System Types

This explanatory note has the task to show what does it mean the term “astatic” regulation and control systems.

In the American and British scientific literature instead of “astatic” regulation and control systems is used the term “System Type”.

Consider the single loop feedback system with input R(s) and output Y(s) as shown at the right.

The system type is determined by the form of the loop (or open-loop) transfer function GH(s). In

general, the form of GH(s) can be written as

where N(s) and D(s) represent polynomials in s, and represents all powers of s that can be factored from the denominator. The number n determines the system type. It is said that the system is a “type n” system. As we will show in the following paragraphs, the value of n determines the "type" of steady-state error the system will have for various inputs.