Abstract:
A proportionally-controllable solenoid including primary and secondary pole pieces that surround an armature axially movable along an axial channel within the pole pieces. The pole pieces are separated axially by an air gap and are conically tapered toward the gap. The leading and trailing faces of the armature are substantially orthogonal to its direction of travel to minimize radial parasitic forces on these faces. Further, the armature is also lengthened such that the trailing face is always outside the axial channel in the secondary pole piece at all points of axial travel to minimize the pullback effect of axial flux vectors on the trailing face.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a Continuation-In-Part of a pending application, U.S. patent application Ser. No. 09/659,183, filed on Sep. 11, 2000 by R. A. Bircann et al. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to electric solenoids; more particularly, to solenoids which are useful as linear actuators of mechanical apparatus, for example, poppet valves; and most particularly, to an improved proportional solenoid actuator having a higher actuation force over an longer range of travel than a comparable prior art solenoid actuator.  
         BACKGROUND OF THE INVENTION  
         [0003]    Solenoids are employed as actuators in many different applications. Within the automotive arts, solenoids are known to be used as linear actuators for, among others, braking systems, transmission systems, and gas management valves such as exhaust gas recirculation valves. In many such applications, the need is for a device to operate in an on/off mode, such as to actuate a valve between open and closed positions. In such solenoid applications, high response speed and high actuation force are generally desirable. In other applications, however, such as in metering the flow of gas through a poppet valve, proportional control of the valve pintle may be required, wherein the axial position of the valve head must be highly controlled at any desired location between fully open and fully closed.  
           [0004]    Force and stroke of a solenoid are interrelated. Force is an important parameter because it overcomes all loads, including parasitic loads of friction, air resistance, mass (inertia), and return spring. Stroke is important, especially for a valve application, because (in the absence of flow passage limitations) the longer the stroke the greater the permissible flow control range of the valve.  
           [0005]    The armature of a solenoid is subject to magnetic forces resulting from the magnetic field formed by the solenoid windings. At any point in the flux field entering or leaving the armature, the flux lines have both axial and radial vector components. In general, radial forces on the armature are parasitic, causing friction, and axial forces are beneficial; in some solenoids, the ratio between parasitic and axial forces may exceed 10:1. However, in proportional control solenoids, sensitive control is achieved by configuring the solenoid such that the pole pieces which magnetically engage the armature surround the armature. Further, the pole pieces may be tapered adjacent to the air gap between them such that magnetic saturation of the pole pieces is progressive as the armature slides within them, enhancing proportional control.  
           [0006]    In a well-known configuration of a solenoid-actuated valve, the valve is opened by the action of the solenoid and is closed by a compressed return spring when the solenoid is de-energized. Therefore, the solenoid must overcome the resistance of the spring plus any pressure differential across the valve. In addition, in known solenoid actuators, the solenoid must overcome a phenomenon known in the art as magnetic “pull-back.” A solenoid armature is not disposed symmetrically axially within the windings but typically has a “leading face,” oriented toward the device to be actuated, recessed within the windings, and a “trailing face,” oriented away from the device to be actuated. In a typical solenoid, when the solenoid is de-energized, the trailing face is at or near the corresponding axial face of the pole piece. When the solenoid is energized, the trailing face of the armature is drawn into the pole pieces. At all stages of axial travel of the armature, the trailing face is subject to axial magnetic flux which creates a parasitic restraining force. The strength of this force is non-linear and varies with the axial position of the armature. This pull-back force must be overcome in any proportional control scheme, and its strength and non-linearity make sensitive proportional control very difficult.  
           [0007]    Further, radial parasitic forces are proportional to the armature area subject to radial flux vectors within the pole pieces. The cylindrical surface of the armature and also its leading (axial) face are both subject to such radial vectors.  
           [0008]    The practical effect of these phenomena, especially in many automotive applications, has been to limit the use of solenoid actuators to applications requiring stroke lengths substantially less than 5 mm and requiring relatively low actuating forces.  
           [0009]    What is needed is a proportionally-controllable solenoid actuator wherein pullback force and radial parasitic forces are minimized to permit longer stroke lengths and higher actuating forces.  
           [0010]    It is a principal object of the present invention to provide a proportionally-controllable solenoid actuator having a long stroke length.  
           [0011]    It is a further object of the invention to provide a proportionally-controllable solenoid actuator having a high actuating force.  
         SUMMARY OF THE INVENTION  
         [0012]    Briefly described, a proportionally-controllable solenoid actuator in accordance with the invention includes primary and secondary pole pieces that surround an armature axially movable along an axial channel within the pole pieces. The pole pieces are separated axially by an air gap and preferably are conically tapered toward the gap to cause progressive saturation with movement of the armature. The armature is substantially squared off on its leading and trailing faces, orthogonal to its direction of travel, to minimize radial parasitic forces on these faces. Further, the armature is also lengthened such that the trailing face is always outside the axial channel at all points of axial travel, to minimize the pull-back effect of axial flux vectors on the trailing face. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    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:  
         [0014]    [0014]FIG. 1 is an elevational cross-sectional view of a solenoid actuated poppet valve assembly in accordance with the prior art;  
         [0015]    [0015]FIG. 2 is an elevational cross-sectional view of an improved proportionally-controllable solenoid actuated poppet valve in accordance with the invention;  
         [0016]    [0016]FIG. 3 is a detailed view of a portion of the solenoid shown in FIG. 2; and  
         [0017]    [0017]FIG. 4 is a graph showing actuating force and control range of both a proportionally-controllable solenoid actuated poppet valve in accordance with the invention and a prior art solenoid actuated poppet valve. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 1, a prior art solenoid actuated poppet valve assembly  10  is shown substantially as disclosed in U.S. Pat. No. 5,433,309 (&#39;309) issued Aug. 22, 1995 to Beck. Valve elements of the disclosure that are not relevant to the present discussion are omitted for clarity. Assembly  10  is shown in the solenoid de-energized position. Assembly  10  includes a valve body  12  supporting a solenoid actuator  14 . Actuator  14  includes a primary pole piece  16 , referred to in &#39;309 as a “magnet core,” disposed coaxially in a well  18  formed in valve body  12  and extending into a guide tube  20  slidably containing an armature  24 . Armature  24  supports a valve pintle  26  extending through an axial bore  28  in primary pole piece  16  for actuating a valve head (not shown) in valve body  12 . A return spring  27  disposed between pole piece  16  and armature  24  urges armature  24  away from pole piece  16  when the solenoid is de-energized. A gap  30  between pole piece  16  and armature  24  defines the length of stroke of actuator  14 . Surrounding guide tube  20  is a secondary pole piece  32  separated from primary pole piece  16 , a windings core  34 , windings  36 , and a housing  38 . Guide tube  20  is disposed in an axial channel  21  formed in secondary pole piece  32 .  
         [0019]    Assembly  10  is said in &#39;309 to be useful for proportional control of fluid flow through a valve. As shown in &#39;309, the valve is open in the solenoid de-energized state and the solenoid actuator must overcome both the return spring and the hydraulic force of the flowing medium. The pressure drop across the valve increases as the valve closes, being maximized when the valve is fully closed and flow is deadheaded. It is recited in &#39;309 that the truncated-cone relationship between armature  24  and pole piece  16 , defining the shape of gap  30 , confers a proportional relationship on the travel of the armature toward and away from the pole piece. However, the driving force for the armature is the axial magnetic attraction between the armature leading face  40  and the primary pole piece face  42 , which force changes non-linearly and with a high and positive slope as a function of the axial dimension of gap  30 . Further, the conical face  40  of armature  24  intercepts radial flux vectors, thus increasing parasitic losses from friction between the armature and the guide tube.  
         [0020]    Further, armature  24  extends beyond the outer axial face  44  of secondary pole piece  32  in the form of an armature extension  46  having a crenelated axial profile  49 . Reasons for the profile are not elaborated in the &#39;309 disclosure; however, profile portions  50 ,  52  appear to be substantially orthogonal to the direction of travel of the armature and are close to face  44  and thus can be expected to contribute to parasitic force losses from magnetic pull-back. In fact, portion  52  in the solenoid rest position is nearly coplanar with pole piece face  44  and thus is particularly vulnerable to pull-back effect. Other, non-orthogonal portions of profile  49  are vulnerable to parasitic force loss from interception of radial flux vectors. Sensitive proportional control can be difficult to achieve with this actuator.  
         [0021]    Referring to FIGS. 2 and 3, an improved proportionally-controllable solenoid actuated poppet valve assembly  10 ′ in accordance with the invention includes an improved solenoid actuator  14 ′ for controlling a valve which may be, for example, an exhaust gas recirculation valve for an internal combustion engine. Actuator  14 ′ includes a conventional coil  36 ′ wound on a core  34 ′ and surrounded by a housing  38 ′. An armature  24 ′ is disposed for sliding axial motion in a guide tube  20 ′. A primary pole piece  16 ′ includes an annular portion  54  extending axially from a base portion  56  and surrounding the inner end of guide tube  20 ′. Portion  54  preferably overlaps slightly the leading edge  40 ′ of armature  24 ′ when the armature is in the rest position, as shown in FIGS. 2 and 3. Preferably, portion  54  is tapered as shown, preferably a conical taper. An improved secondary pole piece  32 ′ is disposed surrounding guide tube  20 ′, similar to the arrangement in the prior art, guide tube  20 ′ being disposed in an axial channel  21 ′ formed in the polepieces  16 ′,  32 ′; however, portion  56  is tapered, opposite to portion  54 , and an air gap  30 ′ is provided therebetween. Tapering the two pole pieces as shown causes magnetic flux lines to be concentrated therebetween, creating an intense magnetic field in gap  30 ′ and extending into armature  24 ′ for axially urging armature  24 ′ toward valve body  12 ′ when the solenoid is energized.  
         [0022]    Referring to armature  24 ′, it can be seen that the armature differs from prior art armature  24  in at least three important ways, all of which serve to increase the working force of the solenoid by decreasing parasitic losses.  
         [0023]    First, leading face  40 ′ is squared off to be substantially orthogonal to the direction of travel  51  of pintle  26 ′, thus minimizing radial flux vector losses. Well  58  for retaining and centering return spring  27 ′ is substantially central to the armature and thus is exposed to primarily axial flux vectors. Leading face  40 ′ may be slightly chamfered or radiused (not shown) where it meets side wall  60  of the armature, to facilitate assembly, without significant performance compromise.  
         [0024]    Second, side wall  60  of the armature is extended beyond outer axial face  44 ′ of secondary pole piece  32 ′ such that the end  62  of side wall  60  does not break the plane of face  44 ′ during the normal operational stroke of the solenoid.  
         [0025]    Third, trailing face  64  of armature  24 ′ is squared off to be substantially orthogonal to the direction of travel  51  of pintle  26 ′, thus minimizing radial flux vector losses. Further, the combination of the length of side wall  60  extending beyond face  44 ′ and the orthogonality of trailing face  64  minimizes parasitic losses from magnetic pull-back.  
         [0026]    Referring to FIG. 4, the benefits of an improved solenoid actuator  14 ′ in accordance with the invention are shown, the benefits being greater proportionally-controllable force over a greater range of actuation. Curve  66  represents force/distance data obtained from a prior art solenoid actuator. Curve  68  represents force/distance data obtained from a solenoid actuator in accordance with the invention. It is seen that the actuating force for curve  68  is consistently and significantly higher than for curve  66  over a solenoid operating stroke of 5.0 mm. However, the force shown in curve  66   a  at strokes greater than 4.64 mm represents a rapidly increasing component of axial flux as the armature approaches the primary pole piece, and such force is not readily controlled proportionally. Therefore, for proportional control, the prior art actuator is limited to strokes of less than 4.65 mm. On the other hand, the improved actuator is proportionally controllable over a stroke of at least 6.00 mm or even greater.  
         [0027]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.