May 2014System Integration & Control Systems Design
SA Instrumentation and Control contributing editor, Michael Brown, recently received some feedback on a loop optimisation article we published in the August 2013 edition. Since it is such a topical experience from a C&I engineer, we decided to publish it.
Mike,
I was reading your article in the August 2013 edition of SA Instrumentation and Control about strangely behaving valves and it reminded me of an experience I had when I was tuning a loop in a DMS plant. It was a simple loop controlling density with water flow, but I did not have a flowmeter to check the flow rates.
An I/P valve manufacturer had been issued with an order to provide a valve of a certain flow range that they did not have in stock, but they could modify the range of another valve by exchanging the seat. What they failed to do, however, was to recalibrate the valve for 4-20 mA: valve fully closed to fully open.
The valve only started opening above 8 mA and, since the loop was quite slow, this caused some problems as you will appreciate. Subsequently the project leader accused me of not knowing what I was doing, since the loop was not controlling the density satisfactorily. I stuck to my guns and said that the valve was not opening when it should.
I checked the density control later when I could open an outlet to see if there was water flowing, and sure enough there was no flow. Now it was my turn to make accusations about who had not done their job. Needless to say there were no apologies.
Loop signature Part 2-4: Feedforward Control: Part 3 Michael Brown Control Engineering
Editor's Choice Fieldbus & Industrial Networking
In the previous articles in this series, the basic theory behind feedforward control was discussed, and it was also shown how to apply feedforward in practice. In this article, it will be shown how well feedforward can work in practice by giving a couple of examples.
Read more...Loop signature Part 2-3: Feedforward Control: Part 2 Michael Brown Control Engineering
Editor's Choice Fieldbus & Industrial Networking
Feedforward control tuning is not nearly as critical as feedback tuning, and fairly simple models are usually fine for the purpose in hand.
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Read more...Loop Signature Part 2-1: Loop signatures and process transfer functions Michael Brown Control Engineering
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The previous series of loop signature articles dealt with the basics of control loop optimisation, and concentrated on troubleshooting and ‘SWAG’ tuning of simple processes. In this new series, consideration will be given to dealing practically with more difficult issues like interactive processes, and with processes with much more complex dynamics.
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