Considerations for Wired versus Wireless Instrumentation

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.

Emerson's Moazzam Shamsi

Moazzam Shamsi
Global Solutions Architect

Wired-vs-WirelessIn 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.

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.

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:

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.

One comment so far

  1. I personally agree using both FOUNDATION fieldbus and WirelessHART is the right architecture for new plants. Many plants that were built years ago before wireless instruments existed are now being modernized by adding plant-wide WirelessHART infrastructure. For a new plant being built it makes sense to put it in right from the start.

    FOUNDATION fieldbus takes the place not only of 4-20 mA but also of the various types of on-off I/O signals such as dry contact, sink and source for DC, as well as relay and Triac for AC. That is, you can mix transmitters, gas chromatographs, control valves, intelligent on-off valves, and two-wire tank gauging systems on the same bus.

    I agree that a bus is typically designed for up to 12 devices but would also like to highlight this is roughly equivalent to 36 conventional I/O as many devices have multiple signals each such as 3 signals on a control or on-off valve, up to 16 signals each for an electric actuator / motor operated valve (MOV), a.s.o.:
    http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf

    If more fieldbus devices have to be added later wires only have to run up to the junction boxes. Additional signals in fieldbus devices can be enabled at a click of a button.

    A fieldbus segment can be, and usually is, far longer than 120 m. The distance from the control system to the junction box can be more than 1000 m. 120 m is the distance from junction box to the device.

    Plants now use couplers with short circuit protection as standard thus ensuring a short at one device does not affect the others

    Most of the time the fieldbus interface card sits in the DCS backplane such that HSE is not required

    Redundant interface cards and power supplies is common, but redundant cable is extremely rare

    Using repeaters, fieldbus can reach several kilometers but since a bus segment can reach more than 1000 m without repeater, a repeater is extremely rare

    When upgrading 4-20 mA to 4-20 mA/HART make sure the cable is shielded twisted pair grounded in one end, meeting capacitance and separation requirements:
    http://en.hartcomm.org/hcp/tech/using/usinghart_wirelength.html

    I personally agree that wireless is an excellent way to add instrumentation in existing plants, and that plant-wide WirelessHART infrastructure is an excellent way to modernize existing plant to improve reliability, maintenance and operation productivity, energy efficiency, and to reduce HS&E incidents and response time. New plants should build in this WirelessHART infrastructure from the very beginning so that it need not be added later. This requires an early involvement of the run & maintain organization in the project. Problem is, the run & maintain organization may not have been established yet when the project is in FEED. Plants need to figure out how to make sure the automation, data collection, and data processing needs of the maintenance and reliability engineers, energy managers, and HS&E officers for a new plant are taken care of at the start of the project. Anyway, even if you have missed this step in FEED in your current project, you can still add WirelessHART in the detail design stage because the infrastructure can be installed plant-wide with minimal wiring.

    WirelessHART is the most cost effective way to add instruments when there is no junction box nearby

    I personally agree that fieldbus and wireless complement each other and modern plant automation should be designed with both. Learn more about WirelessHART for operational excellence from these articles:

    Beyond the Control Room
    http://www.ceasiamag.com/article/beyond-the-control-room/10658

    Second Layer of Automation
    http://www.ceasiamag.com/article/second-layer-of-automation/10354

    Maintenance with a Hart
    http://www.ceasiamag.com/article/maintenance-with-a-hart/9894

    Smart Sensing: Situational Awareness
    http://www.iaasiaonline.com/more.php?id=2222&cat=im

    Are you sending people in harm’s way?
    http://www.iaasiaonline.com/more.php?id=1956&cat=ii

    Wireless for Asset Uptime
    http://www.ceasiamag.com/article/wireless-for-asset-uptime/8689

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