Chapter 6. Review and Outlook

Before proceeding to practical matters that are directly relevant to applications and implementations, let’s summarize the conceptual foundations of feedback control as we have developed them so far.

The Feedback Idea

The idea behind feedback control is simple:

Constantly compare the actual output to the setpoint; then apply a correction in the correct direction and of approximately the correct size.

The comparison and corrections are performed at runtime. Precisely because feedback relies on runtime observations and adjustments, feedback control is capable of responding to unanticipated disturbances.


Feedback is an iterative scheme. That we keep monitoring the output and applying corrections is what makes feedback control feasible. Instead of having to get it “right” in a single step, we need only make things “better” because there is always another chance to fix any outstanding errors.

As a side effect, the ongoing iteration will also make the system robust to change.

Process Knowledge

One benefit of the feedback concept is that it does not require detailed knowledge about the controlled system and its behavior. Only two bits of information are required:

  • We must be able to identify the correct direction for the application of a corrective action. (In other words, we must know whether increasing the input will end up increasing or decreasing the output. This amount of process knowledge is indispensable.)

  • In general, we want to apply the largest possible ...

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