Head Positioning to Servowrite Magnetic Disk Files

Head Positioning to Servowrite Magnetic Disk Files

      This article describes a scheme for remote positioning of
the read/write heads in a magnetic disk file with sufficient accuracy
to allow the writing of servo tracks during the production process.
It can be used without access to the inside of the disk enclosure,
permitting the servo writer to be used in a non-cleanroom
environment.

      The disk file cover will have a small clear plastic window
fitted, aligned with the actuator travel, such that the head mounting
bar is visible throughout the actuator travel.  A small
front-silvered mirror is attached to the top of the actuator below
this window.  The mirror is half occluded by a non-reflective paint
layer or some other feature along a line perpendicular to the
actuator travel. As part of the servowriter in which this scheme is
implemented, an accurate positioning system is located external to
the disk enclosure, travelling along a line which aligns with the
produce actuator.  This external positioning system carries a source
of collimated light which shines through the transparent window in
the file cover.  The half-occluded mirror on the actuator reflects
the light, and the return beam is separated by a half-silvered mirror
arrangement.  The returned image of the reflective/non-reflective
boundary is suitably magnified, and focused onto a photodetector.
Movement of the actuator in one direction will increase the portion
of the detector that is illumin...