Control combined feed-forward/feedback

Fig. 7.70. Combined feed-forward/feedback control of the overhead product stream of a distillation column. Feed composition is the only significant load variable...

First the QR/F ratio is used in conjunction with the temperature controller. A mnltiplier is setup with one input being the feed flow rate and the other input a QR/F ratio that is the output signal of the temperature controller. So the control structure is a combined feed-forward/feedback system. 

Describe which form of control strategy (feed-forward, feedback, cascade, or a combination of y these) best describes how a responsible person drives an automobile. 

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. 

The feedback control in loops 1 and 2 is combined with a feed-forward controller in loop 3 which measures the inlet, temperature T calculates the change in cooling water flow rate Few which is required to bring the reactor temperature Fback to its set point Ts and sends this signal to the feedback controller (the feed-forward controller consists of a model of the process and is therefore not of P-, PI- or PID-type). The feedforward control loop will therefore theoretically eliminate any disturbances in inlet temperature Tv The feedback part of the control system, loop 1, will compensate for any inaccuracies in the feed-forward control model as well as eliminate the effect of other, unmeasured disturbances, e.g. in inlet flow rate Fr... 

Controlling the pressure and flow of a fuel gas alone cannot dehver a constant heat flow if the gas composition varies. The control problems presented by gas composition variations can be overcome by adding a hi-speed calorimeter to monitor heat value and by using a conventional flow orifice to measure flow to an existing burner control system. These two measurements, heat value and flow rate, in combination with the conventional control and feedback loop based on loan, provide the basis for effective, continuous feed forward control of the burning process. 

The feed forward control loop built aroimd the high speed calorimeter (Fig. 3) compensates for changing gas characteristics in advance of combustion. The combination of feed forward control using Wobbe Index plus a process feedback loop assmes the user of correct process heat and makes possible significant reductions in excess air. Those save fuel and do not sacrifice safety or product quality. 

Phase 3. The two types of feedback and feed-forward can be combined as shown in Figure 5.9 (the feed-forward controller sets the setpoint for the feedback controller) or the cascade system as shown in Figure 5.10. 

Step 3. Selection of the control structure, whether open-loop, feedback, feed-forward, or a combination of feed-backward and feedforward control is required. 

Example 21.4 illustrates how feedback control, feed-forward control, or a combination of both can be used to control a process unit. 

An alternative scheme to pure feed-forward control is to use a combination of feedback and feed-forward control strategies. This is illustrated in Figure E21.4rcl and discussed below. 

Example 21.S considers the operation of the combination of feedback and feed-forward control. 

Mg. 31.8. A. Combination of feed-forward and feedback control, B. disturbance correction control. 

Figure 2 illustrates a so-called controlled learning paradigm where the network is supervised so as to work in a one-step-ahead method in the model predicting the object The feedback can come from two sources, depending on the position of switch FFN-RecN. In the FFN position, the artificial neural network is combined into the structure of feed forward network, while in the RecN position, the network structure corresponds to the recurrent network. 

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