Short and long-span track seek control for hard disk drive dual-stage servo actuators

A novel strategy for design of a control system for a dual-stage servo actuator used in hard disk drives is proposed. The control system enables track-following, and both short-span and long-span track seeking. For this, a well-known decoupling structure is employed to disconnect the control of the primary voice coil motor (VCM) actuator from the loop for a secondary high-bandwidth actuator. The design procedure can be summarised in four steps: (i) design of an established non-linear seek-settle-track following controller for the VCM, (ii) design of a linear track following controller for the secondary actuator, (iii) observer design and (iv) design of a compensator to retain global stability and to improve performance. The compensator used in the secondary control loop is suitably combined with a saturation non-linearity to obtain actuator signal boundedness for the secondary actuator. The performance and robustness of the proposed control system is analyzed and demonstrated through simulation. Presented results show good track-following/seeking performance with negligible overshoot, fast settling time and smooth transition between short and long-span track seeking.