JPH039535B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for demagnetizing a magnetic tape, particularly a magnetic tape with a metal reel.

Magnetic tapes used in cassettes, audio, video, electronic computers, etc. are oriented along the length of the tape in order to increase sensitivity, so the magnetic tapes after manufacture have a high residual magnetism. It remains. Additionally, the tape is magnetized for some reason during the tape manufacturing process, which also causes residual magnetism. If residual magnetism remains on the tape, it cannot be used as is.
Therefore, tape manufacturers demagnetize tapes to below a specified level (for example, below -60 dB) before shipping.

Conventionally, as shown in FIG. 1, a magnetic tape 5 is moved between the magnetic poles 2 and 3 of an electromagnet 1 generating an alternating magnetic field H by a conveyor 4 in the direction of arrow A at a predetermined speed V. It was transported and demagnetized. However, in such a demagnetizing method, both sides 5 of the tape 5 parallel to the transport direction A are
The parts a and 5b are well demagnetized, but in the transport direction A
The front and rear portions 5c and 5d, which are orthogonal to each other, are not demagnetized very well. This is because the tape 5 is oriented in the longitudinal direction (arrow B-B' direction) as described above. That is, as shown in FIG. 2a, since the tape 5 moves in the direction of arrow A at a constant speed V with respect to the alternating magnetic field H, the alternating magnetic field H is applied to both sides 5a and 5b of the tape 5 in an oblique direction. Therefore, these portions 5a and 5b are well demagnetized. On the other hand, the direction of the alternating magnetic field H applied to the front and rear portions 5c and 5d of the tape 5 is always vertical (Fig. 2b) regardless of the movement of the tape 5. These front and rear portions 5e and 5d are difficult to demagnetize, and residual magnetism inevitably remains above the specified value.

If such an incompletely demagnetized tape 5 is used, for example, as a music tape and double-speed recording is performed, residual magnetism remains as a noise component because the double-speed recording device is not equipped with a degaussing device. This occurs as periodic noise during playback.

Furthermore, unlike conventional tape, the coercive force Hc is approximately
If the tape is about 300 Oe, it is possible to demagnetize it to a specified level by the demagnetizing method shown in FIG. 1. However, in order to demagnetize a tape with a very high coercive force Hc of approximately 700 Oe, such as recent metal tapes, to the specified level, it is necessary to apply a high magnetic field of approximately 2000 to 2200 Oe using conventional demagnetization methods. It is. When such a high alternating magnetic field H is applied, a large eddy current i (Fig. 1) is generated in the metal reels 6a and 6b, and as a result, the surface temperature of these metal reels 6a and 6b reaches 50°C.
The temperature was higher than 0.degree. C., and there were drawbacks such as the wrapped tape 5 becoming useless due to this high temperature.

The present invention has been made to eliminate the above-mentioned drawbacks, and demagnetizes the tape by rotating a magnetic tape wound on a reel at a predetermined rotational speed and applying a predetermined damped oscillating magnetic field to the tape. The present invention provides a method for demagnetizing a magnetic tape.

The present invention will be described in detail below based on one embodiment of the accompanying drawings.

In FIG. 3, the demagnetizing device 10 has two demagnetizing coils 11 and 12, and these coils 11 and 12 are arranged so that they face each other at a predetermined distance d and have the same axial direction. It is set up. These two coils 11 and 12 are formed to have the same shape and number of turns, and are connected in series to a predetermined demagnetizing power source (not shown). In addition, the two coils 11,
Of course, 12 may be connected in parallel.
The turntable 13 is disposed horizontally and parallel to the axial direction between the coils 11 and 12, and is connected to a drive device 15 via a shaft 14. The drive device 15 is equipped with an induction motor (not shown), etc., and has a predetermined rotation speed, e.g.
The turntable 13 is driven to rotate at a high speed of about 1500 rpm. These coils 11, 12 and drive device 15 are connected to the base 1, respectively.
The arrangement is fixed at 6. Further, the turntable 13, shaft 14, etc. are made of non-ferrous metal members or non-metallic members so as not to impart residual magnetism to the magnetic tape and not to adversely affect the magnetic field H generated by the demagnetizing coils 11 and 12. There is. The turntable 13 is desirably made of a non-metallic material, such as a synthetic resin material, in order to prevent the generation of eddy currents due to the alternating magnetic field for demagnetization and to prevent heat generation.

First, for example, a video magnetic tape 2 is wound around a metal reel 20 on a turntable 13.
After placing and fixing the turntable 13, the turntable 13 is rotated at a predetermined rotational speed, for example, in the clockwise direction C.
(Figure 4) Rotate at high speed. In this state, the tape 21 is parallel to the axial direction of the coils 11 and 12, and therefore the orientation process direction (B
-B′ direction) is also parallel to the axial direction. Next, a predetermined excitation current I is applied to the coils 11 and 12.
This exciting current I has a period, for example, as shown in FIG.
400 msec, and a damped oscillating current with a decay time of 1 sec or less, for example, a damped alternating current. Therefore, the two coils 11 and 12 generate a damped alternating (oscillating) magnetic field H (FIGS. 3 and 4) in accordance with the excitation current I in the axial direction. still,
The maximum value of this attenuated alternating magnetic field H is approximately 2000 Oe. In this way, the magnetic tape 21 is rotated at high speed to align the magnetic tape 21 in the direction B-
By matching B' with the attenuating alternating magnetic field H (FIG. 7), the magnetic tape 21 can be demagnetized to below a specified level. Further, both side surfaces 20a and 20b of the metal reel 20 are always parallel to the direction of the alternating magnetic field H, and therefore, these both side surfaces 20a and 20b are always parallel to the direction of the alternating magnetic field H.
No eddy current is generated in a and 20b, and no heat is generated. Thus, the magnetic tape 21 can be demagnetized without causing any thermal damage.

Although the present embodiment describes the case where two demagnetizing coils are used, the present invention is not limited to this, and one demagnetizing coil may be used. Of course, even in such a case, it goes without saying that the turntable should be placed inside the coil. Further, in this embodiment, the case where the tape is rotated has been described, but the tape may be kept stationary and a rotating magnetic field may be applied from the outside.

As explained above, according to the present invention, it is possible to easily and quickly demagnetize the metal reel below the specified level with a relatively low magnetic field, and the metal reel does not generate heat at all.
No thermal damage to the tape. Further, there are excellent effects such as the ability to easily configure the demagnetizing device.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing a conventional method of demagnetizing a magnetic tape with a metal reel, Fig. 2 a and b are explanatory diagrams of a magnetic field applied to a magnetic tape by the demagnetizing method shown in Fig. 1, and Fig. 3 is a cross-sectional view showing an embodiment of a demagnetizing device used in the magnetic tape demagnetizing method according to the present invention,
FIG. 4 is a plan view of FIG. 3, FIG. 5 is an explanatory diagram of excitation current, FIG. 6 is a diagram showing an example of an attenuating alternating magnetic field for demagnetization, and FIG. 7 is a demagnetization method according to the present invention. FIG. 2 is an explanatory diagram showing the relationship between a magnetic field applied to a magnetic tape and an orientation process of the tape. 11, 12... Demagnetizing coil, 13... Turntable, 15... Drive device, 20... Metal reel, 21... Magnetic tape.

Claims (1)

[Claims] 1. A magnetic tape wound around a metal reel is placed within a magnetic field generated in the internal space of a cylindrical coil, so that the direction of the magnetic field from the coil is parallel to the surface of the metal reel of the magnetic tape. rotating the magnetic tape relative to the coil approximately around the winding center of the magnetic tape, and simultaneously applying a predetermined damped oscillating current to the coil;
A method for demagnetizing a magnetic tape, in which the magnetic tape is demagnetized. 2. The coil is composed of a pair of cylindrical coils arranged coaxially, and the magnetic tape is arranged in the internal space of the coil so as to be sandwiched from both sides by the pair of coils. Method for demagnetizing magnetic tape as described in Section 1.