Magnetic head and disk tester with yaw correction

Details Magnetic head and disk tester with yaw correction Magnetic head and disk tester with yaw correction

1999-02-01

2001-06-05

Strecker, Gerard R. (Department: 2862)

Electricity: measuring and testing

Magnetic

Magnetic information storage element testing

C324S262000, C360S294400

Reexamination Certificate

active

06242910

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to magnetic head and disk testers, and more particularly to testers with improved accuracy in positioning a magnetic head with respect to a disk.
A magnetic head and disk tester is an instrument that is used for testing the characteristics of magnetic heads and disks such as signal-to-noise ratio, pulse width and so on. Each tester includes two main assemblies, i.e., an electro-mechanical assembly that performs movements of the head with respect to the disk, and an electronic assembly that is responsible for measurements, calculations, and analysis of the measured data. The electro-mechanical assembly of the tester is known as the spinstand. The spinstand generally simulates the motions of the head with respect to the disk that occur in, for example, a hard disk drive. Whatever the accuracy of the electronic measurement portion of the tester, the results of measurements will also depend upon the positioning accuracy provided by the spinstand.
An exemplary spinstand
5
of a prior-art head and disk tester is shown schematically from a top view in FIG.
1
A. The spinstand
5
includes a stationary base element
30
that supports the positioning system and the head
12
and disk
10
to be tested. The disk
10
is supported in a preferably horizontal plane in a manner allowing rotary motion of the disk
10
about a spindle axis perpendicular to that horizontal plane. The spinstand
5
includes a coarse positioning system and a fine positioning system arranged in series to effect controlled movement of head
12
with respect to disk
10
. The coarse positioning system positions the magnetic head
12
close to its desired position relative to a magnetic disk
10
. In the illustrated form, the coarse positioning system includes a stepper motor
28
affixed to base
30
. The stepper motor
28
rotationally drives a lead screw
32
that rotates within bearings
24
and engages a nut
34
. Nut
34
is rigidly fixed to a slide
18
so that rotary motion of lead screw
32
effects linear motion of slide
18
along guides (not shown) with respect to base element
30
, along a translation axis X, or X-axis.
The fine positioning system of spinstand
5
resides on slide
18
and effects relatively minor positional changes to the position of head
12
illustrated by the slide
18
. In the illustrated form, the fine positioning system includes a piezo actuator
26
that is disposed between a stop
36
that is rigidly mounted on slide
18
and a deformable (in the direction of x-axis) body
16
also mounted on slide
18
. Two bolts
22
a
and
22
b
are screwed into deformable body
16
through openings in the stop
36
. Piezo actuator
26
is preloaded by springs
20
a
,
20
b
that are compressed between the heads of the bolts
22
a
,
22
b
and the stop
36
. The deformable body
16
at its base is rigidly coupled to slide
18
. The top of body
16
is moveable, in response to the piezo actuator
26
, supports arm
14
, which in turn supports head
12
. Arm
14
is coupled to link
16
a
by a shaft
25
. Body
16
functions as a parallel-link mechanism that is sensitive to the expansions and contractions of piezo actuator
26
to small linear displacements (e.g., 0.001 in) for head
12
, (relative to disk
10
, as supported on base
30
) in addition to the major displacements effected by the coarse positioning system.
FIG. 1B
shows side view of an exemplary form of deformable body
16
in the system of FIG.
1
A. In this form, the deformable body
16
is a parallelogram-structured deformable body comprised of a top and a bottom rigid links
16
a
and
16
b
, disposed in parallel, coupled by two side rigid links
16
c
and
16
d
, wherein flexures are at the junction of link pairs to allow for angular displacement of the elements while substantially maintaining the parallelogram integrity of the structure. With this structure the piezo element
26
drives the uppermost, as shown, or the top link
16
a
of deformable body
16
in the x direction relative to slide
18
(and base
30
), whereby the magnetic head
12
to be tested remains substantially at the same height throughout the range of its displacement.
Movements of the link
16
a
of deformable body
16
are measured by an optical linear encoder
38
a
,
38
b
, as shown in FIG.
1
A. The optical linear encoder
38
consists of a moveable portion
38
a
(i.e., a glass scale) that is rigidly attached to the top link
16
a
of deformable body
16
and a stationary portion
38
b
(i.e., an optical detector) fixed to base
30
. A signal generated by optical detector
38
b
corresponds to movements of top link
16
a
of deformable body
16
relative to base
30
. That signal corresponds to a sum of the linear displacement established by the steppers motor
28
and by the piezo actuator
34
(together with deformable body
16
).
Thus, to achieve high accuracy in linear positioning of head
12
over magnetic disk
10
, the positioning process is split into steps of coarse and fine positioning. The coarse positioning is provided, in part, by the rotation of lead screw
32
by stepper motor
28
. Rotational movement of lead screw
32
is translated into a linear movement of slide
18
by nut
34
. Upon completion of coarse positioning, fine positioning is activated by applying a voltage to piezo actuator
26
from an external power supply (not shown). In a manner known in the art, under the effect of the voltage, actuator
26
changes its linear dimension in proportion to the level of the applied voltage. As a result, the top link
16
a
together with arm
14
and a magnetic head
12
is shifted with respect to magnetic disk
10
in the X direction. The displacement of magnetic head
12
is measured by optical linear encoder
38
and sent to a feedback circuit (not shown) to control the amount of displacement of the deformable body
16
, in a manner known in the art.
During the testing, when the top link
16
a
of deformable body
16
moves arm
14
with magnetic head
12
mounted thereto, an optical linear encoder
38
is used to determine the position of magnetic head
12
. In the prior art, the displacement measured by optical linear encoder
38
is considered to be substantially the same as the displacement of the magnetic head
12
. However, in practice, the top link
16
a
of the deformable body
16
may experience yaw (i.e. rotational displacement about an axis perpendicular to the nominal (horizontal) plane of allowed movement) during the movement. Yaw can occur due to different (asymmetrical) stiffness of the weakened portions (i.e. the flexures) of the deformable body
16
, or due to different stiffnesses of the springs
20
a
and
20
b
.
FIG. 2
shows the effect of the parallelogram-structured deformable body
16
rotating about a point O in the direction indicated by arrow A. As shown, the head
12
moves from an original point P to a point Q. This movement corresponds to a shift X
1
in the X direction, and to a shift Y
1
, in the Y direction. Optical linear encoder
38
a
,
38
b
can only detect movements in the X direction; in this particular case, it detects, a movement of X
2
, which is not equal to X
1
. The difference X
1
-X
2
and the shift Y
1
, cannot be compensated by the normal, prior art feedback circuit, since the yaw component is undetectable. Therefore, the prior art spinstand
5
shown in
FIGS. 1A and 1B
cannot achieve very high positioning accuracy.
This problem of accuracy is solved to some degree in a prior art disk and head tester designated as Model 1701, developed and manufactured by Guzik Technical Enterprises, San Jose, Calif. This tester uses a high-precision micropositioning mechanism that performs fine movements. Although this mechanism operates very efficiently and is advantageous for some applications, it is expensive to manufacture because it requires the use of many interacting parts, relative to, for example, the tester of
FIGS. 1A
,
1
B, and
2
.
Another disadvantage of the prior art spinstand shown in
FIGS. 1A
,
1
B, and
2
is that t

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