Most networks are wired systems based on dozens of proprietary protocols and, more recently, on Ethernet. In an industrial environment, whether it’s manufacturing, process control, transportation, or building automation, these networks are used for monitoring and control in both open-loop and closed-loop control systems. Sensors monitor the physical states of the process. Control signals initiate or control the various parameters of the system. In most cases, the sensor and control points are far from the control system, usually meaning long cables and all of their attendant issues.
This has engineers taking a closer look at wireless networking options, which are more viable than ever before thanks to the wide variety of available technologies. They can offer benefits that were previously unthinkable in most monitor and control operations.
NOT YOUR USUAL OFFICE LAN
Industrial networks differ from the typical office Ethernet local-area networks (LANs), which are religiously administered by their organization’s IT dynasty.
First off, their environment is harsh compared to the comfy setting for most LANs. They’re located in factories, plants, remote buildings, and even outdoors along oil and gas pipelines. Thus, industrial networks are subject to weather, temperature extremes, vibration, chemical fallout, and all sorts of other nasty climates.
Second, industrial networks are usually mission-critical. They’re relied upon for the operation of systems that can’t tolerate any kind of failure or downtime. While an office LAN can go down, delaying an e-mail or Internet search, a failure in an industrial network may shut down a profitable production line, process run, or other 24/7 functions leading to crisis conditions. Reliability must be golden.
Furthermore, industrial networks are subject to noise, moreso than in an office LAN. The noise comes from high-voltage ac lines; the switching of motors, relays, and solenoids; switching power supplies; and various wireless sources. Industrial networks need more than standard noise immunity and protection.
Industrial networking also can be characterized by deterministic operation, where the timing of the various operations is critical. Security may be an issue as well, so operations aren’t compromised by outside nefarious sources or unintentionally by well-meaning employees. Finally, interoperability among multiple different networks is often an issue.
All of these special requirements usually add up to a wired network as the best choice. But with today’s vastly improved wireless technologies, designers can use a wireless solution that meets all of the typical requirements and brings some significant benefits.
WHAT’S IN IT FOR YOU?
Perhaps the greatest benefit of wireless networks is the cost savings, especially if you’re building a new network. Wiring is expensive— copper cables have dramatically increased in price over the years. Industrial installations require conduit and other special wiring considerations to ensure reliability in rough environments. And, as experience has shown, the weak links in most wired networks are the connectors.
Wiring must be installed by licensed electricians or certified technicians at a cost often exceeding $100/hour. Even short runs of twisted pair in a conduit several hundred feet long can cost tens if not hundreds of thousands of dollars and take weeks or months to install.
Despite the cost of the wireless equipment, labor expenses are minimal, and installation time is extremely short. Maintenance costs must also be considered. With no wiring, there’s nothing to maintain except the wireless transceivers. While they do need a battery change every now and then, wireless transceivers are very reliable. In older equipment, that replacement interval was often every few months. Modern wireless systems use very little power and may only need a battery replacement every several years.
Range was a problem with older wireless systems, too, but that’s lessened with recent technologies such as mesh networks. While line-of-sight (LOS) operation is required for most wireless today, signal blockage may still be a problem. Workarounds usually can be found, though. Repeaters, gain antennas, and other solutions are common.
Finally, security may be an issue. Most newer wireless technologies incorporate encryption and other security measures, making it far less of a problem.
SOMETHING FOR EVERYONE
So with all of these benefits, wireless should be the top consideration, especially when installing a new network or replacing or retrofitting an older network. The next step, then, is selecting the wireless technology standard—a decision likely predicated on data rate and distance (Fig. 1).
IEEE 802.15.4 and ZigBee: For short range (less than 30 m), radios based on IEEE 802.15.4 are a good choice. They usually operate in the industrial-scientificmedical (ISM) spectrum from 2.4 to 2.483 GHz, which is available worldwide. The maximum data rate is 250 kbits/s, which is more than adequate for most industrial applications. Other ISM spectrum options include 868 MHz in Europe and 915 MHz in the U.S. at lower data speeds of 20 and 40 kbits/s, respectively.
Built on the 802.15.4 physical-layer (PHY) and media-access-controller (MAC) standard, ZigBee is ideal for mesh networking. That’s because it can signifcantly extend the range and reliability through node relays. ZigBee also is a top choice for industrial sensor networks. The low-dutycycle operation translates into low power consumption and long (like years) battery life.
Wi-Fi, 802.11: Wi-Fi is the main choice of wireless networking in enterprise LANs. It comes in several configurations, with data rates exceeding 100 Mbits/s in some forms. The workhorse 802.11b version is the most widely used, capable of 11 Mbits/s up to a range of 100 m. The 802.11g version supports data rates to 54 Mbits/s at that same range. The most recent version, 802.11n, has yet to be ratified. However, a Draft 2.0 version is now selling and offers rates to 300 Mbits/s using multiple-input/multiple-output (MIMO) antenna technology.
All of these versions operate in the same 2.4-GHz spectrum. The 802.11a version operates in the 5.8-GHz ISM band at a rate to 54 Mbits/s. That band offers less interference and fewer co-existence problems, but its range is slightly shorter.
In the past, the 802.11 wireless standards were less desirable for industrial applications, mainly because of their higher power consumption. But low-power versions from a number of chip suppliers have made 802.11 viable even in sensor or actuator applications, where long battery life is essential for minimum maintenance. When your data-transport application needs high speed and long range, it becomes an excellent choice. It also matches up nicely with the corporate office LAN. The 802.11i security standard goes beyond the usually used WEP, WPA, WPA2, and other encryption standards to ensure a very high level of security in critical links.
Proprietary standards: An interesting alternative in industrial applications is to use a company-specific radio that doesn’t conform to any of the common wireless standards. These radios, which employ unique protocols or wireless versions of wired industrial protocols like Modbus, are often a better choice for some applications. Most still use the ISM spectrum in the 902- to 928-MHz or 2.4-GHz bands. If you don’t have to be compatible with a company LAN or the Internet, they’re a solid option. Usually, they’re a better match for specific applications using proprietary standards like Modbus, Profibus, or HART.
Cellular: For the long range, special cell-phone modules are available for connection to industrial networks. Both cdma2000 of Verizon and Sprint Nextel and GSM/EDGE/WDCMA of AT&T and T-Mobile are available. These standards use the data capability, which is usually fast enough for most industrial monitoring and control applications.
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