US 3708727 A
A high capability, oriented, ceramic permanent magnet is assembled into the magnetic circuit in which is intended to function, and its strength is reduced by demagnetization until it provides the desired flux density for the circuit. This demagnetization is accomplished by encircling the magnet with a demagnetizing coil and applying current pulses to the coil. Control over the degree of demagnetization is achieved by orienting the demagnetization field perpendicular to the direction of polarization of the magnet.
Claims available in
Description (OCR text may contain errors)
United States Patent [1 1 Wielebski et al. I [4 1 Jan. 2, 1973 54] METHOD FOR ADJUSTING THE 3,243,696 3/1966 Lovellet al ..317/1s7.s x STRENGTH OF HIGH ENERGY 2,930,944 3/1960 Yonkers ..311/1s7.s MAGNETS Prima Examiner-J. D. Miller 75 Inventors: W ry 1 zi fi ygfig g gwg Assistant Examiner-Harry E. Moose, Jr.
Attorney-Barry E. Sammons et al.  Assrgnee: Allen-Bradley Company, Milwaukee, W1s.  ABSTRACT Flledi J y 22, 1971 A high capability, oriented, ceramic permanent mag-  Appl. No 165,254 net is assembled into the magnetic circuit in which is intended to function, and its strength is reduced by demagnetization until it provides the desired flux dengl-t -(g 5 sity for the circuit. This demagnetization is accomu n t 6 t 6 t 6 6 a I 6 t I 6 t 6 6 6 6 6 I 6 t 6 t n i a  Field of Search ..317 157.5, 335/284 coil and applying current pulses to the coil Comm, [561 References Cited over the degree of demagnetization is achieved by orienting the demagnetization field perpendicular to UNITED STATE PATENTS the direction of polarization of the magnet.
3,609,611 9/1971 Parnell ..317/1s7.s x 11 Claims, 4 Drawing Figures PuLsE GENERATOR MAGNET STRENGTH(BH PATENTEDJAI 2191s sum 1 or 2 FLUX DENSITY(B)I DE MAGNETIZING FIELD (H INVENTORS' WAYNE H.WIELEBSK| GLEN RAY KWMAW ATTQRNEY mmnznm 2 .m'
3.708.727 sum 2 or z PULSE GENERATOR INVENTORS WAYNE H'WIELEBSKI GLEN RAY ATTORNEY METHOD FOR ADJUSTING THE STRENGTH OF HIGH ENERGY MAGNETS BACKGROUND OF THE INVENTION point in the circuit. Such magnetic circuits typically have air gaps in them and in commercial applications the permeance of an assembled magnetic circuit of a given design can vary considerably due to variations in these air gaps and magnet strength. Such variations in l the permeance of an assembled magnetic circuit results in variations in the flux density at any point in the magnetic circuit produced by a magnet of given strength. In many applications the flux density is critical, and as a consequence, the strength of the permanent magnet must be adjusted after the magnetic circuit has been as sembled in order to obtain the desired flux density (B) in the circuit.
One common method of adjusting the strength, or remanent induction, of a permanent magnet is to first premagnetize the magnet to a strength higher than that required, and then demagnetize the magnet a controlled amount by momentarily applying a demagnetizing field in opposition of the magnetic field produced by the magnet. Apparatus for such a procedure is described in US. Pat. No. 2,930,944 entitled Method and Apparatus for Pulling Down Magnets. In the method of that patent a momentary demagnetizing field is applied to a magnet by injecting a current pulse of damped oscillations through a demagnetizing coil encircling or held alongside the magnet. The extent of the demagnetization is directly related to, and controlled by the amplitude of the injected current pulse. The current pulses are applied with ever increasing amplitude with the time between pulses adjusted to allow the magnet to recoil to its new operating point and allow observation of the effect on the magnet.
This method of adjusting the strength of permanent magnets has proven satisfactory when applied to magnets characterized by demagnetization curves which are rounded, or have steadily changing differential permeability. However, when higher energy magnets of the ceramic or ferrite preoriented or Alnico type are used, attempts to adjust the strength of the magnet by the above described method has proven difficult. More particularly, the demagnetization curve of such high energy magnets characteristically has a relatively straight, horizontal portion extending from the ordinate which curve turns sharply downward to a relatively netization until the demagnetizing field intensity (H) reaches a threshold magnitude determined by the location of the knee in the demagnetization curve, above which magnitude they become highly sensitive to increasing intensities. Consequently, attempts to adjust their strength have been unsatisfactory, and this fact has limited their commercial applicability. It would be desirable to have a method of demagnetizing such high energy i'nagnets in which changes made in the demagnetizing field intensity to adjust their strength were not as critical.
SUMMARY OF THE INVENTION The invention resides in an improved method of adjusting the strength of permanent magnets by the momentary application of a demagnetizing field, wherein the flux lines of the demagnetizing field are applied to 5 the magnet substantially perpendicular to its direction sitivity of the magnet to demagnetization is substantially decreased, thereby, allowing improved control over the adjustment of its strength.
This method is particularly applicable to high energy permanent magnets characterized by intrinsic demagnetization curves having a knee formed by a sudden change in differential permeability, or in other words, having a demagnetization curve with a pronounced rectangular shape. The intrinsic energy of a magnet is equal to the product of the flux density (B) produced by it, times its magnetic field intensity (H). The intrinsic energy product of a magnet depends not only upon its initial strength as measured by its residual flux (8,) and its coercive force (H but also upon the shape of its demagnetization curve and the point on that curve at which it is operated. The maximum theoretical intrinsic energy product possible for a magnet having a fixed residual flux (8,) and coercive force (l-l is the product of these two values. The demagnetizing curve for a magnet having such a theoretical energy product would be rectangular, and the magnet would be operated at the corner, or knee, of the curve where the intrinsic permeability changes abruptly from zero to infinity. Such a magnet, of course, does not exist, however, the demagnetization curves of a number of high energy magnets of both the ferrite or ceramic and Alnico type approach this rectangular shape. It is this type of magnet, having a high energy capability, having a sharp knee in its demagnetization curve, and being difficult to demagnetize in controlled amounts which is termed herein a high capability magnet.
It is the discovery of this invention that the difficulty in adjusting the strength of high capability magnets can be overcome by applying the demagnetizing field substantially perpendicular to the line of polarization of the magnet under adjustment. Although substantially improved results can be obtained by applying the demagnetization field at an oblique angle, optimum desensitization, and therefore greatest control, is obtained when applied perpendicular to the line of polarization. By this method, itbecomes possible to quickly and precisely adjust the strength of a high capability magnet, thus enhancing their commercial applicability to magnetic circuits which require prescribed levels of magnetic flux.
. A general object of the invention is to obviate the ef fects of the characteristic sharp knee in the demagnetization curve of high capability magnets, or in other words, increase the range overwhich the strength of an applied demagnetizing field can be varied to effectively adjust the strength of the magnet. By applying the demagnetizing field at a substantially perpendicular angle, the magnet is effectively desensitized to the demagnetizing field to improve control over the adjustment procedure. The magnitude of the current pulse necessary to demagnetize the magnet a specific amount can thereby be more readily determined and obtained by commercially available equipment, thus making the controlled demagnetization of high energy magnets commercially possible.
Another object of the invention is to provide a method of adjusting the magnitude of the remanent induction of a high capability permanent magnet after it is assembled in the magnetic circuit in which it is intended to function. The demagnetizing coil encircles the magnet positioned in the magnetic circuit and the angle defined by the axis of the demagnetizing coil and the line joining the poles of the permanent magnet is substantially perpendicular. Control over magnet strength is thus maximized and the strength of the demagnetizing field is easily controlled by varying the magnitude of the current pulses flowing through it.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration and not of limitation a preferred embodiment. Such description does not represent the full scope of the invention, but rather the invention may be employed in different arrangements and reference is made to the claims herein for interpreting the breadth of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS.
FIG. 1 is an illustrative demagnetization curve of a high capability magnet;
FIG. 2 is an illustrative graph indicating the results obtained by the invention;
FIG. 3 is a preferred apparatus for applying the method of the invention; and
FIG. 4 is a schematic diagram of a magnet and demagnetizing field applied to it.
DESCRIPTION OF THE PREFERRED EMBODIMENT High capability magnets are manufactured by carefully controlling the manufacture of the magnetic material of which they are composed. For example, ferrite powders may be ground to a fine homogeneous size prior to magnetization. The resulting magnets have magnetic domains which respond in near unison to magnetizing and demagnetizing forces, producing as a result, the rectangular-like characteristic demagnetization curve 1 shown in FIG. 1. Intrinsic curves have been shown in FIG. 1 for purposes of simplifying the following discussion. However, it should be understood that remanent induction curves (not shown in the drawings) provide the true measure of flux density externally of the magnet.
The demagnetization curve 1, is a plot of flux density (B) measured along the ordinant 2, versus field intensity (H) measured along the abscissa 3. The intersection point 4 of the demagnetization curve 1 and the ordinant 2, indicates the residual flux density (B,) of the magnet. The intersection point 5 of the demagnetization curve 1 and the abscissa 3 indicates the intrinsic coercive force (I-I of the magnet. When used, the magnet is inserted into a magnetic circuit having a fixed permeance which is represented in FIG. 1 by the load line 6. The intersection point 7, of the load line 6 and the demagnetization curve 1, represents the operating point of the magnet when no external fields are applied to it. The flux density at this operating point 7 is called the remanent induction (B of the magnet, and is a measure of its strength.
The method of the present invention lowers the strength, or remanent induction (B of the magnet a controlled amount so that it generates a desired flux density in the magnetic circuit. Such desired flux density is represented in FIG. 1 by the operating point 8, on the load line 6. To reach this operating point 8, the magnet is permanently demagnetized, lowering both its residual and remanent induction to produce the demagnetization curve represented by the dashed line 9.
To adjust the magnets strength, it is common practice to momentarily apply an aligned demagnetizing field to it. By controlling the intensity (H) of this demagnetizing field, the magnets strength is controlled. A characteristic feature of high capability magnets is the difficulty of controlling the strength of the magnet by controlling the intensity of the applied demagnetizing field. More particularly, in the demagnetization curve 1 in FIG. 1, the flux density (B) decreases very little for large changes in applied field intensity (H). However, a threshold is reached as shown by the sharp knee in curve 1, above which small changes in field intensity (H) substantially reduce the flux density (B) of the magnet. It is demagnetizing fields above this threshold value which permanently reduce the strength of the magnet and over which small variations in demagnetizing intensities cause large changes in remanent induction. For example, in high capability ferrite magnets operated above this threshold, the remanent induction (B may be reduced by a factor of 30 or more by demagnetizing field variations of as little as eight percent of the coer cive force (H Because high capability magnets are very sensitive in this region, adjusting their strength by controlling the strength of an aligned demagnetizing field is commercially impractical.
In FIG. 2, residual flux density (3,) of a high capability magnet is plotted in curve 10 as a function of demagnetizing field (H momentarily applied in alignment with the field of the magnet. Curve 10, therefore, is a plot of magnet strength versus applied demagnetizing field. It can be seen from curve 10 that the aligned demagnetizing field (H,,) has little permanent effect on the magnet until the field reaches a threshold value and above this value slight increases in the aligned demagnetizing field (H substantially reduces the strength of the magnet. Thus the steep negative slope of the curve 10 indicates a high sensitivity to demagnetization.
An apparatusfro demagnetizing a high capability magnet 11 having the characteristics discussed above is shown in FIG. 3. The magnet 11 is of the ceramic, or ferrite type, and is situated in the magnetic circuit of a reed relay like that disclosed in the copending US. Pat.
application Ser. No. 2023 filed Jan. 12, 1970 and entitled Sealed Contact Relay. The magnetic circuit is comprised of a U-shaped electromagnet 12, having first and second coils 13 and 14 wound around its two vertical legs 15 and 16. A first U-shaped flux finger set 17 extends upward from the leg 15 of the electromagnet l2, and a second U-shaped flux finger set 18 extends upward from the leg 16. The flux finger sets 17 and 18 form a channel in which a cartridge 19 containing a reed switch 20, is located. Magnetic flux generated by the electromagnet l2 flows up one flux finger set, through the sealed reed switch 20, and down the other flux finger set. When the relay is operated in a latching mode, magnetic flux is generated through switch 20 in one direction by the first coil 13, and in the other direction by the second coil 14.
The permanent magnet 11 is located directly beneath the reed switch 20, and is polarized vertically, across its width as shown in FIG. 4. The magnet 11 has four poles; a north pole on the top surface at one end of the magnet, a south pole directly beneath it on the bottom surface, a south pole on the top surface at the other end of the magnet, and a north pole directly beneath it on the bottom surface. The magnets flux pattern is, therefore, vertical in the magnet 11 and immediately adjacent its pole faces. The strength of the permanent magnet 11 is critical when used with reed switches. It must provide a flux strong enough to hold the contacts of the switch 20 closed when actuated by the electromagnet 12, but not strong enough to close the switch 20 itself. Because the holding flux and the permeance of the magnetic circuit varies with the reed switch used, the magnet 1 1 must be adjusted to the particular switch to which it is mated. This adjustment of the permanent magnet 11 is accomplished after it is assembled in the cartridge 19 with the reed switch 20.
Referring again to FIG. 3, a demagnetizing coil 21 is placed around the cartridge 19 and between the flux finger sets 17 and 18. The demagnetizing coil 21 is connected to a pulse generator 22 such as that described in US. Pat. No. 2,930,944 and sold under the trademark Standard Magnatreater. The pulse generator 22 produces a series of alternating current pulses the amplitude of which can be controlled to determine the intensity (H) of demagnetization. The demagnetizing coil 21 generates a field through the magnet 11 in the horizontal direction as indicated in FIG. 4 by the vectors 23. Thus, the demagnetization field 23 is applied to the magnet 11 in a direction perpendicular to the direction in which it is polarized, or perpendicular to the direction of the lines of magnetic flux eminating from the pole faces of the magnet.
By applying the demagnetizing field 23 perpendicular to the field of the magnet 11, control of the demagnetization procedure is substantially increased. Referring to FIG. 2, the curve 24 indicates the effect of a per pendicular demagnetizing field (H) on the strength of the permanent magnet. Instead of demagnetizing the magnet completely over a very narrow intensity range, the perpendicular demagnetizing field completely demagnetizes the magnet over a broad intensity range. This result is indicated in FIG. 2 by the gentle slope of the curve 24 when the threshold value of the demagnetizing field is reached. As a result, the magnitude of the current pulses from the pulse generator 22 can be progressively increased over a significant range of values to produce the desired amount of demagnetization. In other words, referring to FIG. 1, the desired operating point 8 is more easily obtained by applying current pulses of increasing magnitude to the demagnetization coil 21, until the demagnetization curve 9 is obtained.
In the preferred application of the method, the application of the current pulses is delayed to allow the operator time to test the operation of the reed switch 20 between pulses. The magnet 11 is thus demagnetized by alternately applying current pulses to the demagnetization coil 21 with ever increasing strength and observing the operation of the reed switch 20.
The demagnetizing field need not be perpendicular to the field of the permanent magnet to improve control of the demagnetization process. Rather, the curve 24 in FIG. 2 represents an orientation in which maximum control is obtained, and curve 10 represents the opposite extreme. However, as shown by the curves 25, 26 and 27 which respectively indicate the control associated with 30 degree, 60 degree and degree misalignment of the demagnetizing field, significantly improved control is not obtained according to the present method until at least an 80 degree misalignment is obtained.
By implementing the method of the present invention, it is now commercially ,practical to design precision magnetic circuits using high capability magnets. In many applications such magnets are desirable, and the invented method provides an easy and precise means of adjusting their strength after their insertion into the magnetic circuits.
1. In a method of demagnetizing high capability permanent magnets wherein a demagnetization field is momentarily applied to the magnet to reduce its strength, the improvement therein of applying the demagnetization field substantially perpendicular with respect to the'direction of polarization of the magnet to provide greater control over the amount of demagnetization.
2. The method as recited in claim 1, wherein the demagnetization field is applied at an angle of greater than 80 degrees with respect to the direction of polarization of the magnet.
3. The method as recited in claim 1, wherein the high capability permanent magnet is made of ferrite material and is placed in a magnetic circuit prior to being demagnetized by repeated application of said momentary demagnetizing field.
4. A method of constructing a magnetic circuit having a high capability magnet, the steps comprising:
1. assembling the magnetic circuit into its final form;
2. demagnetizing a high capability magnet therein by momentarily applying a demagnetization field substantially perpendicular with respect to the direction of polarization of the magnet; and
3. observing the effect of the demagnetization step on the operation of the magnetic circuit;
wherein steps (2) and (3) are repeated until the desired circuit performance is obtained.
5. The method as recited in claim 4, wherein said magnetic circuit includes a reed switch, the magnet is in magnetic circuit with the reed switch, and the effect of the demagnetization step is observed by operating the reed switch.
6. In a method of adjusting the magnetic strength of a high capability permanent magnet by partial demagnetization, the steps of:
l. applying a demagnetizing field to the magnet in a direction substantially perpendicular to the direction of polarization of the magnet; and
2. increasing the demagnetizing field until the magnet strength is reduced to a desired value.
7. A method of adjusting the magnetic field of a high capability magnet as in claim 6, wherein the demagnetizing field comprises a series of alternating current pulses, there is a delay between pulses in which the magnet is tested, and the pulses are increased in magnitude until the magnet strength is reduced to a desired value.
8. A method of adjusting the magnetic field of a high capability magnet as in claim 6 wherein the demagnetizing field is applied at an angle greater than eighty degrees with respect to the selected direction of polarity.
9. A method of adjusting the magnetic field of a high capability magnet as in claim 7 wherein the demagnetizing field is applied at an angle greater than 80 degrees with respect to the selected direction of polarity.
10. A method of constructing a switch circuit that includes a high capability magnet, the steps of:
l. assembling within a cartridge a reed switch with a pair of lengthwise magnetic reeds, and a high capability permanent magnet extending parallel and alongside said reeds that is magnetized with a direction of polarity crosswise to the length of said reeds for delivering magnetic flux thereto;
2. placing a demagnetizing coil around said cartridge with the coil axis paralleling the length of said reeds and magnet;
3. applying a pulse to said coil to create a demagnetizing field for said magnet that is substantially perpendicular to said direction of polarity;
4. testing the magnetic effect of the magnet upon said reeds; and
5. repeating steps (3) and (4), as necessary, with increasing increments of demagnetizing field strength until desired reed and magnet performance is obtained.
1 1. A method of constructing a switch circuit that includes a high capability magnet, the steps of:
l. assembling a reed switch and a high capability permanent magnet with one another, and with the direction of magnetic polarity of the magnet related to the reed switch to have magnet'flux pass through the reed switch;
2. disposing an adjusting coil in position to apply a magnetic field substantially perpendicular to the direction of polarity of the permanent magnet; and
3. energizing said coil to reduce the magnetic strength of said magnet in said direction of magnetic polarity.