Abstract:
A magnetic bi-stable latch with an upper stator and a lower stator, a rotor between the upper stator and lower stator and adapted for rotation between a first latched position and a second latched position. Each of the stators is a magnetic assembly having at least two inner poles and two outer poles of magnetic material, and at least one stator further having a coil disposed in relation to the inner pole and the outer pole to form an electromagnet. The stators are positioned such that the outer poles of the upper stator align with the inner poles of the lower stator and the inner poles of the upper stator align with the outer poles of the lower stator. The rotor has permanent magnets mounted thereon such that in the first latched position the permanent magnets are aligned with poles of the upper and lower stators.

Description:
TECHNICAL FIELD 
   The present invention generally relates to magnetic latches, and more particularly relates to limited rotation active magnetic devices. 
   BACKGROUND 
   There are certain situations for which a bi-stable latch is particularly suited. For example there is a need for a device that could be used to hold a refrigerator door open or closed, or a deployable appendage deployed or stowed. Another use for the bi-stable latch of the present invention is to provide high speed switching for optical elements. Such a switching mechanism is provided in U.S. patent application Ser. No. 10/103,534 to David A. Osterberg, filed Mar. 20, 2002, and assigned to the assignee of the present invention. Various systems and devices such as, for example; optical test instruments and equipment, include one or more optical elements, which may be provided to implement, for example, optical filtering. In some of these systems, it may be desirable to simultaneously switch one or more optical elements into and out of an optical path. Preferably, this optical element switching operation is performed relatively rapidly. 
   In the past, rapid and simultaneous optical element switching has been accomplished using, for example, a wheel mechanism that is configured to rotate the optical elements into and out of the optical path. In one exemplary wheel mechanism embodiment, the optical elements are arranged around the perimeter of a wheel. As different optical elements are to be moved into and out of the optical axis, a motor or other driver rotates the wheel, stopping when the desired optical element is in the optical path. 
   Although wheel mechanisms generally operate safely, these mechanisms also suffer certain disadvantages. For example, the configuration of many of these wheel mechanisms provides for sequential, rather than random, access to the elements at the edges of the wheel. As a result, the amount of time and energy that may be used to switch one element into the optical path and another optical element out of the optical path can be undesirably high. This may be most pronounced when the wheel is used to move optical elements into and out of the optical paths that are located on opposite sides of the wheel. 
   Another drawback of some known wheel mechanisms is that rapid movement of the wheel can cause disturbances in the system. These disturbances can result in, for example, image blur. This can be a significant factor in applications that implement precise optical system control such as, for example, in satellite applications. To compensate for the disturbances a rapidly moving wheel may cause, some systems may implement long settling periods after wheel movement. Other systems may use complex force compensation and/or isolation mechanisms, which can increase the system complexity and, in some cases, simultaneously decrease system reliability. Moreover, some of these complex mechanisms may also dissipate significant power, which can negatively impact the thermal profile of the system. 
   Hence, there is a need for a switching mechanism that addresses one or more of the above-noted drawbacks. Namely, a switching mechanism that supplies relatively high-speed switching speeds, and/or that dissipates relatively low amounts of power, and/or does not cause significant system disturbances. The present invention addresses one or more of these needs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
   BRIEF SUMMARY 
   A magnetic bi-stable latch with an upper stator and a lower stator, a rotor between the upper stator and lower stator and adapted for rotation between a first latched position and a second latched position is provided. Each of the stators is a magnetic assembly having at least two inner poles and two outer poles of magnetic material, and at least one stator further having a coil disposed in relation to the inner pole and the outer pole to form an electromagnet. The stators are positioned such that the outer poles of the upper stator align with the inner poles of the lower stator and the inner poles of the upper stator align with the outer poles of the lower stator. The rotor has permanent magnets mounted thereon such that in the first latched position the permanent magnets are aligned with poles of the upper and lower stators. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  Is a drawing of one portion of a magnetics assembly usable in the present invention; 
       FIG. 2  is a drawing showing two of the magnetics assemblies of  FIG. 1  together with a plurality of magnets; 
       FIG. 3  is a drawing of a rotor assembly usable in the present invention, together with an optical filter arrangement; and 
       FIG. 4  is a sketch showing a complete but simplified exploded (for clarity) view of the bi-stable latch and the magnetic paths. 
     FIG.  5 A and  FIG. 5B  show, respectively, top views of the planes of contact of the components of the bi-stable latch having two inner poles and two outer poles on each of the upper and lower stator magnetic assemblies. 
   

   DETAILED DESCRIPTION 
   The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
   The device of the present invention will be described in term of a preferred embodiment, but it is understood that other configurations may be used. The device described has two stators and a rotor intermediate the stators. Each stator has a plurality of pole pieces (also called spiders) and is designed with an even number of poles. The number of poles can be selected in accordance with the angular travel required between poles. In this case the preferred embodiment utilizes eight poles and thirty degrees of rotation. More poles decrease the size of the device (for the same latching moment) but reduce the permitted travel of the rotor. 
   The device of the present invention is applied herein to a fast pivot mechanism utilized to latch a pivoted device in one of two positions. The device will be seen to have several advantages over previous designs, for example wheel designs or other magnetic latch designs. First, the latch of the present invention applies a moment around the rotation axis of the rotor rather than a force. Internally the device applies several forces in different directions summing to zero, however they are coupled to apply a pure moment. The unique magnetic path within the device, as will be discussed in some detail later, allows two stators to be used that have external actuation coils. Since the coils are external there are few limitations as to how large they may be. Simply extending the magnetic path of the stators and installing a larger coil will reduce the power and increase the efficiency of the device, by reducing the power necessary to unlatch the device from one of its two positions. Hereinafter the two stators are described as identical stators though they need not be identical. For example, as described above, and in accordance with a preferred embodiment of the invention, two coils are described, one each in association with the two stators. It is possible, however, to have only one coil, associated with only one of the stators, to accomplish the objectives of the invention. 
   For purposes of this detailed description of a preferred embodiment, the structure and operation of the bi-stable latch will be described using all three of  FIG. 1 ,  FIG. 2 , and  FIG. 3  since different components of the latch are shown more clearly in different drawings.  FIG. 1  is a drawing of a magnetics assembly according to the invention. Each of the magnetic assemblies  12  and  14  comprises (FIG.  1  and  FIG. 2 ) outer pole pieces or “spiders”  16  and inner pole pieces or “spiders”  18  and a torroidal coil  20 . Again, as noted previously, the invention may be practiced with the use of only one coil associated with one of the stators. 
   The actual latch of the invention comprises two such magnetic assemblies, a top assembly  12  and a bottom assembly  14  as is shown in FIG.  2 . Between the upper magnetics assembly and the lower magnetics assembly is a rotor assembly  26  (not shown in  FIG. 2  for purposes of clarity, but shown in  FIG. 3 ) that supports a plurality of magnets  30 , preferably permanent magnets, including alternating upper pole (north) pole pieces  32 , lower pole (south) pole pieces  34 , upper pole (south) pole pieces  36 , and lower pole (north) pole pieces  38 . The rotor assembly  26  is attached to a rotor shaft  40  as shown in FIG.  3 . The rotor shaft  40  may be any shaft affixed to the rotor  26  including a shaft that is capably of applying torsion to the rotor  26  when the rotor is in either of its terminal or latched positions, such as a torsion bar or a shaft biased by a torsion spring, for example. The rotor shaft need not be a shaft under torsion, however, as the repulsive effect of the electromagnet, as will be described in more detail later, begins the movement of the rotor from a first position to a second position. The use of a torsion rod, or a rotor shaft under torsion makes the switching from a first latched position to a second latched position faster. 
   Also shown in  FIG. 3  is a holder arm  42  attached to the rotor assembly  26  such that as the rotor assembly rotates, on optical filter or other device  44  is rotated into out of registration with a desired location. The rotor assembly  26  may also comprise a counterweight  46  to assist in providing minimum disturbance to the assembly during motion. 
   The stators  12  and  14  of FIG.  1  and  FIG. 2  are shown as identical, but as previously noted, they need not be. They must, however, have similar magnetic paths. The torroidal coil  20  has two iron pole pieces  16 ,  18  wrapped around it as shown in the FIGS. The pole pieces  16 ,  18  are designed with an even number of poles (here, eight) and joined with the coil  20  to form the stator assembly. As noted, the number of poles can be selected for the angular travel required of the pivotable member  42 . More poles decrease the size (for the same latching moment) but reduce the travel. When the upper stator  22  and the lower stator  24  are joined in the assembly of the latch, one of the stators is rotated one pole so that an inner pole on one stator aligns with an outer pole on the other stator. 
   Each of the stator magnetic assemblies has four pole pairs making eight pole pairs for the two stators. The rotor  26  may be machined from a non-magnetic material such as aluminum and has the same number of pole pairs the spiders, in this case eight. Each of the eight magnets  30 , of course has two poles associated with it so that when the stators and rotor are assembled each magnet aligns with an inner pole of one stator and an outer pole of the other stator. The magnets  30  are installed in the rotor in alternating directions so that when viewed from the top or the bottom the polarities alternate between north and south as shown in FIG.  2  and FIG.  3 . Iron pole pieces  32 ,  34 ,  36 ,  38  previously described are installed at each end of each individual magnet  30  mating with the iron poles of the spiders  16 ,  18 . 
   When assembled the rotor  26  is free to rotate while supported on rotor shaft  40  between the poles of the spiders  16  and  24 . The pole pieces of the spiders  16 ,  18  serve as detents to the rotation of the rotor  26 . The thickness of the poles in this example was designed to allow thirty degrees of free rotation, although as previously noted, the number of stator poles also determines the degree of free rotation of the rotor  26 . 
     FIG. 4  is a schematic diagram showing the magnetic circuit established during the operation of the latch when the rotor is at either of its latched positions. The schematic shows only two each of the upper and lower pole pieces, it being understood that in the preferred embodiment there are eight of each and that any even number of pole pieces may be used depending upon the rotational angle desired, etc. The schematic is also shown as partially exploded for clarity, it being understood that the pole pieces of the upper and lower magnetics assemblies may act as detents to the pole pieces of the rotor magnets to limit the rotation of the rotor. 
   The rotor  26 , or, more precisely, the magnets  30  and poles  32 ,  34 ,  36 , and  38 , complete the magnetic circuit that starts at a rotor magnet  30 , flows through rotor pole piece  36 , then follows the outer pole spider  16 T through one coil  20 T (of the top magnetic assembly  12  in this example) then out the inner pole  18 T across the second magnet  30  (in an additive direction) and through the outer pole  16 B of the bottom magnetic assembly, across the bottom magnetic assembly coil  20 B through the inner pole piece  18 B and then through magnet  30  of that assembly and back out the rotor pole piece  38  to the magnet  30  where it started. As previously noted, it is possible to eliminate one of the coils, in which case the circuit is completed through the pole pieces of the stator that lacks a coil. Since In the preferred embodiment there are four poles (and eight pole pieces) in each of the upper and lower magnetic assemblies, there are four parallel paths through which the magnetic circuit is completed, each path utilizing two upper and two lower pole pieces. The rotor  26 , of course, rotates around rotor shaft  40 . 
   The magnetic reluctance causes the magnets  30  of the rotor  26  to be attracted to the pole respective upper or lower pole pieces of the stators at each end of its travel, generating a bi-stable magnetic detent at two locations. The latch is released by driving a current pulse through the coils  20  in a direction opposing the flux in the iron of the pole pieces  16 ,  18 ,  32 ,  34 ,  36 , and  38 . The opposing flux generated by the electromagnet counteracts the flux of the permanent magnets and, if sufficiently strong, can have a repulsive effect upon the magnets, driving them toward the other latching position. The coils  20  may also be energized in the other direction to release from the opposite detent. Thus a positive pulse causes the latch to switch to one state and the opposite pulse causes it to switch to the other state, the torsion of the rotor shaft in this example providing additional momentum to complete the switch. The circuit could be used without a torsion spring mechanism applied to the rotor shaft, however, but the power consumption would usually be greater in such a configuration as the latching attraction would by necessity be greater. 
   FIG.  5 A and  FIG. 5B  show, respectively, top views of the planes of contact of the components of a bi-stable latch in accordance with the invention, but having two inner poles and two outer poles on each of the upper and lower stator magnetic assemblies. These diagrams, shown with the rotor between detents at the upper and lower poles, show the relative positioning among the various poles of the magnets of the rotor  26  and the upper and lower magnetic assembly spiders  16  and  18 . In  FIGS. 5A and 5B  inner poles  16  and outer poles  18  of the top and bottom magnetic assemblies are shown, as are the pole pieces  32 ,  34 ,  36  and  38  of the rotor magnets  30 . The magnets  30 , of course cannot be seen in these views as they are below the pole pieces. As can be appreciated, as the rotor pole  32 , for example moves toward inner pole  18  ( FIG. 5A ) the pole  18  acts as a detent stopping the rotation of the rotor. Since there are four pole pairs in each of the upper and lower assemblies and eight pole pairs in the rotor, contact is made with all poles simultaneously thus forming four parallel flux paths and two detents. 
   Should active control be desired to allow more precise control a flux sensor, such as a Hall sensor  44  (in FIG.  5 A), may be installed in the gap to sense magnetic flux. This sensor can then be used to control the detent torque since flux density is approximately proportional to output torque. During passive operation i.e., when the coil is not energized, this sensor also gives an indication of the state of the device. The two detent points give a strong positive and negative flux reading while a near-zero flux indication represents a rotor half-way between the detents. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.