Two-Degree-Of-Freedom Control with Dual-stage Actuators for Short-Span Seeking in HDD

In hard disk drive (HDD) servo systems, the short-span track seeking control may be viewed as a reference tracking problem. However, the perfect tracking control (PTC) is difficult to realize, because the voice coil motor (VCM) has a double-integrator behavior with a relative degree of two and it may also have nonminimum-phase (NMP) zero. These characteristics prevent the plant model inversion for the PTC design. A dual-stage HDD servo system uses VCM as the primary actuator and places a secondary micro-actuator piggyback on the VCM. This paper proposes a two-degree-of-freedom (2DOF) control scheme for fast short-span seeking such a system. The control design utilizes the parallel structure of two actuators to relax the restriction of plant order and NMP zero for perfect reference tracking. Experiment results show that the time of short-span seeking can be greatly shortened with the proposed 2DOF control design with dual-stage actuators.