Abstract:
A valve assembly HAS an element movable in a mechanical journal arrangement relative to a valve opening to establish an opening position of a valve, and A control unit for controlling the application of an oscillatory electromagnetic force to the element ( 8 ) having an known peak amplitude to cause the element to execute a sympathetic oscillatory motion as a friction reducing maneuver. The control unit is adapted to control the peak amplitude of the oscillatory electromagnetic force to sequentially apply a number of different peak amplitudes, each during a different measurement period. A monitor detects changes in the opening position of the valve during each of the measurement periods and provides a measure thereof. A processor is connected to the control unit and to the monitor to determine a working amplitude of the oscillatory electromagnetic force at which the detected changes will be minimized and to supply the determined working amplitude to the control unit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of controlling an oscillatory friction-reducing maneuver of a valve element journaled in a bearing for movement to regulate a degree of opening of the valve, and to a valve assembly having an element operable according to the method.  
           [0003]    1. Description of the Prior Art  
           [0004]    In valves for which the degree of opening is automatically regulated with a mechanically journaled element friction in the bearing can cause problems. One of the most common is that frictional forces cause the element to stick in a particular position so that a change in the force applied to the element 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 element against the bearing etc., so that the force which must be applied to the element in order to overcome the frictional force can be unpredictable.  
           [0005]    It is known from U.S. Pat. No. 5,787,924 to provide, in a valve having a mechanically journaled element as described above, a controller which exerts a force on the element to move it to a desired position and then exerts an additional oscillatory force of a predetermined maximum magnitude, a working amplitude, to cause sympathetic oscillation of the element 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 element sticking in the journal arrangement. The working 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 element and the desired position.  
           [0006]    However, the magnitude of the working amplitude of the additional force, or its dependency, is factory pre-set and is typically the same for all valves of a particular type. If set too low it may not overcome the frictional forces which could vary, particularly as the valve ages. If set too high then unacceptably large oscillations in the position of the element may result. A compromise magnitude or dependency must therefore be pre-set which ensures that, for all valves, the valve will operate properly under a large cross-section of conditions.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide a method and an electromagnetic valve operable according to the method in which the working amplitude of the additional force causing the oscillations is adapted to the operating conditions of the valve as well as to the mechanical properties of an individual valve.  
           [0008]    In accordance with the present invention this object is achieved in a method for controlling a friction reducing maneuver of the valve element whereby variations in the magnitude of a property, such as flow or pressure of fluid flowing through the valve, which is affected by a degree of opening of the valve and so is indicative of the degree of movement of the journaled element, is monitored as the maximum amplitude of the force causing the element to perform the friction reducing maneuver is varied. The maximum amplitude of the force is then set to that determined from the monitored variations as providing the minimum variation. Thus the amplitude of the force causing the oscillations need not be pre-set but is made dependent on the prevailing conditions within the valve. This ensures that only sufficient additional force is applied to cause the element to oscillate while minimizing the adverse effects on the fluid passing through the valve.  
           [0009]    Preferably the method includes the step of preventing movement of the journaled element in directions other than that required to establish a degree of opening of the valve. This controls a source of variation in friction and so reduces the number of occasions during the operation of the valve for which the amplitude for minimum variation needs to be determined. Indeed to may be sufficient to carry out this determination only once, for example before delivery, during the lifetime of the valve.  
           [0010]    The aforementioned object also is achieved in accordance with the present invention in a valve, such as an electromagnetic valve, operable according to the above-described method of the present invention and so possessing the advantages associated with that method. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic representation of an electromagnetic valve according to the present invention.  
         [0012]    [0012]FIG. 2 is a block diagram of a portion of a patient ventilator system including a valve according to the present invention.  
         [0013]    [0013]FIG. 3 illustrates a modification to the exemplary valve of FIG. 1 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    As shown in FIG. 1, an electromagnetic valve assembly has a valve  44  with an inlet  2  having an opening  4  that can be opened and closed with a membrane  6 .  
         [0015]    The membrane  6  is elastically resilient. To close the valve  44 , as rod  8  pushes the membrane  6  against the opening  4 . To open the valve  44 , the rod  8  retracts, (upward in the 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 .  
         [0016]    The rod  8  passes through an enclosure  12  and is journaled in bearings  14  and  16  for movement in its longitudinal direction, along a control axis C. The bearings  14 ,  16  are made with close tolerances, since they serve as guides for the rod  8 .  
         [0017]    An anchor  18  made of magnetic material is fixed on the rod  8  and is movable inside a coil  20 . In this manner the anchor  18  and accordingly the rod  8  can be moved back and forth in the rod&#39;s longitudinal direction when an appropriate current is applied to the coil  20 .  
         [0018]    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  can therefore be adjusted by controlling a main current, I, to the coil  20 .  
         [0019]    Instead of having a separate anchor  18  arranged on the rod  8 , at least a part of the rod itself can be made of a magnetic material which serves as the anchor. Alternately, the rod  8  can, at least in part, be coated with magnetic material.  
         [0020]    For safety reasons, it may be preferable to have the rod  8  biased by a biasing element, here by a spring  22 , to press the membrane  6  towards the valve opening  4 , so that the valve  44  closes as soon as the main current I to the coil  20  is shut off. The spring  22  is in the present example housed in an extension  12 ′ of the housing  12 .  
         [0021]    Friction in the bearings  14 ,  16  often causes the rod  8  to “stick” in an opening position. This friction is an unpredictable factor which changes, for example, with the age of the valve assembly, the side of the rod  8  pressing against the bearing etc. It is known to superimpose a smaller, additional current, preferably a sinusoidal, square or triangular wave signal, on the main current I to cause the rod  8  to oscillate around the valve&#39;s opening position as a friction reducing maneuver. Accordingly, the membrane  6  also is oscillated around this opening position. Since the rod  8  is kept constantly in motion, sticking in the bearings  14 ,  16  thus is prevented.  
         [0022]    The frequency of the superimposed signal is lower than the upper limit frequency of the valving assembly but still is high enough for any oscillations induced in the flow to be quickly filtered out in the valve tubing normally connected to the outlet  10 . A suitable frequency may be between about 100 Hz and about 1000 Hz.  
         [0023]    The membrane  6  may be appropriately compressible, so the rod  8  is able to perform an oscillating, translational movement even with the membrane  6  pressed against the valve seat  4 . The membrane  6  will then be alternatingly compressed and expanded. “Sticking” between the valve seat and the valve membrane  6 , which can contribute to the valve  44  becoming “stuck” in the closed position, also can be reduced by imparting motion to the rod  8  as described above.  
         [0024]    It will be appreciated that the valving assembly as described above represents an arrangement well known in the art.  
         [0025]    A controller  24  is provided with a current regulator  28  which operates to control the magnitude of the main current I and the superimposed current. As described below, the magnitude of at least the latter current is, for the present example, controlled in response to a pressure signal from a pressure meter  26  that is located to monitor pressure of fluid flowing through the valve  44  (shown in FIG. 1 connected to the inlet  2 ). It will be appreciated that another sensor for monitoring the degree of opening of the valve  44  may substitute for or be used in addition to the pressure meter  26  and its output signal used in a manner similar to that described below with respect to the output signal of the pressure meter  26 . Examples of such sensors are a flow meter, employed to monitor fluid flow through the valve  44 ; or a position sensor, employed to sense the position of the rod  8 .  
         [0026]    A microprocessor  30  is included within the controller  24  and is adapted to receive a signal indicative of the degree of opening of the valve  44 , which in the present example is a pressure signal from the pressure meter  26 . The microprocessor  30  is further adapted to provide a control signal to the current regulator  28  which acts to modify a peak amplitude A W , so-called “working amplitude” of the superimposed current. The microprocessor  30  is programmed to modify this working amplitude dependent on a calculated magnitude of a temporal deviation of a monitored parameter, in this example pressure, indicative of the degree of opening of the valve. The microprocessor  30  operates, as is described in detail below, to set the working amplitude to the amplitude at which the monitored variations are minimized.  
         [0027]    According to the present example the microprocessor  30  operates during a measurement period to provide a control signal to the current regulator  28  to change the peak amplitude of the superimposed oscillatory current from the present working amplitude A WOld  to a new peak amplitude A 1 . During this measurement period a number M determinations of a variance VAR(j)is made by the microprocessor  30 . Each variance VAR(j) is determined for a number N (which may be different for each M) samples of the pressure signal P(i) according to:  
               VAR        (   j   )       =       [       1     N   -   1              ∑     i   =   1     N            (       P                   (   i   )       -       P   DT          (   i   )         )     2         ]       j   =   1     M             (   1   )                               
 
         [0028]    The signal P DT (i) is the pressure signal P “de-trended” in some sense. Typically a polynomial of suitable degree can be used, simplest is the mean value. In the present example a straight line is used, and an extension of that could be to use a number of lines with waypoints in between, a “train of straight lines”.  
         [0029]    The straight line used here was estimated in least square sense (φ1 is offset and φ2 is slope):  
           P   DT ( i )=φ 1 +φ 2 ·( i−i   m )   (2)  
         [0030]    where  
               ϕ   1     =       1   N            ∑     i   =   1     N          P                   (   i   )                   (   3   )                 ϕ   2     =         ∑     i   =   1     N          P                     (   i   )     ·     (     i   -     i   m       )               ∑     i   =   1     N            (     i   -     i   m       )     2                 (   4   )                 i   m     =       1   N            ∑     i   =   1     N        i               (   5   )                               
 
         [0031]    P can be any measured signal that can be said to represent the degree of opening the valve, for example flow or shaft position. A mean variance, MEAN_VAR, for the M determinations that were made over the measurement period may be calculated as:  
             MEAN_VAR   =       1   M            ∑     j   =   1     M                     VAR        (   j   )                   (   6   )                               
 
         [0032]    Where VAR(j) can be determined for each new incoming sample of P(i), that is the sample widow is moved by one sample for each of the M measurements. Alternatively, VAR(j) is determined for every N samples of P, thereby reducing the determinations by a factor of 1/N.  
         [0033]    The above is repeated for a number, x, of other measurement periods, during each of which the microprocessor 30 provides a different peak amplitude A x , of the superimposed oscillatory current to the current regulator  28 . A corresponding MEAN_VAR, calculated according to the equations (1)-(6) above, is stored in a memory of the microprocessor  30  which is programmed to then determine the peak amplitude A x at which the MEAN_VAR value is minimized and then to supply this to the current regulator  28  as a new working amplitude, A WNew  where it is used to regulate the friction reducing oscillation maneuvers of the rod  8 .  
         [0034]    An example of the use of the valve of FIG. 1 is shown in FIG. 2 which depicts a functional block diagram of a portion of a patient ventilator system that is operated to control the flow of a breathing gas to and from a patient. The valve assembly of FIG. 1 is shown as a unit  32  connected in series to an expiration line  34 , inside a ventilator device  36 . Gas flows from a patient during expiration through the line  34  in a direction indicated by the arrows and, as is common, is vented from the ventilator device  36  either to atmosphere or to a gas recovery system (not shown). In the present example the valve unit  32  acts, in manner well known in the art, as a PEEP valve to control the flow of expiration gas through the expiration line  34  (connected to inlet  2  of the valve of FIG. 1) during an expiration phase of a patient breathing cycle and thereby to regulate the pressure within the airways of a patient (not shown) to establish predetermined PEEP level.  
         [0035]    At the onset of an expiration phase during which a measurement period will be provided, the VAR calculation is disabled for an interval. This interval can be of a specific length in time or be determined by logic of some kind, looking at the signal level of P, for example terminating when the pressure sampled by the meter  26  of FIG. 1 falls below a pre-set level, such as the PEEP level. After this interval the microprocessor  30  of the valve unit  32  (which may alternatively be realized in program coding of a microprocessor control system (not shown) of the ventilator unit  36 ) operates as described above to change the peak amplitude of oscillation from the present working amplitude A WOld  to the new amplitude A 1 , at which it remains during the measurement period, typically for the remainder of the expiration phase. The microprocessor  30  is programmed to then calculate the value MEAN_VAR as described above. This value MEAN_VAR represents the variance for one single breath. The calculation of MEAN_VAR is then repeated for a number of other breaths, each using oscillations of a different peak amplitude, A x . From each of these calculated values of MEAN_VAR and a knowledge of the associated peak amplitude A x  the microprocessor is programmed, using standard algorithms, to determine a value of A x  at which the variance, MEAN_VAR, will be minimized and to provide this value A x  to the current regulator  28  for use as a new working amplitude A WNew  for the friction reducing oscillations. Such a determination may be simply made, for example, by comparing all calculated values MEAN_VAR and selecting the peak amplitude A x  associated with the MEAN_VAR having the smallest value as the new working amplitude A WNew .  
         [0036]    Thus an optimum working amplitude, A WNew , for the valve  44  of the unit  32  can be determined in use without disrupting the provision of ventilatory support by the ventilator unit  36 .  
         [0037]    To provide a more accurate measure, a value MEAN_VAR_BREATHS, may be calculated as an average of several breaths with the same oscillation amplitude A x  as:  
               MEAN_VAR                 _BREATHS                =                  1   NrOfBreaths                       ∑     k              =              1     NrOfBreaths                     mean_var                   (   k   )                   (   7   )                               
 
         [0038]    It was discovered that the degree of friction experienced by the rod  8  changed appreciably if the rod  8  was rotated in the bearings  14 ,  16  in which it was journaled. To minimize this the valve assembly according to the present invention may be modified to inhibit rotation of the rod  8 . An exemplary modification of the valve assembly according to FIG. 1 to prevent rotation is illustrated in FIG. 3. In FIG. 3 the extension  12 ′ to the enclosure  12  is shown with the spring bias element  22  removed for clarity. The section of the rod 8 that passes into the extension  12 ′, through the bearing  14 , is modified by providing extensions or “wings”  38  that slidably locate in complementary grooves  40  in the internal wall  42  of the extension  12 ′.  
         [0039]    Other types of valve bodies, directly positioned by an electromagnet, can be used as well as other means for limiting the degrees of freedom of the movable element  8  to essentially that required to vary the degree of opening of the valve.  
         [0040]    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.