Method of and apparatus for demagnetizing a magnetic material
United States Patent 3895270
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.
US Patent References:
Method of demagnetizing a magnetic record
Folse - November 1960 - 2962560
Magnetic erase means
Young et al. - June 1962 - 3038036
Microwave ferrite-rotator degaussing system
Christianson et al. - June 1963 - 3093774
METHOD FOR REDUCING CORE LOSSES IN SILICON STEELS AND THE LIKE
Houze, Jr. et al. - February 1973 - 3716763
Method of demagnetizing a magnetic record
Folse - November 1960 - 2962560
Magnetic erase means
Young et al. - June 1962 - 3038036
Microwave ferrite-rotator degaussing system
Christianson et al. - June 1963 - 3093774
METHOD FOR REDUCING CORE LOSSES IN SILICON STEELS AND THE LIKE
Houze, Jr. et al. - February 1973 - 3716763
Application Number:
05/465211
Publication Date:
07/15/1975
Filing Date:
04/29/1974
View Patent Images:
Export Citation:
Assignee:
Western Electric Company, Incorporated (New York, NY)
Primary Class:
International Classes:
H01F13/00; H01F13/00
Field of Search:
317/157.5
Primary Examiner:
Hix L. T.
Attorney, Agent or Firm:
Lloyd R. A.
Claims:
1. In an apparatus for demagnetizing a magnetic material core of an electromagnetic coil:
2. In an apparatus as set forth in claim 1, further including:
3. In a system for imparting a magnetic bias to a magnetic medium:
4. In a system as set forth in claim 3, wherein the means for imparting a magnetic bias to the magnetic medium includes:
5. In an apparatus for demagnetizing a magnetic medium:
6. In a system for demagnetizing a magnetic material which is inductively coupled to an electrical induction coil:
7. In a method of demagnetizing a magnetic material core of an electromagnetic coil:
8. In a method as set forth in claim 7, further including:
9. In a method of demagnetizing a magnetic medium:
10. In a method as set forth in claim 9, further including:
11. In a method of demagnetizing a magnetic medium:
2. In an apparatus as set forth in claim 1, further including:
3. In a system for imparting a magnetic bias to a magnetic medium:
4. In a system as set forth in claim 3, wherein the means for imparting a magnetic bias to the magnetic medium includes:
5. In an apparatus for demagnetizing a magnetic medium:
6. In a system for demagnetizing a magnetic material which is inductively coupled to an electrical induction coil:
7. In a method of demagnetizing a magnetic material core of an electromagnetic coil:
8. In a method as set forth in claim 7, further including:
9. In a method of demagnetizing a magnetic medium:
10. In a method as set forth in claim 9, further including:
11. In a method of demagnetizing a magnetic medium:
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of, and apparatus for, demagnetizing a magnetic material, and in particular to a method of, and apparatus for, demagnetizing a magnetic material with a flux field generated by an electrical induction coil having an increasing frequency signal applied thereacross.
2. Description of the Prior Art
Demagnetizing magnetic material is well known in the prior art. For example, in the manufacture of a product employing a magnetic material, such as an electromagnetic coil having a magnetic material core, it is often necessary to demagnetize the magnetic material, prior to transport or storage of the coil, to prevent the attraction thereto of contaminants of a magnetic nature as a result of residual magnetism therein. In the case of a magnetic coil having a magnetic material core, the conventional technique for demagnetizing the core is to apply a constant frequency, sinusoidal signal across the coil, and to then decrease, or modulate, the signal in amplitude until the current flow through the coil is essentially zero. With a constant frequency signal applied across the coil, the impedance of the coil remains essentially constant, and the magnitude of the current flow therethrough decreases as a result of the decreasing amplitude of the signal. This generates within the core a magnetic field of flux which alternates in polarity with the signal, and which decreases in magnitude with the decrease in magnitude of the signal.
This conventional technique has several disadvantages. For example, if a mechanical voltage regulation device, such as a variac, is employed to decrease the amplitude of a demagnetizing signal applied across the coil, any noise or discontinuity in the decaying signal for the purpose of demagnetization can introduce, rather than eliminate, residual magnetism in the magnetic coil. Furthermore, if an electronic circuit, as compared with a mechanical device such as a variac, is employed to eliminate noise in the demagnetizing signal, the output stage of the circuit must initially conduct large currents when the amplitude of the demagnetizing signal is large, and must later dissipate a large amount of power as the amplitude of the demagnetizing signal is decreased. Furthermore, the load impedance exhibited by a magnetic coil may shift rapidly as the magnetic drive field exceeds the coercive force of the core material thereof, and when this occurs the sudden change in current through the coil may modulate the demagnetizing signal in such a way as to introduce residual magnetism into the core. In this case, if a plurality of parallel connected magnetic coils are simultaneously demagnetized, the current modulation caused by an individual one of the coils, as the coercive force of the core material is exceeded by the drive signal, introduces magnetism in all of the coils as a result of the altered drive signal. Also, as the strength of the magnetic field of flux must be changed in small increments to effectively demagnetize a magnetic material, the time required to demagnetize the material may be considerable since the minimum time between each incremental decrease is based upon the period of a cycle of the constant frequency demagnetizing signal.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system for imparting a magnetic bias to a magnetic medium includes circuitry 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 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 as the frequency of the square wave increases from the first to the second frequency value.
Preferably, an iscillator generates an initial signal the cycles of which successively and incrementally increase in frequency and circuitry, which includes 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, is 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. Additional circuitry is 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 which is directly in accordance with the frequency of the initial signal.
The square wave signal is applied to a low-pass filter which produces at an output thereof an alternating polarity demagnetizing signal which has a frequency equal to the frequency of the square wave signal and an amplitude which is essentially constant when the frequency of the square wave signal is less than a predetermined value and which decreases when the frequency of the square wave signal is at least equal to the predetermined frequency value. This demagnetizing signal is applied across the induction coil for generating through the magnetic medium, which is inductively coupled therewith, the 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 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, so that 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 medium incrementally decreases from an initial value to essentially zero to successively and incrementally decrease the magnitude of the magnetic flux within the magnetic medium to demagnetize the magnetic medium.
One specific aspect of the invention contemplates demagnetizing a magnetic material core of an electromagnetic coil. In this case, 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 is applied across the electromagnetic coil. This generates in the magnetic material core 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 alternating voltage wave, the second frequency being of sufficient value to reduce the magnitude of the magnetic flux within the core to essentially zero to demagnetize the core, and the frequencies between the first and second frequency values increasing in frequency in a relation to successively and incrementally decrease the magnitude of the magnetic flux within the core.
Other advantages and features of the invention will be apparent upon consideration of the following detailed description when taken in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates, partially in block diagram form and partially in schematic form, circuitry for demagnetizing a magnetic material in accordance with the present invention;
FIG. 2 shows a waveform of the signal provided at the output of the increasing frequency oscillator of FIG. 1, and
FIG. 3 shows a waveform of the signal provided at the output of the flip-flop of FIG. 1 in response to the signal of FIG. 2 at the input thereof.
DETAILED DESCRIPTION
Referring to FIG. 1 of the drawings, there is shown one embodiment of a circuit which may advantageously be employed to impart a magnetic bias to, or to demagnetize a magnetic material, or a magnetic medium, in accordance with the present invention. In this circuit, to demagnetize a magnetic material, an increasing frequency oscillator 12 applies to an electrical induction coil, through intermediate circuitry, a constant amplitude alternating polarity demagnetizing signal, or voltage wave, the successive cycles of which are frequency modulated to incrementally increase in frequency from an initial low first value to a second and higher value. This generates a field of magnetic flux within the coil, having a polarity which alternates with the alternating polarity of the signal, and having a magnitude inversely in accordance with the frequency of the signal, to demagnetize a magnetic material placed within the field. For the purpose of describing the invention, the magnetic material may be a core 16, and the induction coil may be an inductively wound winding 20 around the core 16, which together form an electromagnetic coil 24. However, it is within the contemplation of the invention that other magnetic mediums may be demagnetized, or have a magnetic bias imparted thereto, by being placed within a field of magnetic flux generated by, or by being inductively coupled with, other types of induction coils, or magnetic flux generation devices, to which a signal of the type generated by the circuit is applied, and reference to a core and a winding of an electromagnetic coil is intended to be illustrative only, and not limiting.
The invention contemplates taking advantage of the increasing impedance exhibited by an induction coil when an increasing frequency signal, or voltage wave, is applied thereacross. In practice, an initially low frequency, constant amplitude signal is applied across the coil 20, and is then increased in frequency while maintaining the amplitude thereof constant. While the frequency of the signal is low, the coil 20 exhibits a relatively low impedance which results in a relatively large alternating current flow therethrough, and therefore in a corresponding relatively large amplitude alternating shift in the field of magnetic flux generated thereby. This field of magnetic flux passes through the core 16 to magnetically switch the material thereof to impart a magnetic bias thereto and, as will be seen, results in the demagnetization of the core upon an increase in frequency of the signal.
As the frequency of the signal increases the impedance of the coil 20 similarly increases, resulting in a decreasing magnitude alternating current flow therethrough, and therefore in corresponding decreasing amplitude shifts in the magnetic flux through the core 16. This occurs since the impedance of an induction coil is directly proportional to the frequency of a signal applied thereacross, and for a constant amplitude signal applied across a coil the current through the coil is inversely proportional to the frequency of the signal.
To achieve optimum demagnetization of the core 16, the frequency of the demagnetizing signal applied to the coil 20 is increased incrementally with each cycle, or with each half cycle, of the signal, until the current flow through the coil, as limited by the impedance of the coil, is essentially zero. This generates through the magnetic material core 16 successive cycles, or excursions, of magnetic flux which alternate in polarity, and which successively decrease in amplitude from a relatively high initial value to a final value which is essentially zero, to effectively demagnetize the core.
In the demagnetization of the core 16, it is necessary that the magnitude of the alternating current through the coil 20, and therefore the magnitude of the magnetic flux excursions which magnetically switch the magnetic material core 16, be incrementally decreased from an initial value to zero. In a conventional demagnetizer where the amplitude, but not the frequency, of the demagnetizing signal is varied, this is accomplished by incrementally decreasing the amplitude of the constant frequency signal from an initial value to zero. In the present invention this is accomplished by incrementally increasing the frequency of a constant amplitude demagnetizing signal from an initial relatively low value to a relatively high value. To minimize the time required to demagnetize the core 16, the frequency of the demagnetizing signal may be increased at an exponential rate, which is possible since it is the incremental decrease in the magnitude of the half cycles of the alternating current through the coil 20 which is of concern, and not the time interval between the incremental decreases. This, of course, allows demagnetization of the core 16 in a shorter time interval than would be attainable with a conventional demagnetizer where, to obtain a uniform incremental decrease in current through the coil with a constant frequency signal applied thereto, it is necessary to linearly decrease the amplitude of the signal applied thereacross at a rate which is dependent upon the period of the cycles of the signal.
More particularly, the increasing frequency oscillator 12 may be of the type disclosed in my copending application Ser. No. 461,082, filed Apr. 15, 1974, or may be comprised of a conventional wide range voltage controlled oscillator circuit which receives as an input the output from a conventional ramp voltage generator circuit. The oscillator 12 generates at an output 28 thereof a signal 32, as shown in FIG. 2, the individual cycles of which have a constant voltage excursion, or amplitude, and the successive cycles of which incrementally increase in frequency from an initial frequency value to a higher frequency value. In other words, successive cycles of the signal 32 have decreasing periods, and the period t N of the Nth cycle of the signal 32 is less than the period t N - 1 of the preceding Nth-1 cycle, and is greater than the period t N + 1 of the succeeding Nth+1 cycle.
The signal 32 at the output 28 of the oscillator 12 is applied through a capacitor 34 to a flip-flop 36 which receives a first operating potential from a source of reference potential 40, and a second operating potential from the juncture between a Zener diode 55 and a resistor 48 which are connected in series between the source of reference potential 40 and a source of negative potential 52. The Zener diode 44 provides a regulated power source for flip-flop 36, and the output of the flip-flop, with the signal 32 at the input thereof, is a square wave signal 56, as shown in FIG. 3, having alternating polarity half cycles which occur in response to each negative going transition 60 of each cycle of the signal 32. In other words, the flip-flop 36 divides the signal 32 by two, and the output of the flip-flop alternately changes polarity with each cycle of the signal 32 at the negative going transition portion 60 thereof. This generates the signal 56 which is an alternating voltage signal, or square wave, having alternating polarity half cycles which successively and incrementally increase in frequency in accordance with the successive and incremental increase in frequency of each successive cycle of the signal 32, and each successive half cycle of the signal 56 has a period which is identical with the period of a corresponding full cycle of the signal 32.
The signal 56 is applied through a resistor 64 to the base of a transistor 68 for controlling the conduction thereof. The transistor 68 is connected at its base to the source of reference potential 40 through a resistor 72, the emitter of the transistor is connected directly to the source of reference potential 40, and the collector thereof is connected to the source of negative potential 52 through a resistor 76.
A diode 80 is connected in series with a Zener diode 84 between the source of reference potential 40 and the source of negative potential 52 through the resistor 76, and is connected at its cathode to the collector of the transistor 68. A resistor 88 is connected between the anode of the diode 80 and the source of negative potential 52. During the positive half cycles of the signal 56 the base of the transistor 68 is positive with respect to the emitter thereof, the transistor 68 is nonconductive, and the Zener diode 84 conducts current from the source of reference potential 40 through the diode 80 and the resistor 76 to the source of negative potential 52, as well as through the resistor 88 to the source of negative potential. At this time, the potential at the collector of the transistor 68 is essentially equal to the potential at the anode of the diode 80. That is, the potential at the collector of the transistor 68 is equal to the relatively negative potential at the anode of the diode 80, as determined by the breakdown voltage of the Zener diode 84, less the forward voltage drop of the diode 80.
During the negative half cycles of the signal 56 the base of the transistor 68 is negative with respect to the emitter thereof, the transistor 68 is conductive, and the potential at the collector thereof is essentially equal in value to the source of reference potential 40. At this time the diode 80 is reversed biased and does not conduct current therethrough as a result of a potential at its anode, as the Zener diode 84 continues to conduct through the resistor 88, which is negative with respect to the potential at its cathode. It is to be appreciated that the resistor 88 provides a conductive path for the Zener diode 84 to maintain the Zener diode conductive, during the time when the transistor 68 conducts, when the Zener diode would otherwide become nonconductive as the potential at the collector of the transistor 68 becomes essentially equal to the source of reference potential 40. This eliminates delay, as a result of the stabilizing time of the Zener diode 84, in reaching the potential to be applied to the collector of the transistor 68 when the transistor becomes nonconductive, and generates at the collector of the transistor 68 a well defined square wave having symmetry in amplitude as well as symmetry in time, minus predetermined incremental decrements, between successive half cycles thereof.
The signal at the collector of the transistor 68 is applied through a low-pass filter, comprising a pair of resistors 92 and 96 and a pair of capacitors 100 and 104, to a power amplifier circuit 108 over a conductor 112. The output from the amplifier 108 is applied as a demagnetizing signal to the coil 20 of the electromagnetic coil 24, and the values of the resistors 92 and 96 and the capacitors 100 and 104 of the low-pass filter are such that the signal on the conductor 112, as applied to the coil 20 through the amplifier 108, has a constant amplitude until the frequency thereof becomes sufficiently high that the impedance of the coil 20 becomes sufficiently great that essentially no current flows therethrough, and thereafter is reduced in amplitude as the frequency thereof continues to increase. That is, as the signal at the collector of the transistor 68, as applied over the conductor 112 through the low-pass filter, increases in frequency from an initial first relatively low frequency value to a second and higher frequency value, at which point essentially no current flows through the coil 20, the lowpass filter provides a constant amplitude signal on the conductor 112 until the frequency of the signal at the collector of the transistor 68 reaches the second frequency value, and thereafter modulates, or decreases, the amplitude of the signal on the conductor 112 by an amount directly in accordance with the frequency value thereof. This effectively and automatically terminates the application of the demagnetizing signal to the coil 20.
The demagnetizing signal, or voltage wave, at the output of the amplifier 108 applied across the coil 20 has a waveform essentially as shown in FIG. 3. That is, the demagnetizing signal is a constant amplitude voltage wave having alternating polarity half cycles which successively and incrementally increase in frequency. This signal generates a field of magnetic flux, with the coil 20, which passes through the magnetic material core 16 for imparting a magnetic bias thereto and which has a polarity determined by the polarity of the signal and a magnitude, or strength, which varies inversely in accordance with the frequency of the signal. Therefore, as each successive half cycle of the demagnetizing signal incrementally increases in frequency the impedance of the coil 20 incrementally increases in value, the current through the coil 20 incrementally decreases in value, and the magnitude of the magnetic flux generated by the coil 20 through the core 16 incrementally decreases in value. This imparts a magnetic bias to the core 16 and results in an incrementl step-by-step demagnetization of the core 16 with each successive half cycle of the demagnetizing signal as the frequency thereof incrementally increases in value with respect to the frequency of the preceding half cycle.
When the frequency of the demagnetizing signal increases to a value where the impedance exhibited by the coil 20 is sufficiently great so that essentially no current flows through the coil 20 the magnitude, or strength, of the magnetic field of flux passing through the magnetic material core 16 is essentailly zero, and the core 16 is demagnetized. At or above this frequency, to eliminate the possibility of introducing residual magnetism into the core 16 upon the termination of the demagnetizing signal applied to the coil 20, the low-pass filter gradually reduces the amplitude of the signal with continued and incremental increases in the frequency thereof to effectively remove the demagnetizing signal from the coil 20 without a sudden change in value thereof. It should be noted that demagnetization of the core 16 is accomplished as a result of the increasing frequency of the demagnetizing signal prior to the time that the amplitude of the signal is reduced by the low-pass filter, and that at the time the low-pass filter becomes operative to reduce the amplitude of the demagnetizing signal demagnetization of the core 16 has been completed.
While one embodiment of the invention has been described in detail, it is understood that various other modifications and embodiments may be devised by one skilled in the art without departing from the spirit and scope of the invention. For example, while the demagnetizing signal has been described as having half cycles which successively and incrementally increase in frequency, it is within the contemplation of the invention to sequentially increase the frequency of a demagnetizing signal upon each occurrence of a group of half cycles or full cycles. Furthermore, while the invention has been described with respect to the demagnetization of a magnetic material core of an electromagnetic coil, it is within the contemplation of the invention that any induction coil capable of generating a field of magnetic flux, or any other type of device which is capable of being energized by an alternating voltage signal for imparting a desired magnetic bias to a magnetic material which varies in strength in accordance with frequency of the signal, may be employed to demagnetize a magnetic material other than a core of an electromagnetic coil, or any other magnetic medium.
1. Field of the Invention
The present invention relates to a method of, and apparatus for, demagnetizing a magnetic material, and in particular to a method of, and apparatus for, demagnetizing a magnetic material with a flux field generated by an electrical induction coil having an increasing frequency signal applied thereacross.
2. Description of the Prior Art
Demagnetizing magnetic material is well known in the prior art. For example, in the manufacture of a product employing a magnetic material, such as an electromagnetic coil having a magnetic material core, it is often necessary to demagnetize the magnetic material, prior to transport or storage of the coil, to prevent the attraction thereto of contaminants of a magnetic nature as a result of residual magnetism therein. In the case of a magnetic coil having a magnetic material core, the conventional technique for demagnetizing the core is to apply a constant frequency, sinusoidal signal across the coil, and to then decrease, or modulate, the signal in amplitude until the current flow through the coil is essentially zero. With a constant frequency signal applied across the coil, the impedance of the coil remains essentially constant, and the magnitude of the current flow therethrough decreases as a result of the decreasing amplitude of the signal. This generates within the core a magnetic field of flux which alternates in polarity with the signal, and which decreases in magnitude with the decrease in magnitude of the signal.
This conventional technique has several disadvantages. For example, if a mechanical voltage regulation device, such as a variac, is employed to decrease the amplitude of a demagnetizing signal applied across the coil, any noise or discontinuity in the decaying signal for the purpose of demagnetization can introduce, rather than eliminate, residual magnetism in the magnetic coil. Furthermore, if an electronic circuit, as compared with a mechanical device such as a variac, is employed to eliminate noise in the demagnetizing signal, the output stage of the circuit must initially conduct large currents when the amplitude of the demagnetizing signal is large, and must later dissipate a large amount of power as the amplitude of the demagnetizing signal is decreased. Furthermore, the load impedance exhibited by a magnetic coil may shift rapidly as the magnetic drive field exceeds the coercive force of the core material thereof, and when this occurs the sudden change in current through the coil may modulate the demagnetizing signal in such a way as to introduce residual magnetism into the core. In this case, if a plurality of parallel connected magnetic coils are simultaneously demagnetized, the current modulation caused by an individual one of the coils, as the coercive force of the core material is exceeded by the drive signal, introduces magnetism in all of the coils as a result of the altered drive signal. Also, as the strength of the magnetic field of flux must be changed in small increments to effectively demagnetize a magnetic material, the time required to demagnetize the material may be considerable since the minimum time between each incremental decrease is based upon the period of a cycle of the constant frequency demagnetizing signal.
SUMMARY OF THE INVENTION
In accordance with the present invention, a system for imparting a magnetic bias to a magnetic medium includes circuitry 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 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 as the frequency of the square wave increases from the first to the second frequency value.
Preferably, an iscillator generates an initial signal the cycles of which successively and incrementally increase in frequency and circuitry, which includes 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, is 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. Additional circuitry is 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 which is directly in accordance with the frequency of the initial signal.
The square wave signal is applied to a low-pass filter which produces at an output thereof an alternating polarity demagnetizing signal which has a frequency equal to the frequency of the square wave signal and an amplitude which is essentially constant when the frequency of the square wave signal is less than a predetermined value and which decreases when the frequency of the square wave signal is at least equal to the predetermined frequency value. This demagnetizing signal is applied across the induction coil for generating through the magnetic medium, which is inductively coupled therewith, the 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 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, so that 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 medium incrementally decreases from an initial value to essentially zero to successively and incrementally decrease the magnitude of the magnetic flux within the magnetic medium to demagnetize the magnetic medium.
One specific aspect of the invention contemplates demagnetizing a magnetic material core of an electromagnetic coil. In this case, 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 is applied across the electromagnetic coil. This generates in the magnetic material core 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 alternating voltage wave, the second frequency being of sufficient value to reduce the magnitude of the magnetic flux within the core to essentially zero to demagnetize the core, and the frequencies between the first and second frequency values increasing in frequency in a relation to successively and incrementally decrease the magnitude of the magnetic flux within the core.
Other advantages and features of the invention will be apparent upon consideration of the following detailed description when taken in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates, partially in block diagram form and partially in schematic form, circuitry for demagnetizing a magnetic material in accordance with the present invention;
FIG. 2 shows a waveform of the signal provided at the output of the increasing frequency oscillator of FIG. 1, and
FIG. 3 shows a waveform of the signal provided at the output of the flip-flop of FIG. 1 in response to the signal of FIG. 2 at the input thereof.
DETAILED DESCRIPTION
Referring to FIG. 1 of the drawings, there is shown one embodiment of a circuit which may advantageously be employed to impart a magnetic bias to, or to demagnetize a magnetic material, or a magnetic medium, in accordance with the present invention. In this circuit, to demagnetize a magnetic material, an increasing frequency oscillator 12 applies to an electrical induction coil, through intermediate circuitry, a constant amplitude alternating polarity demagnetizing signal, or voltage wave, the successive cycles of which are frequency modulated to incrementally increase in frequency from an initial low first value to a second and higher value. This generates a field of magnetic flux within the coil, having a polarity which alternates with the alternating polarity of the signal, and having a magnitude inversely in accordance with the frequency of the signal, to demagnetize a magnetic material placed within the field. For the purpose of describing the invention, the magnetic material may be a core 16, and the induction coil may be an inductively wound winding 20 around the core 16, which together form an electromagnetic coil 24. However, it is within the contemplation of the invention that other magnetic mediums may be demagnetized, or have a magnetic bias imparted thereto, by being placed within a field of magnetic flux generated by, or by being inductively coupled with, other types of induction coils, or magnetic flux generation devices, to which a signal of the type generated by the circuit is applied, and reference to a core and a winding of an electromagnetic coil is intended to be illustrative only, and not limiting.
The invention contemplates taking advantage of the increasing impedance exhibited by an induction coil when an increasing frequency signal, or voltage wave, is applied thereacross. In practice, an initially low frequency, constant amplitude signal is applied across the coil 20, and is then increased in frequency while maintaining the amplitude thereof constant. While the frequency of the signal is low, the coil 20 exhibits a relatively low impedance which results in a relatively large alternating current flow therethrough, and therefore in a corresponding relatively large amplitude alternating shift in the field of magnetic flux generated thereby. This field of magnetic flux passes through the core 16 to magnetically switch the material thereof to impart a magnetic bias thereto and, as will be seen, results in the demagnetization of the core upon an increase in frequency of the signal.
As the frequency of the signal increases the impedance of the coil 20 similarly increases, resulting in a decreasing magnitude alternating current flow therethrough, and therefore in corresponding decreasing amplitude shifts in the magnetic flux through the core 16. This occurs since the impedance of an induction coil is directly proportional to the frequency of a signal applied thereacross, and for a constant amplitude signal applied across a coil the current through the coil is inversely proportional to the frequency of the signal.
To achieve optimum demagnetization of the core 16, the frequency of the demagnetizing signal applied to the coil 20 is increased incrementally with each cycle, or with each half cycle, of the signal, until the current flow through the coil, as limited by the impedance of the coil, is essentially zero. This generates through the magnetic material core 16 successive cycles, or excursions, of magnetic flux which alternate in polarity, and which successively decrease in amplitude from a relatively high initial value to a final value which is essentially zero, to effectively demagnetize the core.
In the demagnetization of the core 16, it is necessary that the magnitude of the alternating current through the coil 20, and therefore the magnitude of the magnetic flux excursions which magnetically switch the magnetic material core 16, be incrementally decreased from an initial value to zero. In a conventional demagnetizer where the amplitude, but not the frequency, of the demagnetizing signal is varied, this is accomplished by incrementally decreasing the amplitude of the constant frequency signal from an initial value to zero. In the present invention this is accomplished by incrementally increasing the frequency of a constant amplitude demagnetizing signal from an initial relatively low value to a relatively high value. To minimize the time required to demagnetize the core 16, the frequency of the demagnetizing signal may be increased at an exponential rate, which is possible since it is the incremental decrease in the magnitude of the half cycles of the alternating current through the coil 20 which is of concern, and not the time interval between the incremental decreases. This, of course, allows demagnetization of the core 16 in a shorter time interval than would be attainable with a conventional demagnetizer where, to obtain a uniform incremental decrease in current through the coil with a constant frequency signal applied thereto, it is necessary to linearly decrease the amplitude of the signal applied thereacross at a rate which is dependent upon the period of the cycles of the signal.
More particularly, the increasing frequency oscillator 12 may be of the type disclosed in my copending application Ser. No. 461,082, filed Apr. 15, 1974, or may be comprised of a conventional wide range voltage controlled oscillator circuit which receives as an input the output from a conventional ramp voltage generator circuit. The oscillator 12 generates at an output 28 thereof a signal 32, as shown in FIG. 2, the individual cycles of which have a constant voltage excursion, or amplitude, and the successive cycles of which incrementally increase in frequency from an initial frequency value to a higher frequency value. In other words, successive cycles of the signal 32 have decreasing periods, and the period t N of the Nth cycle of the signal 32 is less than the period t N - 1 of the preceding Nth-1 cycle, and is greater than the period t N + 1 of the succeeding Nth+1 cycle.
The signal 32 at the output 28 of the oscillator 12 is applied through a capacitor 34 to a flip-flop 36 which receives a first operating potential from a source of reference potential 40, and a second operating potential from the juncture between a Zener diode 55 and a resistor 48 which are connected in series between the source of reference potential 40 and a source of negative potential 52. The Zener diode 44 provides a regulated power source for flip-flop 36, and the output of the flip-flop, with the signal 32 at the input thereof, is a square wave signal 56, as shown in FIG. 3, having alternating polarity half cycles which occur in response to each negative going transition 60 of each cycle of the signal 32. In other words, the flip-flop 36 divides the signal 32 by two, and the output of the flip-flop alternately changes polarity with each cycle of the signal 32 at the negative going transition portion 60 thereof. This generates the signal 56 which is an alternating voltage signal, or square wave, having alternating polarity half cycles which successively and incrementally increase in frequency in accordance with the successive and incremental increase in frequency of each successive cycle of the signal 32, and each successive half cycle of the signal 56 has a period which is identical with the period of a corresponding full cycle of the signal 32.
The signal 56 is applied through a resistor 64 to the base of a transistor 68 for controlling the conduction thereof. The transistor 68 is connected at its base to the source of reference potential 40 through a resistor 72, the emitter of the transistor is connected directly to the source of reference potential 40, and the collector thereof is connected to the source of negative potential 52 through a resistor 76.
A diode 80 is connected in series with a Zener diode 84 between the source of reference potential 40 and the source of negative potential 52 through the resistor 76, and is connected at its cathode to the collector of the transistor 68. A resistor 88 is connected between the anode of the diode 80 and the source of negative potential 52. During the positive half cycles of the signal 56 the base of the transistor 68 is positive with respect to the emitter thereof, the transistor 68 is nonconductive, and the Zener diode 84 conducts current from the source of reference potential 40 through the diode 80 and the resistor 76 to the source of negative potential 52, as well as through the resistor 88 to the source of negative potential. At this time, the potential at the collector of the transistor 68 is essentially equal to the potential at the anode of the diode 80. That is, the potential at the collector of the transistor 68 is equal to the relatively negative potential at the anode of the diode 80, as determined by the breakdown voltage of the Zener diode 84, less the forward voltage drop of the diode 80.
During the negative half cycles of the signal 56 the base of the transistor 68 is negative with respect to the emitter thereof, the transistor 68 is conductive, and the potential at the collector thereof is essentially equal in value to the source of reference potential 40. At this time the diode 80 is reversed biased and does not conduct current therethrough as a result of a potential at its anode, as the Zener diode 84 continues to conduct through the resistor 88, which is negative with respect to the potential at its cathode. It is to be appreciated that the resistor 88 provides a conductive path for the Zener diode 84 to maintain the Zener diode conductive, during the time when the transistor 68 conducts, when the Zener diode would otherwide become nonconductive as the potential at the collector of the transistor 68 becomes essentially equal to the source of reference potential 40. This eliminates delay, as a result of the stabilizing time of the Zener diode 84, in reaching the potential to be applied to the collector of the transistor 68 when the transistor becomes nonconductive, and generates at the collector of the transistor 68 a well defined square wave having symmetry in amplitude as well as symmetry in time, minus predetermined incremental decrements, between successive half cycles thereof.
The signal at the collector of the transistor 68 is applied through a low-pass filter, comprising a pair of resistors 92 and 96 and a pair of capacitors 100 and 104, to a power amplifier circuit 108 over a conductor 112. The output from the amplifier 108 is applied as a demagnetizing signal to the coil 20 of the electromagnetic coil 24, and the values of the resistors 92 and 96 and the capacitors 100 and 104 of the low-pass filter are such that the signal on the conductor 112, as applied to the coil 20 through the amplifier 108, has a constant amplitude until the frequency thereof becomes sufficiently high that the impedance of the coil 20 becomes sufficiently great that essentially no current flows therethrough, and thereafter is reduced in amplitude as the frequency thereof continues to increase. That is, as the signal at the collector of the transistor 68, as applied over the conductor 112 through the low-pass filter, increases in frequency from an initial first relatively low frequency value to a second and higher frequency value, at which point essentially no current flows through the coil 20, the lowpass filter provides a constant amplitude signal on the conductor 112 until the frequency of the signal at the collector of the transistor 68 reaches the second frequency value, and thereafter modulates, or decreases, the amplitude of the signal on the conductor 112 by an amount directly in accordance with the frequency value thereof. This effectively and automatically terminates the application of the demagnetizing signal to the coil 20.
The demagnetizing signal, or voltage wave, at the output of the amplifier 108 applied across the coil 20 has a waveform essentially as shown in FIG. 3. That is, the demagnetizing signal is a constant amplitude voltage wave having alternating polarity half cycles which successively and incrementally increase in frequency. This signal generates a field of magnetic flux, with the coil 20, which passes through the magnetic material core 16 for imparting a magnetic bias thereto and which has a polarity determined by the polarity of the signal and a magnitude, or strength, which varies inversely in accordance with the frequency of the signal. Therefore, as each successive half cycle of the demagnetizing signal incrementally increases in frequency the impedance of the coil 20 incrementally increases in value, the current through the coil 20 incrementally decreases in value, and the magnitude of the magnetic flux generated by the coil 20 through the core 16 incrementally decreases in value. This imparts a magnetic bias to the core 16 and results in an incrementl step-by-step demagnetization of the core 16 with each successive half cycle of the demagnetizing signal as the frequency thereof incrementally increases in value with respect to the frequency of the preceding half cycle.
When the frequency of the demagnetizing signal increases to a value where the impedance exhibited by the coil 20 is sufficiently great so that essentially no current flows through the coil 20 the magnitude, or strength, of the magnetic field of flux passing through the magnetic material core 16 is essentailly zero, and the core 16 is demagnetized. At or above this frequency, to eliminate the possibility of introducing residual magnetism into the core 16 upon the termination of the demagnetizing signal applied to the coil 20, the low-pass filter gradually reduces the amplitude of the signal with continued and incremental increases in the frequency thereof to effectively remove the demagnetizing signal from the coil 20 without a sudden change in value thereof. It should be noted that demagnetization of the core 16 is accomplished as a result of the increasing frequency of the demagnetizing signal prior to the time that the amplitude of the signal is reduced by the low-pass filter, and that at the time the low-pass filter becomes operative to reduce the amplitude of the demagnetizing signal demagnetization of the core 16 has been completed.
While one embodiment of the invention has been described in detail, it is understood that various other modifications and embodiments may be devised by one skilled in the art without departing from the spirit and scope of the invention. For example, while the demagnetizing signal has been described as having half cycles which successively and incrementally increase in frequency, it is within the contemplation of the invention to sequentially increase the frequency of a demagnetizing signal upon each occurrence of a group of half cycles or full cycles. Furthermore, while the invention has been described with respect to the demagnetization of a magnetic material core of an electromagnetic coil, it is within the contemplation of the invention that any induction coil capable of generating a field of magnetic flux, or any other type of device which is capable of being energized by an alternating voltage signal for imparting a desired magnetic bias to a magnetic material which varies in strength in accordance with frequency of the signal, may be employed to demagnetize a magnetic material other than a core of an electromagnetic coil, or any other magnetic medium.