Magnetic Media Erasure System

This invention generally relates to the erasure of information from magnetic storage media systems, both analog and digital types and, more particularly, to magnetic media erasure systems for achieving sufficiently deep AC and/or DC erase to facilitate declassification of U.S. Government classified magnetic media of the high coercivity type.

It is known that DC erasure is necessary prior to AC degaussing to successfully erase National Security Agency Class II or higher coercivity magnetic media. AC degaussing alone will not suffice. Further, it is known that erasure in as many spatial planes as possible (normal, transverse and longitudinal) facilitates erasure of the primary signal and its harmonics to optimum depths. However, existing NSA Class I AC Degausser systems have proven unsatisfactory on high coercivity media, on the order of 750 Oersteds or more, in achieving the minimum targeted bulk erase level of -90 dB below signal saturation that has been set for the declassification of classified magnetic media.

The magnetic media erasure system in accordance with the present invention overcomes the above difficulties by providing at least one pair of magnets, preferably Neodymium Iron Boron alloy, having poles of the same polarity disposed opposite each other so that a "bucking field" is induced. The DC magnetic field thus produced causes the DC erasure of media passed thru the gap between the magnets so that it can now be successfully erased by a Class I degausser. However, an AC erasure effect can be brought about through the relative movement of the media through 2 or more opposite polarity fixed fields. In the case of audio tapes, this movement may be provided by any number of methods, such as passing the tape past magnets disposed between the reels of a reel-to-reel tape machine, by rubbing a magnet block along the tape, by winding the tape onto an empty reel while a small horse-shoe shaped magnet inserted between the sides outward on the accumulating tape, or rotating a magnetic block relative to the tape, or the like. Alternatively, the magnetic media may be dropped through the bucking field. This latter method is particularly useful for the emergency destruction of magnetic media of all types except computer disk packs. For computer disk packs, a wand having magnets of opposite polarity attached to a handle, preferably teflon coated to avoid damaging the magnetic media, may be inserted into the computer disk pack and wiped across the disk surfaces to achieve both DC and AC erase.

Various magnet configurations for generating the desired fields are disclosed. Using a magnetic media erasure system in accordance with the invention, DC erasure to at least -74 dB below the signal saturation level can be consistently achieved, and with some configurations, DC erasure to the NSA standard for declassification of -90 dB below the signal saturation level or better has been achieved.

FIG. 1 is a perspective view of magnetic media erasure system in accordance with a preferred embodiment of the present invention;

FIG. 2 is a perspective view of a magnetic media erasure system using stepped magnet pole segments in accordance with a further preferred embodiment of the present invention;

FIG. 3 is a perspective view showing the bucking field generated by one configuration of three-pole magnets in accordance with another preferred embodiment of the present invention;

FIG. 4A is a top planar view showing the bucking field lines of one configuration for DC and AC erase;

FIG. 4B is a top planar view showing the bucking field lines of one configuration for DC erase only;

FIG. 5 is a side elevational view of a system in accordance with another preferred embodiment of the present invention using relative rotation between the magnets and the magnetic media;

FIG. 6 is a planar top view of a system similar to that shown in FIG. 2, except that the magnet blocks are disposed on either side of the magnetic media, as are the heads in cross-field recording;

FIG. 7 is a planar top view of a system similar to that shown in FIG. 6, except that instead of increasing the spacing between the tape and magnet poles, gradually weaker magnets are employed;

FIG. 8 is a side elevational view of a "drop thru" system in accordance with a further embodiment of the present invention;

FIG. 9 is a side elevational view of another "drop thru" system in accordance with a further embodiment of the present invention.

A magnetic media erasure system in accordance with a first embodiment of the present invention is shown in FIG. 1 and generally designated 10. Throughout the figures, like numerals are used to represent like elements.

System 10, as shown in FIGS. 1 and 4A, includes special high energy permanent magnet 12 having mutually repelling poles 14, 14 and 16, 16 located opposite one another. The magnets used throughout the various embodiments of the present invention are preferably made of rare-earth/iron alloys, such as the Neodymium/Boron/Iron magnet sold by Crucible Materials Corporation of Elizabethtown, Kentucky, under the trade name "Crumax 355", or any even stronger permanent magnet.

The "bucking field" induced by opposing, similar magnetic poles 14, 14 and 16, 16 creates a high energy flux pattern that washes all three spatial planes (normal, transverse and longitudinal) within the magnetic media passed thru it. This effect is not possible when using an attracting field (a north pole opposing a south pole), since such a field passes straight through the pole gap undeflected, causing erasure in only the transverse plane. However, a system utilizing mutually repelling poles as in the present invention causes the magnetic flux lines to collide in the center of the gap and to move away at right angles from the straight line separation of the poles, causing erasure in both the transverse and longitudinal or transverse and normal planes or portions of all three planes. As can be seen from the above, the specific type of erasure thus depends solely on the orientation of the magnets relative to the magnetic media. Hence, a unit magnetic pole within the media would, theoretically, achieve erasure in all three spatial planes, as it passes through this type of field.

AC erasure may be affected without the use of electric power by relative movement of the magnetic media through fixed magnetic fields of opposite polarity created by permanent magnets (not wire coils), causing the media to experience a reversing polarity of magnetic saturation. The cyclic rate of reversal is dependent upon the speed of movement of the media through the stationary fields. Thus, a series of alternating magnetic poles such as shown in FIG. 3 and 4A could produce both AC and DC erasure.

As shown in FIG. 3, the magnets 12 are preferably separated by a distance x, the width of the object to be erased by the particular device. The magnet poles preferably have a width w.gtoreq.2x, a depth d=approximately 20% more than the object to pass under, and a length z.gtoreq.x. Adjacent poles are separated by a distance y.gtoreq.x. The degree of flux penetration in the normal spatial plane has been found to be highly dependent on the w, z and d dimensions.

One preferred configuration of such a system utilizing a series of magnets is shown in FIG. 2. The system utilizes a magnet block 18 of width x, preferably at least 25% wider than the tape, having alternating North and South pole segments 20, 22 disposed along the direction of travel of a magnetic tape and in close proximity to it. The length of the magnet segments gradually decreases to create a "stepped" magnet of gradually diminishing intensity at the tape plane, with a spacer 24 filling the stepped gap formed between the lower edge of the segments and the lower edge of the magnet block 18 which establishes the plane of the tape. The step distances will vary, depending on the tape coercivity, to achieve maximum reduction in signal to noise ratio.

In another preferred embodiment shown in FIG. 5, the system may be adapted to be rotated relative to the magnetic media, although in practice it may be desirable to move the media rather than the magnets. In this embodiment, the system includes a pair of cross-shaped magnet blocks 26 formed by radially extending arms 30. Each magnet block 26 includes a magnet 28 attached in a suitable manner to each radially extending arm 30. While only six magnets 28 can be seen in this view, it is understood that eight magnets are required for symmetry and balance. Variations could use a total of 4 or 16 magnets or the like. In the preferred embodiment, the arms are formed of magnetic steel plates or the like.

A central spindle shaft hole 32 is formed in each of the magnet blocks 26 to permit the magnet blocks to rotate relative to the magnetic media. A standard drive mechanism, either manual or powered, (not shown) may be coupled to a drive spindle (not shown) fitted within shaft hole 32 to effect rotation of the magnet block. A peripheral drive could also be used. The polarity of the magnets alternates from arm to arm so that the desired reversing polarity discussed above for AC erasure can be obtained. More complex eraser is also possible when the blocks 26 rotate at different speeds since this will provide North to South polarization in addition to the bucking fields.

The embodiment illustrated in FIG. 6 is substantially the same as that shown in FIG. 2, except that the magnet pole segments 20, 22 are arranged on both sides of the magnetic media. As in the embodiment illustrated in FIG. 2, the rate of magnetic flux decrease due to the separation distance is dependent on the coercivity of the tape to be erased and the acceptable signal to noise ratio. The arrangement shown in FIG. 6 is particularly useful for erasing tapes that are recorded using heads placed adjacent both the upper and lower sides of the tape so that the information is recorded in a direction normal to the tape.

Alternatively, instead of using stepped segments to increase the tape gap between the magnet blocks, as shown in FIG. 6, the same reduction in field density can be obtained by decreasing the strength of the magnet pole segments along the direction of tape travel, as shown in FIG. 7, by either reducing pole area or selecting lower energy magnets or both.

The magnet pole segments shown in FIGS. 6 and 7 are preferably encased in plastic with a tape slot provided for accommodating passage of the tape therethrough. The entry gap width of the slot is preferably no greater than 1 1/2 times that of the tape thickness to provide tolerance for the tape. Instead of encasing the magnet pole segments in plastic, guide posts may be formed at each end of the tape path to ensure proper alignment of the tape relative to the magnets.

FIGS. 8 and 9 illustrate "drop-thru" systems in which a computer disk 40 or other magnetic media such as tapes and cassettes may be dropped past the magnets to achieve erasure. In the embodiment illustrated in FIG. 8, several layers of magnet pole segments 34 (only one is shown) may be used to achieve varying levels of DC erasure. The embodiment illustrated in FIG. 9 may utilize several layers of alternating North and South magnet pole segments 36, 38 (again, only one layer is shown) to achieve varying levels of AC erasure as well as DC erasure, according to the mechanism discussed previously with respect to the other embodiments.

The foregoing is for illustrative purposes only. Modifications can be made, particularly in size, shape and arrangement of parts, in accordance with the scope of the invention as defined by the appended claims. For example, the magnet configurations of the system can be adjusted to accommodate helically recorded magnetic media.