Performance Comparison between Triple-stage Actuator Systems in HDDs

Abstract

In this paper, we present a comparison study between three triple-stage actuator systems of the magnetic-head positioning system in hard disk drives (HDDs). As of year 2019, we have four kinds of actuators for the magnetic-head positioning system: voice coil motor (“VCM”), “Milli” actuator with two piezoelectric (PZT) elements, “Micro” actuator with two PZT elements, and a “Thermal” actuator that consists of heaters embedded in the magnetic head and move read/write elements a few nanometers in a horizontal direction with thermal expansion. This means that we are able to build three kinds of triple-stage actuator systems: “VCM + Milli + Micro”, “VCM + Milli + Thermal”, and “VCM + Micro + Thermal”. In order to find the best triple-stage actuator system for upcoming 1000 kTPI (track pitch: 25 nanometer) HDD era, we examine case studies and estimate the position error signals and movements of actuators of the track-following control systems under the external vibration in a file server. In this case study, it is shown that achievable disturbance-rejection performances of these triple-stage-actuator systems depend on not only the frequency responses the actuators but also the stroke limitations of the actuators. As a result, we found that the triple-stage actuator system that consist of the VCM, the Milli actuator, and the Micro actuator has the best performance for 1000 kTPI HDD.