U.S. patent application number 10/897882 was filed with the patent office on 2006-01-26 for permanent magnet bulk degausser.
This patent application is currently assigned to Data Security, Inc.. Invention is credited to Robert A. Schultz.
Application Number | 20060018075 10/897882 |
Document ID | / |
Family ID | 35149389 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060018075 |
Kind Code |
A1 |
Schultz; Robert A. |
January 26, 2006 |
Permanent magnet bulk degausser
Abstract
One or more pairs of magnet assemblages (14 and 16) are provided
with magnetized segments (21-30) arranged in a Halbach-like array.
The magnet assemblages (14 and 16) define a gap (18) through which
magnetic data storage media (12) pass in a direction (20) across
the segments (21-30). The magnetized sides (36) of the magnet
assemblages (14 and 16) face each other thereby creating strong
magnetic fields which degauss the magnetic data storage media (12)
passing through the gap (18).
Inventors: |
Schultz; Robert A.;
(Lincoln, NE) |
Correspondence
Address: |
FITCH EVEN TABIN AND FLANNERY
120 SOUTH LA SALLE STREET
SUITE 1600
CHICAGO
IL
60603-3406
US
|
Assignee: |
Data Security, Inc.
|
Family ID: |
35149389 |
Appl. No.: |
10/897882 |
Filed: |
July 23, 2004 |
Current U.S.
Class: |
361/143 ;
G9B/5.028 |
Current CPC
Class: |
G11B 5/0245 20130101;
H01F 13/006 20130101 |
Class at
Publication: |
361/143 |
International
Class: |
H01H 47/00 20060101
H01H047/00 |
Claims
1. An apparatus for erasing magnetic storage media comprising: at
least one pair of magnet assemblages; each magnet assemblage
comprising at least three segments; each segment having a direction
of magnetization approximately perpendicular to the longest
dimension of the segment; at least one magnet assemblage arranged
such that the segments are aligned adjacently with the direction of
magnetization of each successive segment rotated by approximately
90 degrees relative to the previous segment wherein the direction
of magnetization for a segment repeats every fifth segment if the
magnet assemblage includes five or more segments; and a gap defined
by each pair of magnet assemblages such that a magnetic field
created by each assemblage exists at least partially in the
gap.
2. The apparatus of claim 1 wherein at least one segment is an
integral piece.
3. The apparatus of claim 1 wherein at least one segment comprises
a plurality of permanent magnets arranged in at least one row such
that each permanent magnet in the segment has a direction of
magnetization pointing in the same direction.
4. The apparatus of claim 3 wherein at least one permanent magnet
includes a cross section in the shape of a square and a height that
is one half of a length of a side of the square with the direction
of magnetization in the direction of the height of the permanent
magnet.
5. The apparatus of claim 3 wherein at least one permanent magnet
is a neodymium-iron-boron block.
6. The apparatus of claim 3 wherein the direction of magnetization
for each segment is either approximately perpendicular to or
approximately parallel to the gap and each segment with a direction
of magnetization substantially perpendicular to the gap includes at
least two rows of permanent magnets and each segment with a
direction of magnetization substantially parallel to the gap
includes fewer rows of permanent magnets than the segments with a
direction of magnetization substantially perpendicular to the
gap.
7. The apparatus of claim 1 wherein the direction of magnetization
for each segment is oblique relative to the gap.
8. The apparatus of claim 7 wherein at least one pair of magnet
assemblages is aligned such that segments from each magnet
assemblage line up across the gap and that each segment mirrors the
direction of magnetization of the segment directly across the
gap.
9. The apparatus of claim 1 wherein the direction of magnetization
for at least one segment is either approximately perpendicular to
or approximately parallel to the gap.
10. The apparatus of claim 9 wherein each pair of magnet
assemblages includes the same number of segments.
11. The apparatus of claim 10 wherein each pair of magnet
assemblages is aligned such that segments from each magnet
assemblage line up across the gap and that each segment mirrors the
direction of magnetization of the segment directly across the
gap.
12. The apparatus of claim 10 wherein each pair of magnet
assemblages is aligned such that segments from each magnet
assemblage line up across the gap and that each segment with a
direction of magnetization approximately perpendicular to the gap
points in approximately the same direction as the segment across
the gap.
13. The apparatus of claim 10 wherein at least one pair of magnet
assemblages is aligned such that segments from each magnet
assemblage line up across the gap and that each segment mirrors the
direction of magnetization of the segment directly across the gap,
at least one pair of magnet assemblages is aligned such that
segments from each magnet assemblage line up across the gap and
that each segment with a direction of magnetization approximately
perpendicular to the gap points in approximately the same direction
as the segment across the gap, and the gaps for each pair of magnet
assemblages line up such that magnetic media storage may pass
directly from one gap to the other.
14. The apparatus of claim 1 wherein at least one pair of magnet
assemblages is aligned such that segments from at least one magnet
assemblage are offset relative to the segments across the gap.
15. The apparatus of claim 1 wherein at least one magnet assemblage
comprises a Halbach array across the segments.
16. The apparatus of claim 1 further comprising an adjustable frame
structure securing the magnet assemblages such that the width of
the gap between the magnet assemblages can be adjusted.
17. The apparatus of claim 1 further comprising a lateral
adjustable frame structure securing the magnet assemblages such
that the lateral position of the magnet assemblages can be
adjusted.
18. The apparatus of claim 1 wherein the two magnet assemblages of
at least one pair of magnet assemblages have different numbers of
segments.
19. An apparatus for erasing magnetic storage media comprising: at
least two pairs of magnet assemblages; each magnet assemblage
comprising at least three segments; each segment having a direction
of magnetization approximately perpendicular to the longest
dimension of the segment; each magnet assemblage arranged such that
the segments are aligned adjacently with the direction of
magnetization of each successive segment rotated by approximately
90 degrees relative to the previous segment wherein the direction
of magnetization for a segment repeats every fifth segment if the
magnet assemblage includes five or more segments; a gap defined by
each pair of magnet assemblages such that a magnetic field created
by each assemblage exists at least partially in the gap; and at
least one of the at least two pairs of magnet assemblages arranged
with the segments of the first magnet assemblage aligned at 90
degrees relative to the segments of the second magnet assemblage
and with the segments of both magnet assemblages at a 45 degree
angle relative to a magnetic storage media path through each gap
defined by each pair of magnet assemblages.
20. The apparatus of claim 19 wherein the at least one of the at
least two pairs of magnet assemblages are further arranged such
that each of the magnet assemblages are arranged in repulsion.
21. The apparatus of claim 19 further comprising: at least one
additional pair of magnet assemblages disposed on the magnetic
storage media path; each magnet assemblage of the at least one
additional pair comprising at least three segments; each segment of
the magnet assemblages of the at least one additional pair having a
direction of magnetization approximately perpendicular to the
longest dimension of the segment; each magnet assemblage of the at
least one additional pair arranged such that the segments are
aligned adjacently with the direction of magnetization of each
successive segment rotated by approximately 90 degrees relative to
the previous segment wherein the direction of magnetization for a
segment repeats every fifth segment if the magnet assemblage
includes five or more segments; and the at least one additional
pair of magnet assemblages arranged in attraction.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to magnetic degaussers and
more particularly to permanent magnet magnetic degaussers for
erasing magnetic data storage devices.
BACKGROUND
[0002] Magnetic degaussing systems of various kinds are known in
the art. Typically, magnetic fields of varying strength and
direction are applied to the item to be degaussed forcing the
magnetization within the object to change thereby destroying any
patterns therein. Magnetic degaussing systems have become
increasingly important with the increasing use of magnetic data
storage. Data stored magnetically can remain on the storage medium
for long periods of time after its use. For example, a computer
disk's data can be retrieved even after a user has "erased" the
data from the disk because the old data will not be changed until
new data is written over that segment of the disk. If another
person were to obtain the disk, that person may be able to access
information from that disk.
[0003] In the art of bulk degaussing of magnetic data storage
media, electrically powered degaussing systems are commonly used.
For example, laminated steel cores of extruded "U" shapes in
association with electrical windings are generally recognized as
one configuration suitable for erasure of magnetic data storage
media. Similarly, "E" shaped cores may be used. Pairs of such cores
are often configured opposite each other with like poles facing,
although single sided and offset configurations are also known in
the art. Although such configurations are suitable for some
situations, these systems have the disadvantage of needing a power
source to create the fields necessary for magnetic data storage
media erasure.
[0004] More recently, the discovery and improvement of rare earth
permanent magnets have made the generation of magnetic fields of
strengths suitable for bulk media erasure using permanent magnets
practical. Such permanent magnets can be arranged with steel
elements into magnetic circuits that act much like their electric
counterparts. The weight requirements of permanent magnet systems
are about equal to the electric systems. Further, the zero power
input required by permanent magnets offsets higher production costs
as compared to electric systems.
[0005] Another advantage of permanent magnet systems includes the
use of individual elements, which may be off-the-shelf items,
rather than trying to fabricate large elements or permanently
magnetizing a single large shape. For example, it is known that a
total of eight 2-inch by 2-inch by 1-inch neodymium-iron-boron
(NeFeB) blocks, magnetized in the 1-inch direction, can be adhered
by magnetic attraction onto steel plates as groups of four blocks
thereby forming two 2-inch by 8-inch poles, a classic "U" shape
magnet of 8-inch depth. Two such "U" shapes can be configured with
like poles facing in repulsion across a gap suited to passage of
1-inch thick magnetic media. Such an assemblage can apply a
magnetic field with good uniformity and at least 6000 gauss to
every point in a common form factor for magnetic data storage media
passing through that field. It is understood that at least a second
passage of a magnetic storage medium through the field with a
different orientation between the storage medium and the magnetic
field is necessary to impart the desired change within the storage
medium to affect magnetic data storage erasure.
[0006] Despite the advantages of these known permanent magnet
systems, certain drawbacks exist. For instance, magnetic data
storage media are being developed with increasing magnetic
coercivities such that much stronger fields must be applied to
completely erase the media. As such, the 6000 gauss strength
achieved by known permanent magnet bulk degaussing systems is
marginal with respect to the emerging media's coercivities.
[0007] Attempts to increase the strength of the known permanent
magnet bulk degaussing systems by scaling up the systems, however,
quickly lead to diminishing returns. Such scaling of prior art
includes stacking off-the-shelf elements in their direction of
magnetization, placing elements side by side on the steel plates,
stacking and placing elements, or substituting larger custom-made
elements or magnets for the off-the-shelf elements. It is generally
recognized in the art of bulk degaussing that worst case field
strength drives performance and that a measure of nonuniformity in
field strength can be tolerated. It is also known that attempts to
furnish field strengths sufficient for erasure of magnetic storage
media with higher coercivities using various prior art facing "U"
arrangements would require at least a correspondingly increased
amount of NeFeB or other magnetic material plus thick steel
components needed to complete the required magnetic circuit. Such a
system would result in an unacceptable degree of field strength
nonuniformity across the gap. In particular, the diminishing
returns from prior art scaling using NeFeB elements arise due to
flux leakage from NeFeB elements to each other and into the steel
plates where media cannot be placed to affect erasure.
[0008] Additionally, any such scaling results in larger volume,
increased weight, and greater cost. It is well known that in the
assembly of the prior art permanent magnet systems, regions of both
magnetic attraction and magnetic repulsion will arise between
various elements and members. For example, magnets are attracted to
steel plates and to each other when stacked with unlike poles
facing. Conversely, placing magnets adjacent to each other with the
same magnetic direction causes repulsion, as does placing like
poles facing each other across a gap. To counter such forces,
framework members must be added. In the prior devices, a thick
steel plate serves a dual role as a required component of the
magnetic circuit and as one of the framework members, but other
members generally must be of nonmagnetic materials to avoid
undesirable magnetic circuit paths or unnecessary magnetic field
fringing effects. In particular, prior devices require an
attraction-countering member between unlike poles, which
experiences extreme compressive force, and this member cannot be
magnetic steel. These structural requirements only become
aggravated with the scaling of the prior permanent magnet
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The above needs are at least partially met through provision
of the permanent magnet bulk degausser described in the following
detailed description, particularly when studied in conjunction with
the drawings, wherein:
[0010] FIG. 1 is a perspective view of a permanent magnet bulk
degausser embodying features of the present invention;
[0011] FIG. 2 is a side plan view of a Halbach array of square
cross-section permanent magnet elements with directions of
magnetizations shown by arrows;
[0012] FIG. 3 is a perspective view of a preferred permanent magnet
element;
[0013] FIG. 4a is a side plan view of a model of the magnetic
fields created by a pair of magnet assemblages in accordance with
the array of FIG. 2;
[0014] FIG. 4b is a side plan view of a model of the magnetic
fields created by the pair of magnet assemblages illustrated in
FIG. 1;
[0015] FIG. 5a is a graph showing the magnetic flux density along
the gap between a pair of magnet assemblages in accordance with
FIG. 4a;
[0016] FIG. 5b is a graph showing the magnetic flux density along
the gap between a pair of magnet assemblages in accordance with
FIG. 4b;
[0017] FIG. 6 is a perspective view of an alternate permanent
magnet bulk degausser embodying features of the present
invention;
[0018] FIG. 7 is a perspective view of a prior art permanent magnet
bulk degausser;
[0019] FIG. 8 is a side plan view of an alternate permanent magnet
bulk degausser embodying features of the present invention;
[0020] FIG. 9 is a perspective view of a frame structure for use
with various embodiments of the permanent magnet bulk
degausser;
[0021] FIG. 10 is a side plan view of the frame structure of FIG.
9;
[0022] FIG. 11 is a side plan view of an alternate permanent magnet
bulk degausser embodying features of the present invention;
[0023] FIG. 12 is a side plan view of a model of the magnetic
fields created by the pair of magnet assemblages illustrated in
FIG. 11; and
[0024] FIG. 13 is a top plan view of an alternate permanent magnet
bulk degausser embodying features of the present invention.
[0025] Skilled artisans will appreciate that elements in the
figures are illustrated for simplicity and clarity and have not
necessarily been drawn to scale. For example, the dimensions and/or
relative positioning of some of the elements in the figures may be
exaggerated relative to other elements to help to improve
understanding of various embodiments of the present invention.
Also, common but well-understood elements that are useful or
necessary in a commercially feasible embodiment are often not
depicted in order to facilitate a less obstructed view of these
various embodiments of the present invention. It will also be
understood that the terms and expressions used herein have the
ordinary meaning as is accorded to such terms and expressions with
respect to their corresponding respective areas of inquiry and
study except where specific meanings have otherwise been set forth
herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0026] With reference to FIG. 1, there is illustrated a permanent
magnet bulk degausser 10 for erasing magnetic storage media 12. The
apparatus 10 includes a pair of magnet assemblages 14 and 16
arranged so as to define a gap 18 through which magnetic storage
media 12 passes in the direction as indicated by arrow 20 across
each segment 21-25 and 26-30 of the assemblages 14 and 16. By
moving in this direction 20, the magnetic data storage medium 12
passes through the magnetic field created by the magnet assemblages
14 and 16 thereby facilitating erasure of data on the medium 12.
One should note that the magnetic data storage medium 12 can be any
medium including magnetic tape, computer disks, hard drives, and
the like.
[0027] The segments 21-25 and 26-30 are aligned adjacently within
each magnet assemblage 14 and 16 with the direction of
magnetization of each successive segment rotated by approximately
90 degrees relative to the previous segment. More specifically, the
direction of magnetization across successive segments rotates in
the same direction so that the direction of magnetization repeats
within a magnet assemblage only every fifth segment. This
magnetization arrangement is commonly known as a Halbach array. In
a variation on the traditional Halbach array, segments 22 and 24 of
magnet assemblage 14 with directions of magnetization approximately
perpendicular to the gap 18 have two rows of permanent magnets,
whereas segments 21, 23, and 25 with directions of magnetization
approximately parallel to the gap 18 have one row of permanent
magnets.
[0028] The traditional Halbach array ascribed to Klaus Halbach, as
conventionally illustrated in two dimensions in FIG. 2, includes a
linear sequence of adjacent squares 31-35 magnetized such that the
direction of magnetization in each adjacent square rotates 90
degrees with respect to its neighbor, with the direction of
rotation constant from element to element. The arrows designate a
direction of magnetization pointing from magnetic South to magnetic
North; however, this convention may be reversed without affecting
performance as long as the convention is uniformly applied within a
given embodiment. The Halbach array arrangement forms a strongly
magnetic side 36. Neglecting slight imperfections in dimension,
shape, and magnetization, side 38 is largely self-shielding and
nonmagnetic. Such linear arrays can be illustrated as an unlimited
sequence, and the square element construction shown in FIG. 2
typically yields a substantially sinusoidal magnetic field strength
along the direction of the array on the magnetic side 36 of the
array. As such, the magnet assemblages 14 and 16 of FIG. 1 are
arranged with the magnetic side of each assemblage facing the gap
18.
[0029] Preferably each segment 21-30 includes a plurality of
permanent magnets arranged in at least one row such that each
permanent magnet in the segment has a direction of magnetization
pointing in the same direction, substantially perpendicular to the
length of the row. The preferred permanent magnet element 40 as
illustrated in FIG. 3 is a readily available NeFeB block such as a
2-inch square by 1-inch thick block with a direction of
magnetization (as indicated in the figures by an arrow) in the
direction of the block's thickness. Such a magnetization produces a
magnetic North pole on one 2-inch square face of the block and a
magnetic South pole on the opposite 2-inch square face. Neglecting
fringing effects at the ends, each preferred permanent magnet
generates a 2-inch wide field in the magnetized direction. Placing
additional preferred permanent magnets in a row will provide
4-inch, 6-inch, and so on wide fields. As is the case with prior
devices, one additional adjacent magnet suffices to counter
fringing effects.
[0030] One should understand that in three dimensions, such
elements or segments depicted as having a square cross section may
be square plates, cubes, or rods. Similarly, other permanent
magnetic materials may be used. For example, SmCo blocks have
aspect characteristics similar to NeFeB and can substitute for it.
Also, a particular element size is not necessary. For instance,
various segments 21-25 or 26-30 within a magnet assemblage 14 or 16
may have varying sizes and/or shapes. Alternatively, each segment
can be an integral permanent magnet with a magnetization in a
direction substantially perpendicular to the segment's longest
dimension. Also, a complex fixture could magnetize a single large
block into a one-piece magnet assemblage with several differently
magnetized segments of the block.
[0031] Additionally, it is understood that assembling the invention
from individual blocks can introduce acceptable minor field
imperfections due to surface roughness, size and shape tolerance,
and the common practice of plating NeFeB material. Similarly,
introduction of thin nonmagnetic elements such as shims between
permanent magnet elements 40 or segments 21-25 or 26-30 may
introduce some acceptable field imperfections. Likewise, relatively
thin and magnetically soft ferromagnetic materials introduced as
shims between permanent magnet elements 40 or segments 21-25 or
26-30 would hardly disturb the fields.
[0032] FIGS. 4a and 4b model the magnetic flux vectors of two
embodiments where the magnet assemblages are arranged in repulsion
across the gap 18. For objective comparisons, all models disclosed
herein use residual flux density (B.sub.r) of 10,000 gauss. One
skilled in the art will recognize that NeFeB grades are available
with B.sub.r exceeding 13,000 gauss. FIG. 4a demonstrates the
magnetic flux for an embodiment using a traditional Halbach array
as illustrated in FIG. 2 with square segment cross-sections. FIG.
4b models the magnetic flux for the preferred embodiment
non-traditional Halbach array as illustrated in FIG. 1. For both
embodiments, magnetic flux concentrates within the gap 18, and
minimal magnetic flux is present outside the gap 18.
[0033] FIG. 5a illustrates a spatial waveform derived from the
internal field of the magnet assemblage pair of FIG. 4a. It can be
seen that the waveform of FIG. 5a approximates a "windowed"
sinusoid. FIG. 5b illustrates a spatial waveform derived from the
internal field of the preferred embodiment model of FIG. 4b. It can
be seen that the waveform of FIG. 5b has a distinctively triangular
characteristic when compared to the waveform of FIG. 5a.
[0034] The harmonic content above the fundamental as seen in FIG.
5b may be detrimental to some Halbach applications, such as for
particle beam accelerator components. Peak strength, however, is
paramount in the art of erasing magnetic media, and the harmonic
content of the numeric analysis given in FIG. 5 indicates a 4%
stronger field, nearly a 10,000 gauss peak magnetic field, for the
preferred embodiment non-traditional Halbach array when compared to
the traditional Halbach array embodiment. By contrast, prior art
magnetic circuits, such as illustrated in FIG. 7, generate only
about half this strength, and scaling of the prior art magnetic
circuit shown in FIG. 7 by adding additional permanent magnets
fails to achieve the field strengths of the embodiments of the
invention while using a comparable amount of NeFeB. For example,
doubling the NeFeB material in either of two dimensions of the
prior permanent magnet degausser of FIG. 7 increases the magnetic
strength from about half that of the embodiments of FIGS. 4a and 4b
to about 70% of that strength. Doubling NeFeB in both dimensions of
the prior art degausser uses more material than a non-traditional
Halbach embodiment but has several percent less field strength.
[0035] Alternatively, Halbach-like arrays of more or less than five
segments can be utilized. For example, a mirror-imaged pair of
three-segment (as illustrated in FIG. 6) or five-segment (as
illustrated in FIG. 1) assemblages with magnetic sides facing in
repulsion creates fields much like the prior art permanent magnet
facing "U" arrangements (as illustrated in FIG. 7), but each
embodiment offers respectively improving degrees of uniformity of
field. Simulations indicate that the three-segment arrangement of
FIG. 6 nearly doubles the field strength of the prior permanent
magnet arrangement of FIG. 7. A seven-segment arrangement not only
doubles the prior arrangement's strength, but also produces two
magnetic fields of equal strength and opposite direction along a
media path 20.
[0036] In one such alternative embodiment illustrated at FIG. 6, as
few as three segments 64, 65, and 66 can be arranged within a
magnet assemblage 62 in a configuration not generally recognized as
a complete Halbach array, but still effective for erasing magnet
data storage media. The magnet assemblages 60 and 62 of FIG. 6 each
have magnetic sides facing toward the gap 18 through which magnetic
data storage medium 12 passes. The segments 64-66 of magnet
assemblage 62 line up across the gap 18 from the segments 67-69 of
magnet assemblage 60 such that the directions of magnetization of
segments 64-66 mirror the directions of magnetization of segments
67-69 in what is known as an arrangement in repulsion.
[0037] The alternative embodiment of FIG. 6, if built using the
preferred permanent magnet, saves 28% on material cost and weight
as compared to the embodiment of FIG. 1. Although the alternative
embodiment of FIG. 6 also includes less field strength per unit gap
width and slightly less uniformity across the gap, such an
embodiment could be applied, for example, with a narrower gap 18 to
achieve higher strength for future and continually smaller
varieties of magnetic storage media.
[0038] In addition to the field strength and uniformity advantages
of the various embodiments, there is much less need for steel
elements and framing materials when compared to prior permanent
magnet devices. Contrary to the prior permanent magnet devices,
steel is not required for any supporting members or magnetic
circuit elements. Also, any such shielding of the small magnetic
flux leakage of the various embodiments would only be needed for
certain applications such as against compass interference in
airborne or other mobile applications. Typically, thin steel also
suffices to shield against the slight magnetic flux leakages
arising from imperfections in magnet element dimensions and
magnetization. In applications where shielding is not a factor,
nonmagnetic materials having better strength to weight
characteristics can alternatively be used for framing.
Additionally, the repulsive or attractive forces between the magnet
assemblages of the various embodiments are generally reduced in
comparison to prior conventional degaussers. Thus, less extensive
framing support is needed.
[0039] In alternative embodiments, the overall size of the
degausser 10 can be manipulated. For instance, a data processing
operation that depends on erasing a large quantity of
microminiaturized hard disk drives could benefit from a drastically
scaled down version of the invention. In one example, it is now
feasible to issue a personal digital assistant (PDA) for each
patient entering a hospital. Also, each PDA may include an
apparatus for removeably connecting an inexpensive 5 mm thick 4G
Byte disk drive. The PDA could conveniently accompany a patient
anywhere in the hospital (except places like MRI imagers) to
capture all diagnostic and treatment information on the one drive.
Medical records by law, however, must be protected. Thus, by using
a physically smaller embodiment of the invention, such small drives
can be erased after their use by being passed through the degausser
10. The large variety of NeFeB blocks available off the shelf other
than the preferred permanent magnets raises many possibilities for
configurations of the invention.
[0040] Also, Halbach arrays are known with magnetization angles of
less than 90 degrees between segments. Use of multiple thin plate
magnet segments with such reduced angular magnetization yield some
further optimization for certain applications. Such approaches
trade off some loss at additional contact surfaces between segments
for improved harmonic content of the magnetic field profile.
[0041] In yet another embodiment, a pair of mirror-imaged permanent
magnet assemblages 80 and 82 as illustrated in FIG. 8 can be offset
from each other by various degrees, generating a magnetic field
component in the direction across the gap 18. By varying the
offset, a variety of magnetic field directions are produced within
the gap 18. Offset embodiments of the invention can address various
directional erasure characteristics such as perpendicular recording
on hard disk drives.
[0042] In still another embodiment, gap adjustability can be
introduced to trade off field strength against media thickness
capacity. Frame structures for manipulating the magnet assemblages
to adjust the gap width and to offset the assemblages are known,
and an example of such a frame structure 90 is illustrated in FIGS.
9 and 10. Lower plate 92 supports lower magnet assemblage 16. Upper
plate 94 supports upper magnet assemblage 14. Pillars 96 are
rigidly affixed to lower plate 92 by any conventional method. The
pillars 96 include a thick diameter mid-section 98 between upper
magnet assemblage 14 and lower magnet assemblage 16, a smaller
diameter upper portion 100 that slip fits through apertures defined
(not shown) by upper plate 94, and a thick diameter top portion 102
fixedly attached to smaller diameter upper portion 100. The thick
diameter mid-section 98 and top portion 102 of the pillars 96
define the limits of the adjustability of the gap 18. Rods 104
attach to lower plate 92 in a known manner allowing the rods 104 to
rotate within and pull on lower plate 92. At least upper portions
106 of rods 104 have screw threads over the range of adjustability
that mate with threaded holes (not shown) defined by upper plate
94.
[0043] Crank 108 and lower pinion gear 110 rigidly attach to each
other and rotatably attach to upper plate 94. Lower spur gears 112
and tall upper pinion gears 114 also rigidly attach to each other
and rotatably attach to upper plate 94. Upper spur gears 116 attach
rigidly to the partially threaded rods 104. Turning crank 108
causes lower pinion gear 110 to turn lower spur gears 112 that turn
tall upper pinion gears 114, thereby causing upper spur gears 116
and rods 104 to turn. Threaded portions 106 of rods 104 act on
upper plate 94 to selectively raise or lower it, thus affecting the
gap 18 between magnet assemblages 14 and 16 for the passage of
various magnetic storage media with different thicknesses.
[0044] The form of gap adjustment shown in FIGS. 9 and 10 is
illustrative and not limiting. Similar adjustment apparatuses can
be provided for other embodiments of the invention, such as offset
forms, attractive forms, and multiple assemblage pairs set at
angles to a media path. The various forms of the invention can be
combined with each other and with prior art along a media path,
with or without a gap adjustment apparatus.
[0045] Similarly, many prior art applications may be used with the
various embodiments to impart the sufficient exposure of the
magnetic storage media to varying fields necessary to accomplish
complete erasure. As noted above, when trying to erase magnetic
storage media, simply providing a simple linear media path through
a single magnetic field direction is generally recognized as
requiring further media-field variation, such as two passes through
the magnetic field combined with a rotation of the media or field.
Such actions can be performed by a human operator, or by the use of
mechanisms known in the art. Also, various mechanisms can impart a
raster-scan-like motion to the magnetic media path to accomplish
full magnetic exposure of media volume to a smaller magnetic field
volume.
[0046] Alternatively, two or more pairs of permanent magnet
assemblages can provide fields of varying direction along a media
path 20. In one embodiment, one pair of magnet assemblages is
mirror-imaged across the gap with magnetic sides in repulsion such
as the degausser in FIG. 1 forming fields generally in the
direction parallel to the gap 18, and another pair has elements
arranged so the magnetic sides are in attraction such as the
degausser in FIG. 11 forming two fields generally in opposite
directions across the gap 18. FIG. 11 illustrates a pair of magnet
assemblages 118 and 120 arranged in attraction using a basic
permanent magnet element with a direction of magnetization
different from that of the preferred magnet element. Like the
embodiments of FIG. 1 and FIG. 6, the arrangement illustrated in
FIG. 11 can be modified in a number ways including adding or
removing segments or by building the pair of assemblages with
alternative permanent magnet elements.
[0047] FIG. 12 models magnetic flux vectors for the pair of
assemblages 118 and 120 in attraction illustrated in FIG. 11
showing strong flux projecting across the gap 18. The assemblages
118 and 120 are also largely non-magnetic and self-shielding
outside the gap 18. It can be seen that the pair of five segment
assemblages 118 and 120 produces two fields of opposite direction
within gap 18. The strength of each field peaks near 10,000 gauss,
which, like the assemblage pairs arranged in repulsion, constitutes
a significant advance beyond the results achievable with prior
magnetic circuits. Passage of magnetic storage media 12 through a
magnet assemblage pair arranged in attraction before or after
passage through a magnet assemblage pair arranged in repulsion
provides the exposure to varying fields necessary for erasure of
certain varieties of magnetic storage media.
[0048] In yet another embodiment illustrated in FIG. 13, two pairs
of magnet assemblages 122 and 124 with magnetic sides in repulsion,
each of depth approximately 1.4 times an intermediate dimension of
the magnetic storage media 12 size, are provided along a media path
20 and oriented with field directions at 45 degree angles to that
path and at 90 degrees to each other forming a "one pass"
configuration sufficient to erase the magnetic storage media with
one pass through the magnet assemblages. Such placement reduces the
effective width of the field across the path to approximately 70%
of the width achieved in embodiments like FIG. 1 or FIG. 6. Unlike
those embodiments, the embodiment of FIG. 13 need only treat media
12 with a single pass in the orientation shown with longest
dimension aligned in direction of motion 20. The magnetic field
direction varies by 90 degrees with along the path 20 through the
two pairs of assemblages 122 and 124. Embodiments with a single
pair of assemblages generally require two passes, including one
pass with the orientation indicated in FIG. 1 and FIG. 6 with
longest dimension of media 12 perpendicular to direction 20 to
media motion. It can be appreciated that media placement limits 126
reside well clear of the ends of pairs of assemblages 122 and 124
where fringing effects weaken the field strength.
[0049] This embodiment can be further modified to add cross-gap
magnetic fields, forming a "universal" configuration that erases
horizontal and perpendicular hard disk drive media in one pass and
no media rotation. For example, to the configuration of FIG. 13 can
be added the cross-gap field direction of an array pair with
magnetic faces in attraction like that of FIG. 11, forming a
"universal" configuration that erases horizontal and perpendicular
hard disk drive media in one pass and no media rotation. One should
note that not all elements of such multi-gapped embodiments need be
Halbach-like arrays.
[0050] Those skilled in the art will recognize that a wide variety
of modifications, alterations, and combinations can be made with
respect to the above described embodiments without departing from
the spirit and scope of the invention, and that such modifications,
alterations, and combinations are to be viewed as being within the
ambit of the inventive concept.
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