October 2008

Wireless sensors

By Steve Toteda

The information you need to run your industrial plant more safely and efficiently is all around you; the challenge is getting access to it in a reliable, cost-effective way. Today's new standards-based, ultra low-power wireless sensor systems allow you to do just that, with their ability to measure critical parameters like differential pressure, temperature, level, and flow.

You can put wireless sensors almost anywhere with little or no regard for the existing wired infrastructure. They are easy to install and cost only a fraction of wired installations. Equally important, these sensors are simple for plant technicians to use with most deployments boasting an up-and-running network within hours of installation.

Major corporations are using wireless sensing to ensure environmental conditions are safe, monitor critical process parameters and equipment conditions, and gain easy access to the critical performance metrics contained in the sensors themselves.

Wireless essentials

Wireless sensors, the basic elements of a plant solution, create a redundant, fault-tolerant wireless network of connections among themselves. This mesh network, which carries data to a gateway connected directly to the main monitoring and control system, delivers superior reliability compared to alternative solutions that require direct line-of-sight communication between each device and gateway.

Wireless mesh networking uses the industry standard 802.15.4 radio from IEEE. The 16 channels available for individual communications give wireless sensors the ability to choose a different radio channel for each transmission, thereby avoiding interference. Use of the IEEE standard makes straightforward the issue of coexistence with WiFi, another common wireless standard used in a plant. WiFi is also based on an IEEE standard called 802.11, and the IEEE dealt with the potential for coexistence issues in the development of the standard. Extensive testing of multiple applications within the industry has shown these technologies can and do coexist well, even under difficult circumstances.

Wireless has a notorious reputation in the canyons of steel that make up a typical plant, and much of this is well-founded. Most assume the major stumbling block with wireless deployments will come from interference with some other type of radio, and then spend time and money attempting to profile the overall spectrum they will use. However, the real culprit of poor reception or blocked transmissions often comes from the signal bouncing around the plant, canceling itself out if it bounces in just the wrong way.

By channel hopping and taking advantage of the multiple paths in the mesh network, devices and gateways work together to find paths
and channels that optimize data reliability while minimizing power consumption and eliminating points of failure. This gives the network wire-like reliability while avoiding problems with radio-frequency (RF) interference from other radios and from the electromagnetic noise of motors, lights, and other sources common in plant environments. Most vendors report performance metrics that exceed 99.9% data reliability, with numerous real-world examples of 100% over extended periods of time.

The individual sensors in a wireless mesh network are also equipped to transmit and receive data to and from the gateway in a time-synchronized manner. This means sensors share the same clock, knowing precisely when to talk, listen, and most importantly, when to turn themselves off. Time-synchronized communication is critical for conserving battery life, allowing sensors to operate on the equivalent of two AA-batteries for seven to 10 years or more.

As far as security and reliability goes, all measurement and control traffic between sensors is secured, and all messages are encrypted, authenticated, and checked for integrity. Today's wireless standards typically use a proven AES 128-bit cipher to provide authentication and encryption. End-to-end security is an altogether different story, and you'll need to carefully think through selecting equipment that permits a secure wireless link form the filed device all the way to the control room, in most cases passing over unsecured wiring.

Future applications

Companies can realize the potential of process production facilities with the emergence of open standards-based field wireless infrastructure. Typical refineries deal with the harsh environments created by the complex piping of the production facilities through the previously described advanced techniques. Such environments are often quite hostile to RF signals, exacerbating the traditional RF issues of path loss, fading, and multipath.
The low installation cost of wireless makes it tempting for a company to get its feet wet. But plants can realize a bigger cost savings through improved predictive maintenance and better operational performance.

Future advances in wireless sensor technology will further reduce power consumption of the sensors and their wireless networking interface. We are already seeing wireless sensors harnessing the vibration of the machine it is measuring to generate their own power to report on the health of the equipment. This will give more options in where to embed devices.

Here are some tips to consider when selecting wireless sensor products:

  • Make sure to adopt a full mesh networking architecture with channel hopping to ensure wire-like reliability.
  • Choose an automation vendor whose wireless sensors and gateways provide dynamic network optimization to accommodate the inevitable changes in the RF and physical environment of your plant.
  • Choose a solution based on an industry-accepted standard.
  • Insist on security in your wireless solution.
ABOUT THE AUTHOR

Steve Toteda is an executive member of the board of directors of the Wireless Industrial Networking Alliance (WINA) and vice president of product management at Dust Networks in Hayward, Calif.  Contact him at stoteda@dustnetworks.com.