Connecting HART to the Rest of the Plant

Many Plants Are Using HART Technology Only Once or Twice a Year. You Can No Longer Afford to Do This

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There are well over 30 million installed HART devices in the world. Roughly 15% to 20% of them are connected digitally through direct connection to the control system. The rest of the HART population is being used simply as a single PV transmitter or valve positioner over a conventional 4-20m A dc current loop. The HART smart capability of the device is only used for setup and calibration of the instrument. Using HART technology in this way limits its usefulness in the plant to just twice a year.

Automation professionals no longer can afford to do this. There is so much valuable information in HART-enabled devices that the use of that information can seriously affect the profitability of the enterprise. And now, with HART 7 and WirelessHARTTM, there are simple and easy ways to get that information out of the device and into the hands of the engineers, operators and maintenance and operations personnel who need it.

Trapped Inside

The vast majority of HART-enabled devices have only one output connected to the control system—the PV (process variable) that reports on the 4-20 mA loop. Depending on the manufacturer, there is a lot more intelligent information within the device that can be accessed. (See Figure 1.)

Much of that information, such as the other seven cyclically reported process and dynamic device variables, trending information and device status, is useful to the process control system. The broadcast messaging feature, report by exception and time- or condition-based alerts are also important to the process control system and eliminate the need to poll the device for information, as in previous versions of the Protocol.

Other information items stranded inside the transmitter are device configuration, device calibration, the 32-character device tag, loop check data and much more system and process diagnostic information. Some of this is useful to the process control system, but much of it is even more useful for the quality and asset management systems.

Here are some examples of the really useful data that can be found in various types of field devices and final control elements.

Magnetic FlowMeters

  • Flow 
  • Raw Flow 
  • Volumetric Flow
  • Smoothed Flow
  • Positive Total
  • Negative Total
  • Net Total
  • Process and Diagnostic Alarms

Ultrasonic FlowMeters

  • Flow
  • Sound Velocity
  • Positive Total
  • Negative Total
  • Process and Diagnostic Alarms

Vortex FlowMeters

  • Mass Flow
  • Volumetric Flow
  • Velocity
  • Total Flow
  • Process Temperature
  • Vortex Frequency
  • Process and Diagnostic Alarms

Hydrostatic Level Meters

  • Level
  • Volume
  • Pressure
  • Differential Pressure
  • Sensor Pressure
  • Process Temperature
  • Sensor Temperature
  • Process and Diagnostic Alarms

Pressure Transmitters

  • Pressure
  • Differential Pressure
  • Percent Range
  • Electronics Temperature
  • Sensor Temperature
  • Process and Diagnostic Alarms

pH (and other wet chemistry) Transmitters

  • pH 
  • Process Temperature
  • Electrode PerformanceProcess and Diagnostic Alarms

Valve Positioners and Damper Operators

  • Stem Position Percentage
  • Input 
  • Supply Pressure
  • Set Point
  • Deviation from Set Point 
  • Actuator Pressure 
  • Positioner Temperature
  • Process and Diagnostic Alarms
  • Total Strokes

Getting the Data Un-Stranded

So why has this data typically remained stranded? For many years, even though it was available, DCSs and PLC-based control systems were not able to take the data in directly. Companies such as Pepperl+Fuchs and Moore Industries International have made up for this by producing HART interface modules. (See Figure 2).

These interface modules typically have taken the HART data, stripped it off the 4-20mA loop and presented it to the control system in the form of Modbus, Modbus TCP serial digital information or additional 4-20mA input signals.

Many control system manufacturers now have HART-enabled I/O, allowing native HART data directly into the DCS, and most of the major vendors are expected to release this feature for HART 7 devices (backward-compatible to earlier HART Protocol versions) in the next 12 months.

The multi-drop, all-digital network configuration of HART-enabled field devices is also available for this purpose.

But what happens if you have a control system that doesn’t accept HART data or you don’t have room in your marshalling cabinets to install HART Interface Modules? What happens if it just plain costs too much money, and you can’t make the justify the project, even though you know you will save time, save money, and provide better performance to the plant—but you just can’t prove it in advance?

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