Abstract:
The disclosed embodiments include an apparatus and a method for controlling fluid flow. In accordance with certain of the disclosed embodiments, a valve is disclosed that comprises a yoke that encases a coil. The yoke has a gap adjacent to a central aperture of the yoke. A core tube is placed in the central aperture. The core tube has a core slidingly disposed within an interior chamber for controlling a fluid flow rate from a flow inlet to a flow outlet. A current is provided to the coil. Magnetic flux, generated by the coil in response to the current, is directed through the gap to cause the core to move proportionally based on the amount of current provided to the coil for changing the fluid flow rate from the flow inlet to the flow outlet.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This Application claims priority to U.S. Provisional Patent Application Ser. No. 61/122,604 filed on Dec. 15, 2008, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Measurement of a fluid flow-rate is an important practice in a wide variety of applications. For example, in biotechnology or semiconductor applications, the flow rate of gases is critical to growth (of cells, solid-state layers, etc.). Too much flow or too little flow will “spoil” the process result. Flow-measurement can come via several different technologies, such as differential-pressure, thermal-mass-flow, and coriolis-mass-flow, but many others exist as well. 
     However, not only is the measurement of these flows important, but the end-user is frequently extremely concerned about the control of those flow rates. This control is most typically performed via a “modulating” or “proportional” control-valve, passing more or less fluid (gas or liquid) from a source (e.g. tank) to a process/destination. In other words, as the process needs more or less flow, a “setpoint” (desired flow) will be calculated and communicated to a “loop-controller,” then compared to the actual flow (process-measured-variable), and the difference will cause the motion of the valve (more-open, or more-closed). 
     As with flow measurement technologies, there are many techniques to build a control-valve, but they typically involve opening or closing a path for fluid to pass through. Typical constructions are: poppet, ball, needle, pinch, guillotine, etc. Each has its performance advantages and disadvantages, as well as cost differences. So, the unique process-application will determine the best choice. 
     For safety reasons, valves come in several configurations, so that when power is removed from the system, the valve will go open, closed, or will “fail-in-place.” Most often, a normally-closed valve is selected (e.g. stop adding fluid to a process), but a normally-open valve is sometimes specified as well. 
     In the context of low flow measurement and control devices, the fluid quantities passed through the devices are on the order of several cm3/minute of gas, or several grains/hour of liquids. These small quantities are typically used for a critical component, such as dopants, catalaysts, additives, etc. These low flow devices are also typically quite small. Thus, the manufacture of these small devices is quite demanding. Additionally, the opportunity for a process to block these small devices and associated cavities is high. 
     In the low flow context, many of the valve construction types mentioned above are simply impractical to build due to small geometries. For manual adjustments, a needle-valve is sometimes employed. The needle valve typically includes a tapered pin that is inserted into or withdrawn from an orifice. As the pin is withdrawn, more orifice cross-sectional area is available for flow to pass. The typical length of the needle is on the order of a centimeter or more. However, as manual controls give way to automated controls, this long tapered needle requires a large positioner to be moved, which causes the valve to become bulky. 
     Another common low flow valve is a poppet-valve. In a poppet-valve, a flat seat is placed over an orifice to close off flow or lifted a small amount, so that the fluid may flow through an annular ring and then pass through the orifice. Unlike the aforementioned needle-valve, the movement of the poppet, from open to close or vice-versa, represents a very short stroke. Consequently, a solenoid is often the positioner employed with the low flow poppet valves. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the disclosed invention include an apparatus and a method for controlling fluid flow. In one embodiment, a valve for controlling fluid flow is disclosed comprising a mounting body having a fluid inlet and a fluid outlet; a core tube adjacent to the mounting body, the core tube having an interior chamber with a proximal end and a distal end; a core slidingly disposed within the core tube interior chamber, the core having a proximal end, a distal end and a center located between the proximal end and the distal end, the core movable between an open position and a closed position relative to the mounting body, wherein the core distal end engages at least one of the mounting body fluid inlet and mounting body fluid outlet to block fluid flow therethrough when the core is in the closed position; at least one biasing member operable to bias the core in at least one of the open and closed positions; a coil disposed about the core tube, the coil operable to cause the core to move between the open position and the closed position when energized; and a yoke encasingly disposed about the coil, the yoke having an opening adjacent to the core tube, the opening having a center, wherein a distance between the core center and the yoke opening center decreases when the coil is energized. 
     In another embodiment, a yoke for with a valve is disclosed comprising a top yoke hub; a bottom yoke hub; an inner portion housing a coil; an aperture for receiving a core tube, the core tube having a core slidingly disposed within an interior chamber; and a gap between the top yoke hub and the bottom yoke hub, the gap having a center at a midpoint between a first end of the top yoke hub and a second end of the bottom yoke hub, and wherein magnetic flux generated by the coil when energized is directed through the gap to cause the core to move. 
     Still, in yet another embodiment, a method of controlling fluid flow is disclosed comprising encasing a coil in a yoke, the yoke having a gap adjacent to a central aperture of the yoke; placing a core tube in the central aperture of the yoke; the core tube having a core slidingly disposed within an interior chamber for controlling a fluid flow rate from a flow inlet to a flow outlet; providing current to the coil; and directing magnetic flux, generated by the coil in response to the current, through the gap to cause the core to move for changing the fluid flow rate from the flow inlet to the flow outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, cross-sectional view of a normally closed valve assembly showing a valve assembly in a closed position in accordance with certain of the embodiments; 
         FIG. 1A  is an exploded cross-sectional view of a needle in contact with an outlet for stopping fluid flow in accordance with certain of the embodiments; 
         FIG. 2  is a schematic, cross-sectional view of the valve assembly of  FIG. 1  showing the valve assembly in an open position in accordance with certain of the embodiments; 
         FIG. 3  is a schematic, cross-sectional view of a normally open valve assembly showing the valve assembly in an open position in accordance with certain of the embodiments; 
         FIG. 4  is a schematic, cross-sectional view of the valve assembly of  FIG. 3  showing the valve assembly in a closed position in accordance with certain of the embodiments; 
         FIG. 5  is a schematic, cross-sectional view of an alternative yoke assembly in accordance with certain of the embodiments; and 
         FIG. 6  is a schematic, perspective view of the yoke assembly of  FIG. 5  in accordance with certain of the embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of several illustrative embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical structural, mechanical, electrical, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the invention, the description may omit certain information known to those skilled in the art. The following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Referring now to  FIGS. 1 and 2 , an illustrative valve assembly  100  is shown. The valve assembly  100  includes a core  105 , core tube  110 , coil  115 , yoke assembly  120 , and mounting body  125 . The illustrative valve assembly  100  has a circular cross-section when viewed in a top plan view; however, it will be appreciated that the valve assembly  100  may have any suitable cross-section. The mounting body  125  includes an orifice body  150 . The orifice body  150  includes a flow inlet  152  and a flow outlet  154 . The mounting body  125  is operable to fluidly couple the valve assembly  100  to a flow body  800 , such as a fluid conduit for transporting or carrying a fluid from one point to another, such that the orifice body  150  is disposed within a passageway for the flow of fluid whereby the fluid is capable of passing from the flow inlet  152  to the flow outlet  154 . As will be discussed further below, the core  105  is operable to control the amount of fluid that is allowed to pass from the flow inlet  152  to the flow outlet  154  and thereby control the amount and/or rate of fluid that is permitted to pass through the flow body. 
     The core tube  110  encases the core  105 . The core tube  110  is formed from any suitable material that is not ferromagnetic. The core tube  110  may be mounted to the mounting body  125  by any suitable means, including, but not limited to, adhesive, welding, mechanical fasteners, or any other suitable fastening means. Alternatively, the core tube  110  may be integrally formed with the mounting body  125 . In yet another alternative, the core tube  110  may merely rest atop the mounting body  125 . 
     The core  105  includes a proximal end  156  and a distal end  158 . The distal end  158  includes a stop  160  and a needle  162  projecting therefrom. The needle  162  and flow outlet  154  are complimentarily shaped such that movement of the needle  162  relative to the flow outlet  154  permits proportional control of fluid passing from the flow inlet  152  through the flow outlet  154 . For example, in the illustrative embodiment, the needle  162  and flow outlet  154  have complimentary tapered cross-sections whereby the needle  162  may be received within the flow outlet  154  such that when the needle  162  fully engages the flow outlet  154 , the stop  160  rests atop the orifice body  150  and fluid is not permitted to pass through the flow outlet  154 ; this is referred to as the closed position. In other words, the stop  160  provides a means to close the valve and prevent fluid to pass through from inlet  152  to outlet  154 . Alternatively, in some embodiments, fluid flow may be stopped by having a side surface  190  of needle  162  fully contact the sides of outlet  154  as core  105  is moved towards a closed position, e.g. by means of a conical needle into a conical flow outlet hole as illustrated in  FIG. 1A . As will be discussed further below, as the core  105  is moved from the closed position to an open position, the needle  162  is withdrawn from the flow outlet  154  and fluid is permitted to pass from the flow inlet  152  to the flow outlet  154 . As the needle  162  is withdrawn further from the flow outlet  154 , the amount of fluid that is permitted to pass from the flow inlet  152  to the flow outlet  154  increases. While the illustrative embodiment shows the needle  162  and flow outlet  154  as having tapered cross-sections, it will be appreciated that the needle  162  and flow outlet  154  may have any suitable complimentarily shaped cross-sections. Additionally, in the illustrative embodiment, the core  105  is formed from a ferromagnetic material such that the core  105  may be excited and moved by the coil  115  when the coil  115  is energized; however, it will be appreciated that the core  105  may be formed from any suitable magnetic material. Moreover, the core  105  may have one or more grooves or slots  164  to prevent dashpot effects that may otherwise develop as the core  105  is moved between an open and closed position while fluid is passing from the flow inlet  152  to the flow outlet  154 . A dashpot effect may cause core movement resistance due to viscous friction when the core  105  is moving and fluid volumes are displaced. Therefore, grooves  164  in the core  105  may lower the viscous friction to avoid negative effects on fluid control. 
     The core  105  is slidingly and concentrically disposed within the core tube  110  such that the core  105  is movable between an open position and a closed position relative to the orifice body  150 . A first spring  166  is positioned between the proximal end  156  of the core  105  and the upper end of the interior portion of the core tube  110 . A second spring  168  is positioned between the distal end  158  of the core  105  and the mounting body  125  near the upper portion of the orifice body  150 . In the illustrative embodiment, the first spring  166  is a conical compression spring and the second spring  168  is a conical compression spring where the spring constant (k) of the first spring  166  is greater than the spring constant of the second spring  168  such that the core  105  is biased to a closed position when the coil is not energized, i.e., the valve assembly  100  is “normally closed” ( FIG. 1 ). As will be discussed further below, the core  105  remains in a closed position unless or until the coil  115  is energized. In an alternative embodiment, the position of the first and second springs  166 ,  168  may be reversed such that the core  105  is biased to an open position when the coil  115  is not energized, i.e., the valve assembly  100  is “normally open” and remains open unless or until the core  105  is energized. Also, while the illustrative embodiment employs conical compression springs, it will be appreciated that any type or number of resilient members, or biasing members, may be used to bias the core  105  to an open position, a closed position, or any other suitable position. 
     The coil  115  is disposed about the core tube  110 . The coil  115  may be any magnetic inductive coil operable to generate a magnetic field that is capable of moving the core  105  between a closed position and an open position. The coil  115  is at least partially encased by the yoke assembly  120 , which is formed from a ferromagnetic material or any other suitable magnetic material. The coil  115  and yoke assembly  120  may be maintained in position around the core tube via a retaining clip disposed about the top of the core tube and top of the yoke assembly  120 ; however, it will be appreciated that any suitable retaining means may be employed. The yoke assembly  120  includes a top yoke hub  130 , a yoke  135  and a bottom yoke hub  140 . Each of the top yoke hub  130 , bottom yoke hub  140  and yoke  135  are generally tubular bodies and include an aperture for receiving the core tube  110  therein. A portion of each of the yoke hubs  130 ,  140  may extend into the yoke  135 . The top yoke hub  130  and bottom yoke hub  140  may be coupled to the yoke  135  by any suitable means, including, but not limited to, adhesive, welding, mechanical fasteners, or any other suitable fastening means. Alternatively, the yoke hubs  130 ,  140  may abut or rest on or against the yoke  135 . 
     The yoke assembly  120  is arranged such that a gap  145  exists between the top yoke hub  130  and bottom yoke hub  140 . The gap  145  includes a center  172  that is the midpoint between the ends of the portions of the yoke hubs  130 ,  140  that extend into the yoke  135 . The gap  145  is adjacent core tube  110  and permits magnetic flux generated by the coil  115 , when the coil  115  is energized, to bridge the gap in the yoke assembly  120  through the core  105  and causes the core  105  to move. In other words, the yoke assembly  120  directs the magnetic flux generated by the coil  115  through the gap  145  such that when the coil  115  is energized, the center  170  of the core  105  (i.e., the core&#39;s center of gravity) is drawn towards the center  172  of the gap  145 . Proportional control of core  105  is provided by controlling the amount of current provided to the coil  115 . Thus, slight current provided to the coil  115  provides slight movement of the core  105 , and, in one embodiment, when the coil  115  is fully energized, the center  170  of the core  105  and the center  172  of the gap  145  are substantially aligned. In other words, the distance between the center  170  of the core  105  and the center  172  of the gap  145  decreases as the coil is energized. The change in the distance between the center  170  of the core  105  and the center  172  of the gap  145  is in proportion to an amount of current provided to the coil  115 . Therefore, as will be further described, the change in fluid flow from the flow inlet  152  to the flow outlet  154  is in proportion to an amount of current provided to the coil  115  to move the core  105  in the direction of an open position or a closed position. 
     When the valve assembly  100  is normally closed, as in the illustrative embodiment shown in  FIGS. 1 and 2 , the gap  145  is nearer the upper portion  174  of the core tube  110 . Alternatively, when the valve assembly is normally open, the gap is nearer the mounting body  125 . In a normally closed valve assembly, as shown in  FIGS. 1-2 , when the center  170  of the core  105  is aligned with the center  172  of the gap  145 , the valve assembly  100  is fully open. In an alternative embodiment, in a normally open valve assembly, when the center of the core is aligned with the center of the gap, the valve assembly is fully closed. 
     The following description will be made in reference to the normally closed valve assembly  100  of  FIGS. 1 and 2 ; however, it will be appreciated that the valve assembly may be normally open and remain within the scope of the present invention. In use, the mounting body  125  is coupled to a flow body such that the orifice body  150  is introduced to a passageway for the flow of fluid whereby fluid is only permitted to further pass through the flow body by passing from the flow inlet  152  to the flow outlet  154 . In the closed position, the needle  162  is fully engaged with the flow outlet  154  such that the stop  160  rests atop the orifice body  150  and fluid is not permitted to pass from the flow inlet  152  to the flow outlet  154 . 
     As current is provided to the coil  115 , the biasing force provided by the first spring  166  is overcome and the center  170  of the core  105  is pulled towards the center  172  of the gap  145  of the yoke assembly  120  and the needle  162  is drawn away from the flow outlet  154  thereby permitting fluid to pass from the flow inlet  152  to the flow outlet  154 . The second spring  168  begins to extend and serves to maintain the core  105  in a centered position relative to the core tube  110  and coil  115 . Slight increases in current provided to the coil  115  result in slight movement of the core  105  and slight opening of the valve assembly  100 . As the current provided to the coil  115  increases, the core  105  is drawn further from the orifice body  150  the distance between the needle  162  and the flow outlet  154  increases; whereby the amount of fluid that is permitted to pass from the flow inlet  152  to the flow outlet  154  increases. When the coil  115  is fully energized, the center  170  of the core  105  is aligned with the center  172  of the gap  145  of the yoke assembly  120  and the valve is fully open. 
     Referring now to  FIGS. 3 and 4 , an illustrative valve assembly  200  is shown. The valve assembly  200  includes a core  205 , core tube  210 , coil  215 , yoke assembly  220 , and mounting body  225 . The illustrative valve assembly  200  has a circular cross-section when viewed in a top plan view; however, it will be appreciated that the valve assembly  200  may have any suitable cross-section. The mounting body  225  includes an orifice body  250 . The orifice body  250  includes a flow inlet  252  and a flow outlet  254 . The mounting body  225  is operable to fluidly couple the valve assembly  200  to a flow body  800 , such as a fluid conduit for transporting or carrying a fluid from one point to another, such that the orifice body  250  is disposed within a passageway for the flow of fluid whereby the fluid is capable of passing from the flow inlet  252  to the flow outlet  254 . As will be discussed further below, the core  205  is operable to control the amount of fluid that is allowed to pass from the flow inlet  252  to the flow outlet  254  and thereby control the amount and/or rate of fluid that is permitted to pass through the flow body. 
     The core tube  210  encases the core  205 . The core tube  210  is formed from any suitable material that is not ferromagnetic. The core tube  210  may be mounted to the mounting body  225  by any suitable means, including, but not limited to, adhesive, welding, mechanical fasteners, or any other suitable fastening means. Alternatively, the core tube  210  may be integrally formed with the mounting body  225 . In yet another alternative, the core tube  210  may merely rest atop the mounting body  225 . 
     The core  205  includes a proximal end  256  and a distal end  258 . The distal end  258  includes a stop  260  and a needle  262  projecting therefrom. The needle  262  and flow outlet  254  are complimentarily shaped such that movement of the needle  262  relative to the flow outlet  254  permits proportional control of fluid passing from the flow inlet  252  through the flow outlet  254 . For example, in the illustrative embodiment, the needle  262  and flow outlet  254  have complimentary tapered cross-sections whereby the needle  262  may be received within the flow outlet  254  such that when the needle  262  fully engages the flow outlet  254 , the stop  260  rests atop the orifice body  250  and fluid is not permitted to pass through the flow outlet  254 ; this is referred to as the closed position. However, as will be discussed further below, as the core  205  is moved from the closed position to an open position, the needle  262  is withdrawn from the flow outlet  254  and fluid is permitted to pass from the flow inlet  252  to the flow outlet  254 . As the needle  262  is withdrawn further from the flow outlet  254 , the amount of fluid that is permitted to pass from the flow inlet  252  to the flow outlet  254  increases. While the illustrative embodiment shows the needle  262  and flow outlet  254  as having tapered cross-sections, it will be appreciated that the needle  262  and flow outlet  254  may have any suitable complimentarily shaped cross-sections. Additionally, in the illustrative embodiment, the core  205  is formed from a ferromagnetic material such that the core  205  may be excited and moved by the coil  215  when the coil  215  is energized; however, it will be appreciated that the core  205  may be formed from any suitable magnetic material. Moreover, the core  205  may have one or more grooves or slots  264  to prevent dashpot effects that may otherwise develop as the core  205  is moved between an open and closed position while fluid is passing from the flow inlet  252  to the flow outlet  254 . 
     The core  205  is slidingly and concentrically disposed within the core tube  210  such that the core  205  is movable between an open position and a closed position relative to the orifice body  250 . A first spring  266  is positioned between the proximal end  256  of the core  205  and the upper end of the interior portion of the core tube  210 . A second spring  268  is positioned between the distal end  258  of the core  205  and the mounting body  225  near the upper portion of the orifice body  250 . In the illustrative embodiment, the first spring  266  is a conical compression spring and the second spring  268  is a conical compression spring where the spring constant (k) of the second spring  268  is greater than the spring constant of the first spring  266  such that the core  205  is biased to an open position when the coil is not energized, i.e., the valve assembly  200  is “normally open” ( FIG. 3 ). As will be discussed further below, the core  205  remains in an open position unless or until the coil  215  is energized. Also, while the illustrative embodiment employs conical compression springs, it will be appreciated that any type or number of resilient members, or biasing members, may be used to bias the core  205  to an open position, a closed position, or any other suitable position. 
     The coil  215  is disposed about the core tube  210 . The coil  215  may be any magnetic inductive coil operable to generate a magnetic field that is capable of moving the core  205  between a closed position and an open position. The coil  215  is at least partially encased by the yoke assembly  220 , which is formed from a ferromagnetic material or any other suitable magnetic material. The coil  215  and yoke assembly  220  may be maintained in position around the core tube via a retaining clip disposed about the top of the core tube and top of the yoke assembly  220 ; however, it will be appreciated that any suitable retaining means may be employed. The yoke assembly  220  includes a top yoke hub  230 , a yoke  235  and a bottom yoke hub  240 . Each of the top yoke hub  230 , bottom yoke hub  240  and yoke  235  are generally tubular bodies and include an aperture for receiving the core tube  210  therein. A portion of each of the yoke hubs  230 ,  240  may extend into the yoke  235 . The top yoke hub  230  and bottom yoke hub  240  may be coupled to the yoke  235  by any suitable means, including, but not limited to, adhesive, welding, mechanical fasteners, or any other suitable fastening means. Alternatively, the yoke hubs  230 ,  240  may abut or rest on or against the yoke  235 . 
     The yoke assembly  220  is arranged such that a gap  245  exists between the top yoke hub  230  and bottom yoke hub  240 . The gap  245  includes a center  272  that is the midpoint between the ends of the portions of the yoke hubs  230 ,  240  that extend into the yoke  235 . The gap  245  is adjacent core tube  210  and permits magnetic flux generated by the coil  215 , when the coil  215  is energized, to bridge the gap in the yoke assembly  220  through the core  205  and causes the core  205  to move. In other words, the yoke assembly  220  directs the magnetic flux generated by the coil  215  through the gap  245  such that when the coil  215  is energized, the center  270  of the core  205  (i.e., the core&#39;s center of gravity) is drawn towards the center  272  of the gap  245 . Proportional control of core  205  is provided by controlling the amount of current provided to the coil  215 . Thus, slight current provided to the coil  215  provides slight movement of the core  205 , and when the coil  215  is fully energized, the center  270  of the core  205  and the center  272  of the gap  245  are aligned. 
     When the valve assembly  200  is normally open, as in the illustrative embodiment shown in  FIGS. 3 and 4 , the gap  245  is nearer the mounting body  225 . In the normally open valve assembly, as shown in  FIGS. 3-4 , when the center  270  of the core  205  is aligned with the center  272  of the gap  245 , the valve assembly  200  is fully closed. 
     The following description will be made in reference to the normally open valve assembly  200  of  FIGS. 3 and 4 . In use, the mounting body  225  is coupled to a flow body such that the orifice body  250  is introduced to a passageway for the flow of fluid whereby fluid is only permitted to further pass through the flow body by passing from the flow inlet  252  to the flow outlet  254 . In the open position, the needle  262  is fully withdrawn from with the flow outlet  254  such that fluid is permitted to pass from the flow inlet  252  to the flow outlet  254 . 
     As current is provided to the coil  215 , the biasing force provided by the second spring  268  is overcome and the center  270  of the core  205  is pulled towards the center  272  of the gap  245  of the yoke assembly  220  and the needle  262  is drawn towards into the flow outlet  254  towards a closed position. The first spring  266  begins to extend and serves to maintain the core  205  in a centered position relative to the core tube  210  and coil  215 . Slight increases in current provided to the coil  215  result in slight movement of the core  205  and slight closing of the valve assembly  200 . As the current provided to the coil  215  increases, the core  205  is drawn closer to the orifice body  250  the distance between the needle  262  and the flow outlet  254  decreases. This results in a lesser amount of fluid that is permitted to pass from the flow inlet  252  to the flow outlet  254 . When the coil  215  is fully energized, the center  270  of the core  205  is aligned with the center  272  of the gap  245  of the yoke assembly  220  and the valve is fully closed. 
       FIGS. 5 and 6  show an alternative embodiment of a yoke assembly  500  for use in a valve assembly  100 ,  200  similar to that as shown in  FIGS. 1-4 . The upper yoke hub  505  has a tubular body  510  and a flange  515 . The upper yoke hub  505  includes an aperture  520  for receiving a core tube therein. The lower yoke hub  522  has a tubular body  525  and a flange  530 . The lower yoke hub  522  includes an aperture  535  for receiving a core tube therein. The upper and lower yoke hubs  505 ,  522  are connected by a yoke  540  such that a gap  545  exists therebetween. Unlike the yoke  135  of  FIGS. 1-4 , which is generally tubular in shape, the present yoke  540  is a bracket that connects the yoke hubs  505 ,  522  to one another. For example, the yoke  540  may be a piece of sheet metal formed with two 90-degree angles such that the yoke hubs  505 ,  522  may be connected thereby; an upper portion  550  is coupled to the upper yoke hub  505  and a lower portion  555  is connected to the lower yoke hub  522 , wherein the upper and lower portions  550 ,  555  are coupled together via a central portion  560 . However, it will be appreciated that the yoke hubs  505 ,  522  may be connected by any suitable means and a yoke assembly in accordance with the present invention is not limited to employing either a tubular yoke  135  or a bracket yoke  540 . 
     The disclosed embodiments provide several advantages over the existing control-valves. For example, the disclosed embodiments provide for a long stroke actuator, as opposed to a short stroke actuator, such as, but not limited to, a poppet valve. By providing a long stroke actuator, the disclosed embodiments enables precise control of fluid flow in proportion to an amount of current provided to the control valve. In addition, the disclosed embodiments utilize low precision yoke hubs that are mounted in unwetted space (i.e., not in contact with potentially corrosive process fluids). 
     Additionally, because the fail mode of a valve is often not determined until late in the development process, certain of the disclosed embodiments may be converted from a normally-open mode to a normally-closed mode, and vice versa, by simply replacing the top and bottom springs (e.g., first and second springs  166  and  168 ) and the yoke hubs. In some embodiments, the top and bottom springs and/or the top and bottom yoke hubs may simply be swapped to convert from a normally-open mode to a normally-closed mode, and vice versa. In other embodiments, the top and bottom springs and/or the top and bottom yoke hubs may be replaced with different springs (e.g., having a different spring constant (k)) and/or different yoke hubs respectively. Therefore, the disclosed embodiments reduce inventory costs associated with maintaining normally-open and normally closed valves, and increases customer-responsiveness. 
     Further, in certain embodiments, the valve fashioned in a single unit that is easily inserted and replaceable in a mass flow controller. In other words, all of the ‘works’ of the valve come within a single subassembly. This embodiment offers several advantages, such as, but not limited to, removing from the balance of the instrument any critical dimensional features (lowering its cost), as well as easing the development-path to adding embodiments of the disclosed valve onto many disparate products and assemblies. Accordingly, the disclosed embodiments maintain low-cost and high-manufacturability while yielding a precise flow control resolution. 
     Although the present invention and its advantages have been disclosed in the context of certain illustrative, non-limiting embodiments, it should be understood that various changes, substitutions, permutations, and alterations can be made without departing from the scope of the invention as defined by the appended claims. It will be appreciated that any feature that is described in a connection to any one embodiment may also be applicable to any other embodiment.