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Feb
18

Dynamic Reset Limit

I just realized external reset is the key to sustainable manufacturing and in particular abnormal situation prevention and optimization. I knew the PID option “dynamic reset limit” where the external reset signal is a secondary loop PV, variable frequency drive speed, or control valve position, was important. I demonstrated in Deminar # 3 how dynamic reset limit option can eliminate the insidious burst of oscillations in a primary process loop that occurs for large setpoint changes or load disturbances when the primary PID integral action is faster than the secondary loop or final control element can respond.

As I was thinking about how the enhanced PID developed for wireless (DeltaV PIDPlus) would eliminate limit cycles in Valve Position Control , I thought what if I add a set point velocity limit in the analog output block to slow down a transition when the enhanced PID decides a correction needs to be made. Since the dynamic reset limit is automatically enabled when the PIDPlus option is chosen, integral action would not try to make the PID go faster than the velocity limit. Then I thought there is often a need for a “Cautious Approach and Fast Getaway Scenario”, such as we encounter in the prevention of compressor surge, exothermic reactor runaway, RCRA pH violation, and Bioreactor biomass starvation. Normally I would use a kicker because the tuning of the controller for drastically different speeds of actuation is problematic. If I use the dynamic reset limit option I don’t have to tune the controller based on the direction and I don’t need to excessively be concerned about the exact value of the velocity limit. Also, I can change my mind without having to retune the PID.

There are many more examples were an intelligent adaptation of the speed of actuation of the final control element or secondary loop could be beneficial. In general, you want to approach optimums slowly to minimize disruption but as you operate close to the edge, you depend upon a fast recovery to prevent going over the edge. With compressor surge control the edge is literally a cliff. While other applications might not be as dramatic, you can see how the dynamic reset limit could enable a wide spectrum of PID techniques for sustainable manufacturing, which in its broadest definition includes flexibility, operability, and profitability.