Ask Greg McMillan
We ask Greg:
What role do you see dynamic simulation playing in the future of adaptive Cascade Control?
Greg's Response:
Dynamic first principle simulations with instrumentation dynamics included can help find, test, and tune the inner loop for cascade control so that it can correct for disturbances before they significantly affect the outer loop. The outer closed loop time constant needs to be 5 times larger than the inner closed loop time constant to reduce oscillations from interactions between the loops. This is best achieved by fast process and instrumentation dynamics in the inner loop.
Often unrecognized are the rangeability problems of the inner loop measurement especially for differential head and vortex flow meters.
Online dynamic simulations can bumplessly substitute an inferential measurement based on installed valve characteristic and stream operating conditions when the rangeability is exceeded.
Online dynamic simulations can also adapt PID tuning so that the outer closed loop time constant stays 5 times larger than the inner closed loop time constant for increases in process and sensor delays and lags for low production rates and fouling.
A dynamic simulation in a Digital Twin can test the setup and show the value of filtered positive feedback integral action to suppress oscillations. Chapter 11 in my Tuning and Control Loop Performance Fourth Edition (free to download) has the recommendations, test results, and key points for cascade control.