Abstract:
A solenoid for use in a valve such as an exhaust gas recirculation valve for a motor vehicle. The solenoid includes a housing having a coil for generating a first magnetic field. An armature is slidably mounted in the housing. The solenoid further includes a permanent magnet having a second magnetic field, wherein the magnet is located adjacent the armature. In addition, a stator is affixed in the housing for cooperation with the armature and the magnet to form a third magnetic field.

Description:
CROSS REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM 
   This application is a division of Ser. No. 10/253,198 filed Sep. 24, 2002 now U.S. Pat. No. 6,787,946 entitled ACTUATOR HAVING A PERMANENT MAGNET. 
   This application claims the benefit of U.S. Provisional Application No. 60/373,382 filed on Apr. 12, 2002 in the name of Gilles Delaire and Frederic Gagnon and entitled USE OF A MAGNET IN A LINEAR SOLENOID ACTUATOR, which is incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates to evaporative emission control systems for internal combustion engines, and more particularly, to an actuator having a permanent magnet for increasing armature force and displacement. 
   BACKGROUND OF THE INVENTION 
   Many motor vehicles utilize actuators such as solenoids to operate several types of devices. This includes devices such as a fuel cell valve or an exhaust gas recirculation (EGR) valve used in an EGR system. In such systems, the EGR valve is controlled by a circuit in accordance with various engine operating conditions to regulate the amount of engine exhaust gas that is recirculated back into the engine for combustion. This serves to limit the combustion temperature and hence reduce the formation of oxides of nitrogen. 
   Solenoids typically utilize an electromagnet coil to generate a magnetic force which causes an armature to move along an axis. The armature may be part of a mechanism for operating a valve, such as an EGR valve. Referring to  FIG. 1 , an enlarged view of a first magnetic flux density  10  located near portions of a lower stator  12 , upper stator  46 , gap  36 , first armature  14  and coil  16  of a conventional solenoid is shown. It is noted that the configuration shown is substantially symmetrical about an axis of the solenoid and that only one side of the axis is shown for purposes of clarity. The lower stator  12  has a frusto-conical shape having a predetermined geometry and is separated from the upper stator  46  by a gap  36 . The shape of the lower stator  12  along with the size of the gap  36  and other parameters are selected so as to optimize a flux path that forms a part of a magnetic circuit. This provides a desired solenoid characteristic in that the armature force is substantially constant with respect to armature displacement. Referring to  FIG. 1 , the magnetic flux density vectors are oriented in a substantially clockwise configuration in a lower portion  18  of the first armature  14  adjacent to the lower stator  12 . In addition, the magnetic flux density vectors are relatively dispersed along an edge  20  of the lower portion  18 . In this configuration, the magnetic flux density in an upper section  22  of the lower stator  12  ranges from approximately 1597 to 2195 kiloGauss (kGauss). 
   It is desirable that solenoids used in motor vehicles provide increased armature force and increased travel so as to improve controllability and increase flow. However, this would require larger solenoids and the amount of space available in current vehicle engine compartments is limited. 
   SUMMARY OF THE INVENTION 
   A solenoid which includes a housing having a coil for generating a first magnetic field. An armature is slidably mounted in the housing. The solenoid further includes a permanent magnet having a second magnetic field, wherein the magnet is located adjacent the armature. In addition, a stator is affixed in the housing for cooperation with the armature and the magnet to form a third magnetic field. 
   The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, may be best understood by reference to the following description taken in conjunction with accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view of a first magnetic flux density in a conventional solenoid. 
       FIG. 2  is a cross sectional view of a solenoid having a permanent magnet in accordance with the present invention. 
       FIG. 3  is a view of a second magnetic flux density in accordance with the present invention. 
       FIG. 4  depicts first and second curves which show force and displacement properties for a conventional solenoid and for a solenoid of the present invention, respectively. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of  FIGS. 1-4 . 
   Referring to  FIG. 2 , a cross sectional view of an actuator such as a solenoid  24  including a permanent magnet  26  in accordance with the present invention is shown. The solenoid  24  includes a housing  28  having an internal passageway  30  which is in fluid communication with an outlet port  32  and an inlet port  34 . When the solenoid  24  is used in an exhaust gas recirculation (EGR) valve, the inlet port  34  is in fluid communication with engine exhaust gas and the outlet port  32  is in fluid communication with an engine induction system of an internal combustion engine. It is noted that the current invention may be used in other types of devices that use actuators, such as fuel cell valves. 
   The coil  16  is symmetrically disposed about a first axis  40  of the housing  28 . The housing  28  includes a second armature  42  that is affixed to a valve stem  44 . A portion of the second armature  42  is located within an upper stator  46  that is affixed to the housing. The upper stator  46  is separated from the lower stator  12  by the gap  36 . The lower stator  12  is symmetric about the first axis  40  and is located adjacent the second armature  42 . The lower stator  12  includes a first bore  48  and a second bore  50  of reduced size that is bounded by an end wall  52 . The lower stator  12  further includes an axially extending side wall  54 . A tapered wall  56  extends from the side wall  54  toward the first axis  40  and terminates at a tip surface  64  adjacent the magnet  26  to form a substantially frusto-conical configuration. 
   The permanent magnet  26  includes top  60  and bottom  62  surfaces. The magnet  26  is substituted for a portion of the first armature  14  in a conventional solenoid such that the overall size of the second armature  42  and magnet  26  is substantially equivalent to that of the first armature  14 , resulting in a solenoid of substantially the same size. The magnet  26  is positioned in the first bore  48  such that the top surface  60  is adjacent the second armature  42  and substantially colinear with the tip surface  64 . As will be described, the magnet  26  is located so that its magnetic field is added to the first magnetic field  10  generated by coil  16 . This results in a solenoid having a substantially higher armature force within substantially the same solenoid volume. 
   The housing  28  further includes a spring  66  located in the first  48  and second  50  bores between the bottom surface  62  of the magnet  26  and the end wall  52 . A bearing member  68  is affixed within the housing between the end wall  52  and the passageway  30 . The stem  44  extends through the magnet  26 , spring  66 , end wall  52 , bearing member  68  and into the passageway  30 . A bottom end  69  of the stem  44  includes a valve head  70  shaped for cooperation with a valve seat  72  formed in the inlet port  34 . 
   The bearing  68  enables movement of stem  44  along the first axis  40 . This enables movement of the valve head  70  between open and closed positions. In the open position, the valve head  70  is spaced downward from the seat  72  to enable fluid communication between the inlet  34  and outlet  32  ports. In the closed position, the valve head  70  contacts the seat  72  to thus close the inlet port  34  as shown in FIG.  2 . The spring  66  is biased against the bottom surface  62  of the magnet  26  to urge the magnet  26  and thus the valve head  70  to the closed position. Upon energization of the coil  16 , a magnetic field is generated which is sufficient to overcome spring bias to cause downward movement of the second armature  42  and place the valve head  70  in the open position. The housing  28  also includes a connector  74  which serves to transmit electrical power from a power source to the coil  16  for forming the first magnetic field  10 . 
   Referring to  FIG. 3 , an enlarged view of balloon section  76  of  FIG. 2  is shown.  FIG. 3  depicts a second magnetic flux density  78  located near portions of the lower stator  12 , upper stator  46 , magnet  26 , second armature  42  and coil  16  in accordance with the present invention. The top surface  60  of the magnet  26  is located adjacent the second armature  42  so that its magnetic field is added to the first magnetic field  10  generated by the coil  16  to thus form the second magnetic flux density  78 . The flux density vectors are oriented in a substantially counterclockwise configuration in the magnet  26 , second armature  42 , and gap  36 . Further, the magnetic flux density vectors are concentrated at the tip surface  64 . This serves to increase the force on the second armature  42 . In addition, the magnetic flux density in the upper section  22  of the lower stator  12  is approximately 2524 kGauss, resulting in an increased flux density in the upper section  22  over that of conventional solenoids. It is noted that the shape of the lower stator  12 , size of the gap  36  and other associated parameters may be optimized for use with the magnet  26 . 
   Referring to  FIG. 4 , first  80  and second  82  curves depicting force and displacement properties for a conventional solenoid and for the solenoid  24 , respectively, are shown. In particular, it can be seen that armature force for a given armature displacement is substantially increased for the solenoid  24  relative to that of a conventional solenoid. This results in substantially improved valve performance. 
   While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.