EP0597181A1 - Method and device for demagnetizing of magnetic materials - Google Patents

Abstract

The invention assumes that the envelope of a decreasing alternating magnetic field for demagnetizing magnetic materials must be larger than can be achieved by conventional oscillating circuits. Accordingly, a gradually decreasing alternating magnetic field having a large decay factor is generated in an oscillating circuit (C1, L1, L2) in whose air-core coil (L1) the material is held. By specifically recharging the capacitor (C1) from a recharging capacitor (C2) by means of a power supply (LS2), the capacitor voltage (U1) is increased and the energy which is reduced by damping is resupplied synchronously with the oscillation process, so that the envelope E(t) of the alternating field is specifically enlarged and the duration of the decaying alternating magnetic field in the air-core coil (C1) is prolonged. <IMAGE>

Description

The invention relates to a method and a device for demagnetizing or calibrating magnetic materials in a decaying alternating magnetic field of an electrical oscillating circuit with an amplitude that can be influenced. Demagnetization processes are based on the fact that magnets are exposed to an alternating magnetic field with decreasing amplitude. The demagnetization is repeated several times, with the magnetization of the magnet or its working point being measured in between.

DE 3005927 C2 proposes a method in which the magnetic flux density in the air gap of the magnet to be demagnetized is measured and the enveloping amplitude of the alternating field is reduced when the desired value is reached. The method is based on the fact that the amplitude of the alternating magnetic field of an oscillating circuit is forcibly reduced by changing the supply voltage when the resonance frequency is set automatically.

The disadvantages of known demagnetization processes are that the value of demagnetization, especially with shape anisotropic materials (e.g. AINiCo), is not met with sufficient accuracy. In the case of a not inconsiderable number of permanent magnets, there is no stable operating point, although the nominal value is set correctly nominally. The known methods are associated with instabilities which have not hitherto been sufficiently eliminated.

Another disadvantage is that the air coil for receiving the magnet or the device containing the magnet must have a special shape and low ohmic resistance. The last requirement could be met to a limited extent by using silver wire or even superconducting coils, but this is not an economical alternative.

The invention has for its object to provide a method and an apparatus with which magnetic materials can be demagnetized more reliably.

The object is achieved with the characterizing features of the main claim. A device for performing the method is specified in the independent claims. Advantageous refinements can be found in the subclaims. Furthermore, the use of the device for magnetizing is proposed.

The invention is based on the fact that the duration of the declining alternating magnetic field for demagnetization must be particularly long, or that the ratio of successive amplitudes must be greater than can be achieved with conventional oscillating circuits generating an alternating field. The larger the envelope of the alternating field, or the slower the alternating field decays and the more often the magnet experiences the change between positive and negative half-wave of the decreasing magnetic field, the better the magnetic areas of the magnet to be adjusted are set and stabilized. The ratio of the amplitudes of successive equipolar half periods (or the decay constant) should therefore be close to 1.

Amplitude ratios close to 1 naturally lead to long adjustment times. The ratio can advantageously also be set as a constant value between 0.9 and 1.0. On the one hand, there is an upper limit in what is technically achievable, but on the other hand, the adjustment of magnets must be limited to an economical duration.

The underlying principle can be described in that energy is fed into the resonant circuit in synchronism with the magnetic alternating field. The energy is preferably fed in in cycle times with shorter units than half a period of the alternating field.

It is also proposed to feed the energy in such a way that the decay constant of the envelope of the alternating field is as large as possible, that is to say between 0.9 and 1.0.

Without special measures or interventions in an oscillating circuit, the duration and the amplitude ratio of the alternating field are limited due to the existing damping.

The measures according to the invention make it less important for air coils which shape and quality (L / R) they have. Air coils can be used that would work very poorly as inductors in parallel resonant circuits under conventional conditions. When using the invention, particular attention does not have to be paid to the construction volume and the internal resistance or the resulting quality.

In an oscillating circuit, the frequency f = [1 / (LC) - R ² / (4L ² )] ¹¹² produces an alternating magnetic field with an exponentially decaying amplitude A in the air coil. The envelope E (t) of the alternating field is described by:

The sizes mean: L = inductance; C = capacity; R = effective resistance of the resonant circuit; t = time; A _o = initial amplitude; α = R / (2L) = decay constant.

An oscillating circuit, which is used for the magnetization or demagnetization, usually contains, in addition to the magnet whose magnetization is to be changed, further conductive and magnetic materials. Losses due to magnetic reversal and eddy currents occur in them, which dampens the oscillating circuit and the oscillator frequency f1 can no longer be described solely by the formula given above, which only deals with ohmic damping. In the following the amplitude ratio is to be understood as the ratio of two successive positive amplitudes of the alternating field A2 (t + T) / A1 (t). The period is T. The amplitude ratio (or the decay constant a) in resonant circuits with magnetic materials is even less favorable than in resonant circuits with only ohmic losses.

It is proposed in an embodiment of the invention to extend the duration of the uniformly decaying alternating field by recharging at least one capacitor of the resonant circuit in order to supply the energy lost through damping. For certain power ranges, a further capacitor can be connected in parallel with the first capacitor in the resonant circuit, so that several capacitors are also available for receiving the make-up energy.

• 1. Messung und Steuerung der primären Ladespannung des Kondensators und Messung der Ladespannung des Nachladekondensators,
• 2. Messung der Amplitude (und Frequenz) des Wechselfeldes oder solcher Größen, die ihnen proportional sind,
• 3. Dosierung und Synchronisierung der Energieeinspeisung anhand der Messung nach 1., so daß das Wechselfeld einen zeitlich vorgebbaren Verlauf mit besonders großer Abklingkonstante annimmt.

There are various possibilities for recharging the capacitor in the resonant circuit to compensate for the energy lost through damping, which results in several further embodiments. The following measures must be taken to control the recharging of the capacitor:

• 1. measurement and control of the primary charging voltage of the capacitor and measurement of the charging voltage of the recharging capacitor,
• 2. measurement of the amplitude (and frequency) of the alternating field or of quantities which are proportional to them,
• 3. Dosage and synchronization of the energy feed based on the measurement according to 1., so that the alternating field assumes a time-definable course with a particularly large decay constant.

The invention is described in more detail in the single figure. It shows a circuit arrangement with energy supply to extend the alternating field.

A magnet, which is to be demagnetized, is located in a holder in the air coil L1 of an oscillating circuit. As far as the geometric or electrical dimensions of the arrangement allow, the entire device containing the magnet can also be fastened in the holder. The parallel resonant circuit consists of two inductors L1 and L2 and the capacitor C1. The primary energy for the resonant circuit (for the time 0 of the alternating field) is provided once by a high-voltage power supply LS1 by charging the capacitor C1.

To influence the uniform decay of the alternating field with a certain decay constant a, there are provided: a measuring tap U1 on the capacitor C1 for measuring the vibration profile, an energy supply for recharging the capacitor C1 via a second high-voltage power supply LS2 and an electronic control unit SM, with the shape or duration of the magnetic alternating field is controllable synchronously with the oscillation of the resonant circuit. The control unit SM essentially consists of a microprocessor MP, associated program or data memories (e.g. EPROM), an analog-digital converter ADW and an interface unit IOP. The control unit SM can also be implemented overall by a personal computer.

During the charging phase of the capacitor C1 by the high-voltage power supply LS1, the electronic switches S1, S4, S5 and S6 are in the open state. The switch S3 is in position "b". The function of the switch S6 and the diode D1 is described below.

The charging current flows into the capacitor C1 when the power unit LS1 is activated by the control unit SM or the switch S2 is closed. The voltage measurement on the capacitor C1 takes place via an analog-digital converter ADW, via which the voltage U1 is supplied to the control unit SM. The measured voltage is proportional to the amplitude of the alternating magnetic field. When a certain predetermined charging voltage U1 is reached, the power supply unit LS1 is switched off or the switch S2 is opened. When switch S1 is also closed by control unit SM, resonant circuit L1, L2, C1 begins to oscillate.

Frequencies f1 in demagnetizing resonant circuits for larger magnets are in the range from 50 to 250 Hz; a typical value is approximately f1 = 100 Hz. Without additional intervention in the resonant circuit or without energy feed, the amplitude of the alternating field has dropped below 1 percent of the output amplitude after about 10 to 12 periods and is then no longer effective.

To compensate for all ohmic and magnetic damping losses in the resonant circuit and to generate a predetermined decay constant, the capacitor C1 is recharged from the energy of the charging capacitor C2. The voltage U2 at the charging capacitor C2 is tapped and fed to the control unit SM via the analog-digital converter ADW. A sine / square wave converter SR1 detects only positive voltages U1 at the capacitor C1 and controls a switchable square wave generator G1. During the positive half-wave of the voltage across capacitor C1, this generates an in-phase square-wave voltage with a frequency f2 which is substantially greater than the frequency f1 of the alternating voltage across capacitor C1. In the present case, the Fre quenz f2 selected 30 times larger (f2 = 3 kHz). However, the frequency of the square wave voltage f2 can also vary in the range from 10 to 100 times the frequency f1. The rectangular generator G1 triggers a monoflop M1. The sine / square wave converter SR1 generates an in-phase square wave voltage to the voltage at the capacitor C1. In the positive half-wave of the square-wave voltage, the square-wave generator G1 generates a high-frequency square-wave voltage. During the negative half-wave of the capacitor voltage, the output of the square wave generator G1 is "low"; no control signal is given.

The output of monoflop M1 controls the duty cycle of switch S5 via switch S3 in position "b". With each positive edge of the square-wave voltage of the square-wave generator G1, the closing of the switch S5 is triggered (via the monoflop M1). The cycle time Tt is approximately 300 microseconds at the selected frequency f2. The pulse duty factor of switch S5 is preset manually by means of potentiometer TR1, regardless of frequency; the effective recharging current for the capacitor C1 can thus be determined.

The frequency and the duty cycle of the switch S5 are defined in terms of hardware with the three structural units sine / square wave converter SR1, square wave generator G1 and monoflop M1.

Before the demagnetization process is started, the control unit SM charges the capacitor C2 by closing the switch S4. A second high-voltage power supply LS2 is available for this. The capacitance of the charging capacitor C2 is chosen to be approximately twice as large as that of the capacitor C1. The charging voltage U2 of the capacitor C2 must be greater than the starting voltage of the capacitor C1 by a certain factor (K1). A factor K1 = 2 is suggested as a typical value. If the switch S5 is now switched on and off with the positive half-wave of the AC voltage with a certain duty cycle, a certain amount of charge flows from the charging capacitor C2 via the diode D2, the switch S5 and the inductor L2 to the capacitor C1 and charges it synchronously in small steps to the oscillation process, whereby the envelope of the magnetic alternating field decays more slowly than without recharging. The reloading process is preferably repeated for all subsequent positive half-waves.

The inductance L2 is only of minor importance for the operation of the resonant circuit C1, L1, L2, because of its ohmic resistance it even contributes somewhat to the damping. However, it is used to avoid large compensation currents between the charging capacitor C2 and the capacitor C1 when the switch S5 is closed, which would otherwise destroy the switch S5.

Since the frequency f2 of the recharging (determined by the duty cycle of the switch S5) is substantially greater than the oscillation frequency f1 of the resonant circuit C1, L1, L2, the inductance L2 must be smaller in this ratio in order to limit its inductive resistance.

Thanks to the optimal dimensioning of the components and the right choice of reload frequency, it is possible to transfer the reload energy with almost no loss.

The diode D2 between the high-voltage power supply LS2 and the resonant circuit C1, L1, L2 is used so that the resonant circuit remains separated from the capacitor C2 during the negative calf wave. Demagnetization stops when the amplitude of the alternating field has dropped below 1 percent of the initial value.

With the circuit arrangement described, there is no need to recharge the capacitor C1 during the negative half-wave of the alternating magnetic field. However, it is also possible to expand the circuit arrangement in such a way that the capacitor C1 is recharged in the correct phase even during the negative half-wave of the alternating magnetic field. With the symmetrical operation, a direct current component of the alternating field is avoided, so that no zero line shift occurs with complete demagnetization.

When switch S4 is open, capacitor C2 is not recharged. The charge transfer to the capacitor C1 reduces the amount of charge in the charging capacitor C2 approximately exponentially in time. The gradual decrease in the voltage of the charging capacitor C2 is quite desirable since the duration of the alternating field should be finite. The increase in the envelope and extension of the alternating field is determined by the amount of charge in the capacitor C2, the charge voltage U2 of the capacitor C2 (factor K1) and the duty cycle of the switch S5. The duration of the process is defined by the duty cycle. In this simple embodiment, only the charging voltage U2 of the capacitor C2 is controlled by the control unit SM.

The additional effort by using two power supplies is not a disadvantage, because the power supplies can each be dimensioned for the very different charging voltages U1 and U2. In further refinements, it can be provided that only one high-voltage power supply is used instead of two power supplies. Here, the one power supply unit must be designed for the larger charging capacity.

In another possibility, the power supplies can also be switched so that the power supply LS1 charges the capacitor C2 and / or the power supply LS2 recharges the capacitor C1. In the

Figure is indicated the waiver of a power supply, or the simultaneous use of both power supplies for recharging each capacitor with a dashed connection between power supply LS1 and switch S4. In one of these modes of operation - as already described above - the switches S2 and S4 are controlled accordingly by the control unit SM, so that with simultaneous monitoring of the voltages U1 and U2 in phase with the oscillation of the resonant circuit, the charge from the power supply unit LS1 to the capacitor C2 and / or from the power supply LS2 charge flows to the capacitor C1.

In a further embodiment of a demagnetizing resonant circuit, the extensive performance of the microprocessor is used. With different magnetic materials, it is namely advantageous to manipulate as many parameters of the alternating magnetic field as possible. The alternating field can be changed with regard to the following variables: the first half-wave is left out from influencing the alternating field or the beginning of the influencing in an even later half-wave; Raising the decay constant to a fixed value and keeping it constant or generating a time-varying decay constant. The switch S3 is therefore brought into position "a" for this type of manipulation and thus decoupled from the monoflop M1.

The pulse duty factor is no longer specified via the three modules SR1, G1, M1. The synchronization via the control unit SM takes place via the tap GG behind the sine / rectangular converter SR1. The pulse duty factor of the switch S5 can thus be controlled programmatically and all parameters for changing the alternating field of the resonant circuit (in particular pulse duty factor, charging voltages of the capacitors C1 and C2) can be varied freely within limits.

The program can be controlled in such a way that amplitude values are specified in a table in the permanent memory of the microprocessor MP, which are dependent on the prevailing initial conditions (e.g. the charging voltage). By comparing the amplitude-proportional measurement voltage U1 with the setpoints of the envelope E (t) of the alternating field stored in the read-only memory, the pulse duty factor of the switch S5 is controlled in such a way that the envelope E (t) determines predetermined amplitude values or with a decay constant (e.g. a = 0 , 92) follows.

The decaying alternating field can be influenced by energy feed-in by comparing the amplitude-proportional measurement voltage U1 with nominal values of the envelope E (t) of the alternating field calculated in real time by a high-performance microprocessor of the control unit SM and also dependent on the initial conditions.

In an alternative method, the control unit SM is operated in the pulse width modulation mode. For this purpose, a microprocessor with integrated pulse width modulation is used, so that the microprocessor takes over the tasks of the rectangular generator G1 and the monoflop M1. As a result, the advantages of manipulating the duty cycle over time are fully exploited in this operating mode.

In an alternative embodiment it can be provided that the switch S4 remains closed during the demagnetization. The capacitor C2 is continuously recharged from the high-voltage power supply LS2 and the alternating magnetic field can thereby be influenced even longer.

The oscillation cycle is ended when the amplitude A (measured value U1) is less than 1 percent of the initial amplitude (initial measured value). The value of the magnetization of the magnet is finally measured.

As a rule, after the first demagnetization cycle, the target value of the magnetization, which is within a certain target bandwidth (for example ± 10 percent), has not yet been reached, so that further demagnetization cycles must follow. These cycles are carried out according to the principle of successive approximation. The principle is implemented in such a way that the charging voltage of the capacitor C1 is changed in the next cycle by half the difference between the two previous values with a certain sign. A new demagnetization cycle follows the first cycle with a new charging voltage U1.

The circuit arrangement can also be used to magnetize magnets. To operate the circuit arrangement as a magnetizing device, the parts combined with the reference symbol AM (switch S6, control line for switch S6, diode D1 to ground) are added to the resonant circuit. The switches S1 and S4 are set to the "off" position. The switch S6 is turned on by the control unit SM. The capacitor C1 is charged with the switch S2 closed via the power supply unit LS1 until a certain high charging voltage has been set in the capacitor C1. To generate the magnetic pulse, switch S1 is closed. The resulting oscillation is without a negative half-wave, because the diode D1 short-circuits the voltage of the negative half-wave.

Claims (18)

1. Process for demagnetizing magnetic materials in a decaying ma A magnetic alternating field of an electrical oscillating circuit with an amplitude that can be influenced, characterized in that energy is fed into the oscillating circuit (C1, L1, L2) synchronously with the magnetic alternating field. 2. A method for demagnetizing according to claim 1, characterized in that the energy supply takes place in shorter time units than half a period (T / 2) of the alternating field. 3. A method for demagnetizing according to claim 1 or 2, characterized in that energy is fed in such a way that the decay constant (a) of the envelope E (t) = Ao exp (-at) of the alternating field is close to 1.0. 4. A method for demagnetizing according to one of the preceding claims, characterized in that at least one capacitor (C1) of the resonant circuit (C1, L1, L2) is recharged. 5. A method for demagnetizing magnetic materials according to any one of the preceding claims, characterized in that the recharging takes place only during half-waves of the alternating field with the same polarity. 6. A method for demagnetizing according to one of the preceding claims, characterized in that the reloading only begins at the beginning of the second or a later half-wave. 7. A method for demagnetizing according to one of claims 4 to 6, characterized in that the primary charging and recharging of the capacitor (C1) is carried out from a single power supply (LS1). 8. A method for demagnetizing according to one of claims 4 to 6, characterized in that the recharging takes place as charge transfer from a capacitor (C2). 9. A demagnetizing method according to one of claims 1 to 8, characterized in that the energy for the primary charging of the capacitor of the resonant circuit (C1) and the feeding of the recharging capacitor (C2) is supplied from a power supply unit (LS1, LS2). 10. A method for demagnetizing according to one of claims 1 to 8, characterized in that the energy for the primary charging of the capacitor (C1) of the resonant circuit and the feeding of the recharging capacitor (C2) is optionally supplied from one of two power supplies (LS1, LS2) . 11. A method for demagnetizing according to one of the preceding claims, characterized in that influencing the decaying alternating field by feeding in energy by comparing an amplitude-proportional measurement voltage (U1) with setpoints of the envelope E (t) stored in a read-only memory (MP) and dependent on the initial conditions. the alternating field is carried out by a control unit (SM). 12. A method for demagnetizing according to one of claims 1 to 10, characterized in that the influencing of the decaying alternating field by feeding in energy by means of a comparison of an amplitude-proportional measurement voltage (U1) with setpoints of the envelope calculated in real time by a microprocessor (MP) and dependent on the initial conditions E (t) of the alternating field is carried out by a control unit (SM). 13. A demagnetizing method according to one of the preceding claims, characterized in that the decaying alternating field is influenced by energy input from a control unit (SM) in the pulse width modulation mode. 14. A method for demagnetizing according to one of claims 1 to 10, characterized in that the influencing of the decaying alternating field by controlling a switch (S5), controlled by the voltage (U1) on the resonant circuit capacitor, between recharging capacitor (C2) or recharging power supply (LS1, LS2) and capacitor (C1) of the resonant circuit is made. 15. Device for demagnetizing according to the method of claims 1 to 14 consisting of a holder for the magnetic material in an air coil of a resonant circuit, with electrical components inserted into the resonant circuit, with which the course of the alternating field can be changed and with a device for supplying energy in the Oscillating circuit, characterized in that the resonant circuit (C1, L1, L2) has a measuring tap (U1), from which the capacitor voltage (U1) can be fed to a control unit (MP, SM) for controlling the alternating field, the frequency and phase of the voltage (U1) from the measuring tap (U1) U1) the control unit (SM) is fed via a sine / square wave converter (SR1) that the control unit (MP, SM) in cycle times (Tt) that are small compared to the period (T) of the alternating field, the capacitor voltage (U1) compares with values for a given decay constant (a) close to 1.0 and that the control unit (SM) controls a switch (S5) between the recharging capacitor (C2) or recharging power supply (LS1, LS2) and capacitor (C1) of the resonant circuit and thus the alternating field imprints a change that corresponds to a predeterminable course of the envelope E (t) of the decaying alternating field. 16. Device for demagnetizing according to the method of claims 1 to 14 consisting of a holder for the magnetic material in an air coil of a resonant circuit, with electrical components inserted into the resonant circuit, with which the course of the alternating field can be changed and with a device for supplying energy to the Resonant circuit, characterized in that the resonant circuit capacitor voltage (U1) is fed to a sine / square wave converter (SR1), that the output signal of the sine / square wave converter (SR1) is passed to a square wave generator (G1) and a series-connected and with a potentiometer (TR1) adjustable monoflop (M1) acts and that the output signal of the monoflop (M1) determines the duty cycle of a switch (S5) between recharging capacitor (C2) or recharging power supply (LS1, LS2) and capacitor (C1) of the resonant circuit. 17. Device for demagnetizing according to one of claims 15 or 16, characterized in that the control of the switch (S5) between recharging capacitor (C2) or recharging power supply (LS1, LS2) and capacitor (C1) of the resonant circuit with a switch (S3) can be switched is so that the device can be operated either in the operating mode microprocessor control according to claim 15 or in the operating mode fixed value control according to claim 16. 18. Use of the device according to one of claims 15 to 17 for the magnetization of magnetizable materials, characterized in that the resonant circuit (C1, L1, L2) is connected to a switchable ground connection (AM) for limiting the alternating magnetic field to a half-wave in parallel.

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