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Laser Interferometry Technique for Track Following Servo Control of DASD Head Location

IP.com Disclosure Number: IPCOM000049956D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 4 page(s) / 49K

Publishing Venue

IBM

Related People

Levenson, MD: AUTHOR

Abstract

This article describes a variety of laser interferometry techniques for measuring the distance between the slider on which the magnetic heads of a direct-access storage device (DASD) are mounted and a cylindrical reflector concentric with the axis of the disk spindle. The measured distance and velocity can be used to continually update the head position servo and thus allow track following. In each variation, the cylindrical reflector around the spindle and another reflector mounted on the slider form part of the interferometer. Single Frequency Laser Interferometry

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Laser Interferometry Technique for Track Following Servo Control of DASD Head Location

This article describes a variety of laser interferometry techniques for measuring the distance between the slider on which the magnetic heads of a direct-access storage device (DASD) are mounted and a cylindrical reflector concentric with the axis of the disk spindle. The measured distance and velocity can be used to continually update the head position servo and thus allow track following. In each variation, the cylindrical reflector around the spindle and another reflector mounted on the slider form part of the interferometer. Single Frequency Laser Interferometry

The output of a stabilized single frequency laser is split into two beams by the beam splitter B in Fig. 1, with most directed towards the cylindrical reflector and part through an eighth wave plate and towards a reference reflector. Light reflected by the cylinder towards the mirror M on the slider is partially retroreflected back to the cylindrical reflector through a linear polarizer L and into the beam splitter. The beam splitter combines the waves from the cylinder and from the reference surface, as in a Michelson interferometer, and directs the resultant beam through a polarization analyzer A with a small aperture into photodiodes D and E. Changes in the distance between M and the cylinder change the phase of the reflected beam from one arm, causing the intensity at the detectors to vary. Each period of oscillation corresponds to a translation of the slider Lambda/2. The polarization analyzer A is oriented to direct equal amplitudes from the reference reflector and from the slider reflector into each of the two diodes. The beam from the slider is linearly polarized, while that from the reference is circularly polarized. Hence there will be a phase shift of Pi/2 between the fringe patterns detected by diodes D and E. The direction of the motion of the slider can be determined from the relative phases of the oscillating diode signals. The frequency of the oscillation is proportional to the relative velocity of the slider and cylinder. The frequency can be integrated to determine the position.

In operation, the system is first set to track zero, and the slider position is fixed. Eccentricities in the cylindrical reflector will result in a relative velocity that repeats on every rotation of the spindle. This eccentricity is measured and stored in the memory of the microprocessor used to control the servo. It will be subtracted from the velocity readings measured later when the servo is activated. In finding or following another track, the micro-processor integrates the difference between the relative velocity measured for a given disk angle and that found for the circular zero track previously stored. This quantity is then used to update the servo. If the signal is lost, the slider must return to the zero track before searching for the desired track.

Fig. 2 shows a fiber-optic impl...