Patent Publication Number: US-6670875-B2

Title: Magnetically-efficient solenoid for a linear actuator

Description:
CROSS-REFERENCE TO RELATED-APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/184,514, filed Feb. 24, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates to electric solenoids as used in mechanical linear actuators; more particularly, to such solenoids as may be required to operate without regard to orientation; most particularly to such a solenoid having actuation force maximized by minimization of air gaps in the magnetic pathway within the solenoid. 
     BACKGROUND OF THE INVENTION 
     Electric solenoids are well known in electrical engineering and are widely used as actuating components in electromechanical actuators. A typical electric solenoid consists of a plurality of windings of an electric conductor about north and south polepieces. When current is passed through the windings, a characteristic toroidal magnetic field is produced having field lines at the axis which are parallel to the axis. A ferromagnetic armature is slidably disposed in an axial bore in the polepieces. An axial force is exerted by the magnetic field on the armature which tends to displace the armature axially. The strength of such force can be varied by varying the current flowing through the windings. Thus, by attaching the armature to a shaft, a solenoid may be adapted readily to provide linear mechanical actuation of a device to which it is attached. Solenoids are probably the commonest type of such actuators in use today. 
     The maximum force which may be exerted on the armature is in part a function of the axial size and stability of the cylindrical air gap between the armature and the polepieces. Ideally, the thickness of the air gap is zero, but conversely, the armature must not touch the either of the polepieces. Further, the armature is not spontaneously centered in the bore, and non-axial magnetic vectors within the bore destabilize centering of the armature, resulting in unpredictable variances in the size and shape of the air gap and in the corresponding response of the armature. 
     It is known in the art to provide a lubricious, non-magnetic, cylindrical sleeve in the air gap to keep the armature centered in both of the polepieces and to function as a journal bearing to facilitate low-friction motion of the armature. Such a sleeve can reduce the centering problem but in itself still contributes to the thickness of the non-magnetic gap between the armature and the polepieces, thus limiting the maximum actuating force of the solenoid. Such a sleeve also has frictional contact, however small, with the armature over the full length thereof, through both polepieces. 
     Further, because of necessary tolerances between the sleeve and the armature and between the sleeve and the polepieces, the armature may still be unacceptably decentered by gravity if the actuator is used in orientations wherein the actuator axis is inclined more than about 30° from vertical. Thus, prior art solenoid actuators can impose serious engineering design restrictions in their use. 
     Solenoids are inherently inefficient due to their relatively high radial/axial force ratio. Radial forces on the armature exist because the magnetic field within the windings is fully parallel to the axis of the solenoid only at infinite distances from the axial ends of the windings. At all other locations, because of the magnetic fringing field a significant radial component exists which tends to decenter the armature unpredictably and frictionally against the guiding sleeve. Even in solenoids having the best available lubricious coatings of the guiding sleeve, the ratio of radial-to-axial forces can be as high as 10:1. Because only the axial component of force can be utilized to move the armature axially, the radial forces constitute parasitical friction which must be overcome by the device to perform properly. 
     What is needed is an improved, efficient solenoid which may be used in any orientation without loss in effectiveness, wherein the thickness of the gap between the armature and the polepieces is minimized and controlled to be substantially cylindrical and wherein the reluctance of the magnetic circuit is minimized. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved solenoid for providing linear actuation. The outer polepiece of the solenoid is provided with an axial, self-lubricated, non-magnetic journal bearing for supporting an actuating shaft extending coaxially from the solenoid armature. Preferably, the radial tolerance between the diameters of the bearing inner bore and the shaft is as small as in practically possible without inducing significant drag of the shaft in the bearing. This feature permits elimination of that prior art portion of the guiding sleeve extending into the outer polepiece, thereby reducing frictional losses with the sleeve, and reduction in thickness of the air gap between the armature and the outer polepiece. Further, small prior art air gaps between the pole pieces and the housing are eliminated to reduce reluctance of the magnetic circuit. A significant increase in actuating force is realized in comparison with a prior art solenoid actuator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a prior art solenoid actuator; 
     FIG. 2 is a cross-sectional view of a solenoid actuator in accordance with the invention; and 
     FIG. 3 is a cross-sectional view of an actuator in accordance with the invention operationally attached to an exhaust gas recirculation (EGR) valve on an internal combustion engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The benefits afforded by the present invention will become more readily apparent by first considering a prior art solenoid actuator. Referring to FIG. 1, a prior art actuator  10  includes a housing  12  containing first and second pole pieces  14 , 16 , respectively, and a plurality of windings  18  about the polepieces. A ferromagnetic armature  20  is slidably disposed within a stepped first axial bore  21  in the pole pieces. An actuating shaft  22  is axially disposed and retained within armature  20  and extends from housing  12  via a second axial bore  24  in polepiece  16  for connection to work. Step  26  in bore  21  receives a coil spring  28  disposed in compression between step  26  and a well  30  in armature  20  for biasing the armature into the solenoid. A generally cylindrical non-magnetic sleeve  32  surrounds armature  20  and spring  28  for slidably guiding and centering the armature axially of polepieces  14  and  16 . Typically, the sleeve is formed of a non-galling non-ferromagnetic material such as stainless steel or ceramic, and either the sleeve or the armature may be coated with any of various well-known dry lubricants. 
     Referring to FIG. 2, embodiment  34  of an improved half-sleeve solenoid actuator in accordance with the invention comprises several elements analogous to elements in prior art actuator  10 : housing  12 , first and second polepieces  14 , 16 , and windings  18 . Sleeve  32 ′ is limited in axial length to approximately the length of the axial portion  35  of inner or first polepiece  14 . Air gap  36  is shown substantially larger than to scale for illustration purposes; preferably, the distance between outer or second polepiece  16  and armature  20 ′ is on the order of a small fraction of a millimeter to minimize its contribution to magnetic reluctance. A working shaft  22 ′ is press-fit into armature  20 ′. An axial bore  24 ′ in second polepiece  16 , alternative to bore  24  in the prior art actuator, retains a spool bearing  40  for radially supporting shaft  22 ′ in axial motion. As already described, shaft  22 ′ is preferably fitted to the bore in bearing  40  as closely as possible without causing drag on the shaft. Bore  41  in bearing  40  is coated with a permanent dry lubricant such as a fluorocarbon polymer; preferably, bearing  40  is a commercially-available coated non-magnetic metal bearing element, for example, a Norglide bearing available from Saint-Gobain Performance Plastics Corporation, Wayne, N.J., USA, or a Permaglide Plain bearing available from INA Waelzlager Schaeffler GmbH, Herzogenaurach, GERMANY. Preferably, sleeve  32 ′ is also formed from this or a similar material. Preferably, the axial length of bearing  40  is at least 1.5 times the diameter of shaft  22 ′ to minimize wobble of the shaft in the bearing and resulting cocking of the armature in the polepieces. 
     It is important that bearing  40  be formed of non-ferromagnetic material because the bearing also acts as a fixed stop to limit the travel of the armature. If bearing  40  were ferromagnetic, the armature would become magnetically latched to the bearing, interfering with operation of the actuator. 
     Because air gap  36  between armature  20 ′ and polepiece  16  is substantially fixed in size and shape by a combination of sleeve  32 ′ and bearing  40 , as well as being reduced to a minimum thickness, the armature cannot strike the polepieces. Thus, solenoid actuators in accordance with the invention may be used freely without regard to spatial orientation. This feature can be extremely useful, for example, in fitting an EGR valve into the engine compartment of a vehicle. 
     The magnetic circuit in the solenoid passes through polepieces  14 , 16  and housing  12 . Any air gap in the magnetic circuit increases reluctance and, consequently, reduces magnetic flux and force potential. Because of the relatively high reluctance of air, compared to magnetic material in the circuit, significant gains in field strength can be achieved by minimizing or, preferably, eliminating all such gaps. Therefore, a solenoid in accordance with the invention is preferably assembled by “Magneforming,” a proprietary technique of the Maxwell Magneform Company, San Diego, Calif., USA, wherein ferromagnetic components are thrust together under very high forces produced by magnetic fields. In solenoids  10  and  34 , critical interfaces  42 , 44  exist between first polepiece  14  and housing  12  and between second polepiece  16  and housing  12 , respectively. Gaps at these interfaces in improved solenoid  34  may be effectively eliminated, and interface reluctance reduced to substantially zero, through use of the Magneform process, which forces mating components to come into contact with each other in the closest possible relationship short of actual fusion. Magneforming is highly superior to mechanical swaging or staking of the housing to the polepieces as is common in prior art solenoids. 
     The combination of minimal air gap  36 , afforded by centering of the armature in sleeve  32 ′ and bearing  40 , and elimination of air gaps at interfaces  42  and  44 , allows the highest force potential attainable for a solenoid of any given size. 
     Because solenoid  34  may be employed in an actuator in any orientation rather than essentially vertically and shaft-down as in prior art solenoid  10 , a hazard may be created wherein intrusive moisture or condensation is trapped within the actuator, leading to corrosion and failure. Accordingly, drainage preferably is provided from solenoid  34 , for example, via a plurality of inner vents  46  and outer vents  48  radially disposed preferably at 90° spacing in the solenoid. 
     Referring to FIG. 3, embodiment  34  is shown mounted via standoffs  51  onto an EGR valve  52  to form an EGR valve assembly  53  which is bolted to the exhaust manifold  54  and intake manifold  56  of an internal combustion engine. Shaft  22 ′ engages the outer end  58  of the pintle  60  of valve  52  to open and close valve head  62  from valve seat  64  to selectively admit exhaust gases from exhaust manifold  54  into intake manifold  56  to reduce smog emitted by the engine. Of course, if desired, shaft  22 ′ can be continuous like pintle shaft  22  in FIG. 1 between valve head  62  and armature  20 ′, within the scope of the invention. 
     The foregoing description of the preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive nor is it intended to limit the invention to the precise form disclosed. It will be apparent to those skilled in the art that the disclosed embodiments may be modified in light of the above teachings. The embodiments described are chosen to provide an illustration of principles of the invention and its practical application to enable thereby one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, the foregoing description is to be considered exemplary, rather than limiting, and the true scope of the invention is that described in the following claims.