Settling control with reference redesign for dual actuator hard disk drive systems

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Abstract

This paper considers settling control of a recording head for a dual actuator system for hard disk drives consisting of a coarse actuator (voice coil motor, VCM) and a fine actuator (peizoelectric transducer, PZT). The design method proposed in this paper is called the dual actuator reference trajectory re-design (RTRD). In this method, the design is divided into three steps. In the first step, the coarse actuator loop is designed to achieve stability and basic performance. In the second step, the fine actuator path is designed by loop shaping for superior performance of the overall system. Finally, the reference signals are generated for both the coarse and fine actuators by minimizing the square integral of jerk during the transition. The reference signals are updated or redesigned in real time as the recording head approaches the target track for smooth landing and minimal residual vibration. The “soft switching” technique is applied for gradually introducing the fine actuator (PZT actuator), which may shorten the settling time and achieve a smooth settling response. The effectiveness of the proposed approach is evaluated by simulations.

Introduction

A disk drive stores data as magnetic patterns on disk media, which has a thin layer of magnetic materials coated on an aluminum or glass substrate. The data bit’s value of one or zero is represented by the presence or absence of a magnetic transition (magnetization reverse), which can be set (written) or detected (read) by the read/write head (transducers) as they move over it. The schematic for a conventional hard disk drive system (HDD) is shown in Fig. 1. Major elements of the HDD servo system include disks, voice coil motor (VCM), arm and read/write (R/W) head attached at the end of suspension. The spindle motor drives the disk as fast as 10,000 rpm and the VCM moves the arm and suspension system that holds the R/W head.

The hard disk drive (HDD) industry continues to strive for increased areal storage densities and reduced data access times. This necessitates performance improvements of the head positioning system in terms of fast transition from one track to another (track seeking), fast and accurate settling (track settling), and precise track following of the target track (track following). To achieve these improvements, the servo bandwidth of the head positioning system must be increased to lower the sensitivity to disturbances such as disk flutter vibrations, spindle motor run-out, windage, and external vibration. The servo bandwidth, however, is mainly limited by the mechanical resonance of the head positioning system. Dual actuator systems offer one way to enlarge the servo bandwidth. The dual actuator system consists of two actuators: coarse and fine actuators. The coarse actuator is low bandwidth but has a large stroke; the voice coil motor (VCM) is the most popular coarse actuator. The fine actuator is high bandwidth but has a small stroke; The piezoelectric transducer (PZT) is a popular fine actuator.

The conventional HDD servo control method is the mode switching control (MSC) (Yamguchi, 1996). In MSC, the controllers are selected in the order of the seeking controller, the settling controller, and the following controller in moving the R/W head to the target track. This paper is concerned with the design of settling controllers, but the premise is that the seeking and settling controllers have a two degree-of-freedom control structure with the feedback controller and feedforward controller for each of the coarse and fine actuators. It is important to minimize the seeking time. The settling controller plays an important role to achieve this objective. It must make the R/W head land on the target track fast and with no residual vibration.

Research efforts have been devoted in recent years to the design of dual actuator servo systems (Evans, Griesbach, & Messner, 1999; Grochowski & Hoyt, 1996; Horsley, Hernandez, Horowitz, Packard, & Pisano, 1998; ; Horsley, Wongkomet, Horowitz, & Pisano, 1999; Koganezawa, Uematsu, & Yamada, 1999). In dual actuator servo systems, the VCM actuator output and PZT actuator output are not measured separately, but usually only the total sum of the two actuator outputs is available to the servo controller: namely, the dual actuator system is a dual-input/single output (DISO) system and the servo controller is a single input/dual output (SIDO) system. The controller design is a special case of multi-input/multi-output (MIMO) system design. MIMO design techniques such as LQG (Hu, Guo, Huang, & Chen, 1999), H and μ-synthesis (Hernandez, Park, Horowitz, & Packard, 1999) have been applied to the design of dual actuator track following servo system. These techniques usually result in high order controllers even after model reduction, which are hard to implement on actual hardware. The PQ method (Schroceck & Messner, 1998) reduces the MIMO design to two SISO design problems in the frequency domain. The PQ method, however, does not guarantee the stability of the VCM feedback loop. In practice, the VCM feedback loop should be stable so that the disk drive works safely when the PZT actuator is not activated. Seeking and settling controllers for dual actuator systems have been studied by (Kobayashi & Horowitz, 2001; Numasato & Tomizuka, 2001).

This paper extends the work of Numasato and Tomizuka (2001) on the RTRD for the settling control of the dual actuator system. The design of RTRD dual actuator settling system is divided into three steps: (1) the VCM feedback loop is first designed to achieve stability and basic performance, (2) the PZT path is designed for superior performance of the overall system, and (3) the reference trajectories are designed for the VCM and PZT actuators. The third step involves real time redesign of trajectories: namely, they are updated or redesigned as the R/W head approaches to the target track for fast and smooth landing. Numasato and Tomizuka (2001) applied the reference trajectory to the VCM loop only. In this paper, it is shown that the settling time may be shortened by applying the reference input to the PZT actuator as well as the VCM actuator.

The remainder of this paper is organized as follows. Section 2 presents the structure and modeling of the dual actuator system. Section 3 discusses the RTRD design for the settling control of the dual actuator system. Section 4 demonstrates a design example for the RTRD settling design. Section 5 presents some simulation results for RTRD settling. Concluding remarks are given in Section 6.

Section snippets

Dual actuator system model

In this paper, we consider a dual actuator system consisting of a voice VCM as a coarse actuator and a PZT as a fine actuator (Fig. 2). The VCM is a conventional actuator used for commercial hard disk drive systems. The fine actuator is an active suspension, which finely moves the R/W head laterally. The fine actuator is located between the head suspension and the base plate, which is moved by VCM. A slider is attached to the tip of the suspension. The advantage of this moving suspension type

Overall structure of HDD servo system during settling mode

Fig. 5 shows the overall structure of HDD servo system during settling. It consists of the track following or regulation controller, the feedforward controller and the reference trajectory generator for each of the two actuators, VCM and PZT. In the figure, PVCM(s) and PPZT(s) represent the dynamics of VCM and PZT, respectively. The feedback controllers for VCM and PZT are represented by CVCM(z) and CPZT(z), and the feedforward controller by CVff(z) and Cpff(z). EPZT(z) is called the decoupling

RTRD settling design

In this section, we present a design example of the dual actuator setting controllers. The dual actuation system is described by Eqs. (1) and (2) with parameter values in Table 1. The design specification is to achieve an 1.0 kHz or higher bandwidth, an about 40 phase margin and a 4 dB or larger gain margin. The sampling frequency is 10.9 kHz corresponding to 84 sectors each track.

The VCM feedback controller is: CVCM(z)=8.66×104z21.011z+0.1254z2+0.1659z+0.1813

When the PZT controller is

RTRD settling simulation

In the following simulations, TH and Tf are chosen to be 12 samples (1.2 ms) and 15 samples (1.5 ms), respectively. α is linearly increased from 0 to 1 (Fig. 11) within first four samples in the following simulations. This “soft switching” makes sense since PES tends to be large at the beginning of switching from seeking to settling (Numasato & Tomizuka, 2001).

Fig. 12 shows a settling simulation with the VCM initial values [aVCM(0),vVCM(0),PVCM(0)]=[100m/s2,60mm/s,20μm]. The thick solid

Conclusion

In this paper, the reference trajectory re-design (RTRD) scheme was discussed for dual actuator settling control. The most important feature of RTRD is real time computation of reference trajectories as a function of the position, velocity and acceleration of the VCM at the beginning of the settling period. The settling time was shortened by introducing a reference signal for each of VCM and PZT actuators instead of VCM alone. Soft switching was used for the PZT actuator to avoid the excitation

Acknowledgements

This research was conducted at the Computer Mechanics Laboratory (CML) in the Department of Mechanical Engineering, University of California at Berkeley. The authors would like to thank Mr. H. Numasato for suggestions and assistance.

Jiagen Ding was born in Anhui, China, in 1973. He received the BS and MS degrees in engineering from Zhejiang University, Hangzhou, China, in 1994 and 1997, and MS degree in Electrical Engineering and Computer Science and PhD degree in Mechanical Engineering from University of California at Berkeley, Berkeley, USA, in 2003. Since 2003, he is with the Electronic & Photonic System Technologies Laboratory, General Electric Research Center. His current research interests include digital control for

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Jiagen Ding was born in Anhui, China, in 1973. He received the BS and MS degrees in engineering from Zhejiang University, Hangzhou, China, in 1994 and 1997, and MS degree in Electrical Engineering and Computer Science and PhD degree in Mechanical Engineering from University of California at Berkeley, Berkeley, USA, in 2003. Since 2003, he is with the Electronic & Photonic System Technologies Laboratory, General Electric Research Center. His current research interests include digital control for disk drive systems, mechatronic technologies related to medical systems, embedded systems, and wireless sensor network for cargo security.

Shang-Chen Wu was born in Taipei, Taiwan, in 1978. She received the BS degree in Mechanical Engineering from National Taiwan University in 2000. She is currently working towards the PhD degree in the Mechanical Engineering Department, University of California at Berkeley. Since 2001, she has been with the Computer Mechanics Laboratory, University of California at Berkeley, engaged in research projects on hard disk drives servo control. Her research interests include digital motion control, robust control, and signal processing.

Masayoshi Tomizuka was born in Tokyo, Japan, in 1946. He received the BS and MS degrees from Keio University, Tokyo, Japan, and the PhD degree from the Massachusetts Institute of Technology, Cambridge, in 1974, all in mechanical engineering. In 1974, he joined the faculty of the Department of Mechanical Engineering, University of California at Berkeley (UC Berkeley), where he currently holds the Cheryl and John Neerhout, Jr., distinguished professorship chair. His current research interests are optimal and adaptive control, digital control, signal processing, motion control, and control problems related to robotics, machining, manufacturing, information storage devices and vehicles. He served as technical editor of the ASME Journal of Dynamic Systems, Measurement and Control, and an associate editor of the Journal of the International Federation of Automatic Control, Automatica and the European Journal of Control. He is a Fellow of the ASME and the Society of Manufacturing Engineers. He is the recipient of the Charles Russ Richards Memorial Award (ASME, 1997) and the Rufus Oldenburger Medal (ASME, 2002).

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