1. Fundamental assumptions of the first harmonic method

1.1 A reminder about linearity

In engineering terms, a system is said to be linear if it is governed by linear differential equations with constant coefficients, i.e. if its transfer function is a rational fraction in p (p is the Laplace variable).

The pure delay transfer function exp (– τ p ), for which most of the tools used in linear automatics apply, is usually appended to this definition of linearity.

This implies the validity of the superposition principle at all times, which can be stated as follows (figure 1 ):

• if the response of a linear system to an input x (t ) is y (t ), its response to the input k x (t ),...