  A new method of virtual unknown parameter is proposed to synchronize two
different systems with unknown parameters and disturbance in finite time.
Virtual unknown parameters are introduced in order to avoid the unknown
parameters from appearing in the controllers and parameters update laws when
the adaptive control method is applied. A single virtual unknown parameter is
used in the design of adaptive controllers and parameters update laws if the
Lipschitz constant on the nonlinear function can be found, while multiple
virtual unknown parameters are adopted if the Lipschitz constant cannot be
determined. Numerical simulations show that the present method does make the
two different chaotic systems synchronize in finite time.
