Feedforward control examples

Fig. 15. Example of steady-state feedforward controls, where + indicates the summation of signals. Terms are defined in text.

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

The example simulation THERMFF illustrates this method of using a dynamic process model to develop a feedforward control strategy. At the desired setpoint the process will be at steady-state. Therefore the steady-state form of the model is used to make the feedforward calculations. This example involves a continuous tank reactor with exothermic reaction and jacket cooling. It is assumed here that variations of inlet concentration and inlet temperature will disturb the reactor operation. As shown in the example description, the steady state material balance is used to calculate the required response of flowrate and the steady state energy balance is used to calculate the required variation in jacket temperature. This feedforward strategy results in perfect control of the simulated process, but limitations required on the jacket temperature lead to imperfections in the control. 

The success of this control strategy depends largely on the accuracy of the model prediction, which is often imperfect as models can rarely exactly predict the effects of process disturbances. For this reason, an additional feedback loop is often used as a backup or to trim the main feedforward action, as shown in Fig. 2.25. Many of the continuous process simulation examples in this book may be altered to simulate feedforward control situations. 

In this example, the reactor is equipped with a feedforward controller that calculates the flowrate F and the jacket temperature Tj required to maintain the reactor temperature constant for variations in inlet feed concentration CA0 and temperature T0. 

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. 

Figure 11.5a shows a typical implementation of feedforward controller. A distillation column provides the specific example. Steam flow to the reboiler is ratioed to the feed flow rate. The feedforward controller gain is set in the ratio device. The dynamic elements of the feedforward controller are provided by the lead-lag unit. 

There are no inherent linear limitations in feedforward control. Nonlinear feedforward controllers can be designed for nonlinear systems. The concepts are illustrated in Example 11.3. 

Design a feedforward controller for the two-heated-tank process considered in Example 10.1. The load disturbance is inlet feed temperature 7. ... 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

ATP exerts negative feedforward control (contrast with Example 9.7), while ADP exerts positive feedback control. 

Example 21.1 Feedforward Control of Various Processing Units... 

As the examples above have indicated, feedforward control systems can be developed for more than one disturbance. The controller acts according to which disturbance changed value. Therefore, the schematic of Figure 21.1a with several disturbances represents the general case of feedforward control with a single controlled variable. 

The feedforward control of a CSTR, in Example 21.1, indicates that the extension to systems with multiple controlled variables should be rather straightforward. 

The question that arises is How do we design feedforward controllers The reader may have suspected already that conventional P, PI, or PID controllers will not be appropriate. Let us start with an example, the design of feedforward controllers for a stirred tank heater. 

The simplest feedforward controller and the easiest to implement is the steady-state one. As demonstrated in Example 21.2, we use simple steady-state balances for design. How does this modify the design equations (21.9) and (21.10) ... 

Consider again the tank heater of Example 21.2. Under feedforward control only, we have the configuration shown in Figure 21.3b. The design transfer functions are... 

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. 

Feedforward control configuration uses direct measurement of the disturbances to adjust the values of the manipulated variables (Figure 2.5). The objective here is to keep the values of the controlled output variables at desired levels. An example of feedforward control configuration is shown in Figure 1.4. 

This example demonstrates very vividly how important mathematical modeling is for the design of a feedforward control system. In fact, without good and accurate mathematical modeling we cannot design efficient feedforward control systems. 

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