Feedforward-feedback control

The combination of feedforward and feedback control provides a very powerful practical strategy for the control of polymer properties such as composition and molecular weight. Typically it is still fairly difficult to have on-line direct measurements of polymer composition, so the control design has to incorporate the avail- 

Feedforward controller, previously discussed, which maintains the total monomer feed rate and monomer A feed composition at the appropriate set points for the specific polymer grade being produced. The feedforward controller can be easily extended to n monomers by specifying the total monomer reactor feed rate and the feed composition for n — 1 monomers. 

Gas chromatograph feedback controller, which uses the velocity algorithm for digital PID control, discussed in Section 12.4.3.4 to calculate flow correction factors for the monomers from the gas chromatograph measurement of the actual monomer feed composition. This controller provides the necessary integral action so that the offset between the actual reactor monomer feed composition and its set point, which may be caused by flowmeter inaccuracies or other unmeasured disturbances, is niinimized. 

Polymer composition feedback controller, which updates the set points for the reactor monomer feed composition based on the laboratory analysis of a reactor sample. This controller thus provides the necessary integral action so that the offset between the measured composition of the reactor sample and the polymer grade composition is minimized. 

It is important to note that these feedforward and feedback controllers have been designed hierarchically, in the sense that each level in the structure will not activate unless the levels below it are functioning properly. Furthermore, in practice extensive data validation checks must be incorporated so that robust performance can be assured even when the gas chromatograph or laboratory analysis measurements may be unavailable or faulty. 

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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. 

Apply classical controller analysis to cascade control, feedforward control, feedforward-feedback control, ratio control, and the Smith predictor for time delay compensation. 

Figure 10.4. (a) A feedforward-feedback control system, (b) The diagram after moving G vGp. 

Figure 11.5h shows a combined feedforward-feedback system where the feedback signal is added to the feedforward signal in a summing device. Figure 11.Sc shows another combined system where the feedback signal is used to change the feedforward controller gain in the ratio device. Figure 11.6 shows a combined feedforward-feedback control system for a distiltetion column where feed-rate disturbances are detected and both steam flow and reflux flow arc changed to hold both overhead and bottoms compositions constant. Two feedforward controllers are required.

Feedforward-feedback control with udditive signals... 

H.O. Mendez-Acosta, D.U. Campos-Delgado, R. Femat, and V. Gonzalez-Alvarez. A robust feedforward/feedback control for an anaerobic digester. Comput. Chem. Eng., 31 1-11, 2005. 

FIG. 8-34 A block diagram of a feedforward-feedback control system. (Source Sehorg et at., Process Dynamics and Control, 2d ed., Wiley, New York, 2004.)... 

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. 

For further improvement of the control performance the MPC feedforward-feedback control structure is proposed. The nitrate and DO concentrations in the inlet flow of the first anoxic reactor are measured and used for feedforward control, while the nitrate concentration in the second anoxic reactor together with the DO concentration in the last aerated rector are used for feedback control. The combined feedforward-feedback MPC is presented in Fig. 5. 

Desborough, L. Harris, T. Performance Assessment measures for univariate feedforward/ feedback control. Can. J. Chem. Eng. 1993, 71, 605-616. 

A generalized schematic diagram for feedforward/feedback control of particle size in a continuous or semicontinuous crystallizer, such as that in Fig. 7-5, is shown in Fig. 7-6. The measurement device can be most easily envisioned as an in-line particle size analyzer such as a Lasentec focused beam reflectance measurement (FBRM), but it could also be a sampler/ offline device combination (even a plain microscope). The most common variant on this control schematic would be elimination of feedforward control, and perhaps of the fines trap as well. 

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. 

We would expect that a combined feedforward-feedback control system will retain the superior performance of the first and the insensitivity of the second to uncertainties and inaccuracies. Indeed, any deviations caused by the various weaknesses of the feedforward control will be corrected by the feedback controller. This is possible because a feedback control loop directly monitors the behavior of the controlled process (measures process output). Figure 21.7 shows the configuration of a combined feedforward-feedback control system. 

Example 21.4 Feedforward-Feedback Control of the Tank Heater... 

Figure 21.7 Generalized block diagram for feedforward-feedback control.

A close examination of eq. (21.19), which yields the closed-loop process output under feedforward-feedback control, reveals the following characteristics ... 

In Section 21.4 it was claimed that the stability characteristics of a feedforward-feedback control system are affected only by the feedback loop. Explain why. 

V.25 (a) Develop a feedforward-feedback control system for a process with the following transfer functions ... 

Feedforward Feedback Control. If the process exhibits slow dynamic response and disturbances are frequent, then the use of feedforward control with feedback control may be advantageous. Feedforward control differs from feedback control in that the primary disturbance is measured via a sensor and the manipulated variable is adjusted so that, ideally, the controlled variable does not change. As shown in Figure 9.11, the controller calculates the manipulated variable, u, needed to counteract the system upset introduced through the disturbance, d. By taking control action based on measured disturbances, rather than controlled variable error, the controller can reject disturbances before they affect the controlled variable, y. 

Figure 9.12 Schematic diagram of feedforward/feedback control of a continuous crystallizer, /n, = moment ofCSD.

Block diagrams, (a) Openloop. (b) Feedback control, (c) Feedforward control, (d) Combined feedforward/feedback control. 

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