Working in our industry it is impossible to avoid bumping into the HART Communication protocol. Introduced in the mid-1980s by Rosemount as an attempt to find an inexpensive, almost trial way to get more information from smart field instrumentation, it later developed into something that finds its way back even when it seems to have outlived its purpose.
I won’t get into the details as to when, why, and how the HART standard was developed and how it works. It’s common domain information, but in a nutshell it utilizes low-frequency carrier to communicate digitally with intelligent field instruments. For point-to-point connection with an instrument, even the existing 4-20 mA cables can be used – leaving the main variable to the current signal and using digital communications for auxiliary information. It’s the best of both worlds.
Let’s talk about how a practicing controls engineer – especially the process control variety – may use that extra information communicated over HART.
I have my personal favorites. The first one – by far – is the actual valve position feedback that can be obtained from a digital positioner. A single glance at these two variables plotted together may show that your valve is sticking. Moreover, a simple deviation alarm can be set to indicate to the maintenance personnel that the valve performance deteriorated so they can address it even before the operators start complaining about the control loop not performing.
Another perk is the ability to get extra information from a multivariable transmitter. For example, Coriolis mass flowmeters can also report process media density. A DP-based mass meter can report absolute pressure and temperature in addition to the main process measurement.
Granted, the asynchronous nature of HART communications results in relatively large and unpredictable time between individual samples, preventing these extra measurements from being directly used in some control applications. However, Emerson recently introduced a modified PID algorithm (PIDPlus) that artfully addresses this problem. In any case, the extra measurements coming over the digital channel can be used for monitoring, alarming, and for enhancing process calculations and control.
Another user of HART data can be a centralized instrumentation maintenance package, or asset management system. To be practical though, such an application has to rely on the system’s ability to pass-through HART messages from field instruments to the system’s high-speed network where a PC-bound driver unwraps HART messages from Ethernet telegrams and feeds this information to the software application as if it were connected directly to the instrument.
Now, all these benefits – and many more – can only be realized if your system can understand HART. Some control systems, mostly relatively current DCSs, have HART modems built in. Some high-end PLCs and PACs can be specified with HART-enabled I/O modules. If this is not the case in your system, then your options are limited. You can add a HART multiplexor, but it may be difficult to integrate it with the existing field termination. Another option is to add a HART splitter that can split and send several new 4-20 mA signals to the spare I/O points, however this is not suitable for an asset management application, by the way.
In real life, neither of the two options is implemented with any regularity.
However, the introduction of HART7 with WirelessHART in 2007 presented a new opportunity to integrate smart instruments into the control system. Emerson’s Model 1420 Smart Wireless Gateway connects WirelessHART instruments with control systems and data applications. For sensors that don’t support WirelessHART, Emerson offers Smart Wireless THUM that can be retrofitted to many conventional HART instruments to wirelessly transmit HART information without the need to modify field termination cabinets. Other suppliers offer similar wireless add-on devices.
The gateway can be integrated with host systems via Modbus over RS-485 or Ethernet, and optional OPC functionality is available. It doesn’t have to be an Emerson control system. While working for Emerson three years ago, I tested and documented integration procedures for a wide variety of DCS and PLC/PAC brands. It also integrates nicely with Emerson Asset Management System – AMS Suite allowing it to be used with any control system.
Of course, wireless field communications go beyond getting “extra” information from the instruments. It could be used as the main way of connecting field instrumentation to the control system, but that is beyond the scope of this article.
When faced with the old question – how to bring HART data to your control system – you may want to give WirelessHART a try.