Feedforward control error with

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. 

Feed-Forward - Feedback Control. In practical applications, feedforward control is normally used in combination with feedback control. The feedforward part is used to reduce the effects of measurable disturbances, while the feedback part 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 discussed in most standard control text books. 

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

Feedforward Feedback Control. If the process exhibits slow dynamic response and disturbances are frequent, then the use of feedforward control with feedback control may be advantageous. Feedforward control differs from feedback control in that the primary disturbance is measured via a sensor and the manipulated variable is adjusted so that, ideally, the controlled variable does not change. As shown in Figure 9.11, the controller calculates the manipulated variable, u, needed to counteract the system upset introduced through the disturbance, d. By taking control action based on measured disturbances, rather than controlled variable error, the controller can reject disturbances before they affect the controlled variable, y. 

According to industrial practice [31], the combination of feedforward and feedback is the most powerful tool to control processes susceptible to load and setpoint changes the feedforward performs most of the load disturbance rejection task, and the feedback compensates for the modeling error. A feedforward controller built from an approximated model usually provides a significant improvement, in conjunction with feedback. 

It might be possible to construct an extremely complete dynamic model of a process, but any compensator with more than three terms to adjust would be unreasonably difficult to cope with. Furthermore, the purpose of dynamic compensation is to minimize an error which is already transient, so perfection is not really warranted. In most cases, a simple lead-lag function will be perfectly adequate and will be able to reduce the absolute area of the response curve by tenfold or more, distributed uniformly. Figure 8.14 compares the load response of the heat exchanger under dynamically compensated feedforward control with that encountered under feedback control. 

For this example, feedforward control with dynamic compensation provides a better response to the mea jci disturbance than does combined feedforward-fee control. However, feedback control is essential to cop unmeasured disturbances and modeling errors, combined feedforward-feedback control system is preferred in practice. 

The effectiveness of feedforward control depends on the accuracy of the measurement of the upset and the calculation and timing of the corrective action. In order for there to be no error, the corrective action must arrive in the process at the same point and time as the upset with an opposite but equal value to the upset. The disturbance measurement is first filtered to eliminate noise. The timing (dynamic compensation) is then set s ia a lead-... 

Another disadvantage of feedforward control is that all of the possible disturbances and their effects on the process must be known precisely. If unexpected disturbances enter the process when only feedforward control is used, then no corrective action is taken and the errors will build up in the system. If all the disturbances were measurable and their effects on the process precisely known, then a feedback control system for regulatory purposes would not be needed. However, such complete and error-free knowledge is never available, so feedforward is generally combined with feedback, as illustrated in Figure 6.6. The intent of this union is that the feedforward mitigates most of the effects of the principal disturbances and the feedback loops provide residual control and set point tracking. 

The state feedback controller with integration that includes the static feedforward for calculating compressor air flow rate state error minimizes error between desired and actual air flow rate through the compressor. 

The terminology of electromechanical systems has been appropriated to describe both feedback inhibition and feedforward activation as servomechanisms, or automatic devices which control the flow of large amounts of energy with the expenditure of only a minute amount of energy. Implicit in the servomechanism concept is the capacity to sense and correct errors, such capability being automatic, constant, and immediate. 

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