Abstract:
The force motor of the present invention controls the local magnetic field through a uniquely designed mechanical structure of the internal components. The mechanical structure divides the magnetic field in the force motor into three sections. The force produced on the armature by the magnetic field in the first section increases exponentially as the armature approaches the housing. The force produced on the armature by the magnetic field in the second and the third sections, as the armature approaches the housing, counter balances the rise in the force due to the magnetic field in the first section. Thus, a flat F-S curve over a long stroke length is obtained.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This disclosure relates generally to a linear actuated force motor that requires low power input and provides a long proportional stroke. More particularly, this disclosure relates to a technique to control local magnetic field distribution so as to provide a long proportional stroke. 
   2. Description of the Related Art 
     FIG. 1  shows a cross-sectioned view of a conventional force motor. A conventional force motor includes a shaft  1  mounted in bearings  2  that are mounted in a housing  3 . An armature  4  is mounted on the shaft. Two springs  5  and  6  are mounted on the shaft with the armature located between the springs. The springs keep the armature in the neutral position when no net axial force is being exerted on the armature. The armature shaft is free to slide on the bearings in axial directions. A permanent magnet  7  is located at the periphery of the armature. Two coils  8  and  9 , wound in the same direction are located on each side of the permanent magnet. 
   The permanent magnet produces a magnetic field B p . When energized, the coils produce a magnetic field B i . Since the coils are wound in the same direction the magnetic field B i  produced by the coils is in the same direction as the magnetic field B p  on one side of the permanent magnet and in the opposing direction on the other side of the permanent magnet. Thus, the resultant magnetic field on one side of the permanent magnet is B p +B i  and on the other side of the permanent magnet is B p −B i . See  FIG. 2 . The electrical force produced on the armature is proportional to the square of the magnetic field and can be calculated as follows.
 
F=KB 2   Eqn. 1
         Where F=electrical force
           B=Magnetic flux density   K=Constant
 
Using equation 1, the net force on the armature of a force motor when the coils are energized can be calculated as follows:
   
               

                         F   fm     =     K   ⁢     {         (       B   p     +     B   i       )     2     -       (       B   p     -     B   i       )     2       }                     =     4   ⁢           ⁢     KB   p     ⁢     B   i         ⁢                         Eqn   .           ⁢   2               
For a proportional solenoid wherein a coil produces a magnetic field equal to Bi, the net force on the armature can be calculated using equation 1 as follows:
 F ps =KB i   2   Eqn. 3     Now if
 
B p &gt;B i  
 
then
 
4B p &gt;&gt;B i  
 
Therefore
 
F fm &gt;&gt;F
 
Thus, by using a permanent magnet, for a given level of coil energization (i.e. current), the force motor produces larger net force on the armature. Therefore, for a given force requirement the force motor can be operated with lower power input compared to the proportional solenoid. If B p  is assumed to be constant in equation 2, it is clear the net force is proportional to the magnetic field produced by the coils.
 
F fm =CB i   Eqn. 4
   where
       C=4KB p , assuming B p =constant   
       Since B i  is proportional to I   where I is the current supplied to the coils,
       F fm  is proportional to I
 
i.e. the net force on the armature is proportional to the current supplied to the coils.
   
       
   However, B p  can be assumed to be constant only when the armature is in the neutral position. As the armature moves away from the neutral position, B p  changes. When the armature moves, B p  on one side of the armature increases whereas B p  on the other side of the armature decreases. This results in a dramatic increase in the net force on the armature. Thus, in a conventional force motor, the force is proportional to the stroke only within a small range of the stroke, for example 0.01 to 0.03 inches. 
   U.S. Pat. No. 5,787,915 describes a conventional force motor having a permanent magnet and coils. However, it does not teach any means of providing increased proportional stroke. 
   U.S. Pat. No. 3,900,822 (the &#39;822 Patent) describes a conventional proportional solenoid with a conical pole piece on each side of the bobbin. When the solenoid is energized, the armature is pulled to one side and enters into the conical pole piece. The conical pole piece provides a leakage flux path and thereby reduces the increase in the net force on the armature. The proportional solenoid similar to that of the &#39;822 Patent requires higher power input compared to the force motor of the present invention to produce the same amount of force on the armature. 
   The use of a conical pole piece as taught by the &#39;822 Patent does not provide a substantial increase in proportional stroke. Additionally, when a conical pole piece is used, the proportionality and the constancy of the net force on the armature gets worse with increase in current (I) supplied to the coils or when the plunger position changes. 
   SUMMARY 
   None of the above mentioned patents teach a force motor with a long proportional stroke with a flat force versus stroke characteristic (F-S curve) and low power input. 
   The force motor of the present invention overcomes the aforesaid shortcomings of the prior art by controlling the local magnetic field through a uniquely designed mechanical configuration of the internal components. The mechanical configuration divides the magnetic field in the force motor into three sections. In operation, as the armature moves in the axial direction towards the end of the stroke, the force exerted on the armature by a magnetic field in the first section increases exponentially. At the same time, the force exerted by the magnetic field in the third section either has a smaller increase compared to the first section, or decreases. As the armature moves towards the stop, the amount of magnetic flux in the second section increases. The direction of this magnetic field is perpendicular to the armature&#39;s direction of movement and therefore does not produce any force in the direction of the movement thereby reducing the total force on the armature. By adjusting the mechanical parameters associated with the three sections, the net axial force on the armature can be controlled, thereby providing, for a given power level, a flat force vs. stroke curve over a long stroke. 
   It is an object of the present invention to provide a force motor with low power input to achieve a desired force with a flat F-S curve and long proportional stroke when compared to a conventional proportional solenoid. These and other objects are accomplished by providing a housing and an armature movable along an axial direction in the housing wherein the shape of the armature and the housing cooperate to produce a flat F-S curve for the force motor. The invention further contemplates a method of controlling the magnetic field in a force motor to obtain a flat F-S curve by forming a first section having a first magnetic field that produces a force on the armature that increases as the armature approaches the housing and forming a second section and a third section in the force motor. The force on the armature due to the a second magnetic field in the second section and a third magnetic field in the third section, as the armature approaches the housing, counter balances the force on the armature produced by the first magnetic field in the first section to produce the flat F-S curve. 
   Also provided is a housing having an internal wall, a cylindrical extension projecting from the internal wall working as a stop to limit the armature&#39;s movement, and a concave surface formed on the internal wall. An armature supported by the bearing sits in the housing. The armature includes a cylindrical portion connected to a conical section. The shape of the armature and the housing are such that they cooperate to produce a flat F-S curve for the force motor. 
   Further features and advantages will appear more clearly on a reading of the detailed description, which is given below by way of example only and with reference to the accompanying drawings wherein corresponding reference characters on different drawings indicate corresponding parts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS. 
       FIG. 1  is a cross-sectional view of a prior art force motor; 
       FIG. 2  shows a magnetic field produced in the force motor of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the force motor of the present invention; 
       FIG. 4  is a cross-sectional view of another embodiment of the force motor of the present invention; 
       FIG. 5  is an enlarged view of cooperating mechanical structures of the force motor shown as detail E in  FIG. 3 ; 
       FIG. 6  is a conceptual representation of the F-S curve for the three sections formed by the cooperating sections of  FIG. 5 ; 
       FIG. 7  shows F-S curves for a conventional force motor of  FIG. 1  having a greater slope and F-S curves for the force motor of  FIG. 4  which are flat. 
       FIG. 8  shows F-S curves for the force motor of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows a cross-sectional view of the force motor of the present invention.  FIG. 4  shows cross-sectional view of another embodiment of the force motor of the present invention. Force motor  10  includes a shaft  12  which is slidably mounted in bearings  14  and  16 . Armature  18  is firmly mounted on shaft  12 . Springs  22  and  24  are mounted along shaft  12 , one on each side of armature  18 . The assembly of shaft  12 , bearings  14  and  16 , armature  18  and springs  22  and  24  is mounted in a housing  26 . A bobbin  28  is enclosed within housing  26  and is located at the periphery of armature  18 . Bobbin  28  forms three compartments. In the center compartment is located a permanent magnet  32 . Bobbin  28  prevents contaminants from magnet  32  from falling on the armature  18 . Coils  34  and  36  are located one on each side of magnet  32  in the compartments formed by bobbin  28 . 
   Armature  18  is symmetric around the shaft  12  and includes a base  38  connected to a cylindrical portion  42  (see  FIG. 3 ) which in turn is connected to a conical section  44  having cylindrical face  62  (formed by a counter-bore. In embodiment of  FIG. 3 , the large end of the conical section  44  is larger than the cylindrical portion  42 . In the embodiment of  FIG. 4  base  38  is connected to conical section  44  having a cylindrical face  62  which in turn is connected to cylindrical portion  42 . In embodiment of  FIG. 4 , the large end of the conical section  44  is larger than the cylindrical portion  42 . Armature  18  and housing  26  are all made of a ferro-magnetic material that form a magnetic circuit. A stainless steel shim  46  is mounted on cylindrical portion  42  of armature  18 . By varying the thickness of shim  46 , the travel of armature  18  along shaft  12  can be increased or decreased; a thicker shim  46  resulting in a shorter travel distance. Between bobbin  28  and armature  18 , along the periphery of armature  18 , is located a cylindrical copper layer  48  that is firmly attached to the armature  18 . Copper layer  48  induces back EMF to dampen the unexpected movement of the armature caused by vibration, shock, and acceleration. 
   An internal wall  56  of housing  26  is shaped to form a stop  52 . The shape of stop  52  cooperates with the shape of armature  18  to provide control of the magnetic field in the area surrounding the cooperating shapes. Stop  52  includes a cylindrical extension  54  which projects from internal wall  56  of housing  26 . Stop  52  also has a concave conical surface  58  formed on wall  56 . Conical surface  58  corresponds to the conical section  44  on armature  18 . Cylindrical extension  54  corresponds to the cylindrical portion  42  and in cooperation with steel shim  46  determines the maximum stroke length of armature  18 . 
   When coils  34  and  36  are energized by current I, magnetic field B i  is produced. Magnetic field B i  interacts with magnetic field B p  as described previously in reference to the conventional force motor. The action of these two magnetic fields combined produces a net force F fm  on armature  18 . However, as compared to the conventional force motor, the force F fm  for a given I remains constant over a longer stroke length for the reasons explained below. 
   Force motor  10  of the present invention has shaped armature  18  and stop  52 . The magnetic field between armature  18  and stop  52  is divided into three sections.  FIG. 5  is the enlarged view of cooperating mechanical structures of armature  18  and stop  52 . Also shown in  FIG. 5  are the three sections formed by the cooperating mechanical structures.  FIG. 6  shows a conceptual representation of the forces in the three sections formed by the cooperating mechanical structures. 
   The first section is the magnetic field Φ 1  formed between cylindrical portion  42  and internal wall  56 . This is equivalent to a magnetic field inside a solenoid with flat-faced-armature. The characteristics of the force produced by this field are essentially exponential increase when the solenoid is pulled-in towards the stop (see curve A in  FIG. 6 ). 
   The second section is the magnetic field Φ 2  located between face  62  of conical section  44  on the armature  18  and the face  64  of cylindrical extension  54 . As a greater portion of face  62  slides along face  64 , Φ 2  increases. Since Φ 2  is perpendicular to the direction of motion of armature  18 , it does not produce any significant force in the direction of motion. Line B in  FIG. 6  is a conceptual representation of the force produced by Φ 2 , that is about zero all over the stroke length. 
   The third section is the magnetic field Φ 3  located between conical section  44  on armature  18  and the conical face  58  on stop  52 . It is equivalent to a force in a conical-faced-armature solenoid. The characteristics of this force curve produced by Φ 3  is that it is flatter than that of the first section. (See curve C on  FIG. 6  for a conceptual representation). 
   When the armature is pulled-in, the second section of magnetic field Φ 2  takes away the magnetic flux from the first section and the third section. Therefore, the force produced by Φ 1  and Φ 3  is actually reduced due to the increase of leakage flux in the second section, and the force-stoke curves produced by the magnetic field of the first section and the third section drop down (see curve A′ and C′ on  FIG. 6 ). 
   The resultant force F fm  exerted on armature  18  of force motor  10  is the sum of the force represented by curve A′, B, and C′. i.e.
 
 F   fm   =F   Φ1   +F   Φ2   +F   Φ3   Eqn. 5
 
   Thus, by adjusting the cooperating mechanical structures on armature  18  and stop  52 , for example, by varying the shape, size and angles of cooperating mechanical elements, a desired force—stroke characteristics curve can be achieved. Adjustment of force—stroke characteristics may also be done by use of materials with different magnetic properties. A flat F-S curve advantageously allows the use of springs with a smaller spring constant, to have wide range of control and more precise control. 
     FIG. 7  shows F-S curves for a conventional force motor such as shown in  FIG. 1  and force motor  10  of the present invention as shown in  FIG. 4  for comparison.  FIG. 8  shows the F-S curves for the embodiment of the force motor  10  shown in  FIG. 3 . The embodiments shown in  FIG. 3  and  FIG. 4  have a flat F-S curve over the stroke length of 0.0 to 0.065 in. and 0.0 to 0.16 in., respectively while the conventional force motor only has proportional stroke of 0.0 to 0.025 in. The force motors used to obtain the curves had the same external dimensions, used a similar magnet, used similar coils and had the same armature diameter. The only difference between the motors was the presence of cooperating mechanical structures as described previously in reference to force motor  10 . The F-S curves for the conventional force motor are the ones with greater slope and shorter stroke. On the other hand, the F-S curves for the force motor  10  are very much flat over a greatly longer stroke, the proportional stroke length being (0.15 inches) six times the proportional stroke length (0.025 inches) for the conventional force motor. In  FIG. 7 , the substantially constant force is between 0.2 and 2 lbs. with a variation of about 0.2 lbs. maximum for any curve. In  FIG. 8 , the substantially constant force is 0.4 to 5.5 lbs. with a variation of about 1.5 lbs. for any one curve. 
   The invention controls the slope of the F-S curve even if the slope is not driven to zero. As shown in  FIG. 8 , there may be a slight slope. 
   While a preferred embodiment of the invention has been described, various modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims. For example, the local magnetic field may be controlled be varying the shape and size or location of the mechanical configurations in a different manner than described here. The local magnetic field control may also be achieved by using different materials with different magnetic properties.