Patent Document

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a valve assembly and in particular to an assembly of the type in which regulation of the flow of fluid through the valve is achieved by means of translational and/or rotational movement of a shaft to seal or un-seal a valve opening in response to an applied force from an associated force generator. 
   2. Description of the Prior Art 
   In a valve assembly having a mechanically journaled shaft friction in the bearing can cause problems. One of the most common is that frictional forces often cause the shaft to stick in a particular position so that a change in the force applied to the shaft in order to move it to another position can have unpredictable results. Moreover, the magnitude of the frictional force itself is often unpredictable, since it may change with the age of the valve, the pressure of the shaft against the bearing or with movement of the shaft within the bearing in a direction not associated with the sealing or un-sealing of the valve opening, so that the force from the force generator which must be applied to the shaft in order to overcome the frictional force can be unpredictable. 
   In the remainder of the this application a force generator of the solenoid type will be described in more detail in connection with the valve assembly of the present invention. Those skilled in the art will appreciate that the valve assemblies in the description may be modified to apply to other assemblies that employ different force generators acting on the shaft so as to effect the required movement without departing from the invention. 
   It is known from U.S. Pat. No. 5,787,924 to provide a valve assembly in which a shaft is movable along the coil axis of a solenoid coil type force generator. A controller is provided which operates to vary the current through the coil to vary an electromagnetic force exerted on the shaft and thereby cause its translation to a desired position. The controller then operates to impose an additional oscillatory current to exert an additional, oscillatory force of a predetermined maximum magnitude on the shaft and cause a sympathetic oscillation of the shaft about the desired position by amounts sufficiently small so as not to adversely effect the flow control operation of the valve. This continuous oscillatory motion is designed to prevent the shaft sticking in the mechanical journal arrangement. The maximum amplitude of this additional force may even be made to be dependent, in a known manner, on the magnitude of an error signal representing the known position of the shaft and the desired position. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a valve assembly that at least minimizes the problems associated with known valve assemblies, as discussed above. 
   This object is achieved in accordance with the invention in a valve assembly having a valve body with a valve opening therein, a force generator that is energizable to generate force, a shaft on which the force generated by the force generator acts, said shaft being movable relative to the valve opening in response to the force in a direction to regulate a degree of opening of the valve, and a bearing assembly in which the shaft is guided for movement relative to the valve opening, the bearing assembly including a magnetic arrangement configured to produce a magnetic suspension of the shaft. 
   By replacing the mechanical bearing which guides the movable shaft with a magnetic bearing, either active or passive or a combination of the two, the frictional force between the movable shaft and the bearing is effectively eliminated. The need for sophisticated means for controlling friction between the shaft and the bearing is thus obviated. 
   Additionally, the magnetic bearing may be configured to act on the movable shaft to produce a magnetic biasing toward either closing or opening the valve, as desired, in the absence of a force applied to the shaft by the force generator. This has the advantage that magnetic bearing can replace or augment biasing means typically employed in a valve assembly to be used in safety critical applications, such as an inspiration or an expiration flow control valve in a pneumatic circuit of a conventional mechanical ventilator, where for patient safety reasons it is preferable that, respectively, the valve closes or opens in the event of a failure of the force generator. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a valve assembly according to the present invention. 
       FIG. 2  shows in greater detail a magnetic bearing arrangement according to the present invention, employed in the valve assembly of FIG.  1 . 
       FIG. 3  shows a magnetic bearing arrangement usable in a valve assembly according to the present invention to produce a desired magnetic biasing effect. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As shown in  FIG. 1 , an electromagnetic valve assembly has a valve  44  with an inlet  2 , the opening  4  of which can be opened and closed with a membrane  6 . 
   The membrane  6  is elastically resilient. To close the valve  44 , a movable shaft  8  pushes the membrane  6  against the opening  4 . To open the valve  44 , the movable shaft  8  retracts, (upward in FIG.  1 ), whereupon the membrane  6  moves resiliently away from the opening  4 , to allow a flow through the inlet  2 , via the opening  4 , and out through an outlet  10 . 
   The movable shaft  8  passes through an enclosure  12  and is supported in magnetic bearing assemblies  14  and  16  for movement in its longitudinal direction along a movement axis that is, in the present example, the coil axis C (an axis of symmetry of a solenoid coil  20  along which it is wound). The bearing assemblies  14 ,  16  thus serve to guide the movable shaft  8 . 
   An armature  18  made of magnetic material is fixed on the movable shaft  8  and is located inside the solenoid coil  20 . The armature  18  together with the solenoid  20  forms, in the present example, a force generator which when energized is able to move the movable shaft  8 . Thus, when an appropriate current, I, is supplied from an associated current control unit  22  to energize the coil  20  the armature  18  and accordingly the movable shaft  8  can be, in the present embodiment, made to reciprocate parallel to the coil axis, C. It will be appreciated that a so-called “voice coil” arrangement, in which a solenoid is attached to the movable shaft  8  in place of the armature  18  of the present example and a permanent magnet replaces the static solenoid coil  20  of the present example, may substitute for the force generator described above without departing from the invention. 
   The valve&#39;s degree of opening, i.e. the magnitude of the distance between the opening  4 , serving as a valve seat, and the membrane  6  therefore can be regulated by the control unit  22  through varying the current, I, supplied to energize to the solenoid coil  20 . 
   As an alternative to providing a separate armature  18  arranged on the movable shaft  8 , at least a part of the movable shaft itself can be made of a magnetic material that then serves as the anchor. As a further alternative, the movable shaft  8  can be coated, at least in part, with magnetic material. 
   For safety reasons, it may be preferable to have the movable shaft  8  biased by a biasing arrangement, here by a spring  24 , so that the valve  44  returns to a default position as the current I to the coil  20  is shut off. The biasing arrangement  24  in the present example is contained in an extension  12 ′ to the housing  12  and operates to bias the movable shaft  8  to push the membrane  6  against the valve opening  4  and closes the valve  44  in the absence of the current I. This is particularly useful when the valve  44  is used as an inspiration flow control valve in a pneumatic circuit of a conventional mechanical ventilator. In circumstances in which the valve  44  is employed as an expiration flow control valve in a pneumatic circuit of a conventional mechanical ventilator then the biasing arrangement preferably is provided to establish an opening position of the valve in the absence of the current I to the coil  20 . 
   A region of the valve assembly that includes the magnetic bearing assembly  14  employed in the present embodiment is shown in greater detail in FIG.  2 . It will be appreciated that the following description is equally applicable to the magnetic bearing assembly  16 . The bearing assembly  14  includes a first, typically permanent magnet, static magnet  26 . This static magnet  26  is shown in the present example as a ring magnet, either continuous or segmented, attached to a section of the housing  12 , and is provided with a pole face  28  directed toward the movable shaft  8 . Also included in the bearing assembly  14  is a second, typically permanent magnet, magnet  30 , that may also be a ring magnet, mounted on the movable shaft  8  and provided with a pole face  32  directed toward the pole face  28  of the first magnet  26 . In the example of the bearing assembly  14  illustrated in  FIG. 2 , the pole faces  28 , 32  are orientated with the same magnetic pole facing one another. Thus, a radial magnetic repulsive force between the co-operating magnet arrangement  26 , 30  of the bearing assembly  14  procures a magnetic suspension of the shaft  8  within the static magnet  26  to maintain the shaft  8  separated from the pole face  28  of that magnet  26  by a small air gap  34  as the shaft ( 8 ) moves to regulate the degree of opening of the valve  44 . 
   In addition to the radial magnetic force described above an axial magnetic force (that is, parallel to the coil axis C) is also generated by the interaction of the magnetic fields of the co-operating magnet arrangement  26 , 30  of the magnetic bearing assembly  14  (and also for the similar co-operating magnet arrangement of the magnetic bearing assembly  16 ). This axial magnetic force will vary as the movable shaft  8  moves along the axis C. By appropriate selection of magnetic pole  28 , 32  and of the relative axial locations of the magnets  26 , 30 , the axial magnetic force produced by the co-operating magnet arrangement  26 , 30  of the bearing assembly  14  (and also of the assembly  16 ) may be employed to produce the biasing force which, in the exemplary embodiment of  FIG. 1 , is provided by the spring  24 . This provides, as desired, either a default open position or a default closed position of the membrane  6  with respect to the valve opening  4  in the absence of current, I, to the coil  20 . 
   One possible arrangement of magnets that may constitute the magnetic bearing assemblies  14 , 16  of the valve assembly of  FIG. 1  is illustrated in FIG.  3 . This arrangement, as described below, procures an axial magnetic bias force FX acting in a direction along the movement axis (here the solenoid axis C) and, in the present example, acts in a manner equivalent to the bias force generated by the spring  24  of the valve assembly shown in FIG.  1 . With reference to  FIG. 3 , a first bearing assembly  46  has a first magnet  36  fixedly located at a wall of the housing  12  and a second magnet  38  mounted on the movable shaft  8 . The first and second magnets  36 , 38  are disposed so that the same magnetic pole (shown in the embodiment as a north pole, N) of each magnet  36 , 38  is facing one another. Similarly, a bearing assembly  48  has a third magnet  36 ′ fixedly located at a wall of the housing  12  and a fourth magnet  38 ′ mounted on the movable shaft  8 . The third and fourth magnets  36 ′, 38 ′ are disposed so that the same magnetic pole (shown in the embodiment as a north pole, N) on each magnet  36 ′, 38 ′ is facing one another and, in the present example, is selected to be is the same as the facing poles of the other bearing assembly  46 . 
   The magnets  38 , 38 ′ are positioned on the movable shaft  8 , relative to the complementary magnets  36 , 36 ′ that make up the co-operating magnet arrangement of each associated bearing assembly  46 , 48  such that an axial magnetic repulsive force between the co-operating magnet arrangement  36 , 38  and between the co-operating magnet arrangement  36 ′, 38 ′ increases as the shaft  8  moves (in the direction of the larger arrow of  FIG. 3 ) to open the valve  44 . Thus, the axial magnetic bias force, FX, is procured which increases as the valve  44  opens (that is the shaft  8  moves in the direction of the larger arrow) and which acts to bias the shaft  8  always towards closing the valve (that is in the direction of the smaller arrow), even in the absence of a current I through the coil  20 . In this manner the magnetic bearing can replace or augment the biasing arrangement  24  that is present in the embodiment of FIG.  1 . 
   It will be appreciated by those skilled in the art that the direction and magnitude of the magnetic bias force FX may be readily selected through suitable choice of one or both of the orientation of the poles and the relative physical locations of the magnets  36 , 38 ; 36 ′, 38 ′ that constitute the respective magnetic bearing assemblies  46 ; 48 . Moreover, it will be appreciated that although the magnetic bearing assemblies  14 ; 16 , 46 ; 48  described herein all employ only permanent magnet arrangements some or all may be exchanged for electro-magnet arrangements whilst remaining within the scope of the invention as claimed. The use of such electro-magnets has an advantage that magnitudes of one or both the suspension (here radial) and the bias (here axial) magnetic forces may be controlled by varying the current flowing to the electromagnets. 
   Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.

Technology Category: f