GB2069259A - Method and circuit arrangement for demagnetizing permanent magnets - Google Patents

Abstract

A method is described for demagnetizing permanent magnets consisting of circulating through a coil encompassing the magnets d.c. current pulses of alternatively opposed polarity and decreasing amplitude, which pulses are obtained by discharging a capacitor set 5. The demagnetizing circuit comprises two sets of static SCR switches 8 oppositely poled, means for setting the capacitor charging level of threshold, means 25 for counting the number of dischargings necessary for demagnetizing the magnets and for simultaneously decreasing the charging threshold of the capacitors and means 11, 12 for switching the two SCR switch sets for reversing the conduction direction thereof. <IMAGE>

Description

SPECIFICATION Method and circuit arrangement for demagnetizing permanent magnets The present invention generally relates to the systems for demagnetizing permanent magnets used in the industrial field, for example for anchoring workpieces on tool machines and the like, and to a circuit arrangement for demagnetizing said permanent magnets.

Presently the workpiece bearing tables of the tool machines and the like are provided of devices for anchoring the workpiece consisting of a permanent magnet which is magnetized by means of a coil therethrough an electric current flows, upon having suitably located the workpiece on the table therefor.

At the end of the piece working operations, the magnetized permanent magnet which is a part of the workpiece bearing table has to be demagnetized in order to release the workpiece from the table or to anchor said workpiece at a new position.

The demagnetizing operation of the permanent magnets is obtained by several methods, the better whereof is based on subjecting the magnets to progressively decreasing amplitude hysteresis cycles by means of two types of apparatus, that is an alternating magnetic field demagnetizing apparatus and a capacitor discharging demagnetizing apparatus.

The alternating field demagnetizing apparatus comprises a coil therethrough an alternating or ac current flows having a constant RMS value, and effective to provide electromagnetic fields up to 1,000 Oersteds. In this case the maximum strength of the alternating field has to be effective to saturate the magnets and to be successively slowly decreased. The frequency of the demagnetizing field is, usually, the mains frequency (50 Hz) and, in order to reduce the shielding effect due to the eddy currents flowing through the metal magnets, it is possible to decrease said frequency. The demagnetizing is usually carried out by a field having the same direction as the pre-existing magnetization.

On the other hand, the capacitor demagnetizing is based on the concept of obtaining the demagnetizing field by means of an oscillatory discharging of a capacitor set or battery on a demagnetizing coil. By varying the voltage across the capacitors it is possible to vary the demagnetizing field strength or amplitude and, by varying a resistor included in the demagnetizing apparatus, it is possible to vary the frequency of the oscillations and accordingly to decrease the demagnetizing field in the time. Thus, by varying the amplitude of the demagnetizing field and decrementating said field, it is possible to obtain progressively decreasing amplitude hysteresis cycles effective to demagnetize the magnets.

These methods, though they have provided rather satisfactory performances, are however affected by great drawbacks the most important whereof is that they require high current capacity distributing networks, of special design, and, furthermore, they are rather encumbering.

The present invention sets out to provide a method and a circuit arrangement effective to demagnetize permanent magnets, by means of capacitor dischargings, without requiring the use of high current capacity distributing networks and such an apparatus, for carrying out the method, which is of small size and reduced cost.

More specifically the method for demagnetizing permanent magnets according to the present invention is characterized in that it consists of circulating, through a coil encompassing the magnets, d.c.

current pulses of alternatively opposite polarity and decreasing amplitude, said pulses being obtained by the discharging of a capacitor set or battery.

According to one aspect of the present invention, the polarity reversing is obtained by means of static switches consisting of silicon controlled diodes (SCR's) driven by a counter effective to count the number of dischargings of the capacitors, by means of enabling pulses as generated by a monostable circuit.

According to another aspect of the present invention, the decreasing amplitude of the demagnetizing pulses is obtained from the capacitor discharging number counter which, as it is incrementated, causes the setting of the capacitor battery charging level or threshold to vary, starting from the highest level and arriving at the zero level.

The demagnetizing circuit according to the present invention, comprising the conventional current rectifying bridge connected to the mains, and the conventional capacitor battery, is characterized in that between said capacitor battery and the demagnetizing coil there are provided two sets of static switches, oppositely poled, and in that there are provided means for setting said capacitor charging level orthreshold, means for counting a predetermined number of said capacitor dischargings, as necessary for demagnetizing the magnets, and for decrementing, in the same time, said capacitor set charging level down to zero, in such a way that said capacitor discharging pulses have a current strength decreasing to zero, and means for alternatively switching the two static switch sets for alternatively reversing the current conduction direction.

The invention will be hereinafter described in a more detailed way referring to a preferred embodiment thereof, given by way of an indicative and not limitative example, and illustrated in the figures of the accompanying drawing, where: Figure 1 illustrates a block diagram of the circuit arrangement for demagnetizing and magnetizing permanent magnets by the capacitor discharging method or system, by using decreasing intensity current pulses; Figure 2 illustrates a rather schematic diagram of the demagnetizing pulses; and Figure 3 illustrates a rather schematic diagram of the magnetizing pulses.

Referring now to the drawings, Figure 1 illustrates the demagnetizing circuit arrangement according to the present invention which is designed in such a way as to be also effective to provide the magnetizing operation. The mains R is connected to a diode rectifier bridge 1, the d.c. outputs 3,4 whereof lead to a conventional magnetizing-demagnetizing coil 2.

Between the rectifier bridge 1 and the coil 2 there is coupled, across the d.c. lines 3 and 4, a capacitor set or battery 5 (being for simplicity represented by a single capacitor) and, downstream of said capacitor battery 5 there are provided two static switch sets A and B, consisting of silicon controlled diodes SCR's coupled to the circuit in such a way as to provide a current conduction in alternatively opposite directions by means of the leads 6 and 7. The rectifier bridge 1 is coupled to the mains R though a static charging switch 10. The silicon controlled diodes SCR's of the A set are connected to a discharging enabling circuit 11, by means of the leads 8 while the silicon controlled diodes SCR's of the B set are coupled to a discharging enabling circuit 12 by means of the leads 9.

At the ends of the mains R there is also coupled a low voltage power supply 13 provided for supplying the low voltage for operating the discharging driving circuits and for controlling the intensity or strength of said dischargings. The power supply 13 is coupled, by means of a line 16, to a demagnetizing bistable circuit 14, through the interposition of a demagnetizing starting switch 15.The output 17 of the demagnetizing bistable circuit 14 leads to an operation mode selector 18, effective to set the demagnetizing or magnetizing mode of operation and the output whereof 19 is coupled to a ramp source 20 having its output 22 coupled to a non dissipative discharging controlling circuit 21 and driving the charging switch 10 of the capacitor battery, as coupled to the circuit 24 for setting the charging level orthreshold of said capacitors, in such a way as to allow for the comparator 23 to signal and switch off the charging of the capacitors 5 each time the value or level set in the charging level setting circuit 24 is reached.

A counter 25 acts to count the capacitor discharging number and it is connected, from one side, to a monostable circuit 26 coupled to the comparator 23 and operating for providing the square pulses for enabling the discharging and incrementing the demagnetizing counter 25 and, from the other side, to the circuit 24 for setting said capacitor charging level. The counter 25 for demagnetizing the magnets is coupled to the two discharging enabling circuits 11 and 12, thereby the signal transmitted by the counter 25 through the SCR A set enabling circuit 11 orthrough the enabling circuit 12 will causethe capacitor battery 5 to discharge either in one direction or in the other through the coil 2.The output 27 of the counter 25 will supply a reset signal to the demagnetizing bistable 14, which will be disabled thereby switching the demagnetizing operation off.

The circuit as described so far also comprises means effective to alternatively allow for the magnet tization of the permanent magnets and operating according to the conventional method of providing dischargings of like polarity and constant strength.

More specifically, to the line 16 there is also coupled a magnetizing bistable circuit 28 effective to be actuated by a push button 29 and the output whereof is connected to the mode of operation selector 18 for selecting the magnetizing mode of operation. With respect to the remaining part, the circuit is equal to that thereinabove described, with the difference that will be energized, for example, only the set A of SCR switches, thereby providing pulses of a single polarity through the coil 2. To this end a Hall probe 32 is provided effective to detect the number of dischargings of the capacitor battery 5, which probe sends its pulses, through the lead 33, to a demagnetizing discharging number counter 31, which, at the end of its counting period, will provide at the output 34 thereof a resetting signal for the bistable 29 thereby causing the magnetizing operation to end.

Figure 2 schematically illustrates a demagnetizing pulse diagram. On the Y axis there is represented the positive and negative current strength while on the Y axis there is represented the time.

As it can be noted the pulses have alternatively opposite polarity and the intensity or amplitute thereof decreases as the time increases.

It has been found that, by using pulses having a duration or width of 5-20ms it is possible to obtain a demagnetization in about 20 seconds.

Figure 3 schematically illustrates the same diagram, for the magnetizing pulses. As it should be noted, the pulses have the same polarity and the amplitude thereof is constant also in this case.

Also in this case, by using pulses having a width of 5-20ms it is possible to demagnetize the magnets in about 7 s.

The operation for demagnetizing the magnets is as follows: by drawing the necessary power from the monophase mains R( 110-220-380 Volts-50/60Hz), the voltage is rectified by means of the rectifier bridge 1.

The static switch 10, controlled by the phase controlling device 21,will gradually increase the bridge output voltage, thereby gradually and not dissipatively charging the capacitor battery or set 5 (see the ramp from 0 to 300 Volts). The charging level or threshold is controlled by the comparator 23 which will signal and switch off said charging each time the value set in 24 is reached. Said comparator 23 will drive, furthermore, the discharging of the capacitors 5 through the coil 2, thereby disabling in the meanwhile the charging switch 10.

The discharging enabling pulse provided by the monostable 20, in addition to driving the discharging of the capacitors 5, will cause the counter 25 to step up, which counter counts the number of the predetermined capacitor dischargings nescessary for de magnetizing the magnets. These dischargings can varyfrom a minimum of 5 to an undefined max imum. The counter 25, moreover, causes, by means of the enabling circuits 11 and 12, the static switches Aand B, consisting ofthe silicon controlled diodes SCR's, to alternatively switch off. By alternatively switching off the switches A and B, the demagnetiz ing current direction through the coil 2 will be alternatively reversed. Furthermore, as the counter 25 is incremented, it will vary the setting 24 of the capacitor battery 5 charging level or threshold, starting from the highest level and arriving at the zero level. Upon having arrived at the set or predetermined discharging number, the counter 25 will provide an end of cycle signal and will disable the demagnetizing bistable 14. That same circuit, as it is operated as a demagnetizing circuit, discharges pulses of like polarity and constant amplitude through the coil 2. The number of these magnetizing pulses will be counted by the counter 31 which will obtain the information from a Hall probe 32 as coupled to the discharging lead 4.

It should be noted that the circuit thereinabove described has been illustrated only by way of an indicative and not limitative example, being susceptible to all variations and modifications falling within the inventive concept scope.

Claims (10)

1. A method for demagnetizing permanent magnets, characterized in that it consists of circulating through a coil encompassing said magnets d.c.

current pulses of alternatively opposed polarity and decreasing amplitude, said pulses being obtained by discharging a capacitor set or battery.

2. A method, according to claim 1, characterized in that said polarity reversing is obtained by static silicon controlled diode (SCR) switches driven by a counter effective to count the number of said capacitor dischargings by enabling pulses as generated by a monostable element.

3. A method, according to the preceding claims, characterized in that the decreasing amplitude of said demagnetizing pulses is obtained by means of said capacitor discharging number counter which, as it is incremented, causes the capacitor charging level or threshold setting to vary, starting from the highest level and down to zero.

4. A demagnetizing circuit arrangement for carrying out the method according to the preceding claims, comprising a current rectifier bridge, connected to the mains and a capacitor set or battery, characterized in that between said capacitor set and the demagnetizing coil there are provided two static switch sets, as oppositely poled, and in that means are provided for setting the capacitor charging level or threshold, means for counting the predetermined discharging number necessary for demagnetizing said magnets and for simultaneously decrementing to zero said capacitor set charging level or threshold, thereby the discharging pulses of said capacitors are of decreasing current intensity to zero and means for alternatively switching said two static switch sets for alternatively reversing the current conduction direction thereof.

5. A circuit according to claim 4, characterized in that said means for counting said capacitor discharging number consists of a counter which, as it is incremented, causes said capacitor charging level to decrease.

6. A circuit according to claim 4, characterized in that said means for alternatively switching said two static switch sets and alternatively reversing the conduction direction thereof comprise a monostable element or circuit driving said charging number counter and enabling circuitry.

7. A circuit according to claims 4 to 6, characterized in that it comprises means effective to cause said permanent magnets to demagnetize.

8. A circuit according to any claims 4to 7, characterized in that it comprises non dissipative means for charging said capacitor set, thereby reducing the size of the demagnetizing and magnetizing apparatus.

9. A circuit for demagnetizing permanent magnets substantially as hereinbefore described with reference to the accompanying drawing.

10. A method of demagnetising permanent magnets substantially as hereinbefore described with reference to the accompanying drawing.