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Touch down detection method using weigh factors for radial and circumferential components of frictional force Disclosure Number: IPCOM000207622D
Publication Date: 2011-Jun-06
Document File: 3 page(s) / 37K

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The Prior Art Database


Touch down detection can be done using a variety of methods to sense contact. Some of those methods perform better at particular radial locations than others. This disclosure identifies the problem and proposes the solution, with a specific discussion of two touch-down detection methods that use frictional force in different directions to identify head-disk contact. This is not limited to the two specific detection methods discussed, they just provide an easy way to discuss the problem and the solutions. One method uses timing-based signal as the input which represents circumferential component of frictional force. The other method uses VCM (voice coil motor) current signal as the input which represents radial component of frictional force. These two methods have pros and cons in the view point of touch down detection sensitivity. This article offers better sensitivity utilizing a combined hybrid signal of these two methods.

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Page 01 of 3

Touch down detection method using weigh factors for radial and circumferential components of frictional force


Two or more kinds of touch down detection methods based on frictional force have pros and cons at different radial locations (see fig.1 and fig. 2).
1) Timing-based signal does not have very good S/N (=repeatability) at ID (inner diameter)
2) VCM (voice coil motor) current shift is not available at MD (middle diameter) skew zero


Judge touch down according to combination of two kinds of signal, with importance weighting that depends on radial location.

For better S/N, it is effective to introduce weight factors (see fig. 3 equation).

The weight factors are calculated from radius and skew angle
Example factor for timing is "proportional to radius". Example factor for VCM is "proportional to absolute skew angle"

Fig. 1


Page 02 of 3


Timing signal depends on torque (radius)

VCM current depends on skew angle

Disk center


Disk rotation

Head at ID

MD skew zero

VCM pivot

Fig. 2

Pros and cons

Frictional force (blue)

Timing signal (green)

VCM current shift (red)














Page 03 of 3

Fig. 3

Hybrid signal = (weigh factor 1) * (timing signal)^2 + (weigh factor 2) * (VCM current shift)^2

Disclosed by Hitachi Corporation