  A frequency based data-driven control design considering mixed H2/H-infinity
control objectives is developed for multiple input-single output systems. The
main advantage of the data-driven control over the model-based control is its
ability to use the frequency response measurements of the controlled plant
directly without the need to identify a model for the plant. In the proposed
methodology, multiple sets of measurements can be considered in the design
process to accommodate variations in the system dynamics. The controller is
obtained by translating the mixed H2/H-infinity control objectives into a
convex optimization problem. The H-infinity norm is used to shape closed loop
transfer functions and guarantee closed loop stability, while the H2 norm is
used to constrain and/or minimize the variance of signals in the time domain.
  The proposed data-driven design methodology is used to design a track
following controller for a dual-stage HDD. The sensitivity decoupling
structure[16] is considered as the controller structure. The compensators
inside this controller structure are designed and compared by decoupling the
system into two single input-single-output systems as well as solving for a
single input-double output controller.
