Aloysius K. Mok

WirelessHART: Applying Wireless Technology in Real-Time Industrial Process Control

Wireless technology has been regarded as a paradigm shifter in the process industry. The first open wireless communication standard specifically designed for process measurement and control applications, WirelessHART was officially released in September 2007 (as a part of the HART 7 Specification). WirelessHART is a secure and TDMA- based wireless mesh networking technology operating in the 2.4 GHz ISM radio band. In this paper, we give an introduction to the architecture of WirelessHART and share our first-hand experience in building a prototype for this specification. We describe several challenges we had to tackle during the implementation, such as the design of the timer, network wide synchronization, communication security, reliable mesh networking, and the central network manager. For each challenge, we provide a detailed analysis and propose our solution. Based on the prototype implementation, a simple WirelessHART network has been built for the purpose of demonstration. The demonstration network in turn validates our design. To the best of our knowledge, this is the first reported effort to build a WirelessHART protocol stack.

RT-WiFi: Real-Time High-Speed Communication Protocol for Wireless Cyber-Physical Control Applications

Applying wireless technologies in control systems can significantly enhance the system mobility and reduce the deployment and maintenance cost. Existing wireless technology standards, however either cannot provide real-time guarantee on packet delivery or are not fast enough to support high-speed control systems which typically require 1kHz or higher sampling rate. Nondeterministic packet transmission and insufficiently high sampling rate will severely hurt the control performance. To address this problem, in this paper, we present our design and implementation of a real-time high-speed wireless communication protocol called RT-WiFi. RT-WiFi is a TDMA data link layer protocol based on IEEE 802.11 physical layer to provide deterministic timing guarantee on packet delivery and high sampling rate up to 6kHz. It incorporates configurable components for adjusting design trade-offs including sampling rate, latency variance, reliability, and compatibility to existing Wi-Fi networks, thus can serve as an ideal communication platform for supporting a wide range of high-speed wireless control systems. We implemented RT-WiFi on commercial off-the-shelf hardware and integrated it into a mobile gait rehabilitation system. Our extensive experiments demonstrate the effectiveness of RT-WiFi in providing deterministic packet delivery in both data link layer and application layer, which further eases the controller design and significantly improve the control performance.

Wi-HTest: Compliance Test Suite for Diagnosing Devices in Real-Time WirelessHART Network

WirelessHART was released in September 2007 and is the first open wireless communication standard specifically designed for real-time process control applications. It is designed to the same standards as its wired counterpart for reliability and interoperability. To ensure the compliance with the HART Communication Protocol and the adherence to its strict timing requirements, all WirelessHART devices must be thoroughly tested and registered with the HART Communication Foundation (HCF). In this paper, we present Wi-HTest, the test suite designed to exercise WirelessHART devices, thus facilitating compliance assessment. We discuss the detailed architecture of Wi-HTest and highlight several critical features like packet handling with accurate timing control and fault data injection. We also describe a sniffer called Wi-Analys for capturing WirelessHART packets along with their timing information and a post process suite for analyzing the packets. These three tools together provide the complete compliance verification environment for WirelessHART. Based on the test specification developed by HCF, a representative test case is conducted for the purpose of demonstration. This test case in turn shows that Wi-HTest is a novel and efficient test suite for verifying the compliance of real-time WirelessHART devices.

Hardware challenges and their resolution in advancing WirelessHART

The requirements and solutions for industrial wireless mesh networks are much more challenging and complicated than those for the consumer mesh networks. This puts additional stress on existing hardware chips on the market for wireless mesh networks, which started as products marketed towards consumers. The reason why we talk about the IEEE 802.15.4 chips is because most of the industrial wireless mesh network standards converge on IEEE 802.15.4 as the physical and MAC layer standards. In this paper we selectively chronicle the challenges we faced with the IEEE 802.15.4 chips, the products targeted with ZigBee for the consumer market, to advance WirelessHART, the technology targeted for industrial process control. We also describe our resolution over the challenges and offer our wish list for the next generation IEEE 802.15.4 chips.

A complete wirelessHART network

WirelessHART is the first open wireless standard for the process control industry. Previously we demonstrated a three-node prototype network based on an early release of the protocol stack. In this demonstration we build a fully operational WirelessHART sensor network of multiple nodes. We show the creation of the network and the execution of process monitoring applications on the network. This new demonstration network serves as a proof of concept for the revised WirelessHART standard and as a platform for our future research and experiments.

RT-WiFi: real-time high speed communication protocol for wireless control systems

Due to their enhanced mobility and reduced configuration and maintenance cost, wireless control systems (WCSs) are widely used in process and vibration control systems, on medical devices, unmanned vehicles and robotics. However, most literatures in WCSs focus on monitoring and low speed control, and less effort has been made on high speed WCSs. It is because most existing wireless communication protocols cannot provide real-time and reliable communication links with preferable high speed by taking energy saving into consideration.

Improving Control Performance by Minimizing Jitter in RT-WiFi Networks

Wireless networked control systems have received significant attention due to their great advantages in enhanced system mobility, and reduced deployment and maintenance cost. To support a wide range of high-speed wireless control applications, we presented in our prior work the design and implementation of a flexible real-time high-speed wireless communication platform called RT-WiFi. RT-WiFi currently provides up to 6kHz sampling rate and deterministic timing guarantee on packet delivery. While guaranteed delivery latency is essential for networked control, control performance is also impacted by communication jitter and other QoS parameters. To reduce jitter, a flexible network manager is needed to control network-wide scheduling of packet transportation. In this paper, we present an RT-WiFi network manager design and propose efficient solutions for two fundamental RT-WiFi network management problems. To improve control performance in networked control systems, our RT-WiFi network manager is designed to generate data link layer communication schedule with minimum jitter under both static and dynamic network topologies. In order to minimize network management overhead, an efficient data structure called S-tree is invented to manage the communication requests to deal with network dynamics. We have implemented the RT-WiFi network manager, and validated its network and control performance through extensive experiments with a real application.

Robust time delay compensation in a wireless motion control system with double disturbance observers

Time delay is a common phenomenon which can be varying and unknown in many networked motion control systems. Since time delay negatively affects the stability and tracking performance, it needs to be carefully handled in controller design. Moreover, both external disturbance and sensor noise exist in such systems, which makes the controller design more challenging. In this paper, a double disturbance observer (DDOB) structure is proposed to handle time delay, external disturbance, and measurement noise simultaneously. Design of disturbance observers (DOBs) and baseline controllers are elaborated. An RT-WiFi wireless network is developed for high-speed and real-time control applications. The RT-WiFi network is integrated with a DC motor and its performance is examined. Simulation and experimental results are demonstrated to verify the effectiveness of the proposed algorithm.

Time delay compensation in a wireless tracking control system with previewed reference

In this paper, a wireless tracking control problem with varying time delay longer than one sampling interval is discussed, and a preview controller is employed for precise motion control. A delay-dependent system model is first introduced and a reference generator is then employed to model the previewed future reference. The system model is augmented with the reference generator and an optimal controller is synthesized to minimize a quadratic cost function of tracking errors and control inputs. A time-varying Kalman filter is designed for state estimation and feedback control. To make the Kalman filter feasible under long time delay, a linear regression model is proposed for delay estimation based on past measurements. A new wireless protocol called RT-WiFi is developed for high-speed and real-time control applications. Using the delay measurement from the RT-WiFi network, simulation studies is conducted to verify the effectiveness of the proposed algorithm.

Data Link Layer

The data link layer provides the reliable means to transfer data between network nodes by detecting and possibly correcting errors that may occur in the physical layer. This layer has the important task of creating and managing data frames. There are usually two sublayers, Logical Link Control (LLC) layer and Medium Access Control (MAC) layer. LLC layer defines the service to the network layer and MAC defines how the communication medium is accessed by multiple nodes. This chapter lists the services that the WirelessHART data link layer provides to the upper network layer. It further describes in more detail the LLC and MAC layers. We describe the message format and its flow control, error detection, and security. We also describe how to maintain slot synchronization with TDMA, schedule slots to listen for packets from neighbors and propagate packets from the network layer. Please refer to the TDMA Data Link Layer Specification (HCF_SPEC-75) for more details.