Abstract:
A turbine valve control system is provided. The turbine valve control system includes a variable flow metering device, a first sensor, a second sensor, a third sensor, a fourth sensor, memory, and processing circuitry. The variable flow metering device is capable of meeting a plurality of predetermined mass flow rates by varying positioning of the flow metering device. The first sensor is configured for detecting variable positioning of the flow metering device and generating a first output signal that is a function of the positioning of the flow metering device. The second sensor is configured for detecting fluid pressure upstream of the flow metering device and generating a second output signal that is a function of the detected upstream fluid pressure. The third sensor is configured for detecting fluid pressure downstream of the flow metering device and generating a third output signal that is a function of the detected downstream fluid pressure. The fourth sensor is configured for detecting temperature upstream of the flow metering device and generating a fourth output signal that is a function of the detected temperature. The memory includes first computer program code for calculating a combined coefficient of discharge times area that gives a desired flow for a given valve position, upstream pressure, downstream pressure, and temperature for subsonic flow. The memory also includes second computer program code for calculating a combined coefficient of discharge times area that gives a desired flow for a given valve position, upstream pressure, and temperature for sonic flow. The processing circuitry is configured to receive the signals, determine whether the flow is subsonic or sonic, and implement a corresponding one of the first and second computer program codes to calculate a combined coefficient of discharge times area that will generate a desired mass flow rate for the flow metering device. A method is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority in U.S. Provisional Patent Application Ser. No. 60/467,911, entitled “Turbine Valve Control System and Method”, naming as inventor Nathan Todd Miller, which was filed on May 5, 2003, and which is incorporated by reference herein. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention pertains to fuel delivery valves. More particularly, the present invention relates to fuel metering valves and control systems that regulate delivery of fuel to a turbine engine.  
       BACKGROUND OF THE INVENTION  
       [0003]     Liquid and gas fuel metering valves have been used for a number of industrial turbine engine applications. For example, liquid fuel metering valves have been used in numerous marine applications.  
         [0004]     In another case, gas fuel metering valves have been coupled with industrial turbine engines. For example, VG Series gas fuel metering valves such as the VG1.5, sold by Precision Engine Controls Corporation of San Diego, Calif., assignee of the present invention, have a balanced design with a single moving part. However, the gas flow path that extends through such valves deviates substantially from a linear flow path, requiring fuel to transit laterally around 90° lateral corners which reduces efficiency and performance.  
         [0005]     Applications for such fuel metering valves are present in the power industry for generating electrical power with gas turbine engines, for implementation on offshore oil rigs for power generation, on turbine engines in marine applications such as on hovercraft, and in the pipeline industry for related gas turbine engine applications requiring precise fuel metering.  
         [0006]     Many fuel metering techniques require the use of a Coriolis flow meter in combination with a metering valve. However, these flow meters are very expensive and cost-prohibitive, which restricts their adoption for many applications and uses.  
         [0007]     Accordingly, improvements are needed to increase controllable flow accuracy and efficiency from a fuel metering valve to a gas turbine engine, and to reduce cost of implementation. Additionally, improvements are needed in order to easily reconfigure a fuel metering valve to optimize the accuracy and efficiency of fuel delivery over varying ranges of supply pressure. Even furthermore, improvements are needed in the manner in which a fuel metering valve is controlled in order to deliver a desired flow rate of fuel without requiring the utilization of a separate flow meter which can significantly increase cost and complexity.  
       SUMMARY OF THE INVENTION  
       [0008]     A gas turbine valve control system is provided for a gas turbine valve with a displacement sensor for detecting position of the valve, an upstream pressure sensor, an upstream temperature sensor, and a downstream pressure sensor. Sonic and subsonic flow equations are selectively used to determine valve position that achieves a desired fuel mass flow rate for the valve.  
         [0009]     According to one aspect, a turbine valve control system is provided. The turbine valve control system includes a variable flow metering device, a first sensor, a second sensor, a third sensor, a fourth sensor, memory, and processing circuitry. The variable flow metering device is capable of meeting a plurality of predetermined mass flow rates by varying positioning of the flow metering device. The first sensor is configured for detecting variable positioning of the flow metering device and generating a first output signal that is a function of the positioning of the flow metering device. The second sensor is configured for detecting fluid pressure upstream of the flow metering device and generating a second output signal that is a function of the detected upstream fluid pressure. The third sensor is configured for detecting fluid pressure downstream of the flow metering device and generating a third output signal that is a function of the detected downstream fluid pressure. The fourth sensor is configured for detecting temperature upstream of the flow metering device and generating a fourth output signal that is a function of the detected temperature. The memory includes first computer program code for calculating a combined coefficient of discharge times area that gives a desired flow for a given valve position, upstream pressure, downstream pressure, and temperature for subsonic flow. The memory also includes second computer program code for calculating a combined coefficient of discharge times area that gives a desired flow for a given valve position, upstream pressure, and temperature for sonic flow. The processing circuitry is configured to receive the signals, determine whether the flow is subsonic or sonic, and implement a corresponding one of the first and second computer program codes to calculate a combined coefficient of discharge times area that will generate a desired mass flow rate for the flow metering device.  
         [0010]     According to another aspect, a method is provided for measuring gas flow rate into a gas turbine engine. The method includes: providing an adjustable position valve having a known coefficient of discharge and flow area for meeting a predetermined mass flow rate at each position of the valve; sensing position of the valve; sensing pressure upstream of the valve; sensing pressure downstream of the valve; sensing temperature upstream of the valve; determining whether flow through the valve is subsonic or sonic; based on whether the flow is determined to be subsonic or sonic, calculating a coefficient of discharge times area for the valve that provides a desired flow rate through the valve; and positioning the valve to a new position that achieves the calculated coefficient of discharge times area for the valve.  
         [0011]     According to yet another aspect, a turbine valve control system is provided. The turbine valve control system includes means for realizing any of a plurality of predetermined mass flow rates; means for detecting position of the flow metering device and generating a first output signal that is a function of the positioning of the flow metering device; means for detecting fluid pressure upstream of the flow metering device and generating a second output signal that is a function of the detected upstream fluid pressure; means for detecting fluid pressure downstream of the flow metering device and generating a third output signal that is a function of the detected downstream fluid pressure; means for detecting temperature upstream of the flow metering device and generating a fourth output signal that is a function of the detected temperature; means for determining a combined coefficient of discharge times area that gives a desired flow for a given valve position, upstream pressure, downstream pressure, and temperature for subsonic flow and second calculating means for calculating a combined coefficient of discharge times area that gives a desired flow for a given valve position, upstream pressure, and temperature for sonic flow; and means for determining whether flow is subsonic or sonic, and configured to implement a corresponding one of the first and second computer program codes to calculate a combined coefficient of discharge times area that will generate a desired mass flow rate for the flow metering device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Preferred embodiments of the invention are described below with reference to the following accompanying drawings.  
         [0013]      FIG. 1  is an isometric view of a metering valve provided in an application environment for delivering fuel at a controlled rate to a gas turbine engine, according to one aspect of the invention;  
         [0014]      FIG. 2  is an exploded isometric view of the metering valve illustrated in  FIG. 1  depicting assembly and placement of internal components;  
         [0015]      FIG. 3  is a partial breakaway isometric view of the metering valve of  FIGS. 1-2 , with the metering valve positioned upside down and viewed relative to an outlet end;  
         [0016]      FIG. 4  is a vertical centerline sectional view taken through the center of the machine illustrating the internal construction of the metering valve.  
         [0017]      FIG. 5  is a schematic block diagram for the electronics and control system for the metering valve of  FIGS. 1-4 .  
         [0018]      FIG. 6  is a state machine diagram for the metering valve of  FIGS. 1-5 .  
         [0019]      FIG. 7  is a proportional-integral-differential (PID) loop control controlling the metering valve of  FIGS. 1-5 .  
         [0020]      FIG. 8  is a flowchart illustrating steps in a proportional-integral-differential (PID) loop control algorithm for controlling the metering valve of  FIGS. 1-5 .  
         [0021]      FIG. 9  is a flowchart illustrating steps in a flow measurement algorithm for measuring flow through the metering valve of  FIGS. 1-5 .  
         [0022]      FIG. 10  is a flowchart illustrating steps in a flow control algorithm for controlling flow through the metering valve of  FIGS. 1-5 .  
         [0023]      FIG. 11  is a pair of flowcharts illustrating steps for an analog-to-digital converter (ADC) interrupt and foreground operations for ADC input conditioning for the digital signal processor (DSP) of the control system for the metering valve of  FIGS. 1-5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).  
         [0025]     Reference will now be made to a preferred embodiment of Applicants&#39; invention. An exemplary implementation is described below and depicted with reference to the drawings comprising a fuel metering valve and control system for delivering fuel to industrial gas turbine engines. A first embodiment is shown and described below in a configuration with reference generally to  FIGS. 1-11 . While the invention is described by way of a preferred embodiment, it is understood that the description is not intended to limit the invention to this embodiment, but is intended to cover alternatives, equivalents, and modifications which may be broader than this embodiment such as are defined within the scope of the appended claims.  
         [0026]     In an effort to prevent obscuring the invention at hand, only details germane to implementing the invention will be described in great detail, with presently understood peripheral details being incorporated by reference, as needed, as being presently understood in the art.  
         [0027]     A metering valve implementing a valve control system and method of the present invention is described with reference to  FIGS. 1-11  and is identified by reference numeral  10 . Such a metering valve  10  is particularly suited for use with industrial gas turbine engines. Construction and operation of physical components for this exemplary valve are described below with reference to  FIGS. 1-4 . Further details of the exemplary valve are disclosed in U.S. patent application Ser. No. 10/429,092 entitled “Gas Turbine Metering Valve”, naming the inventors as E. Joseph Mares, Nathan Todd Miller, and Mark Robert Huebscher, filed on May 3, 2003, and herein incorporated by reference. A description of the control system for the metering valve and operation are provided with reference to  FIGS. 5-11 .  
         [heading-0028]     A. Valve Construction  
         [0029]     As described below and also as previously incorporated by reference, metering valve  10  comprises a single exemplary metering valve capable of benefiting from control system features of the present invention. However, it is understood that a valve control system and method are provided by the present invention capable of being incorporated in any of a number of flow control devices, including any of a number of presently known control valve constructions that form alternative configurations to metering valve  10  of  FIGS. 1-4 . Metering valve  10  of  FIGS. 1-4  provides but one exemplary implementation, and it is envisioned that other implementations can benefit from the present control system and method as described below in greater detail.  
         [0030]     As shown in  FIG. 1 , metering valve  10  is configured to provide flow control, contamination resistance, and precision control over a wide flow range and within a relatively compact package size. The metering valve is also configured with a control system for use with high-performance, low-emissions, industrial gas turbines that require more than just reliable fuel control in order to optimize gas turbine engine functionality. Such applications demand stable, fast, and accurate fuel flow control for a variety of supply pressures and gases.  
         [0031]     In order to achieve this result, metering valve  10  is configured with a valve housing  11  that is formed by an electronics enclosure assembly  12  and a valve body assembly  14  that are secured together by fasteners (see hollow bolts  204  and  206  in  FIG. 4 ). According to one construction, housing  11  is formed from 6061-T6 aluminum alloy. Additionally, housing assembly  11  includes various O-ring seals  178 ,  180 ,  182 ,  184  and  186 , as shown in  FIGS. 3 and 4 . Metering valve  10  is mated in sealed engagement with an inlet supply pipe  16  and an outlet supply pipe  18  to deliver fuel from inlet supply pipe  16  in a metered and precisely controlled manner out through outlet supply  18  to a turbine engine (not shown) where it is combusted. Inlet supply pipe  16  is secured with fasteners (not shown) through a mounting end plate at a flow inlet  20 , whereas outlet supply pipe  18  is affixed to metering valve  10  via an end plate  42  using similar threaded fasteners, such as individual hex head bolts  40 . Outlet supply pipe  18  is secured in sealing engagement with a flow outlet  22  of metering valve  10 .  
         [0032]     It is understood that inlet supply pipe  16  has an end plate that is similar to end plate  42  of outlet supply pipe  18 , and is secured with fasteners similar to threaded bolts  40  which are received within threaded bores  44  of an outlet end plate  28 . In the case of flow inlet  20 , inlet supply pipe  16  secures with threaded fasteners using a similar end plate within threaded bores. It is further understood that each end plate includes a circumferential groove that extends about the respective flow inlet or outlet into which an O-ring is received for sealing and mating engagement between the respective end plate and an orifice plate assembly  52  (in the case of outlet supply pipe  18 ) and a corresponding circumferential portion of inlet end plate  26  (in the case of inlet supply pipe  16 ).  
         [0033]     Valve body assembly  14  includes a cylindrical valve housing  24  to which inlet end plate  26  and outlet end plate  28  are each affixed at opposite ends using a plurality of threaded, high-strength steel, double hex bolts (or fasteners)  38 . Fasteners  38  are preferably equally spaced apart about the circumference of each end plate  26  and  28 . A corresponding end portion at each end of valve housing  24  includes complementary, corresponding threaded bores  86  configured to receive fasteners  38 . Each end plate  26  and  28  includes a plurality of bores (now shown) that extend completely through the end plate, and are sized to receive each fastener  38  therethrough for threaded engagement within valve housing  24 , such as into a respective, threaded aperture  86 .  
         [0034]     Electronics enclosure assembly  12  includes an electronics housing  30  which is fastened to valve housing  24  using hollow bolts  204  and  206  (as shown in  FIG. 4 ) and a plurality of threaded cap screws (or fasteners) not shown. A cover  32  is affixed atop electronics housing  30  for encasing electronics  33  therein, including electronics that accurately control flow rate through metering valve  10 . More particularly, a plurality of threaded cap screws (or fasteners)  36  are used to secure cover  32  atop electronics housing  30 . Similarly, each cap screw  36  is passed through a through-bore within cover  32  and into a threaded bore  104  within a topmost edge of electronics housing  30  where such fasteners are threadingly received to retain cover  32  atop electronics housing  30 .  
         [0035]     Electronics housing  30  also includes a conduit hole  34  through which a turbine engine explosion-proof conduit is passed therethrough. Conduit hole  34  comprises a ¾″ NPT thread. As shown in  FIG. 3 , an explosion-proof conduit fitting (or union)  35  is threaded into hole  34 . More particularly, an explosion-proof wire harness or conduit is passed through conduit hole  34  and fitting  35 , after which fitting  35  is potted with a sealing cement and filler so as to make conduit hole  34  explosion proof and sealed as the conduit passes therethrough. One form of sealing cement for use in fitting  35  comprises Kwik Cement, sold by Appleton Electric Company, 1701 West Wellington Avenue, Chicago, Ill. 60657. One form of explosion-proof conduit fitting comprises a UNY or UNF union, also sold by Appleton Electric Company, 1701 West Wellington Avenue, Chicago, Ill. 60657.  
         [0036]     Electronics housing  30  of metering valve  10  is constructed as an explosion-proof housing having flame paths. U.S. Pat No. 6,392,322 to Mares, et al., issued May 21, 2002 and assigned to the present assignee, teaches one suitable technique for providing flame paths in an explosion-proof housing. Such construction techniques are also used herein in order to achieve an explosion-proof electronics housing  30  that is suitable for use in a potentially explosive user environment.  
         [0037]     Also shown in  FIG. 1 , a metal name plate  46  is secured atop cover  32  using a plurality of threaded drive screws  48 . Product information for metering valve  10  is then printed on or etched into name plate  46 . Furthermore, a threaded ground hole  50  is also provided within a side wall of electronics housing  30  into which a threaded fastener and a ground strap can be attached thereto for grounding the housing of metering valve  10 . Preferably, ground hole  50  does not pass completely through the side wall of electronics housing  30 .  
         [0038]     According to  FIG. 2 , metering valve  10  is shown in an exploded view to further facilitate understanding of the construction and operation of components contained therein. More particularly, metering valve  10  provides a stable, fast and accurate fuel flow control system extending over a range of supply pressures and gases. Because of the particular design of metering valve  10 , a flow-through design is provided that is capable of automatically compensating for variations in pressure and temperature in order to provide precise fuel flow required for specific gas turbine conditions under which the turbine and valve must operate. The electronics assembly includes a determination of fuel flow measurement based on valve feedback derived from pressure, temperature and displacement sensors in the valve, as discussed below. The valve is programmable for flow versus demand and complete closed-loop fuel control is made possible when using particular interface features. Accordingly, metering valve  10  is capable of being programmable for flow versus demand.  
         [0039]     Metering valve  10  provides a smooth flow-through design by way of an orifice plate assembly  52  that is carried in outlet end plate  28  by way of a female threaded bore  74  (see  FIG. 2 ) including a female threaded portion. An axial mover  56  comprising a linear motor  58  supports and moves a central cylindrical flow tube  60  toward and away from orifice plate assembly  52  in order to regulate flow through the orifice plate assembly  52 . By moving flow tube  60  into engagement with a seal  82  on orifice plate assembly  52  (see  FIG. 3 ), flow is completely stopped at orifice plate assembly  52 , and flow outlet  22  is completely closed. By actuating linear motor  58  to move flow tube  60  towards an upstream position away from orifice plate assembly  52 , an annular gap  136  (see  FIG. 3 ) is formed between the downstream end of flow tube  60  and seal  82  of orifice plate assembly  52 .  
         [0040]     In operation, the flow rate of fuel can be controlled by precisely positioning flow tube  60  relative to orifice plate assembly  52 . The relative position of the downstream end of flow tube  60  and orifice plate assembly  52  can be varied by accurately positioning flow tube  60  relative thereto. Additionally, flow is tailored based upon the specific axial and radial geometry provided on a flow diverter  78  of orifice plate assembly  52 . Flow diverter  78  extends upstream and within flow tube  60  so as to vary the dimension of the annular gap  136  (see  FIG. 3 ) formed therebetween for various positions of the downstream end of flow tube  60  relative to orifice plate assembly  52 .  
         [0041]     As shown in  FIG. 2 , orifice plate assembly  52  comprises a cylindrical orifice plate  76  that includes three crescent-shaped flow apertures  84  that are spaced radially about orifice plate  76 . According to such construction, orifice plate  76  comprises a spider in which three flow apertures  84  are provided between the spokes of such spider. Orifice plate  76  includes a plurality of male threads adjacent an upstream edge that mate in threading engagement within a threaded bore  74  of outlet end plate  28 . A radial outermost portion of orifice plate  76  is received within a complementary bore  72  of outlet end plate  28 . According to one construction, orifice plate  76  is made from Nitronic™ 50, a version of 316 stainless steel (SS).  
         [0042]     One desirable feature of the present metering valve is provided by the ability to replace flow diverter  78  with an alternative flow diverter having a different axial profile by removing threaded fastener  80  which retains flow diverter  78  onto orifice plate  76 . Subsequently, a new, alternatively constructed flow diverter can be mounted upstream and onto orifice plate  76  by re-inserting threaded fastener  80  and threading such flow diverter into engagement therewith. Hence, sensitivity of metering valve  10  can be optimized for different ranges of flow rates by substituting in an optional flow diverter having a desired shape.  
         [0043]     Linear motor  58  of  FIG. 2  includes a motor housing  70  from which a pair of solenoid wires  66  and  68  extends for connection with corresponding electronics  33  within electronics enclosure assembly  12 . A circumferential shoulder  62  is rigidly secured to a location on flow tube  60 . Shoulder  62  helps retain an armature  64  at a precise location along flow tube  60 . Linear motor  58 , in assembly, is received within an internal bore  100  of valve housing  24 .  
         [0044]     In assembly, the double hex bolts  38  extend through outlet end plate  28  and into complementary, corresponding threaded apertures  86  at a downstream end of valve housing  24 . Similarly, double hex bolts  38  extend through corresponding apertures in inlet end plate  26  and into threaded apertures in an upstream end of valve housing  24  (similar to threaded apertures  86  provided at a downstream end of valve housing  24 , but not shown).  
         [0045]     Inlet end plate  26 , as shown in  FIG. 2 , is likewise affixed to an upstream end of valve housing  24  using a plurality of threaded double hex bolts  38 . Inlet end plate  26  is configured to support a displacement sensor  88 , an inlet temperature sensor  94 , and an inlet pressure sensor  96 . According to one construction, displacement sensor  88  comprises a linear variable differential transformer (LVDT)  90  that is carried by an LVDT support plate  92 . According to one construction, temperature inlet sensor  94  comprises a thermistor. Similarly, an outlet pressure sensor  98  is carried on an inner surface of outlet end plate  28 .  
         [0046]     According to one suitable construction, LVDT  90  is a model MHR Schaevitz LVDT sensor sold by Measurement Specialties, Inc. (MSI), 710 Route 46 East, Ste. 206, Fairfield, N.J. 07004. Similarly, temperature inlet sensor  94  comprises a Model H-025-08-1 (Part No. 10K3D612) thermistor sold by BetaTHERM of Shrewsbury, Mass., and headquartered in Galway, Ireland. Furthermore, pressure sensors  96  and  98  each comprise a Model 85 Ultra Stable™ stainless steel pressure sensor manufactured and sold by Measurement Specialties, Inc. (MSI), 710 Route 46 East, Ste. 206, Fairfield, N.J. 07004.  
         [0047]      FIG. 2  also illustrates the detailed construction and assembly of electronics enclosure assembly  12  which is secured atop valve body assembly  14  to form a valve housing assembly  11 . As shown in  FIGS. 2 and 4 , electronics housing  30  is configured to form a substantially rectangular electronics cavity  102  within assembly  12 . An electronics package  101  is physically attached to a bottom surface of electronics cavity  102  using four threaded fasteners  110  that are threaded into engagement with female threads provided within corresponding standoffs  118  that are threaded into the bottom surface of electronics cavity  102 .  
         [0048]     More particularly, electronics package  101  includes a motor driver printed circuit (PC) board  112  and a digital logic printed circuit (PC) board  116 . Boards  112  and  116  are carried in spaced-apart relation using a plurality of tubular spacers  114  that are placed in coincidence within apertures at each of the four corners of each board  112  and  116  and configured to receive threaded fasteners  110  therethrough and into standoffs  118 . Standoffs  118  are first secured within threaded female apertures within a bottom surface of electronics cavity  102 . Standoffs  118  further include female threads sized to receive fasteners  110  at a topmost end for securing electronics package  101  within electronics cavity  102 . Board  112  includes a pair of customer connectors  120  and  122  which will be discussed in greater detail below. Electronics  33  are provided on boards  112  and  116 . Processing circuitry  126  is provided on boards  112  and  116 . Additionally, a pair of powered diode wires  128  and  130  are provided.  
         [0049]     Upon mounting electronics package  101  within electronics cavity  102 , cover  32  is then secured atop housing  30  using a plurality of threaded cap screws  36  which are received through respective clearance through-bores  132  and cover  32 . To facilitate sealing engagement of cover  32  to housing  30 , an O-ring seal  108  is provided within a complementary receiving groove on the bottom of cover  32  positioned to mate with a top sealing surface  106  provided on housing  30  inboard of threaded bores  104  that receive threaded portions of cap screws  36 , in assembly.  
         [0050]     To complete assembly, product name plate  46 , including product and manufacturing information printed or embossed thereon, is affixed atop cover  32  using a plurality of drive screws  48  that pass through holes  54  in plate  46  for threaded securement within corresponding threaded holes  55  provided in corresponding locations of cover  32 .  
         [0051]     Upon assembly, metering valve  10  of  FIG. 2  is configured to receive fuel into flow inlet  20 , meter such fuel by axially positioning flow tube  60  relative to seal  82  and flow diverter  78  of orifice plate assembly  52 , and deliver fuel at a desired rate to a gas turbine engine via three flow apertures  84  that provide flow outlet  22 . The fuel can be gas or liquid. Optionally, the metering valve can be used to deliver a mixture of fuel and air.  
         [0052]     According to  FIG. 3 , a flow tube assembly  134  within metering valve  10  provides for precision fuel flow control over a wide flow range and within a very compact package size through axial displacement of flow tube  60  relative to seal  82  and flow diverter  78  of orifice plate assembly  52 . By properly energizing wire windings  176  of motor winding assembly  172 , electromagnetic force (EMF) lines of flux attract an armature  64  of flow tube assembly  134  towards a pole piece  162 . By adjusting the duty cycle to wire windings  176 , the position of armature  64  (as well as tube  60 ) can be varied such that a frustoconical portion of armature  64  is moved closer towards pole piece  162 , thereby compressing coil spring  142 . When windings  176  are not energized, coil spring  142  drives flow tube  60  into sealing engagement with seal  82  of orifice plate assembly  52 , thereby completely shutting off flow through metering valve  10 .  
         [0053]     As shown in  FIG. 3 , armature  64  has a frustoconical portion that is shaped in complementary relation with pole piece  162  such that maximum attraction of pole piece  162  brings pole piece  162  into proximate nesting relation with the complementary frustoconical portion of armature  64 , thereby moving flow tube  60  away from seal  82  so as to impart a maximum open dimension for flow gap  136 . According to one design, flow gap  136  has a maximum value of one-quarter inch.  
         [0054]     According to one construction, motor winding assembly  172  comprises a bobbin case  174  about which a 17-gauge wire is wound so as to provide wire windings  176 . Motor winding assembly  172 , when energized, generates electromagnetic force (EMF) lines of flux that attract armature  64  and compress spring  142  as wire windings  176  receive an adjusted level of current using a current control loop so as to adjust a duty cycle therethrough. The presence of wire windings  176  between motor housing  70  and pole piece  162  cooperates with armature  64  so as to provide appropriate lines of flux to attract the armature  64  to pole piece  162 .  
         [0055]     In order to determine the relative position of flow tube  60  and the width of circumferential flow gap  136 , a displacement sensor  88  in the form of LVDT  90  detects the position of flow tube  60  relative to inlet end plate  26  in valve housing  24 . Such relative displacement corresponds with the displacement of flow tube  60  relative to orifice plate  76  which corresponds with the dimension of flow gap  136 . Accordingly, fuel is precisely delivered at a desired flow rate by way of flow inlet  20  to flow tube  60  and out through three flow apertures  84  that are provided through orifice plate  76 , as flow tube  60  is spaced away a desired distance from seal  82  via actuation of linear motor  58  corresponding with a specific duty cycle being delivered to wire windings  176 .  
         [0056]     As shown in  FIG. 3 , LVDT  90  comprises a mechanically actuated core  166  that is carried by support plate  92  in fixed relation with flow tube  60 . Accordingly, movement of flow tube  60  can be detected by movement of plate  92  and core  166  relative to coils within a cylindrical coil assembly (or transformer)  168 . Movement of the mechanically actuated core  166  relative to assembly  168  changes reluctance of a flux path between a primary coil and a secondary coil of assembly  168 , thereby generating an output signal related to displacement of flow tube  60 . It is further understood that circuitry is provided for interfacing with LVDT sensor  90  within circuitry provided in electronics package  101  (of  FIG. 2 ).  
         [0057]     As shown in  FIG. 3 , in operation, displacement sensor  88  is configured to detect axial positioning of flow tube  60  relative to a central flow body provided by flow diverter  78  and seal  82  of orifice plate assembly  52 . Fuel which is received upstream via flow inlet  20  passes downstream through flow tube  60 , out and around circumferential flow gap  136 , and out through three arcuate, circumferentially spaced-apart flow apertures  84  within orifice plate  76 . Fuel leaving through flow apertures  84  thereby provide for flow outlet  22 . Subsequently, the precisely metered fuel is delivered to an outlet supply pipe, such as outlet supply pipe  18  depicted in  FIG. 1 .  
         [0058]     According to  FIG. 3 , flow diverter  78  is shaped such that the shape can determine the outlet characteristics, such as flow resolution, provided between flow tube  60 , flow diverter  78 , and seal  82  as fuel is delivered through flow apertures  84  into flow outlet  22 . A threaded fastener  80 , along with a lock washer, is received within an enlarged, recessed bore and a clearance bore in orifice plate  76 , and into a threaded bore that is provided within flow diverter  78 . Securement of threaded fastener  80  into the threaded bore retains flow diverter  78  onto orifice plate  76 . An elevated shoulder is provided in flow diverter  78  and sized sufficiently to securely retain seal  82  in sealing engagement between flow diverter  78  and orifice plate  76  as fastener  80  is secured into flow diverter  78 . Such construction enables a user to easily clean the valve and to change the shape of flow diverter  78 . For example, an alternatively-shaped flow diverter can be substituted for flow diverter  78 .  
         [0059]     As shown in  FIG. 3 , flow tube  60  is carried for axial movement by linear motor  58  in slidable and sealing engagement at the input end and the output end with inlet end plate  26  and outlet end plate  28  of valve body assembly  14 , respectively. More particularly, a dynamic seal  152  is provided adjacent the downstream end of flow tube  60 , as shown in  FIGS. 3 and 4 . According to one construction, dynamic seal  152  is formed from a filled polytetrafluoroethylene (PTFE). Adjacent and upstream of seal  152 , a circumferential bearing  154  is provided. According to one construction, bearing  154  comprises a Rulon™ J bearing. Bearing  154  facilitates axial fore and aft movement of flow tube  60  relative to outlet end plate  28 ; whereas seal  152  provides a sliding seal along the downstream end of flow tube  60  relative to outlet end plate  28 .  
         [0060]     Similarly,  FIG. 3  further illustrates the seal and support components provided for flow tube  60  relative to inlet end plate  26 . More particularly, a wiper seal  156  is provided adjacent an upstream end of flow tube  60  so as to provide a wiping seal between flow tube  60  and inlet end plate  26 . Additionally, a dynamic seal  158  is provided downstream of wiper seal  156  to further facilitate a dynamic seal between flow tube  60  and inlet end plate  26 . Furthermore, a circumferential bearing  160  is provided downstream of dynamic seal  158 , between flow tube  60  and inlet end plate  26 . Bearing  160  provides a sliding bearing surface to facilitate axial fore and aft motion of flow tube  60  relative to inlet end plate  26 . According to one construction, dynamic seal  158  comprises a filled polytetrafluoroethylene (PTFE). According to one such construction, bearing  160  also comprises a Rulon™ J bearing.  
         [0061]     As shown in  FIG. 3 , coil spring  142  is received within a cylindrical groove  170 . Once flow tube  60  is moved to a maximum open-valve position for the metering valve, support plate  92  compresses spring  142  within cylindrical groove  170  to a maximum compressive position. Corresponding with such position, displacement sensor  88 , here LVDT  90 , detects such maximum open position by way of core  166  being displaced maximally within cylindrical coil assembly (or transformer)  168 . When the flow tube is moved to a closed position for the valve assembly, plate  92  moves in a downstream direction as the motor is de-energized, thereby enabling coil spring  142  to drive flow tube  60  to a downstream position as plate  92  (which is circumferentially affixed to flow tube  60 ) pushes flow tube  60  into sealing and seating engagement with seal  82  at an opposite end. Hence, coil spring  142  ensures closure of the valve assembly when the linear motor is not energized. Hence, further benefit is provided in that the valve is closed when power is lost to the drive motor.  
         [0062]     As shown in  FIGS. 3 and 4 , armature  64  has a cylindrical outermost portion that is contiguous with a frustoconical portion. However, a downstream end of armature  64  is undercut, as shown in  FIGS. 3 and 4 . Armature  64 , as shown in  FIG. 4 , has female threads  202  that enable threaded engagement of armature  64  onto flow tube  60  via complementary, corresponding male threads  203  that are provided on flow tube  60 . A circumferential shoulder  62  on flow tube  60  provides an affixation stop point for securing armature  64  in threaded engagement at a fixed location along flow tube  60 , as shown in  FIG. 4 .  
         [0063]     Additionally, a circumferential groove  140  is provided on the radial outermost portion of armature  64  (see  FIG. 4 ) into which an O-ring  139  is first provided and on top of which a seal ring  138  is further provided. As shown in  FIGS. 3 and 4 , seal ring  138  forms a sliding piston-type seal with a bore  69  provided in motor housing  70 . Armature  64  is further secured, after threading, onto flow tube  60  at a fixed position using threaded set screw  165  that is received within a threaded set screw hole  164  of armature  64 . Set screw  165  is threaded into screw hole  164  until set screw  165  engages with an outer surface of flow tube  60 , thereby fixing armature  64  at a desired location on flow tube  60 .  
         [0064]     The provision of seal ring  138  along cylindrical bore  69  provides a further advantage to the present metering valve. According to one construction, seal ring  138  is made of polytetrafluoroethylene (PTFE). More particularly, seal ring  138  provides dampening of flow tube  60  as seal ring  138  and bore  69  cooperate to partition a pair of sealed air chambers  215  and  216  (see  FIGS. 3 and 4 ) downstream and upstream of seal ring  138 , respectively. As shown in  FIG. 4 , movement of armature  64  and seal ring  138  within cylindrical bore  69  provides compression and evacuation on respective opposite sides of seal ring  138  as armature  64  moves so as to change the relative volumes of air chambers  215  and  216 . Such action imparts dampening to sudden motions of flow tube  60  within the metering valve which imparts benefits and stability to fuel flow control by the valve.  
         [0065]      FIGS. 2 and 4  illustrate the provision of a temperature sensor in the form of a thermistor  94  which is provided adjacent an inlet (or upstream) end of flow tube  60  for measuring inlet temperature of fuel into flow tube  60  of metering valve  10 . As shown in  FIGS. 2 and 4 , thermistor  94  is threaded for mounting into a threaded bore  250  provided in inlet end plate  26 . As shown in  FIG. 4 , threaded bore  250  is in fluid communication with a temperature port  244  that communicates with an upstream end of flow tube  60  for detecting upstream temperature at flow tube  60 . To facilitate manufacturing of temperature port  244 , an enlarged port  246  is provided with a threaded female portion for receiving a threaded plug  148 . Plug  148  is used to seal the radial outer end of threaded port  246  and to further facilitate cleaning and maintenance of port  244 .  
         [0066]     In addition to illustrating the position of thermistor  94  and temperature port  244 ,  FIGS. 2, 3  and  4  further illustrate the positioning of an inlet pressure sensor  96  (see  FIGS. 2, 3  and  4 ) and an outlet pressure sensor  98  (see  FIGS. 2 and 4 ). The resulting detected temperature from thermistor  94  and pressures from pressure sensors  96  and  98  are utilized to calculate fuel flow delivery rates through metering valve  10  for both subsonic and sonic flow conditions.  
         [0067]     Inlet pressure sensor  96  is received within a threaded bore  150  which communicates via the pressure port  144  with flow inlet  20 . Pressure port  144  is formed similar to temperature port  244  wherein an enlarged threaded port  146  is first formed and in which a threaded plug  148  is provided to seal threaded port  146  and pressure port  144  after construction.  
         [0068]     Similarly, outlet pressure sensor  98  is threaded into a similar threaded bore  350  which communicates with a pressure port  344 . An enlarged threaded port  346  is used to facilitate construction of pressure port  344 , after which another threaded plug  148  is threaded into sealing engagement therein.  
         [0069]     In operation, temperature port  244  enables thermistor  94  to detect inlet temperature of fuel at flow inlet  20 . Likewise, pressure port  144  enables inlet pressure sensor  96  to detect pressure of fuel at flow inlet  20 . Finally, pressure port  344  enables outlet pressure sensor  98  to detect downstream pressure adjacent flow outlet  22 , or adjacent to the downstream end of flow tube  60 .  
         [0070]      FIG. 4  illustrates the physical attachment of electronics enclosure assembly  12  to valve body assembly  14 . More particularly, a pair of hollow bolts  204  and  206  are used to secure electronics enclosure assembly  12  to valve body assembly  14 . Additionally, hollow bolts  204  and  206  facilitate the passage of wiring from sensors  94 ,  96  and  98  to electronics package  101  within electronics enclosure whereas wires  210  and solenoid wires  66  and  68  pass through hollow bolt  206 . Each bolt  204  and  206  includes a circumferential outer groove  213  in which a helicoil lock  212  is provided to lock each bolt  204  and  206  into the respective threaded surfaces provided in valve housing  24 .  
         [0071]     Also shown in  FIG. 4 , a threaded fastener  214  is used to secure motor housing  70  together with pole piece  162 . Two other threaded fasteners (not shown) are equally spaced circumferentially from threaded fastener  214  shown in  FIG. 4 .  
         [0072]     As shown in  FIGS. 14 , metering valve  10  uses onboard sensors and digital electronics to automatically measure and control mass flow of fuel over a wide range of temperatures and pressures, as described below with reference to  FIGS. 5-11 . The actual fuel flow can be determined with onboard electronics based on feedback signals from sensors in the valve. The metering valve also uses integrated, 24-volt DC (VDC) digital electronics that contain additional inputs and outputs for allowing programmable flow control, closed-loop turbine control, and an array of other options. Analog interfaces are provided within the electronics housing which are user configurable as 4-20 mA (current) or 0-5 VDC (voltage). Real-time health and data monitoring of the metering valve can also be implemented through an isolated RS-232/RS-485 serial interface that enables a user to see mass flow, inlet and outlet pressures, gas temperature and diagnostics.  
         [0073]     The flow tube construction for the metering valve is balanced. Additionally, the flow-through construction is self-cleaning. Even furthermore, the only moving part present within the valve is the moving core that is driven by a direct acting solenoid comprising the armature and flow tube. As a result, prior art techniques of utilizing pneumatics or hydraulics for actuating a valve are eliminated, and their concomitant tendency to leak and break down is eliminated from the design. A fail-safe closing spring along with an easy-to-clean soft seat provides a positive, leak-tight shutoff which further enhances the contamination-resistant design of the metering valve.  
         [0074]     Under experimental tests, it has been determined that the present metering valve design results in improved flow performance because of its smooth, flow-through design. The metering valve has been found to have a 200:1 turn-down ratio and a plus or minus one percent linearity, making such metering valve ideal for use with 1-10 megawatt gas turbines. Even furthermore, the electronics on the metering valve enable a user to program a maximum flow rate and relatively easily achieve such result by way of the incorporated sensors.  
         [heading-0075]     B. Control System and Method  
         [0076]     Details of the valve control system and method of the present invention are disclosed below with reference to  FIGS. 5-11 . The control system uses both sonic and subsonic flow equations to control an exemplary turbine fuel metering valve (of  FIGS. 1-4 ) in order to meet a mass fuel flow that matches a desired demand for a particular turbine engine. Such subsonic and sonic flow equations are implemented using a microcontroller such that the microcontroller can adjust valve fuel flow to match a flow control setpoint. The sensors previously described for measuring flow tube displacement, upstream pressure, downstream pressure, and upstream temperature provide data that is used as input values to the respective flow equations in order to determine valve fuel flow that matches a setpoint value.  
         [0077]      FIG. 5  illustrates electronics  33  forming an electronics system  218  that is provided within the electronics closure assembly of the metering valve described with reference to  FIGS. 1-4 . More particularly, electronics  33  cooperates to provide a control system for the metering valve which includes a 24-volt DC (VDC) electronic power supply line  220 . Power supply line  220  delivers a supply of 24 VDC nominal (16V to 32V) input power with a reverse voltage protection. A four-amp (A) maximum power supply current is provided at 16 volts (V), or 64 watts.  
         [0078]     Analog inputs  222  are also provided to electronics  33 . The analog inputs  222  form part of an analog interface, in combination with analog outputs  234 . Analog inputs  222  comprise software selectable inputs ranging from zero to 20 mA, and zero to 5 volts DC, or variations thereof. The analog outputs  234  comprise software selectable outputs that range from minus 20 to 20 mA, and minus 5 to 5 volts DC, or variations thereof. A first communication port  224  and a second communication port  226  each include communication links that are coupled with an isolated, 16-bit RS-232/RS-485 serial communications interface  272  by way of isolation circuitry  260 . Serial communication interface  272  is provided on digital signal processor (DSP)  256 . Isolation circuitry  260  provides 500 volt AC (714 Vdc) isolation between the case and signal wires provided by communications ports  224  and  226 .  
         [0079]     A fuel inlet  228  provides fuel to metering valve  10  and a fuel outlet  238  delivers the metered fuel in response to operation of electronics  33  which serve to regulate operation of metering valve  10  by operation of solenoid  172 . As shown in  FIG. 5 , metering valve  10  is shown in a simplified schematic form, separated from solenoid  172 . However, it is understood that the associated mechanical components are provided together. Simplified representations of fuel inlet  228 , fuel outlet  238 , and metering valve  10  are depicted herein in order to show their relationship relative to the accompanying electronics  33 .  
         [0080]     Discrete outputs  230  and discrete inputs  232  are provided in association with isolation circuitry  258  for communication with an input/output (I/O) interface  274 . Outputs  230  and inputs  232  comprise optional discrete inputs and outputs. Discrete outputs  230  comprise solid state relay outputs. Discrete inputs  232  comprise optocoupler inputs.  
         [0081]     A 24-volt DC (VDC) solenoid power supply  236  supplies power to a solenoid driver  267  for driving solenoid  172 . Additionally, power conditioning and isolation circuitry  252  conditions and isolates power from power supply  220  for delivery to signal conditioning circuitry  254 , DSP  256 , and signal conditioning circuitry  262 .  
         [0082]     Signal conditioning circuitry  254  conditions input signals for delivery to analog-to-digital (A/D) interface  270  of DSP  256 . Similarly, signal conditioning circuitry  262  serves to condition signals from pulse width modulator (PWM)  276  for delivery to PWM  276 , to analog output  234 , and to displacement sensor  88  (or LVDT  90 ).  
         [0083]     Isolation circuitry  266  conditions signals from PWM  276  for delivery to driver  267 . Similarly, isolation circuitry  264  conditions signals from downstream of driver  267  for delivery to signal conditioning circuitry  254 . Solenoid driver  267  is a high current solenoid driver.  
         [0084]     As shown in  FIG. 5 , an inlet pressure sensor  96  measures upstream pressure, and an inlet temperature sensor  94  measures upstream temperature. Likewise, an outlet pressure sensor  98  measures downstream pressure, relative to metering valve  10 .  
         [0085]     DSP  256  is a Model No. TMS320F2810 manufactured by Texas Instruments. DSP  256  has a 32-bit central processing unit (CPU), with 6.67 nanosecond minimum instruction time, 150 MHz clock speed, 64K by 16 Flash memory, 18K by  16  internal SRAM memory, multiple instruction processing (4 level pipeline), and a watchdog timer.  
         [0086]     DSP  256  has a 12 bit analog-to-digital converter (ADC) with 16 channels. There are two sample-and-hold blocks and a 2 by 8 channel input multiplexer. There are two asynchronous serial communication interfaces (SCIs), a synchronous serial peripheral interface (SPI), an enhanced controller area network (eCAN), and two event manager modules.  
         [0087]     Software and firmware design for the metering valve control system benefits from several software features. First, the valve is configured to be run in multiple modes of operation: a stroke mode, a flow control mode, a flow measurement mode, and a flow limiting mode. Secondly, the processing speed of the DSP (150 MHz) enables fast flow and control calculations. Finally, C/C++ programming language is used for the software which allows for faster software development, provides floating point math functions, and eases software maintenance.  
         [0088]      FIG. 6  illustrates software states for the metering valve control system of the present invention. More particularly, a “POWER-UP/RESET” state is configured to initialize the digital signal processor (DSP) and associated peripherals (such as the LVDT, solenoid driver, solenoid, and circuitry). Additionally, this state performs device tests and data integrity tests. Furthermore, this state transitions to the “SETUP” state, but only if a Setup Command is received.  
         [0089]     A “RUN” state corresponds with four operating modes: a “Run-Stroke Mode”, a “Run-Flow Measurement Mode”, a “Run-Flow Control Mode”, and a “Run-Flow Limiting Mode”. The “Run-Stroke Mode” corresponds with standard metering valve operation. The “Run-Flow Measurement Mode” corresponds with sensor-based determination of flow using flow equations. The “Run-Flow Control Mode” corresponds with position demand being used to set a desired flow. The “Run-Flow Limiting Mode” corresponds with position demand being used to set desired flow limited by turbine acceleration/deceleration tables.  
         [0090]     A “SHUTDOWN” state is entered as a result of critical faults where a selected “RUN” state cannot continue. The “SHUTDOWN” state transitions back to the “RESET” state by way of a power cycle.  
         [0091]     A “SETUP” state is configured to set an operating mode for the metering valve to a stroke mode, a flow measurement mode, a flow control mode, or a flow limiting mode. The “SETUP” state also enables the setting of gas parameters for the fuel being delivered by the valve. Additionally, identification information and flow control loop parameters can be set. Finally, a user can exit the “SETUP” state only by using a reset command, according to one implementation.  
         [0092]     Several control system interrupts are used on the circuitry and software for the metering valve. More particularly, an ADC interrupt is provided to read all A/D inputs and to serve as a main timer interrupt. An SCI transmit interrupt is provided to transmit RS-232/RS-485 serial data. An SCI receive interrupt is provided to receive RS-232/RS485 serial data.  
         [0093]     System timing is used to handle the ADC interrupt and several tasks. First, the ADC interrupt is executed every 12.5 microseconds at an 80 KHz rate, and serves as a timer interrupt. The ADC interrupt sets event flags to execute foreground tasks, and the execution time is 1.47 microseconds. Secondly, an LVDT position task is executed every 100 microseconds at a 10 KHz rate. The LVDT position task is called by the ADC interrupt, and execution time is 0.63 microseconds. Thirdly, a current control task is executed every 100 microseconds at a 10 KHz rate. The current control task runs in the foreground when the event flag is set, and execution time is 100 microseconds. Fourth, a position control task is executed every 100 microseconds at a 10 KHz rate. The position control task runs in the foreground when an event flag is set, and execution time is 100 microseconds.  
         [0094]     Fifth, a flow control task is executed every 20 milliseconds at a 50 Hz rate. The flow control task runs in the foreground when an event flag is set. An estimate of the flow measurement execution time is 24 microseconds, and an estimate of the flow control execution time is 12 microseconds. Finally, a temperature task is executed every 819 milliseconds at a 1.2 Hz rate. The temperature task runs in the foreground when an event flag is set, and an estimated execution time is 16 microseconds.  
         [0095]      FIGS. 7-11  illustrate logic flow for the control algorithms implemented via the control system for the metering valve of  FIGS. 1-5 . More particularly, proportional-integral-differential (PID) control loops include a flow control PID loop, a position control PID loop, and a current control PID loop. Additionally, algorithms include a flow measurement algorithm, a flow control algorithm, and an ADC input conditioning algorithm.  
         [0096]      FIG. 7  illustrates the logic flow for the PID loop control. Step “S1” represents the displacement position of the metering tube in the metering valve as measured from the LVDT. Step “S1” provides inputs to Steps “S2” and “S7”. Step “S2” represents the flow measurement equation used to calculate flow corresponding to the detected position from the LVDT. Step “S3” proceeds to Step “S4”. Step “S3” represents 4-20 milliamp demand as a setpoint input. Step “S4” represents a decision diamond that queries whether the desired control is flow control or position control. When flow control is desired, the process proceeds to Step “S5”. When position control is desired, the process proceeds to Step “S7”. In Step “S5”, a flow control equation is implemented, as described below in greater detail with reference to sonic and subsonic flow conditions. After performing Step “S5”, the process proceeds to Step “S 6 ”.  
         [0097]     In Step “S6”, a flow control PID loop is implemented. The flow control PID loop controls valve flow, has a setpoint input of 4-20 milliamp (0-5 VDC) demand input, and has a status input in the form of a flow measurement equation for both sonic and subsonic flow conditions. The flow control PID loop provides an input to the position control loop PID of Step “S7”. The flow control PID loop uses fixed point math in order to enhance operating speed.  
         [0098]     In Step “S7”, a position control PID loop is implemented in order to control position of the flow tube within the metering valve. The position control PID loop receives one of two setpoint inputs, depending on the operating mode: first, a flow control PID loop output is received for a flow control mode; and second, a 4-20 milliamp demand input is received for a stroke mode. Additionally, a status input is received by the position control PID loop comprising a detected position from the LVDT. The position control PID loop generates an output that provides a solenoid current PID loop input for Step “S9”. The position control PID loop uses fixed point math which enhances speed. After performing Step “S7”, the process proceeds to Step “S8”.  
         [0099]     In Step “S8”, solenoid current is sensed. After performing Step “S8”, the process proceeds to Step “S9”.  
         [0100]     In Step “S9”, a current control PID loop is implemented in order to manage drive current to the solenoid of the metering valve. The current control PID loop receives a set-point input comprising the position control PID loop output. The current control PID loop also receives a status input comprising a solenoid current sense input signal. The current control PID loop generates an output to the solenoid, represented as Step “S10”. The current control PID loop also used fixed point math in order to maximize speed of operation.  
         [0101]      FIG. 8  illustrates a logic flow diagram for implementing a proportional-integral-derivative (PID) loop algorithm for carrying out Steps “S6”, “S7”, and “S9”, of  FIG. 7 . More particularly, Steps “S1” through “S23” detail one suitable PID control loop algorithm usable to implement the PID loop functionality of Steps “S6”, “S7”, and “S9” of  FIG. 7 .  
         [0102]     A proportional integral and derivative (PID) control system is illustrated with reference to  FIG. 8 . More particularly, the control process starts with Step “S1”. In Step “S2”, a reverse acting control loop is implemented wherein error equals setpoint minus output status (or measurement). In Step “S3”, SumError equals plus Error. Steps “S4”-“S9” define a proportional band that provides the amount an input would have to change in order to cause output to move from zero to 100%, or vice-versa.  
         [0103]     In Step “S10”, an integral term is defined. In Step “S 11”, a DeltaOut term is defined. In Step “S12”, a LastOutput term is defined. Furthermore, in Step “S13”, a differential term (Dterm) is defined.  
         [0104]     Step “S14” provides a decision tree wherein, if the setpoint equals the last setpoint, the process proceeds to Step “S15”. If not, the process proceeds to Step “S16”. In Step “S15”, the proportional term (Pterm) is defined. In Step “S16”, the proportional term, the differential term, the last setpoint, and the last output are all set to zero. In Step “S17”, a correction is equated with the proportional term, the integral term, and the differential term, which are added together. In Step “S18”, the output is set equal to the output plus a correction as determined in Step “S17”.  
         [0105]     Step “S19” defines a decision tree wherein, if output is below a minimum, the process proceeds to Step “S20”. If output is not below a minimum, the process proceeds to Step “S21”. In Step “S20”, output is set equal to a minimum. In Step “S21”, a decision tree queries whether the output is greater than a maximum. If the output is greater than a maximum, the process proceeds to Step “S22”. If not, the process proceeds to Step “S23” and returns to start at Step “S1”. In Step “S22”, the output is set to a maximum value and then proceeds to Step “S23” and returns to start at Step “S1”.  
         [0106]      FIG. 9  illustrates a flow measurement algorithm for the flow control system and metering valve described previously. Flow measurement is measured by use of an equation in order to calculate mass flow through an orifice. Flow control and flow measurement algorithm details are provided in a subsequent section, below. As shown in  FIG. 1 , flow measurement is implemented at Step “S1”, where it is initiated. After Step “S1”, the process proceeds to Step “S2” wherein particular constants R (gas constant), K (specific heat ratio), sonic tables, and subsonic tables for coefficient discharge times area versus command interpolation tables are provided. After performing Step “S2”, the process proceeds to Step “S3”. In Step “S3”, the determined constants and the coefficient of discharge times area interpolation tables are input into the controller. In summary, Steps “S1” through “S3” provide a setup mode.  
         [0107]     Subsequent to performing Step “S3”, the process proceeds to a run mode at Step “S4”. More particularly, Step “S4” entails measuring P 1  (upstream pressure), P 2  (downstream pressure), and T 1  (upstream temperature). After performing Step “S4”, the process proceeds to Step “S5” and Step “S6”.  
         [0108]     In Step “S5”, valve position is measured using the LVDT. After performing Step “S5”, the process proceeds to Step “S8” and Step “S10”.  
         [0109]     In Step “S6”, a pressure ratio of output pressure over input pressure is calculated. After performing Step “S6”, the process proceeds to a decision tree at Step “S7”. In Step “S7”, if a pressure ratio (P 2 /P 1 ) is less than 0.53, the process proceeds to Step “S8”. If not, the process proceeds to Step “S10”.  
         [0110]     In Step “S8”, valve position is converted to a coefficient of discharge times area on a sonic, 43-point coefficient of discharge times area of profile. After performing Step “S8”, the process proceeds to Step “S9”.  
         [0111]     In Step “S9”, mass flow is calculated using a sonic flow equation as detailed below.  
         [0112]     After performing Step “S9”, the process proceeds to Step “S12”.  
         [0113]     In Step “S12”, mass flow is output to a flow proportional-integral-differential (PID) loop, as previously detailed with reference to  FIG. 8 . After performing Step “S12”, the process proceeds back to Step “S4”.  
         [0114]     In Step “S10”, valve position is converted to a coefficient of discharge times area based on a subsonic, 43-point coefficient of discharge times area of profile. After performing Step “S10”, the process proceeds to Step “S11”.  
         [0115]     In Step “S11”, mass flow is calculated using the subsonic flow equations as identified below. After performing Step “S11”, the process proceeds to Step “S12”.  
         [0116]     In Step “S12”, mass flow is output to the flow PID loop, as previously described, and the process then returns back to Step “S4”.  
         [0117]      FIG. 10  illustrates the process steps for a flow control algorithm usable with the control system metering valve of the present invention. The process starts at Step “S1”. After performing Step “S1”, the process proceeds to Step “S2”.  
         [0118]     In Step “S2”, the values for constants R, K, coefficient of discharge times area versus command interpolation tables for sonic and subsonic conditions, are determined. After performing Step “S2”, the process proceeds to Step “S3”.  
         [0119]     In Step “S3”, the determined constants and the coefficient of discharge times area interpolation tables are input into the controller for the metering valve. Steps “S1” through Step “S3” provide a setup mode.  
         [0120]     Step “S4” initiates a run mode for flow control of the valve. In Step “S4”, upstream pressure, downstream pressure, and upstream temperature are measured. After performing Step “S4”, the process proceeds to Step “S5” and to Step “S6”.  
         [0121]     In Step “S5”, flow demand is read. After performing Step “S5”, the process proceeds to Step “S8” and Step “S9”.  
         [0122]     In Step “S6”, the pressure ratio of output pressure over input pressure is calculated. After performing Step “S6”, the process proceeds to a decision tree at Step “S7”. If the calculated ratio pressure is less than 0.53, the process proceeds to Step “S8”. If not, the process proceeds to Step “S9”.  
         [0123]     In Step “S8”, flow demand is converted to a coefficient of discharge times area based on the sonic flow equations identified below. After performing Step “S8”, the process proceeds to Step “S10”.  
         [0124]     In Step “S10”, the coefficient of discharge area is converted to a position demand value based on the sonic coefficient of discharge times area interpolation tables. After performing Step “S10”, the process proceeds to Step “S12”.  
         [0125]     In Step “S9”, flow demand is converted to a coefficient of discharge area based on the subsonic flow equation as identified below. After performing Step “S9”, the process proceeds to Step “S11”.  
         [0126]     In Step “S11”, the coefficient of discharge times area is converted to a position demand value based on the subsonic coefficient of discharge times area interpolation tables. After performing Step “S11”, the process proceeds to Step “S12”.  
         [0127]     In Step “S12”, position demand is output to the flow proportional-integral-differential (PID) loop, as identified in  FIG. 8 . After performing Step “S12”, the process proceeds back to Step “S4”.  
         [0128]      FIG. 11  illustrates analog to digital converter input conditioning implemented with the control system. More particularly, Step “S1” entails an analog to digital converter (ADC) interrupt with 12.5 microsecond intervals. After performing Step “S1”, the process proceeds to Step “S2”.  
         [0129]     In Step “S2”, all 16 ADC input channels are read. After performing Step “S2”, the process proceeds to Step “S3”.  
         [0130]     In Step “S3”, each ADC input FIFO buffer is updated, with each having eight elements. After performing Step “S3”, the process proceeds to Step “S4” and returns to normal operation. Steps “S1” through “S4” provide an ADC interrupt.  
         [0131]     Also in  FIG. 11 , a foreground operation is provided starting with Step “SS1”. After performing Step “SS1”, the process proceeds to Step “SS2”.  
         [0132]     In Step “SS2”, an ADC input buffer pointer is incremented from values ranging from zero to 15. After performing Step “SS2”, the process proceeds to Step “SS3”.  
         [0133]     In Step “SS3”, the ADC input FIFO buffer is copied to the ADC average buffer. After performing Step “SS3”, the process proceeds to Step “SS4”.  
         [0134]     In Step “SS4”, an average value is calculated, except highest and lowest values are excluded. After performing Step “SS4”, the process proceeds to a decision tree at Step “SS5”. If the ADC input is in the range of a 4-20 milliamp input, the process proceeds to the decision of Step “SS6”. If not, the process proceeds to Step “SS8”.  
         [0135]     In Step “SS6”, query is raised whether the ADC input is within a hysteresis window. If the ADC input is within a hysteresis window, the process proceeds to Step “SS7” and returns to a normal operation. If not, the process proceeds to Step “SS8”.  
         [0136]     In Step “SS8”, a global ADC input value is updated. After performing Step “SS8”, the process proceeds to Step “SS9”.  
         [0137]     In Step “SS9”, the process returns to a normal operating mode.  
         [heading-0138]     C. Logic Flow Equations  
         [0139]     In order to implement flow control, the metering valve meters mass flow of fuel according to demand. More particularly, the demand signal is proportional to flow. For purposes of implementation, 4 milliamps (13,127 counts) is defined as zero mass flow, corresponding with the valve being closed. Additionally, 20 milliamps (65,636 counts) is defined as maximum flow, corresponding with the valve being fully open. The maximum flow is user selected using set-up software prior to installation. According to an optional implementation, if a discrete RUN command is enabled, the valve will begin controlling flow. As the demand increases, the micro-controller determines the flow control set point, converting analog demand to digital count set point. The micro-controller then adjusts valve fuel flow to match the set point. The following flow algorithms are used:  
         [0140]     Sonic Flow Conditions: Pup/Pdown≧2  
               M   .     =     3600   *   Pup   *   CdA   *       KGc   ZRT       *       (     2     K   +   1       )         K   +   1       2   ⁢     (     K   -   1     )                           M   .     =     mass   ⁢           ⁢   flow   ⁢           ⁢   rate   ⁢           ⁢     (     lbm   ⁢     /     ⁢   hr     )                   Pup   =     Upstream   ⁢           ⁢   Pressure   ⁢           ⁢     (   psia   )                   Cd   =     Coefficient   ⁢           ⁢   of   ⁢           ⁢   discharge                 Z   =     Compressibility   ⁢           ⁢   factor                 R   =     Gas   ⁢           ⁢   Constant   ⁢           ⁢     (     ft   ⁢     -     ⁢   lbf   ⁢     /     ⁢   lbm   ⁢     -     ⁢   R     )                   T   =     Gas   ⁢           ⁢   Temp   ⁢           ⁢     (   R   )                   K   =     Specific   ⁢           ⁢   Heat   ⁢           ⁢   Ratio   ⁢           ⁢     (     Cp   /   Cv     )                   A   =     Metering   ⁢           ⁢   Area   ⁢           ⁢     (     in   ^   2     )                   Gc   =     Gravitational   ⁢           ⁢   Constant   ⁢           ⁢     (     32.2   ⁢           ⁢     ft     sec   ⁢     .   2           )                 
 
         [0141]     Subsonic Flow Conditions: Pup/Pdown&lt;2  
               M   .     =     3600   *   CdA   *         2   ⁢   KGc       R   ⁡     (     K   -   1     )           *     (     Pup     T       )     *       (     Pdown   Pup     )       1   K       *       1   -       (     Pdown   Pup     )       (       K   -   1     K     )                           M   .     =     mass   ⁢           ⁢   flow   ⁢           ⁢   rate   ⁢           ⁢     (     lbm   ⁢     /     ⁢   hr     )                   Cd   =     Coefficient   ⁢           ⁢   of   ⁢           ⁢   discharge                 A   =     Metering   ⁢           ⁢   Area   ⁢           ⁢     (     in   ^   2     )                   K   =     Specific   ⁢           ⁢   Heat   ⁢           ⁢   Ratio   ⁢           ⁢     (     Cp   /   Cv     )                   Gc   =     Gravitational   ⁢           ⁢   Constant   ⁢           ⁢     (     32.2   ⁢           ⁢     ft     sec   ⁢     .   2           )                   Pup   =     Upstream   ⁢           ⁢   Pressure   ⁢           ⁢     (   psia   )                   Pdown   =     Downstream   ⁢           ⁢   Pressure   ⁢           ⁢     (   psia   )                   T   =     Gas   ⁢           ⁢   Temp   ⁢           ⁢     (   R   )                 
 
         [0142]     Pup, Pdown and T are analog inputs provided to the DSP via on-board upstream and downstream pressure transducers and an upstream gas temperature thermistor. Z, R and K are constants that vary from depending on gas fuel medium. These values shall be set in on board, non-volatile memory (EEPROM). Cd and A are functions of valve position, the product of which forms the “effective” metering area. Therefore, a tabular Cd*A (or CdA) vs. valve position table shall be set in on board, non-volatile memory (EEPROM).  
         [0143]      FIG. 12  illustrates one experimental test result for realizing slow demand using the valve and valve control system of  FIGS. 1-11  in order to calculate flow as a turbine is ramped up from a starting condition over time with increasing speed to a final idle position (shown on the right hand of the abscissa). Plot  300  illustrates an experimentally determined fuel flow that was measured continuously from startup to idle using a Coriolis meter. Plot  302  illustrates flow that was calculated using the techniques described with reference to  FIGS. 1-11  wherein four sensors were used instead of using a flow feedback device in order to calculate flow without using a relatively expensive flow detector.  
         [0144]     As shown in  FIG. 12 , plot  300  shows a natural frequency for the Coriolis meter slightly later in time than 12:23. The ordinate (or y axis) shows a unit measure (pounds per hour) of fuel delivered. As can be seen from  FIG. 12 , plot  302  (the present invention) very closely mirrors the performance of plot  300  which was detected using a relatively expensive Coriolis meter as a flow-measuring device which operates the system as a closed loop. The results of such experimental test indicate close performance for the present flow system and method, while eliminating the relatively expensive addition of a flow-measuring device, such as a relatively expensive Coriolis meter. Furthermore, the Coriolis meter reduced an extraneous artifact that occurred at a natural frequency of the meter, as depicted in  FIG. 12 . Such an artifact is undesirable and could affect calculated flow performance.  
         [0145]     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.