Coordinated system with tracking-position and flying-height controls for magnetic heads in HDDs

Abstract

To improve positioning accuracy of a magnetic head in hard disk drives (HDDs), a triple-stage-actuator system was proposed with a thermal positioning actuator for a magnetic head positioning system. This positioning system has three types of actuators: a voice coil motor (VCM), piezoelectric (PZT) actuators, and the thermal positioning actuator. The thermal actuator systems have simple characteristics without mechanical resonant modes even in a high frequency range. As a result, the triple-stage-actuator system enable us to improve the positioning accuracy during a track-following control from conventional dual-stage-actuator systems. However, the thermal positioning actuator causes flying height fluctuations of the magnetic head that could lead to fall of magnetic recording performances. In HDDs, the magnetic head has another heater located in a vertical direction of read/write elements in order to control flying height of the read/write elements. This control system is called thermal flying height control system. In the previous study, we can compensate for the flying-height fluctuations by using a feedforward control scheme of the thermal flying-height control system. However, the feedforward control has little robustness against the plant perturbations. To address this issue, this paper employs a two-degree-of-freedom (TDOF) control scheme for the coordinated control with the thermal positioning control and the thermal flying-height control systems. By using proposed TDOF method, we can compensate for the flying-height fluctuations against the plant perturbations.