Feedforward controller design

We do not yet have all the tools to deal quantitatively with feedforward controller design. When our Russian lessons have been learned (Laplace transforms), we will come back to this subject in Chap. 11. 

We want to use a feedforward controller to make adjustments in steam flow to the reboiler, whenever the feed rate to the column changes, so that bottoms composition is held constant. The feedforward-controller design equation [Eq. (11.28)] gives... 

Modify your feedforward controller design of Prob. 11.10 so that it can handle both feed temperature and feed flow rale changes and uses a feedback temperature controller to trim up the steam flow. 

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. 

V.23 Suppose that the transfer functions xd/F and xofx f for the distillation column of Problem V.22 are not precisely known. Consequently, the feedforward controllers designed in part (a) of Problem V.22 give imperfect control with a certain amount of residual difference between xD and the desired set point. Assume that td = 0.5 min, r = 1 min, and the following new expressions for the actual transfer functions ... 

However, for feedforward controller design, model A should be used because having the correct steady-state gain is more important in feedforward control than the correct dynamics. 

Lewin, D.R., A Simple Tool for Disturbance Resiliency Diagnosis and Feedforward Control Design, CompuL Chem. Eng., 20(1), 13-25 (19%). 

Feedforward control was developed to counter some of these limitations. Its basic premise is to measure the important disturbance variables and then take corrective compensatory action based on a process model. The quality of control is directly related to the fidelity and accuracy of the process model. Two implementations of feedforward control which are widely used in polymer reactor control will be discussed, namely feedforward control design based on steady-state models, and... 

Feedforward Controller Design Based on Steady-State Models 15.3.1 Blending System... 

The starting point for the feedforward controller design is the steady-state mass and component balances that were considered in Chapter 1,... 

FEEDFORWARD CONTROLLER DESIGN BASED ON DYNAMIC MODELS... 

Stea.dy-Sta.teFeedforwa.rd, The simplest form of feedforward (FF) control utilizes a steady-state energy or mass balance to determine the appropriate manipulated variable adjustment. This form of feedforward control does not account for the process dynamics of the disturbance or manipulated variables on the controlled variable. Consider the steam heater shown ia Figure 15. If a steady-state feedforward control is designed to compensate for feed rate disturbances, then a steady-state energy balance around the heater yields ... 

These decoupler design equations are very similar to the ones for feedforward control in an earlier section. In fact, decoupling can be interpreted as a type of feedforward control where the input signal is the output of a feedback controller rather than a measured load variable. 

In contrast, we could have done the derivation using steady state models. In such a case, we would arrive at the design equation for a steady state feedforward controller. We ll skip this analysis. As will be shown later, we can identify this steady state part from the dynamic approach. 

In the last chapter we used Laplace-domain techniques to study the dynamics and stability of simple closedloop control systems. In this chapter we want to apply these same methods to more complex systems cascade control, feedforward control, openloop unstable processes, and processes with inverse response. Finally we will discuss an alternative way to look at controller design that is called model-based control. 

The tools are those developed in Chaps. 9 and 10. We use transfer functions to design feedforward controllers or to develop the characteristic equation of the system and to find the location of Us roots in the 5 plane. 

To design a feedforward controller, that is, to find F, we must know both and The objective of most feedforward controllers is to hold the controlled variable constant at its stea dystate value. Therefore the change or perturbation in should be zero. The output is given by the equation... 

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. ... 

For control purposes, somewhat simplified mathematical models usually are adequate. In distillation, for instance, the Underwood-Fenske-Gilliland model with constant relative volatilities and a simplified enthalpy balance may be preferred to a full-fledged tray-by-tray calculation every time there is a perturbation. In control situations, the demand for speed of response may not be realizable with an overly elaborate mathematical system. Moreover, in practice not all disturbances are measurable, and the process characteristics are not known exactly. Accordingly feedforward control is supplemented in most instances with feedback. In a well-designed system (Shinskey, 1984, p. 186) typically 90%... 

Block Diagram Analysis One shortcoming of this feedforward design procedure is that it is based on the steady-state characteristics of the process and, as such, neglects process (Ramies (i.e., how fast the controlled variable responds to changes in the load and manipulated variables). Thus, it is often necessary to include dynamic compensation in the feedforward controller. The most direct method of designing the FF dynamic compensator is to use a block dir rram of a general process, as shown in Fig. 8-34, where G, represents the disturbance transmitter, (iis the feedforward controller, Cj relates the disturbance to the controlled variable, G is the valve, Gp is the process, G is the output transmitter, and G is the feedback controller. All blocks correspond to transfer fimetions (via Laplace transforms). 

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