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A method and system for controlling fuel flow in a fuel metering system that includes a metering valve and a proportional bypass valve that produces a differential pressure error across the metering valve includes supplying a first fraction of fuel through the metering valve. A second fraction of th

A method and system for controlling fuel flow in a fuel metering system that includes a metering valve and a proportional bypass valve that produces a differential pressure error across the metering valve includes supplying a first fraction of fuel through the metering valve. A second fraction of the fuel is directed through the proportional bypass valve. The differential pressure error produced by the bypass valve is determined, and fuel flow through the supply line is controlled by adjusting the metering valve based at least in part on the determined differential pressure error, and by adjusting the proportional bypass valve to maintain a substantially constant metering valve differential pressure across the metering valve.

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We claim: 1. In a fuel metering system including a metering valve and a proportional bypass valve that produces a differential pressure error across the metering valve, a method of controlling fuel flow in the fuel metering system, comprising the steps of: supplying fuel from a fuel source to a sup

We claim: 1. In a fuel metering system including a metering valve and a proportional bypass valve that produces a differential pressure error across the metering valve, a method of controlling fuel flow in the fuel metering system, comprising the steps of: supplying fuel from a fuel source to a supply line, the supply line having at least an outlet port; directing a first fraction of the fuel in the supply line through the metering valve to the supply line outlet port, the metering valve having a first variable area flow orifice; directing a second fraction of the fuel in the supply line through the proportional bypass valve back to the fuel source, the proportional bypass valve having a second variable area flow orifice; continuously estimating the differential pressure error produced by the bypass valve using an algorithm; and controlling fuel flow to the supply line outlet port by (i) adjusting the area of the first variable area flow orifice based at least in part on the estimated differential pressure error and (ii) adjusting the area of the second variable area flow orifice to maintain a substantially constant metering valve differential pressure across the first variable area orifice. 2. The method of claim 1, further comprising: supplying a fuel flow command representative of a desired fuel flow to the supply line outlet; and adjusting the area of the first variable area flow orifice based additionally on the supplied fuel flow command. 3. The method of claim 2, further comprising: determining a reference differential pressure value that is representative of a desired constant differential pressure across the first variable area orifice; adding the estimated differential pressure error to the desired constant differential pressure error to determine an actual differential pressure value; and adjusting the area of the first variable area flow orifice based on the determined actual differential value and the supplied flow command. 4. The method of claim 1, further comprising: determining a proportional bypass valve position error, wherein the algorithm continuously estimates the differential pressure error based at least in part on the determined proportional bypass valve position error. 5. The method of claim 4, wherein: the proportional bypass valve includes a valve element, a spring, and a diaphragm, the spring having a spring constant and biasing the valve element toward a valve position, and the diaphragm having an area across which the metering valve differential pressure is applied; and the algorithm continuously estimates the differential pressure error from the spring constant, the diaphragm area, and the determined proportional bypass valve position error. 6. The method of claim 4, further comprising: supplying a fuel flow command representative of a desired fuel flow to the supply line outlet; supplying fuel flow from the fuel source to the supply line via a fuel pump; and determining fuel flow through the fuel pump, wherein the algorithm continuously estimates the proportional bypass valve position error based at least in part on the determined fuel flow through the fuel pump and the supplied fuel flow command. 7. The method of claim 6, further comprising: determining fuel pump rotational speed; determining fuel pump discharge pressure; and determining the fuel flow through the fuel pump based at least in part on the determined fuel pump rotational speed and the determined fuel pump discharge pressure. 8. The method of claim 6, further comprising: selecting a reference proportional bypass valve position, the reference proportional bypass valve position representative of a position at which the differential pressure error produced thereby is assumed to be zero, wherein the algorithm continuously estimates the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the supplied fuel flow command, and the reference proportion bypass valve position. 9. The method of claim 8, further comprising: determining fuel pump discharge pressure; determining a fuel control reference pressure, wherein the algorithm continuously estimates the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the supplied fuel flow command, the determined fuel pump discharge pressure, the determined fuel control reference pressure, and the reference proportion bypass valve position. 10. The method of claim 9, further comprising: calculating a pressure difference between the fuel pump discharge pressure and the fuel control reference pressure, wherein the algorithm continuously estimates the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the supplied fuel flow command, the calculated pressure difference, and the reference proportion bypass valve position. 11. A fuel metering system for control fuel flow to a gas turbine engine, comprising: a fuel supply line having an inlet adapted to couple to a fuel source and an outlet adapted to couple to the gas turbine engine; a metering valve positioned in flow-series in the supply line, the metering valve producing a differential pressure thereacross when fuel flows therethrough; a bypass flow line coupled to the fuel supply line upstream of the metering valve for bypassing a portion of the fuel in the supply line back to the inlet; a proportional bypass valve positioned in flow-series in the bypass flow line and configured to control flow therethrough to maintain a substantially constant differential pressure across the metering valve, the substantially constant differential pressure including a differential pressure error produced by the proportional bypass valve; and a control circuit adapted to receive a fuel flow command representative of a desired fuel flow and operable to (i) continuously estimate the differential pressure error using an algorithm and (ii) adjust the metering valve, based at least in part on the continuously estimated differential pressure error and the fuel flow command, to supply fuel through the metering valve at the desired fuel flow. 12. The system of claim 11, further comprising: memory having stored therein a reference differential pressure value that is representative of a desired constant differential pressure across the fuel metering valve, wherein the control circuit is further operable to (i) add the continuously estimated differential pressure error to the desired constant differential pressure error to determine an actual differential pressure value and (ii) adjust the metering valve based on the actual differential value and the supplied flow command. 13. The system of claim 11, wherein the control circuit is operable to: determine a proportional bypass valve position error; and continuously estimate, using the algorithm, the differential pressure error based at least in part on the determined proportional bypass valve position error. 14. The system of claim 13, wherein: the proportional bypass valve comprises a valve element, a spring, and a diaphragm, the spring having a spring constant and configured to bias the valve element toward a valve position, and the diaphragm having an area across which the differential pressure is applied; the memory has stored therein values representative of the spring constant and the diaphragm area; and the control circuit continuously estimates, using the algorithm, the differential pressure error from the spring constant, the diaphragm area, and the determined proportional bypass valve position error. 15. The system of claim 13, further comprising: a fuel pump configured to supply fuel to the supply line, wherein the control circuit is further operable to determine fuel flow through the fuel pump and determine the proportional bypass valve position error based at least in part on the determined fuel flow through the fuel pump and the supplied fuel flow command. 16. The system of claim 15, further comprising: a lag filter coupled to receive the fuel flow command and operable, upon receipt thereof, to supply a filtered fuel flow command, wherein the control circuit is operable to continuously estimate, using the algorithm, the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the filtered fuel flow command, and the determined proportional bypass valve position error. 17. The system of claim 15, wherein: the fuel pump rotates and supplies fuel at a discharge pressure; and the control circuit is further operable to determine (i) fuel pump rotational speed, (ii) fuel pump discharge pressure, and (iii) the fuel flow through the fuel pump based at least in part on the determined fuel pump rotational speed and the determined fuel pump discharge pressure. 18. The system of claim 17, wherein: the memory stores a reference proportional bypass valve position value representative of a position at which the differential pressure error produced thereby is assumed to be zero; and the control circuit continuously estimates, using the algorithm, the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the supplied fuel flow command, and the reference proportion bypass valve position value. 19. The system of claim 18, further comprising: a gearbox disposed between the gas turbine engine and the fuel pump, the gearbox configured to couple rotational drive force supplied from the gas turbine engine to the fuel pump; an engine speed sensor configured to sense a rotational speed of a component in the gas turbine engine and supply an engine speed signal representative thereof, wherein the control circuit is coupled to receive the engine speed signal and is configured to determine the fuel pump rotational speed therefrom. 20. The system of claim 19, further comprising: a boost pump disposed upstream of the fuel supply pump and operable to supply fuel thereto at a substantially constant fuel control reference pressure, wherein the control circuit is further operable to: determine the fuel pump discharge pressure, determine the fuel control reference pressure from the engine speed signal, and estimate, using the algorithm, the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the supplied fuel flow command, the determined fuel pump discharge pressure, the determined fuel control reference pressure, and the reference proportion bypass valve position. 21. The system of claim 20, wherein the control circuit is further operable to: calculate a pressure difference between the fuel pump discharge pressure and the fuel control reference pressure; and estimate, using the algorithm, the differential pressure error based at least in part on the determined fuel flow through the fuel pump, the supplied fuel flow command, the calculated pressure difference, and the reference proportion bypass valve position. 22. The system of claim 20, further comprising: a compressor discharge pressure sensor configured to sense compressor discharge pressure of a compressor in the gas turbine engine and supply a compressor discharge pressure signal representative thereof and an ambient pressure sensor configured to sense ambient pressure around the system and supply an ambient pressure signal representative thereof, wherein: the memory stores (i) a metering valve differential pressure reference value representative of a predetermined design differential pressure across the metering valve and (ii) a fuel nozzle flow number representative of a predetermined flow number associated with one or more engine fuel nozzles, and the control circuit is coupled to receive the compressor discharge pressure signal and the ambient pressure signal and is operable to determine pump discharge pressure from the compressor discharge pressure signal, the ambient pressure signal, the metering valve differential pressure reference value, and the fuel nozzle flow number. 23. The system of claim 22, further comprising: a lag filter coupled to receive the fuel flow command and operable, upon receipt thereof, to supply a filtered fuel flow command, wherein the control circuit is operable to (i) determine a fuel nozzle differential pressure based on the fuel nozzle flow number and the filter fuel flow command and (ii) determine pump discharge pressure from the compressor discharge pressure signal, the ambient pressure signal, the metering valve differential pressure reference value, and the fuel nozzle differential pressure.