Abstract:
A device for metering pressurized fluid in proportion to a supplied input signal to an electric solenoid providing non-proportional magnetic flex that is rectified by a piece-wise non-linear spring assembly. The solenoid, an electromagnetic coil and a movable armature which positions a metering valve as needed to allow the fluid to flow through the device at the desired rate. The non-linear spring assembly includes a pair of flexure springs with four pairs of independent spring fingers. The spring fingers supply a plurality of piece-wise forces opposing movement of the armature caused by magnetic flux generated by the coil so as to provide a non-proportional opposing force to the non-proportional force of the magnetic flux. This allows the metering valve to be moved, and thereby fluid metered, in proportion to the input signal supplied to the coil. A position transducer is mechanically coupled to an elongated axial rod attached to the armature to provide valve position feedback to an electronic control unit for correcting deviations in the actual position of the valve from that desired by the input signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit to provisional application Ser. No. 60/170,880, filed Dec. 15, 1999. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to metering valves and in particular to high precision fluid metering valves.  
           [0004]    Solenoid type metering valves are well known. Such valves include an inductive coil that when energized attracts or repels an armature which in turn moves a valve member to control flow, for example, of hydraulic fluid or fuel. Solenoid type metering valves are beneficial in that they are relatively simple to design and include a limited number of movable components. Proportional solenoid type metering valves are also well known. Proportional valves control flow rate in proportion to the input signal (current) supplied to the coil which is desirable for accurate control of the valve. Conventional proportional valves can perform quite well for many applications.  
           [0005]    However, in precision metering applications, such as when metering fuel and other combustible media to jet and rocket powered vehicles or in applications where the valve is used as an actuator positioning device, the valves must reliably provide consistent and responsive pressure and flow control. In particular, they must be accurate over a wide range of flow rates (high turn-down ratio) and have minimal internal leakage, low power consumption and low hysteresis. They must also be compact.  
           [0006]    Solenoid type metering valves operate by generating magnetic flux which pulls an armature to move the valve. The attractive force of magnetic flux on a metallic body becomes stronger and more non-linear the closer the body is to the source of the flux. Large gaps between the armature and the coil require high current levels and/or prohibitive large coils. Thus, in compact valves where the armature is in close proximity to the coil, the position of the armature, and thereby the valve, will vary non-linearly or non-proportionately with the input signal to the coil. This non-linearity tends to make the valves bi-stable as the air gaps between the armature and the coil decrease. This can lead to large fluid pressure oscillations and undesirable instability making the valve inaccurate and difficult to control with precision. Accordingly, the armature movement must be linearized in some manner. However, this can be complicated because a simple linear spring acting on the armature will not maintain proportional movement throughout its usable range.  
           [0007]    There have been many means of linearizing the force acting on the armature in compact packages. One known means is to use conically shaped openings for the air gaps between the armature and the coil. However, this can impart relatively large side loads on the armature leading to high friction and poor hysteresis. Low-friction guides or suspension systems for the armature can be used to reduce the side loads, such as in U.S. Pat. Nos. 3,861,643 and 4,635,683, however, they add cost and can be difficult to implement. The &#39;643 patent discloses another means of providing a valve by the saturation of the magnetic flux at different sized air gaps in the core of the armature. However, as mentioned, this technique requires a complex frictionless suspension system.  
           [0008]    Accordingly, an improved precision proportional solenoid type metering valve is needed.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an improved proportional solenoid-operated device for accurately metering fluid using a unique piece-wise non-linear spring to rectify induced movement from magnetic flux that is not proportional to the input signal. In particular, the invention includes a housing containing an electromagnet coil for translating an armature to move a metering valve. The movement of the armature, and thereby the metering valve, is linearized to the coil input signal by the non-linear spring applying counteracting forces on the armature in a stepped or piece-wise manner.  
           [0010]    Specifically, the housing defines a valve chamber in fluid communication with an inlet port and an outlet port. The electromagnetic coil in the housing produces magnetic flux varying non-proportionally to an input signal. The armature can be translated by the induced magnetic flux along the stroke axis toward the coil. The metering valve can be moved along the stroke axis by the armature. The non-linear spring communicates with the armature and has a plurality of spring fingers extending radially with respect to the stroke axis to independently engage between the armature and a stationary structure at different points as the armature is translated along the stroke axis. This provides a summing of forces from each spring finger acting on the armature counter to the force induced by the magnetic flux so that positioning the metering valve is more nearly a linear function of the input signal to the coil.  
           [0011]    At least one of the spring fingers has a different thickness so that thicker spring fingers deflect before thinner spring fingers.  
           [0012]    In a preferred form, the non-linear spring is an assembly of flexure springs spaced apart and perpendicular to the stroke axis. Each flexure spring includes four independently flexible spring fingers extending radially outward in the same plane at ninety degrees from each other. The four spring fingers of each flexure are arranged in two pairs of opposite spring fingers each pair extending radially outward at a right angle. Two of the four pairs of spring fingers have tapered tips of decreased thickness defining an angled contact surface for contacting a fixed structure in the housing. As the armature is translated along the stroke axis toward the coil, the non-linear spring provides piece-wise forces acting on the armature by flexure of the four pairs of spring fingers at different portions of the armature stroke by contact of the spring fingers of the first flexure spring with the fixed structure and contact of the spring fingers of the second flexure spring with the corresponding spring fingers of the first flexure spring. This arrangement provides four distinct spring forces applied against the armature at various portions of the stroke.  
           [0013]    An elongated axial rod can be attached to the armature and a compression spring can be disposed about the rod to bias the armature away from the coil. The compression spring provides a fifth distinct spring rate acting against the armature.  
           [0014]    The metering valve assembly includes a generally cylindrical guide fixed to the housing along the stroke axis. The guide has inlet and outlet openings in fluid communication with respective the inlet and outlet ports of the housing. A cylindrical valve member can slide along the diameter of the guide to alternatively block the inlet and outlet openings in the guide and thereby control flow through the device. A valve carrier is disposed about the valve and has an annular flange surface engaging an outer circumferential shoulder of the valve. A compression spring, retained by an annular spring retainer fixed to the guide, is disposed about a portion of the valve to engage the shoulder and bias the valve toward, and the valve carrier in contact with, the armature.  
           [0015]    In another form, the device includes an electronic control unit for controlling the operation of the coil. The device can also include a position transducer electrically coupled to the control unit and having a sensing coil and a metallic transducer element fixed to the elongated rod and disposed axially within the sensing coil of the position transducer. The position transducer provides a feedback signal to the control unit corresponding to the position of the transducer element. The control unit can adjust the input signal supplied to the coil in response to the feedback signal. Preferably, the control unit includes a comparator which compares a commanded metering valve position to an actual metering valve position detected by the position transducer. The control unit adjusts the input signal until the difference between the commanded and actual position is an acceptable value near zero.  
           [0016]    Thus, the present invention provides a device for metering fluid in proportion to an input signal. This is accomplished using a simple and inexpensive non-linear spring assembly having two flexure springs with pairs of spring fingers of different thickness acting independently in a piece-wise manner to oppose armature movement induced by the magnetic flux generated by the coil. The incrementally increasing spring forces produce opposing forces corresponding to the magnetic flux as the distance between the armature and the coil changes. Since the magnetic flux pulling force is not proportional to the input signal to the coil, the resulting force of the non-linear spring assembly provides a counteracting non-proportional force throughout the stroke of the valve, which linearizes the movement of the metering valve assembly so that it is proportional to the coil input signal (current). Moreover, the feedback signal of the position transducer can be used to adjust the input signal to rectify discrepancy between the actual position of the meter and the position corresponding to desired fuel flow. In this way, the device provides for accurate metering suitable for use in precision actuator positioning or fuel metering applications, such as jet and rocket engines.  
           [0017]    These and still other advantages of the present invention will be apparent from the description of the preferred embodiments which follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a side cross-sectional view of the metering device;  
         [0019]    [0019]FIG. 2 is an enlarged side cross-sectional view taken along line  2 - 2  of FIG. 1 showing the metering device with an inlet port closed and a return port open to a valve chamber;  
         [0020]    [0020]FIG. 3 is an enlarged side cross-sectional view similar to FIG. 2 however with the inlet port fully open and the return port closed;  
         [0021]    [0021]FIG. 4 is a front view of a flexure spring assembly used in the metering device;  
         [0022]    FIGS.  5 A- 5 F are partial cross-sectional views taken along path  5 - 5  of FIG. 4 showing the flexure spring assembly in various stages of deflection at various positions of the armature stroke; and  
         [0023]    [0023]FIG. 6 is a block diagram of the metering device of the present invention in an exemplary actuator positioning system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Referring to FIG. 1, the invention provides a fluid metering device  10  having a preferably aluminum housing  12  defining a valve chamber  14  and having an internally threaded end  16  to which is threaded an annular iron alloy bobbin  18  supporting an electromagnetic coil  20  covered by a backing plate  19 . The housing  12  is enclosed by an end cap  22  bolted to a flange  24  of the housing  12 . The housing  12  includes three radially extending inlet  26 , outlet  28  and return  30  ports, in fluid communication with the valve chamber  14 . The ports  26 ,  28  and  30  couple the metering device  10  via fittings  32  to a fluid line between a fluid reservoir and an actuator so as to allow pressurized fluid into and out of the device  10 , as discussed in detail below. The device  10 , as shown in the drawings, is preferably used to meter hydraulic oil to actuators.  
         [0025]    The inlet  26  and return  30  ports communicate with the valve chamber  14  through a generally cylindrical valve guide  34 , which is mounted within an opening  36  of the housing  12  along a stroke axis  38  and sealed by resilient seals  40  disposed in annular grooves  42  in the outer circumference of the guide  34 . The guide  34  includes return  44  and outlet  46  axial bores extending through an outer end  48  of the guide  34  and into the guide  34  different distances. The return axial bore  44  extends proximate an inner end  50  of the guide  34  and is intersected at its end by a radial bore  52 . The inlet axial bore  46  extends axially by a lesser distance and is intersected at an intermediate point by radial bore  54  and at its end by radial bore  56 . The inlet axial bore  44  is threaded at the outer end  48  of the guide and is closed by a suitable sealed threaded stopper (not shown). The intermediate radial bore  54  is in fluid communication with inlet port  26  via an annular channel  58  in the inner diameter of the opening  36 . Flow through the guide  34  and the radial bores  52  and  56  is controlled by a valve  60  which is sized to be capable of simultaneously covering radial bores  52  and  56  so that both the inlet  26  and return  30  ports are closed off at approximately mid-stroke.  
         [0026]    The valve  60  is a generally cylindrical inverted spool shaped member slidably fit around the outer diameter of the guide  34 . The valve  60  has outwardly tapered ends forming a leading metering edge  62  and a trailing metering edge  64 . The valve  60  also has a circumferential shoulder  66  at an intermediate position extending radially outward. The valve  60  is retained in an annular valve carrier  68  having a flange  70  at one end engaging the shoulder  66  of the valve  60  so as to move with the valve  60 . The valve carrier  68  is held off of the inner end  50  of the guide  34  by a boss element  72 . The valve  60  and the valve carrier  68  are biased axially away from the outer end  48  of the guide  34  by a helical compression spring  74  disposed about a portion of the valve  60  and extending between a spring retainer  65  and the shoulder  66 . The spring retainer  65  is an annular perforated aluminum member disposed about the guide  34  allowing flow to by the spring  74  without effecting the spring rate. The guide  34  and valve  60  are preferably a non-corrosive material, such as 300 series stainless steel and the valve carrier  68  is a light-weight, non-corrosive material, such as aluminum. These elements along with spring  74  and spring retainer  65  combine to form a metering valve assembly  76 .  
         [0027]    The position of the valve  60  in the metering valve assembly  76  is controlled by the interaction of the valve carrier  68  and an axially translating armature  80 . An axial boss element  78  of the valve carrier  68  is biased in abutment with the armature  80  by the compression spring  74 . The armature  80  is a magnetically permeable disk, made for example of cold-rolled steel, disposed generally perpendicular to the stroke axis  38  within a ring  82  at the inner diameter of the housing  12  between the valve chamber  14  and the bobbin  18 . The ring  82  is stepped so it includes a circumferential radial surface  84  of greater diameter than the armature  80 . The ring  82  includes a circumferential groove  86  containing a resilient seal  88  for sealing the valve chamber  14  from the coil  20  contained in the bobbin  18 .  
         [0028]    The armature  80  is an annular disk with a stepped central aperture  90  in which is inserted in a stainless steel spring sleeve  92  extending through the center of the coil  20 . The spring sleeve  92  has a stepped end  94  engaging the corresponding surfaces of the aperture  90  and having a decreased outer diameter portion protruding axially past the armature  80 . The stepped end  94  includes axial bores  96  for receiving fasteners (not shown) for joining the spring sleeve  92  to the armature  80  so that it translates with the armature  80 . The spring sleeve  92  also defines an axial cavity  98  in which is disposed a helical compression spring  100  and through which a stainless steel elongated rod  102  extends axially. The rod  102  has a threaded head  104  threaded into an axial bore  106  in the stepped end  94  of the spring sleeve  92 . The spring sleeve  92  fits within a cylindrical center  108  of the bobbin  18 . One end extends through an annular opening  110  in the backing plate  19  and is internally threaded to mate with a plug  112 . The plug  112  has a flanged head  114  that engages the annular opening  110  and an elongated axial tail  116  having an axial bore  118  for receiving the rod  102 . The bore  118  in the tail  116  is sized to accept a cylindrical transducer element  120  made of a suitable iron alloy of greater diameter than the end of the rod  102  and permit the transducer element  120  to translate axially. A sensing coil  122  is disposed about the tail  116 , and thereby the transducer element  120 . The transducer element  120  and sensing coil  122  provide a position transducer for gauging the actual position of the valve  60 .  
         [0029]    This configuration allows the two compression springs  74  and  100  to apply opposing forces on the armature  80 . The springs  74  and  100  provide linear spring rates, however, spring  100  has a higher spring rate than spring  74 . Thus, the armature is biased away from the coil  20  and the valve  60  is biased to close radial bore  56  in the guide  34 , and thereby shut off inlet port  26 , as shown in FIG. 2. In this closed position, radial bore  52  is open so that fuel in the valve chamber  14  can be drawn through the guide  34  and out the outlet port  28  to a return line leading to the fuel tank.  
         [0030]    The coil  20  and armature act like a solenoid such that when the coil  20  is energized by an input signal it creates lines of magnetic flux that interact with the armature  80  by following closed looped paths through gaps  123  around the coil  20  and adjacent to the armature  80 . Non-magnetic reluctors  125 , preferably made of stainless steel, are fit into openings in the bobbin  18  to prevent the flux from bypassing the armature  80 . The magnetic flux provides an attractive force that, in combination with the spring force of spring  74 , is sufficient to overcome the spring force of spring  100  and cause the armature  80  to translate along the stroke axis  38  toward the coil  20 . As the armature  80  translates, spring  74  forces the valve  60  along the stroke axis  38  to open the radial bore  56  and close radial bore  52 . This allows fuel to flow through the inlet port  26  through the proper bores in the guide  34  and out to the valve chamber  14  where it can exit the device  10  through the outlet port  28 , as shown by the arrows in FIG. 1. When the valve  60  travels the full stroke, which is approximately 0.2 inches, the valve is in the position shown by FIG. 3.  
         [0031]    Referring now to FIGS. 2, 3 and  4 , the armature  80  is positioned in close proximity to the coil  20  with a small air gap  123  therebetween. The magnetic flux produced by the coil  20  and acting on the armature  80  is highly non-linear when in such close relation. The armature  80 , and thereby the valve  60 , would ordinarily translate non-linearly or non-proportionally to the input signal supplied to the coil  20 . To prevent this, a piece-wise non-linear spring  124  is mounted to the armature  80  concentric with the stroke axis  38 . The non-linear spring  124  provides stepped or piece-wise forces opposing the armature  80  at different parts of the stroke as it is pulled by the energized coil  20 . As will be explained, the non-linear spring  124 , in combination with spring  100 , provides five distinct spring rates opposing the non-linear force generated by the magnetic flux so that movement of the armature  80 , and thereby the valve  60 , is proportional to the value of the input signal to the coil  20 .  
         [0032]    Referring to FIGS. 2 and 3, the non-linear spring  124  includes two identical flexures springs  126  and  128  spaced apart in tandem substantially perpendicular to the stroke axis  38 . As shown in FIG. 4, each flexure spring is generally a thin disk (approximately 0.018 inches) with a circular opening  130  in the center and four spring fingers  132  extending radially outward at right angles from each other. The spring fingers  132  have three free edges and can be deflected independent of the other spring fingers.  
         [0033]    The flexure springs  126  and  128  are joined to together and to the armature  80  at quarter-round sections  134  between the spring fingers  132 . The quarter round sections  134  have through bores  136  receiving bolts  138  for threading into threaded bores  140  in the armature  80 . A suitable spacer or washer (not shown) is disposed around each bolt  138  to maintain the flexure springs  126  and  128  spaced apart when mounted to the armature  80 . The opening  130  in the flexure spring  126  adjacent the armature  80  fits around the protruding end of the spring sleeve  92 . Four rectangular lugs  142  with bores disposed about the bolts  138  are used to distribute the loads connecting the flexure springs  126  and  128  together. The valve carriage  68  has slots  144  that accommodate the lugs  142  without interfering with the movement of the metering valve assembly  76 .  
         [0034]    The spring fingers  132  are sized so that their tips extend radially past the quarter-round sections  134 . Each flexure spring  126  and  128  has two, oppositely extending spring fingers  132  with tips having oblique contact surfaces  146  tapering away from the coil  20 . Thus, each flexure spring  126  and  128  includes two pair of opposite spring fingers, one pair having a squared tip and the other pair having a tapered tip. The difference in thickness at the midpoint of the contacting surfaces  146  of the tapered spring fingers and the squared spring fingers is approximately 0.002 inches.  
         [0035]    Referring now to FIGS. 2, 3 and  5 A- 5 F, when the coil  20  is not energized, spring  100  biases the armature  80  away from the coil  20  which biases the valve  60  to close off flow from the inlet port so that the device  10  is as shown in FIG. 2. In this position, the spring fingers  132  are not deflected as shown in FIG. 5A and only one spring force is acting against the armature  80 . When the coil  20  is energized with an input signal, the generated magnetic flux begins to pull the armature  80 . As the armature  80  moves through the stroke in this direction, the pair of spring fingers  132  in the first flexure spring  126  with the thicker, squared tips will substantially simultaneously contact the fixed radial surface  84  of the ring  82  mounted to the interior of the housing  12 , as shown in FIG. 5B. As the armature  80  continues in this direction these spring fingers will begin deflecting away from the armature  80  and imparting a second spring force opposing the armature  80 . As shown in FIG. 5C, these spring fingers will continue deflecting as the armature  80  is translated and the tapered spring fingers will abut the radial surface  84  at the oblique contact surface  146  and then impart a third spring force against the armature  80  as they are deflected. Then, as the armature  80  continues to translate toward the coil  20 , a fourth spring force is applied against the armature  80  after the pair of square tipped spring fingers of the second flexure spring  128  contact the squared spring fingers of the first flexure spring  126  and are deflected away from the armature  80 , as shown in FIGS. 5D and 5E. Finally, a fifth spring force is applied against the armature  80  as it continues to translate after the second pair of tapered spring fingers abut the back of the first pair of tapered fingers already deflected away from the armature  80 , as shown in FIG. 5F. In this position, the valve  60  is at the end of the stroke and is position to close off the return port  30  and completely open the inlet port  26 .  
         [0036]    Thus, as mentioned, the compression spring  100  and the flexure springs  126  and  128  combine to provide five distinct linear spring rates opposing the pull of the armature  80 . These five spring rates are selected to provide opposing non-linear forces throughout the entire stroke corresponding to the non-linear pulling forces acting on the armature  80  by the magnetic flux to effectively cancel out the non-linearity so that the position of the valve  60  varies in proportion to the input signal (current) to the coil  20 . This allows the metering device  10  to be used in applications, such as positioning actuators or delivering fuel to jet engines requiring precision control of fluid metering.  
         [0037]    As mentioned, the metering device  10  operates according to the input signal sent to the coil  20 . Referring to FIG. 6, the signal is preferably generated by an electronic control unit  148  in response to a signal from a user interface  150 . The control unit  148  can be any suitable digital processing device, such as an on-board computer, having a suitable memory and I/O interface. In response to an input from the user interface  150 , the control unit  148  can send an input signal of a particular electric current value to the coil  20  to open the valve  60 . Pressurized hydraulic oil, for example, can be pumped from reservoir  154  through fluid line  156  to the inlet port  26 , through the valve chamber  14  and out outlet port  28  (see FIG. 1) to a suitable hydraulic actuator  158 . If the control unit  148  provides a signal to cut back power to the actuator, the valve  60  can be positioned to allow fluid within the valve chamber  14  to be evacuated to the reservoir  154  via return port  30  and return line  160 .  
         [0038]    To make the metering device  10  even more precise, the position transducer provides a feedback signal to the control unit  148  corresponding to the position of the transducer element  120  in the sensing coil  122 , and thereby, the actual position of the valve  60 . The control unit  148  provides the feedback signal to a suitable electronic comparator  162 , which compares the feedback signal, corresponding to the actual position of the valve  60 , to the input signal sent to the coil  20 , corresponding to the commanded position. If the actual position and the commanded position are not the same, or within an acceptable range, the control unit  148  will execute stored algorithms to provide a corrected signal to the coil  20  until the difference is at or near zero.  
         [0039]    A preferred embodiment of the invention has been described herein in detail. The invention may, however, include other aspects not specifically delineated in the aforementioned preferred embodiment. For example, the non-linear spring assembly could take other forms, such as having radially inwardly extending spring fingers or variously sized axially extending spring elements. Moreover, it is mentioned that the device is also suitable for accurately metering liquid fuel to jet engines. In that case, the return port and corresponding passages would be eliminated. Thus, the above in no way is intended to limit the scope of the invention. Accordingly, in order to apprise the public of the full scope of the present invention, reference must be made to the following claims.