Method of and apparatus for demagnetizing a magnetic material

• US 3,895,270 A
• Filed: 04/29/1974
• Issued: 07/15/1975
• Est. Priority Date: 04/29/1974
• Status: Expired due to Term

• Current Assignee: AT&T, Inc.
• Original Assignee: Harry L. Maddox
• Inventors: Maddox, Harry L.
• Primary Examiner(s): Hix, L. T.
• Application Number:
  US05/465,211
• Time in Patent Office: 442 Days
• Field of Search: 317/157.5
• US Class Current: 361/149
• CPC Class Codes: H01F 13/006 Methods and devices for demagnetising of magnetic bodies, e.g. workpieces, sheet material for erasing of information on magnetic record carriers G11B5/00

Abstract

To demagnetize a magnetic material, the material is positioned within a field of magnetic flux generated by an electrical induction coil having applied thereto a constant amplitude, alternating voltage wave, the half cycles of which alternate in polarity and successively and incrementally increase in frequency. The flux alternates in polarity with the polarity of the voltage wave, and the magnitude, or strength, of the flux is in accordance with the current flow through the coil, or is inversely in accordance with the impedance of the coil. As the frequency of the voltage wave incrementally increases from an initial low value to a higher value, the magnitude of the magnetic flux incrementally decreases from an initial high value to a lower value, and the alternating polarity, decreasing magnitude magnetic flux effectively demagnetizes the magnetic material. The invention finds utility in demagnetizing magnetic material cores of magnetic operating coils.

Claims

1. In an apparatus for demagnetizing a magnetic material core of an electromagnetic coil:

means for generating a constant amplitude voltage wave, having half cycles which alternate in polarity and successively and incrementally increase in frequency from a first frequency value to a second and higher frequency value, to be applied across the electromagnetic coil to generate in the magnetic material core thereof an alternating magnetic flux having a polarity in accordance with the polarity of the voltage wave and having a magnitude inversely in accordance with the frequency of the voltage wave, the second frequency being of sufficient value to reduce the magnitude of the magnetic flux within the core to essentially zero, and the frequencies between the first and the second values incrementally increasing in frequency in a relation to successively and incrementally decrease the magnitude of the magnetic flux within the core, to demagnetize the core, and means for applying the voltage wave across the electromagnetic coil while the frequency thereof increases from the first to the second frequency value to demagnetize the core.

2. In an apparatus as set forth in claim 1, further including:

means, operative upon the frequency of the alternating voltage wave reaching and exceeding the second frequency value, for decreasing the amplitude of successive cycles of the voltage wave directly in accordance with the frequency thereof.

3. In a system for imparting a magnetic bias to a magnetic medium:

means for generating a constant amplitude, alternating polarity square wave which increases in frequency from an initial first frequency value to a second and higher frequency value, and means, energizable by the square wave, for imparting to the magnetic medium a magnetic bias having a polarity determined by the polarity of the square wave and a strength which decreases from a first level when the frequency of the wave is at the first frequency value to a second level which is essentially zero as the frequency of the wave increases to the second frequency value.

4. In a system as set forth in claim 3, wherein the means for imparting a magnetic bias to the magnetic medium includes:

an electrical induction coil, energized by the square wave, for generating through the magnetic medium a magnetic field of flux having a polarity determined by the polarity of the square wave and a strength which decreases from the first level to essentially zero as the impedance exhibited by the coil to the square wave increases in value as the frequency of the square wave increases from the first to the second frequency value.

5. In an apparatus for demagnetizing a magnetic medium:

means for generating a frequency signal, the cycles of which successively and incrementally increase in frequency;

means, responsive to the increasing frequency signal, for producing a constant amplitude, alternating polarity voltage wave having a frequency in accordance with the frequency of the increasing frequency signal;

a low-pass filter, responsive to the voltage wave at an input thereof for producing at an output thereof an alternataing polarity demagnetizing signal having a frequency equal to the frequency of the voltage wave and having an amplitude which is essentially constant when the frequency of the voltage wave is less than a predetermined frequency value and which decreases when the frequency of the voltage wave is greater than the predetermined frequency value;

an electrical induction coil for generating a field of magnetic flux through the magnetic medium, and means for applying the demagnetizing signal across the induction coil for generating the field of magnetic flux through the magnetic medium, the polarity of the flux being in accordance with the polarity of the demagnetizing signal and the strength of the flux being inversely in accordance with the frequency of the demagnetizing signal, the strength of the field of flux being essentially zero when the demagnetizing signal is at the predetermined frequency, whereby as the frequency of the demagnetizing signal increases from an initial value to the predetermined value, the strength of the field of flux decreases from an initial value to essentially zero to demagnetize the magnetic medium.

6. In a system for demagnetizing a magnetic material which is inductively coupled to an electrical induction coil:

oscillator means for generating an initial signal the cycles of which successively and incrementally increase in frequency;

means, responsive to the initial signal, for generating a square wave signal having a constant amplitude and a frequency which is directly in accordance with the frequency of the initial signal, the means including a transistor the conduction of which is controlled by the application of the initial signal to the base thereof, and a diode in series with a Zener diode across the emitter-collector of the transistor;

means, connected to the emitter-collector of the transistor and to the juncture between the diode and the Zener diode for (1) biasing the Zener diode into conduction through the diode and for holding the transistor in a nonconductive state during the application of the positive excursions of the initial signal to the base thereof to generate across the emitter-collector thereof a first potential, and (2) biasing the transistor into conduction, and the Zener diode into nonconduction through the diode, during the negative excursions of the initial signal to generate across the emitter-collector of the transistor a second potential, whereby the alternating positive and negative excursions of the initial signal generate across the emitter-collector of the transistor the square wave signal having the constant amplitude from the first to the second potential and a frequency directly in accordance with the frequency of the initial signal;

a low-pass filter, responsive to the square wave signal at an input thereof for producing at an output thereof an alternating polarity demagnetizing signal having a frequency equal to the frequency of the square wave signal and having an amplitude which is essentially constant when the frequency of the square wave signal is less than a predetermined frequency value and which decreases when the frequency of the square wave signal is at least equal to the predetermined frequency value, and means for applying the demagnetizing signal across the induction coil for generating through the magnetic material inductively coupled therewith a magnetic field of flux having a polarity which alternates with the polarity of the demagnetizing signal and a strength which decreases from an initial value when the square wave signal is initially applied thereacross to a value which is essentially zero when the frequency of the square wave signal is equal to the predetermined frequency, whereby as the frequency of the square wave signal incrementally increases from an initial value to the predetermined frequency value the strength of the field of magnetic flux within the magnetic material incrementally decreases from an initial value to essentially zero to successively and incrementally decrease the magnitude of the magnetic flux within the magnetic material to demagnetize the core.

7. In a method of demagnetizing a magnetic material core of an electromagnetic coil:

generating a constant amplitude voltage wave having half cycles which alternate in polarity and successively and incrementally increase in frequency from a first frequency value to a second and higher frequency value, the second frequency value being sufficient, when the voltage wave is applied across the electromagnetic coil, to increase the impedance exhibited thereby sufficiently to reduce the current flow therethrough to essentially zero, and applying the voltage wave across the electromagnetic coil to generate in the magnetic material core thereof an alternating magnetic flux having a polarity in accordance with the polarity of the alternating voltage wave and having a magnitude inversely in accordance with the frequency of the voltage wave, and therefore directly in accordance with the current through the coil, whereby as the frequency of the voltage wave increases to the second frequency the magnitude of the field of flux in the core incrementally decreases from an initial value to essentially zero to demagnetize the core.

8. In a method as set forth in claim 7, further including:

decreasing the amplitude of the voltage wave when the frequency thereof reaches and exceeds the second frequency.

9. In a method of demagnetizing a magnetic medium:

generating a constant amplitude alternating polarity square wave which increases in frequency from an initial first frequency value to a second and higher frequency value;

applying the square wave across an electrical induction coil for generating therewith a field of magnetic flux having a direction determined by the polarity of the square wave and having a magnitude which varies inversely in accordance with the frequency of the square wave, the second frequency value of the square wave being sufficient that the magnitude of the field of flux generated by the induction coil is essentially zero, and positioning the magnetic medium within the generated field of magnetic flux, whereby as the frequency of the square wave increases from the initial first frequency value to the second and higher frequency value, the magnitude of the magnetic flux decreases from an initial value to a value which is essentially zero to demagnetize the magnetic medium.

10. In a method as set forth in claim 9, further including:

decreasing the amplitude of successive cycles of the square wave when the frequency thereof reaches and exceeds the second frequency value.

11. In a method of demagnetizing a magnetic medium:

generating a frequency signal, the cycles of which successively and incrementally increase in frequency;

generating a constant amplitude, alternating polarity voltage wave having a frequency determined by the frequency of the increasing frequency signal;

decreasing the amplitude of cycles of the voltage wave which have a freqUency at least equal to a predetermined frequency, and applying the voltage wave across an electrical induction coil to generate with the coil a field of magnetic flux having a polarity in accordance with the polarity of the voltage wave and a strength inversely in accordance with the frequency of the wave, the strength of the field of flux being essentially zero when the voltage wave is at the predetermined frequency, and positioning the magnetic medium within the generated field of flux, whereby as the frequency of the voltage wave increases from an initial value to the predetermined value, the strength of the field of flux decreases from an initial value to essentially zero to demagnetize the magnetic medium.