EP1791138B1 - Process for degaussing using alternating current pulses in a conductive loop - Google Patents

Abstract

Reproducible, capacitor-free demagnetization of ferromagnetic objects (30) involves using a low-frequency and frequency-modulated alternating current impulse produced by a current control (24), of variable amplitude and alternating current impulse width, in a conductor which may be connected in a capacitor-free manner between the input (27) and an output (28) of the current control. Reproducible, capacitor-free demagnetization of objects with a residual magnetism involves: (1) using a low-frequency and frequency-modulated alternating current impulse produced by a current control, of variable amplitude and alternating current impulse width, in a conductor which may be connected in a capacitor-free manner between the input and an output of the current control; and (2) producing a magnetic field impulse in the conductor which is flexible, completely insulated, unshielded and plastically deformable, and while forming a conductor loop (29) in any shape, is applied around an object to be demagnetized. The ends of the conductor loop are connected to the input and the output of the current control in a capacitor-free. The alternating current impulse of individual, alternatingly poled and symmetrical demagnetization impulses with controlled demagnetization impulse amplitude and alternating current impulse frequency of greater than 1 Hz is fed in, where the temporal course of the demagnetization impulse amplitudes is emulated by a demagnetization curve decaying in a non-exponential manner. The ratio of the smallest demagnetization impulse amplitude to the alternating current impulse amplitude maximum lies >=1:1000, and the conductor loop is removed after completion of the demagnetization of an object. An independent claim is included for a device for the reproducible, capacitor-free demagnetization of object with a residual magnetism.

Description

Technical area

The present invention describes a method for the reproducible capacitor-free demagnetization of objects with residual magnetism by means of at least one low-frequency and frequency-modulated alternating-current pulse of variable amplitude and AC pulse width in a conductor, whereby a magnetic field pulse is generated near the ladder.

The objects may be ferromagnetic pieces of different size and weight. During the production or treatment, the residual magnetism may result from the influence of an external magnetic field or may have been impressed on an object in a targeted manner.

State of the art

For demagnetization of ferromagnetic objects, several methods are known. Early on, the discharge of a charged capacitor of a resonant circuit was exploited for demagnetization, wherein the occurring during the oscillating discharge of the capacitor alternating magnetic field is used on objects in the vicinity of the degaussing coil of the resonant circuit. The disadvantage of this capacitor discharge method lies in the lack of reproducibility of the alternating magnetic field and in the very rapidly decaying current pulse. Depending on the induction of the coil used and the other components used, a varying alternating field pulse results, to which no influence can be taken more, since it is influenced by the parameters of the resonant circuit and is not controlled from the outside.

In US4384313 A further developed method for demagnetization is described, which in turn uses a resonant circuit to which controlled AC pulses are applied. The said resonant circuit is controlled via a complex electronic structure in that a rectifier rectifies the mains voltage and supplies an inverter through the DC voltage, which supplies the resonant circuit from one or more capacitors and the demagnetizing coil with an alternating voltage of variable frequency and amplitude. Since this is a resonant circuit, the frequency of the alternating current used, which induces the alternating magnetic field, is of great importance. The maximum current flow within the resonant circuit and thus within the demagnetizing coil can only be achieved if the phase shift between the applied voltage and the flowing current in the resonant circuit is zero. This phase shift disappears only when the inverter supplies an AC voltage with resonant frequency. Then the impedance, ie the AC resistance of the resonant circuit is minimal and the maximum current and thus the maximum inducible magnetic field within the degaussing coil occur. To detect the phase difference, a phase detector is used to detect the phase shift between the voltage and the current. For this purpose, the current signal is determined via the voltage which drops across a resistor in the resonant circuit. Due to the phase information, an oscillator can set the used inverter specifically to the resonance frequency. When the described setting of the resonant frequency has occurred, the AC amplitude can be ramped down by the inverter, thereby completing the demagnetization process.

In a simpler embodiment, the phase detector is omitted and the frequency of the AC pulse is traversed over a range smaller to greater than the resonant frequency. The resonance frequency is reached in any case for a short time, whereby the maximum possible alternating magnetic field occurs.

Since an AC pulse is used for demagnetization in a resonant circuit whose inductance is changed by the objects to be demagnetized, an electronic control must be used that allows adjustment of the AC pulse frequency. If the resonant frequency is to be tracked regulated, in addition to an electronic component which performs the control of the AC pulse frequency, in addition a component must make the detection of the resonant frequency. It is thus a complicated electronic structure needed to provide the AC pulse to generate the largest possible magnetic alternating field pulse.
Since fixed coils are used, the geometric dimensions of the objects to be demagnetized are limited, since they must be brought into the vicinity of the coils. The most accurate and tight winding of the power cable in a coil has the disadvantage that the cables are strongly heated at high currents and can lead to altered demagnetization curves in continuous operation. The capacitors used also change their properties with greater heat generation, which has effects on the resonant circuit.

To get an insight on how the objects to be demagnetized are arranged relative to the magnetic field becomes EP1465217 directed. There, a transport road is described, on which the objects are transported between a stationary long coil or two stationary coils of a resonant circuit, where the objects dwell for a desired time. The demagnetization is also done here by an AC pulse which is controllable in frequency and amplitude and whose AC pulse amplitude is automatically reduced from a maximum value to zero. The objects are in a homogeneous alternating magnetic field during the demagnetization process, whose field strength is reduced by the alternating current pulse amplitude. Here, too, an inverter ensures the control of the current, which flows through the resonant circuit consisting of the two demagnetization coils and capacitors.

Since a resonant circuit is also used here, the tuning of the AC pulse frequency to the resonant frequency of the resonant circuit is tuned to achieve the maximum current flow in the resonant circuit.
A disadvantage is that the size of the demagnetizable objects is determined by the diameter of the demagnetization coils. Likewise, the weight of the objects is limited by the carrying capacity of the transport road. Thus, a complete demagnetization of a large object, such as a turbine, as a whole is nearly impossible, unless one provides a suitable transport line and coils of suitable diameter. But since the entire demagnetization apparatus is so large and bulky that it must be permanently installed in a workshop, the disassembly and transport of very large object to demagnetizer would be very tedious.

Also in US4360854 a device is described in which the objects to be demagnetized must be moved by coils of large diameter, so that the demagnetization can take place. Despite the large dimensions of the coils, it is not possible here to demagnetize large objects as a whole and in one step. The size of the apparatus makes a mobile use of demagnetization impossible. The objects to be demagnetized must be transported to the demagnetizer to be moved there on moving trolley by the magnetic field of the coils. Here are also special requirements placed on the trolley, which must be designed for large weights. The device described requires mandatory disassembly of demagnetizing components so that they can be guided by the magnetic field. The machines and devices in which there are objects with residual magnetism are therefore shut down for a long time, so that the dismantling, demagnetization and reassembly can take place.

US-A-4,607,310 discloses a method for capacitor-free demagnetization of a magnetic read / write head by means of an AC pulse having adjustable AC pulse frequency of variable amplitude and AC pulse width. Due to the software-controlled control of voltage, frequency and decay behavior (see Sp. 1, Z. 38-43), it should be possible to generate a non-exponential decay curve and an AC frequency greater than 1 Hz (see Figures 2a and 2b). about 10 Hz).

GB-A-1 164 786 discloses a method for capacitor-free demagnetization of a cathode ray tube (or its shadow mask) and US-A-5,995,358 discloses a similar method of demagnetizing a magnetic support plate. Both methods use an AC source of constant frequency, only length and possibly phase of the pulses are modulated.

What these three previously published demagnetization methods have in common is that the applications require a permanent demagnetization option and thus the stationary whereabouts of the demagnetization coil in the object to be demagnetized.

US-A-2703052 discloses a method for demagnetization of hulls, in which an unspecified electric current is sent through a cable wound around the hull.

Presentation of the invention

The present invention has for its object to provide a method which allows small to very large and heavy objects to be demagnetized on site flexibly and without disassembly reproducible. Thus, longer downtime of equipment in which objects are to be eliminated with residual magnetism, avoidable. The objects do not necessarily have to be moved away for demagnetization. The present method does not require transportation of the objects with residual magnetism through transport roads or otherwise, which can lead to complications before and during demagnetization. The dimensions of the demagnetizable objects are not limited by a prefabricated, possibly specially made, demagnetization coil. Due to the few components required and the absence of bulky structures, the process is portable and can be applied in a small space isolated.

The present method is without capacitors and thus without an electronic resonant circuit, so that no Resonance frequency detection and Resonanfrequenzeinstellung by other electronic components is needed. In contrast to the resonant circuit solutions, the present method solves the task at selectable low frequencies from 1 Hz to work, which can not be achieved with the use of capacitors, or only with high demands on the capacitors. The avoidance of capacitors allows the AC pulse to superimpose a constant DC component, which can impress the object with a desired residual magnetism. This is not possible with a resonant circuit solution because the capacitor blocks and charges DC current.

Brief description of the drawings

• Figur 1 zeigt einen benutzten Wechselstromimpuls im I/t Diagramm, wobei aus Gründen der Übersicht nur etwa 20 Perioden aufgezeichnet sind. Figur 2 zeigt zusätzlich einen Wechselstromimpuls dem ein Gleichstromanteil additiv überlagert ist.
• Figur 3 zeigt die Stromsteuerung mit einigen Details und den Anschluss des Leiters unter Bildung einiger Schlaufen um ein Objekt in einer schematischen Darstellung.

The method and apparatus for achieving the object described above will be described below in conjunction with the drawings.

• FIG. 1 shows a used AC pulse in the I / t diagram, for reasons of clarity, only about 20 periods are recorded. FIG. 2 additionally shows an AC pulse to which a DC component is additively superimposed.
• FIG. 3 shows the current control with some details and the connection of the conductor to form some loops around an object in a schematic representation.

description

For demagnetization of components of different thickness alternating magnetic fields are used, which are generated by at least one AC pulse 1 with adjustable AC pulse width 2. As in FIG. 1 the alternating current pulse 1 is a chain of demagnetizing 5 alternating polarity with controllable Entmagnetisierimpulsamplituden 6. The polarity change of Demagnetisierimpulse 5 is done with an adjustable Wechselstromimpulsfre frequency 4. The AC pulse frequency 4 determines the penetration depth of the resulting magnetic field in the material to be demagnetized. Low AC pulse frequencies 4 of a few Hertz lead to large penetration depths. In the present invention operates with AC pulse frequencies 4 greater than 1 Hz. Starting from an AC pulse amplitude maximum 3, the demagnetizing pulse amplitudes 6 are continuously reduced to zero with a controllable decrement. The envelope of the demagnetizing pulse amplitudes 6 will be referred to below as the demagnetizing curve 7. Measurements have shown that it is advantageous for the demagnetization curve 7 to drop as flat as possible and thus slowly. The AC pulse width 2 is usually selected so that about 100 AC pulse periods are passed through in a demagnetization process. Depending on the required depth of penetration of the magnetic field, the alternating current impulse is chosen 4 frequency, whereby the AC pulse width 2 and thus the total time of complete demagnetization is determined.

For complete and reproducible demagnetization very high demands are placed on the shape of the AC pulse 1. On the one hand, it is absolutely necessary for the current zero point to pass through each time after each polarity change after each individual demagnetization pulse 5 linearly and without singularities becomes. Secondly, a high degree of symmetry of the demagnetizing pulses 5 must be achieved.
Third, accurate and reproducible control of small demagnetizing pulse amplitudes 6 in the already severely decayed region of the demagnetizing curve 7 is very important. So it must be achieved a high current resolution. In the present invention, the alternating demagnetizing pulse amplitudes 6 are reproducibly controlled to an amplitude less than one-thousandth of the AC pulse amplitude maximum 3.

The requirements described for the shape of the demagnetization curve 7 are achieved by a current controller 24 described in more detail below.
An inverter 20 generates in the current controller 24 described herein, the low-frequency AC pulses 1 with the requirements described above. This inverter 20 is composed of transistors in a bridge circuit which operates with pulse width modulation. Today, IGBTs (Insulated Gate Bipolar Transistors) are used because they have a high blocking voltage and can switch high currents. For the internal circuit of the inverter 20 but also other circuit concepts (for example, with MOSFETs) are thinkable and executable.

The rectangular pulses generated in the inverter 20 of frequencies greater than 3 kHz are generated by transistors that show no holding current effect. The high fundamental frequency of the inverter 20, which is used for pulse width modulation, allows the control of AC pulses 1 from zero Hz (ie DC) to the power frequency (50Hz or 60 Hz) with high precision. The current controller 24 includes a current sensor 22, which can read the currently flowing current, even at low Demagnetisierimpulsamplituden 6, whereby a closed current control loop is controlled. The signals of the current sensor 22 are replaced by a Programming and readout unit 23 fed back into the inverter 20. Thus, the control of small Demagnetisierimpulsamplituden 6 is to achieve less than one-thousandth of the AC pulse amplitude maximum 3. The inverter internal circuit also ensures that the current zero is traversed absolutely linearly with each polarity change, which is essential for complete demagnetization.

The high inverter internal frequency is in terms of the immission limits, mainly with inductive load at the inverter, in a range in which a lower power loss occurs in the generation of the AC pulses 1, as at lower frequencies. This makes using an inverter 20 more effective than using other sources of power. The current control 24 and thus the demagnetization curve 7 to be traveled, the demagnetizing pulse amplitudes 6 and the alternating current pulse frequency 4 can be programmed via the programming and readout unit 23. This allows the parameters of the AC pulse 1 to be set by connecting a computer to the current controller 24 or by manual programming of the programming and reading unit 23.

For demagnetization, a flexible and fully insulated unshielded conductor of sufficient length in the form of a known stranded cable between an input 27 and an output 28 of the current controller 24 is connected. Care must be taken to ensure that the cable is designed for the maximum voltage of the demagnetization and the AC pulse amplitude maximum 3. Because of high currents and voltages, it is important that the conductor be securely attached to the current controller 24 and not accidentally come off. One possibility consists in a screw connection, wherein the plugs of the conductor, as well as the Connecting sockets (27, 28) of the current control 24 have threads.

To be sure that the conductor is correctly wired and an AC pulse 1 can flow, a conductor monitor 26 is used. This measures the ohmic resistance between input 27 and output 28 of the current controller 24 in the unloaded state, from which it can be seen whether the conductor is correctly connected to input 27 and output 28 of the current controller 24 and whether the cable is in order. Only when the conductor is connected correctly, ie an ohmic resistance is measurable, the AC pulse 1 can be triggered by the current controller 24.

After the conductor has been tested by the conductor monitor 26 and connected to the current controller 24, it is brought close to the object 30 to be demagnetized, so that the object 30 is positioned in the magnetic field resulting from the subsequent current flow. One possibility is to form the flexible conductor into a conductor loop 29 whose shape is variable. In order to be sure that the magnetic field penetrates the object 30, the conductor loop 29 can be laid around the object 30 in at least one loop. In advantageous embodiments of the conductor, this is chosen so long that it can be placed in several loops around the object 30 or formally wound. When forming multiple loops around the object 30 this forms the Leitersch leifenkern.

Also, a collection of objects 30 may be demagnetized in a demagnetization process, when the objects are filled, for example, in a bulk container, which is enclosed by the conductor loop 29 with an arbitrary number of windings.

During the flow of the alternating current pulse 1 through the conductor, an alternating magnetic field is formed, which leads to a random, arbitrary field line distribution due to the type of looping of the conductor. Due to the sometimes very high current flow, the conductor heats up. This effect can optionally be used for current flow monitoring 25. By the current flow monitor 25 can be determined whether the AC pulse 1 has actually flowed through the conductor. This current flow monitoring 25 is carried out with the aid of a resistance measuring device which reads out the instantaneous ohmic resistance of the conductor during the demagnetization process and forwards it to the programming and readout unit 23. Due to the high current amplitudes of more than 100 A during demagnetizing pulses 5, the temperature of the conductor increases, which results in an increased ohmic resistance. This current flow monitor 25 thus provides a measured value which indicates whether the current has flowed through the conductor. In addition, the temperature determination from the measured resistance value allows the protection of the conductor from excessive temperatures.

By using a capacitor-free circuit for the flow of AC pulses 1 through a flexible conductor loop, it is possible the additive field to add a defined, constant and small DC component additively superimposed. A direct current source 21 in the current controller 24 can add the DC component 9 already at the beginning of the AC pulse 1 or start up in the course of the decaying demagnetizing curve 7. Such a, the AC pulse 1 superimposed DC component 9 is used to compensate for the static earth magnetic field. In addition, by superimposing a DC component on the treated object 30, a desired magnetization can be impressed.

Especially with simple and cheap inverters 20, it is possible to reduce the demagnetization voltage resulting in the AC pulse and thus reduce the demagnetizing pulse amplitude. For this purpose, said inverters 20 have a function called controlled engine shutdown. The reproducibility of the demagnetization is not necessarily given when reducing the demagnetization.

LIST OF REFERENCE NUMBERS

1.

AC pulse

Second

AC pulse width (envelope to 0)

Third

AC pulse amplitude maximum

4th

AC pulse frequency

5th

Entmagnetisierimpuls

6th

Entmagnetisierimpulsamplitude

7th

Entmagnetisierkurve

9th

DC component

10th

Demagnetization curve with DC component ≠ 0

20th

inverter

21st

DC power source

22nd

current sensor

23rd

Programming and reading unit

24th

current control

25th

Current flow monitoring

26th

conductor monitoring

27th

Input of the current control

28th

Output of the current control

29th

Ladder speed

30th

object

Claims (12)

1. A method for the reproducible capacitorless demagnetisation of objects with residual magnetism by means of at least one alternating-current pulse (1), produced by a current control (24), with settable alternating-current pulse frequency (4) of variable amplitude and alternating-current pulse width (2) in a conductor which can be connected between an input (27) and an output (28) of the current control (24) in a capacitorless manner, as a result of which a magnetic field pulse is generated in the region of the conductor, characterised in that
   the conductor is flexible, completely insulated, unshielded and deformable and is laid with the formation of a conductor loop (29) in any desired shape around an object (30) to be demagnetised, whereupon
   the ends of the conductor loop (29) are connected to the input (27) and the output (28) of the current control (24) in a capacitorless manner
   and whereupon the alternating-current pulse (1) is fed in from individual, alternatingly poled and symmetrical demagnetising pulses (5) with controlled demagnetising pulse amplitude (6) and alternating-current pulse frequency (4) greater than 1 Hz, wherein the temporal progression of the demagnetising pulse amplitudes (6) is reproduced from a non-exponentially falling demagnetising curve (7), wherein the ratio of smallest demagnetising pulse amplitude (6) to the alternating-current pulse amplitude maximum (3) is at least 1:1000 and the conductor loop (29) is removed after the completion of the demagnetisation of an object (30).
2. The method according to Claim 1, characterised in that the conductor loop (29) is laid with any desired number of windings around the object (30) to be demagnetised with the formation of at least one loop, wherein the object (30) is used as a conductor loop core (30).
3. The method according to Claim 1, characterised in that the conductor loop (29) is laid with any desired number of windings around a bulk container filled with objects (30) to be demagnetised with the formation of at least one loop, as a result of which a plurality of objects (30) can be demagnetised in a demagnetising procedure.
4. The method according to any one of Claims 1 to 3, characterised in that the alternating-current pulse width (2) extends over at least 100 periods of the alternating-current pulse frequency (4), wherein the demagnetising pulse amplitudes (6) are reduced along the demagnetisation curve (7) to zero.
5. The method according to any one of the preceding claims, characterised in that a controlled constant direct-current component (9) is additively laid over the alternating-current pulse (1), which direct-current component is still present after passing through the demagnetisation curve (7) to a demagnetising pulse amplitude (6) of zero and as a result a magnetic field is imparted into the treated object (30).
6. The method according to any one of the preceding claims, characterised in that a conductor monitoring (26) upstream of the flow of the alternating-current pulse (1) measures and evaluates the ohmic resistance between input (27) and output (28) of the current control (24), whereupon the demagnetising procedure is only started in the case of a finite ohmic resistance.
7. The method according to any one of the preceding claims, characterised in that a current-flow monitoring (25) reads the ohmic resistance of the conductor during the passing through of the demagnetisation curve (7).
8. The method according to Claim 7, characterised in that the temperature of the conductor during the demagnetisation is determined from the result of the current-flow monitoring (25).
9. The method according to any one of the preceding claims, characterised in that the demagnetising pulse amplitudes (6) and thus the demagnetisation curve (7) proceeds by means of a reduction of the demagnetising voltage at the inverter (20) by means of a regulated motor shutdown.
10. A device for the reproducible capacitorless demagnetisation of objects with residual magnetism, comprising a programmable current control (24) with which alternating-current pulses (1) with settable alternating-current pulse frequency (4) of variable amplitude and alternating-current pulse width (2) can be generated, a conductor which can be connected between an input (27) and an output (28) of the current control (24), and for the application of the method according to at least one of the Claims 1 to 9, characterised in that the conductor is a flexible commercially available cable which is completely insulated, unshielded and can be deformed to form a conductor loop (29) of any desired shape and can be laid with any desired number of windings around an object (30) to be demagnetised.
11. The device according to Claim 10, characterised in that the programmable current control (24) has a current-flow monitoring (25) which reads the ohmic resistance of the conductor during the passing through of the demagnetisation curve (7).
12. The device according to Claim 11, characterised in that the temperature of the conductor during the demagnetisation can be determined from the result of the current-flow monitoring (25).

Images