelessHART networks under graph routing. We first formulate the network lifetime maximization problem and prove it is NP-hard. Then, we propose an optimal algorithm based on integer programming, a linear programming relaxation algorithm and a greedy heuristic algorithm to prolong the network lifetime of WirelessHART networks. Experiments in a physical testbed and simulations show our algorithms can improve the network lifetime by up to 60% while preserving the reliability benefits of graph routing.

Holistic Cyber-Physical Management for Dependable Wireless Control Systems

Wireless sensor-actuator networks (WSANs) are gaining momentum in industrial process automation as a communication infrastructure for lowering deployment and maintenance costs. In traditional wireless control systems, the plant controller and the network manager operate in isolation, which ignores the significant influence of network reliability on plant control performance. To enhance the dependability of industrial wireless control, we propose a holistic cyber-physical management framework that employs runtime coordination between the plant control and network management. Our design includes a holistic controller that generates actuation signals to physical plants and reconfigures the WSAN to maintain the desired control performance while saving wireless resources. As a concrete example of holistic control, we design a holistic manager that dynamically reconfigures the number of transmissions in the WSAN based on online observations of physical and cyber variables. We have implemented the holistic management framework in the wireless cyber-physicalsimulator (WCPS). A systematic case study is presented based on two five-state plants and a load positioning system using a 16-node WSAN. Simulation results show that the holistic management design has significantly enhanced the dependability of the system against both wireless interferences and physical disturbances, while effectively reducing the number of wireless transmissions.

Impacts of channel selection on industrial wireless sensor-actuator networks

Industrial automation has emerged as an important application of wireless sensor-actuator networks (WSANs). To meet stringent reliability requirements of industrial applications, industrial standards such as WirelessHART adopt Time Slotted Channel Hopping (TSCH) as its MAC protocol. Since every link hops through all the channels used in TSCH, a straightforward policy to ensure reliability is to retain a link in the network topology only if it is reliable in all channels used. However, this policy has surprising side effects. While using more channels may enhance reliability due to channel diversity, more channels may also reduce the number of links and route diversity in the network topology. We empirically analyze the impact of channel selection on network topology, routing, and scheduling on a 52-node WSAN testbed. We observe inherent tradeoff between channel diversity and route diversity in channel selection, where using an excessive number of channels may negatively impact routing and scheduling. We propose novel channel and link selection strategies to improve route diversity and network schedulability. Experimental results on two different testbeds show that our algorithms can drastically improve routing and scheduling of industrial WSANs.