Feedforward control feedback trimming

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. 

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. 

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. 

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. 

Another advantage of this scheme is that it lends itself readily to the application of feedforward control in order to maintain the D/F ratio for measured changes in feed flow. This feedforward signal would be trimmed by the addition of a feedback signal from the column temperature controller. 

Figure 21.8 (a) Temperature response of a stirred tank heater under feedforward control alone, and with feedback trimming (b) corresponding block diagram. 

A control system able to react to these load variations when they occur rather than wait for them to pass through the process before initiating a corrective action would be ideal. The technique is called feedforward control. There are two types of load signals are inputs to the feedforward control system where they compute the set point of the manipulated variable control loop as a function of the measured load variations. The unmeasured load variables pass through the process, undetected by the feedforward system, and cause an upset in the controlled variable. The output of the feedback loop then trims the calculated value of the set point to the correct operating level. In the limit, feedforward control would be capable of perfect control if all load variables could be... 

Regardless which term is trimmed to remove offset, the procedure amounts to manual feedback. Automatic feedback is perfectly capable of effecting the same result, if the controlled variable can be measured with sufficient reliability. (This qualification is significant in that feedforward control is occasionally used because a feedback measurement is unavailable.)... 

Proportional feedback trim is insufficient to eliminate offset, for the same reason that it was insufficient in a conventional control loop. The presence of feedforward control components within the feedback loop Induces no substantial change in the mode settings required of the feedback controller the process is just as difficult to control as it was without feedforward-only the amount of work required of the feedback controller has been markedly reduced. 

In practical applications, feedforward control is normally used in combination with feedback control. Feedforward control is used to reduce the effects of measurable disturbances, while feedback trim compensates for inaccuracies in the process model, measurement errors, and unmeasured disturbances. The feedforward and feedback controllers can be combined in several different ways, as will be discussed in Section 15.6. A typical configuration is shown in Fig. 15.4, where the outputs of the feedforward and feedback controllers are added together and the combined signal is sent to the control valve. 

As mentioned in Section 15.1 and illustrated in Example 15.5, feedback trim is normally used in conjunction with feedforward control to compensate for modehng errors and unmeasured disturbances. Feedforward and feedback controllers can be combined in several different ways. In a typical control configuration, the outputs... 

Alternative ways of incorporating feedback trim into a feedforward control system include having the feedback controller output signal adjust either the feedforward controller gain or an additive bias term. The gain... 

Consider the following example of the feedforward control of a heat exchanger with cascaded feedback trim control (shown in Figure 6.7). The addition of feedback and cascade control serves to eliminate offset due to modelling inaccuracies and other non-measured disturbances. 

Fig. 14. Examples of feedforward (EE) controls having feedback (EB) trim, where L/L = lead/lag element ...

Although the feedforward loop carries most of the load, feedback is still important in its execution. The ultimate regulatory system is a triple cascade-the primary feedback controller trimming the set point of the feedforward loop, which in turn manipulates a cascade flow loop. The symmetry of such a configuration is striking. 

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