[1]Uni'ted States Patent Office 31416,749 3,416,749 MAGNETIC HYSTERESIS APPARATUS James P. ONeill, Playa Del Rey, Calif., assignor to TRW Inc., Redondo Beach, Calif., a corporition of Ohio Filed May 10, 1965, Ser. No. 454,365 10 Claims. (Cl. 244-1) ABSTRACT OF THE DISCLOSURE Magnetic dampin.- is aebieved when a magnetizable member having large ma,-netic hysteresis characteristics is moved through a magnetic field produced by poles of alternating polarity. Maximum damping actioii is achieved for small increments of movement of said magnetizable member and the damping effect is independent of the rate of movement. This invention relates to magnetic hysteresis apparatus for producing a force between two members in resistance to a change in relative position and for dissipating energy as heat generated in ferromagnetic materials when the magnitude and/or orientation of magnetization is chan,-ed due to re!ative motion between a magnetizing device and a variaibly magnetized member. Such magnetic hysteresis apparatus may be used as drives, brakes, and dampers, for example. Iii the usual application of magnetic hysteresis to the damping of an oscillatory motion, such as that required to reduce the librations of a gravity-gradient stabilized satellite, a ring of ferromagnetic material is magnetized by a bar magnet that produces north-to-south polarization clockwise in one semicircular half of the ring and ,counterclockwise in the other half. With the damper arranged to produce relative rotation between the magnet and riiig, magnetic domains in the ring are reversed from c!ockwise to counterclockwise magnetization, and vice versa, with the consequent dissipat-ion of energy in the ring due to magnetic hysteresis. Similar arrangements are used as a clutch, a brake, or a magnetic hysteresis drive. A general object of the invention is to provide magnetic circuits which minimize the change in relative position required to produce the maximum force capability of systems of the general kind described, and thereby to reach the max@imiim energy dissipation as a function of further relative niotion. Another, and related, object of the invention is to minimize the dimensions of the circuits in the direction of the relative motion, and thereby facilitate the use of n-iultipole confi.-Lirations that produce a higher force and dissipate more energy as a result of multiple changes in magnetization. The foregoing and other objects are realized according to the invention throu.-h the provision of magnetic hysteresig circuits comprising a magnetizing device and a magnetizable member so arranged that a minimum of re!ative motion between the device and mem,ber is required to reach the maximum hysteresisderived force resisting this relative motion. Accordingly, minimum rnotion is required to attain the maximum energy dissipation. This invention achieves the property of requiring only a small relative rtiotion for maximum effect by using a close spacing of the poles of the magnetizing device to produce a short reversal distance. The reversal distance may be defined as the distance from the center of Patented Dec. 17, 1968 2 a gap between a pair of poles to the center of the next adjacent gap where the direction of magnetization is reversed. The concept of closely spaced poles cannot be defined in terms of specific diniensions, since the iri:vention applies to any scale of apparatus. Closely spaced poles will generally be separated, however, by gap dimensions that are smaller than the width or length of the magnet associated with the magnetizin- device and either equal to or smaller 10 than the thickness of the magnet. The object of close pole spacing is to bring adjacent gaps, producing a reversal of magnetizing force, as close together as is consistent with the production of some specified hysteresis-derived force resisting relative motion. Therefore, a more basic 15 definition of close spacing is one that minimizes, within the limits of practical complexities of construction, the relative displacement required to attain the specified hysteresisderived force. Close pole spacin.- also facilitates the use of multire20 versal arrangements for attaining higher values of maximum force and dissipation. In a multireversal arran,-ement of the magnetizing device, the first three pole faces forming two gaps of reversed direction of magnetization are followed by additional pole faces forming additional -aps 25 of alternating direction of magnetization. Thus, when the ma.- netiza@ble member is forced through the multiple reversed magnetizin.- fields, each reversal contributes to the total energy dissipation and increases the required force. 30 A feature that results in minimum relative motion to produce full magnetic hysteresis action in certain embodiments of the invention is the reductign of the width and spacing of pole tips to the extent that the pole tips of high-permeability material are saturated. 35 The property of attaining maximum hysteresis drag with minimum relative motion results in a minimum of reversed motion being required for full reversal of the dra.- force, i.e., maximum drag force in the opposite direction. This property is particularly important in the 40 damping of oscillatory systems, since major hysteresis loops to the maximum force-reversal excursion are maintained down to smaller amplitudes; the system oscillation is conseqliently reduced to a very small amplitude before the regime is reached where less damping is obtained 45 from minor hysteresis loops that do not reach the maximum force-reversal excursion. Another feature of the invention is the magnetic shielding provided by certain ei-nbodiments of the multipole magnetizing device in which the outer pole pieces, and the 50 case which forms a magnetic shield, h-ave the same polarity. Further reduction in the stray magnetic field produced by the device is accomplished by certain shaded-pole versions in which the outer poles of a multiple magnetizing device are reduced in strength. Thus, the diminishing re55 versals of the magnetizing field reduce the magnetization of the variably magnetized member where it emerges from the vicinity of the magnetizing device. In the drawing: FIG. 1 is -a front elevation showing one, form of the 60 magnetic hysteresis apparatus according to the invention; FIG. 2 is a plan view of the apparatus of F@IG. I in which the membe@r is a rotating vane; FIG. 3 is an elevation view of the @apparatus in which 65 the magnetizable member is a cylinder; -F@IG. 4 is a graphshowing a bysteresis curve;
[2]3)416,749 3 F,IG. 5 is a plan View showing a modified form of the apparatus provided with magnetic shieldin.-; FIG. 6 is @a sertion taken along line 6-6 of FIG. 5; FIG. 7 is a seciton taken along line 7- 7 of FIG. 5; ,F,IG. 8 is a plan view showing another form of the appa,ratus; FIG. 9 is a section taken along line 9-9 of FIG. 8; FIG. 10 is a section taken along line 10-10 of F-IG. 8; ;FIG. 11 is a front elevation shonving another form of the apparatus; F-IG. 11 is a front elevation showing another form of t-he apparatus; -FIG. 12 is a sectional view showing another form of the apparatus; F-IGS. 13-15 are elevation views showing forms of the apparatus utilizing tapered pole tips; ,FIGS. 16 and 17 are sectional views showing other forms of the apparatus utilizing tapered pole tips; and F,IG. 18 is a perspective view of a satellite structure incorporating magnetic hysteresis dampi-.ig apparatus according to the invention. Referring to the drawin,@, FIG. I shows a pair of mu,ltipole inagnetizing devices 10 and 12 symmetrically facing the opposite sides of a movable hysteresis Inember or sheet 14 of magnetizable material. The device 10 comprises an ordered array of closely spaced magnetizing elements inade up of a plurality of ma.-netic pole pieces 16a-16e interleaved with a plurality of permanent magnets 18a-l$d. Similarly, the device 12 com,prises a plurality of magnetic pole pieces 20a-20e with a plurality of permanent magnets 22a-22d. The pole pieces and magnets may be held together by bonding, for example, or by other mer-hanical means that does not interfere with the magnetir, circuits. A typical selection of materials for the magnetic hysteresis apparatus would include magnets niade of barium ferrite or an aluminum-nickel-cobalt alloy having a very high ability to retain a strong magnetization; and hysteresis member made of a material such as 31/2% chrome steel that is easily magnetized yet capable of dissipating energy when subjected to a cycle of changing magnetization. The arrangement of FIG. 1 may represent a system in which there is linear motion between the sheet 14 and devices 10 and 12. Alternatively, it rnay represent t,lie developed section of a system in which there is rotational motion between the sheet 14 and devices 10 and 12. For example, as shown in FIG. 2, the sheet 14 may comprise a disc or vane rotating or oscillating between the devices 10 and 12, about an axis 24, between the device 10 on one side of the vane 14 and the device 12 on the other side of the vane 14, the device 12 being hidden in the figures by the device 10. In FIG. 3 the sheet 14 is shown as a cylinder rotating or oscillating between the devices 10 and 12 about the axis 24. Referring again to FIG. 1, the @foreed relative motion that occurs during normal operation of the arrangement as a magnetic hysteresis drive, brake or damping system, is guided by @a track, pivot or bearing system, shown as a number of oppositely located rallers 26, that keeps the magnetizable member 14 positioned midway between the magnetizing devices 10 and 12. Adjacent pole pieces of each of the magnetizing devices 10 and 12 are oppositely magnetized by the inagnets. For exwnple, the face of rjaagnet 18a adjacent the pole piece 16a is a north pole, making pole piece 16a a north pole, as shown, and the face of magnet 18a adjacent pole piece 16b is a sotith pole, making pole piece 16b -a south pole. The face of magnet 18b adjacent pole piece 16b is a south pole and the face adjacent p(>Ie piece 16c is a north pole, making pole piece 16c a north pole. Thus pole pieces 16a-16e are altemately magnetized north, south, noirth, south, iand north in that order. 4 Simillarly, pole pieces 20a-20e are also magnetized north, south, north, south, and north in that order, so that opposingly facing pole pieces, such as poie pieces 16a and 20a, are similarly poled. Wiih the magnetizable member 14 fixed in the positions shown, the magnetizin@ units formed by the intOTleaved pole pieces and magnets as above described, induce a given state of magnetization in the member 14. For example, one ma.-netizi,-ig unit is formed by the pole 10 pieces 16a and 16b and the ma;,@net Iga, and anothermagn,-tizing unit is formed by the pole pieces 20a, 20b, -nd magnetic 22a. This first, oppositely facing set of magnetizing units induces a state of magnetization in the adjacent portion of the ferromagnetic sheet 14 such 15 that magnetic domains in t@his portion of the sheet are ma-net,zed with a field produced by a north pole on the left and a south pole on the right. Similarly, the secbnd set of ma.-netizing units are formed by pole pieces 16b, 16c, and magnet 18b, and pole pieces 20b, 20c, and 20 magnetic 22b. But the portion of the ferroma,-netic sheet between these two elements is magnetized with a field of opposite olientation as produced by a south pole on the left and a noith pole on the ri.-ht. The third pair of magnetizing units formed by pole pieces 16c, 16d, and 25 magnet 18c, and pole p@eces 20c, 20d, and magpet 22c create a field oriented as that produced by the first set and finally the fourth pair of iragnetizing units fornied by pole pieces 16d, 16e, and magnet 13d and pole pieces 20d, 20e, and magnet 22d create the again reversed 30 orientation as produced by the second set. When the variably magnetized member 14 is moved from left to right, the magnetizing force impressed on any particle or domain iii the member 14 is changed. For example, as a domain is moved from a position in 35 front of magnet 18b to a posil@io@n in front of ma.anet 18c, the impressed ma.- netizin.- force is reversed, the magnetizing force havin@, passed through zero at the central plane of the symmetrically forcing north poles 16c and 20c. Further motion of the domain to a positi@on in 40 front of magnet 13d reverses the magnetizing force again and completes a full cycle of variable magnetization. Now it is common to all unsaturated magnetic materials that a change in magnetization lags the variation in magnetizing force to produce a hysteresis loop that 45 is a measure of ener.-y loss in the material, By conservation of energy, the energy loss that occurs as the magnetizable member 14 is moved between the magnetizing units in the apparatus shown, is manifest as a force being required to move the member 14 relative to the mag50 netizing devices. Reference is now made to FIG. 4 which is a graph showing how the magnetic induction B in a magnetic material changes as the magnetizing force H is varied. When demagnetized material is subjected to a -radtially 55 increasin- magnetizing force up to Hn,,,, the induction in the material increases from zero to Bm,,,,. If the magnetizing force is then gradually reduced to zero, the inductio-n decreases from Bma. to B, on the vertical axis. This value (B,) is known as the residual induction. 60 If the magnetizing force is reversed in direction and increased in value, the induction in the material is further reduced, and it becomes zero when the demagnetizing force reaches a value of H,, known as the coercive force. A further increase of this negative force causes 65 the induction to reverse direction, becoming -B,,,. at -H If the magnetizing force is reversed and increased from this point to H,,,,,., the change in induction is along curve -Bmax, -Br, 13max. This gives the complete' hysteresis loop. 70 This type of curve applies to all ma,-netic materials, the difference in materials being largely a matter of the values. Materials having a low coercive force are Iowenergy materials, and those having a high coercive force are high-energy materials. These have been commonly 75 known as soft and hard materials, respectively, but the
[3]81416,749 5 terms low-energy and high-energy are more representative of the characteristics of the magnetic materials. As the magnetizable meriiber 14 is moved between the magnetizing devices 10 and 12, the magnetic domains in the member 14 experience chan,-es in magnetization 5 which define a hysteresis loop similar to that shown in FIG. 4. The area under the hysteresis loop is a measure of the energy loss in the member 14. With the action described above @being kept iii mind, it is now possible to more fully describe the force-deflec- 10 tion characteristics of the device. The nature of this action is principally governed 'by the n orth-south-northsouth repetitive arrangement of the pole faces. Neglecting effects of magnetization lag and fringe fields, 1/2 cycle of motion, represented for example by a domain movin.@ 15 from the central plane of magnet 18b to the central plane of magnet 18c is required for the force producing the relative motion to attain its maximum value. Further differential motion thereafter requires this same constant force since each unit deffection causes the same number 20 of magnetic d-omains to complete a cycle or go through the state of magnetic induction where ener.-Y dissipation takes place. As the direction of relative motion is reversed, 1/2 cycle of reversed motion is required (a domain which had arrived at the central plane of magnet 18c 25 now moves back to the central plane of magnet 18b) for a complete reversal of the force such that the same constant max,@Mum force in the opposite direction is required. Magnetization lag in the magnetic member (i.e., the lag in the changes in magnetization behind the varia- 30 tigns in the magnetizing force) wovild cause some increase in the 1/2 cycle of motion to obtain the above mechanical effects; but the increase due to lag would not extend the total required motion to more than one cycle. Because of this behavior, close pole spacing is 35 advantageous in the usual application where a short travel for maximum force is desired; and a small gap between the magnetizing devices and the ma,-netizable meiiiber is used to reduce ftinge fields and obtain maximum benefit from the close pole spacing. 4,0 It will now be appreciated that if the magnetizable member 14 is subjected to oscillatory motion such as that produced 7by the librations of a satellite, the apparatus shown in FIG. 1 will damp the motion by virtue of the energy absorption in the member 14 due to his- 45 teresis losses. Fringe and end effects are minimized and better magnetic shielding is provided'by the configuration:shown in FIGS. 5, 6, and 7. In this and other repetitive multipole ma.-netizing elements, the end effect is reduced by using 50 3, 5, 7 or other odd numbers of pole pieces so that the end pole pieces (e.g., 28a and 28d) have the same polarity. This also allows connecting the odd numbered pole pieces 28a-28d together with a backshield 30 and side shields 32 of a highly permeable material to form a complete 55 shieldin- box as shown. The even numbered pole pieces 34a-34c are spaced from the shields 30 and 32 to avoid short circuiting. The design shown in FIGS. 5-7 illustrates that the space allocated to the shielding box might be completely 60 filled with magnet material, such as magnets 36a-36f. Then, for the 'box volume and pole spacing used, max' mum flux is induced in the magnetizable member 14 and maximum hysteresis drag capability is 6btained. Although only one ma.-netizing device is shown or 65 required in some applications, the duplicate, symmetrically facing device is often warranted to increase the drag, improve the shielding, and provide a central position for the vane where the lateral magnetic attractin,@ forces are balanced. This also provides a plane or planes of sym- 70 metry where the impressed ma.-netization force is zero and thereby insures that a dissipative hysteresis loop is traversed. Should it be required that the magnetig hysteresis apparatus produce minimum external field, the case might 75 6 be extended to cover the magnetizable member at its widest excursion or a shadedpole magnet assembly might be used. If the magnetizable meniber 14 of FIG. 5 is magnetized appreciably by the end magnets 36a and 36b in the repetitive array, it will emerge with residual magnetism. This can be reduced, however, by using the shadedpole arrangement in which the force of the impressed magnetizing cycles are decreased as the ends of the array ;are approached. The reduced strength of the magnets might be accomplislied by any appropriate means but reduced pole spacin.- would be advantageous to the force-reversal characteristics. Another embodiment of the shielded magnetizing element is shown in FIGS. 8, 9 and 10. In this embodiment, the teeth of a U-shaped and a T-shaped element are interleaved to form multiple pole faces. This configuration can be dimensioned for very close pole spacing without using correspondingly short magnets; and furthermore, only two niagnets are used regardless of the number of teeth cut to produce the north-southnorth repetitive pole array. It also illustrates a mo-re efficient utilizati'on of the magnet material by use of a shielding case with less shuntin.- of the field developed by the magnets. Referring to the fl,@ures, the teeth of a slotted, T-section element 38 form multiple pole faces 40 of one polarity that project through slots 42 in a U-shaped element 44, the teeth of which form the multiple pole faces 46 of the other polarity. The magnets 47a and 47b, are oriented so that the element 44, foriiiing the sides and slotted face of the case, is contacted by the magnet faces of one polarity while the central element 38 is contacted by the magnet faces of the other polarity. The width of the pole faces of one polarity on the T-shaped element 38, the width of the adjacent pole faces of the other polarity as formed by the bars between the slots in element 44, and the gap between the pole faces 40 and 46 of opposite polarity can be varied to control the coupling to the magnetizable member 14 and thereby to change the drag force and force reversal characteristics. The shielding box is completed by a U-shaped element 48 which closes the back and the ends of the slotted U-shaped element 44. In this assembly, an efficient utilization of magnet material is illustrated since the case is well separated fr<)m all materials of opposite polarity. A simple embodiment of the repetitive, north-southiiorth ma,-netizing device is shown in FIG. II where this device 50 is a sheet, disk, or tape of hard magnetic material having the permanent magnetic pattern impressed on it. The adjacent magnetizable member 52 of a softer magnetic material is variably magnetized with the consequent dissi-pation of its hysteresis losses as the device 50 and member 52 are forced into Telative motion. The draa force can be increased, not only by increasing the length of the magnetizilidevice 50 and the number of the reversals in the direction of motion, but also by expanding the pair to a stack of alternate devices and members. Furthermore, in this as well as in other embodiments of the invention, the drag might be increased by allowing some Coulomb-friction drag between the device 50 and member 52. In this case the magnetic attraction between the device 50 and member 52 might be used as part or all of the normal force between the friction surfaces. FIG. 12 illustrates the use of any of the former repetitive, north-south magnetizilig members as a configuration of revolution for producing a torque when the ma.-netizing and the magnetized members are siibjected to relative rotary motion. The configuration of revolution is applicable for a circular disk -or conically shaped magnetizable member as well as for a cylindrical member, however, in the cylindtical configuration the magnetic attraction forces are balanced out without the necessity of having a magnetic drive member on both sides of the magnetizable member. Referring again to FIG. 12, the magnetizable member
[4]7 54 is in the form of a thin-walled cylindrical shell that rotates about its axis to establish relative motion with respect to the remainder of the assembly which forms the magnetizing device 56. This latter device 56 is composed of four types of elements arranged symmetrically around the ceiitral magnetizable member 54. Pole piece wedges 58a, 58b . . . 58h of one polarity have pole tips facin.the central member 54 while their outer surfaces contact an exterior shielding cylindrical shell 60. Pole piece wedges 62a, 62b . . . 62h of the other polarity face the central member 54 in a similar manner but their outer surfaces are isolated from the shielding case or shell 60. The latter wedges 62a-62h are grooved at their outer surface to avoi leakage to the shell 60. The two types of pole pieces alternate circumferentially while bein.- separated by the intervenin.- magnets 64 which are oriented to establish the opposite polarities for the altern-,tte pole tips. In FIG. 13, an embodiment of the iiivention is shown in whi ' ch the variably magnetizable member 66 is subjected to a sin.-le forced reversal of magnetization; but the motion reqtiired for this reversal has been minimized. The ma.-netizing devices 68 and 70 differ from the configuration of FIG. 5 in that the many reversals are sacrificed in favor of closer pole spacing in the direction of relative motion. This allows maximum pote@itialities for obtaining a short travel for reversal of the hysteresis dra.@ force. The shortest reversal distance is obtained by reducing the pole spacing to such aii extent that th-. -required dra.- force can only be obtained by operating high permeability pole tips at saturation. As shown in FIG. 13, each of the magnetizing devices 68 and 70 has an element 7Z that acts as the shielding case which wraps around the assembly to terminate in the two outer pole tips of common polarity. The narrow pole piece 74 of opposite polarity is held by the ma-nets 76a and 76b between the two closely spaced outer pole tips. The tips of the pole pieces 72 and 74 are tapered or beveled to impress a narrow north-sOL[th-nortli ma-netizing field on the magnetizable member 66. In one variation of this configtiration, shown in FIG. 14, the outer pole pieces 77 are applied as an overlay after the shieldin,@ case 79, magnets 76a and 76b and center ;pole piece 74 are assembled. This allows variation of the magnetic gap to be used more conveniently in the adjustment of the hysteresis drag developed. FIG. 15 illustrates both the repetitive use of the minimum-gap-width members similar to that shown in FIGS. 13 and 14 and the confl-uration-of-rotation arrangement r@ used for prodlicing a to que during relative rotation. The thin-walled cylindrical shell 78 used as the variably ma,@netizable member would require a minimum of reversed relative rotation for reversal of the torque. A.-ain, as was the case for FIG. 12, this configuration-of-revolution arrangement is also applicable for a circular disk or conically shaped magnetizable member. In FIG. 15, the shielding case and the pole piece of one polarity are combined as one element 80. The element 80 is suitably cut or otherwise shaped to form a plurality of tapered poie faces 82 circumferentialy arran.-ed around the shell 78. The ma.-nets 84, disposed in openings 85 in the element 80 hold the narrow tapered pole pieces 86 of opposite polarity between the gaps in the tapered pole faces 82. The reversal of the magnetization of the cylindrical shell 78 takes place in the narrow limits of the north-south-north pole spacing. In FIG. 16 is shown an embodiment of the invention in which the narrow pole spacing of FIG. 13 is retained but the multi-element assembly is intended for linear relative motion. Such an assembly might be used as a damping member attached between two points in a spacecraft structure or mechanism. Referring to FIG. 16, a cylindrical ma.-netizing device 87 is disposed within a hollow cylindrical magnetizable member 88. A magnet assembly rod 90, preferably of 3)4161749 8 nonrilagnetic material, clamps the erid pole pieces 92 at each end of a stacked array of circular plates or disks which form the magnetizing device 87. The device 87 includes end pole pieces 92 and intermediate pole pieces 94 of the same polarity, the narrow pole pieces 96 of the opposite p<)Iarity, and the magnets 98. The rims of the pole pieces 92, 94, and 96 are suitably shaped to form tapered pole faces which converge towards restricted areas of the magnetizable member 88. The magnetizable member 88 is 10 operated by a rod 100 attached to a plate 102 closing one end of the @member 88. With the pole pieces 94 and 96 in contact with themagnetizable member 88, as shown, some friction drag would be added to the ma.-netic hysteresis drag, but the friction could be minimized when 15 desired by guiding the reciprocating motion by a lowfriction system such as that provided by separate flexure members connecting the magnetizable member 88 and the assemby rod 90. Alternatively, the member 88 may be provided with longitudinal slits 103, as shown, to per20 mit the member 88 to flex as it is -moved relative to the device 87. FIG. 17 shows apparatus similar to that of FlG. 16 except that the magnetizing device 87a is formed of rectatigular elements disposed between two flat rectangular 25 ma.@netizable members 105. The magnetizin- device includes end pole pieces 92a and intermediate pole pieces 94a of the same polarity, narrow pole pieces 96a of the opposite polarity, and magnets 98a. The two ends of pole pieces 92a, 94a, and 96a in contact with the magnetizable 30 members 105 are stiitably beveled to form the closely spaced poles. FIG. 18 shows a magnet hysteresis damper incorporated in a satellite structure utilizing a gravity gradient stabilization system. Only those portions of the satellite 35 strLiCtLire are shown wliich aftord an understanding ol" the dampin- mechanism. The s,@itellite strijeture includes an inertia boom 104 fixed to a rotor 106 and having a longitudinal axis 108 at right angles to the axis 110 of rotation of the rotor 106. 40 A vane 112 of ferromagnetic material is clamped to the boom 104 and rotor 106 with its plane at right angles to the rotation axis 110 of the rotor 106. The vane 112 is shaped in the form of a segment of a circle having its center coinciding with the Totation axis 110. 4,5 A pair of ma-netizin@ devices 68 and 70 are Iocated on opposite sides of the vane 112 adjacent to the periphery thereof. The magnetizing devices 68 and 70 are fixed relative to the rotational axis 110, as indicated schematically by ground lines. 50 The magnetizing devices 68 and 70 may comprise one of the number of arrangements already shown and described. However, the arrangement of FIG. 13 is shown for illustration. When the satellite structure is in orbit, the librations of 55 the satellite cause the boom 104 to oscillate about the rotational axis 110. As the boom 104 oscillates the vane 112 also oscillates between the magnetizing devices 68 and 70. The energy of oscillation is absorbed in th-. ferro60 magnetic vane 112 through the me@hanism of hysteresis damping as abovedescribed, so as to cause the oscillations to stop. For any of the embodiments of the invention, it is intended that complete magnetic shielding of the ma-netiz65 able member as well as the magnetizing members be provided where necessary either to protect the dampin. device itself from external fields or to prevent stray fields from being produced by the device. The various embodiments of the invention all make use of a magnetization lag in the magnetizable hysteresis 70 member that takes place ivhen some movement causes a change in the magnetizing field. After sufficient change in the vicinity of any magnetic domain, the stress causes a change of state that results in dissipation of energy. The force-displacement characteristics of the different forms 75 of apparatus all show a force buildup with displacement
[5]3)416)749 9 as the magnetic lag is developed; and then the force is limited to some constant value after the displacement reaches the point where further increments cause equal numbers of domain to reach the stress causing dissipation <)f energy. The value of the force opposing further motion is such that the energy input is equal to the dissipation. For the various magnetic domains in the magnetizable member, the applied magnetic field vector might vary in magnitude, in direction, or in both ma.-nitude and direction. It is necessary that at least part of the variation be at magnitudes smaller than that value which -causes saturation. Above saturation, there is no hysteresis loss due to changes in either magnitude or direction of the applied field; consequenty motions effecting such changes do not result in an opposing force. Excess field strength should therefore be avoided, especially for hysteresis devices in which the field change is primarily that of rotation. For the usual designs where both magnitude and direction are changing, excess field strength is not a problem. Furthermore, the configurations with symmetrically facing magnetizing members as shown in FIGS. I and 13 have planes of zero magnetizing force through which all the hysteresis material passes. This insures that a dissipative hysteresis loop is traversed regardless of the maximum value of the field strength applied. The embodiments of the invention in which an exclusive property or privilege is
