Patent Publication Number: US-2022213804-A1

Title: Drive system health monitor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/812,644, filed Mar. 9, 2020, which claims the benefit of U.S. patent application Ser. No. 15/788,020 filed Oct. 19, 2017, and issued as U.S. Pat. No. 10,590,796, issued Mar. 17, 2020, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The subject matter disclosed herein generally relates to measurement systems and, more particularly, to a method and an apparatus for drive system health monitoring of a gas turbine engine. 
     Gas turbine engines typically include a compressor, a combustor, and a turbine, with an annular flow path extending axially through each. Initially, air flows through the compressor where it is compressed or pressurized. The combustor then mixes and ignites the compressed air with fuel, generating hot combustion gases. These hot combustion gases are then directed from the combustor to the turbine where power is extracted from the hot gases by causing blades of the turbine to rotate. The rotation also drives rotation of a fan that provides thrust under various operating conditions. 
     Multiple drive shafts may be used to link rotation of various stages of the turbine, compressor, and fan. Monitoring systems can be used to measure conditions within a gas turbine engine for monitoring degradation that may lead to a future servicing event, as well as identify maintenance conditions. Engine vibrations are typically monitored using accelerometers that can detect vibrations in one or more axis. However, accelerometers may not readily detect all desired conditions of rotating components that can be monitored for potential maintenance events. 
     BRIEF DESCRIPTION 
     According to one embodiment a drive system of a gas turbine engine includes a first drive shaft and a second drive shaft operable to rotate within the gas turbine engine, a first sensor operable to detect rotation of the first drive shaft, a second sensor operable to detect rotation of the second drive shaft, a drive gear system coupled to the first drive shaft and the second drive shaft, and a processing system coupled to the first sensor and the second sensor. The processing system is operable to determine a timing variation based on output of the first sensor and output of the second sensor, determine a torsional deflection between the first drive shaft and the second drive shaft based on the timing variation, and detect a health status of the drive system based on the torsional deflection. The health status identifies whether a fault condition of the drive gear system is detected based on the torsional deflection. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the processing system is operable to perform a frequency domain analysis based on output of the first sensor. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the processing system is operable to identify a dominant mode as a shaft frequency of the first drive shaft and a lower amplitude frequency domain component as a torsional mode of the first drive shaft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the shaft frequency is used to identify an operating mode of the gas turbine engine, and trending of the torsional mode is determined based on the operating mode of the gas turbine engine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the processing system is operable to perform a frequency domain analysis based on output of the second sensor and identify a dominant mode as a shaft frequency of the second drive shaft and a lower amplitude frequency domain component as a torsional mode of the second drive shaft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the shaft frequency is used to identify an operating mode of the gas turbine engine, and trending of the torsional mode is determined based on the operating mode of the gas turbine engine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the processing system is operable to track the timing variation between a first position indicator associated with the first drive shaft and a second position indicator associated with the second drive shaft. 
     According to another embodiment, a gas turbine engine includes a drive system including a first drive shaft operable to drive a fan of the gas turbine engine and a second drive shaft operable to be driven by a turbine of the gas turbine engine. The gas turbine engine also includes a first sensor operable to detect rotation of the first drive shaft, a second sensor operable to detect rotation of the second drive shaft, and a processing system coupled to the first and second sensors. The processing system is operable to identify an operating mode of the gas turbine engine, determine a trend of a torsional mode of the drive system based on the operating mode of the gas turbine engine, and detect a health status of the gas turbine engine based on the torsional mode. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the processing system is operable to perform a frequency domain analysis based on output of the first sensor and identify a dominant mode as a shaft frequency of the first drive shaft and a lower amplitude frequency domain component as a torsional mode of the first drive shaft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the shaft frequency is used to identify the operating mode of the gas turbine engine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the processing system is operable to perform a frequency domain analysis based on output of the second sensor and identify a dominant mode as a shaft frequency of the second drive shaft and a lower amplitude frequency domain component as a torsional mode of the second drive shaft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where the shaft frequency is used to identify the operating mode of the gas turbine engine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where a drive gear system is coupled between the first drive shaft and the second drive shaft, and the health status identifies whether a fault condition of the drive gear system is detected. 
     According to a further embodiment, a method of monitoring a drive system in a gas turbine engine is provided. The method includes detecting rotation of a first drive shaft via a first sensor operably coupled to a processing system, detecting rotation of a second drive shaft via a second sensor operably coupled to the processing system, identifying an operating mode of the gas turbine engine, and determining, by the processing system, a trend of a torsional mode of the drive system based on the operating mode of the gas turbine engine. The method also includes detecting a health status of the drive system based on the torsional mode. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include performing, by the processing system, a frequency domain analysis based on output of the first sensor. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include identifying a dominant mode as a shaft frequency and a lower amplitude frequency domain component as a torsional mode of the first drive shaft, where the shaft frequency is used to identify the operating mode of the gas turbine engine. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include performing a frequency domain analysis based on output of the second sensor and identify a dominant mode as a shaft frequency of the second drive shaft and a lower amplitude frequency domain component as a torsional mode of the second drive shaft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where a timing variation is tracked based on a first position indicator associated with the first drive shaft and a second position indicator associated with the second drive shaft. 
     In addition to one or more of the features described above or below, or as an alternative, further embodiments may include where a drive gear system is coupled between the first drive shaft and the second drive shaft, and the health status identifies whether a fault condition of the drive gear system is detected. 
     A technical effect of the apparatus, systems and methods is achieved by using one or more speed sensors to determine torsional modes of a drive system in a gas turbine engine as described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
         FIG. 1  is a partial cross-sectional illustration of a gas turbine engine, in accordance with an embodiment of the disclosure; 
         FIG. 2  is a schematic illustration of a drive system of a gas turbine engine, in accordance with an embodiment of the disclosure; 
         FIG. 3  is a frequency response plot of speed sensor data, in accordance with an embodiment of the disclosure; 
         FIG. 4  is a timing diagram of phonic wheel pulse trains, in accordance with an embodiment of the disclosure; 
         FIG. 5  is a block diagram of a processing system, in accordance with an embodiment of the disclosure; and 
         FIG. 6  is a flow chart illustrating a method, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
       FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flow path B in a bypass duct, while the compressor section  24  drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The exemplary engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided, and the location of bearing systems  38  may be varied as appropriate to the application. 
     The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a speed change mechanism, which in exemplary gas turbine engine  20  is illustrated as a geared architecture  48  to drive the fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure compressor  52  and high pressure turbine  54 . A combustor  56  is arranged in exemplary gas turbine  20  between the high pressure compressor  52  and the high pressure turbine  54 . An engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The engine static structure  36  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
     The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. It will be appreciated that each of the positions of the fan section  22 , compressor section  24 , combustor section  26 , turbine section  28 , and fan drive gear system  48  may be varied. For example, gear system  48  may be located aft of combustor section  26  or even aft of turbine section  28 , and fan section  22  may be positioned forward or aft of the location of gear system  48 . 
     The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about five 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present disclosure is applicable to other gas turbine engines including direct drive turbofans. 
     A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet (10,688 meters). The flight condition of 0.8 Mach and 35,000 ft (10,688 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)]{circumflex over ( )}0.5. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 m/sec). 
     Referring now to  FIG. 2 , a drive system  100  of the gas turbine engine  20  of  FIG. 1  is depicted. In the example of  FIG. 2 , the drive system  100  includes a first drive shaft  102   a  coupled between the fan  42  and the fan drive gear system  48  (also referred to as a drive gear system  48 ). The drive system  100  also includes a second drive shaft  102   b  coupled between the low pressure turbine  46  and the fan drive gear system  48  such that at least one drive shaft  102   a,    102   b  is operable to rotate within the gas turbine engine  20 . A first phonic wheel  106   a  is coupled to the first drive shaft  102   a,  and a second phonic wheel  106   b  is coupled to the second drive shaft  102   b.  A first speed sensor  108   a  is operable to detect rotation of the first phonic wheel  106   a  indicative of rotation of the first drive shaft  102   a.  Similarly, a second speed sensor  108   b  is operable to detect rotation of the second phonic wheel  106   b  indicative of rotation of the second drive shaft  102   b.  The first phonic wheel  106   a  may also be referred to as a fan phonic wheel, including a number of teeth  110   a.  Similarly, the second phonic wheel  106   b  may also be referred to as a low pressure turbine phonic wheel, including a number of teeth  110   b.  In some embodiments, the number of teeth  110   a  of the first phonic wheel  106   a  is the same as the number of teeth  110   b  of the second phonic wheel  106   b.  In other embodiments, the number of teeth  110   a  of the first phonic wheel  106   a  is different than the number of teeth  110   b  of the second phonic wheel  106   b.    
     The teeth  110   a  of the first phonic wheel  106   a  may induce a detectable signal at the first speed sensor  108   a  as each tooth passes in close physical proximity to the first speed sensor  108   a  (e.g., through electro-magnetic communication). Similarly, teeth  110   b  of the second phonic wheel  106   b  may induce a detectable signal at the second speed sensor  108   b  as each tooth passes in close physical proximity to the second speed sensor  108   b.  The teeth  110   a  of the first phonic wheel  106   a  and the teeth  110   b  of the second phonic wheel  106   b  can be substantially regularly spaced. However, one of the teeth  110   a  of the first phonic wheel  106   a  and one of the teeth  110   b  of the second phonic wheel  106   b  can be physically offset or be physically extended to create a detectable position indicator for each of the first and second phonic wheel  106   a,    106   b,  e.g., a once-per-revolution indicator. 
     In the example of  FIG. 2 , the first drive shaft  102   a  and the second drive shaft  102   b  are mechanically coupled through the fan drive gear system  48 . Rotation of low pressure turbine  46  drives rotation of the second drive shaft  102   b  and drives rotation of the first drive shaft  102   a  through the fan drive gear system  48  to rotate the fan  42 . A gear ratio of the fan drive gear system  48  can result in the first drive shaft  102   a  rotating at a slower speed than the second drive shaft  102   b.  Thus, a phonic wheel pulse train induced by rotation of the first phonic wheel  106   a  and detected by the first speed sensor  108   a  can transition slower than a phonic wheel pulse train induced by rotation of the second phonic wheel  106   b  and detected by the second speed sensor  108   b  as both the first and second drive shafts  102   a,    102   b  rotate. 
     In embodiments, a processing system  112  is coupled to at least one of the first and second speed sensors  108   a,    108   b.  In the example of  FIG. 2 , the processing system  112  is coupled to both the first and second speed sensors  108   a,    108   b.  The processing system  112  is operable to detect one or more phonic wheel pulse trains, determine a torsional mode based on the one or more phonic wheel pulse trains, and record one or more trends of the torsional mode indicative of a health status of the drive system  100 . The processing system  112  may use one or more signal processing techniques to determine torsional modes based on speed sensor signals from the first speed sensor  108   a  and/or the second speed sensor  108   b.    
     As an example, the processing system  112  can perform individual shaft torsional analysis on a per shaft basis by separately analyzing data from each of the first speed sensor  108   a  and the second speed sensor  108   b.  The processing system  112  can perform a frequency domain analysis of a phonic wheel pulse train from the first speed sensor  108   a  or the second speed sensor  108   b  and identify a dominant mode  302  as a shaft frequency and a lower amplitude frequency domain component  304  as a torsional mode as depicted in the frequency response plot  300  of  FIG. 3 . The dominant mode  302  tracks with respect to rotational speed of the respective drive shaft (e.g., the first drive shaft  102   a  or the second drive shaft  102   b ) being monitored. The torsional mode has a lower amplitude than the dominant mode  302  and does not directly correlate to drive shaft rotational speed. The torsional mode represents a resonance due to oscillations in shaft loading. Frequency content in speed sensor signals, such as that depicted in frequency response plot  300 , can be produced using a Fourier transform, a wavelet-based transform, or other known techniques. The same or similar frequency analysis can be performed with respect to the first drive shaft  102   a  and the second drive shaft  102   b.    
     The shaft frequency can be used to identify an operating mode of the gas turbine engine  20 , and trending of the torsional mode can be performed based on the operating mode of the gas turbine engine  20 . For example, a speed range and transition sequence between speed ranges (as well as one or more other parameters) can be used to identify whether the gas turbine engine  20  is operating at ground idle, flight idle, max cruise, take-off, max power, or another known operating mode. The processing system  112  can collect a buffer of speed sensor values over a collection period for each of the first and second speed sensors  108   a,    108   b.  The buffered speed sensor data can be analyzed to determine whether the collection period was substantially steady state and did not include an operating mode transition, for instance, the speed did not vary by more than a predetermined steady state threshold. If the collected speed data was determined to be steady state, the torsional mode can be determined, for instance, using a frequency domain transform as previously described. The torsional mode can be tracked with respect to the identified operating mode of the gas turbine engine  20 . Trending of changes in the torsional mode can be performed on an operating mode basis to determine whether changes are indicative of increased shaft fatigue. For example, a rate of change of the torsional mode above a change threshold may be used to trigger an indicator, such as a health status. The health status can be set to initiate one or more actions, such as an inspection event, a maintenance event, additional monitoring events, and/or other events internal or external to the gas turbine engine  20 . Further, unlike typical accelerometer based vibration monitoring systems, the torsional mode is not located at the shaft frequency where the dominant mode  302  is located and can be independent of the precise value of the shaft frequency. 
     Although the frequency response plot  300  is depicted with only two frequency components, it will be understood that additional frequency components (not depicted) may also be captured in the frequency response plot  300 . For example, there may be spectral spreading and/or harmonics depending upon filtering and alignment between spectral bins of the frequency domain transform and the actual frequencies observed. 
     As another example, the processing system  112  can observe a first phonic wheel pulse train  402  and a second phonic wheel pulse train  404  and track a timing variation between a first position indicator  406  of the first phonic wheel  106   a  and a second position indicator  408  of the second phonic wheel  106   b  as depicted in the timing diagram  400  of  FIG. 4 . In the example of  FIG. 4 , the first position indicator  406  is a rising edge of an offset tooth of the first phonic wheel  106   a,  and the second position indicator  408  is a rising edge of an offset tooth of the second phonic wheel  106   b.  A time difference  410  between the first and second position indicator  406 ,  408  can be tracked for variations indicative of torsional resonance through the fan drive gear system  48  of  FIG. 2 . The processing system  112  can determine a torsional deflection between the first drive shaft  102   a  with respect to the second drive shaft  102   b  based on the timing variation between the first position indicator  406  of the first phonic wheel  106   a  and the second position indicator  408  of the second phonic wheel  106   b.  As the gear ratio of the fan drive gear system  48  results in different rotational speeds of the first and second phonic wheel  106   a,    106   b,  multiple revolutions of the first and second phonic wheel  106   a,    106   b  can be tracked in groups to account for relative positional variations. For example, if the gear ratio of the fan drive gear system  48  results in a 2.3:1 speed reduction, then 10 revolutions of the first drive shaft  102   a  corresponds with 23 revolutions of the second drive shaft  102   b,  and the expected alignment positions can be compared to the observed alignment positions. However, the expected alignment positions need not be used, as the pattern of changes in the time difference  410  at a particular speed can be tracked to monitor torsional deflection. Similar to the frequency domain example of  FIG. 3 , the torsional deflection can be tracked with respect to the operating mode of the gas turbine engine  20 , and the health status can be set to identify whether a fault condition of the drive gear system  48  is detected based on the torsional deflection. 
     Referring now to  FIG. 5 , an example of the processing system  112  is shown in greater detail. The processing system  112  includes a memory  502  which can store executable instructions and/or data associated with control and/or diagnostic/prognostic systems of the gas turbine engine  20  of  FIG. 1 . The executable instructions can be stored or organized in any manner and at any level of abstraction, such as in connection with one or more applications, processes, routines, procedures, methods, etc. As an example, at least a portion of the instructions are shown in  FIG. 5  as being associated with a control program  504 . 
     Further, as noted, the memory  502  may store data  506 . The data  506  may include, but is not limited to, values to support detecting the operating mode of the gas turbine engine  20 , commands for various actuators, lookup tables, sensor data, communication data, or any other type(s) of data as will be appreciated by those of skill in the art. One or more speed sensor buffer  516  and/or torsional mode trends  518  can be stored in the memory  502  and may be part of or separate from the data  506 . The instructions stored in the memory  502  may be executed by one or more processors, such as a processor  508 . The processor  508  may be operative on the data  506 , speed sensor buffer  516 , and/or torsional mode trends  518 . 
     The processor  508  can be any type or combination of computer processors, such as a microprocessor, microcontroller, digital signal processor, application specific integrated circuit, programmable logic device, and/or field programmable gate array. The memory  502  is an example of a non-transitory computer readable storage medium tangibly embodied in or operably connected to the processing system  112  including executable instructions stored therein, for instance, as firmware. 
     The processor  508 , as shown, is coupled to one or more input/output (I/O) devices through an I/O interface  510 . For example, the I/O interface  510  can be operable to receive speed sensor signals  512   a,    512   b  from the first and second speed sensors  108   a,    108   b  of  FIG. 2 . The processor  508  can also communicate with one or more other systems (not depicted) using a communication interface  514  to send and receive messages on one or more communication buses  515 , which may include transmitting a health status  517  based on torsional values captured in the torsional mode trends  518 . The processing system  112  may further include other features or components as known in the art. For example, the processing system  112  may include one or more transceivers and/or devices configured to transmit and/or receive information or data from sources external to the processing system  112 . For example, in some embodiments, the processing system  112  may be configured to receive information over a network (wired or wireless) or through a cable or wireless connection with one or more devices remote from the processing system  112  via the communication interface  514 . The information received can stored in the memory  502  (e.g., as data  506 ) and/or may be processed and/or employed by one or more programs or applications (e.g., program  504 ) and/or the processor  508 . 
     The processing system  112  can also include one or more counters  520  and/or timers  522 . The counters  520  can be used, for example, to track the teeth  110   a,    110   b  of the first and second phonic wheel  106   a,    106   b  and assist in determining the location of the first and second position indicator  406 ,  408 , for instance, based on tooth-to-tooth timing variations observed via timers  522 . The timers  522  can also support observations of the time difference  410  between the first and second position indicator  406 ,  408 . 
     Although the processing system  112  is depicted as a single system, it will be understood the portions of the processing system  112  can be distributed between multiple processing circuits, including multiple instances of the processor  508 , memory  502 , and the like. 
     Referring now to  FIG. 6  with continued reference to  FIGS. 1-5 .  FIG. 6  is a flow chart illustrating a method  500  for health monitoring of the drive system  100  in a gas turbine engine  20 , in accordance with an embodiment. At block  602 , a phonic wheel pulse train indicative of rotation of at least one drive shaft is detected via a speed sensor operably coupled to processing system  112 , such as the first phonic wheel pulse train  402  of the first drive shaft  102   a  detected by the first speed sensor  108   a  or the second phonic wheel pulse train  404  of the second drive shaft  102   b  detected by the second speed sensor  108   b.    
     At block  604 , the processing system  112  determines a torsional mode of the at least one drive shaft based on the phonic wheel pulse train. The processing system  112  may perform a frequency domain analysis of the one or more phonic wheel pulse trains as previously described in reference to  FIG. 3 . The frequency domain analysis can include identifying a dominant mode  302  as a shaft frequency and a lower amplitude frequency domain component  304  as the torsional mode. The shaft frequency can be used to identify an operating mode of the gas turbine engine  20 . For example, the operating mode can be identified as one of: ground idle, flight idle, max cruise, take-off, max power, and any other flight condition known to one of skill in the art. In some embodiments, the processing system  112  can track a timing variation between a first position indicator  406  of the first phonic wheel  106   a  on the first drive shaft  102   a  and a second position indicator  408  of the second phonic wheel  106   b  on the second drive shaft  102   b.  A torsional deflection between the first drive shaft  102   a  with respect to the second drive shaft  102   b  can be determined based on the timing variation (e.g., changes in time difference  410 ) between the first position indicator  406  of the first phonic wheel  106   a  and the second position indicator  408  of the second phonic wheel  106   b.    
     At block  606 , one or more trends of the torsional mode indicative of a health status  517  of the drive system  100  are recorded, for instance, in torsional mode trends  518 . As previously described, trending of the torsional mode can determined based on the operating mode of the gas turbine engine  20 . The health status  517  identifies whether a fault condition of the drive gear system  48  can be detected based on the torsional deflection. 
     While the above description has described the flow process of  FIG. 6  in a particular order, it should be appreciated that unless otherwise specifically required in the attached claims that the ordering of the steps may be varied. 
     The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.