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.