Feedforward control and feedback

Generic Control Strategies. The two generic strategies for process control are feedback and feedforward control. Most process control strategies are based on one or a combination of these strategies (1 3). 

It should also be noted that the analyses of delay times discussed above do not distinguish between feedback and feedforward control. In practice, disturbances may often be measured with a smaller delay than the delay between the disturbance and any of the constrained outputs. This allows feedforward control to make a contribution to achievable control performance which should be considered explicitly. 

The MFC algorithm used for the control of the wastewater treatment unit uses the combination of feedback and feedforward control design in order to reduce the effects of the large time constants shown by the process. 

Flow rate and oxygen consumption changes have been superimposed on the capillary-tissue process block as automatic feedback and feedforward control loops (see Figure 2). 

The experimental curve in Figure 3 demonstrates overshoot in the tissue oxygen response. It was determined previously (22) that a term representing pure delay along with the steady-state blood flow vs. arterial oxygen tension data would cause overshoot. In this investigation it was found that a first-order time constant delay would also produce overshoot. Therefore, since exact controller mechanisms are not being postulated, the flow controlled dynamics used in this study include pure delay and time constant lag. To consider the problem of sensor location, feedback and feedforward control loops were superimposed on the capillary-tissue model. 

Describe the steps that you would go through in designing a control system for maintaining the pH of the liquid in a stirred tank (see Figure QI.2) at a desired value What questions must you resolve Develop both feedback and feedforward control configurations for this system. 

Returning to the tank heater example, we realize that we can use a different control arrangement to maintain T = Ts when T, changes. Measure the temperature of the inlet stream T, and open or close the steam valve to provide more or less steam. Such a control configuration is called feedforward control and is shown in Figure 1.4. We notice that the feedforward control does not wait until the effect of the disturbances has been felt by the system, but acts appropriately before the external disturbance affects the system, anticipating what its effect will be. The characteristics of the feedback and feedforward control systems will be studied in detail in subsequent chapters. 

The use of control structures is an advanced flowsheeting feature. Flowsheet controllers are particularly useful in plant operation. An important application is the simulation of the steady state behaviour of SISO controllers. With respect to control action we can distinguish between two basic types, feedback and feedforward control. 

Figure 24.3. Feedback and feedforward control systems. In feedback control, a measuring instrument obtains information at the output of a process, the signal obtained is compared to a set point, and the difference (or result) is applied to a final actuator. The result is ultimately detected by the measuring instrument and closed-loop control results. In feedforward control a measuring instrument obtains information at the input of a process, the signal obtained is again compared to a set point, but now the result is applied to an actuator that controls another input to the process. The result is not detected by the measuring instrument and open-loop control results. Courtesy of the Foxboro Company.

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. 

There are two key mechanisms in control theory to keep variations between limits feedback and feedforward control (Goodwin et al, 2001). With feedback control, a system maintains its homeostatic action through adjusting the system according to monitored output properties. 

As mentioned earlier, successful operation of a process requires that key process variables such as flow rates, temperatures, pressures, and compositions be operated at or close to their set points. This Level 3a activity, regulatory control, is achieved by applying standard feedback and feedforward control techniques (Chapters 11-15). If the standard control techniques are not satisfactory, a variety of advanced control techniques are available (Chapters 16-18). In recent years, there has been increased interest in monitoring control system performance (Chapter 21). 

Two physical examples, a continuous blending system and a distillation column, have been used to introduce basic control concepts, notably, feedback and feedforward control. We also motivated the need for a systematic approach for the design of control systems for complex processes. Control system development consists of a number of separate activities that are shown in Fig. 1.10. In this book we advocate the design philosophy that for complex processes, a dynamic model of the process should be developed so that the control system can be properly designed. 

The basic process control system (BPCS) consists of feedback and feedforward control loops that regulate process variables such as temperatures, flow rates, liquid levels, and pressures. Although the BPCS typically provides... 

In Chapter 11 it was shown that the roots of the characteristic equation completely determine the stabihty of the closed-loop system. Because Gf does not appear in the characteristic equation, the feedforward controller has no effect on the stability of the feedback control system. This is a desirable situation that allows the feedback and feedforward controllers to be tuned individually. 

In industrial plants, large numbers of process variables must be maintained within specified limits in order for the plant to operate properly. Excursions of key variables beyond these limits can have significant consequences for plant safety, the environment, product quality, and plant profitability. Earlier chapters have indicated that industrial plants rely on feedback and feedforward control to keep process variables at or near their set points. A related activity, process monitoring, also plays a key role in ensuring that the plant performance satisfies the operating objectives. In this chapter, we introduce standard monitoring techniques... 

A simple example that illustrates the roles of feedback and feedforward control in nature is the heat shock response exhibited by simple bacteria (El-Samad et al., 2006), as illustrated in Fig. 24.1. When the organism experiences an increase in temperature, it leads to the misfolding of protein, which disrupts a number of... 

Figure 24.1 Feedback and feedforward control loops that regulate heat shock in bacteria (modified from El-Samad, et al., 2006) (positive feedback is common in biological systems).

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