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Apr
28

Common Control Myths – Old and Unimproved

I dug up the following myths from my April 2006 Control Talk column in Control Magazine. I am into recycling and going green. In fact these myths may be a bit moldy.

(26) Auto tuners can compute controller tuning settings with an accuracy of more than one significant figure. Act surprised when unmeasured disturbances, load changes, valve stick-slip, and noise cause each result to be different. Look forward to the opportunity to play bingo with the second digit.

(27) You can just dump all your historical data into a neural network and get wonderful results. Forget about the same stuff that cause auto tuners to have problems and use variables drawing straight lines because anything that smooth or well controlled must be important. Use the controlled variables (process variables) instead of the manipulated variables (controller outputs). Don’t try to avoid extraneous inputs or identification of the control algorithm instead of the process. If you want to purse a career in data processing, use every variable.

(28) Models can predict a process variable that is not measured in the field or lab. Great way to spur creativity in training a neural network and validating a first principal model plus it has the added bonus of the model never being wrong. Wait till your customers figure out something is wrong with the composition of your product. Discount as hearsay any suggestions that even the best models need periodic correction.

(29) To reduce variability in process outputs (temperatures and compositions), keep all the process inputs (flows) constant. Keep believing that you can fix both the process inputs and outputs and don’t accept the notion that process control must transfer variability from process outputs to process inputs to compensate for disturbances.

(30) Positioners should not be used on fast loops. This was true for the good old days of pneumatic positioners and analog controllers. Surely, digital positioners with tuning settings and digital control system scan times can’t make the original theoretical concerns less important than the practical issues of real valves. If you would rather believe the controller outputs are the actual valve positions, and just want valve problems to slip by, save some bucks on your project and only put positioners on slow loops. Just don’t stick around for start up.