Feedback loop, inferred

Given Fig. 10.11 and classical control theory, we can infer the structure of the control system, which is shown in Fig. 10.12. That is, we use two controllers and two feedback loops, where for simplicity, the measurement and actuator functions have been omitted. 

The basic control scheme used for the perfect control analysis, a single inline feedback loop, gives 10 times the allowable concentration variation at the exit of the second tank for the predicted worst case. Including feedforward reagent addition would be insufficient by itself to give the tenfold improvement required, due to a 20% error in the estimated load (inferred from pH), so an additional in-line ratio feedback controller was added between the two tanks. As an additional actuator was therefore available at no extra cost, lead-lag feedforward from the load error at the first controller to the second controller actuator was added. To reduce feedforward dynamic mismatch, the lag was set to approximately the residence time of the first tank. The lead constant was added to the design parameters. 

Delays may not be directly observable, but may need to be inferred. Depending on where in the feedback loop the delay occurs, different control algorithms are required to cope with the delays [25] dead time and time constants require an algorithm that makes it possible to predict when an action is needed before the need. Feedback delays generate requirements to predict when a prior control action has taken effect and when resources will be available again. Such requirements may impose the need for some type of open loop or feedforward strategy to cope with... 

Control actions will generally lag in their effects on the process because of delays in signal propagation around the control loop an actuator may not respond immediately to an external command signal (called dead time) the process may have delays in responding to manipulated variables (time constants) and the sensors may obtain values only at certain sampling intervals (feedback delays), lime lags restrict the speed and extent with which the effects of disturbances, both within the process itself and externally derived, can be reduced. They also impose extra requirements on the controller, for example, the need to infer delays that are not directly observable. 

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