Electricity: measuring and testing – Magnetic – Magnetic information storage element testing
Reexamination Certificate
1999-02-01
2001-05-22
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic information storage element testing
C318S652000, C324S210000
Reexamination Certificate
active
06236201
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to magnetic head and disk testers, and in particular, to a method and apparatus for improving the accuracy of the mechanisms that position a magnetic head with respect to a magnetic disk in a magnetic head and disk tester.
A magnetic head and disk tester is an instrument that is used for testing characteristics, such as signal-to-noise ratio, track-to-track error, of magnetic heads and disks. In some cases, a disk (e.g., a computer hard drive disk) may be tested with a known and calibrated head and in other cases a magnetic head may be tested with a calibrated disk. A magnetic disk and head tester consists of two main portions, a mechanical portion that performs movements of the head with respect to a disk supported by the tester, and an electronic portion that is responsible for measurement, calculation, and analysis of data. The mechanical portion of the tester is known as a spinstand. The quality of the results of tests performed using such a magnetic head and disk tester depends at least in part upon the positioning accuracy provided by the spinstand for the head with respect to the disk.
A typical magnetic head and disk tester of the prior art is shown schematically in
FIGS. 1 and 2
, and is described in U.S. Pat. No. 5,382,887 to Guzik, et al. In the description below, spinstands are described in which a carriage (and attached head) is selectively moved along an axis in a horizontal plane, where the head is moved with respect to a horizontally supported disk, rotatable about a vertical axis. While these vertical and horizontal reference directions are used in the exemplary spinstands, other orientations may be used in other embodiments.
The illustrated spinstand
10
includes a base
50
having a disk spindle
46
extending vertically (as illustrated) therefrom. The spindle
46
supports a disk
42
in a horizontal (as illustrated) plane, in a manner permitting controlled rotation of the disk about a vertical axis.
A carriage
30
is slidably supported on a plurality of rails
22
,
24
which are rigidly mounted to a base
50
, whereby carriage movement can occur along a horizontal axis (X-axis). A head support element
44
is secured to carriage
30
and includes a magnetic read-write head
40
mounted at its distal end. A drive assembly includes (1) a coarse positioner, for effecting gross motion of the carriage
30
along rails
22
,
24
, and (2) a fine positioner, for effecting minor motions of the carriage
30
. The movement of carriage
30
results in a corresponding movement of the magnetic head
40
to desired positions over a magnetic disk
42
. The magnetic head
40
is moveable in a radial direction relative to the disk
42
, such movement facilitating testing of a disk or a head.
In this prior art systems, the linear position of the carriage
30
relative to base
50
and thus the relative position of the magnetic read-write head
40
to the disk
42
, is measured using two linear encoders
12
and
14
that are symmetrically mounted to the carriage
30
on opposite sides of the X-axis. That is, a first encoder
12
is mounted on a right lateral side of the carriage
30
and a second encoder
14
is mounted on a left lateral side of the carriage
30
. Outputs of each encoder
12
and
14
are supplied to an arithmetic unit
52
which determines the position of the magnetic read-write head
40
using these outputs. Each encoder is substantially comprised of two parts. One part is secured to the base
50
, and so is stationary. The other part is affixed to carriage
30
, and so is moveable relative to the stationary part of the encoder. The measurement of the relative movement of the moveable part of the encoder with respect to the stationary part of the encoder is used to determine the movement and position of the head
40
relative to the disk
42
.
The coarse positioner of the spinstand includes a stepper motor
32
affixed to base
50
, a lead screw
34
, a nut
36
on the lead screw
34
and a block
38
which is mounted on rails
26
and
28
. The nut
36
is attached to the block
38
. The lead screw
34
and nut
36
are used to transfer the rotary movement of the stepper motor
32
to linear movement of the block
38
on rails
26
,
28
along X-axis.
The fine positioner of the prior art spinstand
10
includes carriage
30
which is mounted on rails
22
and
24
and moves along the X-axis, and a piezoelectric actuator
48
. The piezoelectric actuator
48
is mounted between the block
38
and the carriage
30
. The actuator
48
is responsive to voltages applied thereto, to change its dimension in the x-direction, direction, which in turn results in displacement of the carriage
30
with respect to block
38
.
In operation, the coarse positioner is able to move the carriage
30
over relatively long distances, but remains limited in linear resolution to the degree that it cannot position the magnetic read-write head
40
with the accuracy required to adequately test current heads and disks. The magnetic recording technology today requires spinstands that can position the magnetic read-write head
40
with an accuracy of about 10 nm or better, thus the need for the fme positioner. The piezoelectric actuator
48
has a much shorter movement range than the coarse positioner, and can position the carriage
30
with the required accuracy of 10 nm. A typical piezoelectric actuator
48
would be PZT-5H produced by Morgan Matroc Inc., Ohio, U.S.A. This unit has a 15 micrometer range and is able to create movement with steps shorter than 10 nm.
The prior art methods used to test the magnetic read-write heads and disks include positioning the magnetic read-write head
40
a number of times with very small displacements that require the accuracy of the fine positioner. These small movements require extreme accuracy in the fine positioner. During these movements, it is common, for the magnetic read-write head
40
to be moved from a predetermined position to a new position, and then return to the first position. A signal read by the magnetic read-write head
40
can reveal any mismatch between the intended position, and the actual position of the magnetic-read-write head
40
. The difference in position due to this forward and backward movement of the carriage
30
is called “mechanical hysteresis” and it is possible to detect this hysteresis by measuring the amplitude of the signal read by the magnetic read-write head
40
.
There are many causes for mechanical hysteresis, and two of the most common ones in a spinstand are the yaw and pitch of the carriage
30
. The term “yaw” refers to angular motion of the carriage
30
about a vertical axis. The term “pitch” refers to angular motion of the carriage
30
about a horizontal axis which is perpendicular to the x-axis.
The position of the carriage
30
is measured using the linear encoders
12
,
14
during each coarse and fine positioning movement. In an ideal case, the linear encoders are mounted in close proximity to the magnetic read-write head
40
, and therefore would measure its actual position. In reality, due to mechanical, limitations, it is often not possible to position the linear encoders this way. Rather, the encoders are mounted away from the head. This orientation leads to errors in the measurement of the position of the head, due to the pitch and yaw motions of the carriage. Therefore, a plurality of linear encoders need to be used to best determine the actual position of the head. In U.S. Pat. No. 5,382,887, granted Jan. 17, 1995, and assigned to the assignee of the present invention, the yaw motion of a carriage (and attached head) is detected by placing a linear encoder
12
and
14
on each side of the carriage, parallel to the direction of the movement of the carriage and symmetrically with respect to the center line (X-axis) of the carriage. Using the difference in the readout of the two linear encoders
12
and
14
, the amount of yaw that occurs during the positioning movements can be determined. However, there is no current meth
Guzik Nahum
Karaaslan Ufuk
Kilicci Cem
Guzik Technical Enterprises
Knox Amber C.
McDermott & Will & Emery
Snow Walter E.
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