  This paper develops adaptive output tracking control schemes with the
reference output signal generated from an unknown reference system whose output
derivatives are also unknown. To deal with such reference system uncertainties,
an expanded adaptive controller structure is developed to include a
parametrized estimator of the equivalent reference input signal. Without using
the knowledge of the reference system transfer function and equivalent input,
both are the critical components of a traditional model reference adaptive
control (MRAC) scheme, the developed new MRAC schemes designed for various
cases plant and reference model uncertainties, ensure completely parametrized
error systems and stable parameter adaptation, leading to the desired
closed-loop system stability and asymptotic output tracking.
