Abstract:
A flow controller having a first and second stages of regulation. The first stage is a pressure regulation stage that maintains the pressure within an intermediate chamber (formed by a series of interconnected passageways and cavities) within a predetermined range above the pressure in an outlet port. The second stage maintains the flow rate within a predetermined range about a target flow rate. Both stages sample the pressure in the outlet port and automatically adjust the flow of fluid to ensure that fluctuations in pressure at the inlet and outlet ports do not affect the flow rate. The flow rate is set and controlled by a user determined setpoint. The pressure is regulated by one or more feedback systems. The feedback systems used may be mechanical or electromechanical based on a particular operating environment.

Description:
RELATED APPLICATIONS 
     This is a continuation application of U.S. Ser. No. 09/805,708 filed Mar. 13, 2001, U.S. Pat. No. 6,467,505, which is based on U.S. Provisional Application Ser. No. 60/239,716 filed Oct. 11, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates to systems and methods for controlling the flow of pressurized fluid and, more specifically, to such systems and methods that allow precise control of flow of fluid from a source having high, variable, and/or unknown pressure. 
     BACKGROUND OF THE INVENTION 
     In many disciplines, a pressurized fluid must be supplied in precise quantities. Usually, the quantity of fluid supplied is controlled by regulating the flow of the fluid. Fluid flow is independent of conduit size, supply pressure, and the like, and controlling the flow rate ensures that a precise quantity of the fluid is delivered where required. 
     The present invention is of particular significance when used to control the flow of gasses at relatively low flow rates. The present invention also has application to other fluids such as liquids and to relatively large flow rates. The scope of the present invention should thus be determined with reference to the claims appended hereto and not the following detailed description. 
     One example where the quantity of a gas supplied must be precisely controlled is the delivery of a gas to a medical patient. In this context, a gas is mixed with air supplied to the patient through a ventilator to obtain a desired effect. If too little gas is supplied to the patient, the desired effect may not be obtained. On the other hand, too much gas may be toxic to the patient. Other examples where precise quantities of gas must be supplied include scientific and medical testing, industrial processing, and scuba diving. 
     A primary impediment to maintaining a constant flow of gas is that the pressure at which the gas is supplied may be unknown or variable. Often, the source of the pressurized gas is a pressurized tank or compressor. The pressure of the fluid supplied by either of these sources can fluctuate significantly. For example, as the quantity of gas within a pressurized tank decreases, the pressure of the fluid flowing from the tank will decrease. Accordingly, in many systems in which the flow of a gas is important, the flow rate must be measured and monitored and the system adjusted as necessary to maintain the flow rate within predetermined limits. 
     RELATED ART 
     U.S. Pat. No. 4,015,626 issued to the present Applicant discloses a valve assembly for maintaining constant flow rates. This valve assembly comprises a housing that defines upstream and downstream chambers, a movable wall assembly arranged between these chambers, a spring located in the downstream chamber that acts on the movable wall, a bicycle valve located in the upstream chamber such that its control stem engages the movable wall, and coiled high resistance tubing connected between the chambers. Changes in the pressure in the downstream chamber allow the movable wall to move and operate the bicycle valve control stem to open or close the bicycle valve to control the flow of fluid flowing through the valve assembly. The spring may be adjusted to obtain different flow rates. The tubing functions as a pressure reducing restriction and to average the flow rate of fluid passing therethrough. 
     The valve assembly disclosed in the &#39;626 patent is relatively complex and expensive to manufacture. Also, the pressure drop through the high resistance tubing plays an important role in determining the range of flow rates that may be obtained. Altering the pressure drop through the tubing to obtain different flow rate ranges would be difficult because different tubing (i.e., change of length or passageway size) must be used. The length of the high resistance tubing supplied renders the system relatively large and increases the likelihood of failure. And this valve assembly is designed to operate only at relatively low (80 to 150) psi. 
     U.S. Pat. No. 6,026,849 issued on Feb. 22, 2000, to the present Applicant also discloses a regulated flow controller that operates using a source having high, variable, and/or unknown pressure. The systems and methods disclosed in the &#39;849 patent solved many problems related to supplying fluids, and in particular gasses, at constant flow rate. 
     However, the need exists for improvements to regulated flow controllers such as are disclosed in the &#39;849 patent that allow this technology to be applied to specific environments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an end elevation view of a first embodiment of a flow controller constructed in accordance with, and embodying, the principles of the present invention; 
     FIG. 2 is a section view of the flow controller of the first embodiment taken along lines  2 — 2  in FIG. 1; 
     FIG. 3 is an enlarged section view showing details of the flow controller of the first embodiment as depicted in FIG. 2; 
     FIG. 4 is a section view of the flow controller of the first embodiment taken along lines  4 — 4  in FIG. 2; 
     FIG. 5 is a section view of a second embodiment of a flow controller constructed in accordance with, and embodying, the principles of the present invention; 
     FIG. 6A is a section view depicting details of an input stage of the second embodiment of the flow controller as depicted in FIG. 5; and 
     FIG. 6B is a section view depicting details of an output stage of the second embodiment of the flow controller as depicted in FIG.  5 . 
     FIG. 7 is a section view of a third embodiment of a flow controller constructed in accordance with, and embodying, the principles of the present invention; 
     FIG. 8 is a section view of a fourth embodiment of a flow controller constructed in accordance with, and embodying, the principles of the present invention; 
     FIG. 9 is a block diagram view of a control system that may be used in connection with the flow controller of FIG. 8; 
     FIG. 10 is a section view of a fifth embodiment of a flow controller constructed in accordance with, and embodying, the principles of the present invention; 
     FIG. 11 is a block diagram view of a control system that may be used in connection with the flow controller of FIG. 10; 
     FIG. 12 is a section view of a sixth embodiment of a flow controller constructed in accordance with, and embodying, the principles of the present invention; and 
     FIG. 13 is a block diagram view of a control system that may be used in connection with the flow controller of FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. First Embodiment 
     Referring now to FIG. 1 of the drawing, depicted therein at  20  is a flow controller system constructed in accordance with, and embodying, the principles of the present invention. The flow controller system  20  of the first embodiment may be manufactured relatively inexpensively and is designed to operate reliably in a variety of configurations depending upon such factors as the expected range of input pressure and the desired range of flow rates. 
     The exemplary flow controller system  20  comprises a housing assembly  22  comprising a first end member  24 , a second end member  26 , an intermediate member  28 , and first and second diaphragm members  30  and  32 . The first and second end members  24  and  26  are attached to the intermediate members  28  using bolts  34  to form the housing assembly  22 . So assembled, the first diaphragm member  30  is arranged between the first upper member  24  and the intermediate member  28  to define an input chamber  40  and a first regulator chamber  42 . The second diaphragm member  32  is arranged between the second upper member  26  and the intermediate member  28  to define an output chamber  44  and a second regulator chamber  46 . 
     Referring now to FIG. 4, it can be seen that the exemplary intermediate member  28  is milled to define first and second end cavities  50  and  52 . The exemplary first and second end cavities  50  and  52  are similar and each comprises an outer portion  54 , a threaded intermediate portion  56 , and an inner portion  58 . 
     First and second valve assemblies  60  and  62  are arranged substantially within the first and second end cavities  50  and  52 , respectively. Except for dimensional differences, the exemplary valve assemblies  60  and  62  are similar in construction. 
     In particular, the first valve assembly  60  comprises a first valve seat member  70 , a first O-ring  72 , a first valve stem assembly  74 , and a first valve spring  76 . The second valve assembly  62  comprises a second first valve seat member  80 , a second O-ring  82 , a second valve stem assembly  84 , and a second valve spring  86 . 
     The first and second valve seat members  70  and  80  are threaded into the threaded portions  56  of the first and second end cavities  50  and  52 , respectively. The O-rings  72  and  82  are retained within the threaded portions  56  of the end cavities  50  and  52  by the valve seat members  70  and  80 . The first O-ring  72  is thus arranged in the threaded portion  56  of the first end cavity  50  to prevent fluid flow between the threaded and inner portions  56  and  58  of the cavity  50 . The second O-ring  82  is similarly arranged in the threaded portion  56  of the second end cavity  52  to prevent fluid flow between the threaded and inner portions  56  and  58  of that cavity  52 . 
     The exemplary valve stem assemblies  74  and  84  are identical and each comprises a stem member  90  and a valve member  92 . The exemplary stem member  90  is a T-shaped part the cross-bars of which are embedded within the valve member  92 . 
     The stem members  90  of the valve stem assemblies  74  and  84  extend through and are supported by first and second stem openings  110  and  112  formed in the valve seat members  70  and  80 , respectively. In addition, the valve members  92  of the valve stem assemblies  74  and  84  are arranged adjacent to valve seat surfaces  114  and  116  formed on the valve seat members  70  and  80 . 
     The valve stem assemblies  74  and  84  move a short distance relative to the valve seat members  70  and  80  such that the valve members  96  engage or disengage from the valve seat surfaces  114  and  116 . When the valve members  96  are disengaged from the valve seat surfaces  114  and  116 , fluid is allowed to flow through first and second valve passageways  120  and  122  formed in the valve seat members  70  and  80 , respectively. But when the valve members  96  engage the valve seat surfaces  114  and  116 , fluid flow through the first and second valve passageways  120  and  122  is prevented. 
     The first and second valve springs  76  and  86  are arranged in the inner portions  58  of the first and second end cavities  50  and  52 , respectively, to bias the valve members  92  of the valve stem assemblies  74  and  84  towards the valve seat surfaces  114  and  116 . 
     The valve assemblies  60  and  62  are thus normally closed valves. Further, the valve assemblies  60  and  62  open substantially linearly in response to first and second valve control forces applied to upper members  124  and  126  of the valve stem assemblies  74  and  84 . 
     The upper members  124  and  126  of the valve stem assemblies  74  and  84  are located in the input chambers  40  and  44 . First and second valve plates  130  and  132  are located in the input chambers  40  and  44  between the valve stem upper members  124  and  126  and the first and second diaphragm members  30  and  32 , respectively. First and second piston members  140  and  142  are located in the first and second regulator chambers  42  and  46  on the other side of the first and second diaphragm members  30  and  32  from the first and second valve plates  130  and  132 . First and second regulator springs  144  and  146  are also located in the regulator chambers  42  and  46  to bias the first and second piston members  140  and  142  towards the valve plates  130  and  132  through the diaphragm members  30  and  32 . The diaphragm members  30  and  32  are flexible and thus allow forces on the piston members  140  and  142  to be transferred to the valve plates  130  and  132 . 
     Referring now back to the construction of the housing assembly  22 , FIG. 3 shows that a connecting passageway  150  extends between the inlet chamber  40  and the inner portion  58  of the second end cavity  52 . Fluid thus may flow from the inlet chamber  40  to the outlet chamber  42  through the second valve assembly  62  when the second valve assembly  62  is open. 
     FIG. 4 shows in broken lines an inlet passageway  152  that extends from an inlet port  154  to the inner portion  58  of the first end cavity  50 . Fluid may thus flow from the inlet port  154  to the inlet chamber  42  through the first valve assembly  62  when the first valve assembly  62  is open. 
     FIGS. 3 and 4 show a restriction passageway  156  that allows fluid to flow from the outlet chamber  42  to an outlet passageway  158  through a restriction chamber  160 . A restriction member  162  is located in the restriction chamber  160 . The outlet passageway  158  in turn allows fluid to flow from the restriction passageway  156  through the restriction member  162  to an outlet port  164 . 
     FIG. 4 also shows that first and second sampling passageways  170  and  172  allow fluid to flow from the outlet passageway  158  to the first and second regulator chambers  42  and  46 , respectively. 
     FIG. 4 further shows that the restriction chamber  160  is formed by an access hole  180  milled into the intermediate member  28 . The access hole  180  is threaded to allow an elongate plug  182  to be attached to the intermediate member  28 . The plug  182  defines a plug chamber  184  through which fluid flows from the restriction passageway  156  to the restriction chamber  160 . A first plug O-ring  186  is arranged to ensure that fluid flows from the restriction passageway  156  only through the restriction member  162  in the restriction chamber  160 . A second O-ring  188  is arranged to prevent fluid from exiting the system  20  through the access hole  180 . 
     In use, fluid flows from the inlet port  154  to the outlet port  164  along a main fluid path as follows: fluid in the inlet port  154  flows into the inlet chamber  40  through the inlet passageway  152 ; fluid then flows from the inlet chamber  40  to the outlet chamber  44  through the first valve assembly  60 , the connecting passageway  150 , and the second valve assembly  62 ; fluid in the outlet chamber  44  then flows to the outlet port  164  through the restriction passageway  156 , the restriction member  162  in the restriction chamber  160 , and the outlet passageway  158 . 
     Fluid in the outlet passageway  158  flows to the first and second regulator chambers  42  and  46  through the sampling passageways  170  and  172 . The combination of the fluid pressure in the regulator chambers  42  and  46  and biasing force applied by the regulator springs  144  and  146  creates control forces that control the valve assemblies to maintain substantially a substantially constant fluid flow rate through the flow controller  20  that is substantially independent of upstream or downstream pressure changes. 
     Referring for a moment back to FIG. 2, depicted at  190  therein is a pressure adjusting assembly that adjusts the control force applied by the second regulator spring  146  to the upper member  126  of the second valve stem assembly  84 . The pressure adjusting assembly  180  allows the flow rate of fluid through the system  20  to be adjusted. While the exemplary pressure adjusting assembly  180  is manually operated, a powered pressure adjusting assembly could be used, as will be discussed below, that would allow the flow rate to be controlled remotely and/or automatically. 
     The design of the housing assembly  22  allows the flow controller system  20  easily to be configured for different environments. In particular, the intermediate member  28 , which is a relatively complicated part, can be manufactured in a standard form. The standard intermediate member  28  can then be configured for a specific environment by appropriate selection of the first and second valve seat members  70  and  72  and the restriction member  162 . 
     The valve seat members may be manufactured in different forms, each having a valve passageways with a different effective cross-sectional area. The effective cross-sectional area of the valve passageway for a given first or second valve seat member is selected based on the expected range of upstream and downstream fluid pressures. A system designer will select the appropriate valve seat member for a given environment. If two different valve seat members are fabricated each defining a different cross-sectional area, the system designer may select from among four effective configurations of the flow controller system. More valve seat members will provide more effective configurations of the flow controller system. 
     The restriction member  162  creates a predetermined pressure drop between the restriction passageway  156  and the outlet passageway  158 . The magnitude of this pressure drop may be altered by selecting different restriction members and/or using two or more restriction members in various combinations. The characteristics of the flow controller system of the present invention may thus be altered by appropriate selection of the restriction member  162 . 
     The flow controller system  20  thus forms a general purpose flow controller that may easily be embodied in different configurations depending upon the environment in which the system  20  is to be used. 
     II. Second Embodiment 
     Referring now to FIG. 5 of the drawing, depicted therein at  220  is a second embodiment of a flow controller system constructed in accordance with, and embodying, the principles of the present invention. The flow controller system  220  of the second embodiment is designed to operate reliably in a variety of configurations depending upon such factors as the expected range of input pressure and the desired range of flow rates. While the flow controller system  220  may have application in many environments, it is of particular significance in clean environments in which the risk of contamination of the fluid must be minimized. 
     The exemplary flow controller system  220  comprises a housing assembly  222  comprising a first upper member  224 , a second upper member  226 , a base or intermediate member  228 , and first and second diaphragm assemblies  230  and  232 . 
     The diaphragm assemblies  230  and  232  are similar to each other and each comprise first and second diaphragm members  234  and  235 . The first diaphragm assembly  230  further comprises a first diaphragm plate  236  and a first diaphragm sleeve  237 ; the second diaphragm assembly  232  also further comprises a second diaphragm plate  238  and a second diaphragm sleeve  239 . 
     The diaphragm members  234  and  235  are welded to the diaphragm plates  238  and  239 . The diaphragm members  234  are additionally welded to the diaphragm sleeves  237  and  239 , while the diaphragm members  235  are secured between the diaphragm sleeves  237  and  239  and the intermediate member  224 . The first and second upper members  224  and  226  are threaded onto the base member  228  to form the housing assembly  222 . 
     So assembled, the first diaphragm assembly  230  defines an input chamber  240  and a first regulator chamber  242 . The second diaphragm assembly  232  defines an output chamber  244  and a second regulator chamber  246 . Welds and seals are employed to ensure that flow between the various chambers described herein are only through the passageways or flow paths defined below. 
     The exemplary base member  228  is milled to define first and second end cavities  250  and  252 . The exemplary first and second cavities  250  and  252  are similar and each comprises a threaded outer portion  254 , a threaded intermediate portion  256 , and an inner portion  258 . 
     First and second valve assemblies  260  and  262  are arranged substantially within the first and second end cavities  250  and  252 , respectively. Except for dimensional differences that will be noted below, the exemplary valve assemblies  260  and  262  are similar in construction. 
     In particular, the first valve assembly  260  comprises a first valve seat member  270 , a first O-ring  272 , a first valve stem assembly  274 , and a first valve spring  276 . The second valve assembly  262  comprises a second first valve seat member  280 , a second O-ring  282 , a second valve stem assembly  284 , and a second valve spring  286 . 
     The first and second valve seat members  270  and  280  are threaded into the threaded portions  256  of the first and second end cavities  250  and  252 , respectively. The O-rings  272  and  282  are retained within the threaded portions  56  of the end cavities  50  and  52  by the valve seat members  70  and  80 . The first O-ring  72  is thus arranged in the threaded portion  256  of the first end cavity  250  to prevent fluid flow between the threaded and inner portions  256  and  258  of the cavity  250 . The second O-ring  282  is similarly arranged in the threaded portion  256  of the second end cavity  252  to prevent fluid flow between the threaded and inner portions  256  and  258  of that cavity  252 . 
     The exemplary valve stem assemblies  274  and  284  are identical and each comprises a stem member  290  and a valve member  292 . The exemplary stem member  290  is a T-shaped part the cross-bars of which are embedded within the valve member  292 . 
     The stem members  290  of the valve stem assemblies  274  and  284  extend through and are supported by first and second stem openings  310  and  312  formed in the valve seat members  370  and  380 , respectively. In addition, the valve members  292  of the valve stem assemblies  274  and  284  are arranged adjacent to valve seat surfaces  314  and  316  formed on the valve seat members  270  and  280 . 
     The valve stem assemblies  274  and  284  move a short distance relative to the valve seat members  270  and  280  such that the valve members  296  engage or disengage from the valve seat surfaces  314  and  316 . When the valve members  296  are disengaged from the valve seat surfaces  314  and  316 , fluid is allowed to flow through first and second valve passageways  320  and  322  formed in the valve seat members  270  and  280 , respectively. But when the valve members  296  engage the valve seat surfaces  314  and  316 , fluid flow through the first and second valve passageways  320  and  322  is prevented. 
     The first and second valve springs  276  and  286  are arranged in the first and second regulator chambers  242  and  246 , respectively. As will be described in detail below, the valve springs  276  and  286  bias the valve members  292  of the valve stem assemblies  274  and  284  towards the valve seat surfaces  314  and  316 . 
     The valve assemblies  260  and  262  are thus normally closed valves. Further, the valve assemblies  260  and  262  open substantially linearly in response to first and second valve control forces applied to upper portions  324  and  326  of the valve stem assemblies  274  and  284 . 
     The upper portions  324  and  326  of the valve stem assemblies  274  and  284  are located in the input chambers  240  and  244  and are rigidly connected to the diaphragm plates  236  and  238 , respectively. The diaphragm plates  236  and  238  are in turn connected to first and second regulator plates  330  and  332  located in the first and second regulator chambers  242  and  246 . 
     First and second regulator springs  340  and  342  are also located in the regulator chambers  242  and  246  to bias the first and second regulator plates  340  and  342  towards the diaphragm plates  236  and  238  through the valve springs  276  and  286 . 
     The first regulator plate  330 , first diaphragm plate  238 , and first valve member  274  thus are rigidly connected to each other to form a first slide assembly  344 . Similarly, the second regulator plate  332 , second diaphragm plate  239 , and second valve member  284  are rigidly connected to each other to form a second slide assembly  346 . 
     The slide assemblies  344  and  346  are suspended by the first and second stem openings  310  and  312  formed in the valve seat members  370  and  380 , the diaphragm members  234  and  236 , the valve springs  276  and  286 , and the first and second regulator springs  340  and  342 . The diaphragm members  234  and  236  and springs  276 ,  286 ,  340 , and  342  are flexible and allow the slide assemblies  344  and  346  to move. In addition, the valve members  274  and  284  slide relative to the valve set members  370  and  380 . Accordingly, the slide assemblies  344  and  346  move within the housing assembly  222  such that forces on the diaphragm members  234  and  236  open and close the valve assemblies  260  and  262 . 
     Referring now back to FIG. 5, a connecting passageway  350  extends between the inlet chamber  240  and the inner portion  258  of the second end cavity  252 . Fluid thus may flow from the inlet chamber  240  to the outlet chamber  242  through the second valve assembly  262  when the second valve assembly  262  is open. 
     An inlet passageway  352  extends from an inlet port  354  to the inner portion  258  of the first end cavity  250 . Fluid may thus flow from the inlet port  354  to the inlet chamber  240  through the first valve assembly  262  when the first valve assembly  262  is open. 
     A restriction passageway  356  allows fluid to flow from the outlet chamber  242  to an outlet passageway  358  through a restriction chamber  360 . A restriction member  362  is located in the restriction chamber  360 . The outlet passageway  358  in turn allows fluid to flow from the restriction passageway  356  through the restriction member  362  to an outlet port  364 . 
     A first bypass passageway  370  formed in the diaphragm plate  238  allows fluid to flow from the inlet chamber  240  to a first bypass chamber  372  defined by the first diaphragm assembly  230  and the housing assembly  222 . A second bypass passageway  374  formed in the base member  224  allows fluid to flow from the first bypass chamber  372  to a second bypass chamber  246  defined by the second diaphragm assembly  232  and the housing assembly  222 . A sampling passageway  378  connects the outlet passageway  358  to the second bypass chamber  374 . A bypass member  380  is located within the first bypass passageway  370  to create a pressure differential between the inlet chamber  240  and the first bypass chamber  372 . 
     In use, fluid is introduced into the inlet port  354  and then flows through the system  220  along a main flow path as follows. Fluid in the inlet port  354  flows into the inlet chamber  240  through the inlet passageway  252 . Fluid then flows from the inlet chamber  240  to the outlet chamber  242  through the first valve assembly  260 , the connecting passageway  250 , and the second valve assembly  262 . Fluid in the outlet chamber  242  then flows to the outlet port  364  through the restriction passageway  356 , the restriction member  362  in the restriction chamber  360 , and the outlet passageway  358 . 
     A small amount of fluid in the inlet chamber  240  flows along a bypass path through the bypass member  278  in the first bypass passageway  370  and into the first bypass chamber  372 . This fluid continues along the bypass path through the second bypass passageway  374  and into the second bypass chamber  376 . The fluid then further continues along the bypass path from the second bypass chamber  376  to the outlet passageway  358  through the sampling passageway  378 . 
     The pressure within the first and second bypass chambers  372  and  376  is thus the same as the pressure within the outlet passageway  358 . The combination of the fluid pressure in the bypass chambers  372  and  376  and biasing force applied by the regulator springs  344  and  346  creates control forces that control the valve assemblies  260  and  262  to maintain substantially a substantially constant fluid flow rate through the flow controller  20  that is substantially independent of upstream or downstream pressure changes. However, fluid does not accumulate in the bypass chambers or bypass passageways because a small amount of the fluid will flow along the bypass path during normal operation of the system  220 . 
     Referring for a moment back to FIG. 5, depicted at  390  therein is a pressure adjusting assembly that adjusts the control force applied by the second regulator spring  346  to the second valve stem assembly  284 . The pressure adjusting assembly  390  allows the flow rate of fluid through the system  220  to be adjusted. While the exemplary pressure adjusting assembly  390  is manually operated, a powered pressure adjusting assembly could be used that would allow the flow rate to be controlled remotely and/or automatically. 
     The design of the housing assembly  222  allows the flow controller system  220  appropriate for use in situations in which contact between the springs and the controlled fluid is avoided and fluid is not allowed to collect anywhere within the system  220 . 
     As with the system  20  described above, the valve seat members may be manufactured in different forms, each having a valve passageways with a different effective cross-sectional area. The effective cross-sectional area of the valve passageway for a given first or second valve seat member is selected based on the expected range of upstream and downstream fluid pressures. A system designer will select the appropriate valve seat member for a given environment. If two different valve seat members are fabricated each defining a different cross-sectional area, the system designer may select from among four effective configurations of the flow controller system. More valve seat members will provide more effective configurations of the flow controller system. In addition, the valve seat members of the system  20  may be the same as, and used in, the system  220 . 
     The restriction member  362  creates a predetermined pressure drop between the restriction passageway  356  and the outlet passageway  358 . The magnitude of this pressure drop may be altered by selecting different restriction members and/or using two or more restriction members in various combinations. The characteristics of the flow controller system of the present invention may thus be altered by appropriate selection of the restriction member  362 . 
     III. Third Embodiment 
     Referring now to FIG. 7, depicted at  420  therein is yet another exemplary flow controller system constructed in accordance with, and embodying, the principles of the present invention. The flow controller system  420  of the third embodiment is similar to the flow controller system  220  described above. In the interests of brevity and clarity, the flow controller system  420  will be described primarily to the extent that it differs from the flow controller system  220 . 
     The flow controller system  420  is designed to be electrically controlled so that the system  420  may be operated remotely and/or as part of a larger system having a centralized controller. 
     The exemplary flow controller system  420  comprises a housing assembly  422  comprising a first upper member  424 , a second upper member  426 , a base or intermediate member  428 , and first and second valve assemblies  430  and  432 . The first and second upper members  424  and  426  are threaded onto the base member  428  to form the housing assembly  422 . 
     The first and second valve assemblies  430  and  432  are arranged below first and second regulator chambers  434  and  436 , respectively, defined by the housing assembly  422 . The exemplary valve assemblies  430  and  432  are similar in construction and each comprises a valve spring  440 , a valve member  442 , and a valve seat surface  444 . The valve springs  440  bias the valve members  442  towards the valve seat surfaces  444  such that the valve assemblies  430  and  432  are normally closed. 
     The valve assemblies  430  and  432  open substantially linearly in response to first and second valve control forces applied to first and second regulator plates  450  and  452 . The regulator plates  450  and  452  are suspended within the regulator chambers  434  and  436  between the valve springs  440  and first and second regulator springs  454  and  456 . The regulator springs  454  and  456  bias the valve assemblies  430  and  432  into the open position against the force of the valve springs  440 . 
     In use, fluid is introduced into an inlet port  460  and then flows through the system  420  along a main flow path to an outlet port  462  as follows. A small amount of fluid flows along a bypass path such that the combination of the fluid pressure along the bypass path and the biasing force applied by the regulator springs  454  and  456  creates control forces that control the valve assemblies  430  and  432  to maintain substantially a substantially constant fluid flow rate through the flow controller  20 . Under proper conditions, the fluid flow rate through the flow controller  20  is substantially independent of upstream or downstream pressure changes. 
     In the exemplary system  420 , the control forces applied to the valve assembly  432  are generated at least in part by a control system  470  a portion of which is mounted on the second upper member  426 . In particular, the control system  470  comprises a motor assembly  472 , an actuator member  474 , and a housing  476 . The housing  476  supports the motor assembly  472  above the second regulator chamber  436 . A threaded portion of the actuator member  474  extends through a threaded actuator opening  478  in the second upper member  426 . The actuator member  474  engages the second regulator plate  452  to apply a control force that opposes the force applied to the regulator plate  452  by the second regulator spring  456 . 
     The actuator member  474  is operatively connected to the motor assembly  472  such that operation of the motor assembly  472  causes axial rotation of the actuator member  474 . The actuator member  474  engages the threaded actuator opening  478  such that axial rotation of the member  474  is translated into axial displacement of the member  474  towards and away from the second regulator plate  452 . Operation of the motor assembly  472  can thus increase or decrease the control force applied to the regulator plate  452 . 
     As is apparent from the foregoing discussion of the flow controller system  220 , increasing or decreasing the control force applied to the regulator plate  452  will vary the rate of fluid flow along the main flow path through the controller  420 . 
     The motor assembly  472  is or may be a conventional stepper motor that is controlled by a USER SETPOINT INPUT signal to cause axial rotation of the actuator member  474 . With the USER SETPOINT SIGNAL at a desired value, the system  420  will maintain constant fluid flow at a rate corresponding to that desired value. If the value of the USER SETPOINT SIGNAL changes to a second desired value, the fluid flow through the system  420  will change to a second flow rate corresponding to the second desired value. 
     Accordingly, a controller that generates the USER SETPOINT SIGNAL can change the desired flow rate as required at a given point in time. Once that desired flow rate is set, the system  420  will operate as described above with reference to the system  220  to maintain a constant flow rate even with changing upstream and downstream pressures. 
     The exemplary motor assembly  472  further comprises a rotary encoder that generates a ROTARY POSITION OUT signal that corresponds to the angular position of the actuator member  474 . The ROTARY POSITION OUT provides feedback to the controller that generates the USER SETPOINT SIGNAL for more precise control of the motor assembly  472 . 
     IV. Fourth Embodiment 
     Referring now to FIG. 8, depicted at  520  therein is yet another exemplary flow controller system constructed in accordance with, and embodying, the principles of the present invention. The flow controller system  520  of the fourth embodiment is similar to the flow controller system  220  described above. In the interests of brevity and clarity, the flow controller system  520  will be described primarily to the extent that it differs from the flow controller system  220 . 
     The flow controller system  520  is designed to be electrically controlled so that the system  520  may be operated remotely and/or as part of a larger system having a centralized controller. 
     The exemplary flow controller system  520  comprises a housing assembly  522  comprising a first upper member  524 , a second upper member  526 , a base or intermediate member  528 , and first and second valve assemblies  530  and  532 . The first and second upper members  524  and  526  are threaded onto the base member  528  to form the housing assembly  522 . 
     The first and second valve assemblies  530  and  532  are arranged at least partly within a regulator chamber  534  and a regulator cavity  536 , respectively, defined by the housing assembly  522 . The exemplary valve assembly  530  comprises a valve spring  540 , a valve member  542 , and a valve seat surface  544 . The valve spring  540  biases the valve member  542  towards the valve seat surface  544  such that the valve assembly  530  is normally closed. The exemplary valve assembly  532  comprises a valve member  546  and a valve seat surface  548 . The valve assembly  532  is closed when the valve member  546  engages the valve seat surface  548 . 
     The first valve assembly  530  opens substantially linearly in response to a first valve control force applied to a regulator plate  550 . The regulator plate  550  is suspended within the regulator chamber  534  between the valve spring  540  and a regulator spring  552 . The regulator spring  552  biases the valve assembly  530  into the open position against the force of the valve spring  540 . 
     In use, fluid is introduced into an inlet port  560  and then flows through the system  520  along a main flow path to an outlet port  562  as follows. A small amount of fluid flows along a bypass path to control the first valve assembly  530  by applying a first control force thereon. The pressure of the fluid flowing along the bypass path is further used to control second valve assembly  532 . The valve assemblies  530  and  532  are controlled to maintain substantially a substantially constant fluid flow rate through the flow controller  520 . Under proper conditions, the fluid flow rate through the flow controller  520  is substantially independent of upstream or downstream pressure changes. 
     In particular, the exemplary system  520  further comprises a control system  570 . The control system  570  comprises a piezo-electric actuator  572  (FIG. 8) and a control circuit  574  (FIG.  9 ). The piezo-electric actuator  572  is conventional and comprises a shaft  575  that moves along a shaft axis according to an ACTUATOR CONTROL signal applied to inputs  576  of the actuator  572 . The shaft  575  is in turn operatively connected to the valve member  546  such that movement of the shaft  575  moves the valve member  546 . The exemplary actuator  572  is bolted to a collar member  578  that forms a part of the housing assembly  522  and is held in place by the second upper member  526 . 
     As shown in FIG. 9, the control circuit  574  generates the ACTUATOR CONTROL signal based on a SETPOINT signal and one or more pressure signals PRESSURE 1  and PRESSURE 2 . The control circuit  574  also generates a SCALED OUTPUT signal that, when the system  520  is calibrated, corresponds to the flow rate of fluid through the system  520 . 
     In the exemplary flow controller system  520 , the actuator  572  and control circuit  574  form an electromechanical feedback system that replaces one of the mechanical feedback systems employed by the flow controller systems  20 ,  220 , and  420  described above. 
     In particular, the pressure signals PRESSURE 1  and PRESSURE 2  correspond to the pressures upstream and downstream of a restriction member  580  employed by the system  520 . The difference between the PRESSURE 1  and PRESSURE 2  signals is the differential pressure across the restriction member  580 . 
     In the exemplary system  520 , the PRESSURE 1  and PRESSURE 2  signals are preferably sampled at any one of a number of sampling locations. These sampling locations may be spaced along the bypass path, at the output port, or the like are possible. The sensors should be isolated from the main and bypass flow paths when the fluid is sensitive to contamination. 
     The control circuit  574  may comprise a microprocessor operating under control of a software program. In this case, appropriate digital-to-analog and analog-to-digital converters are used to translate between analog signals employed by components peripheral to the microprocessor and digital signals and commands that are processed within the microprocessor. The software program will be customized for a particular application, but will in general implement algorithms and logic necessary to generate the ACTUATOR CONTROL signal as described below. 
     Alternatively, the control circuit  574  may be a dedicated analog or hybrid digital/analog circuit that directly implements the algorithms and logic necessary to generate the ACTUATOR CONTROL signal. The decision of whether to use a software controlled microprocessor or a dedicated analog or hybrid circuit will depend upon such factors as the size, cost, and performance characteristics of the system  520 . 
     In the exemplary system  520 , the ACTUATOR CONTROL signal is an analog voltage that controls the piezo-electric actuator to move the valve member  546  as necessary to maintain fluid flow through the system  520  at rate determined by the SETPOINT signal. 
     In certain circumstances, the mechanical feedback systems employed by the flow controllers  20 ,  220 , and  420  described above will continuously oscillate between closed and open valve positions to maintain constant fluid flow. In contrast, the control system  570  of the flow controller  520  will operate the valve assembly  532  such that the valve assembly  532  is closed, partly open, or fully open as necessary to maintain constant fluid flow. The ability of the valve assembly  532  to be held in a partly open position can allow the control system  570  to exist in a steady state without the oscillations that may be encountered with mechanical feedback systems. 
     In many environments, a continuously oscillating valve assembly is acceptable; in other environments, allowing the valve assembly to reach a steady state is preferred, and the system  520  may be the implementation of choice. 
     In some situations, the algorithm implemented by the control circuit  574  may operate independent of fluid temperature. In other situations, the fluid temperature may be monitored to improve the constancy of fluid flow through the system  520 . In particular, the exemplary control circuit  574  generates the ACTUATOR CONTROL signal further based on a TEMP signal corresponding to the temperature of fluid flowing through system  520 . The TEMP signal allows the control circuit algorithm to factor in the state of the fluid flowing through the system  520 . The state of the fluid flowing through the system affects the mass flow and viscosity of the fluid. Knowledge of the state of the fluid may be important for calibration in some situations. 
     V. Fifth Embodiment 
     Referring now to FIG. 10, depicted at  620  therein is yet another exemplary flow controller system constructed in accordance with, and embodying, the principles of the present invention. The flow controller system  620  of the fifth embodiment is similar to the flow controller system  220  described above. In the interests of brevity and clarity, the flow controller system  620  will be described primarily to the extent that it differs from the flow controller system  220 . 
     The flow controller system  620  is designed to be electrically controlled so that the system  620  may be operated remotely and/or as part of a larger system having a centralized controller. 
     The exemplary flow controller system  620  comprises a housing assembly  622  comprising a first upper member  624 , a second upper member  626 , a base or intermediate member  628 , and first and second valve assemblies  630  and  632 . The first and second upper members  624  and  626  are threaded onto the base member  628  to form the housing assembly  622 . 
     The first and second valve assemblies  630  and  632  are arranged below first and second regulator chambers  634  and  636 , respectively, defined by the housing assembly  622 . The exemplary valve assembly  630  comprises a valve spring  640 , a valve member  642 , and a valve seat surface  644 . The valve spring  640  biases the valve member  642  towards the valve seat surface  644  such that the valve assembly  630  is normally closed. The exemplary valve assembly  632  comprises a valve member  646  and a valve seat surface  648 . The valve assembly  632  is closed when the valve member  646  engages the valve seat surface  648 . 
     The first valve assembly  630  opens substantially linearly in response to a first valve control force applied to a regulator plate  650 . The regulator plate  650  is suspended within the regulator chamber  634  between the valve spring  640  and a regulator spring  652 . The regulator spring  652  biases the valve assembly  630  into the open position against the force of the valve spring  640 . 
     In use, fluid is introduced into an inlet port  660  and then flows through the system  620  along a main flow path to an outlet port  662  as follows. A small amount of fluid flows along a bypass path to control the first valve assembly  630  by applying a first control force thereon. The pressure of the fluid flowing along the bypass path may further be used to control the second valve assembly  632 . The valve assemblies  630  and  632  are controlled to maintain substantially a substantially constant fluid flow rate through the flow controller  620 . Under proper conditions, the fluid flow rate through the flow controller  620  is substantially independent of upstream or downstream pressure changes. 
     In particular, the exemplary system  620  further comprises a control system  670 . The control system  670  comprises a solenoid actuator  672  (FIG. 10) and a control circuit  674  (FIG.  11 ). The solenoid actuator  672  is conventional and comprises a winding  676  and a solenoid member  678  that moves along a solenoid axis according to an ACTUATOR CONTROL signal applied to inputs  680  of the actuator  672 . The solenoid member  678  is supported within the second regulator chamber  636  by a solenoid spring  682 . The solenoid member  678  is in turn operatively connected to the valve member  646  such that movement of the solenoid member  678  moves the valve member  646 . 
     As shown in FIG. 11, the control circuit  674  generates the ACTUATOR CONTROL signal based on a SETPOINT signal and one or more pressure signals PRESSURE 1  and PRESSURE 2 . The control circuit  674  also generates a SCALED OUTPUT signal that, when the system  620  is calibrated, corresponds to the flow rate of fluid through the system  620 . 
     In the exemplary system  620 , the PRESSURE 1  and PRESSURE 2  signals are preferably sampled at sampling locations along the bypass path to prevent the main fluid flow from coming into contact with the pressure sensors that generate the signals PRESSURE 1  and PRESSURE 2 . Again, other sampling locations, such as directly at the output port  662 , are possible with fluids less sensitive to contamination. 
     In the exemplary flow controller system  620 , the actuator  672  and control circuit  674  form an electromechanical feedback system that replaces one of the mechanical feedback systems employed by the flow controller systems  20 ,  220 , and  420  described above. 
     In particular, the pressure signals PRESSURE 1  and PRESSURE 2  correspond to the pressures upstream and downstream of a restriction member  690  employed by the system  620 . The differential pressure across the restriction member  690  is used to control the valve assemblies  630  and  632 . These pressures may be measured at sampling locations spaced along the bypass path, at the output port, or the like. The sensors should be isolated from the main and bypass flow paths when the fluid is sensitive to contamination. 
     The control circuit  674  may comprise a microprocessor operating under control of a software program or a dedicated analog or hybrid digital/analog circuit. The software program or hybrid circuit implements algorithms and logic necessary to generate the ACTUATOR CONTROL signal. 
     In the exemplary system  620 , the ACTUATOR CONTROL signal is an analog current that passes through the windings  676  and induces movement of the solenoid member  678  and thus the valve member  646  as necessary to maintain fluid flow through the system  620  at rate determined by the SETPOINT signal. 
     Like the control system  570  of the flow controller system  520  described above, the control system  670  of the flow controller  620  will operate the valve assembly  632  in closed, partly open, or fully open positions as necessary to maintain constant fluid flow. The ability of the valve assembly  632  to be held in a partly open position can allow the control system  670  to exist in a steady state without the oscillations that may be encountered with mechanical feedback systems. In certain environments, allowing the valve assembly to reach a steady state is preferred, and the system  620  may be the implementation of choice. 
     In some situations, the algorithm implemented by the control circuit  674  may operate independent of fluid temperature. The fluid temperature is monitored in the exemplary system  620  to improve the constancy of fluid flow. In particular, the exemplary control circuit  674  generates the ACTUATOR CONTROL signal further based on a TEMP signal corresponding to the temperature of fluid flowing through system  620 . The TEMP signal thus allows the control circuit algorithm to factor in the state of the fluid flowing through the system  620 . 
     VI. Sixth Embodiment 
     Referring now to FIG. 12, depicted at  720  therein is yet another exemplary flow controller system constructed in accordance with, and embodying, the principles of the present invention. The flow controller system  720  of the sixth embodiment is similar to the flow controller system  220  described above. In the interests of brevity and clarity, the flow controller system  720  will be described primarily to the extent that it differs from the flow controller system  220 . 
     The flow controller system  720  is designed to be electrically controlled so that the system  720  may be operated remotely and/or as part of a larger system having a centralized controller. 
     The exemplary flow controller system  720  comprises a housing assembly  722  comprising a first upper member  724 , a second upper member  726 , a base or intermediate member  728 , and first and second valve assemblies  730  and  732 . The first and second upper members  724  and  726  are threaded onto the base member  728  to form the housing assembly  722 . 
     The first and second valve assemblies  730  and  732  are arranged below first and second regulator chambers  734  and  736 , respectively, defined by the housing assembly  722 . The exemplary valve assembly  730  comprises a valve member  740  and a valve seat surface  742 . The exemplary valve assembly  732  similarly comprises a valve member  744  and a valve seat surface  746 . The valve assemblies  730  and  732  are closed when the valve members  740  and  744  engage the valve seat surfaces  742  and  746 , respectively. 
     In use, fluid is introduced into an inlet port  760  and then flows through the system  720  along a main flow path to an outlet port  762  as follows. A small amount of fluid flows along a bypass path. The pressure of the fluid flowing along the bypass path is used to control the first and second valve assemblies  730  and  732  to maintain substantially a substantially constant fluid flow rate through the flow controller  720 . Under proper conditions, the fluid flow rate through the flow controller  720  is substantially independent of upstream or downstream pressure changes. 
     In particular, the exemplary system  720  further comprises a control system  770 . The control system  770  comprises first and second solenoid actuators  772  and  774  (FIG. 12) and a control circuit  776  (FIG.  13 ). The solenoid actuators  772  and  774  are conventional and each comprise a winding  776  and a solenoid member  778  that moves along axes defined by the actuators  772  and  774 . A FIRST ACTUATOR CONTROL signal and a SECOND ACTUATOR CONTROL signal are applied to inputs  780  of the actuators  772  and  774  to control movement of the solenoid members  778 . The solenoid members  778  are supported within the first and second regulator chambers  734  and  736  by solenoid springs  782 . The solenoid members  778  are in turn operatively connected to and move the first and second valve members  740  and  744 . 
     As shown in FIG. 13, the control circuit  776  generates the ACTUATOR CONTROL signals based on a SETPOINT signal and one or more pressure signals PRESSURE 1  and PRESSURE 2 . The control circuit  776  also generates a SCALED OUTPUT signal that, when the system  720  is calibrated, corresponds to the flow rate of fluid through the system  720 . 
     In the exemplary system  720 , the PRESSURE 1  and PRESSURE 2  signals are generated by pressure sensors arranged at appropriate sampling locations. The sensors should be isolated from the main and bypass flow paths when the fluid is sensitive to contamination. Again, other sampling locations, such as directly at the output port, are possible with fluids less sensitive to contamination. 
     In the exemplary flow controller system  720 , the actuators  772  and  774  and control circuit  776  form electromechanical feedback systems that replace the mechanical feedback systems employed by the flow controller systems  20 ,  220 , and  420  described above. 
     In particular, the pressure signals PRESSURE 1  and PRESSURE 2  correspond to the pressures upstream and downstream of a restriction member  790  employed by the system  720 . The difference between the PRESSURE 1  and PRESSURE 2  signals represents the differential pressure across the restriction member  790  and is used to control the valve assemblies  730  and  732 . 
     The control circuit  776  may comprise a microprocessor operating under control of a software program or a dedicated analog or hybrid digital/analog circuit. The software program or hybrid circuit implement algorithms and logic necessary to generate the ACTUATOR CONTROL signals as described below. 
     In the exemplary system  720 , the ACTUATOR CONTROL signals are analog currents that pass through the windings  776  and induce movement of the solenoid members  778  and thus the valve members  740  and  744  as necessary to maintain fluid flow through the system  720  at a rate determined by the SETPOINT signal. 
     Like the control systems  570  and  670  of the flow controller systems  520  and  620  described above, the control system  770  operates the valve assemblies  730  and  732  in closed, partly open, or fully open positions as necessary to maintain constant fluid flow. The ability of the valve assemblies  730  and  732  to be held in a partly open position can allow the control system  770  to exist in a steady state without the oscillations that may be encountered with mechanical feedback systems. Allowing both valve assemblies to reach a steady state may be preferred, in which case the system  720  may be the implementation of choice. 
     In the exemplary system  720 , the exemplary control circuit  776  generates the ACTUATOR CONTROL signals further based on a TEMP signal corresponding to the temperature of fluid flowing through system  720 . The use of the TEMP signal is, however, not essential to any implementation of the present invention. 
     VII. Additional Considerations 
     In the foregoing embodiments, certain of the components have been described as they are used in the examples given. The properties of many of these components can be changed depending upon the circumstances to “tune” the flow controllers for a particular use. 
     For example, the various springs used, interior chambers defined by the valve stems, and inlet and outlet openings in the valve stems must be selected based on the type of fluid, expected inlet pressures, and desired flow rates. 
     In addition, the materials used for the various components must be selected based on the pressures and types of fluids expected. For example, for air at low pressures, plastic may be used for many of the components. For caustic fluids and higher pressures, steel or stainless steel may be used. 
     In systems using one or more electromechanical feedback systems, the algorithm implemented by the control circuit, whether using discrete components or software, will also be created for a given set of predetermined parameters. 
     In the interests of brevity, the Applicant has described only six exemplary flow controller systems that may be constructed in accordance with the principles of the present invention. One of ordinary skill in the art will, however, recognize that certain features of these systems may be arranged in combinations that yield still more implementations of flow controllers constructed in accordance with the principles of the present invention. 
     In particular, the first and second exemplary flow controller systems  20  and  220  may be characterized as canister and plate configurations, respectively, because of overall appearance of the housing assemblies thereof: the housing assembly  22  comprises a generally cylindrical base or intermediate member  28 , while the housing assembly  222  comprises a generally planar base or intermediate member  228 . 
     The third through sixth embodiments  320 ,  420 ,  520 ,  620 , and  720 , disclose the use of one or more electromechanical feedback systems in the context of a plate configuration. However, such electromechanical feedback systems may be used in the context of a canister configuration, as well. In addition, the electromechanical feedback systems may be used in combination with each other and/or other types of feedback systems as desirable for a given environment. 
     A designer may design a particular implementation by initially determining the operating environment in which the flow controller system is to be used. The operating environment will include the properties fluid itself, the expected range of fluid input and output pressures, the ambient conditions, the tolerance for error, and the like. The designer may also consider commercial factors such as cost. 
     Based on the operating and commercial environments, the designer will initially decide on the basic physical structure of the flow controller housing by selecting from among a plurality of basic structure types such as canister, plate, or the like. 
     The designer will also select the material from which the housing assembly and other components are made based on the operating and commercial environments; if the fluid is a corrosive liquid or gas, the housing assembly may be made of stainless steel or other material that resists corrosion. The selected material may be plastic if the commercial environment dictates that the flow controller system is to be disposable. 
     The designer will then decide on the type of feedback regulator system that may be used in the input and output regulators. Generally, the designer may elect to use a mechanical or electrical feedback system for either or both regulators. If an electrical feedback system is used, the designer may elect to use a rotational device such as a stepper motor or a translating device such as a piezo-electric actuator or a solenoid actuator. 
     When the operating and commercial environments and physical structure of the flow controller system are determined, the algorithm necessary to implement the physical structure in the particular environment is determined. In a mechanical feedback system, this step will include selecting the type and arrangement of springs and other control devices employed. In an electrical feedback system, this step will include the design of an appropriate feedback loop to obtain the desired operational characteristics. 
     Accordingly, the invention may be embodied in forms other than those described herein without departing from the spirit or essential characteristics of the invention. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.