Abstract:
An apparatus comprises a rotatable shaft and first and second targets coupled to the rotatable shaft so as to rotate therewith, a first probe for transmitting a first transmission signal to the first target and receiving a first response signal from the first target, and a second probe for transmitting a second transmission signal to the second target and receiving a second response signal from the second target. The apparatus further comprises a processor operatively coupled to the first and second probes for determining a torsional displacement of the shaft based on the first and second response signals received by the first and second probes, respectively. The processor then determines a torque imposed on the rotatable shaft based on its torsional displacement.

Description:
[0001]    The present invention relates to a system and method for measuring torque on a rotating shaft and particularly to a system and method for measuring torque on a rotating load coupling shaft for driving a power generator.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various machines, such as a gas turbine and/or a steam turbine, may be used to drive a load such as a power generator. In particular, a gas turbine and/or a steam turbine may be used to rotate a magnet within a stator to generate electric power. The power generator includes a shaft which is connected to the rotating magnet and which itself is connected to a large connecting shaft (also called a load coupling shaft) rotated by one or more turbines. The connecting shaft is typically large and stiff, thereby resulting in very small torsional displacements (strains) when a torque is imposed on the connecting shaft. A measurement of torque transmitted through the connecting shaft-is often made to determine the power output of the machine(s) rotating the connecting shaft.  
           [0003]    The torque imposed on the connecting shaft has been measured in the past using strain gauges. However, the accuracy of torque measurements provided by strain gauges often does not meet engineering requirements because the uncertainty of such measurements is rather large as compared to the strains measured.  
           [0004]    Accordingly, there remains a need in the art to measure torque on a rotating shaft, such as a rotating load coupling shaft for driving a power generator, with a high degree of accuracy. The present invention satisfies this need. For example, the present invention is capable of measuring torque of a rotating shaft within a +/−0.5% accuracy.  
           [0005]    A known digital light probe system, developed by GE Aircraft Engines, has been used for several applications in the past including measuring compressor rotating blade vibratory displacements.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0006]    In one exemplary embodiment of the invention, an apparatus and method comprises a rotatable shaft, at least one first target coupled on the rotatable shaft so as to rotate therewith, at least one second target coupled on the rotatable shaft so as to rotate therewith, a first probe for transmitting a first transmission signal to the first target and receiving a first response signal from the first target, a second probe for transmitting a second transmission signal to the second target and receiving a second response signal from the second target; and a processor operatively coupled to the first and second probes for determining a torsional displacement of the shaft based on at least the first and second response signals received by the first and second probes, respectively.  
           [0007]    The processor may determine a torque imposed on the rotatable shaft based upon its torsional displacement. The processor may determine the torsional displacement based on the difference in time between when the first response signal is received by the first probe and when the second response signal is received by the second probe.  
           [0008]    A magnet of a power generator may be coupled to the rotatable shaft to rotate therewith. At least one of a gas turbine and a steam turbine may rotate the rotatable shaft.  
           [0009]    The first and second probes may be formed by laser probes and the first and second targets may include a reflective material so that the first transmission signal is a laser light signal and the first response signal is a laser light signal formed from a reflection of the first transmission signal by the first target and the second transmission signal is a laser light signal and the second response signal is a laser light signal formed from a reflection of the second transmission signal by the second target. The first and second targets may be coupled to the rotatable shaft on opposite axial ends thereof.  
           [0010]    Another first target may be coupled on the rotatable shaft so as to rotate therewith. The first probe transmits the first transmission signal to the another first target and receives another first signal from the another first target, and the processor determines a vibration displacement of the rotatable shaft based on the first signal and the another first signal received by the first probe. The torsional displacement of the shaft may be determined based (at least in part) on the vibration displacement of the shaft. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a diagram illustrating, inter alia, a cross sectional view of a rotating shaft in a simple cycle configuration whose torque is measured in accordance with an exemplary embodiment of the present invention.  
         [0012]    [0012]FIG. 2A is a diagram illustrating signals received by two different laser light probes from a rotating shaft having no measurable torque imposed thereon.  
         [0013]    [0013]FIG. 2B is a diagram illustrating signals received by two different laser light probes from a rotating shaft having a measurable torque imposed thereon.  
         [0014]    FIGS.  3 A- 3 C are diagrams illustrating an exemplary method for calculating torque of a rotating shaft based on its torsional displacement (circumferential twist).  
         [0015]    [0015]FIG. 4 is a diagram illustrating, inter alia, a cross sectional view of a rotating shaft in a combined cycle configuration whose torque is measured in accordance with an alternative exemplary embodiment of the present invention.  
         [0016]    [0016]FIG. 5 is a perspective view of the combined cycle configuration illustrated in FIG. 4 (viewed from the reverse side of FIG. 4).  
         [0017]    [0017]FIG. 6 is a diagram illustrating, inter alia, a cross sectional view of a rotating shaft in a simple cycle configuration whose torque is measured in accordance with another alternative exemplary embodiment of the present invention.  
         [0018]    [0018]FIG. 7 is a cross sectional view taken from line  7 - 7  in FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 illustrates a shaft  20  that serves as a load coupling shaft in accordance with an exemplary embodiment of the invention. Shaft  20  is connected at one axial end  24   a  to shaft  42  of gas turbine  40  and connected at the other axial end  24   b  to a rotatable shaft  62  of power generator  60 . Accordingly, shaft  20  forms a portion of a simple cycle configuration in the exemplary embodiment illustrated in FIG. 1.  
         [0020]    Shaft  20  is rotated by gas turbine machine  40 . In turn, the rotational force provided by gas turbine machine  40  is transmitted to rotatable shaft  62  of power generator  60 . Rotatable shaft  62  of power generator  60  is connected to a magnet  64  which rotates with rotatable shaft  62  (and hence with shaft  20 ) within a stator (not shown) of power generator  60  to generate electric power.  
         [0021]    Shaft  20  includes a hollow area  22  and one or more passageways  26  leading to hollow area  22 . Wires  38  extend through passageways  26  and hollow area  22  to carry signals to and/or from a RF telemetry system  36 . RF telemetry system  36  is capable of rotating along with shaft  20  and transmits/receives signals to/from, for example, power generator  60  through wires  38  or wirelessly through a transmitting antenna of the RF telemetry system  36 .  
         [0022]    A pair of targets  32  and  34  are bonded on an outer surface of shaft  20 . Targets  32  and  34  may be mounted on opposite axial ends of shaft  20 . For example, in the exemplary embodiment illustrated in FIG. 1, targets  32  and  34  are separated along the axial direction by approximately 80 inches. The respective radii of the outer surface on which targets  32  and  34  are bonded are approximately 11 and 22 inches, respectively. While FIG. 1 illustrates targets  32  and  34  being bonded on the outer surface of shaft  20  at different radii, targets  32  and  34  could alternatively be mounted on an outer surface of shaft  20  at the same radii. Each of targets  32  and  34  may be formed by a pair of highly reflective tapes which are each capable of intensifying and reflecting a light signal which is incident on the tape. Each of the targets  32  and  34  may be aligned at the same circumferential position or be circumferentially offset from one another.  
         [0023]    A pair of low power laser light probes  12  and  14  are positioned at an angle which is perpendicular to shaft  20 . Laser light probes  12  and  14  may be made of fiber optic cables for transmitting and receiving laser light signals. The tips of laser light probes  12  and  14  which are closest to shaft  20  are approximately 0.05 inches from the outer surface of shaft  20 . Laser light probes  12  and  14  are aligned in the same axial planes as targets  32  and  34 , respectively.  
         [0024]    Laser light probes  12  and  14  are connected to processor  10 . Processor  10 , as will be discussed in more detail below, is capable of calculating a torsional displacement (circumferential twist) of rotating shaft  20  based upon measurements taken by laser light probes  12  and  14  and calculating a torque imposed on shaft  20  based on its torsional displacement. Processor  10 , may be implemented by, for example, General Electric Aircraft Engine (GEAE) digital light probe system.  
         [0025]    Target  33  is bonded on an outer surface of shaft  20  and may be formed by a metal. Like targets  32  and  34 , target  33  rotates along with shaft  20 . Target  33  rotates underneath probe  13  once per revolution of shaft  20 . Probe  13  may be, for example, an eddy current probe which detects the presence of (metal) target  33 . A signal from probe  13  is triggered and sent to processor  10  once during every revolution of shaft  20  as target  33  passes underneath and is detected by probe  13 . The trigger signal provided from probe  13  enables processor  10  to establish a reference zero timing for signals received by laser probes  12  and  13  in every revolution of shaft  20 . Accordingly, a time measured from the reference zero time to the time laser probe  12  or  14  receives a signal is started when probe  13  transmits a trigger signal to processor  10  in every revolution. In cooperation with target  33 , probe  13  thus forms a “one per revolution sensor.” The operation of probe  13  and target  33  also provide the necessary information to allow processor  10  to calculate the rotational speed of shaft  20 . Specifically, the rotational speed of shaft  20  may be determined by ω=2×n×(1/time difference between two consecutive trigger signals sent from probe  13 ).  
         [0026]    In operation, gas turbine  40  will rotate shaft  20 , which will in turn rotate shaft  62  of power generator  60 . The rotation of shaft  62  enables magnet  64  to rotate within a stator of power generator  60  to generate electric power.  
         [0027]    As shaft  20  rotates, targets  32  and  34  will once pass underneath laser light probes  12  and  14  upon every revolution of shaft  20 . The laser light signals transmitted by laser light probes  12  and  14  will be incident on targets  32  and  34 , respectively, as those targets  32  and  34  pass underneath probes  12  and  14 . Targets  32  and  34  will intensify and reflect the transmitted laser light signals incident on targets  32  and  34 . The reflected laser light signals, which effectively form response laser light signals (i.e., laser light signals formed in response to the transmitted laser light signals incident on targets  32 ,  34 ) are received by laser light probes  12  and  14  which then send corresponding signals to processor  10 . Processor  10  determines and records the precise time at which the laser light signal reflected from target  32  is received by probe  12  and the precise time at which the laser light signal reflected from target  34  is received at probe  14 . The difference between the respective reception times of the reflected laser light signals by probes  12  and  14  may then be detected. For example, a difference of time of as small as approximately 10 nanoseconds may be detected.  
         [0028]    The difference in time between the laser light signal receptions by probes  12  and  14  will change as different levels of torque is applied to rotating shaft  20 . After processor  10  has determined the difference in time, processor  10  can then determine an angular torsional displacement of shaft  20 . As an example, the torsional displacement measured in radians may be calculated, assuming the circumferential positions of targets  32  and  34  on shaft  20  are the same (i.e., targets  32  and  34  are circumferentially aligned), by multiplying (Δt×ω) where Δt is the time difference between the receptions of laser light signals by probes  12  and  14  and ω is the rotational speed of shaft  20 . The rotational speed ω of shaft  20  may be determined from the operation of probe  13  and target  33  as discussed above.  
         [0029]    [0029]FIGS. 2A and 2B are diagrams illustrating the reception of laser light response signals received by laser light probes  12  and  14  resulting from laser light signals transmitted from laser light probes  12  and  14  being reflected by targets  32  and  34 , respectively, when two different levels of torque are imposed on rotating shaft  20  (again assuming that targets  32  and  34  have the same circumferential position). In particular, FIG. 2A is a diagram which illustrates laser light signals received by laser light probes  12  and  14  when no (measurable) torque is imposed on rotating shaft  2 . As can be seen from FIG. 2A, the times at which the respective laser light signals are received by laser light probes  12  and  14  are simultaneous. Accordingly, there is no torsional displacement on shaft  20  (i.e., shaft  20  has not been twisted) as a result of the rotational force imposed on the shaft  20  since Δt, the time difference between receptions of laser light signals by laser light probes  12  and  14 , is 0 seconds. Of course, if targets  32  and  34  are bonded to shaft  20  at circumferentially offset positions, a time difference which depends at least on the rotational speed of shaft  20  would be expected when there is no torsional displacement of shaft  20 .  
         [0030]    In contrast to FIG. 2A, FIG. 2B is a diagram illustrating laser light signals received by laser light probes  12  and  14  when a measurable torque is imposed on shaft  20 . In particular, because of the torque imposed on shaft  20 , shaft  20  will have a torsional displacement (i.e., circumferential twist). Targets  32  and  34  which were previously circumferentially aligned therefore become circumferentially offset from one another so that the respective laser light signals reflected by targets  32  and  34  are received by laser light probes  12  and  14  at different times. This difference in time Δt may be multiplied by the rotational speed of the shaft ω to calculate the torsional displacement in radians.  
         [0031]    As illustrated generally in FIGS.  3 A- 3 C, processor  10  may then calculate the torque imposed on rotating shaft  20  based on its calculated torsional displacement in a highly accurate manner (e.g., with +0.5%). For example, the torque may be calculated from the torsional displacement using a finite element model analysis. Power generated by gas turbine  40  may be determined based on the calculated torque.  
         [0032]    In particular, torque on shaft  20  may be calculated from the torsional displacement as follows. If shaft  20  comprises a uniform material at a constant temperature and its cross-sectional area is uniform and constant over its entire length, then torque may be calculated using the closed form solution:  
       τ   =         (   θ   )          (   G   )          (   j   )         (   L   )                             
 
         [0033]    where τ=torque on shaft, θ=torsional displacement in radians (angle change measured by probes  12 ,  14  and calculated by processor  10 ), G=shear modulus of the material of shaft  20  (available in engineering handbooks), j=polar moment of inertia and L=axial distance between probes  12  and  14 . The polar moment of inertia (j) is the inherent stiffness of shaft  20  and can be calculated by  
       j   =         (   π   )          (     R   4     )       2                           
 
         [0034]    for a solid circular cross section where R=radius of shaft  20 .  
         [0035]    The torque calculation becomes more complex to precisely determine if any one or more of the following occur:  
         [0036]    (1) Shear modulus (G) changes along the length and/or radial direction (e.g., due to temperature changes of the shaft material or use of a different material).  
         [0037]    (2) If the cross-sectional area of shaft  20  is not uniform (e.g., keyway notch)  
         [0038]    (3) If the cross-sectional area is not constant along the length of shaft  20 .  
         [0039]    Items (2) and (3) affect the polar moment of inertia (j) calculation. While a combination of shaft design features (items (1) and (3) above) make it virtually impossible to accurately convert torsional displacement to torque using hand calculations (see FIG. 3A), Finite Element Analysis (FEA) can be utilized to accurately to make this calculation with great precision. Specifically, a Finite Element Model (FEM) is created that captures the shaft geometry, material properties, and boundary conditions. A necessary boundary condition is an arbitrary torque load applied parallel to the shaft centerline. The FEA is performed on the FEM and the result is a distribution of torsional displacement along shaft  20  as can be seen in FIG. 3B. The amount of torsional displacement between the two axially spaced probes  12  and  14  is readily available by FEA post processing. This is accomplished by taking the arbitrary torque value used in the FEM and dividing it by the calculated torsional displacement value determined from processor  10 . This is the constant that relates torsional displacement to torque as shown in FIG. 3C. Thus, the torque carried by shaft  20  in operation can be calculated by taking the torsional displacement determined by processor  10  and multiplying by the FEA calculated constant.  
         [0040]    While shaft  20  illustrated in the exemplary embodiment of FIG. 1 is rotated by a gas turbine  40 , those skilled in the art will appreciate that shaft  20  may alternatively be rotated by another machine such as a steam turbine, nuclear power generator or internal combustion engine. Moreover, although shaft  20  transmits the rotational force exerted on it from gas turbine  40  to rotate a magnet  64  in power generator  60 , those skilled in the art will appreciate that shaft  20  can be alternatively connected to drive other loads. For example, shaft  20 , once rotated by a machine such as turbine  40 , can be used to drive other loads such as rotating a propeller on a vehicle.  
         [0041]    FIGS.  4 - 5  illustrate another exemplary embodiment of the present invention. Reference numbers corresponding to parts previously described for previous embodiments will remain the same. Only the differences from previous embodiments will be discussed in detail. While FIG. 1 illustrates shaft  20  as part of a simple cycle configuration, FIGS.  4 - 5  illustrate shaft  20  as part of a combined cycle configuration. Specifically, shaft  20  illustrated in FIGS.  4 - 5  is rotated by gas turbine  40  while steam turbine  50  imposes a rotational force on shaft  62  of power generator  60 . Axial end  24   a  of shaft  20  is connected to shaft  42  of gas turbine  40  and axial end  24   b  of shaft  20  is connected to shaft  52  of steam turbine  50 . Gas turbine  40  rotates shaft  42  to rotate shaft  20  and, in turn, shaft  20  rotates shaft  52  of steam turbine  50 . Thus, the torque imposed on shaft  20  by gas turbine  40  is transmitted to shaft  52  which then imposes a torque on shaft  62 . Shaft  62  is thus subject to the combined rotational forces from steam turbine  50  and gas turbine  40 . Magnet  64  of power generator  60  thus rotates as a result of rotational forces provided by steam turbine  50  and gas turbine  40 .  
         [0042]    As discussed in the exemplary embodiment of the FIG. 1, as shaft  20  is rotated by gas turbine  40 , laser light signals transmitted from laser light probes  12  and  14  are reflected by targets  32  and  34 , respectively, as they revolve and pass underneath probes  12  and  14 . The laser light signals reflected from targets  32  and  34  are received by laser light probes  12  and  14  and their respective times of arrival measured. Processor  10  then calculates the difference in the time at which laser light signals are received by laser light probes  12  and  14  to determine a torsional displacement and then determines a torque imposed on shaft  20  based upon its torsional displacement. Power generated by gas turbine  40  can be calculated from the determination of torque.  
         [0043]    FIGS.  6 - 7  illustrate another exemplary embodiment of the present invention. Again, reference numbers corresponding to parts previously described for pervious embodiments will remain the same. Only the differences from previous embodiments will be discussed in detail. FIGS.  6 - 7  illustrate multiple targets passing underneath each of light probes  12 ,  14 . Specifically, two (or more) targets  32 ,  32   a  pass underneath light probe  12  and two (or more) targets  34 ,  34   a  pass underneath light probe  14  upon rotation of shaft  20 .  
         [0044]    As shaft  20  twists when it is loaded, targets  32  and  34  will be displaced from one another as discussed above. These targets  32  and  34  will also be displaced from one another if shaft  20  vibrates. The displacement from shaft vibration can be measured through the use of additional targets  32   a  and  34   b . By assessing the time of arrival of at least one of the sets of targets  32  and  32   a  (or  34  and  34   a ) within one revolution of shaft  20  and comparing it to the expected time of arrival based on the actual distance between the targets  32  and  32   a  and the rotational speed of shaft  20 , the displacement from vibration can be calculated. For example, if targets  32  and  32   a  are circumferentially offset from one another by 180° (see FIG. 7), the respective times of arrival of signals detected by probe  12  is expected to be one-half of the time required for one complete rotation. The time for a complete rotation may be determined through the operation of probe  13  and target  33  as discussed above. The displacement of shaft  20  due to its vibration may then be determined by the difference between the expected time difference and the actual time difference that respective response signals from targets  32  and  32   a  are detected by probe  12  and/or the difference between the expected time difference and the actual time difference that respective response signals from targets  34  and  34   a  are detected by laser light probe  14 . The total torsional displacement may thus be determined by adding the displacement caused by the vibration and the load displacement (i.e., the torsional displacement caused by the rotational force imposed on shaft  20 ). Accordingly, by bonding additional targets  32   a  and/or  34   a  to shaft  20  and detecting response signals therefrom utilizing laser probes  12  and/or  14 , a correctional value may be determined for the torsional displacement resulting from the rotational force imposed on shaft  20 . Accuracy in the torsional displacement measurement may therefore be enhanced.  
         [0045]    While FIGS.  6 - 7  illustrate adding additional targets  32   a ,  34   a  onto shaft  20  as part of a simple cycle configuration, those skilled in the art will appreciate that targets  32   a ,  34   a  may also be added to a shaft  20  as part of a combined cycle configuration as illustrated in FIGS.  4 - 5 .  
         [0046]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.