Abstract:
An electromagnetic actuator includes first winding and second windings passing through a first core and a second core, respectively. The first and second cores are arranged such that the first and second core slots form a gap their respective cores between the first and second windings and the first and second permanent magnets and the first and second core slots have a slot opening width (a), the first and second magnets have a magnet height (h) measured along the central axis that the first and second cores have a core height (H) measured along the central axis and h&gt;a; H&gt;h; and H−h&lt;0.5 H.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to actuators and, in particular, to a constant force, short-stroke electromagnetic actuator. 
         [0002]    A linear actuator is an actuator that creates motion in a straight line, in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery valves and dampers, and in many other places where linear motion is required. A short-stroke electromagnetic actuator is an electromechanical energy conversion device, which converts the electrical energy into mechanical energy of short-distance linear motion. 
         [0003]    There are several manners in which an actuator can be formed. One is to convert a rotary motion in to a linear motion. Another is to apply a current to a winding surrounding a permanent magnet. Application of a current causes the magnet to move and this motion, in turn, causes a plunger attached to the magnet to move to deliver linear motion. However, in such cases, the amount of force provide may be dependent on the location of the magnet relative to the winding and the force is not constant. An electromagnetic actuator that delivers a constant force, regardless of plunger position would be well received by industry. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to one aspect of the invention an electromagnetic actuator is disclosed. The electromagnetic actuator includes an outer housing having a central axis and a plunger contained at least partially within the outer housing and configured to move along the central axis, the plunger including an outer layer and first and second permanent magnets disposed within the outer layer and arranged such they are magnetized in opposite directions and perpendicular to the central axis. The electromagnetic actuator also includes a first core on a first side of the central axis and outside of outer layer, the first core having a first core slot and a second core outside of outer layer on a second side, opposite the first side, of the central axis, second core having a second core slot, and a first winding and second winding, the first winding passing through the first core and the second winding passing through the second core. In this embodiment, the first and second cores are arranged such that the first and second core slots form a gap their respective cores between the first and second windings and the first and second permanent magnets and the first and second core slots have a slot opening width (a), the first and second magnets have a magnet height (h) measured along the central axis that the first and second cores have a core height (H) measured along the central axis. In this embodiment, h&gt;a; H&gt;h; and H−h&lt;0.5 H. 
         [0005]    Also disclosed is a method of forming an electromagnetic actuator that includes: forming a plunger having a central axis, the plunger including an outer layer and first and second permanent magnets disposed within the outer layer and arranged such they are magnetized in opposite directions and perpendicular to the central axis; disposing a first core assembly on a first side of the central axis and outside of outer layer, the first core assembly including a first core having a first core slot and a first winding passing through the first core; and disposing a second core assembly on a second, opposite side of the central axis and outside of outer layer, the second core assembly including a second core having a second core slot and a second winding passing through the second core. In this embodiment, wherein the first and second cores are disposed such that the first and second core slots form a gap their respective cores between the first and second windings and the first and second permanent magnets and the first and second core slots have a slot opening width (a), the first and second magnets have a magnet height (h) measured along the central axis that the first and second cores have a core height (H) measured along the central axis and h&gt;a; H&gt;h; and H−h&lt;0.5 H. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a perspective view of an actuator according to one embodiment; 
           [0009]      FIG. 2  shows a side view of an actuator according to one embodiment; 
           [0010]      FIG. 3  is cross-section of  FIG. 2  taken along  3 - 3 ; 
           [0011]      FIG. 4  is cross-section of  FIG. 2  taken along  4 - 4 ; 
           [0012]      FIG. 5  shows an embodiment of a core that may be used in an actuator according to one embodiment; 
           [0013]      FIG. 6  shows a simplified version of the actuator shown in  FIG. 3  illustrating the flux lines when no current is supplied to it; 
           [0014]      FIGS. 7A-7B  shows the actuator is illustrated in  FIG. 6  and the resulting flux lines when current is applied ( FIG. 7A ) and after the plunger has moved a distance x ( FIG. 7B ) and; 
           [0015]      FIG. 8  shows a graph of the force-position characteristic at constant current. 
       
    
    
       [0016]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Shown in  FIG. 1  is a perspective view of an electro-magnetic actuator  100  according to one embodiment. The actuator  100  includes an outer-housing  102 . The outer housing  102  includes top  104 , first side and a first end  108 . The actuator includes a plunger  120  that extends outwardly from housing  102 . As illustrated, the plunger  120  extends outwardly from an optional housing extension  122 . While not visible in  FIG. 1 , the extension may include a linear bearing. In general, application of a current/voltage to the actuator  100  causes the plunger  120  to move in either the −x or +x direction depending on the polarity of the applied current. The x direction is measured from the middle of location of the middle of the plunger  120  when no current is applied. When conditions shown below are implemented, the force produced at an end of the plunger  120  is, in general, constant, i.e., practically independent of the position + or −x of the plunger  120 . Such an actuator  100  may be used to control a fuel valve in one embodiment. A fuel valve is shown by dashed box  150 . 
         [0018]    Shown in  FIG. 2  is a side view of the electro-magnetic actuator  100 . The actuator  100  includes an outer-housing  102 . The outer housing  102  includes top  104 , first side  106 , first end  108  and a second end  110 . The actuator includes a plunger  120  that extends outwardly from housing  102 . As illustrated, the plunger  120  extends outwardly from an optional housing extension  122  that may include a linear bearing. In general, application of a current/voltage to the actuator  100  causes the plunger  120  to move in either the −x or +x direction depending on the polarity of the applied current. 
         [0019]      FIG. 3  shows a cross-section taken along section line  3 - 3  in  FIG. 2  of an embodiment an electro-magnetic actuator  100  and  FIG. 4  shows a cross-section taken along section line  4 - 4  in  FIG. 2 . The following discussion refers to both figures. 
         [0020]    In this embodiment, the plunger  120  includes an outer layer  302  that is formed of a non-ferromagnetic material. Examples include aluminum, brass, bronze, stainless steel, plastics, etc. 
         [0021]    Disposed with the outer layer  302  are first and second permanent magnets (PMs)  304   a ,  304   b . Each of the magnets  304  are generally flat and have generally rectangular sides and tops and bottoms. From  FIG. 3  the rectangular nature of the top and bottoms is clear and the end rectangular shapes are shown in  FIG. 4 . The PM&#39;s  304   a ,  304   b  are magnetized in opposite directions and the direction from N-S is perpendicular to the direction of motion x. 
         [0022]    The space between ends  310 ,  312  of the plunger  120  and the PM&#39;s  304   a ,  304   b  is filled with by a non-ferromagnetic materials such as aluminum, brass, bronze, stainless steel, plastics, etc. 
         [0023]    A linear bearing  308  is located within the outer housing  120 . In one embodiment and as illustrated in  FIG. 3 , the linear bearing  308  is located within the housing extension  122 . As illustrated, the actuator  100  includes  4  separate ferromagnetic cores  320   a ,  302   b ,  320   c ,  320   d . Each core includes a slot  322   a ,  322   b ,  322   c ,  322   d . One or more of the cores  322  may be laminated, made of sintered powder or even made of solid steel, because the actuator is fed with DC current during operation. 
         [0024]    It shall be understood that the cores on each side of plunger  120  (e.g.  320   a ,  320   b  and  320   c ,  320 ) may be replaced with a single core having slots. For example, cores  320   a  and  320   b  may be combined to form a single core  500  with slots  502  and  504  as shown in  FIG. 5 . The same is also true of cores  320   c  and  320   d . In  FIG. 5 , the core  500  includes a central arm  506  around with winding  360  is wrapped. That is, the winding  360  surrounds the central arm  506 . 
         [0025]    Regardless of the configuration of the core(s), the cores on a particular side (where “side” refers to being disposed on either the left or right side of center line or central axis  350 ) of the plunger  120  include a winding  360  wrapped around adjoining sides of the cores on that side. The windings  360  can be made of round or rectangular wire or can be wound using a copper or aluminum foil. 
         [0026]    As shown in  FIG. 3 , winding  360   a  is on the right side of center line  350  and wraps around adjacent sides of cores  320   a  and  320   b  and winding  360   b  is on the left side of the center line  350  and wraps around adjacent sides of windings  320   a  and  320   b  and winding  360   b . In one embodiment, neither winding  320   a ,  320   b  surrounds the plunger  120 . That is, either winding  320   a ,  320   b  completely encircles an of the permanent magnets  304   a ,  304   b . 
         [0027]    The slots  322  all define an opening that exposes the windings to an adjacent magnet. Stated differently, the slots all define an opening in the core between the windings and the magnets as well as the center line. The outer housing  106  may a width, w, and the cores may a have a length, L, as shown in  FIG. 4 . 
         [0028]    Application of a direct current to both windings  360   a ,  360  be will cause the plunger  120  to move in a first direction (e.g., +x) and application of current of an opposite polarity will have the opposite effect. 
         [0029]    By following the requirements related to the sizing of slots  322 , the magnets and the cores  320 , embodiments may provide a constant force regardless of displacement in the x direction. In one embodiment the slot opening width, a, defines a width of the slots  322  and a magnet height, h, is the height of the magnet taken from 0 in the +x direction and H is the height of the core  320 . With these conventions, the constant force may be created when: 
         [0030]    h&gt;a; 
         [0031]    H&gt;h; and 
         [0032]    H−h&lt;0.5 H 
         [0000]    are all met. 
         [0033]    It shall be understood that in the embodiment of  FIG. 5 , the actual core height is shown as  2 H but is core height of H used, the above requirement still hold. 
         [0034]      FIG. 6  shows an example of a system (taken along line  3 - 3 ) according on one embodiment and related flux lines  600  when no current is applied to the windings. This illustration includes only the PMs  304   a ,  304   b , the cores  320  and the windings  360 . In this state, only the PMs  304  are producing flux. 
         [0035]    Contrasting  FIG. 6  to  FIGS. 7A and 7B  which, respectively, shown the flux lines that exist when a DC current is applied to the windings when the PMs have displace a distance in the +x direction. In particular, in  FIGS. 6, 7A and 7B , the flux lines shown are formed off of a simulation where w=36 mm, the cores are separated by 8 mm, L=30 mm, H=24 mm, h=18 mm, a=4 mm, the width of the PMs=5 mm, the magnetomotive force MMF is 5000 Aturns (ampturns), and the PMs formed of NdFeB grade 35 PMs. 
         [0036]      FIG. 8  shows force plotted against the position of plunger in the x direction.  4 . The constant force F=160 N=16.3 kG=36 lbf is in the interval − 2 &lt;x&lt;+2 mm. At 5000 Aturns the force density is 365.26 N/kg=35.25 kG/kg=82.11 lbf/kg. 
         [0037]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.