Degaussing technique

To degauss cassettes of magnetic tape, a magnetic field is applied to the magnetic material with a flux density of at least 1,000 gauss and at an angle between 20 degrees and 50 degrees from the horizontal of the magnetic tape for a time period of at least one second and the field is alternated at a frequency of at least ten hertz. The magnetic material may be rotated while it is in the field.

BACKGROUND OF THE INVENTION 
This invention relates to methods and apparatuses for erasing information 
from a magnetic recording medium. 
In one class of methods and apparatuses for erasing information from 
magnetic recording media, the recording medium, which may be a magnetic 
tape wound about a reel, is subjected to a varying or alternating 
electromagnetic field to randomize the magnetic particles on the magnetic 
material. 
In one prior art method and apparatus in this class, the magnetic tape is 
moved into an electromagnetic field that is applied in each of a plurality 
of different directions, one direction at a time, such as by first 
applying a vertically oriented field followed by a longitudinally oriented 
field. Techniques of this type are described in U.S. Pat. Nos. 4,730,230 
and 4,751,608. 
In another prior art technique, the magnetic tape is carried by a conveyor 
over a rotating electromagnet that has pole faces parallel to each other 
in the same plane underneath the conveyor belt. Thus, the electromagnet 
rotates a time varying electromagnetic field with it to cause the time 
varying electromagnetic field to pass through the magnetic material in the 
tape at a plurality of different angles. This type of prior art device is 
disclosed in U.S. Pat. No. 4,639,821. 
Still another prior art apparatus and technique of this class includes a 
conveyor that carries a magnetic tape into a rotating magnetic field. The 
rotating magnetic field is substantially parallel with the conveyor and is 
in the plane of the magnetic tape. It is created by electromagnetic poles 
on both sides of the conveyor belt, energized in such a way that similar 
polarities oppose each other on opposite sides of the tape with the phases 
of the poles on each side of the conveyor changing in synchronism to cause 
the field to rotate. Thus, north electromagnetic poles face each other on 
opposite sides of the tape and south magnetic poles face each other on 
opposite sides of the tape and the north and south poles alternate with 
each other in the same plane on the same sides of the tape. The poles 
rotate in synchronism. 
The prior art degaussing techniques provide erasure that is satisfactory 
for some purposes but do not erase to the extent desired for other 
applications. In general, the systems which apply vertical and 
longitudinal fields separately have the disadvantage of moving the energy 
back and forth between even and odd harmonics of the recorded signal. This 
reduces the effectiveness of the erasure. Rotational fields by themselves 
do not improve the depth of erasure to the extent needed for some 
applications when practiced as described in the aforementioned prior art 
references. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a novel degaussing 
apparatus and method. 
It is a further object of the invention to provide a novel technique for 
applying an alternating current, electromagnetic field to a magnetic 
medium. 
It is a still further object of the invention to provide a technique for 
increasing the depth of erasure of information recorded on a magnetic 
medium over other techniques for erasing recorded information from a 
magnetic medium without increasing the magnetic flux density. 
In accordance with the above and further objects of the invention, a 
magnetic field vector is applied through a tape cassette at an angle of 
between 20 to 50 degrees to the direction of the orientation of magnetic 
domains on the medium, or of course, the supplement of the angles in this 
range. For the common longitudinally recorded magnetic tape, the field 
vector is between 20 to 50 degrees from the longitudinal axis of a strip 
of the tape. 
For a cassette having this orientation of recorded information, the field 
may be applied at an angle of between 20 to 50 degrees to the larger flat 
sides of the cassette. Generally, if this is done, the field is rotated 
about an axis that is perpendicular to the flat sides of the cassette so 
that all sections of the tape receive a vector with a proper orientation 
at some time during the rotation of the field. The alternating field is 
applied at a sufficient magnetic flux density to change the orientation of 
individual magnetic domains or particles and thus randomize the 
orientation. 
The field is applied parallel to a plane that is: (1) at an acute or obtuse 
angle to the longitudinal direction of the tape; and (2) at an obtuse or 
acute angle to the side-to-side direction of the tape. Thus, the field 
vector is not perpendicular to the direction of magnetization (which is 
generally along the longitudinal axis of the tape) nor is the field vector 
in the direction of magnetization. However, it has vertical and 
longitudinal components that are simultaneously present at the same point 
in the tape. 
The angle is selected so that when resolved into components, the magnetic 
flux density has a vertical component nearly equal to or less than that of 
the longitudinal component. Keeping the ratio of vertical component to 
longitudinal component (the tangent of the vector angle) between 1.2 and 
0.36 prevents energy transfer between even and odd harmonics of the 
recorded signal during degausing and results in even erasure of all 
harmonics. Preferably, angle is selected so that the signal fundamental 
and all harmonics are reduced below a desired degauss level. In the 
preferred embodiment, the angle is approximately 45 degrees from the 
horizontal and the reel is rotated as it is moved linearly through the 
field. 
It can be understood that the degausser of this invention has the advantage 
of providing a greater amount of erasure with the same flux density than 
other techniques which apply fields in different directions in different 
stages or apply them directly or predominantly in the direction of 
magnetization or normal to the direction of magnetization.

DETAILED DESCRIPTION 
In FIGS. 1 and 2, there is shown a fragmentary, simplified sectional view 
of a degausser 10 in the process of receiving and degaussing a magnetic 
tape cassette 12 containing magnetic tape for degaussing. The degausser 10 
includes a source of a magnetic field 16, a conveyor assembly 14 for 
delivering the magnetic tape cassette 12 into the magnetic field 16, and a 
system shown at 18A and 18B for causing the field 16 and/or the cassette 
12 to rotate with respect to each other. FIG. 1 shows the tape 12 entering 
the degaussing apparatus and FIG. 2 shows the tape 12 within the field. 
The source 16 of the magnetic field is positioned with respect to the 
conveyor so that when the cassette 12 is moved into the field, the source 
16 creates a field at an angle to the side of the cassette 12 that is in 
the range of 20 to 50 degrees, and in the preferred embodiment, 45 or 
fewer degrees. This angle is referred to in this specification as the 
"effective angle". FIG. 1 shows one effective angle from one side and FIG. 
2 shows a different effective angle that is the supplement of the 
effective angle shown in FIG. 1. It is desirable for the domains of the 
tape to receive peak o substantially a peak field strength at both the 
effective angle and its supplement. 
In using the effective angle, a magnetic field vector is applied through 
the tape at an angle of between 20 to 50 degrees from the longitudinal 
side edges of the tape. The alternating field is applied at a sufficient 
magnetic flux density to change the orientation of individual magnetic 
domains or particles and thus randomizes the orientation. 
The field is applied parallel to a plane that is: (1) at an acute or obtuse 
angle to the longitudinal direction of the tape; and (2) at an obtuse or 
acute angle to the side-to-side direction of the tape. Thus, the field 
vector is not perpendicular to the direction of magnetization (which is 
generally along the longitudinal axis of the tape) nor is the field vector 
in the direction of magnetization. However, the field vector has 
components that are simultaneously present in the perpendicular and 
longitudinal directions. 
The effective angle is selected so that when resolved into components, the 
magnetic flux density has a vertical component nearly equal to or less 
than that of the longitudinal component. More generally, the ratio of the 
vertical component to the longitudinal component (the tangent of the 
vector angle) is preferably between 1.2 and 0.36. This reduces energy 
transfer between even and odd harmonics of the recorded signal during 
degausing and results in even erasure of all harmonics. The angle is 
selected so that during degaussing, the fundamental and all harmonics are 
reduced below a desired level. 
In the preferred embodiment, the angle is approximately 45 degrees from the 
horizontal and the reel is rotated as it is moved linearly through the 
field. At 45 degrees, the vertical and longitudinal field components are 
in equal proportions at a given point in space. Unlike other degaussers in 
which the magnetic field is applied at a number of angles but at different 
points in space or time resulting in ineffective degaussing of all 
harmonics without consideration of the best angle, the degausser 10 
applies the magnetic vector at the effective angle preferably all of the 
time, but at least a sufficient proportion of the time the degausser 
operates on the magnetic material to be efficient such as for example at 
least 30 percent of the time the tape is in the field. 
Of particular significance is that the field be at the effective angle for 
the domains to be erased for at least one and one-half cycles and that it 
be at the effective angle during at least one peak of the alternating 
current as the tape and field are separated such as when a cassette is 
moved from a field by a conveyor. The reduction of the field strength from 
the fringes of the field at the time the tape and field are being 
separated is particularly effective in erasing the tape. 
The conveyor assembly 14 in the embodiment of FIG. 1 includes a belt 20, a 
first roller 22 and a second roller 24. Either or both of the rollers are 
drive rollers with the belt 20 passing over them as an endless belt 
capable of moving the cassette 12 or a series of such cassettes along the 
top run of the belt 20. Of course, any other means for moving the 
cassettes with respect to the field may be used. 
The source of the magnetic field 16 includes first an second 
electromagnetic assemblies 30 and 32, respectively. One of the assemblies 
32 in the embodiment of FIG. 1 is located close to the top run of the belt 
20 and beneath the belt 20 s that the cassette 12 passes over it and the 
other electromagnetic assembly 30 is located above the top run of the belt 
20 at an elevation high enough so that the cassette 12 may pass 
therebetween. 
The first electromagnetic assembly 30 includes a first coil 34, a first 
core leg 36, a bridging portion 38, a second coil 40 and a second core leg 
42. The first and second coils 34 and 40 are respectively wound around the 
first and second downwardly extending vertical legs 36 and 42 to create a 
field through them with the bridge 38 connecting the legs 36 and 42 to 
provide a closed magnetic path, to hold them in position, and to mount 
them to the means for rotating 18B in fixed relationship with respect to 
each other. The first core leg 36 ends in a pole face 54 and the second 
core leg 42 ends in a pole face 56. The legs 36 and 42 are of high 
permeability metal to concentrate the field at the pole faces 54 and 56 
for extension downwardly through a cassette 12 to the second 
electromagnetic assembly 32. 
The second electromagnetic assembly 32 is constructed in a manner similar 
to the first assembly 34 and includes a first coil 44, a first upwardly 
extending, core leg portion 46, a bridge 48, a second coil 50 and a second 
downwardly extending, core leg portion 52. The first leg 46 ends in a pole 
face 58 and the second leg 52 ends in a pole face 60. The core legs 36, 
42, 46 and 52 are parallel to each other with the pole faces 58 and 60 
being positioned near the bottom of the bottom rung of the belt 20 and the 
pole faces 54 and 56 being located at a sufficient height so that the 
cassette 12 passes under them when carried into position by the belt 20. 
The pole faces of opposite polarity on opposite sides of the top run of the 
belt 20 are off-set from each other so that a field extending between the 
pole face 54 and 58 and the field extending between the pole face 56 and 
60 are at an angle to the cassette 12. That angle is between 20 and 50 
degrees so that if the components of the field are resolved into 
components that are vertical to the cassette 12 and horizontal to it, the 
vertical component would be 0.36 to 1.2 times the magnitude of the 
longitudinal component. Thus, the critical angle is formed for the 
magnetic vector. 
The bridges 38 and 48 are preferably of high permeability material and the 
windings wound in a direction so that the poles alternate north to south. 
In this case, the poles 58 and 60 are of opposite polarity and the pole 58 
has the opposite polarity from the pole 54 so that if 54 is north, 58 is 
south. In this embodiment, the reluctance path between the poles 54 and 56 
and the reluctance of the path between the poles 58 and 60 should be much 
greater than the reluctance of the path between the poles 54 and 58 and 
the poles 56 and 60. This may be accomplished by spacing the poles 54 and 
56 and the poles 58 and 60 much further from each other than the poles 54 
and 58 and the poles 56 and 60 are spaced from each other. 
The rotating system portions 18A and 18B are identical to each other and 
each includes a motor driven shaft which serves to rotate the 
electromagnetic assemblies in synchronism to maintain the effective angle 
between the poles 54 and 58 and 56 and 60. For this purpose, the shaft 19B 
and 19A are connected off-center to the bridges 38 and 48, respectively, 
to support the bridges and thus, the core legs and windings as they are 
rotated. 
The shafts 19A and 19B are parallel to each other and perpendicular to the 
bridges 48 and 38 so that the pole faces 54, 56, 58 and 60 remain parallel 
to the belt 20. Power in this embodiment is electrically connected to the 
windings through slip rings 21B and 21A connected to a source of A.C. 
power such as the source 23 shown connected to the slip rings 21B with the 
shaft 19B adapted to slide wit respect to the slip rings and the slip 
rings being electrically connected to the windings. 
In FIG. 3, there is shown a simplified top view of the embodiment of FIG. 1 
showing the cassette 12 being carried by the belt 20 into a position where 
it is between the top positioned coils 34 and 40 and the bottom coils 44 
and 50. The windings and conveyor are conventional. The belt 20 and 
electromagnets 30 and 32 may be fabricated in the manner described in 
connection with U.S. Pat. No. 4,897,759. 
Either the cassette 12 or the electromagnetic assemblies 30 and 32 may be 
rotated since it is the motion between the two that is significant and 
similarly the tape 12 may be moved between the assembly or the assembly 
moved over the tape 12. Moreover, instead of physically rotating the 
electromagnets 30 and 32, a rotating magnetic field may be created, such 
as that disclosed in U.S. Pat. No. 4,423,460, provided the angular 
direction of the field as it passes through the cassettes is maintained at 
the effective angle. 
In FIG. 4, there is shown a simplified elevational sectional view of 
another embodiment 10A of degausser positioned to erase a cassette 12 
having a means for moving the cassette 12 and degaussing field with 
respect to each other for erasing information from the magnetic tape, a 
means for rotating the tape with respect to the field, and a source of a 
magnetic field adapted to be applied at an angle to the tape for efficient 
demagnetization thereof. In this embodiment, the means for moving the 
cassette and degaussing field with respect to each other is similar to a 
file cabinet drawer 74 positioned to move with respect to a source of a 
magnetic field. The drawer 74 in this embodiment includes a drawer door 
82, a drawer frame 84 and a drawer roller assembly 86. 
The drawer frame 84 supports a rotating means 80 and is connected to the 
door 82 for moving on the drawer rail and roller assembly 86. The source 
of the magnetic field 16 (not shown in FIG. 3) is mounted to be stationary 
with respect to the frame 84 so that the means for rotating the cassette 
12 is moved between the first and second electromagnetic assemblies 30A 
and 32A for demagnetization. 
A means 80 for rotating the cassette 12 includes a pan mounted for rotation 
within the frame 84 by bearings 87 such as along the rim of a circular 
opening 89 in the bottom of the frame 84. To rotate the cassette 12, a 
drive motor 90 is connected by a belt 92 to the rim of the pan 96 for 
rotating about the idler pulley 94 so as to turn the pan. The tape mounts 
within the pan 96 and is rotated therewith within the field 16. The first 
and second electromagnetic assemblies 30A and 32A are mounted to the side 
of the cabinet by frame members 70 and 72 (not shown in FIG. 3). 
In FIG. 5, there is shown a plan view of the degausser 10A showing the 
manner in which cassette 12 is mounted for rotation with the pan 96. The 
pan 96 as best shown in FIG. 4, is positioned to be rotated by the motor 
90, idler pulley 94 and belt 92 to rotate the cassette 12. The 
electromagnetic assemblies 30A and 32A are identical to those in the 
embodiment of FIGS. 1 and 2 except that they are mounted to be stationary 
rather than rotatable. 
In each of these embodiments, the tapes may be rotated or not. Slightly 
better demagnetization is obtained by rotating the tapes, probably because 
the intensity of the field at the effective angle is evenly distributed by 
the rotation over the tape around the reel during the rotation. Otherwise, 
the orientation of the tape within the field could cause some portions to 
receive less activation than others. 
In operation, a cassette or other holder for magnetic tape, is placed 
either on the conveyor belt 20 in the embodiments of FIGS. 1 and 2 or in 
the pan 96 in the embodiments of FIGS. 3 and 4. The cassette, or plurality 
of cassettes, are then moved linearly into an alternating magnetic field 
which alternates at a frequency of approximately 60 hertz in a field of 
magnitue proportional to the magnetic coercivity of the cassette tape of 
other medium to be degaussed. Satisfactory results, however, have been 
obtained for a reduction of 95 decibels below saturation of a 750 oersted 
coercivity tape using a field vector of only 2700 gauss. 
While a 60 hertz field is generally used, other frequencies may be used 
within the range of 10 to 400 hertz. The magnetic flux density should be 
at least two times the tape or medium coercivity. For todays media, the 
minimum field should be about 1,000 gauss. Generally the higher the field, 
the better the depth of eraser, but other factors such as inductive 
heating effects and power loss that increase with higher flux limit the 
field strength in practical designs. 
The tape is rotated in the rotating pan or the magnet assemblies are 
rotated. The rate of rotation of the tapes, when used, may be as low as 40 
revolutions per minute or higher. The linear movement of the tape, such as 
on a conveyor belt with a plurality of tapes being moved, may be at any 
convenient speed such as 0.3 inches per second but should be in a range of 
0.1 inches per second to two inches per second. The rates of rotation and 
linear speed are interdependent and are selected prior to the degaussing 
in conjunction with the depth of erasure needed, the intensity of field 
that is to be applied and the coercivity of the magnetic medium. 
From the above description, it can be understood that the degausser of this 
invention has several advantages, such as: (1) it is relatively economical 
and fast in operation because it applies a single field at an effective 
angle; and (2) it provides a greater level of erasure for the same 
magnetic flux density than other techniques. 
Although a preferred embodiment or the invention has been described with 
some particularity, many modifications and variations in the invention are 
possible in the light of the above teachings. Therefore, it is to be 
understood that, within the scope of the appended claims, the invention 
may be practiced other than as specifically described.