Feedforward control scheme

Manual control implies that the operator makes the changes in manipulated variable, which is used occasionally. Feedback control connotes that the manipulated variable is changed automatically in response to the error between the set point and controlled variable. The second approach serves as the basis for most automatic control schemes. Feedforward control is a technique where the manipulated variable is changed as a function of a disturbance variable. Both feedback and feedforward control are discussed in later sections of this chapter. 

The basic control scheme used for the perfect control analysis, a single inline feedback loop, gives 10 times the allowable concentration variation at the exit of the second tank for the predicted worst case. Including feedforward reagent addition would be insufficient by itself to give the tenfold improvement required, due to a 20% error in the estimated load (inferred from pH), so an additional in-line ratio feedback controller was added between the two tanks. As an additional actuator was therefore available at no extra cost, lead-lag feedforward from the load error at the first controller to the second controller actuator was added. To reduce feedforward dynamic mismatch, the lag was set to approximately the residence time of the first tank. The lead constant was added to the design parameters. 

The second level is the advanced and predictive control. These are two different control schemes that work at the same level. Information is transmitted horizontally and vertically in this (and upper) level. More elaborated control strategies as selective control, ratio control, feedforward control are implemented. In this second level implicit as well as explicit (heuristic and first principles based) models are used to generate the action. The action is the set point (goal) to achieve at the lowest level. Prediction horizon is (in the case of model predictive control) of tens of movements. 

It is clear that our first reaction is to use a composition analyzer to measure the concentration of pentane in the distillate and then using feedback control to manipulate the reflux ratio, so that we can keep the distillate 95% in pentane. This control scheme is shown in Figure 2.2a. An alternative control system is to use a composition analyzer to monitor the concentration of pentane in the feed. Then in a feedforward arrangement we can change the reflux ratio to achieve our objective. This control... 

Part V (Chapters 19 through 22) deals with the description, analysis, and design of more complex control systems, with one controlled output. In particular, Chapter 19 introduces the concept of feedback compensation with Smith s predictor, to cope with systems possessing large dead times or inverse response. Chapter 20 describes and analyzes a variety of multiloop control systems (with one controlled output) often encountered in chemical processes, such as cascade, selective, and split-range. Chapter 21 is devoted exclusively to the analysis and design of feedforward and ratio control systems, while Chapter 22 makes a rather descriptive presentation of adaptive and inferential control schemes why they are needed and how they can be used. 

The turbulent flow control can be implemented through different control schemes [1]. They are predetermined open-loop control, reactive feedforward open-loop control, and reactive feedback closed-loop control. The physical arrangement of sensors and actuators depends on the nature of control schemes and flow characteristics. The criteria for selection of appropriate actuators have been discussed in next section. It should be noted that the actuators require parasitic power supply for its operation. This fact should be taken into consideration for overall performance analysis of actuators. 

For proper combination tower control, it would appear that a feedforward computer-controlled system is required since conventional automatic control seems to lag behind heat-loss effects caused by coke drum warm-up and switch-over. Many refiners have implemented such advanced control schemes with good success. 

The constructive method, which is considered as a major breakthrough in control theory, was developed in the last decade. As it stands, the method is intended for feedback control design, and its application to the batch motion case requires the nominal output to be tracked and a suitable definition of finite-time batch motion stability. In a more applied eontext, the inverse optimality idea has been applied to design the nominal motion of homo [11] and copolymer [12] reactor, obtaining results that are similar to the ones drawn from direct optimization [4]. The motion was obtained from the recursive application of the process dynamical inverse [13], and the inverse yielded a nonlinear SF controller [9, 10] that was in turn used to specify a conventional feedforward-feedback industrial control scheme. However, the issues of motion stability and systematized search were not formally addressed. 

Due to the above-mentioned difficulties with manual and temperature-based feedback controllers, the control systems based on product moisture content have been investigated and applied in many graindrying installations in recent years. Forbes et al. compared different control strategies based on the product moisture content for commercial corn-drying units [49]. Three control schemes were studied (1) a model-based feedforward controller for which the corn-drying process is represented by an exponential-decay-type model with the corn-drying characteristics lumped into a single parameter (2) a feedback... 

Fig. 15 Feedforward control scheme for sectored IPMC (Fleming et al. 2012) ( lOP Publishing. Reproduced with permission from lOP Publishing. All rights reserved)...

The bottom half of the figure shows the material balance control scheme in which the top quality is controlled by the distillate draw-off However, the draw-off does not affect the top quality but rather the level. The level in turn affects the reflux flow, which subsequently affects the top quality. This means that there is a severe degree of mutual interaction between the control loops. It was found that the control structure of Fig. 34.8b would result in an oscillatory behavior of the quality control loop for feed flow changes of +10%. Only addition of a feedforward loop from distillate flow to reflux would stabilize the quality control loop. In that case, the distillate flow changes were subtracted from the reflux flow changes calculated by the level controller. After addition of feed-forward control, the response of the top and bottom qualities were similar to the responses of the energy balance control scheme for this situation with virtually no deviation from setpoint of the top quality and a bottom quality response similar to Fig. 34.7. 

We will begin with combinations of level control and feedforward compensation for applications where material-balance control is in the direction opposite to flow. Then we will consider schemes in which material-balance control is in the direction of flow. Unfavorable schemes—those that are hard to design or to make work—will be pointed out their use should be avoided unless no suitable option is available. 

For any of the control systems discussed—Figure 19.2, 19.3, or 19.4— assume that top composition and bottom composition are held constant. We wish to find the changes in Lk, D, B, and V, required to hold compositions constant in the face of a feed composition change, Azp. This information could be used to design feedforward compensators to minimize transioit changes in terminal compositions. The variables chosen for feedforward compensation will depend on which feedback control scheme is used—Figure 19.2,19.3, or 19.4. 

Finally, we consider which process variables to measure. Clearly, the three CVs should be measured. It is also desirable to measure the three MVs because this information is useful for controller tuning and troubleshooting. If large and frequent feed disturbances occur, measurements of disturbance variables F and xp could be used in a feedforward control strategy that would complement the feedback control scheme. It is not necessary to measure Tp because sensible heat changes in the feed stream are typically small compared to the heat fluxes in the evaporator. 

The feedforward control scheme in Fig. 15.3 can provide better control of the liquid level. The steam flow rate is measured, and the feedforward controller... 

Fig. 15.9. We wish to design a feedforward control scheme to maintain exit composition jc at a constant set point Xsp, despite disturbances in inlet composition, Suppose that inlet flow rate w and the composition of the other inlet stream X2 are constant. It is assumed that x is measured but that x is not. If x were measured, then feedback control would also be possible. The manipulated variable is inlet flow rate W2- The flow-head relation for the valve on the exit line is given by w = CyVfL Note that the feedforward controller has two input signals the xi measurement jci, and the set point for the exit composition Xj p.

An alternative feedforward control scheme for the blending system is shown in Fig. 15.10. Here the feedforward controller output signal serves as a set point to a feedback controller for flow rate W2. The advantage of... 

Feedfoward control is normally implemented in conjunction with feedback control. Tuning procedures for combined feedforward-feedback control schemes have been described in Section 15.7. For these control configurations, the feedforward controller is usually tuned before the feedback controller. 

It is desired to design a feedforward control scheme in order to control the exit composition X4 of the two-tank blending system shown in Fig. E15.13. Flow rate 2 can be manipulated, while disturbance variables, and X5, can be measured. Assume that controlled variable X4 cannot be measured and that each process stream has the same density. Also, assume that the volume of liquid in each tank is kept constant by using an overflow line. The transmitters and control valve have negligible dynamics. 

Evaluate the use of enhanced single-loop control strategies, including feedforward, ratio, cascade, and selective control schemes. 

B.l. Evaluate the use of advanced single-loop control strategies, including feedforward, ratio, cascade, and selective control schemes. In reviewing the plant processing and control objectives, a variable that needs... 

A control scheme that predicts the effect of a load change and takes corrective action before the controlled variable is affected, Q.g. feedforward control. 

The effect of the disturbance on the controlled variable These models can be based on steady-state or dynamic analysis. The performance of the feedforward controller depends on the accuracy of both models. If the models are exac t, then feedforward control offers the potential of perfect control (i.e., holding the controlled variable precisely at the set point at all times because of the abihty to predict the appropriate control ac tion). However, since most mathematical models are only approximate and since not all disturbances are measurable, it is standara prac tice to utilize feedforward control in conjunction with feedback control. Table 8-5 lists the relative advantages and disadvantages of feedforward and feedback control. By combining the two control methods, the strengths of both schemes can be utilized. 

Lee, M., and Park, S., A new scheme combining neural feedforward control with model predictive control. AIChE J., 38, 193 (1992). 

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