A Return to Single-Loop-Integrity

Control in the field is becoming mainstream once again – just as the ISA SP50 fieldbus committee predicted it would be. CONTROL's regular contributor Dave Harrold looks at the integrity of the control loop.

By Dave Harrold

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The algorithm for production gas estimation applies to natural producing, gas-lifted, ESP-lifted, and pumped-oil wells and helps reduce the number of well tests required, instantaneously identifies well production changes, supports the creation of well lift performance curves, and facilitates continuous production optimization.

Like the standard function block examples presented earlier, these are but a few examples of control and calculation solutions being implemented using FF’s flexible function blocks.

Available on the Fieldbus Foundation’s Web site are additional flexible function block control solutions described in Function Block Capabilities in Hybrid/Batch Applications. Included in this collection of innovative application descriptions – some complete with implementation solutions – are:

  • Multivariable matrix control
  • Variable-speed drive control
  • Four discrete valve control
  • Extended PID with AutoTuner
  • Fermentation Zymolysis control
  • Distillation startup and shutdown
  • Integration path for a legacy system and multiple field HART® interface
  • Process cooling water system
  • Cleanroom makeup air unit
  • Packaged batch distillation control

A less obvious application solution facilitated by FF’s flexible function block is the ability to use OPC (OLE for process control) to help integrate legacy systems connected to different fieldbus protocols.

Another example described in some detail in the Function Block Capabilities in Hybrid/Batch Applications document titled Integration path for a legacy system and multiple field HART interface uses a commonly encountered two-tank storage level application to illustrate how FF’s flexible function blocks can help integrate legacy systems.

The example anticipates that control problems may include a mixture of fieldbus protocols and conventional 4-20mA instrumentation, the example makes use of hard and soft gateways, OPC, HSE, and H1 devices.

A Modbus/RTU is integrated using an HSE "hard" gateway device and Profibus-DP is integrated using a "soft" gateway based on OPC servers for Fieldbus and Profibus in conjunction with an OPC bridge.

To ensure communication integrity, a FF flexible function block hosts watchdog logic for both FF and Profibus communication protocols. The watchdog logic continuously validates the integrity of the OPC link, initiating a pre-defined “safe” action in case of defined communication faults.

Though all of the above examples may not be appropriate for all process control applications, each illustrates the capabilities that FOUNDATION fieldbus function blocks are able to deliver.

Conclusion
In the beginning, the justification to install FOUNDATION fieldbus was predicated on savings from simplified design, installation, and commissioning – still legitimate justifications. However, as the SP50 committee predicted, FF’s ability to host control-in-the-field also restores the much sought single-loop integrity philosophy thus creating a more reliable, less complex control system implementation.

While no one really knows how much CIF is actually implemented, three things are known; 1) FOUNDATION fieldbus is installed in most of the top companies in every processing industry all over the world; 2) top companies became and remain top companies because they make full use of their technology investments; and 3) FOUNDATION fieldbus is now mainstream.

Instantiation Basics

Instantiation allows users to create an instance of one particular function, such as a level control strategy, give it a generic name (e.g., LIC-xx1), and then use instantiation tools to produce more refined, deployable versions that include variables such as tag (e.g., LIC-101, LIC-201, etc.), descriptions, I/O channels, etc.


  About the Author

Dave HarroldDave Harrold is co-founder of the AFAB Group, and a technical writer with 38 years of experience in process control and automation systems. He can be reached by email at AFABGroup@indy.rr.com.
 

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