The problem of control

A system is a set of components (mechanical, electrical, biological, etc.) interconnected to perform a certain function. If its behavior varies over time, we speak of a dynamic system. A system has input and output variables. The latter can be measured. The input variables can influence the system's evolution, i.e. the output variables. When they can be voluntarily modified, they are called control variables. Independently of these variables, the system may have internal variables that cannot be accessed; these are called state variables.

Controlling a system on the basis of measurements made on output variables is called feedback. It requires...