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More than one way to go unwired

Sept. 24, 2009
Not all wireless links are based on Ethernet. Serial wireless can be a simpler alternative.

Say the words “wireless link” today and most technologists start thinking about Ethernet wireless protocols. Ethernet has certainly become ubiquitous. But it is not necessarily the best way to implement a link either to remote facilities or mobile platforms.

Authored by:
Leland Teschler
[email protected]
Key points:
• Wireless serial connections are still an option for simple low-bandwidth communication needs.
• Wireless Ethernet comes in handy for general-purpose industrial needs and can be extended with phone line access if need be.
National Instrument Corp.,
Omron Corp. serial wireless,
Phoenix Contact Inc.,
Trusted Wireless application note,
WiFi security encryption overview,

Wireless connections that don’t involve transferring a lot of data can often get by with a radio link rather than an Ethernet connection. The typical term for such systems is a wireless serial-communication system. These systems typically communicate using spread-spectrum radio signals in the ISM 900-MHz or 2.4-GHz band. The configuration often used is that of one master station connected with a number of slave stations which contain a combination of input and output points.

Examples of such systems include one from Omron Electronics LLC, Schaumburg, Ill., called the WT30 and Trusted Wireless systems from Phoenix Contact Inc., Middletown, Pa. Omron officials say such systems often find use on mobile applications such as AGVs and on factory equipment that gets moved around and reorganized frequently. Here wireless setups can be alternatives to delivering data signals via slip rings or installing guide lines in a factory floor. The Trusted Wireless equipment operates in a similar manner though it incorporates a means of getting to remote facilities via phone lines.

Of course, serial links of this nature have their limitations — you wouldn’t want to use them for streaming YouTube videos. Similarly, data from an industrial vision systems would overwhelm a serial link. Serial systems like that from Omron operate at 115,000 bps compared to about 54 million bps for WiFi. But the lower rate is adequate for handling ordinary machine I/O. Omron puts the upper limit of its system at 1,000 I/O points.

Communication range for systems of this nature range vary. Simple serial-wireless systems may cover 60 to 300 m depending on whether obstructions are a factor. Some WiFi systems, equipped with the proper antennas, can cover two to 20 miles. And it’s possible to extend the range through use of repeaters, basically receiver/transmitters that receive one set of signals and rebroadcast them. In some cases an I/O point can be used to serve as a repeater.

Costs of wireless systems can vary as well. Omron says its serial-wireless systems run about half the cost of industrially hardened WiFi systems. And security is less of a concern. Tips on hacking into WiFi systems are easy to come by on the Web, not so for serial systems. Still, serial systems incorporate a level of security. For example, the Omron system gives each slave unit a unique identifier that is registered with the master unit before communication can begin. And the master only acknowledges preregistered slaves.

Remote links

Wireless systems frequently are the method of choice for communicating with remotely located assets such as tank farms. In some cases, though, it may be impractical to cover the whole distance with a radio. So phone lines may be a way to handle such situations. But the use of public phone lines has its drawbacks. If something goes wrong on the lines, users are at the mercy of the utility to get the system up and running.

“We’ve seen a large number of people moving away from leased lines because when your lines go down, you are not a priority for the phone company,” says Phoenix Contact’s wireless product specialist Ira Sharp. “We have done a lot of retrofits moving people from leased lines to wireless connectivity. The terrain and length of the link will determine the best kind of wireless technique.”

Phone lines also have their own bandwidth limitations. Use of digital-subscriber-line (DSL) technology could be one means of overcoming such problems. Sharp says Phoenix is seeing more such installations in Europe and expects a pick up in U.S. demand as well.

Wireless applications that demand a significant information bandwidth are almost exclusively WiFi today. The range of a WiFi transceiver tends to be less then that of serial systems — about 1,000 ft is typical. That range can be extended through use of WiFi nodes serving as repeaters. A WiFi repeater is often just two WiFi routers connected together in the same cabinet, one receiving, the other transmitting. Alternatively, a few vendors make self-contained store-and-forward WiFi nodes that will function as a repeater.

Cautionary tales

Application engineers who have set up industrial wireless systems say there are several aspects of these systems that frequently trip up users. One is an underspecification of system resources given the number of wireless nodes involved. In general, industrial wireless networks usually expand. So it better to plan for significantly more wireless nodes than go into initial installations.

Use of high-gain antennas can also pose a few problems. Antennas that come standard with most wireless equipment are omnidirectional, exhibiting the same effective radiated power in all directions. High-gain antennas concentrate their ERP in the direction of most interest. “High gain antennas are useful and necessary in many applications although it should be understood that with high gain comes greater difficulty in aligning the antennas,” says Phoenix Contact’s Ira Sharp. “Increasing the gain of an antenna reduces the main lobe which does make them more difficult to aim and install. But this directionality can also be used to your advantage as this limited radiation angle reduces the accessibility of the wireless system from outsiders and thus promotes security. You can use directional antennas to your advantage by confining the RF energy to your plant.”

Security should be less of a concern in WiFi systems that use a modern encryption standard called WPA2. This standard, released about two years ago, has proven to be nearly bulletproof. (See “Bulletproof,” Machine Design, Oct. 23, 2008.) Difficulties may arise, though, because many factory networks are a patchwork of equipment, some installed before the most up-to-date encryption standards came into use. Legacy encryption standards such as WEP and WPA have vulnerabilities which are widely discussed on the internet and can be broken. Judicious use of antenna patterns won’t cure such vulnerabilities but can be a means of limiting network access to inside the plant. MD

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