Abstract:
A system for controlling angular position of stator blades including: a mechanism for calculating a set angular position of the blades according to one of speeds; and a module for correcting the set position including: a mechanism for determining the angular position of the blades; a mechanism for measuring fuel flow rate of the turbine engine; a memory unit in which consecutive angular positions of the blades are combined with the fuel flow rates of the turbine engine measured at the angular positions; and a mechanism for determining a correcting angle according to the difference between the fuel flow rates measured between two consecutive angular position of the blades. A method for optimizing the common angular position can utilize the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of gas turbine engines comprising at least two spools and comprising one or more stator stages in which the blades are variable-pitch blades. 
     The object of the invention is to optimize the angular position of said stator blades so as to reduce the fuel consumption when the turbine engine operates at steady speed. “Steady speed” means an engine speed at which the thrust provided by the engine is substantially constant over time. 
     2. Description of the Related Art 
     As an example, each spool of a twin-spool gas turbine engine comprises at least one compressor and one turbine mounted downstream of said compressor. By convention, in the present application, the terms “upstream” and “downstream” are defined relative to the direction of travel of the air in the turbine engine. Traditionally, a compressor comprises several rotor stages in order to accelerate and compress an air stream travelling from upstream to downstream in the engine. In order to straighten out the air flow after acceleration, a stator stage is arranged directly at the exit of each rotor stage. 
     A stator stage takes the form of a fixed wheel, extending axially, with radial stator blades mounted on the periphery of the stator wheel. In order to optimize the straightening of the air flow by the stator stages downstream of the rotor stages, it is possible to modify the angular orientation of the stator blades, the blades being called variable-pitch blades. Accordingly, the turbine engine comprises a system for controlling the angular position of the stator blades of the compressor. 
     Conventionally, with reference to the schematic  FIG. 1A , the angular position of the stator blades of a twin-spool turbine engine M is determined mainly as a function of the rotation speed of the high-pressure rotor N 2  and of the temperature at the entrance of the compressor T 25 . Accordingly, the control system comprises means  20  for computing a set value VSV CAL  of the angular position of the blades on each stator wheel for a given rotation speed of the rotor N 2 . The computed set value VSV CAL  is transmitted to a control actuator  6  arranged to modify the current angular position of the stator blades of the turbine engine M. 
     The computing means  20  are programmed by mathematical laws that have been previously determined in order to suit an “average” engine which is neither too recent (new engine straight out of the factory), nor too “worn” (ready for overhaul). 
     In practice, the real engine does not correspond to the “average” engine for which the mathematical laws have been computed. The mathematical laws of the current systems take account of the margin requirements of the engine (margins of robustness to aging, margins of dispersion from engine to engine, margins of fouling, etc.). The result of this is that the angular position of the blades is not optimized for the real engine but robust both for a new or for a degraded engine. 
     A solution would be to modify the mathematical laws so that the parameters of engine wear and the dispersions between engines are taken into account. However, this solution is difficult to apply, the parameters being numerous and difficult to model. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to remedy these drawbacks, the applicant proposes a system for controlling the angular position of variable-pitch stator blades of a turbine engine compressor comprising at least two spools, each with a rotation speed (N 1  and N 2  respectively), for a turbine engine operating at steady speed, the system comprising:
         means for computing an angular set position of the blades as a function of one of the speeds (N 1 , N 2 ) and   a module for correcting the set position comprising:
           means for determining the angular position of the blades;   means for measuring the fuel flow rate of the turbine engine;   a memory in which the successive angular positions of the blades are associated with the fuel flow rates of the turbine engine measured at said angular positions; and   means for determining a correcting angle, said means being arranged to compute the correcting angle as a function of the difference between the fuel flow rates measured between two successive angular positions of the blades.   
               

     The system according to the invention advantageously makes it possible to determine an angular position of the blades that optimizes the consumption of fuel by the turbine engine. The applicant has determined that the fuel flow rate of the turbine engine, at a given steady speed, is a function of the angular position of the blades and that this function has a minimum locally. In other words, by locally varying the angular position of the blades, it is possible to determine to what extent it is necessary to modify the current angular position of the blades to limit the fuel flow rate. The correction module of the invention makes it possible to supplement a conventional system for controlling the angular position of the blades in order to improve the performance of the engine at a given steady speed. 
     Unlike the prior art, in which the law for determining the angular position of the blades is static for all the engines without taking account of the dispersion of the engine parameters or its wear parameters, the system according to the invention allows an adjustment of the angular position of the blades as a function of the state of the engine. Instead of listing all the parameters of wear or of dispersion of the engine and obtaining multiple complex mathematical laws, the applicant measures directly the impact of a variation in angle on the consumption of fuel. 
     By virtue of the invention, the theoretical set position computed on the basis of a mathematical model corresponding to an “average” engine is corrected. Such a system may be simply incorporated into an existing turbine engine. This new formulation of the problem to be solved makes it possible to determine an optimum value of angular position of the blades. 
     Preferably, the system comprises an adder arranged to compute an optimized set position by adding the correcting angle to the set angular position. This therefore corrects the set value to take account of the consumption of fuel. 
     Also preferably, the system comprises an actuator arranged to control the angular position of the blades as a function of the optimized set position. The current angular position is thus modified by the actuator to “follow” the optimized set position. 
     Also preferably, the correction module comprises means for checking the state of the turbine engine and means for inhibiting the correction of the current angular position of the blades, the inhibition means being activated if the state of the turbine engine is not suitable for a correction of the angular position of the blades. 
     The inhibition means are activated if the state of the turbine engine is not suitable for a correction of the angular position of the blades. The inhibition means make it possible, on instruction from the checking means, to prevent a modification of the angular position of the blades which could endanger the turbine engine or which would not be suitable for its operating state. 
     Preferably, the correction module comprises means for limiting the value of the correcting angle, said means being arranged to limit the value of the correcting angle in order to remain within a risk-free operating range. 
     The invention also relates to a turbine engine comprising a control system as described above. 
     The invention also relates to a method for optimizing the current angular position of stator blades of a turbine engine compressor comprising at least two spools each rotating at a speed (N 1 ; N 2 ), for a turbine engine operating at steady speed, a method in which:
     a) the reference fuel flow rate of the turbine engine is determined at a reference angular position of the blades;   b) the current fuel flow rate of the turbine engine is determined at the current angular position of the blades;   c) a correcting angle is computed as a function of the difference between the reference fuel flow rate and the current fuel flow rate so as to reduce the fuel flow rate;   d) said correcting angle is added to a previously computed set position so as to obtain an optimized set position;   e) the current angular position of the blades is modified so that it corresponds to the optimized set position.   

     Preferably, steps (a) to (e) are iterated by using as the reference angular position in step (a) the current angular position of step (b) of the previous iteration. 
     Advantageously this makes it possible to optimize the angular position of the blades “step by step” which ensures an optimization that is precise and has no harmful side effects such as the occurrence of transients. 
     Again preferably, the correcting angle is computed by an optimization method, preferably by a method of steepest descent of the fuel function F defining the fuel flow rate of the turbine engine relative to the angular position of the blades. 
     The fuel function F allows a local minimum which ensures the convergence of the optimization method. It may occasionally be convex which ensures the existence of an optimum angular position. 
     Still preferably, the value of the correcting angle is limited in order to remain within a risk-free operating range (overspeed, surge, temperature rise, etc.). 
     According to another embodiment of the invention, the state of the engine is checked and the modification of the current angular position of the blades is inhibited if the state of the turbine engine is not suitable for a correction of the angular position of the blades. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VEIWS OF THE DRAWINGS 
       The invention will be better understood with the aid of the appended drawing in which: 
         FIG. 1A  represents a system for controlling the angular position of the blades according to the prior art; 
         FIG. 1B  represents a system for controlling the angular position of the blades with a module for correcting the angular position according to the invention; 
         FIG. 2  represents a schematic diagram of a first embodiment of a system of angular control of the stator blades of a turbine engine arranged to compute a correcting angle; 
         FIG. 3  represents a schematic diagram of a second embodiment of a control system with correction inhibition means; 
         FIG. 4  represents a schematic diagram of a third embodiment of a control system with means for limiting the value of the correcting angle and 
         FIG. 5  is a curve representing the change in the fuel flow rate of the engine as a function of the angular position of the stator blades of the engine, for a determined steady speed of the engine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A system for controlling the angular position of the stator blades of the HP compressor of a turbine engine according to the invention is shown in  FIG. 1B  for a twin-spool engine; a low-pressure LP spool with a rotation speed N 1  and a high-pressure HP spool with a rotation speed N 2 . With the aid of a throttle, the engine is controlled by indicating to it the desired thrust; the thrust is directly linked to the speed of the low-pressure LP spool. Thus, a thrust setting imposes a rotation speed setting N 1   DMD  of the LP spool. For the purposes of clarity, the reference N 1 , relating to the rotation speed of the LP spool, will be used also for the engine thrust because of the direct link between these two parameters. Similarly, the reference N 1  may correspond to other parameters having a direct link with the engine thrust, in particular the parameter EPR corresponding to “Engine Pressure Ratio” that is well known to those skilled in the art. 
     Conventionally, the turbine engine comprises means  20  for computing the set angular position of the stator blades VSV CAL  as a function of the rotation speed N 2  of the high-pressure body and of the temperature of the high-pressure (HP) spool T 25 . The computing means  20  are programmed by mathematical laws well known to those skilled in the art that make it possible to compute a set angular position VSV CAL  as a function of the rotation speed N 2  of the HP spool. 
     The control system according to the invention also comprises a module  1  for correcting the set position VSV CAL  of the stator blades of the engine M. The correction module  1  makes it possible to determine a correcting angle VSV CORR  optimizing the consumption of fuel. The control system also comprises an adder S arranged to receive as an input the computed setting value VSV CAL  and the correcting angle VSV CORR  in order to transmit as an output an optimized setting value VSV NEW  corresponding to the total of its two input parameters (VSV CORR , VSV CAL ). The control system also comprises a control actuator  6  which modifies the current angular position of the blades VSV COU  as a function of the optimized setting value VSV NEW . 
     Still with reference to  FIG. 1B , the control system comprises a module  31  for estimating the fuel necessary to maintain the rotation speed N 1 , also called the correcting network, which receives as an input the speed setting N 1   DMD  corresponding to a desired rotation speed, that is to say to a desired level of thrust. The control system also comprises a fuel control device  30  controlled by the correcting network  31  and capable of modifying the fuel flow rate as a function of the effective rotation speed N 1   EFF  of the engine, measured for example by a tachometric sensor. 
     If the fuel flow rate supplied to the engine M does not make it possible to achieve the requested thrust (N 1   EFF  is lower than N 1   DMD ), the correcting network  31  determines the command to be applied to the fuel control device  30  to increase the fuel flow rate supplied to the engine M and thus compensate for the difference between the desired speed N 1   DMD  and the effective speed N 1   EFF . 
     With reference to  FIG. 2 , the correction module  1 , according to a first embodiment of the invention, comprises means  2  for determining the angular position of the blades VSV, said means being known per se, taking the form for example of position sensors, and means  3  for determining the fuel flow rate of the turbine engine WFM at a given angular position of the blades VSV. These means  3  for determining the flow rate may be either direct—they take the form for example of a sensor mounted upstream of the injectors of the turbine engine combustion chamber—or indirect—the linear position of an element closing off the section of passageway of a fuel pipe of the turbine engine is measured for example, the dimensions of the section being known. As a general rule, these determination means  2 ,  3  are activated continuously in order to constantly monitor the angular position of the blades and the fuel consumption. 
     The correction module  1  also comprises a memory  4  in which the successive angular positions of the blades VSV are associated with the fuel flow rates of the turbine engine WFM measured at said angular positions VSV. Over time, the memory  4  of the correction module  1  is supplemented by said determination means  2 ,  3 . In practice, the memory  4  retains only a certain number of value pairs (VSV, WFM), the oldest pairs being replaced by more recent pairs. As an example, the memory  4  comprises at least two pairs: a pair of current values (VSV COU , WFM COU ) and a pair of previous values, called reference values (VSV REF , WFM REF ). 
     In the present case, there is a limitation to the operation of the engine at a steady speed, the thrust supplied by the engine being substantially constant over time. As an example, in steady operation, the rotation speed N 1  is constant or the parameter EPR is constant. At steady speed, it is advantageously possible to monitor the change in the fuel flow rate WFM COU  as a function of the value of the angle of the stator blades VSV COU  by analyzing the discrete function, hereinafter called the fuel function F, defined by the pairs of the memory  4  of the control system  1 . 
     For an operation of the turbine engine at constant rotation speed N 1 , also called “iso N 1 ”, the applicant has studied the fuel function F, defining the fuel flow rate WFM COU  relative to the angular position of the blades VSV COU , and has determined that this fuel function F is locally convex and therefore that there is an angular position of the blades for which the consumption of fuel is lowest, this optimum angular position being referenced VSV OPT .  FIG. 5  shows the fuel function F and the optimum angular position for a determined steady speed of the engine. 
     The angular position VSV OPT  is called the optimum position of the engine for two reasons. First, it is optimum with respect to the determined steady speed of the engine, the optimum angular position varying as a function of the given speed. Secondly, it is optimum with respect to the engine as such, the angular position VSV OPT  being defined “to suit” for the engine by naturally taking account of its state of wear and of the manufacturing dispersion. In other words, according to the manufacturing margins and the variations associated with the installation, a given engine does not have exactly the same behavior as another engine of the same series, resulting in that each engine has an optimum angular position VSV OPT  that is specific to it. 
     The correction module  1  also comprises means  5  for determining a correcting angle VSV CORR , said means being arranged to compute the correcting angle VSV CORR  as a function of the difference between the fuel flow rates measured between two successive angular positions of the blades. In other words, the correcting angle VSV CORR  is not computed by analysis of the intrinsic parameters of the engine but by optimization of the desired result so as to obtain the lowest possible fuel consumption WFM OPT . 
     Accordingly, the means  5  for determining the correcting angle VSV CORR  are arranged to determine a local minimum of the fuel function F at iso N 1  and this is done knowing only a few values of this function (the last successive angular positions). The means  5  for determining the correcting angle VSV CORR  are in this instance programmed by an optimization function, the function of which is to determine the correcting angle VSV CORR  while limiting its value. Specifically, if the current angular position of the blades VSV COU  is modified by a correcting angle of VSV CORR  with too high a value, transients occur in the engine which could damage it. 
     The principle of the optimization consists in locally having the current angular position of the blades vary, in measuring the impact of this angular variation on the effective fuel flow rate in order to learn a lesson therefrom on how to modify the current angular position. 
     The optimization function according to the invention therefore makes it possible to improve the efficiency of the engine in a safe way by limiting the occurrence of transients. The optimization function will be described for a method of steepest descent but other optimization methods would also be suitable, such as an optimization by the least squares method etc. The method of steepest descent makes it possible to optimize the angular position in a simple manner. 
     With the pairs of values (VSV COU , WFM COU ; VSV REF , WFM REF ) stored in the memory  4 , the method of steepest descent computes the value of the gradient of the fuel function F at the current angular position of the blades VSV COU  relative to its previous angular position VSV REF . The direction of convergence of the fuel function F is thus deduced therefrom. By linear optimization, a correcting angle VSV CORR  is computed as a function of the value of the gradient at the current angular position VSV COU  and of a saturated increment SAT 1  and of a convergence rate μ, the convergence rate μ being chosen so as to produce a compromise between a rapid convergence to the optimum angular position VSV OPT  and a protection against the occurrence of transients in the turbine engine. 
     By virtue of the optimization function, the value of the correcting angle VSV CORR  is deduced therefrom which must be added to the set position VSV CAL  in order to obtain the optimized setting value VSV NEW . The control actuator  6  makes it possible to modify the current angular position of the blades VSV COU  in order correspond to the optimized set position VSV NEW . The optimized set position VSV NEW  does not necessarily correspond to the optimum angular position VSV OPT  because a considerable modification of the current angular position VSV COU  could cause the compressor to surge. Preferably, the optimization is carried out progressively, by iterations. 
     By virtue of the optimization of the angular position of the blades, the engine is adjusted to a given speed with a lower fuel flow rate. With reference to  FIG. 1B , the fuel control device  30  commands the correcting network  31  to retain the same speed N 1  despite the modification to the behavior of the HP spool due to the modification of the current angular position of the blades. This therefore produces fuel savings. 
     Preferably, with reference to  FIG. 4 , the correction module  1  comprises means  9  for limiting the value of the correcting angle VSV CORR  arranged to limit the correcting angle by a threshold of gradient saturation SAT 2  so as to prevent the occurrence of oscillations during a modification of the current angular position of the blades VSV COU . This also makes it possible to check the speed of convergence of the optimization method. The saturation function SAT 2  and the saturated increment SAT 1  may be used together or independently. 
     As an example, the steepest descent optimization method may obey the mathematical relation reproduced below:
 
 VSV   CORR ( t )=−SAT1[Gradient F( VSV   COU )×μ]+VSV CORR ( t− 1)
 
 VSV   CORR ′( t )=sign VSV   CORR ( t ))*min(| VSV   CORR ( t )|, SAT2)
 
 VSV   NEW ( t )= VSV   CAL ( t )+ VSV   CORR ′( t )
 
     In order to initiate the optimization procedure, it may be necessary to very slightly modify the current angular position of the blades in order to carry out the optimization and begin the process. It is then said that the optimization method is initiated by “excitation” of the system. The initialization may also result from a mathematical model indicating the direction of variation of the angular position of the blades VSV leading to a reduction in the fuel flow rate WFM. 
     According to a preferred embodiment of the invention, with reference to  FIG. 3 , the correction module  1  comprises inhibition means  7  arranged to cancel out the value of the computed correcting angle VSV CORR  by the means  5  for determining the correcting angle. This makes it possible to prevent a correction of the angular position of the blades by the control actuator  6  when the engine is not operating at steady speed. 
     It goes without saying that the limitation means  9  and the inhibition means  7  could be used in one and the same control system  1 . 
     In this embodiment, the inhibition means  7  take the form of an “OR” logic gate connected to means  8  for measuring the state of the engine, that is to say “its state of health”. As an example, the means  8  for measuring the state of the engine comprise:
         means for storing events of the surge type. If a surge has been detected during the life of the turbine engine, the logic is inhibited by the inhibition means  7 .   means for measuring the temperature margin of the exhaust gases, called the EGT “Exhaust Gas Temperature” margin parameter, relative to a predetermined margin. If there is an insufficient margin, the logic is inhibited by the inhibition means  7 .   means for estimating the state of the compressor of the turbine engine by sensors for measuring the coefficients of flow rate and efficiency of the high-pressure compressor. These coefficients, representing the state of the engine, are compared with predetermined threshold values relative to a “healthy” engine, that is to say an engine in good condition. If the threshold is exceeded, the logic is inhibited by the inhibition means  7 .   means for measuring the stability of the engine, said means being arranged to measure values such as for example the speed of the LP spool (N 1   EFF ), the speed of the HP spool (N 2 ) and their variance. In the event of a transient, the logic is inhibited by the inhibition means  7 .       

     Similarly, if the pilot of the aircraft wishes to accelerate or decelerate the engine by acting on the throttle, the correction is inhibited and the angular position of the blades is not optimized. This check is carried out by means, not shown, for monitoring engine transients. 
     The invention also relates to a method for optimizing the current angular position of stator blades of a turbine engine compressor comprising at least two spools each rotating at a speed, for a turbine engine operating at steady speed, a method in which:
     a) the reference fuel flow rate WFM REF  of the turbine engine is determined at a reference angular position VSV REF  of the blades;   b) the current fuel flow rate WFM COU  of the turbine engine is determined at the current angular position VSV COU  of the blades;   c) a correcting angle VSV CORR  is computed as a function of the difference between the reference fuel flow rate WFM REF  and the current fuel flow rate WFM COU  so as to reduce the fuel flow rate;   d) the correcting angle VSV CORR  is added to the set position VSV CAL  so as to compute an optimized set position VSV NEW ;   e) the current angular position VSV COU  of the blades is modified so that it corresponds to the optimized set position VSV NEW .   

     Preferably, steps (a) to (e) are iterated by using as the reference angular position VSV REF  in step (a) the current angular position VSV COU  of step (b) of the previous iteration. 
     As shown in  FIG. 5 , the angular position of the blades VSV COU  is optimized after each iteration (I 1 , I 2 , I 3 ) so as to minimize the consumption of fuel. Advantageously this makes it possible to come close to the optimum angular position VSV OPT  optimizing the consumption of fuel at a given speed, while preventing the occurrence of transients likely to disrupt the engine in the event of a sudden change in the angular position of the blades. 
     Again preferably, the stability of the engine speed is tested and the modification of the current angular position VSV COU  of the blades is inhibited if the stability test fails, as described in the control system according to the invention.