Chapter 2. Feedback Systems
The method we employed in the previous chapter was based on the feedback principle. Its basic idea can be simply stated as follows.
Continuously compare the actual output to its desired reference value; then apply a change to the system inputs that counteracts any deviation of the actual output from the reference.
In other words, if the output is too high, then apply a correction to the input that will lead to a reduction in output; if the output is below the reference, then apply a correction to the input that raises the value of the output.
The essential idea utilized by the feedback concept is to “loop the system output back” and use it for the calculation of the input. This leads to the generic feedback or closed-loop architecture (see Figure 2-1). This should be compared to the feedforward or open-loop architecture (Figure 2-2), which does not take the system output into account.
Basing the calculation of the next input value on the previous output implies that feedback is an iterative scheme. Each control action is intended only to take the system closer to the desired value, which it does by making a step in the right direction. No special effort is made to eliminate the difference between reference and output entirely; instead, we rely on the repetition of steps that merely reduce the error.