FR2475283A1 - METHOD AND DEVICE FOR DEMAGNETIZING PERMANENT MAGNETS - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS A PROCESS AND A DEVICE FOR DEMAGNETIZING AND MAGNETIZING PERMANENT MAGNETS, INCLUDES TWO GROUPS OF STATIC SWITCHES A AND B AGENCY TO HAVE OPPOSED CONDUCTION DIRECTIONS, MEANS 24 FOR DISPLAYING THE LEVEL OF CHARGE, MEANS 25 FOR COUNTING THE NUMBER OF DISCHARGES NECESSARY FOR DEMAGNETIZATION AND AT THE SAME TIME DECREMENT THE LEVEL OF THE CHARGE OF THE CAPACITORS, AND MEANS 26 FOR SWITCHING THE TWO GROUPS OF SWITCHES A AND B AND INVERTING THEIR DIRECTION OF CONDUCTION.

Description

The present invention relates to a method of demagnetization of ai-

  permanent mants used in industry, for example, for fixing workpieces to machine tools as well as a device for setting

work of this process.

  Machine tool workpiece tables frequently have

  workpiece fastening devices with a permanent magnet

  gnétisé by means of a coil crossed by an electric current after the establishment of the part on said table. After machining the part, the magnet

  permanent magnetized which constitutes a part of the table, must be demagneti-

  dried to allow the piece to be removed from the table and fixed in a

new position.

  The demagnetization operation of the permanent magnets can be performed

  the most effective is based on subjugation

  magnets with a hysteresis cycle having a progressively decreasing amplitude

  and on the use of two types of device: a field demagnetizer device

  alternating and a demagnetizer by discharging capacitors.

  The alternating-field demagnetizer is generally made up

  by a coil traversed by an alternating current having the value of the voltage

  constant service, able to generate fields beyond 1000 Oerstëd.

  In this case, the largest value of the alternating field must be able to saturate the magnets and must then be reduced slowly. Indeed,

  if the reduction in the amplitude of the applied field was too rapid, it is not

  is not possible to obtain complete demagnetization. The frequency of the demagnetizing field is normally the frequency of the electrical network (50Hz), but

  it can be reduced to reduce the effect of eddy currents developing

  pant in the metal magnets. Demagnetization is normally carried out

  killed by a field having the same direction as the pre-existing magnetization.

  The demagnetizer by charging capacitors is designed on

  the principle of obtaining the demagnetizing field by means of the os-

  flickering of a capacitor bank on a demagnetizer coil.

  By varying the voltage of the capacitors, the amplitude of the demagnetizing field can be varied and by varying the value of a resistance contained in the device.

  demagnetizer you can vary the frequency of the oscillations and thus decrease it

  mentation of the demagnetizing field over time. Therefore, by changing the am-

  fullness of the demagnetizing field and by decrementing said field, we can obtain

  hysteresis cycles having a progressively decreasing amplitude which

  complete the demagnetization of the magnets.

  Even if these systems have achieved satisfactory performance,

  39 health, they nevertheless have significant drawbacks, the main

  pal is that they require high capacity distribution networks of

  current difficult to achieve and often bulky.

  The present invention proposes to produce a device based on

  a process for demagnetizing permanent magnets, by discharging condensate

  which does not require a high capacity distribution network

  and making it possible to produce small demagnetization devices

  low cost for industrial applications.

  More particularly, the process of demagnetization of magnets per-

  manents according to the present invention is characterized in that one circulates

  1st, in a coil surrounding the magnets, direct current pulses having an opposite polarity and a decreasing amplitude, said pulses

  being obtained by discharging a capacitor bank.

  According to one embodiment of the method of the present invention, the polarity reversal is obtained by static switches constituted

  by controlled diodes (SCR), controlled by a counter of the number of discharges

  ges of the capacitors, by means of release pulses generated by a

monostable circuit.

  The decreasing amplitude of the demagnetization impulses is obtained

  naked by the counter of the number of discharges of the capacitors which makes it change

  the predisposition or display of the charge level of the condensing battery

  sators, starting from the highest level to reach the zero level.

  The demagnetization circuit according to the present invention, comprising a current rectifier bridge connected to the network and a capacitor bank is characterized in that it comprises, between the capacitor bank and the demagnetization coil two groups of static switches having opposite directions of conduction, means for displaying the level of the charge of said capacitors, means for counting the number of predetermined discharges

  and necessary for demagnetization, and to decrement at the same time the ni-

  charge calf displayed capacitors down to zero, so that

  the capacitor discharge pulses have a current

  decreasing rant to zero, and a means for alternating switching of the two groups of static switches to alternately reverse the direction

current conduction.

  According to another embodiment of the present invention, the demagnetization circuit includes means for making it possible to magnetize

  demagnetized permanent magnets.

  The invention will be described in more detail with reference to a preferred embodiment and to the accompanying drawing in which Figure 1 shows a block diagram of the demagnetization and magnetization circuit of permanent magnets according to the invention, and the fi -ures 2 & 3 respectively represent a schematic diagram

  demagnetization and magnetization pulses.

  Referring to the drawings, Figure 1 shows the dema circuit.

  gnétisation of the present invention also allowing the magnetization of ai-

  permanent demagnetized mantles. The network R is connected to a diode rectifier bridge.

  des whose DC outputs 3 and 4 are connected to a magnetization-demagnetization coil 2. Between the rectifier bridge 1 and the coil 2 is connected a capacitor bank 5 (shown schematically by a single capacitor) connected at the terminals of the direct current lines 3 and 4 and, downstream of this battery are connected two groups of static switches A and B, formed by SCR-controlled diodes, mounted in the circuit so as to allow the conduction of current according to opposite directions opposite by means of conductors 6 and 7. The rectifier bridge 1 is connected to the network R by means of a static charge switch 10. The SCR controlled diodes of group A are connected to a discharge release circuit 12 by

conductors 9.

  At the terminals of the network R, a low voltage power supply 13 is also connected, designed to supply the low voltage necessary for the power supply.

  circuits for controlling the discharges and the intensity of said discharges.

  This power supply 13 is connected by a line 16 to a bistable circuit for

  magnetization 14 via a controlled interlock switch

  during the beginning of demagnetization. The output 17 of the bistable demagnetization circuit

  tisation 14 is connected to an operating mode selector 18, which functions

  operates to display or select the demagnetization or magnetization operating mode, its output being connected to a ramp generator 20, the output 22 of which, applied to a non-dissipative discharge control circuit 21, controls the load switch 10 of the capacitor bank 5. Furthermore, the ramp generator 20 is connected to a comparator 23 of the charge level of the capacitor bank, itself connected to a display circuit 24 of this level, so that the comparator 23 is able to signal and interrupt the charging of the capacitors 5 each time the displayed value is obtained

  in the charge level display circuit 24.

  A counter 25 counts the number of discharges of the capacitors and is connected, on one side to a monostable circuit 26 connected to the comparator 23 and provided

  to produce rectangular pulses of discharge release and

  incrementation of the demagnetization counter 25 and, on the other side to the display circuit 24 of the charge level of the capacitors 5. The counter 25 for 39 demagnetization is connected to the two discharge release circuits 11 and 12

  and consequently the signal supplied by the counter 25 through the release circuit.

  tion 11 for diode group A, or through the release circuit 12

  will terminate the discharge of the capacitor bank 5 in one direction or the other through the coil 2. An output 27 of the counter 25 will provide a reset signal to the bistable demagnetization circuit 14 which will be disarmed

  to interrupt the demagnetization cycle.

  In the circuit described above, means have also been provided for

  alternately allow the magnetization of the magnets operating according to the mete

  conventional method for the generation of discharges having the same polarity and

  constant current intensity.

  Indeed, line 16 is connected to a magnetizing bistable 28 ap-

  te to be activated by a pusher 29 and having its output connected to the operating mode selector 18 to select the magnetization operation. Compared to the remaining part, the circuit is identical to that described above, the difference being that only group A will be controlled by SCR switches, and therefore there will only be pulses having a polarity through the coil 2. For this purpose, a Hall probe 32 is provided for detecting the number of discharges from the capacitor bank 5 which sends its pulses, by means of the conductor 33, to a counter 31 capable of counting the number of discharges

  for magnetization and which, after completing its counting cycle provides

  ra at its output 34 a reset signal to the bistable circuit 29 which controls

the end of the magnetization cycle.

  FIG. 2 schematically represents the pulses of de-

  magnetization. The ordinate axis represents the intensity of the positive current or

  negative and the X axis represents time in seconds. As you can see

  quer, these pulses have alternately an opposite polarity and their intensity decreases with time. It has been found that by using pulses having a

  duration of 5-20ms, demagnetization can be obtained in 20 s. about.

  Figure 3 schematically shows the same diagram

  but for magnetization pulses. As can be seen, these im-

  pulses have the same polarity and their intensity is always constant. Even in this case, by using pulses having a duration of 5-20ms, we obtain

magnetization in 7 s. about.

  The operation for demagnetization is as follows:

  Before the necessary energy from the single-phase distribution network R (110-220-380V 50-60 Hz), the voltage is rectified by means of the rectifier bridge 1. The static switch 10 controlled by the phase control device 21 will increase by 'a

  gradually the voltage of the bridge and will thus charge the condensing battery

  39 teurs 5 in a gradual and non-dissipative manner (see the ramp from O to 300 V).

  The level of the charge is controlled by the comparator 23 which will signal and stop

  will stop said charge each time the value is obtained in 24. The comparison

  tor 23 will also control the discharge of capacitors 5 in coil 2 in

  simultaneously deactivating the load switch 10.

  The discharge release pulse provided by the monostable 26 controls the discharge of the capacitors and will further increment the counter 25

  the number of predetermined discharges necessary for demagnetization.

  These discharges can vary from 5 to an undefined maximum number. The counter opens, by means of release circuits 11 and 12, alternately, the

  static switches A and B formed by controlled diodes (SCR). In pro-

  alternately referring to the opening of switches A and B, the direction of flow

  The demagnetization current of coil 2 will alternately be reversed.

  se. The counter 25, when its count increases, causes the display of the charge level of the capacitor bank 5 to vary, starting from the highest level and arriving at the zero level. When the counter reaches the predetermined number of discharges, it provides a signal for the end of the demagnetization cycle and deactivates the demagnetization bistable 14. This same circuit, when it functions as a magnetizer, causes the discharge

  of pulses having the same polarity and a constant amplitude on the wound 2.

  The number of magnetization pulses is counted by the counter 31 which

  collects the information by means of a Hall 32 probe placed on the fault conductor

  charge. Of course, the preceding circuit is described by way of example not

  limiting and, as a result, is susceptible to numerous modifications and variations

  within the reach of ordinary skill in the art without going beyond the scope of this

26 invention.

Claims (8)

  1. A method of demagnetizing permanent magnets, characterized in that direct current pulses having an alternately opposite polarity and a decreasing amplitude are circulated in a coil surrounding the magnets, said pulses being obtained by the discharge of 'a capacitor bank. 2. Method according to claim 1, characterized in that the polarity reversal is obtained by means of static switches formed by diodes   controlled (SCR) controlled by a counter of the number of condensate discharges   tores by means of release pulses generated by a monostable.   3. Method according to any one of the preceding claims,   characterized in that the decreasing amplitude of the demagnetization pulses is obtained by the counter for the number of discharges of the capacitors which, by increasing its counting, varies the predisposition of the charge level of the capacitor bank, starting from the highest level high to level zero.   4. Demagnetization device for the implementation of the se-process   lon any one of the preceding claims, comprising a bridge straight-   current generator connected to the electrical network and a capacitor bank,   characterized in that it comprises between the capacitor bank (5) and the box   demagnetization box (2), two groups of static switches arranged (A and B) having opposite directions of conduction and means (24) for displaying or adjusting the charge level of the capacitors, means (25) for counting the number of predetermined discharges necessary for demagnetization and for decreasing, at the same time, the displayed charge level of the capacitors to zero, so that the discharge pulses of said capacitors are able to present a decreasing current intensity until arriving   with zero and a means (26) for alternately switching the two groups of inter-   static switches to alternately reverse the direction of conduction of the cou-   rant. 5. Device according to claim 4, characterized in that the means for counting the number of discharges of the capacitors (5) comprise a counter (25) which, by increasing its counting, causes the level of charging of capacitors.   6. Device according to claim 4, characterized in that the means for alternately switching the two groups of static switches (A and B) to reverse their direction of conduction comprises a monostable circuit   37 (26) controlling the counter of discharges and by release circuits.   7. Device according to any one of claims 4 to 6, charac-   terized in that it comprises means (28) for magnetizing said magnets per- manents.   8. Device according to any one of claims 4 to 7, charac-   terized in that it includes non-dissipative means for charging the capacitor bank, to make it possible to reduce the size of the apparatuses   7 of demagnetization and magnetization.