Feedforward control references

In most process plant situations where feedforward control is appropriate, a combination of the feedforward and feedback control is usually used. The feedforward portion reduces the impact of measured disturbances on the controlled variable while the feedback portion compensates for model inaccuracies and unmeasured disturbances. This control strategy is referred to as feedforward control with feedback trim. 

The tuning of the controller in the feedback loop can be theoretically performed independent of the feedforward loop (i.e., the feedforward loop does not introduce instability in the closed-loop response). For more information on feedforward/feedback control appications and design of such controllers, refer to the general references. 

From the design equations (21.9) and (21.10) it is clear that a feedforward controller cannot be a conventional feedback controller (P, PI, PID). Instead, it should be viewed as a special-purpose computing machine. This is the reason it is sometimes referred to as a feedforward computer. 

Ratio control is a special type of feedforward control where two disturbances (loads) are measured and held in a constant ratio to each other. It is mostly used to control the ratio of flow rates of two streams. Both flow rates are measured but only one can be controlled. The stream whose flow rate is not under control is usually referred to as wild stream. 

Chapter 21. Chapter 7 in Shinskey [Ref. 3] is again an excellent reference for the practical considerations guiding the design of feedforward and ratio control systems. It also discusses the use of feedforward schemes for optimizing control of processing systems. Good tutorial references are the books by Smith [Ref. 2], Murrill [Ref. 8], and Luyben [Ref. 9]. The last one has a simple but instructive example on the nonlinear feedforward control of a CSTR. 

Feedback control only takes action after an error has developed. The error must be large or persistent before feedback control makes a large correction for an upset. Feedforward control has the noble idea of calculating a corrective action to compensate for an upset before an error appears. The most common load upsets (disturbances) are influent feed and composition, flow feedforward is widely used and is often referred to as ratio control since the reagent flow is ratioed to the influent flow. The... 

Owing to this nonlinearity, as well as to the problems mentioned earlier, some exothermic reactors are controlled with advanced control techniques, such as feedforward, model reference or adaptive control [11]. 

B gives input AGC, in which the gain and maximum output level are simultaneously adjusted by the volume control but where the input-referred compression threshold is unaffected. A separate trimmer adjustment is normally provided for the compression threshold. Control point A for the volume control gives an example of output AGC, in which the volume control simultaneously adjusts the gain and compression threshold, and a separate trimmer is used to set the maximum output level. Another option is feedforward compression, in which the detector is driven directly by the microphone signal a delay in the amplified signal relative to the control signal can then be used to reduce the attack overshoot in the compression circuit [Verschuure and Dreschler, 1993],... 

R.D. Bartusiak, C. Georgakis, and M.J. Reilly. Nonlinear feedforward/feedback control structures designed by reference synthesis. Chemical Engineering Science, 44 1837-1851, 1989. 

The most popular tool for monitoring single-loop feedback and feedforward/feedback controllers is based on relative performance with respect to minimum variance control (MVC) [53, 102[. The idea is not to implement MVC but to use the variance of the controlled output variable that would be obtained if MVC were used as the reference point. The variation of the inflation of the controlled output variance indicates if the process is operating as expected or not. Furthermore, if the variance with a MVC is larger than what could be tolerated, this indicates the need for modification of operating conditions or process. 

Figure 18 shows the displacement response of IPMC subjeeted to the exponential reference under feedforward/feedback control for 20 min (1200 s). It can be seen that with this control method, the IPMC is able to maintain a steady position for the entire period. As seen in Fig. 18, under open loop condition, the IPMC begins to relax after approximately 10 s. In case of feedforward/feedback control, there was no indication of back relaxation for a period 120 times longer. 

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