To improve positioning accuracy in a head-positioning control system for hard disk drives, we have developed a triple-stage-actuator system on a spin-stand tester. In this system, the first stage is a VCM actuator for moving a head-stack assembly, the second stage is a PZT actuator for moving a suspension, and the third stage is a thermal actuator for moving read/write elements. The frequency response of the thermal actuator showed that the thermal actuator system has no mechanical resonant mode. Therefore, this head-positioning system with a thermal actuator can control the head position in high frequency range without negative impact from mechanical resonances. As a result, the servo bandwidth of the proposed triple-stage-actuator system can be higher than that of the conventional dual-stage-actuator system which consists of the VCM and the PZT actuators. This improvement is similar to the improvement from the single-stage -actuator system to the dual-stage-actuator system. Experimental results on the spin-stand tester showed that the proposed control system can dramatically improve the positioning accuracy during a track-following control.
To increase the recording density of hard disk drives (HDDs), we developed a push-pull multi-layered piggyback PZT actuator that enables fine positioning by a dual-stage servo system. This PZT actuator consists of 31-mode push-pull multi-layered PZT strips and a head suspension. It generates a 1.4-lm effective radial head displacement at 5 V. This displacement is twice that of conventional piggyback actuators. The main resonance frequency of the actuator is higher than 9 kHz, its lifetime is longer than five years, and it has a self-latch property. These features mean that the developed actuator can meet all the requirements for implementation in HDD servo systems, including a track density of 100 kTPI (kilo-tracks per inch). The actuator was implemented in two types of HDDs (A-type and B-type), which reduced the repeatable and non-repeatable positioning errors (by 40 to 45% and 28 to 34%, respectively).
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