Robust Repetitive Perfect Tracking Control of HDDs Based on Re-learning Scheme

Abstract

This paper presents an improved method of a repetitive controller based on perfect tracking control (PTC) in order to reject the variable repeatable runout (RRO) of hard disk drives (HDDs). The author's group proposed repetitive PTC (RPTC) with a switching mechanism. RPTC is realized by using periodic a signal generator (PSG) and PTC. The PSG produces a feedforward signal from the periodic disturbance, and the PTC generates a control input to cancel the periodic error in the steady state. However, we have not considered the difference between the RROs of tracks. This paper proposes a re-learning scheme by taking into account the correlation of adjacent tracks. Finally, the advantages of RPTC using the proposed scheme are demonstrated through simulations and experiments using a hard disk drive equipment with discrete track recording media (DTR). Head-positioning controllers of hard disk drives (HDDs) are generally composed of two control modes: the track- seeking mode and the track-following mode. High-speed po- sitioning and small residual vibration are important in the track-seeking mode, and the disturbance rejection perfor- mance is important in the track-following mode. Digital two-degree-of-freedom controllers generally have two samplers for the reference signal r(t) and the output y(t), and one holder on the input u(t). Therefore, there exist three time periodsTr, Ty ,a ndTu which represent the sampling time of r(t), y(t), and u(t), respectively. In the case of HDDs, the position error is detected by the discrete servo signals em- bedded in the disks. Therefore, the output sampling period Ty is decided by the number of these signals and the rotation frequency of the spindle motor. However, it is possible to set the control period Tu shorter than Ty because of the recent de- velopment of microprocessor. Thus, the servo system can be regarded as a multirate control system where Tu ≤ Ty. Then, many multirate controllers have been proposed both for the track-seeking mode and the track-following mode (1)-(3) . Recently, magnetic printing media, discrete track record- ing media (DTR) and pre-formatted media with perpendicu- lar anisotropy are studied for high track density as new gen- eration media (4) . These new generation media can record servo information in the manufacturing process of disks al- though the conventional disk uses servo track writers after disks are installed in drives. This improved technology can provide high recording density compared with conventional disks. However, it is known that the servo information of these pre-formatted media generally has large decentering from the rotating center. Therefore, higher track-following