The invention relates to a method for the automatic setting of the resonant frequency on demagnetizing different objects in demagnetization installations; according to the introductory part of the independent patent claim.
Residual magnetism in objects is becoming an increasing problem with today's use of materials for mechanical components, and the widespread use of sensitive electronic components and circuits. The collection of ferromagnetic particles on the edges of parts which have a residual magnetism is particularly unfavorable. Such particles in the micrometer range may only be brushed off, wiped off, blown off or washed off when the residual magnetism is eliminated. The residual magnetism present in objects has become a central quality criterion for suppliers of parts of steel or other material which is ferromagnetic to a lesser or greater extent, to machinery and car manufacturers. In particular, with mass-production of parts, the costs are reduced by way of modern manufacturing methods, such as for example electromechanical conveying, clamping, chucking, driving and likewise, and also on account of the material selection. The other side of the coin however is that other risks occur due to this, such as residual magnetism.
One known method for demagnetizing objects uses an open magnetic circuit, for example with a bar magnet or a magnetic yoke, and with a coil through which a constant alternating current flows. The magnetic circuit is applied to the object to be demagnetized and the alternating current is switched on. Thereupon, the magnetic circuit is pulled away from the object slowly by hand. The size and weight of the magnetic circuit is limited with this device. The demagnetization procedure is greatly influenced by environmental conditions. The demagnetization is incomplete and may not be perfectly reproducible.
With a further known method, according to DE 3718936 A1, one operates with a coil tunnel consisting of a large coil, through which alternating current constantly flows. The object to be demagnetized is pulled through the stationary magnetic field of the coil tunnel. By way of this, the object is subjected to the magnetic field firstly in an increasing manner and then in a decreasing manner. The demagnetization effect is limited however. The effect may be improved by a matching alignment of the objects to be demagnetized, but as a whole may hardly be reproduced in a perfect manner. The consumption of electrical energy and the cooling requirement are extremely high due to the continuous operation of the coil. Only a small part of the magnetic field is really utilized for the demagnetization effect.
In a further method, according to EP 1465217, the ferromagnetic objects as a whole are completely demagnetized in that they remain spatially fixed in the magnetic field of a coil for a certain time, and thereby are subjected to an alternating field of a decaying amplitude. Thereby, the alternating field of the coil is produced in a variable manner with regard to frequency and amplitude by an electronic supply source. During the time of presence of the objects in the coil, the alternating field is brought to zero from a maximum value in a continuously decreasing manner. The objects are then demagnetized to such an extent that residual magnetism may no longer be measured. The course of the demagnetization takes place in a cycled manner. This method has always proven its worth with objects to be demagnetized which are always of the same type, and also provides for the complete demagnetization.
It is particularly advantageous to supplement the demagnetization coil with a capacitor into a series oscillation circuit. For this, the capacitor is connected in series with the coil and a pulse-width-modulated inverter of the usual construction type is applied for the supply. With this, the inductive wattless power of the coil is compensated, and the supply source is relieved. This however presupposes an operation in the condition of the resonance of the oscillation circuit, which means that the frequency of the supply must correspond to the resonant frequency of the oscillation circuit. An additional problem arises from this, which is the fact that with a different loading of the coil due to the objects to be demagnetized, its inductance and thus also the resonant frequency of the oscillation circuit also changes.
If, as previously explained, the resonant frequency is not known exactly from the beginning, the operation at the point of resonance is not ensured, and the course of the demagnetizing current is dependent on the exposure of the coil to the objects to be demagnetized. The quality of the demagnetization procedure is therefore different from charge to charge and may not be controlled in an exact manner.
A method which avoids this disadvantage is described in DE 30 05 927 A1. Thereby, the frequency of the supply voltage of the coil at maximum frequency amplitude is shifted slowly from a starting value over the whole range of the possible resonant frequency which is unknown from charge to charge, and subsequently the voltage is reduced with a constantly reducing amplitude (
All these described methods assume that the actual resonant frequency of the oscillation circuit charged with an object is not known. The frequency of the voltage source is either set in a fixed manner or is determined by a time program which is determined in a fixed manner. With the method of the fixed setting exists the danger that one accepts a considerable divergence and does not begin with the maximal current. In the programmed frequency course, at all events one runs through resonant frequency of the charged system, but one requires much time. For this reason, the method is less efficient in energy consumption and leads to an excess heating of the coil.
It is therefore the object of the invention to find a method which does not have the above disadvantages.
The object is achieved in that the resonant frequency of the oscillation circuit with the demagnetization coil charged with any objects, may itself be determined in the shortest of times in an automatic and exact manner, so that a demagnetization procedure, for example according to EP 1465217, may begin directly with the exact resonant frequency. With this, one also ensures that the actual current values precisely follow the set current nominal value over the complete demagnetization process.
The advantage of the invention lies in the fact that no unnecessary time is required for passing through the resonant frequency for the beginning of the demagnetization procedure. The throughput of the demagnetization installation is significantly increased by way of this. Leadtime is no longer wasted for the demagnetization of a charge or of an object, and the procedure begins directly after the charging of the oscillation circuit with the correct resonant frequency. Thus a cycled demagnetization of different charges with respect to mass, material and configuration with correspondingly different resonant frequencies may be carried out without unnecessary delay and corresponding energy consumption.
The invention will be dealt with in the context of the drawings. There are shown in:
a the auxiliary coil for the measurement and setting of the resonant frequency;
b the measurement of the actual inductance of the demagnetization coil with an auxiliary coil;
a the frequency control loop of the demagnetization oscillation circuit;
b the dependence on the admittance and phase angle in the vicinity of the resonance point;
c phase position of voltage and current at the exit of the inverter; and
d shows the course of the current with self-oscillation of the frequency controller.
The course of the current I during the time t is represented in
An alternative procedure according to
The preferred manner according to the invention (
The control loop 30 of the supply of the demagnetisation coil L is represented in
The dependence on admittance and phase angle with a charged coil is evident from
The phase position of the voltage U and the current I at the exit of the inverter is evident from
The course of the current with the leading transient oscillation phase Δt−e of the frequency controller at a reduced current I-red is evident from
The method may however also be applied with a coil subjected to continuous current (tunnel-demagnetizer) by way of constantly retaining the frequency in the resonance point and tracking it, during the passage of the material through the coil. For this, the coil of the tunnel-demagnetizer is fed with continuous current, wherein the frequency is automatically held in the resonance point and tracked, during the passage of the material through the coil.
Number | Date | Country | Kind |
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05 027030.5 | Dec 2005 | EP | regional |