I2MTC 2009 - International Instrumentation and Measurement Technology Conference Singapore, 5-7 May 2009
An Innovative Distributed Instrument for WirelessHART Testing P. Ferrari, A. Flammini, D. Marioli, S. Rinaldi, E. Sisinni A. Taroni Dept. of Electronics for Automation Carlo Cattaneo University University of Brescia Corso Matteotti, 22 – 21053 – Castellanza (VA), Italy Via Branze, 38 – 25123 – Brescia, Italy
[email protected]
Abstract—In the last few years solutions adopted in industrial communication have been deeply changed thanks to the adoption of technologies borrowed from completely different areas. In particular, the advent of hybrid wired/wireless networks promises to greatly improve efficiency and scalability. However, their success will depend on the availability of standard solutions, that ensure multivendor compatibility. Recently, the WirelessHART specifications have been released, making it the only available standard for wireless networking in the industrial field. In this paper, authors present an innovative distributed instrument that will make easier the debug and the development of WirelessHART devices and networks. Keywords- WirelessHART, realtime systems, wireless sensor network, low power system, synchronization
I.
INTRODUCTION
The world of industrial communications shows increasing interest toward wireless fieldbuses, that is the use of wireless communications to interconnect devices at field levels: sensors; actuators; instruments; controllers; and so on. Besides some proprietary solutions, some standards are emerging, like WirelessHART or ISA100 [1]. The goal of both proposals is to establish a wireless communication standard for process automation applications. The more known ZigBee, on the contrary, seems unsuitable for this application field as it has not been specifically designed for reliable, real-time, cyclic communications. Although ZigBee, WirelessHART and ISA100 use the same Physical level of IEEE802.15.4, they differ a lot concerning Medium Access Control (MAC) level, practically impeding the use of common devices and tools. Our work focuses on WirelessHART (WH), that is a mesh solution adopting frequency agility and power adaption to improve communication reliability. The WH specifications are available since september 2007, but instruments specifically designed for commissioning or diagnostics of WH systems are still lacking. HART consortium has proposed a sort of sniffer [2], also called Wi-Analys and still in the development stage, that is able to monitor simultaneously more frequency channels in order to support frequency agility and some companies proposes similar solutions for multistandard analysis, like Perytron-C [3]. If we suppose to install a mesh wireless network in a real industrial plant, the idea of a single-probe instrument shows some limits. In fact, only one-hop network can be analyzed, since the area coverage of the instrument itself is on the same order of the
area coverage of a single device. In addition, a diagnostic instrument should be able to simultaneously analyze several parts of the plant, to better characterize and adjust the mesh behavior. The ability of a WH node to tune the transmitting power can be effectively used only if there is an instrument that is able to simultaneously measure the quality of communication in several points of the plant. In addition, as it will be clear in the following section, a distributed diagnostic instrument is necessary to help the Network Manager in designing the best graph routing. As we said, even if the physical layer is the same of IEEE802.15.4, traditional distributed protocol analyzer, like the Q51 from Exegin [4] or the 2400-SNA from Daintree [5], cannot be used since they are not able to simultaneously hear all the available channels. This paper is structured as follows. In the Section II a brief resume of WH characteristics is reported. In Section III, the architecture of the new distributed instrument, that can be used for both diagnostic and commissioning of WH systems, is detailed. In Section IV, the probe implementation is discussed and in Section V some experimental results are reported. Finally, some concluding remarks are highlighted. II. THE WIRELESS HART STANDARD WirelessHART (WH) is an extension of the well-known and widespread wired HART protocol; it preserves backwards compatibility and offers new possibilities thanks to greater flexibility and scalability of wireless networking. As previously stated, it is mainly devoted to the process automation; for this reason WH supports applications that have a minimum cycle times on the order of seconds. It is a time-synchronised, ultra low-power, mesh wireless fieldbus. In order to maximize reliability, it uses frequency diversity, time diversity, and spatial diversity, beside allowing transmitting power adaption. The WH specifications follow the OSI layers, and contain a PHYsical, Data Link (that includes Medium Access Control) and NetWorK layers. The Transport and APPlication layer are the same for both wired and wireless HART. A WH network contains different kind of devices (logical and or physical): •
one and only one Security Manager, that distributes encryption keys to the Network Manager of each network;
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one and only one active Network Manager, whose aim is to form the network, schedule resources, configure routing paths etc…
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at least one Gateway, whose aim is to interconnect field devices with the plant automation system;
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several field devices that are connected with the process, i.e. devices with sensors and actuators.
There can be also devices that have no connections with the process but have only communication facilities; they are routers, handheld devices (used for commissioning and/or maintenance purposes) and adapter (used to connect legacy hardware with the wireless network). With regards to the physical layer, the working group has adopted the IEEE802.15.4-2006; i.e., physical devices compliant with this standard can be used to implement WH nodes. This implies that modulation schemas are exactly the same but all other protocol layers are different; this means that there is no compatibility between these standards and interoperability has to be realized at the application layer. The main tasks of the MAC (Medium Access Control) protocol are: slot synchronization; identification of devices that need to access the medium; propagation of messages received from the upper layer; propagation of packets coming from neighbors. The MAC is based on an hybrid use of Time Division Multiple Access (TDMA) and Slow Frequency Hopping approaches. The time is organized into repetitive structures called Superframe, that is repeated continuously; each Superframe is made up of a fixed number of timeslots 10ms wide. For this reason, it is very important that all nodes participating to the network share the same sense of time; synchronization is achieved thanks to pair-wise exchange of time information within data messages and their acknowledges. In fact, begins of all transmitted data packets is well known, as imposed by the Network Manager. However, when the network is setup for the first time, it is not configured, i.e. it does not contain any information about how data must be transferred among nodes. This means that the Network Manger has not yet define Links within the Superframe. The opportunities for device to device communications is dictated by the existence of a Link between them; the Link includes a reference to neighbors that are permitted to communicate with the device (unicast and multicast transactions). Furthermore, the slot number within the Superframe, the direction of the communication (transmit/receive), link characteristics (e.g., shared/dedicated), and the initial communication channel are also specified. WH supports only 15 of the 16 channels of IEEE802.15.4 (the last one is avoided), and a black list can be created by the Network Manager to improve coexistence. The active channel index CH is easily computed since every node must track the absolute slot number ASN (i.e. the progressive number associated with slots started when the network is formed) and knows its channel offset CO (CH=(ASN+CO)moduleNAC, where NAC
