Two-degree-of-freedom control scheme for flying-height and tracking-position controls with thermal actuators 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. In this system, a magnetic head has a heater located in a horizontal direction of read/write elements as the thermal positioning actuator. By using this structure, the control system can move the position of read/write elements of the magnetic head in a horizontal direction with thermal expansion induced by the heater with an electric current. The thermal actuator systems have simple characteristics without mechanical resonant modes even in a high frequency range. This means that the thermal actuator is good for positioning control in high frequency range. As a result, the triple-stage-actuator system enable us to improve the positioning accuracy during a trackfollowing 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 TFC 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 TPC and the TFC systems. By using proposed TDOF method, we can compensate for the flying-height fluctuations against the plant perturbations. Simulation results showed that the proposed method is able to improve robust performance against the plant perturbations.

High-bandwidth controller design for dual-stage actuator system in hard disk drives

“…The controllers of the VCM and micro-actuator are designed based on the frequency responses of the actuator. The transfer functions of the VCM and micro-actuator can be defined as (Atusmi, 2018)…”

AFC with gain adjustment in control system in HDDs for compensation of speaker‐induced vibration

“…The controller design is thus manageable. In the present study, the controllers C VCM ( z ) and C PZT ( z ) were designed based on previous studies . The frequency responses of the controller C VCM ( z ), D ( z ) and C PZT ( z ) are shown in Fig.…”

“…The error from the ideal parameter λ idl is described as δλ = λ idl − λ(k ). The difference equation with respect to λ(k ), as presented in (23), is derived by subtracting λ idl from both sides of (16).…”

“…In the proposed AFC, the step size parameter λ is updated based on the algorithm (16). Figure 8 shows the time response of the external vibration d (t), Fig.…”

“…Decoupling controller design is a well‐known controller design method for the dual‐stage actuator in head positioning systems . Figure shows the block diagram of the control system designed by using the decoupling controller design method.…”