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Velocity Balancer Disclosure Number: IPCOM000015332D
Original Publication Date: 2001-Dec-23
Included in the Prior Art Database: 2003-Jun-20
Document File: 5 page(s) / 137K

Publishing Venue



Disclosed is a method to reduce head seek profile asymmetry of hard disk drive

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Velocity Balancer

Disclosed is a method to reduce head seek profile asymmetry of hard disk drive


< Problem >

Seek profile asymmetry (inner seek and outer seek profile difference) is one of well-known problem in the field of HDD servo study. This problem causes increase of seek time and difficulty of servo parameter tuning.

It is regarded that seek profile asymmetry is caused by following factors which varies according to head position. (The value differs whether current position is inner side or outer side of the disk)

1. Dragging force
2. Tension of flexible cable
3. VCM magnet flux
4. Actuator skew angle

Fig. 1 shows an example of seek profile asymmetry. In this figure, 2 different velocity lines are observed, one is for inner seek and the other for outer seek.


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Fig.1 Example of seek asymmetry profile

Generally, HDD servo seek is performed by 3 different controls, which are seek mode, settling mode and following mode. These are replaced according to the distance between current position and target position.


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Fig.2 3 different servo control mode according to the distance to target

When seek mode, actuator is accelerated with DAC max. (fixed acceleration.) However, the above 4 factors such as dragging force etc., which deccelerate actuator, are different whether head position is on inner side or outer side. Therefore, seek asymmetry is observed.

If this asymmetry continues until seek <-> settling boundary, it causes unstable behavior at seek <-> settling boundary (A part in fig.1). This is due to the difference of the initial velocity(v0) when actuator flies into settl...