The paper considers the problem of designing
Adaptive Feedforward Control (AFC) systems for uncertain
SISO linear systems perturbed by multi-sinusoidal disturbances
of known frequencies. The proposed approach removes the
longstanding assumption that either the sign of the real part or
the imaginary part of the transfer function of a stable plant at
the frequency of excitation be known for AFC to be applicable,
which is referred to in this paper as an SPR-like condition.
Notable features of the solution are that persistence of excitation
is not required and stability analysis tools based on averaging
are avoided; hence, the requirement of an exponentially stable
equilibrium for the closed-loop system is circumvented.
