Model-Based Control of Fast Processes

Online Visitor Asks About Applications That You Wish You Could Use APC on, but Can't Do Because of the Speed, Cost and Complexity of Traditional Approaches to Advanced Control

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For example, we've written generic control algorithms for multiple-input, adaptive feed-forward, decoupling, and "smart" PID in the TDC 3000 Application Module and other DCSs. If designed properly, the feedforward and decoupling algorithms provide a form of "model-predictive" control, and the "smart" PID algorithm actually provides feedback control that is superior to conventional MPC. (Remember, the only mechanism for feedback control with conventional MPC is by adjusting the bias between the predicted and actual values of the control variables at each execution—sort of integral-only control.)

These algorithms can be executed every 1-2 sec, if needed. Versions of these algorithms will obviously also run in Foxboro I/A, DeltaV, Experion, Yokogawa and any other DCS that provides the control engineer with the ability to implement user-written programs of medium complexity.

If you'll send me a sketch of the dual-vaporizer system, showing all the control valves and process measurements, I will be happy to suggest a control solution that could be implemented in the DCS—assuming you have a DCS that provides some programming capability.

Dr. James R. (Jim) Ford, PE

A: This seems to be a general question, so here is a general answer. I too think that as long as the controller can calculate appropriate MV action and the final control element can act faster than the process, then control should be good. But, there are two qualifiers: appropriate and faster.

Appropriate: If the missile aerodynamic model is generated for mid-tunnel performance and doesn't include near-wall drag, then control may deteriorate near the wall. In the chemical industry, MPC applications deteriorate when the process moves away from the operating conditions that generated the model, and enter a region that changes the nature of influences.

Appropriate Controller Tuning: A nonlinear process would need the controller to be tuned differently for steering in one direction than another as the process industry also experiences processes that move faster in one direction than another, or in one operating condition than in another. Gain scheduling is a common solution. If the model is appropriate, then in MPC, either the move suppression factor, or the trajectory damping factor could be adjusted dependent on the conditions.

Faster:  A rule of thumb is that there should be 10 control actions within one process time constant. This could be expanded to include delay or dead time and the final element dynamics. The control influence (controller plus final element) should have a response time (delay plus three time constants) that is at least one-tenth of the process response time (delay plus three time constants).

Further, if there are secondary controllers, they should have a response time that is at least one-fifth of the primary controller. The rule needs to guide control frequency and final element dynamics in the operating conditions that lead to the fastest process response.

R. Russell Rhinehart, PhD

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