Feed forward control

Feedback control can never be perfect as it only reacts to the disturbances which are already measured in the system output. The feed-forward method tries to eliminate this drawback by an alternative approach. Instead of using the process output, the measured variable is taken as the measured inlet disturbances and its effect on the process is anticipated via the use of a model. The action is taken on the manipulated variable using the model to relate the measured variable at the inlet, the manipulated variable and the process output. The success of this control strategy depends largely on the accuracy of the model prediction, which is often imperfect as models can rarely predict the 

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A fixed-bed reactor for this hydrolysis that uses feed-forward control has been described (11) the reaction, which is first order ia both reactants, has also been studied kiaeticaHy (12—14). Hydrogen peroxide interacts with acetyl chloride to yield both peroxyacetic acid [79-21-0] and acetyl peroxide... 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

A reflux reduction of 15% is typical. Improved control achieves this by permitting a reduction in the margin of safety that the operators use to handle changes in feed conditions. The key element is the addition of feed-forward capabiUty, which automatically handles changes in feed flow and composition. One of the reasons for increased use of features such as feed-forward control is the reduced cost of computers and online analy2ers. 

In this short initial communication we wish to describe a general purpose continuous-flow stirred-tank reactor (CSTR) system which incorporates a digital computer for supervisory control purposes and which has been constructed for use with radical and other polymerization processes. The performance of the system has been tested by attempting to control the MWD of the product from free-radically initiated solution polymerizations of methyl methacrylate (MMA) using oscillatory feed-forward control strategies for the reagent feeds. This reaction has been selected for study because of the ease of experimentation which it affords and because the theoretical aspects of the control of MWD in radical polymerizations has attracted much attention in the scientific literature. 

Figure 2.36. Feed-forward control with additional feedback loop.

Determine the transfer functions of the feed-forward control scheme assuming linear operation and negligible distance-velocity lag throughout the process. Comment on the stability of the feed-forward controllers you design. 

From this equation the steady state, feed-forward control value of F can be calculated to compensate for variations in CA0 with respect to time and based on the setpoint values for k and CA. 

CA may be substituted from the steady state mass balance to give the steady state feed-forward control equation for jacket temperature Tj... 

Where the pathway generates an allosteric regulator, the opportunity for feedback or indeed feed-forward control arises. Feedback control is exerted by a product of the pathway and acts to switch off a key control enzyme when the concentration of the product reaches a threshold. This indicates the cell contains enough of that particular... 

Feed-forward control is more likely to be focused on a reaction occurring at or near the end of a pathway. Compounds produced early in the pathway act to enhance the activity of the control enzyme and so prevent a back log of accumulated intermediates just before the control point. An example of feed-forward control is the action of glucose-6-phosphate, fructose-1,6-bisphosphate (F-l,6bisP) and phosphoenol pyruvate (PEP), all of which activate the enzyme pyruvate kinase in glycolysis in the liver. 

Finally, if these sensors are necessary under a process control point of view, and by taking into account the fact that in order to install two sensors, one in the input and the other at the output, is not financially possible, the limited knowledge of the bioprocess and their dubious character encourage control engineers to consider feedback control rather than feed-forward control. A sensor, if it is available, will preferably be installed at the output of the process rather than at the input. 

Two main control schemes exist feedback control and feed-forward control (Fig. 6). In feedback control (by far the most common), system performance is monitored, deviations from desired conditions are quantified, and controlled variables are modified to return the system to the desired state. In feed-forward control, process inputs are monitored. As they deviate from desired values, their effect on the system is predicted, and controlled variables are modified to minimize their effect. Feedback control is safer, since it guarantees performance by controlling it directly, but it is also slower corrective action is taken only after the perturbation has affected process performance. Feed-forward control is faster it acts on input deviations as soon as they are detected. However, it is riskier if the detected... 

A second pattern of allosteric control may be referred to as precursor activation or feed-forward control. 

The first involves various feedback and feed forward control mechanisms associated with metabolic pathways. Here a chemical present in the cell, usually an end product of a metabolic sequence, will influence the activity of an enzyme at the beginning of the pathway. This is usually achieved by the presence of an allosteric enzyme whose properties are significantly changed by effector molecules (Fig. 5.31). 

Forcing function is a term given to any disturbance which is externally applied to a system. A number of simple functions are of considerable use in both the theoretical and experimental analysis of control systems and their components. Note that the response to a forcing function of a system or component without feedback is called the open-loop response. This should not be confused with the term open-loop control which is frequently used to describe feed-forward control. The response of a system incorporating feedback is referred to as the closed-loop response. Only three of the more useful forcing functions will be described here. 

Flo. 7.68. Feed-forward control of the composition of the overhead product stream of a distillation column where feed composition is the only load variable... 

A typical arrangement is illustrated in Fig. 7.68 where the variations in feed composition are measured by a suitable composition analyser (An) (see Section 6.8). The signal from the analyser is fed directly to the feed-forward controller, the output of which is cascaded on to the set point of the reflux flow controller (see also Section 7.13). If the transfer functions relating feed composition Xp, reflux flowrate R and overhead product composition xD are known, then we can write ... 

GFf(s) is the transfer function of the feed-forward controller which is obtained from the known transfer functions Gt(s) and G2(j) using equation 7.162. 

Substantial effort in modelling and/or experimental measurement is required in order to derive GFFA(s) and GFFB(s). Due to errors in determining the individual transfer functions (GM(s), G 2(s), etc.), to errors in measurement, and to load variables which have not been accounted for in the models, feed-forward compensation can never be perfect, and considerable drifting of the controlled variable(s) can occur. On the other hand, the two variable feed-forward control model expressed by equation 7.165 automatically takes into account any interaction between the reflux and steam flow control loops (see also Section 7.15). 

Fig. 7.69. Simultaneous feed-forward control of overhead and bottoms compositions in the face of variations in feed composition...

The positive power of the exponent in Gff(s) implies that the feed-forward controller requires values of the load disturbance measured at some future time. This is clearly impossible. 

Imperfections in feed-forward control can often be overcome by the addition of suitable feedback action. A typical design is shown in Fig. 7.70 where any variations in xd which occur bring the feedback control loop into action. The reflux flow is shown on flow control in cascade with the boiling temperature of the liquid at an appropriate point within the column. The inner (or slave) flow controller maintains... 

WARDLE, A. P. and Wood, R. M. /. Chem. E./European Federation of Chem. Eng. Int. Symp. on Distillation (1969) Session VI, 66. Problems of application of theoretical feed-forward control models to industrial scale fractionating plant. 

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