Servo Controller Design for Triple-Stage Actuator in HDD to Compensate for High-Frequency Fan Vibration

Hard disk drives (HDDs) used for data servers are exposed to various vibrations. These vibrations are mainly caused by the cooling fan that controls the temperature inside the server. The vibration affects the positioning accuracy of magnetic heads that read/write digital data. This is an obstacle to obtaining high data transfer rates. To compensate for vibration, the control system for the magnetic heads must have higher frequency control bandwidth as the spectra of these vibrations have peaks up to 10 kHz. In current HDDs, the control system is a triple-stage system consisting of a voice coil motor (VCM), a milliactuator, and a microactuator. In this article, we propose a design strategy for a triple-stage control system to compensate for vibration. The design strategy addresses unstable controllers in the control system. The gain of the feedback controllers of the VCM and milliactuators is increased to expand the control bandwidth. Although these feedback loops become an unstable system due to the increase in the controller gain, the controller for the microactuator is designed to stabilize the overall feedback loop. As a result, the control system achieves high control bandwidth. The performance of the proposed control system is calculated based on the compensation of the vibration caused by the data server. The proposed design can improve positioning accuracy by approximately 48%–58% compared with the conventional controller design.