Abstract:
A structural integrity monitoring system includes a structure, at least two vibration monitoring devices mounted to the structure. Each of the at least two vibration monitoring devices outputs a vibration response signal. The structural integrity monitoring system also includes a controller operatively connected to each of the at least two vibration monitoring devices. The controller is configured to calculate a predicted vibration response based upon a vibration mode shape and the vibration response signal. The controller then compares the predicted vibration response against a measured vibration response to detect changes in the structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to the art of monitoring systems and, more particularly, to a structural integrity monitoring system. 
         [0002]    Removing a piece of equipment, such as a turbomachine, generator, or the like, from service in order to perform routine inspections, or to conduct vibration testing to assess structural integrity, is very costly. The costs accrue not only in terms of lost revenue and actual disassembly, but the inspections and tests are themselves costly. However, failure to perform the inspections, and particularly the tests, could lead to significant and hence even more expensive failures. That is, in general, turbomachines, generators, and the like, are subjected to strenuous loads, harsh operating environments and conditions that, over time, lead to deterioration of certain components. Failure to detect this deterioration can result in machine failure. Significant failures of the machine would far outweigh the costs associated with inspection and testing. 
         [0003]    One method employed to detect deterioration is to test structural integrity of system components. Online vibration monitoring is commonly used to detect structural integrity defects. However, when using online vibration monitoring, it is sometimes difficult to determine if changes in vibration (increasing or decreasing) are the result of changing loads on the structure, expected changes in structure, or the result of deterioration of the structure. Other types of structural integrity tests include modal vibration testing. 
         [0004]    Modal vibration testing of structures typically requires that the structure to be tested be removed from service. In one such testing technique, actuation devices are permanently mounted to the structure. The actuation devices are operated to mechanically excite the structure. Once excited, vibration measurements are taken and compared against baseline measurements to check the integrity of the structure. Still other forms of vibration testing include setting vibration alarm levels and shut down levels. However, such systems cannot distinguish between vibrations that occur as a result in changes in loads from vibrations that occur as a result of changes in the structure. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    According to one aspect of the invention, a structural integrity monitoring system includes a structure, at least two vibration monitoring devices mounted to the structure. Each of the at least two vibration monitoring devices outputs a vibration response signal. The structural integrity monitoring system also includes a controller operatively connected to each of the at least two vibration monitoring devices. The controller is configured to calculate a predicted vibration response based upon a vibration mode shape and the vibration response signal. The controller then compares the predicted vibration response against a measured vibration response to detect changes in the structure. 
         [0006]    According to another aspect of the invention, a method of determining structural integrity of a structure includes measuring at least one mode shape from the structure, collecting vibration data from at least two vibration sensors mounted to the structure, calculating a predicted vibration response of the structure based on the at least one measured mode shape and the collected vibration data, comparing the predicted vibration response to a measured vibration response of the structure to establish a comparison factor, detecting changes in the structure based upon the comparison factor to determine integrity of the structure, and graphically illustrating the integrity of the structure on a display. 
         [0007]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  is an upper right perspective view of a generator including a structural integrity monitoring system in accordance with an exemplary embodiment; 
           [0010]      FIG. 2  is front elevational view of the generator of  FIG. 1 ; 
           [0011]      FIG. 3  is a flow diagram illustrating a method of monitoring structural integrity in accordance with an exemplary embodiment; and 
           [0012]      FIG. 4  is a flow diagram illustrating a method of monitoring structural integrity in accordance with another exemplary embodiment 
       
    
    
       [0013]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    With reference to  FIGS. 1-2 , a structure, shown in the form of a generator, is indicated generally at  2 . Of course, it should be understood that the structure can take on a variety of forms. Generator  2  includes a housing  4  having arranged therein a stator  6  having an end winding  8  and a rotor (not shown). Stator  6  includes a plurality of vibration monitoring devices  20 - 25  mounted to end winding  8  for detecting vibrations that occur during operation of generator  2 . Additional vibration monitoring devices (not shown) are mounted to portions of stator  6  as well as other areas of generator  2 . 
         [0015]    Each vibration monitoring device  20 - 25  includes at least one vibration sensor. In the exemplary embodiment shown, vibration monitoring device  20  includes a first or radial vibration sensor  30  and a second or axial vibration sensor  31 . Similarly, vibration monitoring device  21  includes a radial vibration sensor  34  and an axial vibration sensor  35 , vibration monitoring device  22  includes a radial vibration sensor  38  and an axial vibration sensor  39 , vibration monitoring device  23  includes a radial vibration sensor  42  and an axial vibration sensor  43 , vibration monitoring device  24  includes a radial vibration sensor  46  and an axial vibration sensor  47 , and vibration monitoring device  25  includes a radial vibration sensor  50  and an axial vibration sensor  51 . Vibration monitoring devices  20 - 25  can take the form of displacement sensors, accelerometers, strain gauges, velocity sensors, and the like. 
         [0016]    In accordance with the exemplary embodiment shown, generator  2  includes a structural integrity monitoring system  60  that monitors, in real time, the structural integrity of stator  6 . Of course it should be understood that structural integrity monitoring system  60  could be employed to monitor other areas, components of generator  2 . Structural integrity monitoring system  60  includes a controller  61  operationally linked to each of the plurality of vibration monitoring devices  20 - 25 . As will be discussed more fully below, controller  61  received baseline modal shape data  64  and real time vibration monitoring data  66 . Controller  61  employs modal shape data  64  and real time vibration monitoring data  66  to determine a structural integrity indicator  74  of stator  6  at end winding  8 . Structural integrity indicator  74  is selectively output to a display  78  operatively connected to structural integrity monitoring system  60 . Based upon structural integrity indicator  74 , structural integrity monitoring system  60  activates an alarm  81  and/or a shutdown system  84 . 
         [0017]    Reference will now be made to  FIG. 3  in describing a method  200  employed by structural integrity monitoring system  60  to determine structural integrity of stator  6 . Initially, baseline modal shape data  64  is determined for stator  6  as indicated in block  204 . More specifically, a modal analysis test is performed on stator  6 . The modal analysis test includes exciting the stator  6  and measuring the vibration response signals at vibration monitoring devices  20 - 25 . Stator  6  is impacted with an instrumented hammer or an electromagnetic or hydraulic shaker that serve as a known excitation input signal. Both the known excitation input signal (force from hammer or shaker) and vibration response signal measured at vibrations monitoring device  20 - 25  are measured in the time domain. The signals are transformed into the frequency domain using a Fast Fourier transform (FFT) and divided (response/input) to create a transfer function for each vibrations monitoring device  20 - 25 . Using standard modal analysis methods, natural frequencies and mode shapes are extracted from the transfer function. The natural frequencies and mode shapes are input as mode shape data  64  to structural integrity monitoring system  60 . 
         [0018]    Once baseline data is obtained in block  204 , generator  2  is brought into service and vibration signals are captured, in real time, though vibration monitoring devices  20 - 25  as indicated in block  206 . The vibrations sensed are based on load conditions as indicated in block  208 , and other external influences such as forces due to rotor imbalance or vibrations transmitted from other adjacent components, as indicated in block  210 . The vibration signals are measured as indicated in block  220  and a one by one prediction is performed on the vibration signals as indicated in block  230 . The one-by-one prediction calculates a predicted vibrations level for each of the plurality of vibration monitoring devices  20 - 25  by eliminating, one-by one, a column of [u] and a row of [v] in the matrix outlined in Equation 1.1. A new set of coefficients [C} are calculated based on the remaining elements in the matrix. The new coefficients [C] are a prediction of the contribution from each mode in matrix [u]. The new set of coefficients [C] is used to predict the vibration level for the particular vibration monitoring device  20 - 25 , i.e., the particular element [v] eliminated. The predicted value is then compared to the corresponding measured modal shape data to vibration level in block  220  to determine a comparison factor. The use of modal shape data to predict vibration level for each element [v] eliminated is valid of any load applied or any excitation from external conditions as long as the mode shapes of the structure have not changed. A significant difference between vibration level at any sensor and measured vibration level at that sensor can only occur if the mode shapes of the structure have changes, indicating a change in the structure has occurred. This method allows for real time, continuous monitoring of structural integrity. By monitoring, in real time, structural integrity, any changes are detected early allowing for a quick response to address any required maintenance issues. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         [ 
                         
                           
                             
                               
                                 u 
                                 11 
                               
                             
                             
                               
                                 u 
                                 12 
                               
                             
                             
                               
                                 u 
                                 13 
                               
                             
                             
                               
                                 u 
                                 14 
                               
                             
                             
                               … 
                             
                             
                               
                                 u 
                                 
                                   1 
                                    
                                   n 
                                 
                               
                             
                           
                           
                             
                               
                                 u 
                                 21 
                               
                             
                             
                               
                                 u 
                                 22 
                               
                             
                             
                               
                                 u 
                                 23 
                               
                             
                             
                               
                                 u 
                                 24 
                               
                             
                             
                               … 
                             
                             
                               
                                 u 
                                 
                                   1 
                                    
                                   
                                       
                                   
                                    
                                   n 
                                 
                               
                             
                           
                           
                             
                               
                                 u 
                                 31 
                               
                             
                             
                               
                                 u 
                                 32 
                               
                             
                             
                               
                                 u 
                                 33 
                               
                             
                             
                               
                                 u 
                                 34 
                               
                             
                             
                               … 
                             
                             
                               
                                 u 
                                 
                                   1 
                                    
                                   n 
                                 
                               
                             
                           
                           
                             
                               … 
                             
                             
                               … 
                             
                             
                               … 
                             
                             
                               … 
                             
                             
                               … 
                             
                             
                               … 
                             
                           
                           
                             
                               
                                 u 
                                 
                                   m 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                             
                             
                               
                                 u 
                                 
                                   m 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                             
                             
                               
                                 u 
                                 
                                   m 
                                    
                                   
                                       
                                   
                                    
                                   3 
                                 
                               
                             
                             
                               
                                 u 
                                 
                                   m 
                                    
                                   
                                       
                                   
                                    
                                   4 
                                 
                               
                             
                             
                               … 
                             
                             
                               
                                 u 
                                 mn 
                               
                             
                           
                         
                         ] 
                       
                       × 
                       
                         [ 
                         
                           
                             
                               
                                 C 
                                 1 
                               
                             
                           
                           
                             
                               
                                 C 
                                 2 
                               
                             
                           
                           
                             
                               
                                 C 
                                 3 
                               
                             
                           
                           
                             
                               
                                 C 
                                 4 
                               
                             
                           
                           
                             
                               … 
                             
                           
                           
                             
                               
                                 C 
                                 n 
                               
                             
                           
                         
                         ] 
                       
                     
                     = 
                     
                       [ 
                       
                         
                           
                             
                               v 
                               1 
                             
                           
                         
                         
                           
                             
                               v 
                               2 
                             
                           
                         
                         
                           
                             
                               v 
                               3 
                             
                           
                         
                         
                           
                             … 
                           
                         
                         
                           
                             
                               v 
                               m 
                             
                           
                         
                       
                       ] 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         or 
                          
                         
                           
 
                         
                         [ 
                         u 
                         ] 
                       
                       × 
                       
                         [ 
                         C 
                         ] 
                       
                     
                     = 
                     
                       [ 
                       v 
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1.1 
                 
               
             
           
         
       
     
         [0019]    The comparison factor is graphically output to display  78  for evaluation. If the comparison factor indicates that the predicted value does not match the corresponding measured vibration data, then the measured mode shape no longer represents the actual vibration response indicating that stator  6  has undergone a change in structural integrity. Detecting a change in structural integrity, structural integrity monitoring system  60  sends a signal to alarm  81 . If the change in structural integrity exceeds a predetermined value structural integrity monitoring system  60  signals shut down system  84  to halt operation of generator  2 . The predetermined value that initiates either an alarm or shutdown is based upon the particular structure being monitored, operating conditions, experience, normal changes, and noise in the measured vibration signal. 
         [0020]    Reference will now be made to  FIG. 4  in describing a describing a method  300  employed by structural integrity monitoring system  60  to determine structural integrity of stator  6  in accordance with another exemplary embodiment. In a manner similar to that described above, initially, baseline modal shape data  64  is determined for stator  6  as indicated in block  304 . More specifically, a modal analysis test is performed on stator  6 . However, in contrast to the modal analysis test preformed above, the modal analysis test in accordance with the exemplary embodiment utilized the operational vibrations of generator  2 . More specifically in place of external excitation forces, generator  2  is started and load forces, as indicated in block  306  and other external influences, as indicated in block  308  are detected by vibration monitoring devices  20 - 25  as indicated in block  310 . Vibration monitoring devices output vibration response signals that are measured in the time domain as indicated in block  320 . The signals are transformed into the frequency domain using a Fast Fourier transform (FFT). Using standard output only modal analysis methods, natural frequencies and mode shapes are extracted from the measured vibration levels. The natural frequencies and mode shapes are input as mode shape data  64  to structural integrity monitoring system  60 . 
         [0021]    Once baseline data is obtained in block  304 , generator  2  is brought into service and vibration signals are captured, in real time, though vibration monitoring devices  20 - 25  as indicated in block  310 . In a manner similar to that described above, the vibrations sensed are based on the load conditions as indicated in block  306 , and other external influences, as indicated in block  308 . The vibration signals are measured as indicated in block  320  and a one by one prediction is performed on the vibration signals as indicated in block  330 . In a manner also similar to that described above, the one-by-one prediction calculates a predicted vibration level for each of the plurality of vibration monitoring devices  20 - 25  by eliminating, one-by one, a column of [u] and a row of [v] in the matrix outlined in Equation 1. A new set of coefficients [C} are calculated based on the remaining elements in the matrix. The new coefficients [C] are a prediction of contribution from each mode in matrix [u]. The new set of coefficients is used to predict the vibration level for the particular vibration monitoring device  20 - 25 , i.e., the particular element [v] eliminated. The predicted value is then compared to the corresponding measured vibration level in block  220  to determine a comparison factor. 
         [0022]    The comparison factor is graphically output to display  78  for evaluation. If the comparison factor indicates that the predicated value does not match the corresponding measured vibration level data point, then the measured mode shape no longer represents the actual vibration response indicating that stator  6  has undergone a change in structural integrity. Detecting a change in structural integrity, structural integrity monitoring system  60  sends a signal to alarm  81 . If the change in structural integrity exceeds a predetermined value, structural integrity monitoring system  60  signals shut down system  84  to halt operation of generator  2 . 
         [0023]    At this point it should be understood that the exemplary embodiments provide a system and method that provides real-time structural integrity monitoring of a structure subjected to either steady state dynamic forces or transient dynamic forces. By employing mode shape data as a structural model, external vibrations due to changing load conditions and the like are filtered out allowing structural integrity monitoring system  60  to monitor changes in the structural integrity of a structure during operation creating savings in both operational and maintenance costs. It should also be understood that while shown and described in connection with testing structural integrity of generator components, structural integrity monitoring system  60  can be employed in connection with a wide range of structures. 
         [0024]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.