Feedbackfeedforward Combination Control

The fractional control signal (XB) to the valves is a function of the characterized flow signal f(F'). Therefore, the total output signal to the split-ranged valves (XB) becomes:

2(feedback output)

Figure 7.40.26 illustrates a feedforward control system arrangement in which the flow characterization of the influent wastewater flow signal and the use of the dead-band adjustment feature previously discussed (Figure 7.40.3) are shown. A system as outlined in Figure 7.40.26 has demonstrated the need for the combination of feedforward-feedback control, because each type of control was tested individually and was found to be unsatisfactory.

A combination of feedforward-feedback on one side of neutrality and conventional feedback on the other is also possible. The nature and characteristics of the problem to

FIG. 7.40.26 Three-valve feedforward pH control system. aIf pHC-2 is provided with a dead band, it will be inactive when the trimming controller (pHC-1) alone is operating.

REACTION TANK 1 REACTION TANK 2

FIG. 7.40.25 Cascade residence control of pH. ©1999 CRC Press LLC

FIG. 7.40.26 Three-valve feedforward pH control system. aIf pHC-2 is provided with a dead band, it will be inactive when the trimming controller (pHC-1) alone is operating.

be solved will indicate the nature of the solution.

Figure 7.40.27 shows how pH feedforward is used to rapidly position a large valve for major upsets. In this configuration an integral-only valve position controller (VPC) slowly optimizes the large valve position to keep the trim valve in the middle of its throttling range. Since the feedback manipulation of the big valve must be slow to prevent interaction between the two valves, the pH feedforward action provides a performance edge of rapid action for major upsets. Other methods to coordinate the movement of a large valve and a trim valve involve the use of output tracking strategies between two pH controllers, dedicated and tuned for each valve, so that both controllers are not in service at the same time.

Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment