Considerations for Wired versus Wireless Instrumentation
emersonautomationexperts.com/2015/industrial-internet-things/considerations-for-wired-versuswireless-instrumentation/
March 12,
2015
If you have an instrumentation and automation project for a new installation you are
faced with many technology selection decisions.
For instance, instrumentation based on digital communications technologies, which
provide robust diagnostics and predictive capabilities such as Foundation fieldbus, have
been with us for nearly two decades. And wireless instrumentation, built on standards
such as WirelessHART (IEC 62591 – 2010) and ISA-100 (IEC 62734 – 2014) are being put to
use.
In an Intech magazine article, Wired versus wireless trade-offs,
Emerson’s Moazzam Shamsi explores some considerations for
your technology evaluation.
When analyzing the use of wired or wireless instrumentation for
existing plants, Moazzam notes that the existing infrastructure
is factored in and:
…the choice hinges on a straight comparison of the two
technologies and the application of the solution that makes
sense.
Moazzam Shamsi
Global Solutions
Architect
Without the infrastructure, this cost of installation must be
considered.
For wired devices, the Foundation fieldbus standard provides:
In practice, 12 to 16 instruments are typically installed on a fieldbus segment.
Concerning the length of the segment:
Typically, up to 12 devices can be installed on a fieldbus segment up to 120 m long. If the
process unity has more than 12 instruments, a second or third segment can be installed.
To avoid problems on a segment such as short circuit:
…many plants install a segment protector or device coupler, allowing multiple
instruments to connect at one location. The device coupler is installed in an enclosure
near the process unity. Connections to the individual instruments are called spurs.
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The segment includes a fieldbus power supply to power the devices on the segment and
an H1 card connecting the segment to the distributed control system. The fieldbus
power supply and H1 card are integrated in some systems. Moazzam notes that
segments can be redundant for critical
process units and made intrinsically safe
with barriers for hazardous locations.
He offers guidance for where to use
fieldbus:
…a process unit containing many flow,
pressure, temperature, level,
multivariable, and other instruments, all
within a reasonable distance of one
another.
Segment design tools simplify segment
planning. Moazzam shares that for
existing installations undertaking a
modernization project:
…HART can use existing 4-20 mA wiring from older instruments to carry digital signal
information to the device couplers. The device couplers can be installed in the old
marshalling cabinet, saving a considerable amount in wiring and labor costs. Although
HART does not have performance levels comparable to newer fieldbus technologies, it is
the least costly wired digital option, and often sufficient from an operational standpoint.
Wireless devices:
…do not require communication wiring and related infrastructure…the vast majority use
battery power and thus operate without wires. Wireless networks communicate data back
to host systems securely and reliably, and can be applied to both control and monitoring
applications.
He describes how wireless devices, networks, and gateways to the control systems
operate. You can get some of this background in earlier posts, such as Designing
Communications Reliability into the WirelessHART Standard and Tips and Tricks in
Deploying Wireless Field Networks.
Moazzam highlights the advantages of wireless devices:
…they can be installed virtually anywhere in an efficient, timely, and cost-effective
manner….require no wired infrastructure or local power supply, so they can be far away
from a process unit’s wired fieldbus and power wiring.
He sums up the advantages of Foundation fieldbus and WirelessHART respectively:
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Foundation fieldbus
Widespread support among instrument suppliers
Supported by almost every automation system supplier
Familiar to maintenance personnel
Excellent for control
Decades of operating experience
Best for multivariable instruments and multipoint devices
Digital data shows raw measured signals
Extensive diagnostics
Less wiring than 4–20 mA
WirelessHART
Significant reduction in installation complexity
Inexpensive
Reduces space, weight, and power requirements
Easy to expand to accommodate changes and additions
Excellent for monitoring applications
Can be used for control in some circumstances
Best for hard-to-reach locations
No need to modify existing wired infrastructure
Can reduce the need for fixed asset inspections, reference to IEC60079 Part 17
Allows measurements to be made where fieldbus is not practical or feasible
Read the full article for examples of process manufacturers and producers achieving
results with both technologies and more on the considerations on the right technologies
to select for your project.
You can also connect and interact with other wireless and project experts in the Wireless
and Plan & Design groups in the Emerson Exchange 365 community.
Posted Thursday, March 12th, 2015 under Industrial IoT.
Tags: fieldbus power, Foundation Fieldbus, Foundation fieldbus, IEC 62591, ISA-100,
Moazzam Shamsi, Solve & Support, WirelessHART
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