Terry described an innovative technique of using the PID block for combining split ranged control and valve position control (see Terry's Oct 16 entry). This technique eliminates the limit cycle at the split range point caused by the increase in nonlinearities and the decrease in resolution imposed by backlash, backfilled pipes and dip tubes, rangeability limits, and friction particularly associated with starting a flow from zero. This technique also eliminates the conceptual and tuning problems with valve position control. People tend to confuse valve position control with valve positioners or digital valve controllers. The tuning of the integral-only controller for valve position control is much more critical than most people realize to prevent interaction but provide a fast enough response to reject large load upsets. The best quantitative analysis I have seen on the severity of the tuning issues with "valve position control" is the article by Cheng-Ching Yu and William L. Luyben titled "Analysis of Valve-Position Control for Dual-Input Processes" (Ind. Eng. Chem. Fundam. Vol. 25, No. 3, 1986 pp 344-349).
Instead of a special network for PID control, a standard Model Predictive Control (MPC) block can be configured to eliminate the need for split ranged control and valve position control. The MPC is simply set up for two manipulated variables (MV), one controlled variable, and one optimization variable. The optimization variable is the manipulated variable that provides the finest control (e.g. set point of the fastest and most precise control valve or variable speed drive). The optimization objective is to gradually return the "fine" MV to a mid range (e.g. 50%) after helping the "coarse" MV reject a load upset or minimize overshoot of a new set point. To insure the optimization takes a back set to tight regulation and set point response of the controlled variable, the "penalty on error" (PE) of the optimization variable is decreased (e.g. optimization variable PE=0.1).
When the MV have different process dynamics, the advantage of MPC is greater. By the automatic identification and incorporation of the MV dynamics in MPC, better feedback, feedforward, and constraint control is possible. The longer term view of the MPC also makes it less sensitive to resolution limits. Additionally, the "maximum MV rate" parameter can be written to zero when the controlled variable is close enough to set point to eliminate the limit cycle from the "coarse" MV. The following white paper discusses in more detail this use of a MPC to eliminate split ranged control and valve position control. The article titled "A Fine Time to Break Away from Old Valve Problems", in the November 2005 issue of Control magazine provides more background and a perspective.
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