Legacy technology: Easy ways to use HART on installed devices
One of the great lost resources is the fact that so few pieces of HART enabled instrumentation can deliver all their capabilities. HART instrumentation is everywhere, and is used on a huge variety of products. You probably have some in your plant if you have any electronic pressure, flow, level, or temperature devices made in the last 15 years or so. It's also used on valve positioners and other types of actuators. Whether you knew it or not, most of these are actually multi-variable sensors, and with some very simple equipment, you can unlock the additional data without making disruptive or expensive changes to your control systems.
If you're not sure what HART data is, it's a second signal group that comes down the 4-20 mA wire with the process variable. It uses a digital format that is piggy-backed on the analog, and it carries a whole lot of useful information if you can get at it. If all you're using is the single analog process variable, you'll never even know it's there. That digital signal carries secondary variables, can trigger alarms, and helps with calibration/configuration. For example, a simple pressure sensor might also tell you the process liquid temperature, transmitter temperature, express the reading as percentage of total range, and maybe a few other parameters. Individual manufacturers decide what those auxiliary functions are. You might be amazed how much data you can get from something sophisticated like a Coriolis flowmeter. It's all there as part of the HART package, if you can read it.
Sophisticated control systems and fieldbus architectures use this data in stride, but if you have a single analog connection to an I/O card, the information is going to waste. However, with some very simple equipment, you can read it, and you won't have to rewire things, add Ethernet or anything complicated. There are devices called loop interfaces or monitors that pull the digital signal off the line and convert it to analog. They are available from several suppliers (link below):
One of these devices can be added to the line carrying the primary variable to your control system without disturbing any other connections. The simple versions convert the digital signals into multiple 4-20 mA analog channels that you can feed to another readout device or I/O point for your system. Moore has a range of choices, including units with onboard readouts in addition to analog outputs. (See graphic.)
The biggest advantage is that you can try this without a huge investment or disrupting existing equipment. An interface can be moved from place to place as you determine the best uses for them in your process. Having additional process information without adding instruments could be a major help in your control efforts.
This diagram shows how a loop interface can break out additional process variables and diagnostic information from HART enabled devices. Image courtesy of Moore Industries.
— Control Engineering Daily News Desk
Peter Welander , process industries editor
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