EP1796113A1 - Resonant frequency automatic adjustment to demagnetize different objects in demagnetizing installations - Google Patents

Abstract

Either the admittance of the oscillation circuit charged with the object is measured by way of separate measurement coil with small measurement current. The frequency in the oscillation circuit for demagnetization impulse is set to the value evaluated. The resonant frequency of the oscillation circuit charged with the object is evaluated and controlled at time duration of 5 periods during increase of the demagnetization current. The frequency in the oscillation circuit is continuously tracked to the resonance point.

Description

The invention relates to a method for automatically adjusting the resonant frequency during demagnetization of different objects in demagnetizers according to the preamble of the independent claim.

With today's use of mechanical component materials and the wide use of sensitive electronic components and circuits, residual magnetism in articles is becoming an increasingly important problem. Particularly unfavorable is the accumulation of ferromagnetic particles at the edges of parts which have a residual magnetism. Such particles in the micrometric range can only be stripped off, wiped off, blown off or washed off when the residual magnetism has been eliminated. The residual magnetism present in articles becomes a central quality criterion for suppliers of parts made of steel or other, more or less pronounced ferromagnetic material, to the machine and automobile manufacturers. Modern production methods, such as electromagnetic conveying, clamping, holding, driving and the like, and also by material selection, especially in the mass production, the cost is reduced. However, one often deals with other risks such as residual magnetism.

A known method for demagnetizing objects uses an open magnetic circuit, for example with a bar magnet or a magnetic yoke, and with a coil, which is traversed by a constant alternating current. The magnetic circuit is applied to the object to be demagnetized and the alternating current is turned on. Then the magnetic circuit is slowly pulled away from the object by hand. In a device for this method size and weight of the magnetic circuit are limited. The demagnetization process is strongly influenced by ambient conditions. The demagnetization is incomplete and not perfectly reproducible.

In a further known method, after DE 3718936 A1 , is worked with a coil tunnel consisting of a large, constantly flowing by alternating current coil. The object to be demagnetized is pulled through the stationary magnetic field of the coil tunnel. As a result, the object is the magnetic field first increasingly, then exposed to decreasing. The demagnetizing effect is limited. The effect can be improved by appropriate alignment of the objects to be demagnetized, but overall is hardly perfectly reproducible. Due to the continuous operation of the coil, the consumption of electrical energy and the need for cooling are extremely high. Only a small part of the magnetic field is really exploited for the demagnetizing effect.

In another process, after EP 1465217 , the ferromagnetic objects as a whole are completely demagnetized by remaining locally fixed in the magnetic field of a coil during a certain time and thereby being exposed to an alternating field of decaying amplitude. In this case, the alternating field of the coil with respect to frequency and amplitude is generated variably by an electronic supply source. During the residence time of the objects in the coil, the alternating field is brought from a maximum value continuously decreasing to zero. The objects are now demagnetized so far that no residual magnetism is measurable. The demagnetization process takes place cyclically. This method has proven itself in applications with objects to be demagnetized always the same type and also provides the most complete demagnetization.

It is particularly advantageous to supplement the demagnetizing coil with a capacitor to form a series resonant circuit. The capacitor is connected in series with the coil, and for powering a pulse width modulated inverter of conventional design is used. This compensates for the inductive reactive power of the coil and relieves the supply source. However, this requires an operation in the state of resonance of the resonant circuit, that is, the feeding frequency must match the resonant frequency of the resonant circuit. This results in an additional problem in that, when the coil is subjected to different loads by objects to be demagnetized, its inductance, and therefore also the resonant frequency of the resonant circuit, changes.

If, as stated above, the resonance frequency is not exactly known in advance, the operation at the resonance point is not ensured, and the course of the demagnetizing current depends on the application of the coil to the objects to be demagnetized. Thus, the quality of the demagnetization process from batch to batch is different and not exactly manageable.

One method that overcomes this disadvantage is in DE 30 05 927 A1 described. The frequency of the supply voltage of the coil at maximum frequency amplitude of a starting value is slowly shifted over the entire range of possible, but unknown from batch to batch resonance frequency, and then the voltage in the known manner with steadily decreasing amplitude reduced (Figure 1). A disadvantage of this Method is that for the approach to the resonance frequency a lot of time and thus energy is needed. The control of the coil voltage also has the disadvantage that the corresponding current depends on the ohmic resistance of the coil and thus on the temperature of the coil. Therefore, this method also does not guarantee exactly the same effect of the demagnetizing magnetic field from batch to batch.

All of these described methods assume that the actual resonance frequency of the resonant circuit loaded with an object is not known. The frequency of the voltage source is either fixed or determined by a fixed time program. With the method with the fixed setting, there is a risk that a considerable deviation is accepted and that it does not start with the maximum current. In the programmed frequency response, one goes through the resonant frequency of the loaded system in any case, but requires a lot of time. The method is therefore less efficient in energy consumption and leads to unnecessary heating of the coil.

The object of the invention is therefore to find a method which does not have the above disadvantages.

The object is achieved by automatically and accurately determining the resonance frequency of the resonant circuit loaded with arbitrary objects with the demagnetizing coil itself in a very short time, so that a demagnetization process, for example according to EP 1465217 , can begin immediately with the exact resonance frequency. This also ensures that the actual current value precisely follows the specified current setpoint throughout the demagnetization process.

The advantage of the invention is that no unnecessary time is required to travel through the resonance frequency for the beginning of the demagnetization process. The throughput of the demagnetization system is thus considerably increased. For the demagnetization of a batch or an object no lead time is wasted, and the process begins immediately after loading the resonant circuit with the correct resonant frequency. Thus, a clocked demagnetization of different batches in terms of mass, material and configuration can be performed with correspondingly different resonant frequencies without undue delay and energy consumption.

Fig. 1

Durchfahren des Resonanzpunktes mit der Frequenz der Speisespannung gemäss Stand der Technik;

Fig. 2a

Hilfsspule zum Messen und Einstellen der Resonanzfrequenz;

Fig. 2b

Messung der aktuellen Induktivität der Entmagnetisierspute mit einer Hilfsspule;

Fig. 3a

den Frequenzregelkreis des Entmagnetisierungs-Schwingkreises;

Fig. 3b

Abhängigkeit von Admittanz und Phasenwinkel in der Umgebung des Resonanzpunktes;

Fig. 3c

Phasenlage von Spannung und Strom am Ausgang des Inverters;
und

Fig. 3d

zeigt den Stromverlauf mit Einschwingen des Frequenzreglers.

The invention will be discussed in connection with the figures. Show it:

Fig. 1

Traversing the resonance point with the frequency of the supply voltage according to the prior art;

Fig. 2a

Auxiliary coil for measuring and adjusting the resonance frequency;

Fig. 2b

Measurement of the current inductance of the demagnetizing spider with an auxiliary coil;

Fig. 3a

the frequency locked loop of the degaussing oscillating circuit;

Fig. 3b

Dependence on admittance and phase angle in the vicinity of the resonance point;

Fig. 3c

Phasing of voltage and current at the output of the inverter;
and

Fig. 3d

shows the current curve with settling of the frequency controller.

FIG. 1 shows the course of the current I during the time t, according to the method according to FIG DE 30 05 927 A1 , The current profile I is compared to the optimal current profile I-soll. In a first period of time 11, the demagnetization takes place in an uncontrolled manner with the passage of the resonance point at any point 12. A controlled demagnetization takes place only in the range of a second period of time with regulated current. The drop of the curve D is regulated only in section 12 and so also reproducible. The frequency is reduced from an increased output frequency fa to a demagnetizing frequency fm. In doing so, the resonant frequency fr is reached at any point in between.

An alternative route according to FIG. 2a operates with an auxiliary coil of small cross-section, which determines the admittance of the auxiliary coil 21, and thus the inductance of the demagnetizing coil 20 and, after further conversion, the corresponding resonant frequency. This does not generate additional power dissipation in the coil and can be solved with reasonable effort on electronics. However, the auxiliary coil 21 must be isolated from the degaussing coil 20 for the highest occurring voltage in the demagnetizing cycle, and the detected resonance frequency is inaccurate in that it is determined at very low currents where the. Permeability of the material to be demagnetized at most is still less effective. For this purpose, a test is carried out with the loaded coil before each demagnetization. By means of the auxiliary coil 21, the resonant frequency is determined with a small test current Up. Subsequently is started with the maximum demagnetizing current to resonant frequency of the demagnetization with controlled demagnetization current U.

The preferred way according to the invention (FIGS. 3a, 3b, 3c, 3d) operates with a control loop which automatically keeps the output frequency of the inverter at the resonance point of the resonant circuit. It is based on the measurement of the phase position of voltage and current at the output of the inverter. If the injected frequency is higher than the resonant frequency, the resonant circuit behaves inductively, i. the current follows the voltage behind, and the difference in phase of current and voltage is negative. If the injected frequency is lower than the resonance frequency, the resonant circuit behaves capacitively and the current leads the voltage. The corresponding phase angle is positive. The sampling of the phase angle takes place in the inverter itself, by the zero crossings and their direction of current and voltage are determined. The two signals are in the inverter anyway for the current loop, i. to control the power level, needed. They are therefore available without extra effort. A frequency tracking due to the phase angle requires only an additional subprogram, which is solved purely by software.

In the figure 3a, the control circuit 30 of the supply of the demagnetizing coil L is shown. The resonant circuit consists of a capacitor C, a resistor R and the applied coil L. With the measuring points for current 31 and voltage 32 is detected at both the zero crossing of the current 37 and the zero crossing of the voltage 38. From the time difference of these zero crossings 37 and 38, the phase angle 33 can be determined. Then a corresponding correction signal 34 is issued to the frequency generator 35. The frequency generator 35 now controls the inverter 36 at the corrected frequency.

FIG. 3b shows the dependence on admittance and phase angle when the coil is loaded. The frequency f is located on the x-axis and the admittance Ad and the phase angle Pw are located on the y-axis. At the resonant frequency fr of the charged coil, the phase angle between voltage and current at the feed point of the resonant circuit passes through the zero point. This is the resonance point. Only when the coil is energized with this resonance frequency is a full and reproducible demagnetization possible.

FIG. 3c shows the phase position of voltage U and current I at the output of the inverter. Δt is the time difference between the two zero crossings of voltage and current. Now regulate the frequency of the inverter to Δt = 0, or just the phase angle to Φ = 0.

From Figure 3d, the current waveform with the leading transient phase .DELTA.t-e of the frequency controller at reduced current I-red can be seen. It can be clearly seen that the resonant frequency of the charged coil is reached during the build-up of the current immediately before the actual demagnetization process. This happens within a time of about 5 to 10 periods. The current rises to the maximum setpoint I-max in order to EP 1465217 ) down the demagnetization curve.

However, the method can also be used with a coil (tunnel demagnetizer) which is acted upon by continuous current, in that the frequency during the passage of the material through the coil is continuously kept at the resonance point and tracked. For this purpose, the coil of the tunnel demagnetizer is supplied with continuous current, wherein the frequency is automatically held and tracked during the passage of the material through the coil in the resonance point.

Claims (4)

A method for adjusting the resonant frequency for demagnetizing objects in the region of a coil, wherein an object is within an alternating field during a residence time of a certain duration, and wherein the coil is part of a resonant circuit, which is energized by an inverter, and wherein demagnetizing the current is controlled, and wherein the resonant circuit is brought from an output frequency to the resonant frequency of the resonant circuit loaded with the object, before the current of the resonant circuit is reduced in accordance with a demagnetizing function of a target current to a final current,
characterized,
that
either the admittance of the resonant circuit loaded with the object is measured by means of a separate measuring coil with a small measuring current, after which the frequency in the resonant circuit for the demagnetizing pulse is set to the value determined therefrom
or
the resonant frequency of the resonant circuit loaded with the object is determined and regulated during a period of at least 5 periods during the startup of the demagnetizing current, after which the frequency in the resonant circuit is continuously tracked to the resonance point. A method according to claim 1, characterized in that the zero crossings of voltage (32) and current (31) of the resonant circuit are detected and from the time difference of these zero crossings of the phase angle (33) is determined, whereupon a corresponding correction signal (34) to a frequency generator ( 35), which sets the frequency of the current controlled by the inverter current of the resonant circuit to the resonant frequency. A method according to claim 1, characterized in that first a test current is applied to the auxiliary coil, whereupon the resonant frequency of the loaded resonant circuit is determined, after which the test current of the auxiliary coil is turned off and the supply of the resonant circuit is started at resonant frequency. A method according to claim 1, characterized in that the coil is a tunnel demagnetizer and is supplied with continuous current, wherein the frequency is automatically held and tracked during the passage of the material through the coil in the resonance point, by the zero crossings of voltage (32) and current (31) of the resonant circuit are detected and from the time difference of these zero crossings of the phase angle (33) is determined, whereupon a corresponding correction signal (34) to a frequency generator (35) is given, which is the frequency of the current controlled by the inverter of the resonant circuit to the resonant frequency ..

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