Abstract :
A simple stability criterion based on the return difference is established: Draw the phase plot of the return difference 1 + GH(j¿) vs. ¿, if and only if ¿(¿)¿¿(0) = (n + 2m) ¿/2, the system is stable, where n is the number of poles of GH(j¿) at the origin and m is that in the right half plane; ¿(¿) and ¿(0) are the phase angles of "1 + GH(i¿)" at ¿ = ¿ and at ¿ = 0, respectively.
