JPH08172014A - Method of demagnetizing material - Google Patents

Abstract

PURPOSE: To demagnetize a magnetic material thoroughly regardless of the size and shape of an object by applying a field of first polarity to a work piece for a first time interval and then applying a field for a shorter time interval while reversing the polarity continuously. CONSTITUTION: A demagnetization coil 3 is connected with a DC current polarity reversal unit 4 operable under the control of a control computer 5. A DC current is fed from a current source 6 to the unit 4 A field of first polarity is applied for a first time interval to a work piece and then the polarity is reversed continuously before a field is applied for a shorter time period to the work piece. In other words, current is fed reversely for a short time interval of pT (p is a coefficient smaller than 1) after first magnetization thus reversing the field. Subsequently, the field is reversed again and applied for a time interval p<2> T, p<3> T,..., for example, until p<n> T becomes shorter than 1 sec.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to the degaussing of materials, especially ferromagnetic materials having a relatively thick cross section. The method is not limited by the size and shape of the object and is effective for all magnetic materials regardless of their magnetized state and orientation.

[0002]

2. Description of the Related Art Conventionally, the most general method for demagnetizing a magnetic body has been to apply an alternating magnetic force (MMF) having a reduced amplitude. Usually, a coil in which alternating current (generally drawn from the main power line of 50 or 60 Hz) flows is used, and the current is gradually reduced to almost zero, or the intensity of magnetic flux induced from the coil is sufficiently reduced. Either the object and the coil are pulled apart by increasing the distance between them. While this conventional method is satisfactory for small objects and thin materials, it only partially demagnetizes large objects and thick materials. In detail,
This technique results in an apparent demagnetization with little or no magnetic field from the object. However,
Degaussing is not global and only affects a limited depth. This shunts the internal magnetic field and apparently eliminates magnetism when measured externally,
Often, an enclosed area near the surface of the object is provided. However, when an object is welded or cut to remove edges, such trapped internal magnetic fields are generally exposed.

Other prior art techniques generate magnetic fields that are equal but opposite in direction, with almost no apparent magnetic field outside.
Or it intentionally magnetizes the object so that it disappears altogether. This method is satisfactory especially for simple-shaped objects such as simple tubes, and when existing magnetism is uniform. However, if the object is irregular in shape, is very thick, has a varying cross-section, or is non-uniform or irregular, as is commonly the case for surfaces with thick magnetization. In case, it is not easy to apply. In these situations, the most satisfactory approach is to completely demagnetize the material involved.

A demagnetizing field is provided to further develop the alternative method first mentioned above and to obtain a frequency lower than that which is expediently obtained from the power line or from an electronic oscillator operating at lower frequencies such as below 10 Hz. It is obtained with a coil that carries a unidirectional current that is periodically reversed. This method is more satisfactory than the one obtained at the line frequency in that it can demagnetize a thicker cross section. However,
The same restrictions exist regarding the shape of the objects involved. Therefore, for an extremely thick or irregular cross section, apparent external demagnetization can be performed due to the surface effect or the near surface effect that masks the internal magnetization.

In connection with welding with electric arcs or electron beams, internal magnetisation results in unacceptable deflection. This is the Curie point (in the case of some ferromagnets, especially if the components to be joined melt at the mating surface).
It is generated by heating above about 700 ° C.), disturbs the local surface magnetic field distribution, and exposes the internal magnetism in the deep region.

[0006]

According to one aspect of the invention, a method of at least partially degaussing a workpiece includes at least applying a magnetic field of a first polarity to the workpiece for a first period of time. , Then consisting of a first phase in which the polarity is continuously reversed and a magnetic field of shorter duration is applied.

Therefore, not only the surface or near surface regions, but also the magnetized material is desired, especially when the magnetized material is of a considerable thickness above 50 mm or even above 150 mm. Use the degaussing method to degauss to the depth. The method produces an electromagnetic field capable of completely penetrating all areas of the workpiece in a manner that reduces the time required for degaussing over conventional methods.

Therefore, first, a unidirectional magnetic field is applied from a suitable electromagnetic coil surrounding the object for a considerable time T, for example 100 seconds. This time is 1 depending on the thickness
It can be 000 seconds or more. Single solenoid
A coil may be used, or one or more flat coils may be used depending on the shape of the object and its overall size. For example, if two circular flat coils of average diameter D are set about 0.5D apart, the axial magnetic field between the coils in free space will have the same total excitation, diameter D, and total length D. Equal L
No more than 10% less than the axial magnetic field of one solenoid coil. Thus, the object can be conveniently placed between the two coils, or within an equivalent solenoid coil if it is smaller than the inner diameter.

After the first magnetization, the current is reversed and this is changed to p
Short time such as T (where p is any coefficient less than 1,
The magnetic field is reversed by applying for a suitable number, eg between 0.5 and 0.99). The time reduction rate is determined by the magnetic properties of the material, but for most ferromagnetic materials, p = 0.9 can be arbitrarily set. The magnetic field is reversed again and applied for times p ² T, p ³ T etc. until p ⁿ T is eg less than 1 second.

At this point, the degaussing cycle must be completed satisfactorily during the second stage. There are three possible strategies that can be used alone or in combination. Conventionally, the components and the degaussing coil are gradually separated to reduce the magnetization induced in the components each time the magnetic field is reversed. This technique is well known. Secondly, the time frame can be further shortened by the coefficient p as before. The initial rate of change of the current I _(t) of the degaussing coil having the inductance L is defined as p ⁿ T

[Equation 2] Therefore, when related to the power supply voltage V, I _(t) does not reach the normal value I _M. The peak current reaches a negligible value, ie I _M / C (where C is 100), as p ⁿ T becomes smaller and smaller. at this point,
The degauss cycle is complete. This can be expressed as the following equation.

(Equation 3) Although the inductance L changes depending on the component to be demagnetized, it can be appropriately assumed to be the minimum value (that is, when there is no component to be demagnetized) without significantly affecting the result.

In the third approach, it is substantially zero or less than, for example, less than 1% of the initial maximum magnetic field current I _M , as is usual in demagnetizing small or thin components. Until, the amplitude of the maximum magnetic field current I _t is reduced in a substantially constant period (frequency). The reduction factor may be a nominal constant reduction of the amplitude i, or a nominally constant ratio q, where q ⁿ is, for example, 0.01 in the next cycle of the nominally constant period (frequency). The peak current is set to qI _M , q ² I _M, etc. until the value becomes less than 1.

[0012]

Some examples of methods and apparatus according to the present invention will be apparent from the following description and figures.

FIG. 1 shows a simple linear solenoid 2 having a substantially rectangular shape.
3 shows the distribution of magnetic field strength along the centerline 1 of the. In this example, the diameter (D) is 5 m and the length (L) is 4.5 m.
That is 0.9D. 222 turns and 800A
, The magnetic field strength along the centerline 1 is about 17 kA / m at the end of the solenoid and increases to about 27 kA / m at the center. In this case, excitation is 40
The length is about kA / m. This excitation and field strength in free space is sufficient to magnetize the ferromagnetic material located in the solenoid space to about 50% of the saturation field strength.

However, in general, the existence of the surrounding space leads to self-demagnetization, and the residual magnetic field becomes about half or less of the saturation magnetic field strength, so that the material of the simple block to which a strong magnetic field is applied is completely saturated. It should be noted that this is not possible.

FIG. 2 shows the corresponding field strengths (curves i, i) at the central axes of two short coils, nominally 5 m in diameter and spaced approximately 4 m, 3 m, 2 1/2 m, and 2 m, respectively.
i, iii and iv). In this example, the magnetic field strengths are nominally 111 turns each and are calculated for a coil carrying 800A of current. It can be seen that the magnetic field strength becomes almost uniform along the center line when the distance is about the coil diameter, and the amount is about 23 kA / m. For objects of more complex shape than would be appropriate if placed either in the solenoid or between the two short coils, other similar coils could be used with the axis perpendicular to the axis of the solenoid or pair of flat coils described above, Alternatively, it can be used in an appropriate direction. According to the invention, the degaussing field is applied for a long time frame so that it penetrates sufficiently deep into the thick material, and then for a short time. This effect is shown in Figure 3.
, The magnetic field generated deep in the material is a function of both the distance (X) and the period (t) in which the magnetic field is applied, for the same magnetic field strength B ₀ . With sufficient time, it is possible to magnetize any depth to a magnetic field strength close to that applied. If the applied magnetic field of the same constant value B ₀ is subsequently reversed, the period is shortened, preferably nominally at a constant rate (p), and the subsequent times t ₂ , t ₃ , t ₄ , t ₅ are pt, p ^2.
It corresponds to t, p ³ t, p ⁴ t, etc. As shown, at a known depth X, this results in a reversing magnetic field of decreasing strength until the reversing magnetic field strength at distance X decreases to a low value in a sufficiently short time.

During the first stage, the applied excitation current is initially constant (apart from the rise and fall, which is mostly controlled by the associated coil self-inductance), and has a reduced time as shown in FIG. Is applied for. The initial time is usually over 100 seconds or, in the case of large structures, over 1000 seconds. The minimum time frame is mostly the EMF of the applied power source that controls the self-inductance of the coil and the maximum rate of rise of the current in the coil.
Determined by Upon completion of the first stage, reversal of polarity keeps the frequency constant during the second stage, as shown,
Continued with reduced amplitude. Second for large coils
Set a maximum time of about 1 second at the beginning of the stage, then
It is advantageous to reduce the current in the coil either nominally to, for example, only 1% of the initial current, either by a controlled increase or a controlled function. This sequence of time controlled decrease and amplitude controlled decrease is shown in FIG.

During the first stage, the proportion of the period between successive degaussing currents, which does not appear to change the magnetic properties, is (1
-LogK) ² (where K is the desired reduction in magnetic field strength). Usually, the ratio to a value of K of nominally 0.95 to 0.7 ranges from 1.1 to about 2. Practically, this range is 1.2 to 1.
The range of 3 is preferable.

0.9 corresponding to a nominal 1.25 time ratio
A typical order of periods for K is shown in the table of FIG. The degaussing cycle pulse time (t _n ) is given by:

[Equation 4]

The total time T of the main cycle is given by the following equation.

(Equation 5)

The time to complete the cycle is about 5 seconds. The total time of the degaussing cycle (excluding the switching time) is 4.6 minutes.

It should be noted that this saves a considerable amount of time compared to using constant low frequencies or long-term cycles. In this example, any 12
The total time for a step is 5 times the longest period involved, ignoring the switching time required to reverse the magnetic field current.
It is about double. This savings over the whole period becomes more pronounced the longer the initial period. Therefore, for very thick materials that require an initial magnetization time of, for example, 1000 seconds, the total degaussing time will be only a few hours instead of one or several days.

A typical control sequence is shown in FIG. 6, which, together with the programmable power supply (FIG. 7), provides the required degaussing system for the provided coil. As shown in FIG.
The degaussing coil 3 is connected to a DC current polarity reversing unit 4 whose operation is controlled by the control computer 5. DC current is supplied from the current source 6 to the unit 4.
The solenoid coil described in FIG. 1 or 2 is 0.1
It has an inherent inductance of about 5H and 600V
When it is supplied, it has a rise time of about 0.2 seconds. The three-phase thyristor bridge can be used in the unit 4 only as an on / off switch for a long time frame, for example up to 10 seconds. After that, it is preferable to switch the proper phase sequence for an operating time of 1 second or less. The phase amplitude in thyristor control can then reduce the amplitude of the current, for example as is well known in the field of resistance welding.

The control computer 5 performs the steps shown in FIG. First, the computer 5 is provided with the maximum current value ( _IM ) defined above, the initial penetration thickness (δ), the constant (k) relating to the material to be demagnetized, and the initial value (K) (steps 30-33). . The computer 5 then calculates in step 34 the value of the initial period (t ₀ ) according to the following equation: t ₀ = δ ² / k ²

The operator then starts the process, step 35, which is followed by the computer 5 instructing the unit 4 to flow a current with a maximum value I _M from the current source 6 for a calculated time t _0. (Step 3
6). After the time t ₀ has elapsed, the computer 5 reverses the polarity of the current supplied to the coil 4 in the unit 4 (step 37) and calculates a new period t _{n + 1} according to the following equation:

(Equation 6) However, initially n = 0.

In step 39, the computer 5 compares the latest period t _{n + 1} calculated in step 38 with a preset minimum time period t _min , and the calculated period is t
If not shorter than _min , the process returns to step 36.

If the time period has been reduced to less than t _min (which can be set to 0.1 seconds, for example), the process proceeds to step 41.
Proceed to and start the second stage (shown in FIG. 4). In step 41, the initial current I ₀ is in the coil 3 for the period t _min.
, Then the polarity of the current is reversed by the unit 4 in step 42. The computer 5 calculates a new value of current I _{M + 1} according to the following equation: I _{M + 1} = KI _M (step 43) However, initially M = 0.

The calculated latest value I _{M + 1} is calculated in step 44.
At a minimum current value I _min (usually 2% of I ₀ ) and if the calculated value is less than I _min , the second stage continues and the process returns to step 41. Otherwise,
The degaussing cycle is complete and the operator is requested to indicate if the cycle should be repeated (step 4).
5). If so, the process returns to step 35; otherwise, it proceeds to step 46 and the operator indicates whether new parameters are needed. If parameters are needed, the process returns to step 30,
Otherwise, it ends.

In some cases, especially for large structures, it may not be necessary or practical to degauss the entire structure involved. For example, in FIG.
As shown in FIG.
If the coil is designed to be degaussed, it is possible to degauss only the boss by placing the coil 12 around the boss, as described above. Of course, there is a small amount of self-magnetization due to the magnetic fields present in other parts of the structure, but in general this is an unacceptable local magnetisation, especially when deeply degaussing objects such as bosses, and deeply. It never happens. Alternatively, to further reduce the degree of degaussing, the long structure is shunted with another magnetic body that causes the intrinsic magnetic field to be bypassed with respect to the boss or component to be degaussed, and, if desired, within the shunt. Other magnetic field coils can be used that cancel the residual magnetization of some of the structures.

However, as noted above, component 1
According to the invention with a group of coils 13 covering each of the main parts of the whole 4 as shown in FIG. 9, it is possible to degauss complex shaped objects to a satisfactory extent. In some cases, the coils can all be excited together, but in other cases, the coils can be energized in pairs or sequentially to provide the required degree of flux and flux reversal in the separated portions of the structure. Preferably.

In the case of a structure that creates a closed circuit for a magnetic field,
It is often sufficient to apply it to coil 15 to induce a magnetic field in the closed circuit as shown in FIG. Such coils have traditionally been split into mating halves for easy coupling, rather than being individually wound for each such structure. Of course, in this case, there is no self-magnetization effect, which induces a much higher magnetic flux density and saturates the material at the beginning of the degaussing cycle according to the invention,
Thereafter, it is generally preferable to reduce the time interval and thus the magnetic flux level as described above.

In an alternative configuration to that shown for closed circuits (FIG. 10), a flexible or laminated degaussing mechanism is shown in FIG.
The solenoid coil 16 is installed as shown in FIG.
Excites the arc-shaped or laminated magnetic yoke 17. This laminated yoke 17 can be terminated with suitable pole pieces 18 to match the dimensions of the object 19 to be degaussed, and therefore
Objects 19 larger than the dimensions of the coil 16 can be treated according to the present invention.

In the case of concave or irregularly shaped objects, it is preferable to improve the match between the magnetic properties of the component and the surrounding or integral space. For this reason, a finely divided ferromagnetic material having the same general characteristics as the parent object is appropriately suspended in a flexible container or bag 20 to satisfy such discontinuity, and as shown in FIG. Such a magnetic path can be provided. Alternatively, a machined component 21 of the required dimensions can be inserted into such a discontinuity to minimize air gaps and associated spaces of low or unit permeability (Fig. 12b). .

Other variations of the degaussing technique (not shown) can be realized when using the variation of the molded laminated yoke of FIG.

In this variant, the yoke contains magnetic field sensing elements, such as Hall effect devices, search coils, etc., for measuring the magnetisation of the yoke. The degaussing coil itself is connected to both AC and DC power supplies, which can be used alone or in combination. This device can be used in many modes. In the first mode, it can be used to determine the presence of a weak or remnant magnetic field in the component or part thereof.
First, the yoke is attached to the workpiece and a small AC signal is applied to the degaussing coil. If the net mean magnetization measured at the yoke is significantly increased, this is an indication of the presence of the weak or residual magnetic field originally present in the workpiece. The source of this effect becomes clear by examining FIG. The first point α represents the magnetic state of the area between the ends of the yoke. When a small AC current is applied, this produces a small AC magnetic field of ± ΔH at the workpiece. This causes the trajectory of the magnetic state of the material to move rapidly into a small hysteresis loop centered on A '(FIG. 13B) due to the shape of the initial magnetisation curve (FIG. 13A). The shape of the initial magnetization curve shown in FIG. 13A is typical of many ferromagnetic steels. It can be seen that there is only one point in the initial magnetization curve where there is no net increase in the average magnetic field when a small ± ΔH magnetic field is applied. This is the case when the initial magnetic state A is at point B = H = 0. It can also be seen that the increase in the B magnetic field measured by the magnetic field sensing element is indicative of the degree of remanent magnetization that was initially present.

In the second mode of operation, a DC current is passed through the magnetizing coil, producing a magnetic field opposite to the residual magnetic field. The required amount is estimated by considering the results of the tests performed in Mode 1. The current is applied for a sufficient time to allow the magnetic field to penetrate to the desired depth of the material. Once the current has been reduced to zero, another test in Mode 1 is performed to evaluate the residual magnetic field remaining and the process repeated if necessary.

In the third mode of operation, AC and D
The C-currents are simultaneously passed in a controlled, automated sequence, leaving the component or part of it measurably demagnetized.

Using a device of this type: i) measuring the residual magnetic field locally present in the component and controlling the effective depth of the measurement by controlling the frequency of the applied AC current, ii. ) Degaussing the magnetic field again by controlling the applied time to degauss the component either wholly or locally, and iii)
Both measurement and degaussing can be combined in an automatically controlled cycle.

[Brief description of drawings]

FIG. 1 is a diagram of the axial magnetic field strength distribution of a simple approximately square solenoid coil (also shown).

FIG. 2 is a diagram of an axial magnetic field strength distribution of two flat coils with different intervals.

FIG. 3 is a diagram showing the penetration of a magnetic field into a ferromagnet during different periods.

FIG. 4 is a diagram showing a typical sequence of an applied magnetic field (coil current) according to the present invention.

FIG. 5 is a table of a typical sequence of time periods.

FIG. 6 is a diagram of suitable computer flow control.

FIG. 7 is a diagram of an overall system according to an example of the present invention.

FIG. 8 is a diagram showing a configuration of a degaussing portion of the expanded structure.

FIG. 9 is a diagram showing a configuration for demagnetizing a structure having a complicated shape.

FIG. 10 is a diagram showing a configuration for demagnetizing a ring-shaped or box-shaped structure.

FIG. 11 is a view in which an assembled flexible laminated yoke is used for a soft magnetic body.

FIG. 12 is a view of using a magnetic bridge piece to more evenly distribute a magnetic flux.

FIG. 13 is a diagram showing a source of an increase in apparent B magnetic field when an AC magnetic field is applied.

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[Procedure amendment]

[Submission date] July 13, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

FIG.

[Fig. 2]

[Figure 3]

[Figure 5]

[Figure 4]

[Figure 7]

[Figure 8]

[Figure 9]

[Figure 10]

FIG. 11

[Figure 6]

[Fig. 12]

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[Procedure amendment]

[Submission date] October 19, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Figure 5

[Correction method] Change

[Correction content]

FIG. 5 is a chart of an exemplary sequence of time periods.

Claims (11)

[Claims] 1. A first step of applying a magnetic field of at least a first polarity to a workpiece for a first period of time, and then applying a magnetic field of continuously reversed polarity and of shorter duration. A method of at least partially degaussing a work piece. 2. The length of each period is p ⁿ T, where n is the number of the period, p is a coefficient less than 1, and T is the length of the first period. The method according to Item 1. 3. Method according to claim 2, characterized in that p is in the range 0.5-0.99. 4. The method according to claim 4, further comprising a second step of applying a magnetic field following the first step, wherein the polarity is continuously reversed at a constant frequency, but the amplitude is continuously reduced. Item 4. The method according to any one of items 1 to 3. 5. Method according to claim 4, characterized in that the amplitude of the current of the m + 1st cycle of the second stage is given by I _{m + 1} = KI _m , where K is a constant. . 6. The method according to claim 4, characterized in that the minimum current during the second stage is set to 2% of the maximum current during the second stage. 7. The method according to claim 1, wherein the magnetic field is applied by an electric coil. 8. The method according to claim 1, wherein the first time period is at least 100 seconds. 9. The method of claim 8, wherein the first time period is at least 1000 seconds. 10. Each period (t _{n + 1} ) in the first stage is expressed by Method according to any one of claims 1 to 9, characterized in that it is provided by: 11. The method according to claim 1, wherein the shortest period during the first stage is approximately 0.1 seconds.