Patent Publication Number: US-2006011245-A1

Title: Solenoid-operated valve

Description:
INCORPORATION BY REFERENCE  
      This application is based on and claims priority under 35 U.S.C. sctn. 119 with respect to Japanese Application No. 2004-207500 filed on Jul. 14, 2004, Japanese Application No. 2004-279204 filed on Sep. 27, 2004, Japanese Application No. 2005-168621 filed on Jun. 8, 2005, and Japanese Application No. 2005-168622 filed on Jun. 8, 2005, the entire content of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a solenoid-operated valve of the type that a spool of a valve section is operated upon movement in the axial direction of a plunger of an electromagnetic drive section and in particular, to a solenoid-operated valve which is suitable for use to control fluid pressure applied to a clutch mounted as a part in an automatic transmission.  
      2Discussion of the Related Art  
      In general a solenoid-operated valve is used for controlling fluid pressure applied to a clutch which is a part of an automatic transmission equipped in a motor vehicle. Heretofore, as solenoid-operated valves of this type, there has been known one described in a Japanese unexamined, published patent application No. 11-287348 (1999-287348). In the known solenoid-operated valve, a core that is magnetized by a solenoid is provided with an annular tapered portion at its rear end. The tapered portion has an outer surface so inclined that the magnetic attraction force hardly varies as a front end of a plunger is attracted in the tapered portion due to the influence of the inclination of the suitable degrees of the tapered portion. The characteristics that the magnetic attraction force hardly varies as the front end of the plunger moves in the tapered portion is effective to enhance the damping force which acts on the plunger, and thereby to prevent the clutch from being vibrated by the pressure fluctuation when a high electric current is applied to the solenoid and there is no flow of high pressure fluid from the outlet port to the clutch.  
      In a Japanese unexamined, published patent application No. 2000-274546 (P2000-274546A), an annular cylindrical portion is projected from the rear end of the annular tapered portion to increase the magnetic attraction force when the front end of the plunger is at the end of the annular cylindrical portion.  
      In a solenoid-operated valve, when a low electric current is applied to the solenoid so that a low fluid pressure is supplied to the clutch, it is effective to increase the opening degree in communication of an inlet port to which constant pressure fluid is introduced with the outlet port in order to enhance the responsivety of the clutch. On the other hand, the characteristics that the magnetic attraction force hardly varies as the plunger moves is effective to enhance the damping force which acts on the plunger, and thereby to prevent the clutch from being vibrated by the pressure fluctuation when the high increasing electric current is applied to the solenoid. Therefore, such solenoid operated valve is desired that supplies the pressure regulated fluid to the clutch, wherein the responsivety of the clutch is high when a low electric current is applied to the solenoid, and the clutch is reluctant to vibrate when the increasing high electric current is applied to the solenoid.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a primary object of the present invention to provide an improved solenoid-operated valve which is capable of enhancing the responsivety of a device that is supplied fluid pressure from the solenoid-operated valve while an electric current applied to the solenoid is in a low current range, and also capable of preventing the device from vibrated by the fluid pressure fluctuation while an electric current applied to the solenoid is in a high current range.  
      Briefly, according to the present invention, there is provided a solenoid-operated valve which comprises a valve sleeve and a spool slidably received in the valve sleeve for regulating the pressure of fluid supplied thereto, a stator having a core provided with an annular projecting portion, a plunger received in the stator to be slidably guided in an inner bore formed in the stator and being movable into the annular projecting portion, a spring for resiliently urging the spool toward the plunger, an solenoid for magnetizing the stator to attract the plunger against the resilient force of the spring so as to move the plunger into the annular projecting portion. The annular projecting portion is formed with a tapered portion whose sectional area becomes smaller as a section is closer to the rear end thereof and a thin cylindrical portion that projects from the rear end of the tapered portion. A step that has an end surface is formed on the outside of the annular projecting portion between the rear end of the tapered portion and the thin cylindrical portion.  
      With this configuration, since the step is formed on the outside of the annular projecting portion between the tapered portion and the thin cylindrical portion, the tapered portion can be formed as it has an inclination of any suitable degrees, wherein the magnetic attraction force hardly varies as the plunger moves in the tapered portion. Accordingly, the damping force which acts on the plunger is enhanced, to prevent the clutch from being vibrated by the pressure fluctuation when the high increasing electric current is applied to the solenoid. And, the annular cylindrical portion can be formed thin enough to enhance the responsivety of the clutch when a low electric current is applied to the solenoid so that a low fluid pressure is supplied to the clutch. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
      The foregoing and other objects and many of the attendant advantages of the present invention may readily be appreciated as the same becomes better understood by reference to a preferred embodiment of the present invention when considered in connection with the accompanying drawings, wherein like reference numerals designate the same or corresponding parts throughout several views, and in which:  
       FIG. 1  is a longitudinal sectional view of a solenoid-operated valve in the first embodiment according to the present invention;  
       FIG. 2  is an enlarged sectional view showing around a tapered portion of a core;  
       FIG. 3  is a longitudinal sectional view of a clutch that is supplied fluid pressure from the solenoid-operated valve;  
       FIG. 4  is a graph showing the characteristics of a relationship between a magnetic attraction force and a position of the plunger;  
       FIG. 5  is a graph showing the characteristic of the fluid pressure that increases as the time passes;  
       FIG. 6  is a graph showing the characteristics of the fluid pressure that change as an electric current applied to a solenoid increases;  
       FIG. 7  is an enlarged sectional view showing around a tapered portion of a core in the second embodiment;  
       FIG. 8  is an enlarged sectional view showing around a tapered portion of a core in the third embodiment;  
       FIG. 9  is an enlarged sectional view showing around a tapered portion of a core in the forth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Hereinafter, a solenoid-operated valve in the first embodiment according to the present invention will be described with reference to  FIG. 1 . The solenoid-operated valve  10  in this particular embodiment is composed of an electromagnetic drive section  11  and a spool valve section  12  which is fixed to an end of the electromagnetic drive section  11 . The electromagnetic drive section  11  is composed primarily of a cover  14 , core  15 , a yoke  16 , a solenoid  17  and a plunger  18 . The spool valve section  12  is composed primarily of a valve sleeve  19  and a spool  20  slidably received in the valve sleeve  19 .  
      The cover  14  which takes a cylindrical shape with a bottom (i.e., cup shape) accommodates the yoke  16  and the core  15  therein. The cover  14 , the yoke  16  and the core  15  are made of magnetic material. The core  15  is provided with a flange  21  in the opening portion of the cover  14  and a cylindrical portion  22  which extends from the flange  21  towards the bottom of the cover  14 . The yoke  16  is provided with a flange  23  in the bottom portion of the cover  14  and a cylindrical portion  24  which extends from the flange  23  towards the opening portion of the cover  14 .  
      The cylindrical portion  22  of the core  15  and the cylindrical portion  24  of the yoke  16  are fit in a stainless ring  25  made of non-magnetic material, so that the core  15  and the yoke  16  are held in axial alignment with leaving an air gap therebetween that magnetically separates the end surfaces of the cylindrical portions  22  and  24 . The plunger  18  made of magnetic material is slidably fit in a through hole formed in the yoke  16  on its axis.  
      The flange  23  of the yoke  16  is fit in the bottom portion of the cover  14 , and the flange  21  of the core  15  is fit in the opening portion of the cover  14 , so that an annular space  26  is formed around the cylindrical portions  22  and  24  between flanges  23  and  21 . A bobbin  27  of a solenoid  17  is fixedly fit in the annular space  26 .  
      The core  15  is provided with a stepped through hole on its axis, whose large-diameter hole  30  has an appropriate length thereby to form an annular projecting portion  31  at the rear end thereof. The diameter of the large-diameter hole  30  is a little larger than the diameter of the plunger  18  so that the front end of the plunger  18  may move into the large-diameter hole  30 . And, the length of the large-diameter hole  30  is a little longer than the maximum distance that the plunger  18  moves from a retracted position where its rear end surface abuts on the inner bottom surface of the cover  14  as shown in  FIG. 1 .  
      As shown in detail in  FIG. 2 , the annular projecting portion  31  is composed of a tapered portion  32  whose sectional area becomes smaller as a section is closer to the rear end of the tapered portion  32  and a thin cylindrical portion  33  that projects from the rear end surface of the tapered portion  32  toward the yoke  16 . The thickness of the wall of the cylindrical portion  33  is 0.3˜0.5 millimeters. The diameter of the rear end of the tapered portion  32  is larger than an outer diameter of the thin cylindrical portion  33  to form a step  34  having an end surface perpendicular to the direction of the axial movement of the plunger  18 .  
      By forming the step  34  on the outside of the annular projecting portion  31  between the tapered portion  32  and the thin cylindrical portion  33 , it becomes possible that the tapered portion has an inclination of any suitable degrees, in spite of forming the thin cylindrical portion having the wall of the appropriate thickness. In other words, the rear end diameter of the tapered portion  32  is not required to be the same as the outer diameter of the thin cylindrical portion  33 .  
      The annular projecting portion  31  functions to flow flux between the core  15  and the plunger  18  in the magnetic circuit constituted by the core  15 , the plunger  18 , the yoke  16 , the cover  14  and the solenoid  17 . The core  15 , the yoke  16  and the cover  14  constitute a stator  13 .  
      The valve sleeve  19  slidably receiving a spool  20  therein is arranged in abutting contact with the flange  21  of the core  15  in the opening portion of the cover  14 . The valve sleeve  19  is secured to the electromagnetic drive section  11  in axial alignment therewith by caulking the opening end portion of the cover  14  with a flange of the valve sleeve  19  being in abutting contact with the flange  21  of the core  15 . The core  15  and the yoke  16  accommodated in the cover  14  are axially secured between the bottom of the cover  14  and the flange of the valve sleeve  19  with intervening the stainless ring  25 .  
      The valve sleeve  19  is provided therein with a first valve hole  35 , a second valve hole  36  larger in diameter than the first valve hole  35  and a spring-accommodating hole  37  communicating with the second valve hole  36 , which are coaxial with the core  15  and the plunger  18 .  
      The spool  20  is provided with a first land portion  41  and a second land portion  42  that are fit in the first valve hole  35 , and a third land portion  43  that is fit in the second valve hole  36 . The second land portion  42  and the third land portion  43  are adjacent to each other to form a step portion  44  therebetween. The step portion  44  is in an annular groove that is formed between the first valve hole  35  and the second valve hole  36  thereby to define a feedback chamber. A feed back port  45  which communicates with the feedback chamber is formed radially of the valve sleeve  19 .  
      The first land portion  41  and the second land portion  42  are connected with each other by a small-diameter portion  46  with making an appropriate axial space therebetween. An annular groove  47  that faces the small-diameter portion  46  is formed on an interior surface of the first valve hole  35 . An outlet port  48  that communicate with the annular groove  47  is formed at the axially mid position of the valve sleeve  21 . The outlet port  48  is in communication with the feedback port  45  through a conduit, not shown. A discharge port  49  that is connected to a reservoir and a inlet port  50  that is connected to a fluid supply source are radially formed in the valve sleeve  21  at respective sides of the outlet port  48 . The discharge port  49  and the inlet port  50  open to the valve hole  35  at respective positions where the opposite end surfaces of the first and second land portion  41  and  42  are located. The valve sleeve  21  has a drain port  51  that opens to the spring-accommodating hole  37 . A rod portion  52  which is formed to protrude from a rear end of the spool  20  extends passing through the stepped through hole of the core  15  and abuts on the front end surface of the plunger  18 .  
      The opening of the spring-accommodating hole  37  is closed by a plug  53  screwed into the forward end of the valve sleeve  21 . A spring  54  is interposed between the spool  20  and the plug  53  to urge the spool  20  resiliently rearwards with the rod portion  52  abutting on the plunger  18 . Thus, in the inoperative state, the plunger  18  is kept at the retracted position where the rear end surface thereof abuts on the inner bottom surface of the cover  14 . As shown in  FIGS. 1 and 2 , when the plunger is at the retracted position, the rear end of the annular projecting portion  31  of the core  15  and the front end of the plunger  18  are axially coincide with each other.  
      Pressure fluid controlled at a constant pressure by a regulator valve, not shown, is supplied to the inlet port  50  from the fluid supply source. The outlet port  48  is connected with a pressure chamber provided in a clutch  60  of an automatic transmission through a supply line  61 , as shown in  FIG. 3 . The clutch  60  is a device that is supplied the fluid pressure from the solenoid-operated valve  10 . The clutch  60  is composed of a piston  63  that is moved in response to the fluid pressure introduced into the pressure chamber and multiple clutch plates  64  that are in friction-engagement with each other when pressed by the piston  63 . The piston  63  is urged by a resilient force of a spring  65  exerted thereon to be separated from the clutch plates  64 , and is moved against the resilient force of the spring  65  when the fluid pressure introduced into the pressure chamber of the clutch  60  to press the clutch plates  63 .  
      An operation of the solenoid-operated valve  10  in the first embodiment according to the present invention will be explained. When the solenoid is not excited, the spool  20  is urged by the spring  54  to move the plunger  18  rightward as viewed in  FIG. 1 , so that the plunger  18  and spool  20  is kept at the retracted position where the rear end surface thereof abuts on the inner bottom surface of the cover  14 . In this inoperative state, the outlet port  48  communicates with the discharge port  49  but is interrupted to communicate with the inlet port  50  by the second land portion  42  of the spool  20 .  
      When an electric current is applied to the solenoid  17 , the stator  13  is magnetized in proportion to the magnitude of the electric current applied thereto, and thereby to the plunger  18  is attracted toward the core  15  together with the spool  20  against the resilient force of the spring  54 . In proportion to the moving amount of the spool  20 , the second land portion  42  thereof increases the opening degree in communication of the inlet port  50  with the outlet port  48  and the first land portion  41  decreases the opening degree in communication of the outlet port  48  with the discharge port  49 . Accordingly, the fluid pressure P introduced to the pressure chamber of the clutch  60  from the outlet port  48  is increased, so that the clutch  60  is engaged with the friction force generated on the clutch plates  64  in proportion to the magnitude of the electric current applied to the solenoid  17 .  
      The fluid pressure P from the outlet port  48  is also introduced to the feedback chamber through the feedback port  45  to act on the step portion  44  formed between the second land portion  42  and the third land portion  43 . A feedback force that is the product of the fluid pressure P multiplied by the difference in area between the second land portion  42  and the third land portion  43  acts on the spool  20  in the same direction where the resilient force of the spring  54  acts thereon.  
      In the solenoid operated valve  10 , the plunger  18  and the spool  20  are held at a balanced position where a magnetic attraction force with which the core  15  attracts the plunger in proportion with the electric current applied to the solenoid  17  balances with the sum of the resilient force of the spring  54  and the feedback force exerted on the spool  20 , whereby the fluid pressure P is controlled by the magnitude of the electric current applied to the solenoid  17 .  
      The fluid pressure P is calculated with an equation of F(Ix)=P×S+k(a+L−x), wherein; I is an electric current applied to the solenoid  17 , k is the spring constant of the spring  54 , L is the maximum distance that the plunger  18  and the spool  20  move between the retracted position and the most advanced position where the spool  20  abuts on the plug  53 , x is an actual distance that the plunger  18  and the spool  20  are apart from the most advanced position, S is the difference in area between the second land portion  42  and the third land portion  43 , a is the initially compressed amount of the spring  54 , and F(Ix) is a magnetic attraction force that is exerted on the plunger  18  when an electric current I is applied to the solenoid  17  and the plunger  18  is apart distance x from the most advanced position.  
      In the first embodiment, as the thin cylindrical portion  33  is formed at the rear portion of the annular projecting portion  31 , the saturated degree of the thin cylindrical portion  33  with a magnetic flux becomes high even when the electric current applied to the solenoid  17  is low, wherein the front end of the plunger  18  positions in the range Ra axially corresponding to the thin cylindrical portion  33  as shown in  FIGS. 2 and 4 , the magnetic attraction force increases as the actual distance x decreases as a graph in  FIG. 4  shows its characteristics. And, when the electric current applied to the solenoid  17  is high, wherein the front end of the plunger  18  positions in the range Rb axially corresponding to the tapered portion  32  as shown in  FIG. 2 , the magnetic attraction force hardly varies as the actual distance x decreases due to the influence of the inclination of the suitable degrees of the tapered portion  32  as the graph in  FIG. 4  shows its characteristics.  
      The plunger  18  and the spool  20  are held at the balanced position where the magnetic attraction force exerted on the plunger  18  balances with the sum of the resilient force of the spring  54  and the feedback force exerted on the spool  20 . And, while the piston  63  is moved in response to the fluid pressure (hereafter referred to simply as “piston moving state”), the fluid pressure P from the outlet port  48  becomes lower than that from the outlet port  48  while the piston  63  stops after pressing the clutch plates  64  to frictionally engage the clutch  60  (hereafter referred to simply as “piston stopping state”).  
      Therefore, in the piston moving state, the balanced position of the plunger  18  changes along a chain line A in  FIG. 4  as the magnetic attraction force varies, and in the piston stopping state, the balanced position of the plunger  18  changes along a two-dot chain line B in  FIG. 4 . This also indicates that the plunger  18  is attracted more distance from the retraced position in the piston moving state than in the piston stopping state when the same electric current is applied to the solenoid  17 .  
      The graph shown in  FIG. 4  indicates the characteristics of the relationships between the magnetic attraction forces and the actual distances x with the electric current applied to the solenoid  17  being changed as a parameter, wherein the left end and the right end of the horizontal axis correspond respectively to the most advanced position and the retracted position of the plunger  18 . The lines A and B respectively show the respective relationship between the magnetic attraction force generated in response to the electric current applied to the solenoid  17  and the actual distance x that the plunger  18  at the balanced position is apart from the most advanced position in the piston moving state and in the piston stopping state.  
      When an electric current is applied to the solenoid  17 , the plunger  18  and the spool  20  are first moved to the balanced position in the piston moving state, wherein the fluid pressure P 1  is supplied from the outlet port  48  to the pressure chamber of the clutch  60 . After the piston  63  abuts on the clutch plates  64 , the plunger  18  and the spool  20  are moved to the balanced position in the piston stopping state, wherein the fluid pressure P increases to the pressure P 2  to make the clutch plates  64  frictionally engage with each other. A graph shown in  FIG. 5  indicates such characteristics of the fluid pressure P that increases as the time T passes. The fluid pressure P 1  in the piston moving state is lower than the fluid pressure P 2  in the piston stopping state by pressure difference  P(=P 2 −P 1 ). A graph shown in  FIG. 6  indicates the characteristics of the fluid pressure in the respective states, which respectively change as the electric current increases.  
      The pressure difference  P is calculated with an equation of  P=(− F(Ix)+k x)/S, wherein;  F(Ix) is the difference between the magnetic attraction forces exerted on the plunger  18  with the electric current being applied to the solenoid  17  in the respective states,  x is the difference between the actual distances x of the plunger  18  at the balanced positions in the respective states, and k is the spring constant of the spring  54 .  
      While the electric current applied to the solenoid  17  is in a low current area, wherein the front end of the plunger  18  at the balanced position in the piston stopping state positions in the range Ra, the magnetic attraction force exerted on the plunger  18  at the balanced position in the piston stopping state is smaller than that exerted on plunger  18  at the balanced position in the piston moving state. And, while the electric current applied to the solenoid  17  is in a high current area, wherein the front end of the plunger  18  at the balanced position in the piston stopping state positions in the range Rb, there is little difference between the magnetic attraction forces exerted on the plunger  18  at the balanced positions in the respective states. A graph in  FIG. 4  shows such characteristics.  
      If there is the pressure difference  P, the difference k x between the actual distances x of the spool  20  at the balanced positions in the respective states increases in accordance with the difference  F(Ix) between the magnetic attraction forces exerted on the plunger  18  at the balanced positions in the respective states, that is, the balanced position of the spool  20  in the piston moving state is so shifted as to make the second land portion  42  of the spool  20  increase the opening degree in communication of the inlet port  50  with the outlet port  48 . Therefore, when the electric current is in the low current range, wherein there is the difference between the magnetic attraction forces in the respective states, the flow rate of the fluid introduced into the pressure chamber of the clutch  60  through the inlet and outlet port  50 ,  48  increases to enhance the responsivety in operation of the clutch  60 .  
      The clutch  60  tends to vibrate when increasing high electric current is applied on the solenoid  17  to exert increasing magnetic attraction force on the plunger  18 , and thereby to supply a high fluid pressure to the clutch in the piston stopping state. In the first embodiment, as an angle θ that is made by the nearly horizontal line and the inclined two-dot chain line is large as indicated in an ellipse Z in  FIG. 4 , the damping force acting on the plunger  18  and spool  20  is enhanced while the electric current applied to the solenoid  17  is in the high current range. The nearly horizontal line indicates the magnetic attraction force as the actual distance x decreases, and the inclined two-dot chain line indicates the sum of the resilient force of the spring  54  and the feedback force. When the electric current applied to the solenoid is increased to make the fluid pressure high, the plunger  18  tends to overrun the balanced position in the piston stopping state. However, the damping force that acts on the plunger  18  and the spool  20  to move back to the balanced position becomes large as the angle θ becomes large, and thereby the damping capability of the plunger  18  and spool  20  becomes high. Accordingly, the clutch  60  is prevented from being vibrated by the fluid pressure fluctuation.  
      As there is provided the step  34  having an end surface on the outside of the annular projecting portion  31  between the tapered portion  32  and the thin cylindrical portion  33 , the tapered portion  32  can be formed as it has an inclination of any suitable degrees, wherein the magnetic attraction force hardly varies as the plunger  18  moves in the tapered portion  32 . And the wall of the annular cylindrical portion  33  can be formed thin enough to enhance the responsivety of the clutch  60  that is supplied fluid pressure from the solenoid-operated valve  10 .  
      Referring to  FIG. 7 , an annular projecting portion  31  of a core  15  of the second embodiment is shown. The annular projecting portion  31  is composed of a tapered portion  32  and a thin cylindrical portion  33 . In the second embodiment, a step  134  that has a slightly inclined end surface is formed between a rear end of the tapered portion  32  and the thin cylindrical portion  33 . Therefore, the tapered portion  32  can be formed with an inclination of any suitable degrees.  
      In the third embodiment, as shown in  FIG. 8 , an annular projecting portion  31  of a core  15  also is composed of a tapered portion  32  and a thin cylindrical portion  33 . The thin cylindrical portion  33  in the third embodiment has a wall that is about twice as thick as the wall of the thin cylindrical portion  33  in the first embodiment. By thickening the wall of the cylindrical portion  33 , it is prevented that the cylindrical portion  33  is deformed by a cutting force applied by a tool while machining the cylindrical portion  33  with a tool in mass-producing cores. But as the wall of the cylindrical portion  33  is thickened in the third embodiment, the saturated degree of the thin cylindrical portion  33  with a magnetic flux becomes lower than that in the first embodiment. A step having an end surface is not formed on the outside of the annular projecting portion  31 .  
      In the third embodiment, the core  15  is provided on its axis with a stepped through hole, whose large-diameter hole  30  is provided with an opening potion  33   a  on the inside of the cylindrical portion  33 . An inner diameter of the opening portion  33   a  is a little larger than that of the other portion of the large-diameter hole  30 . Accordingly, an air gap C 1  between the opening portion  33   a  of the cylindrical portion  33  and the plunger  18  that moves therein is larger than an air gap C 2  between the other portion of the large-diameter hole  30  and the plunger  18 . An increased amount of the gap C 1  between the opening portion  33   a  and the plunger  18  is so determined that the increase of magnetic reluctance due to the increase of the gap C 1  cancels the decrease of the saturated degree of the cylindrical portion  33  with a magnetic flux due to the increase of the thickness thereof. Therefore, the characteristics of the magnetic attraction force that increases as the plunger  18  moves into the cylindrical portion  33  in the third embodiment is substantially the same as that in the first embodiment.  
      Referring to  FIG. 9 , an annular projecting portion  31  of a core  15  of the forth embodiment is shown. The annular projecting portion  31  also is composed of a tapered portion  32  and a thin cylindrical portion  33 . The thin cylindrical portion  33  in the forth embodiment also has a thicker wall and an opening potion  33   a  on the inside thereof. In the forth embodiment, a step  34  that has an end surface perpendicular to the direction of the axial movement of the plunger  18  is formed on the outside of the annular projecting portion  31  between the rear end of the tapered portion  32  and the thin cylindrical portion  33 . Therefore, the tapered portion  32  can be formed with an inclination of any suitable degrees.  
      Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.