Manage legacy system assets with HART

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. Using HART data and diagnostics with legacy instrumentation is one choice. And with WirelessHART devices beginning to hit the market, there are more options than ever.

05/01/2008


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. Using HART data and diagnostics with legacy instrumentation is one choice. And with WirelessHART devices beginning to hit the market, there are more options than ever.

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.

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 to gather data 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. 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.

Wireless device options

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 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:

  • Honeywell 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 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.



Author Information

Peter Welander is process industries editor for Control Engineering. He can be reached at PWelander@cfemedia.com


WirelessHART instruments

If you’ve been chomping at the bit to get some actual WirelessHART instruments in your plant, your wait may soon be over. Emerson process management is now taking orders for these new devices and claiming bragging rights for being the first to do so. Search “Listen in” atop www.controleng.com to find “Listen in: First WirelessHART devices about to hit the market” and you’ll hear exclusive comments from Bob Karschnia, vice president of technology for Emerson’s Rosemount division.



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