Selecting device-level communication protocols

Dear Control Engineering: After looking at some recent articles about wireless field instrument deployments and new flowmeters with Ethernet connections, I wonder how device manufacturers decide which options they will offer with specific types of instrument and transmitters.

Companies that make process instrumentation are driven first and foremost by what they believe will sell, so they pay very close attention to customer demand (or at least they should). There are also logistical issues that enter in as well.

Considering the Ethernet connectivity question, that is something that we have been watching for some years. Ethernet networks continue to move closer to the manufacturing floor, but they rarely punch through the PLC barrier. In other words, networks where PLCs are communicating with field devices rarely use Ethernet. They use cheaper analog (4-20 mA) or simpler digital (fieldbus, Modbus) communication more suited to the limited amounts of data the devices produce. The new flowmeters with Ethernet connections are Coriolis and magnetic designs that generate multiple process variables and diagnostic information, so they fall in the complex end of the instrumentation spectrum. I suspect many companies will be interested to see how these fare in the marketplace.

The types of devices that are available with integrated wireless transmitters seem to be following two approaches. First, devices that are relatively simple and suited to battery power. If you think back a few years, some of the early wireless devices tended to be things like temperature or pressure sensors. A thermocouple is ideal for running on a battery because it has minimal outside power requirements. The second approach concentrates on devices that are needed in geographically spread out applications. That’s one reason why wireless radar level gages are emerging. These are needed in tank farm applications where it is costly to run long distance cables.

Furthermore, if you start analyzing various offerings, you’ll find a high correlation between devices needed in oil refineries and those that communicate using Foundation fieldbus.

New technologies are helping reduce the necessity of being concerned about such things. Devices are emerging that can be reconfigured for different communication methods (e.g., FCI gas flowmeter). Companies like Emerson and Invensys have introduced field wiring and marshalling systems that allow users to mix-and-match devices using modular and software configurable connectivity. This evolution will likely continue and offer many new options as companies consider approaches for new projects.

Peter Welander, pwelander@cfemedia.com

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