Xiuming Zhu

Reliable and Real-Time Communication in Industrial Wireless Mesh Networks

Industrial wireless mesh networks are deployed in harsh and noisy environments for process measurement and control applications. Compared with wireless community networks, they have more stringent requirements on communication reliability and real-time performance. Missing or delaying of the process data by the network may severely degrade the overall control performance. In this paper, we abstract the primary reliability requirements in typical industrial wireless mesh networks and define three types of reliable routing graphs for different communication purposes. We present efficient algorithms to construct them and describe the recovery mechanisms in the event of component failures. Based on these graphs, data link layer communication schedules are generated to achieve end-to-end real-time performance. We demonstrate through extensive experimental results that our algorithms can achieve highly reliable routing, improved communication latency and stable real-time communication in large-scale networks at the cost of modest overhead in device configuration.

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.

MBStar: A Real-time Communication Protocol for Wireless Body Area Networks

In this paper, we report on the design and implementation of MBStar, a higher-frequency, real-time, reliable, secure protocol for wireless body area networks(WBAN). As in most proposals for body sensor networks, MBStar adopts the star topology for communication, and is designed to support a message rate as high as 400 Hz, which to the best of our knowledge, is the highest among low-power wireless communication protocols implemented at the present time. The physical layer of MBStar utilizes 802.15.4 DSSS compatible radio for which a higher-frequency, reliable, TDMA MAC layer is built. There is a simple application layer designed for security on top of it. MBStar utilizes public/private key encryption for provisioning devices and does not involve any human configuration before device join. Considering the resource limit of most embedded systems, the TDMA requirement of computing a shared global communication schedule presents a practical problem since it may not be feasible for all the devices to communicate in a long hyper-period while the communication schedule between devices is being created or modified as devices depart and rejoin. We solve this problem by keeping only the global hyper-period schedule on the gateway side, with each device being configured with a shorter, local period. Then, retransmission is employed to resolve any conflicts between the devices. Our strategy has the property that, given any fixed task set, the minimal average number of retransmissions is independent of any communication scheduling algorithm, and the EDF (Earliest Deadline First) is optimal for our communication architecture. Finally, we present experimental results that demonstrate that MBStar is an effective protocol for wireless body area networks.

Control over WirelessHART network

It has been observed that the history of industrial process control development is also a history of reducing the number of wires necessary for effecting the control. Control over wireless is the end of this evolution. Wireless control faces a lot of challenges such as security, reliability, feedback latency, battery longevity, etc. In this paper we report some experience with implementing control over wireless. The platform we use is the WirelessHART mesh network, the first international industrial wireless control standard. We describe a full implementation of the standard and study the issues and solutions in its application. Our data suggest that WirelessHART technology is up to the challenge of wireless control.

Design of a network-based mobile gait rehabilitation system

In this paper, a network-based mobile gait rehabilitation system is proposed for improved mobility and tele-rehabilitation. In this system, an Internet-based body sensor network is employed for health monitoring and a local highspeed wireless network is developed to achieve precise control of the gait rehabilitation device, which is a compact rotary elastic actuator (cRSEA). A new wireless protocol called MBStarPlus is proposed to achieve a high sampling rate and real-time communication. To deal with packet loss in the wireless network, a modified linear quadratic Gaussian (LQG) controller is combined with a disturbance observer (DOB) to control the cRSEA over the wireless network. Successful system integration and promising experimental results validate both the system design and proposed control algorithm. The plan for clinical testing with the proposed system is discussed.

WirelessHART and IEEE 802.15.4e

WirelessHART, the first international industrial wireless standard (IEC 62591), is built on top of the IEEE 802.15.4 standard. Both standards have progressed since the WirelessHART incarnation. WirelessHART has gone through a major release, added support for discrete devices, and lately turned attention to wireless control. While WirelessHART is still based on IEEE 802.15.4-2003, IEEE 802.15.4 has progress to the latest version of IEEE 802.15.4e, with lots of new features that used to be exclusively in WirelessHART. In this paper we study the relationship between these two international standards. We look at how WirelessHART influenced IEEE 802.15.4e, what impact the latest IEEE 802.15.4e has on wirelessHART, how people could benefit from both standards, and what the future holds for us.

Measuring WirelessHART against wired fieldbus for control

Wireless applications in process automation started in areas where wireless sensors provide rich process information to the automation systems. Shortly after WirelessHART became the international standard, both ISA100.11a and WirelessHART claimed that their respective wireless technology could be applied to control as well. Although both standards provide provisions for supporting control over wireless, actual installations of wireless control systems have been slow to be adopted. Due to peoples wariness of using wireless in combination with the serious nature of control, what is needed is a demonstration of performance and reliability of control when being executed across wireless media. A good starting point is a comparison between a wireless control system and its wired counterpart. In this paper we first study if we can establish a WirelessHART control loop the same way as a wired one; then we study if our WirelessHART control loop could achieve the same execution rates and reliability as a wired Foundation Fieldbus control loop. The experiment affirms that, in most likely cases, control over a WirelessHART network could be as good as control over a wired fieldbus.

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.

A Location-Determination Application in WirelessHART

WirelessHART is an emerging wireless communication standard that is targeted at the real-time process control industry.An example application of wireless communication in an industrial process control plant is the location of field engineers. The capability to locate personnel is a safety critical issue in process control plants because of high risks posed by toxic chemicals and other hazards. This paper presents the design, implementation and evaluation of a location-aware application built upon WirelessHART. The aim of this application is to locate a mobile device (and thus the person carrying the device) via the deployed WirelessHART network. The application is a software-based – no device modifications are required. Consequently, it is applicable to any WirelessHART network. In this application, both the mobile device (a handheld device or a badge carried by a worker) and field devices (attached to the plant process) periodically send health reports of their neighbors to the network manager. The network manager analyzes these reports and discards the untrustworthy pairs through comparison. Next, the network manager feeds the average received signal indications to a well-trained radio propagation model to derive the location. To evaluation our solution, several preliminary experiments are carried out and the results are very promising, with a median error less than 4 meters, which is good enough for the industrial requirement. To the best of our knowledge, this is the first attempt to develop location-aware application in WirelessHART networks.