CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority under 35 USC §119 to EP Application Serial No. 07113363.1, filed Jul. 27, 2007, entitled “METHOD AND SYSTEM FOR NON-CONTACT SENSING OF THE TORQUE OF A SHAFT,” the entire contents of which is hereby incorporated by reference. 
       BACKGROUND 
       [0002]    This invention relates generally to sensing the torque of a rotating shaft, and more specifically to a method and system for non-contact sensing of the torque of a shaft. Strings of prime movers and loads comprising turbine generators, electrical motors or compressors for example include drive trains of connected shaft assemblies to transmit torque. Torsional oscillations can occur in the shafts of the drive trains particularly where the mass of the moving parts is large or if the shaft has large moments of inertia combined with low damping. Relatively small amounts of power can excite a resonant torsion or oscillation in such drive trains with low damping. Excitation may be provided for example by one or both of mechanical and electrical means. Mechanical excitations can be caused by sudden changes in mechanical loading over an excitation frequency range for example. Electrical excitations can be caused by switching a load of a generator on or off or if an electric network exchanges significant energy with a generator at one or more of the natural frequencies of the shaft systems for example. 
         [0003]    Significant damage can be caused to the drive train and couplings thereof by resonant torsional oscillations. Repairs and downtime of the drive train are extremely expensive and inconvenient. 
         [0004]    Various systems and methods are available for damping torsional vibrations in a rotating drive train. These systems generally rely on a torque sensor in order to determine the torque of the rotating drive train and take appropriate action to suppress the resonant torsional oscillations. U.S. Pat. No. 7,173,399 discusses an integrated torsional mode damping system and method for a current source convertor that dampens torsional vibration by modulating active power through an inverter or rectifier. 
         [0005]    Many oil and gas and energy applications require measurement of the torque of a relatively large diameter shaft. Typically magnetostrictive torque sensors for such applications employ a sensing method in which a magnetic field is generated in the sensor by passing electric current through an excitation coil. This magnetic field permeates the shaft and returns back to a pick-up coil of the sensor. The output of the pick-up coil is an electrical signal that depends on the total magnetic reluctance of this loop. Part of the total magnetic reluctance is established by the air gap between the coils and the shaft and part of the total magnetic reluctance is established by the shaft itself with the magnetic reluctance of the shaft changing as a function of torque on the shaft. However, the output of such torque sensors suffer from a problem known in the art as “shaft run-out” in which a non-harmonic periodic disturbance signal appears in the output signal caused by inhomogeneities in the magnetic reluctance of the shaft. The shaft run-out phenomenon masks the torque sensor output signal and can lead to the provision of inaccurate results. 
         [0006]    The period of the shaft run-out signal varies with the rotational speed of the shaft making it far harder to eliminate in variable speed applications such as electrically driven compressors. 
         [0007]    A possible method of reducing the shaft run-out phenomenon from a variable speed drive shaft is to use two magnetostrictive sensors, installed 180° apart circumferentially around the rotating drive shaft. Summing the torque signals from both sensors enables the disturbances caused by shaft inhomogeneities to be reduced whilst being able to sense the magnetic reluctance of the shaft changing as a function of torque. However, the requirement for two sensors increases the size and cost of the arrangement and has limited reliability since if either sensor should fail, shaft run-out cannot then be reduced. 
       BRIEF DESCRIPTION 
       [0008]    Torque measurements from a rotating shaft suffer from periodic disturbances known as “shaft run-out.” These are caused by inhomogeneities in the shaft such as frozen mechanical stresses or anisotropic magnetic properties and can make it difficult to identify the underlying torque signal. A method and system for removing or reducing the effect of shaft run-out is disclosed comprising measuring the torque of a rotatable shaft at a position around the circumference of the shaft and correlating the measured torque signal with data indicative of the material properties of the shaft at corresponding positions around the circumference of the shaft. A corrected torque signal is then generated in which the effect of the shaft inhomogeneities are reduced or removed providing a far clearer torque signal. 
         [0009]    According to one embodiment of the present invention there is provided a non-contact torque sensing system for a rotatable shaft of a drive train, the system comprising: a torque sensor arranged to provide a signal indicative of the measured torque of a rotating shaft; a position sensor to provide a signal indicative of the rotational position of the shaft to be associated with the torque signal from corresponding positions on the circumference of the shaft; and a controller to correlate the signal indicative of the torque with data indicative of material properties at corresponding circumferential positions around the shaft and to generate a corrected torque signal. 
         [0010]    As the shaft run-out periodic signal is caused by inhomogeneities in the magnetic reluctance of the shaft, such as frozen mechanical stresses or anisotropic magnetic properties of the shaft material, by applying data indicative of the material properties of the shaft being measured, the sensed torque signals can be modulated accordingly to remove or reduce the effects of shaft run-out. Furthermore, only a single torque sensor is required, reducing costs and size requirements. If two sensors are provided, then very high levels of reliability can be achieved because even if one sensor fails, the other will still be able to provide the required signals. 
         [0011]    The data indicative of the magnetic properties around the circumference of the shaft may be combined with or subtracted from portions of the measured torque signal relating to corresponding circumferential positions of the shaft. The controller may be arranged to sample the measured torque signal of the rotating shaft at regular intervals to provide a series of torque values and to subtract data indicative of the material properties of the shaft from the series of sampled torque values relating to corresponding circumferential positions of the shaft. 
         [0012]    The stored data indicative of the material properties around the circumference of the shaft are preferably torque measurements obtained with no external torque applied to the shaft. 
         [0013]    The position sensor may provide a signal indicative of the rotational position of the shaft in a number of ways. For example, the position sensor may be arranged to detect one or more magnetic markers at predetermined circumferential locations around the shaft as it rotates in order for the rotational position of the shaft to be determined. An advantage of the use of magnetic markers is that the position signal can be detected with the same pick-up coils as used for torque sensing. Alternatively the position sensor may be arranged to determine the signal indicative of rotational position of the shaft from a component connected to the rotatable shaft or a component connected to a drive train in which the rotatable shaft is provided. Such a component may be a stator or an encoder for example. 
         [0014]    According to another embodiment of the present invention there is provided a torsional damping system for a drive train comprising a plurality of interconnected rotatable shafts, the damping system including: a torque sensing system according to the first aspect of the present invention for sensing the torque of the drive train; and a damping controller for using the generated drive train torque for detecting a presence of torsional vibration on the drive train corresponding to a natural frequency of a shaft of the drive train and for generating a damping control signal for damping torsional vibrations. 
         [0015]    According to another embodiment of the present invention, there is provided a torque sensing method for reducing errors in torque sensed in a rotatable shaft of a drive train, the method comprising: measuring the torque of a rotatable shaft at a position around the circumference of the shaft; correlating the measured torque signal with data indicative of the magnetic properties of the shaft at corresponding positions around the circumference of the shaft; and generating a corrected torque signal. 
         [0016]    According to another embodiment of the present invention there is provided a torsional damping method for a drive train comprising a plurality of interconnected rotatable shafts, the damping method comprising: sensing torque in a rotatable shaft in accordance with the third aspect of the present invention; and using the generated corrected torque signal for detecting a presence of torsional vibration on the drive train corresponding to a natural frequency of a shaft of the drive train and for generating a damping control signal for damping torsional vibrations. 
     
    
     
       DRAWINGS 
         [0017]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0018]      FIG. 1  shows an example of inhomogeneities in a shaft such as frozen mechanical stresses or anisotropic magnetic properties; 
           [0019]      FIG. 2  shows a torque sensor positioned adjacent to a shaft; 
           [0020]      FIG. 3  shows a number of traces, the first being the actual torque on a shaft and the second and third showing output signals from a torque sensor illustrating the periodic shaft run-out spikes caused by inhomogeneities in the shaft; 
           [0021]      FIG. 4  illustrates an embodiment of the present invention; 
           [0022]      FIG. 5  illustrates another embodiment of the present invention; and 
           [0023]      FIG. 6  illustrates an output signal from a torque sensor before an embodiment of the present invention is applied to the signal (with periodic shaft run-out spikes) and after an embodiment of the present invention has been applied to the signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows a perspective view of a rotatable drive shaft  10 . The shaft  10  may for example, be coupled with one or more other rotatable drive shafts to form a drive train to transmit torque along a string of prime movers and loads such as turbine generators, electrical motors or compressors for example. The end face  11  of the shaft  10  shows a number of inhomogeneous areas  12 ,  13 ,  14  such as frozen mechanical stresses or anisotropic magnetic properties of the shaft material that leads to inhomogeneities in the magnetic reluctance of the shaft  10 . 
         [0025]      FIG. 2  shows a side view of the rotatable shaft  10  with a torque sensor  20  provided at a point around the circumference of the shaft  10 . Although any suitable torque sensor  20  may be used, in this example the torque sensor includes a primary or excitation coil  21  through which an electric current is passed to generate a magnetic field that permeates into the shaft  10  and returns back to measurement or pick-up coils  22 . In this example there are four pick-up coils  22  equiangularly spaced 90° apart around the excitation coil  21 . The measuring concept is based on the anisotropic magnetostrictive effect in ferromagnetic materials. The permeability for the magnetisation in the direction of compressive stress is different in comparison with the direction of tensile stress. The sensor measures this difference. The output of each pick-up coil  22  is an electrical signal that depends on the total magnetic reluctance of this loop. Part of the total magnetic reluctance is established by the air gap between the coils  21 ,  22  and the shaft  10  and part of the total magnetic reluctance is established by the shaft  10  itself with the magnetic reluctance of the shaft changing as a function of torque on the shaft. A shaft run-out periodic disturbance signal may appear in the output signal caused by the inhomogeneities  12 ,  13 ,  14  in the magnetic reluctance of the shaft. 
         [0026]      FIG. 3  illustrates a series of traces. The upper trace  31  illustrates an ideal torque signal that would be obtained from a rotatable shaft without any inhomogeneities in the magnetic reluctance of the shaft  10 . This shaft run-out free torque signal could be measured using a strain gauge attached to the shaft for example. In contrast, the two lower traces  32 ,  33  illustrate an actual trace that would typically be generated by a torque sensor  20  with a series of periodic shaft run-out spikes caused by inhomogeneities  12 ,  13 ,  14  in the shaft  10 . As can be seen, these shaft run-out spikes mask the actual torque signal  31  making the torque signal unclear. 
         [0027]      FIG. 4  illustrates an embodiment of a torque sensing system in accordance with the present invention. One or both longitudinal end portions of the shaft may be connected to other rotatable shafts (not shown) to form a drive train. As shown in  FIG. 2 , the torque sensor  20  is provided at a point on the circumference of the shaft  10  and has a connector  23  to provide a signal indicative of the measured torque of the rotating shaft  10  to a controller  40 . A position sensor  41  provides a signal indicative of the rotational position of the shaft  10  to be associated with the torque signal from corresponding positions on the circumference of the shaft. The signal from position sensor  41  is provided via connector  42 . The controller  40  correlates the signal indicative of the torque with data indicative of material properties at corresponding circumferential positions around the shaft. This correlation may be performed by any suitable processing means such as a CPU  43 . The data indicative of material properties at corresponding circumferential positions around the shaft may be stored in a memory such as a look up table  44  which may also be provided in the controller  40 . The controller is arranged to generate a corrected torque signal  45  in which the effects of shaft run-out are eliminated, or at least reduced by removing the component of the signal caused by the inherent inhomogeneous properties of the shaft. 
         [0028]    Alternatively a “pattern recognition scheme” based on the known data in the memory can be used to detect shaft position. For example one rotation of the shaft or period can be assumed to end after every second dynamic maximum detected by the torque sensor  20 . 
         [0029]    The corrected torque output signal  45  may be used in any suitable way such as in a torsional damping system for a drive train of which the rotatable shaft  10  forms a part. A damping controller (which may form part of controller  40 ) may use the generated corrected torque signal  45  for detecting a presence of torsional vibration on the drive train corresponding to a natural frequency of a shaft of the drive train and may then generate a damping control signal for damping torsional vibrations of the drive train in any suitable way as is known to those skilled in the art, such as is disclosed in U.S. Pat. No. 7,173,399 for example. 
         [0030]    The data indicative of the inhomogeneous metallic properties of the shaft  10  may be torque measurements from sensor  20  from around the circumference of the shaft  10  when no external torque is applied to the shaft. Such measurements may be made during commissioning of the sensor system and stored in the memory  44  for subsequent operational use. 
         [0031]    The position sensor may be arranged to detect the rotational position of the shaft  10  for association with the corresponding portions of the torque signal in any suitable manner. For example, the position sensor  41  may be arranged to detect the passage of one or more magnetic markers at predetermined circumferential locations around the shaft as it rotates in order for the rotational position of the shaft to be determined. Alternatively, the position sensor may form a part of an existing component associated with or connected to the rotatable shaft  10  or to a drive train of which the rotatable shaft  10  forms a part such as a stator of a machine connected to the drive train or an encoder connected to the drive train. Such components such as a stator or an encoder are typically present on drive trains of connected shaft assemblies and so output signals from such a component could easily be employed by the controller  40  to be indicative of the rotational position of the shaft for association with the torque signal relating to corresponding circumferential positions around the shaft  10 . 
         [0032]    Although the torque sensor  20  and position sensor  41  are shown connected to the controller  40  with physical connectors  21 ,  42  respectively such as cables, it will be appreciated by the person skilled in the art that communication between the sensors  20 ,  41  and controller  40  may be performed by any suitable means such as radio transmitters for example. 
         [0033]      FIG. 5  shows a more detailed example of a torque sensing system. As can be seen, the signal from torque sensor  20  is arranged into portions  50   a  corresponding to one revolution of the shaft  10  as indicated by dotted lines  50 . The arrangement into portions corresponding to one revolution of the shaft is performed by the controller  40  in conjunction with the signal from the position sensor  41 , which in this example is a shaft encoder signal. The shaft position signal  42  is used to select the start position of each one revolution portion  50   a  of the torque signal  21  to correspond to the start position (x 0 ) of the material properties of the shaft  10  stored in memory  44 . The torque signal arranged or marked to indicate the beginning of data relating to each revolution of the shaft  10  is then passed through a low pass filter  51  provided in the controller  50 . The low pass filter  51  is an anti aliasing filter used to reduce signal information with higher frequency than the sampling device is able to resolve. The low pass filter  51  may also reduce the effect of high frequency noise. The torque signal is then sampled using the sample and hold circuit  52 . The frequency (f 0 ) of the sampling is dependent upon the rotational speed of the shaft  10  that is calculated by CPU  43  from the shaft encoder signal  42 . The frequency (f 0 ) of the sampling is selected such that the number of sample values corresponds to the amount of data in the memory  44  and is provided by the following equation: 
         [0000]    
       
         
           
             
               f 
               0 
             
             = 
             
               
                 rotational 
                  
                 
                     
                 
                  
                 speed 
                  
                 
                     
                 
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                 of 
                  
                 
                     
                 
                  
                 shaft 
                  
                 
                     
                 
                  
                 
                   ( 
                   
                     revs 
                      
                     
                       / 
                     
                      
                     minute 
                   
                   ) 
                 
               
               
                 number 
                  
                 
                     
                 
                  
                 of 
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                 stored 
                  
                 
                     
                 
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                 values 
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                 in 
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                  
                 memory 
               
             
           
         
       
     
         [0034]    The subtractor  53  then subtracts the stored material properties provided in memory  44  from the sampled torque signal provided by sample and hold signal  52 . The output of the subtractor  53  may be pulsed  54  and low pass filtered  55  to reduce noise caused by digital conversion and processing of the data, known as reconstruction low pass, before being output as a corrected torque signal  45 . 
         [0035]    The torque signal is preferably measured and processed continuously. Using data blocks representing one revolution would add time delay to the processed torque signal, but could be applied in cases where the time delay is acceptable. The shaft position signal  42  is used to trigger the real-time control to position a look-up table pointer and to calculate the current revolution speed in order to adjust the sampling frequency. The sampling frequency adjustment is one approach to accommodate variable speeds of the shaft. Another approach would be for example to sample with constant, high frequency during operation and to calculate mean values of the revolution speed proportional to the number of samples stored in the memory or look-up table. 
         [0036]      FIG. 6  illustrates a typical waveform  60  produced by a simple torque sensor that clearly demonstrates the periodic disturbances  60   a  caused by shaft run-out. Plot  61  illustrates data relating to inhomogeneities of a shaft  10  stored in the memory  44  and which corresponds to one revolution of the shaft  10 . The start and end points of the shaft material property data may be determined from signals  50  corresponding to the passage of a marker on the circumference of the shaft  10  or by the data signal crossing a horizontal axis  61   a  through the data twice. Waveform  62  illustrates a corrected torque signal  45  after the inherent inhomogeneities of the shaft as indicated by data  61  from memory  44  are subtracted from signal  60  by the controller  40 . The removal of the spikes  60   a  caused by shaft run-out from these experimental results can be clearly seen such that the torque on the shaft can be determined far more easily and precisely. 
         [0037]    As explained above, the torque sensing system  40  as illustrated in particular in  FIGS. 4 and 5  may be connected to a shaft  10  that forms part of a drive train. The corrected torque signal  45  may then be used in a torsional damping system for detecting the presence of torsional vibration on the drive train at a natural frequency of a shaft within the drive train and for generating a corresponding damping control signal such that the torsional vibrations may be damped. 
         [0038]    Many modifications may be made to the examples described above while still falling within the scope of the present invention. For example, the controller  40  may be provided by a processing means of another or higher-level system in which the provision of a shaft run-out corrected torque signal forms a part. As explained above, any suitable method of producing a position signal may be used, such as sensing a marker on the circumference of the shaft  10 , deriving the signal from a component connected to the rotatable shaft  10 , determining when an output signal crosses a horizontal axis twice or using a pattern recognition scheme for example.
Torque measurements from a rotating shaft suffer from periodic disturbances known as "shaft run-out." These are caused by inhomogeneities in the shaft such as frozen mechanical stresses or anisotropic magnetic properties and can make it difficult to identify the underlying torque signal. A method and system for removing or reducing the effect of shaft run-out is disclosed comprising measuring the torque of a rotatable shaft at a position around the circumference of the shaft and correlating the measured torque signal with data indicative of the material properties of the shaft at corresponding positions around the circumference of the shaft. A corrected torque signal is then generated in which the effect of the shaft inhomogeneities are reduced or removed providing a far clearer torque signal.