Abstract:
The disclosed embodiments include a mass flow controller for controlling a flow of a fluid. In one embodiment, the mass flow controller comprises an inlet for receiving the fluid; a flow path in which the fluid passes through the mass flow controller; a mass flow meter for providing a signal corresponding to mass flow of the fluid through the flow path, the mass flow meter having a bypass through which a majority of fluid flows; a turning vane positioned upstream of the bypass for generating a more uniform fluid flow; an adjustable valve for regulating the flow of the fluid out of an outlet of the mass flow controller; and a controller configured to apply a valve control signal to adjust the adjustable valve to a desired valve position to control the flow of the fluid out of an outlet of the mass flow controller.

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application Number PCT/US2014/015461 filed Feb. 9, 2014 and claims priority to U.S. Provisional Application No. 61/777,212 filed Mar. 12, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the operation of a mass flow controller (MFC) or a mass flow meter (MFM). 
     2. Discussion of the Related Art 
     Many industrial processes require precise control of various process fluids. For example, in the semiconductor industries, mass flow controllers are used to precisely measure and control the amount of a process fluid that is introduced to a process chamber. The term fluid is used herein to describe any type of matter in any state that is capable of flow. For example, it is to be understood that the term fluid may apply to liquids, gases, vapors, and slurries comprising any combination of matter or substance to which controlled flow may be of interest. 
     The maximum flow rate measured by a typical thermal mass flow sensor as utilized in a mass flow controller or a mass flow meter is typically limited to approximately 10 sccm depending on the sensor design. Consequently, all MFCs and MFMs utilize a flow bypass so the device can obtain higher flow rates. The perfect bypass would maintain a constant ratio. In other words, total flow for a MFC with a 10:1 flow bypass ratio would be 11 sccm when the sensor is reading 1 sccm and 110 sccm when the sensor is reading 10 sccm. Unfortunately, bypass ratio is affected by the bypass design, fluid properties and the flow path upstream and downstream of the sensor/bypass assembly. So the bypass ratio will change as the flow rate is increased. The change in flow ratio is commonly referred to as “bypass non-linearity.” Such non-linearity degrades the instrument range or turndown and therefore the instrument accuracy. High flow bypass designs typically exhibit extremely poor linearity. 
     Accordingly, the disclosed embodiments seek to provide one or more solutions to the above problems. In particular, the inventors determined that most of the non-linearity is due to entrance affects at the bypass inlet. Thus, the inventors recognized that any improvement in bypass linearity is valuable because it will improve accuracy on process gas and improve the range of the instrument. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present invention are described in detail below with reference to the attached figures, which are incorporated by reference herein and wherein: 
         FIG. 1  is a diagram that illustrates components of a mass flow controller in accordance with the disclosed embodiments; 
         FIG. 1A  is a diagram that depicts the speed contour of a flow meter device without an inlet mesh in accordance with the disclosed embodiments; 
         FIG. 1B  is a diagram that depicts the speed contour of a flow meter device with an inlet mesh in accordance with the disclosed embodiments; 
         FIG. 2  is a diagram that illustrates a turning vane with a flow mixer in accordance with the disclosed embodiments; 
         FIG. 3  is a diagram that illustrates the turning vane of  FIG. 2  in an assembly in accordance with the disclosed embodiments; 
         FIG. 4A  is a diagram that depicts the speed contour of a flow meter device that does not have a turning vane and mixer in accordance with the disclosed embodiments; 
         FIG. 4B  is a diagram that depicts the speed contour of a flow meter device that includes a turning vane and mixer in accordance with the disclosed embodiments; 
         FIG. 5A  is a diagram that depicts the pressure contour of a flow meter device that does not have a turning vane and mixer in accordance with the disclosed embodiments; 
         FIG. 5B  is a diagram that depicts the pressure contour of a flow meter device that includes a turning vane and mixer in accordance with the disclosed embodiments; 
         FIG. 6  is a graph depicting the difference in turndown between a flow meter device with a bypass mesh versus a flow meter with a bypass mesh plus a turning vane and mixer in accordance with the disclosed embodiments; 
         FIG. 7  is a diagram that illustrates an example of a turning vane in accordance with the disclosed embodiments; 
         FIG. 8  is a diagram that illustrates another example of a turning vane in accordance with the disclosed embodiments; 
         FIGS. 9A-9E  are diagrams that illustrate various configurations of a turning vane in an assembly in accordance with the disclosed embodiments; and 
         FIGS. 10A and 10B  illustrate yet another example of a turning vane in an assembly in accordance with the disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed embodiments and advantages thereof are best understood by referring to  FIGS. 1-10  of the drawings, like numerals being used for like and corresponding parts of the various drawings. Other features and advantages of the disclosed embodiments will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within the scope of the disclosed embodiments. Further, the illustrated figures are only exemplary and are not intended to assert or imply any limitation with regard to the environment, architecture, design, or process in which different embodiments may be implemented. 
       FIG. 1  illustrates components of a mass flow controller  100  in accordance with the disclosed embodiments. Mass flow controller  100  includes a step  110 , which is the platform on which the components of the mass flow controller are mounted. A thermal mass flow meter  140  and a valve assembly  150  are mounted on the step  110  between a fluid inlet  120  and a fluid outlet  130 . The valve assembly  150  includes a control valve  170 . The thermal mass flow meter  140  includes a bypass  142  through which typically a majority of fluid flows and a thermal flow sensor  146  through which a smaller portion of the fluid flows. 
     Thermal flow sensor  146  is contained within a sensor housing  102  (portion shown removed to show sensor  146 ) mounted on a mounting plate or base  108 . Sensor  146  is a small diameter tube, typically referred to as a capillary tube, with a sensor inlet portion  146 A, a sensor outlet portion  146 B, and a sensor measuring portion  146 C about which two resistive coils or windings  147 ,  148  are disposed. In operation, electrical current is provided to the two resistive windings  147 ,  148 , which are in thermal contact with the sensor measuring portion  146 C. The current in the resistive windings  147 ,  148  heats the fluid flowing in measuring portion  146  to a temperature above that of the fluid flowing through the bypass  142 . The resistance of windings  147 ,  148  varies with temperature. As fluid flows through the sensor conduit, heat is carried from the upstream resistor  147  toward the downstream resistor  148 , with the temperature difference being proportional to the mass flow rate through the sensor. 
     An electrical signal related to the fluid flow through the sensor is derived from the two resistive windings  147 , 148 . The electrical signal may be derived in a number of different ways, such as from the difference in the resistance of the resistive windings or from a difference in the amount of energy provided to each resistive winding to maintain each winding at a particular temperature. Examples of various ways in which an electrical signal correlating to the flow rate of a fluid in a thermal mass flow meter may be determined are described, for example, in commonly owned U.S. Pat. No. 6,845,659, which is hereby incorporated by reference. The electrical signals derived from the resistive windings  147 , 148  after signal processing comprise a sensor output signal. 
     The sensor output signal is correlated to mass flow in the mass flow meter so that the fluid flow can be determined when the electrical signal is measured. The sensor output signal is typically first correlated to the flow in sensor  146 , which is then correlated to the mass flow in the bypass  142 , so that the total flow through the flow meter can be determined and the control valve  170  can be controlled accordingly. The correlation between the sensor output signal and the fluid flow is complex and depends on a number of operating conditions including fluid species, flow rate, inlet and/or outlet pressure, temperature, etc. 
     A bypass  142  may then be mounted to the sensor, and the bypass  142  is tuned with the known fluid to determine an appropriate relationship between fluid flowing in the mass flow sensor and the fluid flowing in the bypass at various known flow rates, so that the total flow through the flow meter can be determined from the sensor output signal. In some mass flow controllers, no bypass is used, and the entire flow passes through the sensor. The mass flow sensor portion and bypass  142  may then be mated to the control valve and control electronics portions and then tuned again, under known conditions. The responses of the control electronics and the control valve are then characterized so that the overall response of the system to a change in set point or input pressure is known, and the response can be used to control the system to provide the desired response. 
     In addition, the mass flow controller  100  may include a pressure transducer  112  coupled to flow path at some point, typically, but not limited to, upstream of the bypass  142  to measure pressure in the flow path. Pressure transducer  112  provides a pressure signal indicative of the pressure. 
     Control electronics  160  control the position of the control valve  170  in accordance with a set point indicating the desired mass flow rate, and an electrical flow signal from the mass flow sensor indicative of the actual mass flow rate of the fluid flowing in the sensor conduit. Traditional feedback control methods such as proportional control, integral control, proportional-integral (PI) control, derivative control, proportional-derivative (PD) control, integral-derivative (ID) control, and proportional-integral-derivative (PID) control are then used to control the flow of fluid in the mass flow controller. A control signal (e.g., a control valve drive signal) is generated based upon an error signal that is the difference between a set point signal indicative of the desired mass flow rate of the fluid and a feedback signal that is related to the actual mass flow rate sensed by the mass flow sensor. The control valve is positioned in the main fluid flow path (typically downstream of the bypass and mass flow sensor) and can be controlled (e.g., opened or closed) to vary the mass flow rate of fluid flowing through the main fluid flow path, the control being provided by the mass flow controller. 
     In the illustrated example, the flow rate is supplied by electrical conductors  158  to a closed loop system controller  160  as a voltage signal. The signal is amplified, processed and supplied using electrical conductors  159  to the valve assembly  150  to modify the flow. To this end, the controller  160  compares the signal from the mass flow sensor  140  to predetermined values and adjusts the control valve  170  accordingly to achieve the desired flow. 
     Although  FIG. 1  depicts that the mass flow controller  100  includes a thermal mass flow sensor, the mass flow controller  100 , in accordance with the disclosed embodiments, may utilize other types of mass flow sensors including a Coriolis type sensor. An advantage of using a Coriolis-based sensor is that it is capable of determining mass flow independent of temperature, flow profile, density, viscosity, and homogeneity. 
     Additionally, the disclosed embodiments may also be utilized in a mass flow meter. A mass flow meter differs from a mass flow controller, as described above, in that it lacks the control functions. Instead, the mass flow meter is used merely to measure or determine fluid flow rate. 
     As stated above, the disclosed embodiments seek to improve bypass linearity because doing so will improve accuracy on process gas and improve the range of the instrument. For the purpose of this disclosure the term “turndown” is used as a measurement of the linearity of the bypass design. It is defined as the total flow at 100% sensor output divided by the total flow at 2% sensor output. Therefore, a perfectly linear bypass design would have a turndown of 50. Acceptable instruments can be made with turndowns less than the ideal 50, but the lower the turndown, the lower the accuracy potential of the instrument. 
     Currently, inlet screens (also referred to as a “mesh”) are typically used to “flatten” out the velocity profile at the entrance to the bypass  142 .  FIGS. 1A and 1B  respectively depict the speed contour of a flow meter device without an inlet mesh and a flow meter device with an inlet mesh. However, even with an inlet mesh, the disclosed embodiments provide a dramatic improvement over the current use of inlet screens by adding a turning vane. For example, with reference now to  FIG. 2 , in one embodiment, a turning vane  200  with a flow mixer  206  installed upstream of an inlet screen (not depicted) is used to generate a more uniform flow so that the turndown value can reach the max value for a given bypass design and given mesh. 
       FIG. 3  illustrates the turning vane of  FIG. 2  in an assembly. One advantage of this turning vane design is that it can increase the turndown value by about 15% or more depending upon a given bypass design. The turning vane with a flow mixer more uniformly redistributes the fluid mass, so that more of the bypass mesh is used during maximum flow conditions. In the end, the turndown value reaches the max value for a given bypass design and a given mesh. 
     As depicted in  FIG. 2 , the turning vane  200  includes one or more vanes and a flow mixer  206 . In one embodiment, the vanes and flow mixer are made of metal. In one embodiment, a vane consists of a flat section  202  upstream and a curved surface  204  downstream. The vanes form diffusers that distribute the flow across the available inlet area. The number, size, and shape of the vanes may vary within different embodiments. For example, in one embodiment, the angle of the curved surface  204  within a plurality of vanes of a turning vane will gradually increase from a bottom vane to a top vane as depicted in  FIG. 2 . 
     In one embodiment, the flow mixer  206  is a piece of coarse metal mesh, which can be part of the turning vane  200  or a separate element. The location of the turning vane  200  and the flow mixer  206  may be adjusted to reach the highest performance. The shape of the openings on the mixer can be an angular shape, such as star, circle, rectangle or a round shape such as a circle, oval, or ellipse. The dimensions can vary for the specific purposes. 
       FIGS. 4-7  depict an example of a simulated performance test of the turning vane  200  depicted in  FIG. 2 . In the depicted figures, a uniform flow of nitrogen is specified at the inlet (e.g., 6 LPM, 60 LPM, 120 LPM, etc.) of the flow meter at 25° C. The pressure drop is generated at the inlet and outlet of the sensor tube. During the simulations, the flow inside the sensor tube is not simulated because the flow through the sensor is at the order of magnitude of 0.005% of the total flow rate and has a negligible effect on the overall flow. The average pressure is measured at the locations of the inlet and outlet of the sensor tube. The verification has been conducted by comparing the numerical solutions with the experimental results so that the pressure drops in the simulations are in a good range of accuracy. 
       FIGS. 4A and 4B  depict the difference in speed contour between a flow meter device that does not have the turning vane and mixer ( FIG. 4A ), and a flow meter device that includes the turning vane and mixer ( FIG. 4B ). 
       FIGS. 5A and 5B  depict the difference in pressure contour between a flow meter device that does not have the turning vane and mixer ( FIG. 5A ), and a flow meter device that includes the turning vane and mixer ( FIG. 5 b   ). 
     Table 1, presented below, is a chart that indicates the difference in flow rate at 2 sccm and at 10 sccm, and the difference in turndown for a flow meter device that does not have the turning vane and mixer, and a flow meter device that includes the turning vane and mixer. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Flow rate @ 
                 Flow rate @ 
                   
               
               
                 Device Type 
                 0.2 sccm 
                 10 sccm 
                 Turndown 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Regular flow meter 
                 13.25 
                 274.01 
                 20.68 
               
               
                 with bypass 
               
               
                 mesh 
               
               
                 Regular flow meter 
                 15.13 
                 358.28 
                 23.68 
               
               
                 with bypass 
               
               
                 mesh plus turning 
               
               
                 vane and mixer 
               
               
                   
               
             
          
         
       
     
       FIG. 6  is a graph depicting the difference in turndown between the two devices represented in Table 1. 
       FIG. 7  illustrates another embodiment of a turning vane  700  consisting of a single vane. In the depicted embodiment, the turning vane  700  is employed upstream of an existing inlet screen/mesh  710  that is located at the entrance of the bypass  720 . Although not depicted, in certain embodiments, the turning vane  700  may also include a flow mixer, as either an integrated or separate component, of the turning vane  700 . Additionally, the angle, size (e.g., width and height), and number of vanes for the turning vane  700  may vary in different embodiments. 
     For instance,  FIG. 8  illustrates another example of a turning vane  800  with a flow mixer  810  in accordance with the disclosed embodiments. In this embodiment, the turning vane  800  consists of three vanes  802 ,  804 , and  806 . In certain embodiments, the dimensions (e.g., length, width, height, thickness, etc.) of the three vanes  802 ,  804 , and  806  may vary from one another or may vary between different designs. Each of the vanes may  802 ,  804 , and  806  may comprise of one or more metal sections coupled together or alternatively, may comprise of a single metal piece. Additionally, as depicted, in some embodiments, the uppermost vane  802  may include a large flat section abutting the flow mixer (as compared to the embodiment depicted in  FIG. 2 ) that may further help distribute the flow across the available inlet area. 
       FIGS. 9A-9E  illustrate various configurations of a turning vane in an assembly in accordance with the disclosed embodiments. For example,  FIGS. 9A and 9B  illustrate different support structures that may be employed with a turning vane in accordance with the disclosed embodiments. For instance,  FIG. 9A  illustrates that in one embodiment, a portion of the curved section of a turning vane may be straightened to allow for partial flow through the turning vane and provide additional support for the turning vane.  FIG. 9B  employs a different design that uses a single vertical support leg for providing additional support for the turning vane.  FIG. 9C  depicts a similar embodiment to that of  9 B except that the turning vane abuts the inlet screen in the assembly.  FIG. 9D  depicts an embodiment in which a vertical support leg can be cut from a portion of the curved section similar to that of  FIG. 9A .  FIG. 9E  illustrates a similar embodiment to that of Figure D, but without the flat portion of the vane abutting the inlet screen. Although not depicted, each of the depicted embodiments may employ the use of a flow mixer as described above. 
       FIGS. 10A and 10B  illustrate another embodiment of a turning vane  1000  in an assembly in accordance with the disclosed embodiments. In the depicted embodiment, the turning vane  1000  includes a pair of support legs  1010  for providing a support structure when installed in an assembly as shown in  FIG. 10B . 
     While specific details about the above embodiments have been described, the above descriptions are intended merely as example embodiments and are not intended to limit the structure or implementation of the disclosed embodiments. 
     Accordingly, the disclosed inventions provide various embodiments of a turning vane that improves bypass linearity in a mass flow controller or mass flow meter, and thereby improving accuracy on process gas and the range of the instrument. 
     Those skilled in the art will recognize that the present teachings are amenable to a variety of modifications and/or enhancements. While the foregoing has described what is considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein. Such modifications are intended to be covered within the true scope of the present teachings. 
     The terminology used herein is for describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” and/or “comprising,” when used in this specification and/or the claims, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of one or more other features, elements, components, and/or groups thereof. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described to explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. The scope of the claims is intended to broadly cover the disclosed embodiments and any such modification.