Abstract:
A bistable electromagnetic actuator including: a tube; a stator arranged outside of the tube; and a rotor mounted in the tube so as to be displaceable along the longitudinal axis, the rotor at least partially comprises one or more of a paramagnetic and a ferromagnetic material and can be reversibly displaced between a first position and a second position by applying an electromagnetic field; wherein the stator comprises two o ring permanent magnets, a coil for producing the electromagnetic field, and a back-iron element having two stator pole shoes; and the two ring permanent magnets comprise hard magnetic particles that are embedded in a plastic matrix.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation of PCT/EP2015/054539 filed on Mar. 4, 2015, which is based upon and claims the benefit to DE 10 2014 204 736.6 filed on Mar. 14, 2014, the entire contents of each of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    Field 
         [0003]    The present application relates to a bistable electromagnetic actuator, such as for use in a surgical instrument, comprising a stator arranged outside of a tube, and a rotor mounted in the tube so as to be displaceable along the longitudinal axis, the rotor at least partially comprises a paramagnetic and/or ferromagnetic material and can be reversibly displaced between a first position and a second position by applying an electromagnetic field, wherein the stator comprises two, which can be oppositely axially poled, ring permanent magnets, a coil for producing the electromagnetic field, and a back-iron element having two stator pole shoes; as well as a surgical instrument. 
         [0004]    Prior Art 
         [0005]    Bistable electromagnetic actuators have a rotor that is held in one of two extreme positions in a permanent magnetic field, and can be moved out of one stable position into the other stable position by switching an electromagnetic field. This can for example actuate switches. In the case of surgical instruments, such as endoscopes, these compact actuators can be used for example to change a focus or enlarge an optical system, or to change a direction of viewing. This is accomplished in that an optical component is moved by the actuator, wherein the optical component is located in or on the rotor of the actuator. 
         [0006]    A linear motor for optical systems such as endoscopes is known from DE 10 2008 042 701 A1. The motor has a stator with two permanent magnets which are polled in the same direction and magnetically connected to each other with a back-iron element. A coil is arranged between the magnets. On the side next to each magnet, a pole shoe is magnetically connected to the back-iron element. The motor rotor comprises a yoke consisting of a soft magnetic material that magnetically engages with the permanent magnet of the stator. By supplying current to the coil, the rotor can be moved in the longitudinal direction from a resting position. 
         [0007]    The rotor according to DE 10 2008 042 701 A1 consists of a tubular, soft magnetic element so that a strong force must be expended to move the rotor from one position into the other position due to the resulting friction of the tubular rotor against the tube. Furthermore, the linear motor according to DE 10 2008 042 701 A1 is comparatively large. 
         [0008]    Conventional actuators use permanent magnets that can also be designated permanent magnet rings, for example consisting of magnetic materials such as NdFeB or SmCo. These magnetic materials are brittle and therefore very sensitive to mechanical loads. With endoscopes, it is additionally desirable to use minute magnets and small outer diameters. This makes the production of the ring permanent magnet very involved. In addition, the ring permanent magnets are very easy to damage when being installed and must be specially protected even in an installed state to prevent damage. 
         [0009]    Furthermore, it is important to use two ring permanent magnets that only have a minor dimensional deviation from each other since otherwise an excessive deviation in symmetry can arise in the actuator which for example can produce malfunctioning or poor functioning of the actuator. For example in one of the bistable positions, less holding force can arise on one side than in the other bistable position, which may not be desirable. This can be avoided by sorting conventional ring permanent magnets into pairs that, for example, are manufactured by sintering and then ground to size in order for example to install if possible magnets with the same dimensions and the same magnetic properties in an endoscope. 
       SUMMARY 
       [0010]    It is an object to provide a compact bistable electromagnetic actuator and a surgical instrument with a correspondingly bistable electromagnetic actuator that are produced economically and enable symmetrical forces in both bistable positions. 
         [0011]    Such object can be achieved with a bistable electromagnetic actuator, such as for use in a surgical instrument, comprising a stator arranged outside of a tube, and a rotor mounted in the tube so as to be displaceable along the longitudinal axis, which rotor at least partially comprises a paramagnetic and/or ferromagnetic material and can be reversibly displaced between a first position and a second position by applying an electromagnetic field, wherein the stator comprises two, which can be oppositely axially poled, ring permanent magnets, a coil for producing the electromagnetic field, and a back-iron element having two stator pole shoes, the ring permanent magnets comprising magnetic particles that are embedded in a plastic matrix. 
         [0012]    Through the use of ring permanent magnets which comprise hard magnetic particles embedded in a plastic matrix, very precise production of ring permanent magnets is enabled. The ring permanent magnets can consist of hard magnetic particles embedded in a plastic matrix. 
         [0013]    The ring permanent magnets can be produced in a batch, such as by means of an injection molding procedure, and can be produced with only very slight structural deviations since they are produced using the same process parameters. Nearly identical ring permanent magnets are therefore producible within a lot of manufactured ring permanent magnets. Furthermore, such ring permanent magnets that are manufactured from a compound of a polymer material and magnetic particles possess better mechanical properties. These are namely less brittle than corresponding magnets consisting of the solid material. 
         [0014]    Furthermore, the magnets can also be very efficiently and economically manufactured in large quantities through a corresponding injection molding technique. The hard magnetic particles can consist of NdFeB or SmCo. Furthermore, the plastic matrix can consist of a polymer. The plastic matrix can have a polyphenylene sulfide (PPS), or respectively consist thereof. This is a high-temperature-resistant thermoplastic plastic. As an alternative, a polyamide such as a polyamide 12 can be used. 
         [0015]    The ring permanent magnets can be manufactured in an injection molding method. The ring permanent magnets can be injection molded with any geometry and subsequently also magnetized. 
         [0016]    An opening or passage penetrating the ring permanent magnet in a longitudinally axial direction for at least one coil wire of the coil can be provided in at least one ring permanent magnet. This allows the bistable electromagnetic actuator to be produced with a very small diameter since feeding the at least one coil wire around the ring permanent magnet, i.e., either from the outside or from the inside of the coil wire to the coil, is avoided. 
         [0017]    With such embodiment of the bistable electromagnetic actuator, it is possible to provide a very symmetrical configuration of the ring permanent magnet by using two ring permanent magnets which each have an opening or passage for at least one coil wire of the coil that penetrates the ring material in the longitudinal axial direction. The weakening of the magnet is then namely symmetrical. In this case, however, only one opening or penetration in a magnet must serve to accommodate a coil wire of the coil. It is unnecessary for the opening or penetration to be provided with a coil wire of the coil in the other ring permanent magnet. 
         [0018]    An opening or passage penetrating the stator pole shoe in a longitudinally axial direction for at least one coil wire of the coil can be provided in at least one stator pole shoe. The coil wire of the coil may thereby be guided through a stator pole shoe as well as through a ring permanent magnet. Both stator pole shoes can have an opening or passage that runs through the stator pole shoe in the longitudinal axial direction. 
         [0019]    In the embodiments in which both ring permanent magnets and/or both stator pole shoes have an opening or passage for at least one coil wire of the coil, the opening or passage can be flush in a longitudinal direction with the respective opening or passage of the other ring permanent magnet and/or with the other stator pole shoe. 
         [0020]    The outer diameter of the ring permanent magnets can be less than or equal to 5 mm, such as being less than 3 mm and less than 2.5 mm. The wall thickness of the ring permanent magnets, i.e., in the radial and/or axial direction, can be less than 0.6 mm, such as being less than 0.4 mm and less than 0.3 mm. The ring permanent magnets can be each arranged on one side of the coil. 
         [0021]    The back-iron element with the stator pole shoes can comprise the coil, and the stator pole shoes can be arranged on both sides of the coil between the coil and the ring permanent magnets, wherein the rotors have two rotor pole shoes, wherein one axial width of the stator pole shoes can be less than one axial width of the rotor pole shoes. 
         [0022]    The coil current and power loss in the coil can thereby be minimized by increasing the efficiency of the coil. This can be achieved by the geometry of the actuator elements. The geometry can result from the fact that the back-iron element with the stator pole shoe no longer encloses both the coil and ring magnets as disclosed in DE 10 2008 042 701 A1, but only encloses the coil and the ring magnets are arranged outside of the stator pole shoes. Axially magnetized magnetic rings can be used for this purpose since a radially-arranged soft iron of the back-iron element is not needed on the magnetic rings. For this reason, the stator can be realized in a smaller radial installation space. Since the stator pole shoes are arranged between the permanent magnets and coil, a greater coil efficiency results since the pole shoes are connected directly to the back iron. The axial length of the stator and hence the axial length of the rotor as well can thereby be reduced. 
         [0023]    Since the rotor itself has rotor pole shoes, it has a central, radial tapering, and a pole shoe is therefore formed on each of its ends. The rotor therefore only contacts the tube at the locations of the pole shoes and no longer the entire surface. Accordingly, the friction can be reduced between the rotor and the tube in which the rotor is arranged. This increases the switching efficiency since less friction resistance must be overcome. In addition, the negative influence of, for example, straightness errors or curves are reduced due to the smaller fit that is decreased to two small contact surfaces, or respectively contact lines. 
         [0024]    Overall, this yields a favorable efficiency of the coil, or respectively actuator, and an effective balance between holding force and switching force. 
         [0025]    If the axial width of the stator pole shoes is less than an axial stroke of the actuator between the first position and the second position, major differences between the holding force and switching force can be achieved. 
         [0026]    The rotor with the rotor pole shoes can possess an overall length in the axial direction that is greater than the outer distance between the stator pole shoes in an axial direction. A distance between the axial middle planes of the rotor pole shoes can be greater than a distance between the axial middle planes of the stator pole shoes. Due to such features, the balance between the holding force and switching force can be easily adjusted, and the switching force can be increased. 
         [0027]    When the stator pole shoes have the same axial width between each other, and/or the rotor pole shoes have the same axial width between each other, and/or the stator(s) and/or the rotor(s) is/are formed symmetrically across a plane of symmetry, a symmetrical actuator structure is realized in an axial direction such that, in the two end positions, or respectfully in the first position and the second position, the same holding forces predominate and the same switching forces are used for changing the position of the rotor in the actuator. In addition, only some of the cited geometric dimensions can be designed symmetrical. When the actuator is subjected to a constant load during operation, for example from one side, it can be possible to brake up the overall symmetry of the actuator in an axial direction and effectuate greater holding force and/or switching force in one position than in the other position. 
         [0028]    The rotor can lie against a stop in the first and/or second position. The stop can be arranged such that the force exerted on the rotor in this position by the permanent magnets presses or draws the rotor toward the stop against which the rotor lies. 
         [0029]    In one development in an end position, such as the first or second position, the rotor pole shoe can be arranged at the end position to at least partially cover in an axial direction the stator pole shoe opposite the rotor pole shoe, wherein a middle plane of the rotor pole shoe arranged at the end position can be arranged beyond a middle plane of the stator pole shoe opposite the rotor pole shoe in an axial direction toward the end position. This concerns the rotor pole shoe, or respectively stator pole shoe, that is arranged closer to the momentary end position in an axial direction. In the case on an endoscope, this would be the distal pole shoes of the stator and rotor in the distal end position. These lie opposite each other. In the proximal end position, these are the proximal pole shoes of the stator and rotor. These also lie opposite each other. 
         [0030]    An end position for the rotor pole shoe not arranged at the end position can at least partially cover in an axial direction the stator pole shoe opposite the rotor pole shoe, wherein a middle plane of the rotor pole shoe not arranged at the end position can be arranged beyond a middle plane of the stator pole shoe opposite the rotor pole shoe in an axial direction toward the end position. In an endoscope, these can be the proximal pole shoes of the rotor and stator, and vice versa, in the distal end position of the rotor. 
         [0031]    These two configurations, individually or together, mean that very stable and strong holding force can be realized with little effort when current is supplied to the coil in the respective end position due to the favorable conduction of the magnetic flux. In addition, the switching force acting on the rotors is significantly increased. 
         [0032]    Finally, a surgical instrument, such as an endoscope, is also provided having the above-described bistable electromagnetic actuator. Since the actuator can be built very small, it can also be implemented in an endoscope having a narrow endoscope shaft. 
         [0033]    Further features will become apparent from the description of embodiments together with the claims and the included drawings. Embodiments can fulfill individual characteristics or a combination of several characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The embodiments are described below, without restricting the general concept, using exemplary embodiments with reference to the drawings, wherein we expressly refer to the drawings with regard to all details that are not explained in greater detail in the text. In the figures: 
           [0035]      FIG. 1  illustrates a schematic cross-sectional view of an actuator, 
           [0036]      FIG. 2  illustrates a schematic sectional representation of a ring permanent magnet, 
           [0037]      FIG. 3  illustrates a schematic representation of another ring permanent magnet, and 
           [0038]      FIG. 4  illustrates yet another schematic sectional representation of a ring permanent magnet. 
       
    
    
       [0039]    In the drawings, the same or similar types of elements and/or parts are provided with the same reference numbers so that a re-introduction is omitted. 
       DETAILED DESCRIPTION 
       [0040]      FIG. 1  shows a cross-section of a bistable electromagnetic actuator  1 . The actuator  1  is substantially rotationally symmetrical about the central axis  4  and only one-half of the actuator  1  is depicted. The entire section of the actuator  1  results from mirroring across the central axis  4 . 
         [0041]    In the following, the actuator  1  will be described as if it were a surgical instrument, that is, within an endoscope with a distal end and a proximal end. The distal direction in  FIG. 1  is to the right, and the proximal direction is to the left. 
         [0042]    Arranged radially outside of a tube  2  is a stator  10  which has two ring magnets  12 ,  14  that are axially magnetized in opposite directions so that the south pole of the magnets oppose each other in  FIG. 1 . When integrated in an endoscope, the ring magnet  12  is a distal ring magnet, and the ring magnet  14  is a proximal ring magnet. 
         [0043]    A cylindrical coil  16  is symmetrically arranged between the ring magnets  12  and  14 , wherein an also cylindrical back-iron element  18  consisting of a soft magnetic material is also arranged radially to the outside of the coil  16  and abuts the ring magnets  12 ,  14  flush radially to the outside. The back-iron element  18  terminates distally in a distal stator pole shoe  20  and proximally in a proximal stator pole shoe  22 . The back-iron element  18  and stator pole shoes  20 ,  22  can be formed as a single part or from different parts that are all soft magnetic. The distal and proximal pole shoes  20 ,  22  are arranged between the coil  16  and the distal and proximal ring magnets  12 ,  14 . Overall, this yields a flush, terminal surface radially to the outside. The stator  10  according to  FIG. 1  is symmetrical across a plane of symmetry  24  in an axial direction. 
         [0044]    Radially to the inside of the tube  2 , the actuator  1  according to  FIG. 1  has a rotor  30  that can consist entirely of a soft magnetic material. This rotor  30  tapers in the middle and terminates at a distal rotor pole shoe  32  and a proximal rotor pole shoe  34 , wherein the distal rotor pole shoe  32  basically lies opposite the distal stator pole shoe  20 , and the proximal rotor pole shoe  34  basically lies opposite the proximal stator pole shoe  22 . The rotor  30  tapers in the middle so that it leaves a gap  36  open to the tube  2 . Since the rotor  31  only contacts the tube  2  with the inner surfaces of the pole shoes  32 ,  34 , the friction is reduced on the one hand, and a non-tilting arrangement of the rotor  30  in the tube  2  is ensured on the other hand. The rotor  30  is symmetrical across a plane of symmetry  38  in the axial direction. 
         [0045]    The distal and proximal movements of the rotor  30  are restricted by a distal stop  44  and a proximal stop  46 , respectively. In contrast to the rotor  30  that is arranged so as to be axially movable within the tube  2 , the stops  44 ,  46  are fixed in an axial direction. 
         [0046]      FIG. 1  shows a situation in which the rotor  30  is held in a first position  6  by the permanent magnets  12 ,  14  in which the rotor  30  abuts the proximal stop  46 . The second position would be the one in which the rotor  30  abuts the distal stop  44 . 
         [0047]    If a change in the position of the rotor  30  is desired from the first position  6  into the second position  8 , current is supplied to the coil  16 , and the magnetic field electromagnetically generated by the coil  16  passes through the back-iron element  18  and the stator pole shoes  20 ,  22 , as well as through the tube  2  into the pole shoes  32 ,  34  of the rotor  30 , as well as to the permanent magnetic fields of the ring permanent magnets  12 ,  14 . In this case, this magnetic field generated by the coil  16  is oriented such that it supports the magnetic field generated by the ring magnet  14  and counteracts the magnetic field generated by the ring magnet  12 . Since the geometry of the proximal rotor pole shoe  34  depicted in  FIG. 1  completely covers the proximal stator pole shoe  22 , a very efficient magnetic flux is realized in this case, and a strong switching force is exerted on the rotor  30 . At the same time, the holding force exerted by the distal ring magnet  12  is reduced. After switching, i.e., after the rotor  30  reaches the second position  8 , the supply of current to the coil  16  is interrupted, and it accepts the holding force of the permanent magnetic field of the ring magnet  14 . 
         [0048]    The bistable electromagnetic actuator has ring magnets  12  and  14  that can comprise hard magnetic particles which are embedded in a plastic matrix. This allows the production of the ring permanent magnets to be very easy, economical and basically identical. In the embodiment according to  FIG. 1 , an opening, or respectively passage  40 , is schematically indicated in the proximal ring permanent magnet, or respectively proximal ring magnet  14 , through which a coil wire  42  is guided to the coil  16 . Correspondingly for reasons of symmetry, the distal ring magnets  12  can also have a corresponding opening or passage  40  that, however, is not shown. This opening, or respectively passage  40 , can be arranged at a location in the ring magnet  12 , or respectively  14 , such that the symmetry across the axis  4  is interrupted with regard to the opening, or respectively passage  40 . In addition, the proximal stator pole shoe  22  in the exemplary embodiment according to  FIG. 1  also has a corresponding penetration  40  through which the coil wire  42  is guided. This can keep the diameter of the bistable electromagnetic actuator very small, and the coil wire  42  is furthermore guided protected to the coil. 
         [0049]      FIG. 2  schematically portrays a sectional view of a proximal ring magnet  14 . An opening  41  is shown such that the ring magnet material is completely absent in one region. The coil wires  42  and  43  can then be guided through this region. 
         [0050]      FIG. 3  shows another schematic sectional representation of a ring magnet  14 . The opening  41  is configured to be significantly smaller in this case. Magnetic material is still provided around the opening, although from the outside. 
         [0051]      FIG. 4  shows another schematic sectional representation of a ring magnet  14  in which a passage  40  is provided. This passage can be provided while injection molding, or subsequently drilled. Both ring magnets  12  and  14  can be provided with equivalent openings  41 , or respectively an equivalent passage  40 . A corresponding passage or a corresponding opening  41  can also be provided in one or both stator pole shoes  20 ,  22 . 
         [0052]    In the production of ring magnets in an injection molding method comprising hard magnetic particles embedded in a plastic matrix, only a very small deviation in the size of the respective ring magnets is anticipated within one batch. Deviations as such can only arise from the shrinkage of the material after injection molding, wherein the shrinkage of one batch is substantially the same. In addition, the shrinkage of the mixture consisting of the plastic matrix and hard magnetic particles is reduced since there are also magnetic particles, and in principle, only the plastic matrix shrinks during hardening. Corresponding ring permanent magnets that consist of a plastic matrix and hard magnetic particles were produced which, for example, have an outer diameter of 2.26 mm, an inner diameter of 1.7 mm, and a width of 0.35 mm. Maximum deviations from magnet to magnet of 5 μm were found. With magnets that are exclusively produced from hard magnetic materials such as NdFeB or SmCo, the deviations are approximately 20 μm given correspondingly large ring magnets. 
         [0053]    While there has been shown and described what is considered to be preferred embodiments, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1  Actuator 
           2  Tube 
           4  Central axis 
           6  First position 
           8  Second position 
           10  Stator 
           12  Distal ring magnet 
           14  Proximal ring magnet 
           16  Coil 
           18  Back-iron element 
           20  Distal stator pole shoe 
           22  Proximal stator pole shoe 
           24  Plane of symmetry of the stator 
           30  Rotor 
           32  Distal rotor pole shoe 
           34  Proximal rotor pole shoe 
           36  Gap 
           38  Plane of symmetry of the rotor 
           40  Passage 
           41  Opening 
           42 ,  43  Coil wire 
           44  Distal stop 
           46  Proximal stop