To be very abstract, a system is something that gets an input and has an output. The numbers of inputs and outputs can vary so we will differentiate between a Single Input Single Output System (SISO-System) and a Multiple-Inputs Multiple-Output Systems (MIMO-System).

Figure 1: SISO System (image by author)

In systems engineering, everything can be broken down into 3 basic elements or any combination of them. These elements are the P-, the I- and D-element. The indices P, I and D correspond to the proportionality of the system's output to input. The output y of a system is proportional

  • directly to the input u (P).
  • to the integral of the input u (I).
  • to the derivative of the input u (D).

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