; Wireless devices often require
fewer changes to drawings and
less engineering resources. Fewer
review meetings result in faster
project completions with less people involved.
; Wireless can be installed in locations where wired devices cannot—in hard-to-reach locations,
areas hazardous to plant personnel or where power doesn’t exist,
running wires is not allowed or is
prohibitively expensive. These
wireless devices have features
similar to intrinsically safe (IS)
instruments and PDAs that are
already in use in these locations.
; Hazardous area approvals and
certifications are less complicated, especially in locations that
require explosion-proof ratings.
Because wiring and conduits into
the device are eliminated, IS certifications are superior; they are
by their very nature energy-limit-ed and will not be a source of igni-
tion. No source of ignition means
no containment is required as
well.
; Wireless devices operate on low
voltage and low current, can easily be adapted to external power
and can eliminate battery maintenance concerns. These power
sources can also be IS certified.
Installing to use local power is
significantly less costly than running I/O wiring back to a control
room. Another option for power
sought by some customers, especially in remote locations, is solar
panels.
The speed of wireless is getting closer
to wired; monitoring devices are avail-
able with one-second update rates.
When choosing the appropriate update
rate for the device, battery life must be
considered. The transmission of data is
getting smarter through sampling the
position and only sending updates when
the valve position actually changes,
which speeds up the process and lowers
overall energy consumption.
RELIABILITY
An early concern with wireless technologies was that they were not as reliable
as wired technologies. This was because
early wireless technology was strictly
point-to-point; picture a couple of tin
cans with a string between them. If you
did not have a straight line, also called
line-of-sight, between the two devices,
the chances of solid communications
were eliminated. Also, line-of-site solutions require costly site surveys. Those
problems have been addressed.
Today’s technology—mesh networking—provides 99.9% reliability (for
WirelessHART technology). With the
advent of mesh networking, the tin can
scenario was replaced by a spider-web-like network for communication that
eliminates the need for site surveys. The
distances between instruments in a WirelessHART mesh network is 200 meters,
for example.
Wireless valve communication offers many advantages, but can sometimes be confusing. There are many ways to set up a system, and they are governed by different standards. How do you choose? Here’s a brief summary. The layout or topology of a wireless network defines which nodes talk to which other nodes. Any wireless system has to have both end devices (mounted on valves, actuators or sen- sors) and a gateway to connect to a control system or an oper- ator display. The end devices are generally very low power bat- tery-operated units that spend most of their time asleep to save energy. The gateway to the rest of the world is generally powered by the AC mains. A system in which the end devices talk only to the gateway is called a “star” (think of the gate- way as the center and the end devices as the points). Since the nd devices have fairly short transmitting range, this tends to limit the physical size of the system. If the end devices can hear each other and pass along any messages they get to the next node in the system until they reach the gateway (in other words, they can act as routers), the system is called a “mesh.” A mesh can be much larger geo- graphically than a star because end devices only have to be heard by the nearest other end device, not by the gateway. In addition, a message can take any path it must to get to its des- tination; if one node fails or its signal is blocked, the message automatically finds a new route. Some systems have what amounts to a mesh of routers, Understanding Wireless Standards
with each router acting as the center of its own star; this is
called a “hybrid” or “star cluster” network.
Wireless systems are governed by a number of different
standards, with varying topologies and other characteristics
and with support from different groups of vendors. At the lowest level, where such things as operating frequencies and
methods of transmission are decided, most of them follow the
IEEE 802.15.4 standard. But not all do, and even those that
follow IEEE 802.15.4 aren’t necessarily compatible. There
are systems that follow ISA 100 (specifically ISA 100.11a).
There are WirelessHART systems. There are ZigBee systems.
All of them work, but not with each other.
One system that doesn’t comply with IEEE 802.15.4 is
Bluetooth. This is used a lot for things like cell phone earpieces
and laptop computer mice, but there are industrial uses for it
as well, including setting up and configuring smart valve actuators.
When considering wireless it’s important to ask a few questions: Will the wireless system connect to an existing Profibus,
Foundation fieldbus or Modbus network? Will it connect to
Ethernet? Would a mesh, star or hybrid layout be best? How
much area will it cover?
For more detail on wireless standards and topologies, take
a look at the article “Sorting out wireless standards for smart
valves and actuators” on ValveMagazine.com.
