Decoupling control system

Decoupling Control Systems Decoupling control systems provide an alternative approach for reducing control loop interactions. The basic idea is to use additional controllers called decouplers to compensate for undesirable process interactions. 

As an illustrative example, consider the simplified block diagram for a representative decoupling control system shown in Fig. 8-41. The two controlled variables Ci and Co and two manipulated variables Mi and Mo are related by four process transfer functions, Gpn, Gpi9, and pie, Gpii denotes the transfer function between Mi... 

Figure 1. Schematic of closed loop, decoupled control system.

The output of a feedback controller is an unknown variable. If it were known, or predictable with sufficient accuracy, the controller would not be needed. But the output eventually finds its correct level, and in so doing, solves for all the unknowns in the process. Thus the controller output contains more information regarding the manipulated variables than either measurements or set points. The set point will not indicate the presence of a load change, for example. So for the most part, controller outputs will be combined to perform decoupling. The general configuration of a coupled process and decoupling control system would appear as shown in Fig. 7.8. 

Consider how a decoupling control system might be designed for the blending process of Example 7.4. The mathematical model is solved for OTi and mt in terms of F and x ... 

FIG 7.9. The decoupling control system should be the image of the process. 

Classify the five controlled variables appearing at the beginning of the section Decoupling Control Systems. If the composition of only one product needed to be controlled, what could be done with the extra manipulated variable ... 

Heavy distillate is manipulated to control interface level in the decanter. Because reflus flow is less than that of light distillate, it can be accurately manipulated for composition control decanter level is then controlled by manipulation of light distillate. Vapor and reflus flow interact in their effect on bottoms composition. They can be determined from feed composition by V = F Zc Zb), L = F 8zc 2zb), Two bottoms-composition controllers are necessary, their outputs ms and me taking the place of the unknown feed compositions in the previous equations. Then the decoupling control system manipulates heat input and reflus with a forward loop from feed rate V = F 9m.c — ms), L = F 8nic — 2m ). 

One type of decoupling control system for a 2 X 2 process and a 1-1/2-2 control configuration is shown in Fig. 18.9. Note that four controllers are used two conventional feedback controllers, and Gc2, plus two decouplers, T12 and T2i. The input signal to each decoupler is the output signal from a feedback controller. In Fig. 18.9, the transfer functions for the trans-... 

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