High-bandwidth high-accuracy rotary microactuators for magnetic hard disk drive tracking servos

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This paper reports on the design, fabrication, and testing of an electrostatic microactuator for a magnetic hard disk drive (HDD) tracking servo. First, the design requirements for a microactuator in this application were investigated. These include high Z-directional stiffness, low in-plane stiffness, high structural aspect ratio, large output force, high area efficiency, low cost, and mass production by a batch process. A novel area-efficient rotary microactuator design was devised, and microactuators were successfully fabricated using innovative processing technologies, such as high-aspect-ratio polymer etching and thick metal electrodeposition. The fabricated microactuator has a structural thickness of 40 μm with a minimum gap/structure width of approximately 2 μm (aspect ratio of 20 :1). The microactuator's frequency response was measured and it was determined that it can be modeled as a second-order linear system, up to the 26-kHz frequency range. Moreover, the microactuator will enable the design of a servo system that exceeds a 5-kHz servo bandwidth, which is adequate to achieve a track density of more than 25 kilotrack per inch (kTPI). The microactuator/slider assembly was also tested on a spinning disk, with its position controlled by a proportional integral derivative controller using the magnetic position error signal written on the disk. A position accuracy of about 0.05 μm was observed when the servo controller was turned on. This result confirms that this microactuator can be used in a servo system which is capable of more than 25 kTPI. Continuous-time dual-stage servos were designed and simulated using the μ-synthesis technique. A sequentially designed single-input/single-output and a multi-input/multi-output control design method have been shown to be capable of meeting prescribed uncertainty and performance specifications. © 1998 IEEE.