Improve Control with Regulatory Level Transformations

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: One of the best things a control engineer can do to help Operations, as well as advanced control implementers, is to implement fundamental control techniques at the regulatory level. Having well-tuned (not necessarily "over-tuned) PID loops, as well as properly installed valves and positioners, at the regulatory level is important. One feature that normally goes unnoticed is the ability to characterize the PID output to the valve (e.g. output characterization block). I have one particular client who is a guru at using this technique and it has substantially settled out the process (with or without advanced control).

: Let me give an example. Many valves, e.g. louvers, act highly non-linear. Maybe they go off control at 60%. Well if the controller ever gets saturated it takes forever to get the controller valve back within a controllable range; thus the temperature will fall substantially below setpoint. With the output characterization algorithm the controller will "whip" through the deadband and the overall control will be greatly improve.

: While it is true you can do some of this "transformation" at the higher level, it is much better at the lower level. If you do it at the lower level, the responses are then linear (if implemented correctly) for the upper advanced control.

: Try it and you will be glad you did!!!


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