Feedforward and Ratio Control

There are many advanced strategies in classical control systems. Only a limited selection of examples is presented in this chapter. We start with cascade control, which is a simple introduction to a multiloop, but essentially SISO, system. We continue with feedforward and ratio control. The idea behind ratio control is simple, and it applies quite well to the furnace problem that we use as an illustration. Finally, we address a multiple-input multiple-output system using a simple blending problem as illustration, and use the problem to look into issues of interaction and decoupling. These techniques build on what we have learned in classical control theories. 

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. 

Chapter 21. Chapter 7 in Shinskey [Ref. 3] is again an excellent reference for the practical considerations guiding the design of feedforward and ratio control systems. It also discusses the use of feedforward schemes for optimizing control of processing systems. Good tutorial references are the books by Smith [Ref. 2], Murrill [Ref. 8], and Luyben [Ref. 9]. The last one has a simple but instructive example on the nonlinear feedforward control of a CSTR. 

In Fig. 8.7c the ratio of the two flows is changed by the output of a composition controller. This system is a combination of feedforward and feedback control. Finally in Fig. %.ld a feedforward system is shown that measures both the flow rate and the composition of the disturbance stream and changes the flow rate of the manipulated variable appropriately. The feedback controller can also change the ratio. Note that two composition measurements are required, one measuring the disturbance and one measuring the controlled stream. 

Once a conceptual control structure has been developed and the plant has been decomposed into subsystems, the control design procedure reverts to a traditional bottom-up approach. However, there are good reasons to treat the different control activities in a multilevel hierarchy, as shown in Fig. H.l. The first task in Step III is to identify the essential controllers, those that are absolutely required. The safety and regulatory levels in Fig. H.l enable safe and stable operation of the plant. The advanced control functions are handled at Level 3 and keep the controlled variables close to their optimum set points through standard methods such as cascade, ratio, feedforward, and multivariable control. Level 4 in Fig. H.1 considers the real-time optimization of the process operations. The purpose of control at this level is to choose operating conditions that meet overall objectives in an economically optimum fashion. 

Ratio control and multiphcative feedforward control, in general, are subject to the same considerations. Ratio control can be of a steady-state or a dynamic form. It is often implemented using a setpoint as the load variable when the load variable has a controller associated with it and the controller is in auto mode. 

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

A host of gadgets and software are available to perform a variety of computations and logical operations with control signals. For example, adders, multipliers, dividers, low selectors, high selectors, high limiters, low limiters, and square-root extractors can all be implemented in both analog and computer systems. They are widely used in ratio control, in computed variable control, in feedforward control, and in override control. These will be discussed in the next chapter. 

In practice, many feedforward control systems are implemented by using ratio control systems, as discussed in Chap. 8. Most feedforward control systems are installed as combined feedforward-feedback systems. The feedforward controller takes care of the large and frequent measurable disturbances. The feedback controller takes care of any errors that come through the process because of inaccuracies in the feedforward controller or other unmeasured disturbances. Figure 11.4d shows the block diagram of a simple linear combined fe forward-/ feedback system. The manipulated variable is changed by both the feedforward controller and the feedback controller. 

The variables indicated by an asterisk ( ) were assigned a priori. The feedforward (FF) trim controller gain is dimensionless as both the input and the output are ratios of flows. The CSTR controller gains are in m /h reagent per m /h acid at maximum flow per pH. Some of the tuning parameters were on bounds, but the associated Lagrange multipliers did not indicate a significant incentive to rerun the optimization. 

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. 

Starting with Part IV, our main concern will be How can we control a process in order to exhibit a certain desired response in the presence of input changes First, we will study the most common control configuration, known as feedback, which we touched upon very briefly in Chapter 2. Then, in Parts V and VI we will discuss additional control configurations such as feedforward, cascade, ratio, override, split range, and multivariable. 

Although feedback control is the type encountered most commonly in chemical processes, it is not the only one. There exist situations where feedback control action is insufficient to produce the desired response of a given process. In such cases other control configurations are used, such as feedforward, ratio, multivariable, cascade, override, split range, and adaptive control. 

In this section we study the characteristics of feedforward control systems and describe the techniques that are used for their design. In the final section we examine a special case of feedforward control, ratio control. 

Ratio control is a special type of feedforward control where two disturbances (loads) are measured and held in a constant ratio to each other. It is mostly used to control the ratio of flow rates of two streams. Both flow rates are measured but only one can be controlled. The stream whose flow rate is not under control is usually referred to as wild stream. 

Flow controllers set the rates of both streams, one being under flow-ratio control. In principle, either caustic soda or dilution water can be the master stream, with the other following it to maintain the ratio. Blending is controlled by a feedforward system, ultimately reset by the product concentration or density. Feedback from caustic concentration measurement (usually by density) could be used for final adjustment, but the concentration of the hypochlorite solution is the more important variable. The simple flow-ratio controller mentioned here can be replaced by a multi-stream version that allows use of other streams in addition to the principal 50% NaOH and dilution water. A cooler downstream of the mixing point removes the heat of dilution. The standard design is a titanium plate exchanger, which can also provide turbulence to complete the mixing process. Chlorine joins the diluted caustic in the reactor. Its rate of addition is controlled by an oxidation-reduction potential (ORP) instrument. The reaction mass recirculates from a collection tank around the system to reduce the increase of temperature across the reactor and to promote turbulence. The net production is removed from the tank, normally under level control. 

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