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May 2009

Process Automation

Wireless networking in plant

The advantages of a mesh network are redundancy, increased total distance, and removal of the line-of-sight restriction

By Dick Caro

The reduced cost of Ethernet-based networks is driving this fast, low-level, and low-cost technology to the field and shop floor.

Another Ethernet side effect is the application of wireless technology in the Wi-Fi group of wireless protocols. Wi-Fi is essentially wireless Ethernet. Any higher-level application layer and user layer can communicate via Wi-Fi at data rates up to 600 Mbps, without knowledge of the fact that it is on a radio link.

Wi-Fi is the most common wireless technology in 2008; however, it has significant problems for operation in the electrically noisy environment of a process plant or the shop floor in a manufacturing factory.

The popular Wi-Fi-a/b/g standards can achieve a theoretical maximum of 54 Mbps using one of the 2.4 GHz channels. However, the not yet finalized Wi-Fi-n standard, which has a maximum rated specification of 600 MHz, is even more interesting.

Wi-Fi-n allows the bonding of several radio channels, including the channels in the 5 GHz, band to achieve its high speed, but is not due to be finalized until the end of 2009.

The feature of Wi-Fi-n that is most appealing to industrial use is its adoption of multiple input, multiple output (MIMO) technology. MIMO has the demonstrated potential to eliminate the adverse effects of reflections that cause multipath distortion appearing as signal fade. MIMO achieves improved reception through detecting the multipath signals and either eliminating them or phase shifting them to amplify the received signal.

Early experiments suggest using Wi-Fi-n can achieve excellent behavior in both process plants and factories notorious for their "canyons of steel," the cause of poor performance of Wi-Fi-a/b/g.

Both commercial and industrial versions of "pre-N" devices are already widely being sold. Of course, use of wireless technology brings problems of security and privacy to industrial networks, which was never much of an issue before.

HART to handheld terminal

Wireless technology provides excellent solutions to the problem of the high cost of industrial wiring and also provides an ultimate barrier to electrical surges introduced to field equipment through field wiring.

The cost of these wireless advantages is the difficulty of supplying power to field devices, which previously got power from the same cable used to conduct the data exchange between the field device and a host system.

Although there were a few wireless field sensors offered for sale in 2008, it is likely the potential cost reduction of avoiding wire/cable installation and maintenance will provide an expanding market for wireless sensors in the future. Additionally, users are always finding new applications for industrial measurements and controls that leverage wireless devices. These applications were often not economic when they required wired connection.

By the end of 2008, a few WirelessHART process field transmitters were available commercially. In addition, a simple device to convert wired HART transmitters and valve positioners to the WirelessHART protocol is available.

WirelessHART is a de facto standard supported by the HART Communications Foundation, and it is an IEC PAS (publicly available specification). An IEC working group to develop an international standard for WirelessHART has approval.

WirelessHART uses the IEEE 802.15.4:2006 standard modified to hop among the 15 or 16 frequencies (channels) specified by that standard in the 2.4 GHz ISM (industrial, scientific, and medical) band. The slot time is constant for the entire network, usually 10 ms. Transmission encrypts via a 128-bit key to achieve a high degree of security.

Field devices are all part of a mesh network with a secure method of building and repairing the mesh. The advantages of a mesh network are redundancy, increased total distance, and removal of the line-of-sight restriction.

A simple transport layer ensures end-to-end message delivery and confirmation when required. Like wired HART, HART commands poll data access, including all maintenance functions of the WirelessHART network. WirelessHART devices may also be set to transmit data using a publishing method. WirelessHART devices are provisioned (network setup) through a wired connection.

WirelessHART came to the fore as a simple wireless network for field instrumentation, and to enable a wireless method to access diagnostic data in HART instruments installed in the past.

There are probably more than 25 million of these HART instruments that can only provide their digital diagnostic data to a hand-held terminal, since the control systems to which they are connected cannot access that data over the connecting 4-20mA wire. WirelessHART is an answer to this problem.

Standardization for wireless

The ISA100 standards committee has been developing a comprehensive standard for industrial wireless communications. The first release from this organization is ISA100.11a, intended for process data acquisition and limited control needs in the process industries.

While the ISA100.11a standard has achieved ISA and ANSI standard status, it has not yet passed international (IEC) standardization. The fact that WirelessHART and ISA100.11a address the same market is apparent to many users, and the IEC has taken heed as well.

They are technically similar, but very different in detail, causing many users to request the standardization people to merge them. ISA100.12 is already in business to achieve convergence between WirelessHART and the ISA100.11a specification.

At press time, the standardization path for wireless process control and factory automation networks was not yet final. Resolution between WirelessHART and ISA100.11a may finish in 2009, but standardization is still unresolved.

ISA100.11a also uses the same IEEE 802.15.4:2006 standard in a way similar to that of WirelessHART by hopping among the 15 or 16 channels in the 2.4 GHz ISM band. ISA100.11a has far more options to adapt its network for a wide variety of applications including segmentation of the network and peer-to-peer messaging.

Unlike WirelessHART, each network segment may use a different hopping pattern and its own allocated time slot, which allows large networks to form where segments may overlap.

The definition of field routers reduces the number of hops required to reach the host device, and to bridge geographically separated network segments.

ISA100.11a also uses mesh networking, but allows devices at the edge of the network to not route messages for other devices. This can increase security by preventing unauthorized devices to access plant networks from outside the plant. It can also reduce the cost of devices by making them simpler. Additionally, ISA100.11a uses Internet-conforming IP addressing to make data from field devices addressable remotely.

The Transport Layer implements secure end-to-end message delivery, and confirmation uses Internet-conforming User Datagram Protocol messages.

Upgradeable to ISA standard

The Application Layer of ISA100.11a is completely object-oriented, in which data in field devices can be addressed using IEC 61804 standard EDDL protocol. For networks not using this standard, one may encapsulate and tunnel messages to the requesting host device.

ISA100 has already agreed to extend its work in several ways:

• New physical layers (chip technology)
• A wireless or wired backbone (backhaul) network
• Factory-automation sensor network applications
• High security or trustworthy applications
• RFID and location-based services

ZigBee is an organization specifying additional higher layer protocols to use the same wireless IEEE standard, numbered 802.15.4. It can, by design, operate on the shop floor and avoid interference with Wi-Fi. It is also low cost, requires little power, and can transport Ethernet messages. Although ZigBee may operate with a star topology like Wi-Fi, it also allows operation in a mesh network topology.

By the time of our next report on the technology, we should have a much clearer picture of future industrial automation networks. Wireless offers too many advantages to ignore and will tend to change all of the existing networks we know so well to "legacy" status.

However, you cannot install standard wireless networks today, but you may purchase WirelessHART devices and other devices that claim to be upgradable to the ISA100.11a standard.

Wireless networks of any kind do not yet exist for factory automation sensors.

ABOUT THE AUTHOR

Dick Caro is an ISA Life Fellow. He worked on the ISA/IEC Fieldbus committee to develop the communications standard. His new book is Automation Network Selection: A reference manual, 2nd edition, ISA Press 2009. His e-book, Wireless Networks for Industrial Automation, is in its fourth edition.