Feedforward variable

U4 Feed flow to the MX tower a feedforward variable manipulated by a level controller on the Toluene tower. 

Vaporization rate in the gas fired reboiler a feedforward variable manipulated by a separate controller for the reboiler. 

The feedforward control strategy (Fig. lb) addresses the disadvantages of the feedback control strategy. The feedforward control strategy measures the disturbance before it affects the output of the process. A model of the process determines the adjustment ia the manipulated variables(s) to compensate for the disturbance. The information flow is therefore forward from the disturbances, before the process is affected, to the manipulated variable iaputs. 

The primary advantage of the feedforward over the feedback control strategy is that corrective action is initiated before the controlled variable is upset. Feedforward control, however, has its own drawbacks, ie, variables used to characterize the disturbances must be measurable a model of the response of the controlled variable to the disturbance must be available (when the feedforward strategy is used alone, the control performance depends on the accuracy of the model) and the feedforward control strategy does not compensate for any disturbance not measured or modeled. 

In most process plant situations where feedforward control is appropriate, a combination of the feedforward and feedback control is usually used. The feedforward portion reduces the impact of measured disturbances on the controlled variable while the feedback portion compensates for model inaccuracies and unmeasured disturbances. This control strategy is referred to as feedforward control with feedback trim. 

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 ... 

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. 

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. 

Feedforward Control A reedfoi ward system uses measurements of disturbance vai iables to position the manipulated variable in such a way as to minimize any resulting deviation. The disturbance... 

Feedforward Control If the process exhibits slow dynamic response and disturbances are frequent, then the apphcation of feedforward control may be advantageous. Feedforward (FF) control differs from feedback (FB) control in that the primary disturbance or load (L) is measured via a sensor and the manipulated variable (m) is adjusted so that deviations in the controlled variable from the set point are minimized or eliminated (see Fig. 8-29). By taking control action based on measured disturbances rather than controlled variable error, the controller can reject disturbances before they affec t the controlled variable c. In order to determine the appropriate settings for the manipulated variable, one must develop mathematical models that relate ... 

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. 

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. 

Some plants have been using computer control for 20 years. Control systems in industrial use typically consist of individual feedback and feedforward loops. Horst and Enochs [Engineering h- Mining]., 181(6), 69-171 (1980)] reported that installation of single-variable automatic controls improved performance of 20 mineral processing plants by 2 to 10 percent. But interactions among the processes make it difficult for independent controllers to control the circuit optimally. 

Closed loop systems inelude either a feedbaek or feedforward, eontrol loop or both to eontrol the plant. In a feedbaek eontrol loop, the eontrolled variable is eompared to a set point. The differenee between the eontrolled variable to the set point is the deviation for the eontroller to aet on to minimize the deviation. A feedforward eontrol system uses the measured load or set point to position the manipulated variable in sueh a manner to minimize any resulting deviation. 

To counter probable disturbances, we can take an even more proactive approach than cascade control, and use feedforward control. The idea is that if we can make measurements of disturbance changes, we can use this information and our knowledge of the process model to make proper adjustments in the manipulated variable before the disturbance has a chance to affect the controlled variable. 

We will continue with the gas furnace to illustrate feedforward control. For simplicity, let s make the assumption that changes in the furnace temperature (T) can be effected by changes in the fuel gas flow rate (Ffuei) and the cold process stream flow rate (Fs). Other variables such as the process stream temperature are constant. 

This equation provides us with a model-based rule as to how the manipulated variable should be adjusted when we either change the set point or face with a change in the load variable. Eq. (10-5) is the basis of what we call dynamic feedforward control because (10-4) has to be derived from a time-domain differential equation (a transient model). 3... 

Figure 10.3. A feedforward control system on a major load variable with measurement function GMl and feedforward controller GFF.

If there is more than one load variable, we theoretically could implement a feedforward controller on each one. However, that may not be good engineering. Unless there is a compelling reason, we should select the variable that either undergoes the most severe fluctuation or has the strongest impact on the controlled variable. 

Here, we use L to denote the major load variable and its corresponding transfer function is GL. We measure the load variable with a sensor, Gnu., which transmits its signal to the feedforward controller GFF. The feedforward controller then sends its decision to manipulate the actuating element, or valve, Gv. In the block diagram, the actuator transfer function is denoted by G v. The idea is that cascade control may be implemented with the actuator, Gv, as we have derived in Eq. (10-1). We simply use G v to reduce clutter in the diagram. 

Since we do not have the precise model function Gp embedded in the feedforward controller function in Eq. (10-8), we cannot expect perfect rejection of disturbances. In fact, feedforward control is never used by itself it is implemented in conjunction with a feedback loop to provide the so-called feedback trim (Fig. 10.4a). The feedback loop handles (1) measurement errors, (2) errors in the feedforward function, (3) changes in unmeasured load variables, such as the inlet process stream temperature in the furnace that one single feedforward loop cannot handle, and of course, (4) set point changes. 

Of the other two load variables, we choose the process stream flow rate as the major disturbance. The flow transducer sends the signal to the feedforward controller (FFC, transfer function GFF). A summer (X) combines the signals from both the feedforward and the feedback... 

When close control is desired, usually the variable that is to be closely controlled is monitored and no changes are made until the measurement differs from what is desired. This is feedback control. It obviously is not an ideal system, since the controller can only react to changes. A better system would be one that anticipates a change and takes corrective action that ensures an unvarying output. This is a feedforward control system. This type of control is very advantageous when the input variables have a wide range of variation. 

Since it is impractical to measure everything that may affect the output variables, even when feedforward control is used feedback control is also included. Figure 7-10 shows how a feedforward system might be used on a waste neutralizer. The purpose of the waste neutralizer is to make certain that the streams leaving the plant are neutralized. First, all the streams are combined together and the feed rate and... 

Feedforward control. The basic idea is shown in Fig. 1.8. The disturbance is detected as it enters the process and an appropriate change is made in the manipulated variable such that the controlled variable is held constant. Thus we begin to take corrective action as soon as a disturbance entering the system is detected instead of waiting (as we do with feedback control) for the disturbance to propagate all the way through the process before a correction is made. 

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 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. 

I The basic notion of feedforward control is to detect disturbances as they enter the process and make adjustments in manipulated variables so that output variables are held constant. We do not wait until the disturbance has worked its way through the process and has disturbed everything to produce an error signal. If a disturbance can be detected as it enters the process, it makes sense to take immediate action in order to compensate for its effect on the process. 

Figure 11.4c shows the f forward control system. The load disturbance still enters the process through the G/, ) process transfer function. The load disturbance is also fed into a feedforward control device that has a transfer function. The feedforward controller detects changes in the load and makes changes in the manipulated variable. ... 

Thus the transfer function of a feedforward controller is a relationship between a manipulated variable and a disturbance variable (usually a load change). 

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... 

The advantage of feedforward control over feedback control is that perfect control can, in theory, be achieved. A disturbance will produce no error in the controlled output variable if the feedforward controller is perfect. The disadvantages of feedforward control are ... 

We must know how the disturbance and manipulated variables affect the process. The transfer functions and must be known, at least approximately. One of the nice features of feedforward control is that even crude, inexact feedforward controllers can be quite effective in reducing the upset caused by a disturbance. 

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. 

Figure 11.7 shows some typical results of using feedforward control. A first-order lag is used in the feedforward controller so that the change in the manipulated variable is not instantaneous. The feedforward action is not perfect because the dynamics are not perfect, but there is a significant improvement over just feedback control. 

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