Abstract:
Automatic control of the thrust of an aircraft engine may include: (1) calculating a first difference between a measured speed and a reference speed of an aircraft to determine an intermediate term and (2) calculating a second difference between the intermediate term and a previously selected corrector term. If the second difference is greater than a threshold value, then the intermediate term is selected as a current corrector term. Otherwise, the previously selected corrector term is selected. The selected corrector term and an equilibrium term, which produces an equilibrium rating of the engine in the absence of disturbances, are summed to produce a control value, and the control value is applied to the engine.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process and device for automatically controlling the thrust of at least one engine of an aircraft during a phase of horizontal flight at stabilized speed, in particular during a phase of cruising flight of a transport airplane. 
     More precisely, it applies to the case where the thrust of the engine or engines is controlled automatically by a standard autothrust system. 
     BACKGROUND OF TEE RELATED ART 
     It is known that, in this case, the autothrust system automatically regulates the rating of the engines, in a continuous manner, as a function of the deviation between the measured speed of the aircraft and the preset speed. Such an autothrust system therefore makes it possible to produce a stabilized (on the preset speed) speed of the aircraft. 
     However, by reason of the generation of a continuous (or permanent) variation of the rating of the engine or engines, which gives rise to a permanent variation of the sound frequencies emitted by the engines (the sound frequencies varying of course with the rating of the engines) this autothrust system therefore has the drawback of causing a permanent variation in the noise emitted by the engine or engines, thereby reducing acoustic comfort, in particular of the passengers of the aircraft. However, on a transport airplane in particular, the comfort of the passengers is essential during a phase of horizontal flight at stabilized speed such as a cruising phase, which may last several hours. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to remedy these drawbacks. It relates to a process for automatically controlling the thrust of at least one engine of an aircraft during a phase of horizontal flight at stabilized speed, which process makes it possible to improve the acoustic comfort of the passengers of the aircraft. 
     For this purpose, according to the invention, said process according to which the thrust of the engine is automatically controlled by applying a control value thereto which corresponds to the value of a predetermined control parameter representative of the rating of said engine, and according to which the following set of steps is carried out automatically and repeatedly:
     a) an actual speed corresponding to the actual value of the speed of the aircraft is measured;   b) a preset reference speed corresponding to the speed of the aircraft, representative of the control value obtained from the previous set of steps, is determined;   c) a first difference between said actual speed and said preset speed is calculated;   d) an intermediate term dependent on said first difference is determined for said control parameter, said intermediate term making it possible to obtain a corrector term;   e) the sum is computed of said corrector term and of an equilibrium term which produces an equilibrium rating of the engine in the absence of disturbances so as to obtain said control value; and   f) the control value thus obtained is applied to said engine,
 
is noteworthy in that in step d):
       a second difference between said intermediate term of the present set of steps and the corrector term of the previous set of steps is calculated;   this second difference is compared with a predetermined threshold value; and   one selects as corrector term for the present set of steps, that is used in particular in step e):
           said intermediate term of the present set of steps, if said second difference is greater than said threshold value; and   said corrector term of the previous set of steps, if said second difference is less than or equal to said threshold value.   
           
       

     Thus, as long as the second difference is less than or equal to the threshold value, the same corrector term is retained so that the control value is not modified (the relatively stable equilibrium term being modified only when it is appropriate to vary the equilibrium rating, that is to say generally the preset speed). The control value (and thus the control of the rating or of the thrust) is therefore modified only when said second difference (which depends on said first difference) exceeds said threshold value, that is to say when this is necessary for the feedback control of the speed, given the deviations (first difference) between the actual speed and the preset speed of the aircraft. Consequently, by virtue of the present invention, the rating of the engine is not regulated in a continuous (or permanent) manner, but it is regulated in a discrete manner. This results in a very appreciable improvement in the acoustic comfort of the passengers of the aircraft, since the sound frequencies of the noises emitted by the engines are no longer modified permanently. 
     Advantageously, said selected corrector term is filtered before using it in step e), thereby enabling a modification of the rating of the engine to be rendered less abrupt as appropriate and hence enabling the acoustic comfort of the passengers to be improved still further. 
     Within the framework of the present invention, said predetermined control parameter is the speed of rotation N 1  of the engine or the engine pressure ratio EPR of said engine. 
     In the first case (control parameter: speed of rotation N 1 ), said threshold value is advantageously equal to 0.5% of the preset value of the speed of rotation of the engine. This threshold value, as well as the time constant of the filter for filtering the corrector term, may be determined empirically, in particular with the aid of physiological studies performed on passengers of the aircraft. 
     Moreover, in said first case, advantageously, in step d), said intermediate term is determined by computing the sum:
         of a first term which is proportional to said first difference; and   of a second term which:
           corresponds to the integration of said first difference if the following conditions α and β are satisfied:
               α) said first difference is greater than a predetermined value; and   β) said actual speed does not diverge from said preset speed; and   
               is equal to zero, if at least one of the above conditions α and β is not satisfied.   
               

     The present invention also relates to a method of control of the thrust of at least one engine of an aircraft during a phase of horizontal flight at stabilized speed, said method comprising at least one first process of control of thrust of standard type. 
     According to the invention, said method is noteworthy in that it moreover comprises a second process of control of thrust, in that said second control process corresponds to the process specified above, in that account is taken of the results of said first control process under normal operation, and in that one automatically switches into said second control process so as to take account of the results of the latter when at least one of a plurality of predetermined conditions is satisfied. 
     Advantageously, said predetermined conditions comprise at least the following conditions:
         the actual speed of the aircraft is stabilized, being to within a predetermined value, equal to the preset speed;   the conditions of calculation of said equilibrium term are valid;   an autothrust function of the aircraft is engaged in speed holding mode; and   an automatic pilot of the aircraft is active in altitude holding mode.       

     The present invention also relates to a device for automatically controlling the thrust of at least one engine of an aircraft during a phase of horizontal flight at stabilized speed. 
     According to the invention, said device of the type comprising:
         means for measuring an actual speed corresponding to the actual value of the speed of the aircraft;   means for determining a preset speed corresponding to the aircraft&#39;s speed representative of a control value;   means for calculating a first difference between said actual speed and said preset speed;   means for determining, for a control parameter, an intermediate term dependent on said first difference, said intermediate term making it possible to obtain a corrector term;   means for computing the sum of said corrector term and of an equilibrium term which produces an equilibrium rating of the engine in the absence of disturbances so as to obtain a control value; and   means for applying the control value thus obtained to said engine,
 
is noteworthy in that it moreover comprises:
   means for calculating a second difference between said intermediate term and, a previously recorded corrector term;   means for comparing this second difference with a predetermined threshold value;   means for selecting as corrector term:
           said intermediate term, if said second difference is greater than said threshold value; and   said previously recorded corrector term, if said second difference is less than or equal to said threshold value; and   
           means for recording the selected corrector term.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures of the appended drawing will elucidate the manner in which the invention may be embodied. In these figures, identical references designate similar elements. 
         FIG. 1  is the schematic diagram of a device in accordance with the invention. 
         FIG. 2  diagrammatically shows a first embodiment of a calculation unit of a device in accordance with the invention. 
         FIG. 3  diagrammatically shows a second embodiment of a calculation unit of a device in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The device  1  in accordance with the invention and represented diagrammatically in  FIG. 1  is an autothrust device and is intended for automatically controlling the thrust of at least one engine  2  of an aircraft, in particular of a transport airplane, during a phase of horizontal flight at stabilized speed, in particular during a phase of cruising flight for a transport airplane. 
     This device  1  comprises:
         a calculation unit  3 A,  3 B for automatically determining a control value of the rating of the engine or engines  2  of the aircraft; and   standard means  4  for regulating the engine, which are connected by a link  5 A,  5 B to the calculation unit  3 A,  3 B, and which automatically apply the control values determined by said calculation unit  3 A,  3 B to said engine  2 , as illustrated diagrammatically by a chain dotted line  6  in FIG.  1 .       

     It is known that, depending on the engine manufacturer, the control of the rating of an engine  2  is expressed:
         either in the form of a (control) parameter N 1  which represents the speed of rotation of the engine  2 ;   or in the form of a (control) parameter EPR (“Engine Pressure Ratio”) which represents the engine pressure ratio of said engine  2 .       

     These two parameters N 1  and EPR, when they are used as control value of the engine  2 , make it possible to vary its thrust. There is therefore no fundamental difference between these parameters N 1  and EPR when considering the control of the thrust of the engine  2 . Consequently, the solution in accordance with the invention and presented hereinafter on the basis of the parameter N 1  applies equally to the parameter EPR. 
     However, according to the invention, in a general manner, whatever the parameter N 1  or EPR considered, the device  1  automatically and repeatedly carries out the following set of steps a) to f):
     a) it measures an actual speed Vc corresponding to the actual value of the speed of the aircraft;   b) it determines a preset reference speed Vctgt corresponding to the speed of the aircraft, representative of the control value obtained from the previous set of steps a) to f) [that is to say carried out previously (earlier) with respect to the present set of steps];   c) it calculates a first difference between said actual speed Vc and said preset speed Vctgt;   d) it determines, for said control parameter N 1  or EPR considered, an intermediate term dependent on said first difference, said intermediate term making it possible to obtain a corrector term;   e) it computes the sum of said corrector term and of an equilibrium term which produces an equilibrium rating of the engine  2  in the absence of disturbances so as to obtain said control value, said steps a) to e) being implemented by the calculation unit  3 A,  3 B; and   f) it applies, with the aid of the means  4 , the control value thus obtained to said engine  2 .   

     According to the invention, in said step d), said device  1 :
         calculates a second difference between said intermediate term of the present set of steps and the corrector term of the previous set of steps;   compares this second difference with a predetermined threshold value S; and   selects as corrector term for the present set of steps, that it uses in particular in said step e):
           said intermediate term of the present set of steps, if said second difference is greater than said threshold value S; and   said corrector term of the previous set of steps, if said second difference is less than or equal to said threshold value S.   
               

     Thus, as long as the second difference is less than or equal to the threshold value S, the same corrector term is retained so that the control value is not modified (the relatively stable equilibrium term being modified only when it is appropriate to vary the equilibrium rating, that is to say generally the preset speed). The control value (and thus the control of the rating or of the thrust) is therefore modified only when said second difference (which depends on said first difference) exceeds said threshold value S, that is to say when this is necessary for the feedback control of the speed, given the deviations (first difference) between the actual speed Vc and the preset speed Vctgt of the aircraft. Consequently, by virtue of the present invention, the rating of the engine or engines  2  is not regulated in a continuous (or permanent) manner, but it is regulated in a discrete manner. This results in a very appreciable improvement in the acoustic comfort of the passengers of the aircraft, since the sound frequencies of the noises emitted by the engine or engines  2  are no longer modified permanently. 
     In a first embodiment represented in  FIG. 2 , the calculation unit  3 A comprises a calculation assembly  7  comprising:
         a first unit  8  for determining in a standard fashion, as specified hereinbelow, an equilibrium term N 1 eq (equilibrium preset) which corresponds to the equilibrium engine rating, in the absence of disturbance of the flight conditions;   a second unit  9  for determining a corrector term ΔN 1 f, as specified hereinbelow; and   a summator  10  which is connected by links  11  and  12  respectively to said first and second units  8  and  9  and which computes the sum of said equilibrium term N 1 eq and of said corrector term ΔN 1 f in such a way as to obtain a control value N 1 tgt which is transmitted to the means  4  by the link  5 A (means  4  which therefore apply this control value N 1 tgt to the engine  2 ).       

     As may be seen in  FIG. 2 , the second unit  9  comprises a calculation means  13  which calculates the difference ΔVc between:
         the preset speed Vctgt of the aircraft, which is received from a standard means  14 ; and   the aircraft&#39;s actual speed Vc which is measured by a standard sensor  15 , and which may possibly be filtered.       

     Said difference ΔVc emanating from the calculation means  13  is transmitted to a calculation means  16 , which filters it, for example with a time constant of five seconds, so as to obtain a filtered difference DVc. 
     Said second unit  9  furthermore comprises a summator  17  which calculates an intermediate term ΔN 1  by computing the sum:
         of a first term calculated by a calculation means  18  which, for this purpose, multiplies said filtered difference DVc by a predetermined coefficient KF; and   of a second term which:
           corresponds to the integration (by a calculation means  19 ) of the product (afforded by a calculation means  20 ) of said difference DVc and of a predetermined gain, when particular conditions specified hereinbelow relating to the speed Vc are fulfilled; or   is equal to zero (the value “zero” arising from a memory  21 ), when these particular conditions are not fulfilled.   
               

     According to the invention, these particular conditions correspond to the following conditions:
         the difference DVc is greater than a predetermined value, for example 0.5 knots; and   the actual speed Vc does not diverge significantly from the preset speed Vctgt.       

     To this end, a switch  22 , which is controlled by a control means  23  comprising said particular conditions, is arranged between, on the one hand, the outputs of the calculation means  16  and of the memory  21  and, on the other hand, the input of the calculation means  20 . 
     According to the invention, said second unit  9  furthermore comprises a subsystem  24  which includes:
         a calculation means  25  for calculating a second difference Δ 2  between said intermediate term ΔN 1  and a previously recorded corrector term ΔN 1 mem;   a comparator  26  for comparing this second difference Δ 2  with a predetermined threshold value S;   a means of selection  27  for selecting as corrector term ΔN 1 f:
           said intermediate term ΔN 1 , if said second difference Δ 2  is greater than said threshold value S; and   said previously recorded corrector term ΔN 1 mem, if said second difference Δ 2  is less than or equal to said threshold value S,   
           said means of selection  27  transmitting the corrector term ΔN 1 f thus selected to the summator  10 , preferably after filtering by a filtering means  28 ; and   a memory  29  for recording the selected corrector term, the value of which will be used subsequently by the comparator  26 .       

     These characteristics make it possible, advantageously, to keep ΔN 1 mem stable so long as its value remains close (deviation less than the threshold value S) to the value ΔN 1  which would make it possible to carry out the feedback control of the speed Vc to the preset Vctgt. Thus, the rating of the engine or the engines  2  is not modified continuously, but only when necessary for the feedback control of the speed, given the deviations allowed between the latter and its preset. This results in an improvement to the acoustic comfort of the passengers. 
     According to a preferred embodiment of the invention, the threshold value S is substantially equal to 0.5% of the preset value N 1 prst of the speed of rotation N 1  of the engine  2 . This value N 1 prst is determined empirically, in particular with the aid of physiological studies performed on passengers. 
     According to another preferred embodiment of the invention, the filtering means  28  which exhibits a time constant τ is intended for softening the variations in the engine rating during step-wise alterations in ΔN 1 mem, thereby also improving the acoustic comfort of the passengers. Preferably, the time constant τ is equal to five seconds. This value is also determined empirically by means of physiological studies performed on passengers. 
     It will be noted moreover that the equilibrium term N 1 eq can be calculated in standard fashion on the basis of known laws that are integrated into the first unit  8 . 
     By way of illustration, this equilibrium term N 1 eq can be calculated on the basis of the following equations: 
             N1eq   =     N1R   *       T   ⁢           ⁢     t   /   288.15                       with   ⁢           ⁢     {           N1R   =       f   1     ⁡     (     FNR   ,   M     )                   FNR   =     Fn   *     101325   /   Pt                   Fn   =     m   *   g   *     (       sin   ⁢           ⁢   γ     +       (     Cx   /   Cz     )     *   cos   ⁢           ⁢   γ       )                             and   ⁢           ⁢     {           Cx   =     f   ⁡     (       Cz   2     ,   M     )                   Cz   =       (     m   *   g   *   cos   ⁢           ⁢   γ     )     /     (     0.7   *   Ps   *   Sr   *     M   2       )                           
 
in which the following parameters are used:
         Fn: the thrust of the engine  2  (N);   m: the weight of the aircraft (kg);   g: the acceleration due to gravity (≈9.81 m/s 2 );   γ: the slope of the aircraft (rd);   M: the Mach number;   Ps: the static pressure (Pa);   Sr: a reference area (m 2 );   Cx: the coefficient of drag;   Cz: the coefficient of lift;   Tt: the total temperature (degrees Kelvin); and   Pt: the total pressure (Pa).       

     It will be noted moreover that, in the case of the EPR parameter, there is no reduced value such as the value N 1 R. Hence, the equilibrium term EPReq is calculated directly, in a known manner, as a function of FNR and of M: EPReq=f 2  (FNR, M). 
     In another embodiment  3 B represented in FIG.  3  and corresponding to an application of the embodiment  3 A, the device  1  comprises, in addition to the previously described calculation assembly  7 :
         a calculation assembly  30  which implements a standard process of thrust control, such as one exists on a known autothrust system;   a means of switching  31 , whose input is connected to the outputs (by links  5 A and  32 ) of said calculation assemblies  7  and  30 , and which makes it possible to switch between said two calculation assemblies  7  and  32  so as to transmit to its output (via the link  5 B which is connected to the means  4  of  FIG. 1 ) either the value provided by the calculation assembly  7 , or the value provided by the calculation assembly  30 ; and   a control unit  33  which automatically controls said switch  31 , as illustrated by a chain-dotted link  34 , as a function of predetermined conditions.       

     According to the invention, the device  1  takes account of the results of said standard control process (implemented by the calculation assembly  30 ) under normal operation, and it automatically switches into said control process in accordance with the invention (calculation assembly  7 ) so as to take account of the results of the latter process when at least one of a plurality of predetermined conditions is satisfied. 
     In a preferred embodiment, said predetermined conditions comprise at least the following conditions:
         the actual speed Vc is stabilized, being to within a predetermined value, equal to the preset speed Vctgt;   the conditions of calculation of said equilibrium term N 1 eq are valid;   an autothrust function of the aircraft is engaged in speed holding mode; and   an automatic pilot of the aircraft is active in altitude holding mode.