Abstract:
A method for calibrating a torsion torquemeter including: placing a torsion torquemeter in a first state; performing first measurements determining first and second angular offsets, and measuring torque output by the power shaft using a reference torquemeter; placing the torsion torquemeter in a second state; performing second measurements determining the first and second angular offsets, and measuring torque output by the power shaft; placing the torquemeter in a third state; performing third measurements determining the first and second angular offsets, and measuring the torque output by the power shaft; placing the torquemeter in a fourth state; performing fourth measurements determining the first and second angular offsets, and measuring the torque output by the power shaft; calibrating the calculation unit based on the first, second, third, and fourth measurements.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns devices for measuring the torque transmitted by an engine shaft, for example a shaft of an aircraft turbomachine. 
         [0002]    The present invention more particularly concerns a method for calibrating a torsion torquemeter preferably intended to be mounted in a helicopter turbomachine. 
         [0003]    It is to be recalled that measurement of the rotational torque of a shaft is of particular importance in the field of helicopter engines since it generally provides one of the essential piloting data items taken into account by a pilot. Once the rotor of a helicopter has reached constant speed, the power supplied by the latter is exclusively dependent on torque. 
         [0004]    Document FR 2 931 552 describes a torquemeter based on the measurement of torsional deformation of the shaft, this deformation in particular being a function of the transmitted torque. 
         [0005]    The torsion torquemeter described in this document comprises:
       a power shaft intended to transmit rotational torque about an axis, said power shaft being provided with a first wheel carrying a first and second series of angular reference points;   a reference shaft having a first end fastened to one end of the power shaft and a second end provided with a second wheel carrying third and fourth series of angular reference points, the angular reference points of the first and third series being parallel to one another, whilst the angular reference points of the second and fourth series are parallel to each other whilst being inclined relative to the reference points of the first and third series;   a measuring device to measure a first angular offset between two angular reference points respectively belonging to the first and third series of angular reference points, and a second angular offset between two angular reference points respectively belonging to the second and fourth series of angular reference points;   a calculation unit to determine a value of the torque output by the power shaft, based in particular on the first and second angular offsets measured by the measuring device.       
 
         [0010]    To calculate the torque supplied by the power shaft, the calculation unit must previously determine the temperature of the shaft using data provided by the measuring device. Once the temperature has been determined, the torque value is determined using a database previously stored in a memory of the calculation unit, this database containing torque values corresponding to several values of angle deformation for different temperatures. 
         [0011]    The prior art torsion torquemeter therefore requires knowledge of the temperature of the power shaft, and of a behavioural model of the material of the power shaft. 
       OBJECTIVE AND SUMMARY OF THE INVENTION 
       [0012]    It is one objective of the present invention to propose a method for calibrating a torsion torquemeter of the aforementioned type so that the calibrated torquemeter is able to compute the torque without having to determine the temperature of the power shaft. 
         [0013]    According to the method subject of the invention:
       the torsion torquemeter is placed in a first state in which the value of the torque output by the power shaft lies within a first predetermined torque range, the shafts of the torsion torquemeter then being at a first temperature;   a first set of measurements is performed whereby the first and second angular offsets are determined, and the torque output by the power shaft is measured using a reference torquemeter;   the torsion torquemeter is placed in a second state in which the value of the torque output by the power shaft lies within a second predetermined torque range, the second predetermined torque range differing from the first predetermined torque range, the shafts of the torsion torquemeter then being substantially at the first temperature;   a second set of measurements is performed whereby the first and second angular offsets are determined, and the torque output by the power shaft is determined using the reference torquemeter;   the torquemeter is placed in a third state whereby the value of the torque output by the power shaft lies within the second predetermined torque range, the shafts of the torsion torquemeter being brought to a second temperature different from the first temperature;   a third set of measurements is performed whereby the first and second angular offsets are determined, and the torque deliver by the power shaft is measured using the reference torquemeter;   the torquemeter is placed in a fourth state whereby the value of the torque output by the power shaft lies within the first predetermined torque range, whilst the shafts of the torsion torquemeter are substantially at the second temperature;   a fourth set of measurements is performed whereby the first and second angular offsets are determined, and the torque output by the power shaft is measured using the reference torquemeter;   the calculation unit is calibrated from the first, second, third and fourth sets of measurements.       
 
         [0023]    The first, second, third and fourth sets of measurements allow determination of calibration data, which are subsequently used by the calculation unit to compute the torque value such as measured by the torsion torquemeter. 
         [0024]    Once calibrated, the calculation unit of the torsion torquemeter determines the value of the torque from the first and second angular offsets and from the calibration data. 
         [0025]    The calibration method is particularly quick and easy to perform since it only requires four sets of measurements and two temperature-stabilized torque values. Said calibration can be carried out in less than 30 minutes by an experienced operator. 
         [0026]    In addition, the calibration method of the invention is preferably implemented when the torquemeter is mounted in the turbomachine. Calibration can therefore easily be conducted on a test bench without dismounting the turbomachine. 
         [0027]    It was additionally found that a torquemeter calibrated by implementing the method of the invention has better precision than the prior art torquemeter. In practice, the precision obtained is only very slightly below the precision of the reference torquemeter used for calibration. 
         [0028]    Once calibrated, the torsion torquemeter is capable of providing a precise torque value without it being necessary to determine the temperature of the power shaft. As a result, it is no longer necessary to have knowledge of the behavioural model of the material of the power shaft as a function of temperature in order to be able to calculate the torque transmitted by the power shaft. 
         [0029]    According to one preferred embodiment, the second predetermined torque range is higher than the first predetermined torque range, whilst the second temperature value is higher than the first temperature value. 
         [0030]    Therefore, preferably, the second set of measurements is conducted substantially at the same temperature as the first set of measurements. 
         [0031]    The third set of measurements is conducted within the same range as the second set of measurements. The fourth set of measurements is conducted substantially at the same temperature as the third set of measurements and within the same torque range as the first set of measurements. 
         [0032]    By “substantially” is preferably meant a tolerance of plus or minus 5 to 20%. 
         [0033]    Preferably, the second set of measurements is performed soon after the first set of measurements e.g. less than one minute after the first set of measurements so that the temperature of the shafts, via thermal inertia, does not have the time to increase further to an increase in torque. The increase in torque, between the first and second predetermined torque values, is obtained by increasing the speed of the turbomachine which causes heating thereof and hence a rise in temperature of the shafts. 
         [0034]    Similarly, the fourth set of measurements is performed soon after the third set of measurements e.g. less than one minute after the third set of measurements so that the temperature of the shafts, via thermal inertia, does not have the time to decrease further to a decrease in torque. 
         [0035]    Also, the first predetermined torque range is preferably but not exclusively in the region of a first mean torque value of between 0 and 20% of the maximum torque that the turbomachine equipped with the torsion torquemeter is able to produce. 
         [0036]    The second predetermined torque range is preferably but not exclusively in the region of a second mean torque value which is higher than 80% of said maximum torque. 
         [0037]    As non-limiting example, the extent of the first torque range corresponds to the first mean value plus or minus 10%. Similarly, as non-limiting example, the extent of the second torque range corresponds to the second mean value plus or minus 10%. 
         [0038]    To improve on the precision of calibration, it is waited until the temperature of the shafts of the torquemeter is stabilized at the second temperature value before conducting the third set of measurements. 
         [0039]    The inventors have ascertained that a time of about 15 minutes could prove to be sufficient for the temperature of the shafts to stabilize at the second temperature value. 
         [0040]    Advantageously, the first, second, third and fourth sets of measurements are used to determine calibration data which are stored in a memory of the calculation unit. 
         [0041]    This is preferably a re-write memory so that, if necessary, several calibrations of the torsion torquemeter can be performed throughout the lifetime of the turbomachine. 
         [0042]    Advantageously, the first and second wheels are phonic wheels, and the angular reference points are teeth. 
         [0043]    In addition, the measurement device is preferably formed by a single magnetic sensor. 
         [0044]    The invention also relates to a calibrated torsion torquemeter comprising:
       a power shaft intended to transmit rotational torque about an axis, said power shaft being equipped with a first wheel carrying a first and second series of angle reference points;   a reference shaft having a first end attached to one end of the power shaft and a second end provided with a second wheel carrying third and fourth angular reference points, the angular reference points of the first and third series being parallel to each other, whilst the angular reference points of the second and fourth series are parallel to one another whilst being inclined relative to the reference points of the first and third series;   a measuring device to measure a first angular offset between two reference points respectively belonging to the first and third series of angular reference points, and a second angular offset between two reference points respectively belonging to the second and fourth series of angular reference points;   a calculation unit to determine a torque value output by the power shaft on the basis of the first and second angular offsets measured by the measuring device, the calculation unit comprising a memory to store calibration data obtained by implementing the method of the invention, the torque value such as determined by the calculation unit being a function of the first and second angular offsets and of said calibration data.       
 
         [0049]    Finally, the present invention pertains to a turbomachine comprising a torsion torquemeter calibrated according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]    The invention will be better understood on reading the following description of one embodiment of the invention given as a non-limiting example, with reference to the appended drawings in which: 
           [0051]      FIG. 1  is a half axial section view of a torsion torquemeter calibrated according to the invention; 
           [0052]      FIG. 2  shows the relative position of the teeth of the first and second phonic wheels of the torquemeter in  FIG. 1 , and illustrates the first and second angular offsets measured by the sensor of the torquemeter in  FIG. 1 ; 
           [0053]      FIG. 3  illustrates the installation for implementing the calibration method of the invention; 
           [0054]      FIG. 4  illustrates the steps of the calibration method according to the invention; and 
           [0055]      FIG. 5  illustrates a turbomachine equipped with the torsion torquemeter calibrated by implementing the method of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0056]      FIG. 1  illustrates one preferred embodiment of the torsion torquemeter  10  which is intended to be calibrated by implementing the method of the invention. 
         [0057]    The torsion torquemeter  10  comprises a hollow power shaft  12  intended to transmit rotational torque about its axis A. It is this torque that it is sought to measure. 
         [0058]    In the example in  FIG. 1 , the power shaft  12  comprises a pinion  14  at its first end  12   a,  and a driving member  16  arranged at its second end  12   b  opposite the first end. Evidently it is possible for the ends of the power shaft to be differently equipped. 
         [0059]    Also, the power shaft  12 , close to its first end  12   b,  carries a first wheel, here a phonic wheel  18 , which is coaxial and comprises a plurality of angular reference points, in this case teeth. 
         [0060]    The torsion torquemeter  10  further comprises a reference shaft  20  extending axially inside the power shaft  12  to which it is attached via its first end  20   a  close to the first end  12   a  of the power shaft  12 , whilst its second end  20   b  is free. The second end  20   b  of the reference shaft  20 , opposite its first end  20   a,  carries a second wheel, here of phonic type  22 , which is concentric with the first phonic wheel  18 . The second phonic wheel  22  carries a plurality of angular reference points, in this case teeth, which extend radially through openings made in the power shaft  20 . An alternative configuration is described in FR 2 931 552. 
         [0061]    Facing the first and second phonic wheels  18 , 22  there is arranged a single magnetic sensor  26  which is capable of generating an electric signal on each passing of a tooth in front of it, said signal then being sent to a calculation unit  28  intended to determine the value of the torque transmitted by the power shaft  12 . 
         [0062]    The first phonic wheel  18  comprises a first and second series of angular reference points, namely a first series of teeth D 1  identical to each other, and a second series of teeth D 2  identical to each other, whilst the second phonic wheel  22  comprises a third series of teeth D 3  identical to each other and a fourth series of teeth D 4  identical to each other. 
         [0063]    The first and second phonic wheels are arranged such that the teeth D 1 ,D 2  of the first phonic wheel  18  angularly alternate with the teeth D 3 , D 4  of the second phonic wheel  22 . 
         [0064]    As can be seen in  FIG. 2 , the teeth D 1 , D 3  of the first and third series are parallel to each other, whilst the teeth D 2 , D 4  of the second and fourth series are parallel to one another whilst being inclined relative to the teeth of the first and third series. 
         [0065]    In this  FIG. 2 , the distribution of the teeth is illustrated in an orthoradial direction OR of the power shaft. It will therefore be understood that the white teeth D 1  and D 2  belong to the first phonic wheel, whilst the black teeth belong to the second phonic wheel. 
         [0066]    The teeth D 1  and D 3  lie at an angle γ 1  relative to the axial direction of the power shaft, whilst the teeth D 2  and D 4  lie at angle γ 2  different from γ 1 . 
         [0067]    The magnetic sensor  26  is a measuring device which allows the determining of a first angular offset α m  between two teeth D 1 , D 3  respectively belonging to the first series of teeth and the second series of teeth. The magnetic sensor  26  also allows the measurement of a second angular offset α m  between two teeth D 2 , D 4  respectively belonging to the second series of teeth and the fourth series of teeth. 
         [0068]    The calculation unit  28  of the torsion torquemeter  10  is programmed to compute a torque value TQ from the first and second angular offsets α m  and β m  measured by the magnetic sensor  26  and from calibration data stored in a memory  29  of the calculation unit  28 . 
         [0069]    The torque value TQ is obtained for example, but not exclusively, using the following mathematical formula. 
         [0000]    
       
         
           
             
               
                 
                   
                     TQ 
                      
                     
                       ( 
                       
                         
                           α 
                           m 
                         
                         , 
                         
                           β 
                           m 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         β 
                         m 
                       
                       - 
                       
                         β 
                         off 
                       
                       + 
                       
                         
                           [ 
                           
                             
                               ( 
                               
                                 
                                   β 
                                   m 
                                 
                                 - 
                                 
                                   β 
                                   off 
                                 
                               
                               ) 
                             
                             - 
                             
                               ( 
                               
                                 
                                   α 
                                   m 
                                 
                                 - 
                                 
                                   α 
                                   off 
                                 
                               
                               ) 
                             
                           
                           ] 
                         
                         * 
                         
                           
                             TQ 
                              
                             
                                 
                             
                              
                             
                               0 
                               β 
                             
                           
                           
                             
                               TQ 
                                
                               
                                   
                               
                                
                               
                                 0 
                                 α 
                               
                             
                             - 
                             
                               TQ 
                                
                               
                                   
                               
                                
                               
                                 0 
                                 β 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             R 
                             0 
                           
                           + 
                           
                             [ 
                             
                               
                                 ( 
                                 
                                   
                                     β 
                                     m 
                                   
                                   - 
                                   
                                     β 
                                     off 
                                   
                                 
                                 ) 
                               
                               - 
                               
                                 α 
                                 m 
                               
                               - 
                               
                                 α 
                                 off 
                               
                             
                             ) 
                           
                         
                         ] 
                       
                       * 
                       
                         1 
                         
                           
                             TQ 
                              
                             
                                 
                             
                              
                             
                               0 
                               α 
                             
                           
                           - 
                           
                             TQ 
                              
                             
                                 
                             
                              
                             
                               0 
                               β 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0070]    Where:
       α m  and β m  represent the first and second angular offsets measured by the sensor;   R 0 , α off , β off , TQ0 α  and TQ0 β  are the calibration data.       
 
         [0073]    This formula is stored in a processor of the calculation unit. 
         [0074]    The calibration data R 0 , α off , β off , TQ0 α  and TQ0 β  are obtained on completion of the calibration method of the invention which will now be described with reference to  FIGS. 3 and 4 . 
         [0075]    It is therefore appreciated that the torsion torquemeter is fully characterized once the calibration data R 0 , α off , β off , TQ0 α  and TQ0 β  are known. Therefore the object of calibration is to compute the values R 0 , α off , β off , TQ0 α  and TQ0 β . 
         [0076]      FIG. 3  illustrates the installation to implement the calibration method of the invention. In this case it is a calibration bench  50 . 
         [0077]    The torsion torquemeter  10  when calibrated is mounted in a turbomachine  52 , this engine being arranged on the calibration bench  50 . 
         [0078]    The end of the power shaft  12  is attached to a reference torquemeter  54  external to the turbomachine  52 . 
         [0079]    The reference torquemeter  54  therefore provides a reference measurement TQ m  of the torque output by the power shaft  12  when a set torque TQ c  is requested of the turbomachine. It will be understood that if the torquemeter is not calibrated, the value TQ m  does not exactly correspond to the set value TQ c . 
         [0080]    With reference now to  FIG. 4 , a detailed explanation will be given of the implementation of the method of the invention. 
         [0081]    First the torsion torquemeter is placed in a first state E 1  in which the value of the torque output by the power shaft  12  lies within a first predetermined torque range P 1  substantially centred around a first mean torque value C 1 . In this first state E 1 , the power and reference shafts of the torsion torquemeter are at a first temperature T 1 . In this non-limiting example, the first mean torque value C 1  is equal to 10% of the maximum torque which the turbomachine is able to produce, whilst the first temperature is about 80° C. The limits of the first range in this example are plus or minus about 10% of the first mean torque value. 
         [0082]    When the torquemeter is in the first state E 1 , a first set of measurements is performed in which the first and second angular offsets α m1  and β m1  are determined by means of the magnetic sensor  26 , and the torque TQ m1  output by the power shaft is measured using the reference torquemeter  54 . 
         [0083]    The speed rate of the turbomachine is increased by increasing the set value TQ c  so as to place the torquemeter in a second state E 2  (arrow F 1 ) in which the value of the torque output by the power shaft  12  lies within a second predetermined torque range P 2  substantially centred around a second mean torque value C 2  higher than the first mean torque value C 1 . A second set of measurements is then rapidly performed whereby the first and second angular offsets α m2  and β m2  are determined using the magnetic sensor  26 , and the torque TQ m2  output by the power shaft is measured using the reference torquemeter  54 . The second set of measurements is rapidly carried out so that in the second state the temperature of the shafts of the torsion torquemeter remains substantially equal to the first temperature T 1 . 
         [0084]    In this example, the second mean value C 2  is substantially equal to 80% of the maximum torque which the turbomachine is able to produce. 
         [0085]    The limits of the second range in this example lie within about plus or minus 10% of the second mean torque value. 
         [0086]    After completing the second set of measurements, it is waited until the engine heats so that the torsion torquemeter is brought to a third state E 3  (arrow F 2 ) wherein the shafts of the torsion torquemeter are at a second temperature T 2  higher than temperature T 1 , whilst the value of the torque output by the power shaft  12  remains within the second torque range. In other words, the torque value remains substantially equal to the second mean torque value C 2 . 
         [0087]    After waiting a few minutes for the shafts of the torsion torquemeter  10  to stabilize at the second temperature T 2 , a third set of measurements is performed whereby the first and second angular offsets α m3  and β m3  are determined by means of the magnetic sensor, and the torque TQ m3  output by the power shaft is measured using the reference torquemeter  54 . 
         [0088]    The speed of the turbomachine is then reduced so as to bring the torquemeter to a fourth state E 4  (arrow F 3 ) wherein the value of the torque output by the power shaft  12  lies within the first torque range P 1 . The value of the torque is substantially equal to the first mean torque value C 1 , whilst the shafts of the torquemeter are still substantially at the second temperature T 2 . 
         [0089]    When the torquemeter is placed in this fourth state, a fourth set of measurements is performed whereby the first and second angular offsets α m4  and β m4  are determined by means of the magnetic sensor, and the torque TQ m4  output by the power shaft is measured using the reference torquemeter  54 . The fourth set of measurements is performed soon after the first predetermined torque range P 1  has been reached so that the temperature of the shafts remains substantially equal to the second temperature T 2 . 
         [0090]    Evidently, the cycle presented herein is not limiting and it could be conducted in reverse order or in a different order provided that four sets of measurements are performed allowing the calibration data to be calculated. 
         [0091]    The torsion torquemeter, more specifically the calculation unit, is then calibrated using the first, second, third and fourth sets of measurements. 
         [0092]    In other words, the aforementioned calibration data are determined on the basis of the first, second, third and fourth sets of measurements. 
         [0093]    For example: 
         [0000]    
       
         
           
             
               R 
               0 
             
             = 
             
               
                 
                   β 
                   
                     m 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 - 
                 
                   β 
                   
                     m 
                      
                     
                         
                     
                      
                     1 
                   
                 
               
               
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 - 
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     1 
                   
                 
               
             
           
         
       
     
         [0094]    After which the angular offsets are calculated: 
         [0000]    
       
         
           
             
               β 
               off 
             
             = 
             
               
                 β 
                 
                   m 
                    
                   
                       
                   
                    
                   2 
                 
               
               - 
               
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 * 
                 
                   R 
                   0 
                 
               
             
           
         
       
       
         
           
             
               α 
               off 
             
             = 
             
               
                 α 
                 
                   m 
                    
                   
                       
                   
                    
                   2 
                 
               
               - 
               
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 * 
                 
                   R 
                   0 
                 
               
             
           
         
       
       
         
           
             
               
                 TQ 
                  
                 
                     
                 
                  
                 
                   0 
                   β 
                 
               
               
                 
                   TQ 
                    
                   
                       
                   
                    
                   
                     0 
                     α 
                   
                 
                 - 
                 
                   TQ 
                    
                   
                       
                   
                    
                   
                     0 
                     β 
                   
                 
               
             
             = 
             
               
                 
                   B 
                   * 
                   
                     TQ 
                     
                       m 
                        
                       
                           
                       
                        
                       4 
                     
                   
                 
                 - 
                 
                   A 
                   * 
                   
                     TQ 
                     
                       m 
                        
                       
                           
                       
                        
                       3 
                     
                   
                 
               
               
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 - 
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     4 
                   
                 
               
             
           
         
       
       
         
           
             
               1 
               
                 
                   TQ 
                    
                   
                       
                   
                    
                   
                     0 
                     α 
                   
                 
                 - 
                 
                   TQ 
                    
                   
                       
                   
                    
                   
                     0 
                     β 
                   
                 
               
             
             = 
             
               
                 B 
                 - 
                 A 
               
               
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 - 
                 
                   TQ 
                   
                     m 
                      
                     
                         
                     
                      
                     4 
                   
                 
               
             
           
         
       
       
         
           
             Where 
              
             
               : 
             
           
         
       
       
         
           
             A 
             = 
             
               
                 
                   β 
                   4 
                 
                 - 
                 
                   
                     TQ 
                     
                       m 
                        
                       
                           
                       
                        
                       4 
                     
                   
                   * 
                   
                     R 
                     0 
                   
                 
               
               
                 
                   β 
                   4 
                 
                 - 
                 
                   α 
                   4 
                 
               
             
           
         
       
       
         
           
             B 
             = 
             
               
                 
                   β 
                   3 
                 
                 - 
                 
                   
                     TQ 
                     
                       m 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   * 
                   
                     R 
                     0 
                   
                 
               
               
                 
                   β 
                   3 
                 
                 - 
                 
                   α 
                   3 
                 
               
             
           
         
       
     
         [0095]    These calibration data are then stored in the memory  29  of the calculation unit  28  so that they can be used by the calculation unit to calculate the torque value TQ, using formula (1) given above. 
         [0096]    In  FIG. 5 , a turbomachine  100  is illustrated equipped with a torsion torquemeter  10  calibrated using the method of the invention.