Tutorial: HART data/diagnostics for legacy instrumentation
By Control Engineering Staff -- Control Engineering, 4/17/2008
From time to time we’ve discussed ways you can gather secondary variables and diagnostic information from HART-enabled instrumentation. Now the number of options you have to put this technology to work is getting a whole lot bigger.
As a quick review, HART is a protocol that sends and receives information to instrumentation and devices via a digital signal laid on top of the analog process variable. This information involves secondary process variables and diagnostic functions. To read and use the digital data, you need a device that can decode it. Many, if not most of the analog instruments in your plant probably have HART capability.
Some control systems have HART-enabled I/O capabilities that make this very simple. But if you don’t have that, there are other ways besides walking around with a hand-held communicator.
A number of manufacturers make wired HART interfaces that can be placed in parallel with existing cables to read the secondary variables. (We discussed this in greater detail in March, 2007.) There are also multiplexing devices that can gather data from a group of devices on multiple channels (some up to 64 channels) if all the wiring comes to one location. These tend to be slow in that they have to read each device in sequence, so this approach is normally used where data is not time-sensitive.
While these approaches work, the more difficult question can be, what do you do with the data? How do you get it from the monitoring device to the place where you need the information? A legacy control system that has no capability to gather diagnostic data probably has no way to digest it either. That means you are looking at setting up a secondary asset management system that operates in parallel with your control system. Doing this with hard wiring can get complicated and expensive, which may explain why so few people do it.
The new alternative that brings much promise is wireless technology. Consider this possibility: Let’s say you want to implement an asset management system without upgrading your DCS, or you just want to keep a closer eye on some key devices. So you select a group of those instruments within a process unit that are HART-enabled, but from which you are not gathering diagnostic data. A wireless transmitter on each of those devices can send the HART data back to a gateway without disturbing your existing wiring to the operating control system. The main process variable still goes down the wire just as it always has. The wireless gateway gathers up all the diagnostic data in one place, regardless of how the devices are wired. Now that WirelessHART devices are beginning to hit the market, this approach is getting more attention.
Emerson Process Management calls its add-on diagnostics and PV transmitter a “THUM,” and it operates on the wireless network just like another device using that protocol. Airsprite has a similar device coming that can turn any 4-20 mA instrument into a full wireless node or just send the diagnostics.
While WirelessHART is new, it isn’t the only way to approach the situation. There have been wireless systems capable of communicating HART data for some time. For example:
Honeywel Process Solutions has demonstrated transmitters within its OneWireless platform that can mimic wired communication protocols, including HART and fieldbusses. This can work with or without the wired connection.
MacTek has a portable communicator that sends HART data via Bluetooth wireless, and there are others.
The two key benefits of this approach are that you can pick and choose which devices you want to monitor, and all this can take place without any disturbance to your existing networks. Moreover, once a wireless gateway is in operation, you can add new pieces of instrumentation without any wiring at all.
No matter how old your control system is, you can add the benefits of asset management in a way that will minimize costs and operational impact. The question will be if users see wireless as the final push to make them take action and deploy this important money-saving technology.
—Peter Welander, process industries editor, peter.welander@reedbusiness.com,
Process Instrumentation & Sensors Monthly
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