Preparation of NSC Project Reports
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! "# $ % & ! ' ( ) * + , - ) * + . / 0 1 "223 ' 4 5 6 # 7 8 9 : ( ) * + , - ) * + ; < & = > ? @ A ( ) B C D E F ? @ A , - ) ! G = H . "223 ' I . & J 4 K L M N O P H Q O R P H S O R T U "2 2 VW X ; Y Z [ Y 2This research investigates the design and implementation of an active vibration isolation method, and the improvement it brings to the track-following and fine seeking in the optical disk drives. A control scheme is introduced to switch the adaptation between the track-following and fine seeking process. That leads to an efficient compensation of periodic disk runout vacillations both in track-following and seeking, thereby eliminating tracking errors as well as direction hysterics of the pick-up head. The proposed control strategy applies to the variable rotational speeds of constant angular and linear velocity modes in optical disk drives.
: frequency adaptive control, active vibration isolation, optical disk drive
The high data transfer and short access time are two most important performance indices of the optical disk drive (ODD). Various attempts have been made to achieve these objectives such as to increase the disk rotational speed and to implement a fast
seeking control. However, the increment of disk rotational speed to improve the data transfer rate raises up many serious problems in the servo control system, which controls the position and velocity of the laser spot relative to the rotating disk. Especially, the periodic vibration caused by deficiencies in track geometry and eccentric rotation of the disk is instinctive to all the optical disks and deteriorates the track-following and seeking servo system.
In general, to move the laser spot on the correct track of the disk, the pick-up head (PUH) assembly is maneuvered by two control strategies: seeking control and track-following control [1]. The seeking control moves the laser spot rapidly to a target track containing the information to be read, while the track-following control makes the laser spot follow the track closely in spite of the runout disturbance. However, the pull-in ability at the beginning of track-following is not always preserved due to the limitation in the bandwidth of the track-following system and the external runout disturbance. Furthermore, track miscounting occurs due to the risk of failure in generating the track-cross signals for fast PUH movement. In addition, Stan et al. [2] showed that, even for a given seeking length, the counts of track-crossed differed between the outside- and inside-oriented seeking. As a result, the locked track could not guaranteed to be the destination after one fast seeking operation and therefore, additional correction of short seeking becomes an imperative for accurate positioning. Thus, a two-stage seeking mechanism composed of coarse seeking and fine seeking is the most reliable method used in conventional optical disk drives. However, owing to the serious vibration disturbance caused by the high rotational speed of the disk and the limited
bandwidth of the fine seeking, the velocity of the laser spot usually deviates from the profile during its slow velocity region. It usually leads to either a time-consuming fine seeking operation or a track-following failure at the end of the fine seeking.
In this research, a combined seeking and track-following servo structure is proposed with the capability of remarkably reducing the tracking error in the track-following
process and improving the stable
performance in seeking control in the high speed operation of rotational disks.2
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Frequency Adaptive Control Technique
Frequency adaptive control technique is a novel mechanism dealing with the vibration cancellation for both the CAV and CLV spindle modes of the optical disk drives with multi-playing speeds [3,4]. It is realized in a plug-in configuration as shown in Figure 1, in which the proposed algorithm of FACT is illustrated in Figure 2. ) (s Cf Gf ( s) ) ( s Gc ) ( s Cc ) (t xf ) (t xc ) (t xd ) (t xr ) (t e f u ) (t v
Figure 1. Plug-in frequency adaptive controller for track-following.
n a n b ) (t e Y J i Y i Y X J i X i X ∂ ∂ − = + ∂ ∂ − = + µ µ ) ( ) 1 ( ) ( ) 1 ( FSF Low Pass + n x n y t n ω cos t n ω sin ) (t v X X +
Figure 2. Algorithm of the plug-in FACT The variables TRO and FMO are the control inputs to Gf(s) and Gc(s), respectively. The
feedback controller Cf(s) and Cc(s) are
designed so that the closed-loop system is stable and the laser spot follows the track trajectory xr(t) in the presence of the external
runout vibration xd(t). Among the signals
xr(t), xf(t), xc(t), xd(t) and e(t), only the
tracking error signal e(t) is measurable and is
commonly abbreviated as TES.
3.1 Periodic Runout Identification
Let be the fundamental frequency of e(t), then the periodic tracking error e(t) can be expressed as ) sin cos ( ) ( 1
∑
= + = M n n n n n t b t a t e ω ω (2)where M is the highest harmonic order, n
=n1, and an and bn are unknown variables
to be identified. The time series e(k) depicts the sampled values from the N equal-spaced points per period of e(t). Define ) 1 ( ) 1 ( 2 2 1 1 1 1 1 ) ( − − − − − − − + + + + = − − = N N n N n N n N n N N n z W z W z W z W z z H L where exp( 2 ) N n j Wn N π = and n=0,1,L,N−1.
The input e(k) through each elementary filter Hn(z) produces an output expressed as
) ( 2 ) ( ) ( ) ( n n n n j N k e z H k
α
β
ξ
= = + (3)Then, the parameters an and bn describing the
measurable signal e(t) in equation (2) are then identified on-line through
− = + = t t b t t a n n n n n n n n n n ω β ω α ω β ω α cos sin sin cos (4) Adaptation Formulation
The transfer function from the adaptation signal v(t) to the tracking error e(t) can be expressed as ( ) 1 G C W s G v e f f f ≅ + = (5)
The frequency response of W(s) at n can be
denoted as [Wr(n) + j Wi(n)], where
Wr(n) and Wi(n) are the real and
imaginary parts of W(n), respectively.
Define the output v(t) as ) sin cos ( ) ( 1
∑
= + = M n n n n n t y t x t v ω ω (6)the on-line adaptation of variables xn and yn
++++ −−−− −−−− ==== ++++ ++++ −−−− ==== ++++ ] ) ( ) ( [ ) ( ) ( ] ) ( ) ( [ ) ( ) ( n n r n n i n n n n i n n r n n b ω W a ω W µ i y i y b ω W a ω W µ i x i x 1 1 (7) with n=1, 2,…, M.
CD Driver Altera 10KAFPGA
8 8
Figure 3. Experimental setup 3.3 Implementation of FACT
A single ALTERA EPF10KA FPGA device is used to implement the track-following controllers Cf(s) and Cc(s) as
well as the FACT algorithms as shown in Figure 3. The experimental results of the vibration rejection in the track-following system are explained in the following. Figure 4 illustrates the TES waveforms around 4 revolutions of the disk from the original feedback system; where ch1 represents the TES with a large peak-to-peak amplitude corresponding to a maximum 0.1µ position error, while its FFT
waveform depicted in ch2 shows that the TES be dominated by the first three harmonics. After the implementation of the FACT, the waveforms of TES are shown in Figure 5 where the first three harmonics are cancelled successfully.
TES
FFTof TES
113Hz 226Hz 339Hz 452Hz
Figure 4. The waveforms of TES without FACT function for CAV-24X.
TES
FFTof TES
113Hz 226Hz 339Hz 452Hz
Figure 5. The waveforms of TES with FACT added in for CAV-24X
4. Active Free Vibration Configuration The proposed FACT provides a promising solution to the vibration cancellation by a simple setup linking the fine seeking and track-following subsystems. Figures 6 and 7 show the block diagrams of the proposed configuration. ) (s Kf Gf ( s) ) ( s Gc ) ( s Cc ) (t vc ) (t ve ) (t vf ) (t vr ) (t vs f u c u ) (t v ) (t vd
Figure 6. FACT controller for fine seeking.
Y J i Y i Y X J i X i X ∂ ∂ − = + ∂ ∂ − = + µ µ ) ( ) 1 ( ) ( ) 1 ( n x n y t n ω cos t n ω sin ) (t v X X + Track-following Adaptation ON Fine Seeking Adaptation OFF
Figure 7. Adaptation ON for track- following while Adaptation OFF for fine seeking.
The FACT was applied to the track- following control phase, and successfully compensated the disk runout with drastically reduced tracking errors. Then, the adaptation function is turned off as soon as the system is switched from the track- following to the fine seeking process, and the well-tuned coefficients xn and yn in equation
(7) obtained during the track-following are retained to cancel the disk vibration during the fine seeking. In this way, by adding the output v(t) during the fine seeking, the disk vibration source vd(t) will be completely
cancelled, resulting in a vibration free environment.
During the fast CAV-24X mode, examined is the fine seeking operation moving outward for 8 tracks to the target track and the operation begins at the instant when the disk vibration has a maximum inward velocity as shown in Figure 8. It is clear that, owing to the significant inward vibration velocity, the direction of seeking goes inward as soon as the seeking operation is initialized. As a result, the extra effort has to be made until the lens is led to the
accurate direction and the target is eventually reached.
Seeking outward for 8 tracks
outward inward outward
TES TEZC
RFZC
Figure 8. Fine seeking to outward for 8 tracks at CAV-24X without FACT.
In contrast to such an unpleasant case, however, a better result is obtained by means of FACT. The relative movement of lens with respect to the disk maintains in-phase with the disk vibration as if there were no vibration, and a steady and regular seeking motion is achieved at the high disk speed as shown in Figure 9. outward 8 tracks TES TEZC RFZC
Figure 9. Fine seeking to outward for 8 tracks at CAV-24X with FACT.
Due to the periodicity of the sinusoidal vibration, there are two maximum vibration velocity instances for each rotation of the disk. In order to show the vibration-free mechanism is a feasible scheme to deal with the maximum vibration velocity, a seeking operation experiment is conducted where the seeking period covers the entire interval of one complete disk rotation. By the
implementation of FACT, figure 10 shows a uniform movement of the fine seeking without vibration, where the seeking period is same as the time interval of one complete disk rotation when the disk is running at the speed of CAV-24X.
TES TEZC
RFZC
About one turn of disk rotation
Figure 10. Vibration free result when the seeking period equals to the time interval of one complete disk rotation for disk running at CAV-24X.
5. Conclusion
This research presents an active vibration compensation method for the track-following and fine seeking control in the optical disk drives. The vibration compensation performs well in the fine seeking, and the PUH operates as if there were no runout vacillation. Extension of the same application to the CD-ROM, CD-RW or DVD-ROM drives is also expected.
K. C. POHLMNN 1992 The Compact
Disc Handbook. A-R Editions, Inc.
[2] S. G. STAN, H. VAN K EMPEN, G. L EENKNEGT, and T. H. M. AKKERMANS 1998 IEEE Transactions on Consumer Electronics 44, 178--186. Look-ahead seek correction in high-performance cd-rom drives.
J. J. LIU and Y. P. YANG 2002
Proceedings of the 41st Conference on Decision and Control, Las Vegas, Nevada, Frequency adaptive control technique of compact disk drives for rejecting periodic vibration.
[4] J. J. LIU and Y. P. YANG will publish on Control Engineering Practice, Frequency adaptive control technique for rejecting
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