Patent Publication Number: US-2023160834-A1

Title: Optical turbine engine blade damage detector

Description:
BACKGROUND 
     The present disclosure relates to a turbomachine and, more particularly, to a damage detection system and algorithm configured to inspect a rotor of the turbomachine during operation. 
     Damage or wear of turbomachinery rotors are substantial contributors to the performance and operational life of the turbomachine. Aircraft engines are turbomachines that can be damaged by foreign object debris or FOD. Large objects, for example a large bird strike, can cause significant changes that can be detected by monitoring systems, such as shaft vibration. Smaller objects can cause damage that can be difficult to detect visually. Regular inspection is performed to determine if any such damage has occurred such that maintenance can be performed. 
     SUMMARY 
     A damage detection system, in accordance with an exemplary embodiment of this disclosure, includes an emitter, a receiver, and a controller. The emitter has an orientation to emit a plurality of light pulses along an emission axis intersecting a plurality of blades of a turbomachine rotor. The receiver has a first field of view intersecting the emission axis to define a first interrogation volume through which the plurality of blades rotates during operation of the turbomachine. The controller includes a processor and computer-readable memory encoded with instructions that, when executed by the processor, cause the system to emit the plurality of light pulses from the emitter toward the plurality of blades as the blades rotate within the turbomachine. The system collects, at the receiver, a plurality of light returns scattered by the plurality of blades within the interrogation volume. The controller determines an amplitude change of the first light returns and outputs an indication of damages based on the amplitude change detected within the light returns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    and  FIG.  2    are schematic representations of exemplary damage detection systems. 
         FIG.  3 A  is an exemplary emitter that incorporates a lens to produce structured light emissions. 
         FIG.  3 B  is an exemplary emitter that utilizes a mirror to scan a surface of a blade. 
         FIG.  4    is a flow chart describing a damage detection algorithm. 
     
    
    
     DETAILED DESCRIPTION 
     As disclosed herein, a damage control system for a turbomachine includes one or more emitters, one or more receivers, and a controller. During operation of the turbomachine, rotor blades pass through an interrogation volume defined about an intersection of an emitter and receiver field of view. Emitters transmit visible and/or infrared light using a continuous beam or intermittent pulses, and receivers collect back scatter, forward scatter, and/or side scatter light returns. Light returns collected over a period of time define one or more light return amplitude profiles, which are analyzed for light amplitude changes. The controller compares light amplitude changes to one or more damage criteria and output an indication of blade damage when the light amplitude change satisfies the damage detection criteria. Accordingly, the damage detection system monitors a turbomachine rotor in situ and during operation of the turbomachine to identify blade damage that would otherwise be undetected by analyzing primary turbomachine parameters such as rotational speed, operational pressures and temperatures, and vibration. 
       FIG.  1    is a schematic representation of damage detection system  10  installed onto turbomachine  12  to inspect rotor  14 , which includes multiple blades  16  arranged circumferentially about rotational axis  18 . Damage detection system  10  includes at least one emitter  20 , at least one receiver  22 , and controller  24 . Emitter  20  and receiver  22  are mounted to turbomachine  12  within a field of view of rotor  14  and arranged such that emitter line of sight  26  (or emission axis) intersects receiver line of sight  28  to define interrogation volume  30  about the intersection. 
     As shown, turbomachine  12  is a bypass gas turbine engine for an aircraft and rotor  14  is a fan rotor. However, the technique disclosed herein are applicable to other aircraft engine types, such as turboprop engines and turboshaft engines, as well as non-aircraft applications including industrial gas turbine engines and other turbomachinery. 
     Receiver  22  can be co-located with emitter  20  or mounted near emitter  16  such that receiver  22  intercepts light scattered in a direction opposite an emission direction (i.e., back scatter). In other arrangements, receiver  22  can be spaced from emitter  20  such that receiver  22  intercepts light scattered in a direction along an emission direction (i.e., forward scatter). When receiver  22  is positioned at right angles to the emission direction, receiver  22  collects light returns scattered at or near right angles to the emission direction (i.e., side scatter). Between back scatter and side scatter locations and between side scatter and forward scatter receiver positions, different proportions of back scatter, forward scatter, and side scatter light returns can be collected by receiver  22 . Typically, light returns forming an angle between  135  degrees and  180  degrees with the emission direction are considered to be backscatter. Light returns that form an angle between 45 and 135 degrees relative to the emission direction are classified as side scatter while returns parallel to the emission direction or forming an angle between 0 degrees and 45 degrees of the emission direction are considered forward scatter. 
     Other factors can affect the proportion of back scatter, side scatter, and forward scatter at a given location such as surface roughness, surface features, discontinuities, and/or other surface topography. Accordingly, while receivers  22  may initially receive predominately back scatter, side scatter, or forward scatter light returns, the type of light returns collected by receiver  22  can change based on a change in surface features. For instance, wear of blades  16  or damage to blades  16  from foreign object debris (FOD) may cause a change to blade topography that produces a commensurate change in back scatter, forward scatter, and/or side scatter amplitude. 
     In other implementations of damage detection system  10 , multiple emitters  20  and/or receivers  22  are used to inspect rotor  14  of turbomachine  12 . In one instance, a single emitter  20  can be associated with multiple receivers  22 , each receiver  22  arranged to intercept light scattered by rotor  14  that originates from emitter  20 . As shown in  FIG.  1   , receivers  22 A,  22 B, and  22 C are positioned to receive light returns from emitter  20 . Receiver  22 A is collocated with emitter  20  to receive primarily backscatter light returns. Receiver  22 B is positioned along the emission direction to receive forward light scatter returns. Receiver  22 C has an intermediate position between receiver  22 A and receiver  22 B to receive side scatter returns. 5 In yet another embodiment, each independent channel of a multi-channel receiver  22  (or line camera) can interrogate a different radial station of blades  16  in order to collect light returns from a single emitter  20  or multiple emitters  20 . 
     Each emitter  20  of detection system  10  can be, for example, a light emitting diode (LED) or a visible and/or infrared laser. Accordingly, light produced by emitters  20  can be incoherent or coherent. Receivers  22  are photoelectric sensors that detect light amplitude scattered by a target (e.g., blade  16 ) and output a signal representative of the collected light amplitude. In some embodiments, receivers  22  can be avalanche photo-diodes (APD) and/or multi-channel photon counters (MPPC). Additionally, receivers  22  can be used in conjunction with emitters  20  to determine a time of flight of light collected at receivers  22 . Further, emitters  20  can transmit light continuously or intermittently as light pulses. Where emitters  20  produce light pulses, the light pulse frequency can be varied to increase or decrease inspection resolution as necessary for the particular application. For example, shorter light pulses illuminate a smaller area of blade  16  when blades  16  are rotating at speed, which can increase sensitivity for smaller surface features of blades  16 . Further, shorter light pulses allow receivers  22  to attain higher sampling rates, if desired. Applied to rotor  14  of turbomachine  12  in operation, the light pulse frequency can be at least a multiple of a product equal to a rotational speed of rotor  14  times a number of blades  16  assembled onto rotor  14  in order to ensure multiple observations of each blade  16 . In other embodiments, the light pulse frequency of one or more emitters  20  can be less than the rotational speed of rotor  14  times a number of blades  16 . In such embodiments, controller  24  synchronizes the emitter light pulses to a rotational position of blades  16  based on a signal provided by a rotational position sensor or derived from a rotational speed sensor. Accordingly, each light pulse illuminates one or more target locations of blades  16 , even though rotor  14  may complete multiple rotations between samples. 
     In any of the foregoing examples, emitters  20  and/or receivers  22  can be mounted directly to or flush with a surface of turbomachine  12 . In other instances, one or more of emitters  20  and receivers  22  are mounted to turbomachine  12  remote from the surface, light pulses and light returns traversing between emitter  20  and/or receiver  22  via a fiber optic cable. Fiber optic cables can be bifurcated, containing two or more discrete fiber optic paths such that a single cable can be shared between an emitter and receiver, between multiple emitters, or between multiple receivers. Further, any of the foregoing examples can include lens  23  to focus and/or filter light transmitted by emitters  20  or to collect and filter light intercepted by receivers  22 . 
     Controller  24  includes one or more processors  32  and system memory  34  that stores damage detection routine for inspecting blades of a turbomachine rotor to determine the presence of blade damage due to wear and/or foreign object debris (FOD) damage. Processor  32  executes damage detection algorithm as described in further detail below. Examples of processor  32  can include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other equivalent discrete or integrated logic circuitry. System memory  34  can be configured to store information within controller  24  during operation as well as any inspection criteria necessary for distinguishing damaged regions of blades  16 . System memory  34 , in some examples, is described as computer-readable storage media. In some examples, a computer-readable storage medium can include a non-transitory medium. The term “non-transitory” can indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium can store data that can, over time, change (e.g., in RAM or cache). System memory  34  can include volatile and non-volatile computer-readable memories. Examples of volatile memories can include random access memories (RAM), dynamic random-access memories (DRAM), static random-access memories (SRAM), and other forms of volatile memories. Examples of non-volatile memories can include, e.g., magnetic hard discs, optical discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories. 
     In some examples, processor  32  and system memory  34  are collocated in a control unit, which itself can be collocated with other components of damage detection system  10 . In other examples, any one or more components and/or described functionality of controller  24  can be distributed among multiple hardware units. For instance, in some examples, controller  24  can be incorporated into an electric engine control (EEC) unit or a full-authority digital engine control (FADEC) to perform functions other than those required by damage detection system  10 . In other examples, controller  24  can be a module discrete from other aircraft or engine control modules, which may be collocated with or remote from other aircraft or engine control modules and/or other components of damage detection system  10 . In general, though illustrated and described below as an integrated hardware unit, controller  24  can include any combination of devices and components that are electrically, communicatively, or otherwise operatively connected to perform functionality attributed herein to controller  24 . 
     Additionally, controller  24  can include drive circuit  36 , analog-to-digital converter (ADC)  38 , and alarm  40 . Drive circuit  36  is an electrical circuit constructed to regulate the voltage and/or current supplied to emitter  20 . Drive circuit  36  can be designed to produce light from one or more emitter  20  at a target frequency. Additionally, the design of drive circuit  36  can facilitate continuous light emissions from emitters  20  or supply intermittent power to emitter  20  such that emitter produces light pulses  20  at the target frequency. In some instances, receivers  22  produce an analog voltage and/or current in proportion to the amplitude of light collected, an analog-to-digital (ADC) converter  38  can be used to convert these analogue signals into digital signals for use by the controller  24 . Furthermore, controller  24  can include alarm  40  for outputting a signal indicative of blade damage to another system. For instance, detection systems  10  incorporated into aircraft can output an indication of blade damage to an electric engine controller (EEC), a full authority digital engine controller (FADEC), or another engine and/or aircraft system. Alternatively, alarm  40  can output an indication of blade damage to a display so that an operator can take corrective action. 
       FIG.  3 A  depicts an exemplary emitter  20  that includes lens  23 A for illuminating a line of light along blade  16 . Lens  23 A includes curved inlet surface  42  and flat outlet surface  44 . In an emission direction from inlet surface  42  to outlet surface  44 , lens  23 A diverges such that a cross-sectional area of outlet surface  44  exceeds a cross-sectional area of inlet surface  42 . Light entering inlet surface  42  refracts through lens  23 A to produce a fanned light structure emitted from outlet surface  44 . This fanned light structure illuminates a line along the surface of blade  16 , which can be collected by one or more receivers  22 . 
       FIG.  3 B  depicts another exemplary embodiment of emitter  20  that includes mirror  23 B. Mirror  23 B rotates about axis  46  to thereby redirect light produced by emitter  20  to illuminate path P on a surface of blade  16 . Receiver  22  collects light scatter from path P for each blade  16  passing through the inspection volume associated with emitter  20  and receiver  22 . 
       FIG.  4    is a flow chart describing damage detection algorithm  100  stored within and executed by controller  24 . Damage control algorithm  100  includes steps  102 ,  104 ,  106 ,  108 ,  110 ,  112 ,  114 , and  116 . The sequence depicted is for illustrative purposes only and is not meant to limit damage detection algorithm  100  in any way as it is understood that the portions of the algorithm can proceed in a different logical order, additional or intervening portions can be included, or described portions of the algorithm can be divided into multiple portions, or described portions of the algorithm can be omitted without detracting from the described above. 
     In step  102 , controller  24  causes each emitter  20  of damage detection system  10  to emit light toward one or more blade  16  during operation of turbomachine  12 . Emitted light can be produced continuously to illuminate interrogation volume  30 . Alternatively, emitter  20  may produce light intermittently, emitting light pulses at a target frequency and pulse width (or light pulse duration). As discussed above, damage detection system  10  includes at least one emitter, and may include more than one emitter  20  arranged to illuminate the one or more interrogation volume through which blades  16  rotate during operation of turbomachine  12 . Where multiple emitters  20  are utilized by damage detection system  10 , damage detection algorithm  100  may cause each emitter  20  to produce light simultaneously such that receivers  22  collect light transmitted from each emitter  20 . In other embodiments, the proximity and/or configuration of emitters  20  and receivers  22  may benefit by transmitting light from each emitter  20  sequentially such that at any given instant in time, only one or a subset of emitters  20  produces light. Each emitter  20  can illuminate a single spot of blade  16 . In other embodiments, emitter  20  can include lens  23 , distributing light emissions to thereby illuminate a line of blade  16  as shown in  FIG.  3 A . In still other embodiments, emitter  20  can scan along path P of blade  16  as shown in  FIG.  3 B . 
     Subsequently in step  104 , one or more receivers  22  collect light scattered from blade  16 . Depending on the relative positions of emitters  20  and receivers  22 , receivers  22  can collect predominately back scatter light returns, forward scatter light returns, or side scatter light returns. Upon receiving light return amplitude data from each receiver  22 , controller  24  stores light return amplitude data. Steps  102  and  104  repeat for a period of time enabling controller  24  to record light amplitude data as a function of time for each receiver  22  of damage detection system  10 . From the light amplitude data, controller  24  determines a light amplitude difference in step  106  by comparing the light return amplitude at different time steps. 
     Calculating the light amplitude change can be augmented by including one or more of the following steps. For example, the orientation of emitter  20  relative to blades  16 , and/or the number and spacing of blades  16  within turbomachine  12 , may permit light emissions to pass between adjacent blades  16 . However, because the relative distance between emitter  20  and blades  16  as well as the distance between blades  16  and receivers  22  remains fixed, time of flight data can be used to identify a subset of light returns associated with one or more blades  16 . Accordingly, in some embodiments, controller  24  determines a time of flight for each light pulse transmitted from emitter  20  and collected by receiver  22  in step  108  by comparing the time step when emitter  20  produces a light pulse and the time step when receiver  22  collects the emitted light pulse. Using this time-of-flight data, controller  24  may exclude light returns having a time of flight greater than a threshold value, indicating a light pulse returning from behind the intended blade inspection region. In other instances, controller  24  can exclude light returns having a time of flight greater than an upper threshold and less than a lower threshold to exclude returns reflected from objects or features of turbomachine  12  in front of the intended target region. For embodiments utilizing multiple emitters  20  to illuminate different radial stations of blades  16 , or embodiments utilizing lens  23 A to illuminate a linear region of blades  16 , time-of-flight data within upper and lower thresholds can be further divided into subranges. Each subrange can be associated with a radial station of blades  16 . Using any of the foregoing light return subsets, controller can determine a light amplitude change in step  106 . 
     In another embodiment, controller  24  can identify a subset of light returns associated with each rotation of rotor  14  in step  110 . For example, controller  24  can identify a repeating pattern of light amplitude peaks and/or troughs in the light return amplitude signal. In other examples, individual rotations can be identified by counting a predetermined number of peaks and/or troughs, each peak or trough associated with one of blades  16 . Alternatively, light returns associated with each rotation can be identified by subdividing the light returns signals accordingly to a time interval calculated as a function of rotational speed. In other embodiments, a first set of light returns are collected during multiple rotations of rotor  14  to establish a baseline light amplitude profile. During subsequent rotations of rotor  14 , a light amplitude profile derived from a second set of light returns can be compared to the baseline profile. In each instance, controller  24  may determine a light amplitude difference by comparing light return amplitudes from different sequential or non-sequential rotations in step  106 . 
     Furthermore, the light return amplitude calculation can be improved by first measuring an ambient light level in step  112 . In this case, one or more of receivers  22  collect light at a time preceding or between light emissions from emitters  20 . Without first emitting a light pulse from emitter  20 , the light amplitude determined from receivers  22  is representative of the ambient light level, which may change during operation of turbomachine  12 . Using the ambient light level, the light return amplitude data can be corrected or prior to determining a light amplitude difference in step  106 . Accordingly, variances of the light amplitude data due to changes in ambient light amplitude can be reduced or eliminated. 
     Once controller determines a change in light amplitude associated with one or more of receivers  22 , controller  24  compares each calculated light amplitude change to a damage criterion in step  114 . For instance, the damage criterion can be a threshold amplitude change in one or more of back scatter light returns, forward scatter light returns, and side scatter light returns. The threshold amplitude change is selected to exceed a light amplitude variation due to signal noise. Where surfaces of blades  16  are initially smooth, the light return profile may include predominately forward light scatter associated with spectral reflection of the blade surface accompanied by a relatively small amount of back scatter and/or side scatter. Once blades  16  with smooth surfaces become damaged, the damaged region may produce a diffused reflection of light pulses characterized by increased back scatter light returns and/or side scatter light returns accompanied by decreased forward scatter light returns. Contrastingly, the surface roughness of some coated blades  16  can produce predominately diffuse reflectance in an undamaged state. Damage to such blades  16  may expose uncoated material. In this instance, damage to this blade can be detected by deviations from the baseline light returns, in which the backscatter, side scatter, and forward scatter light returns may be used to characterize the type of damage. 
     If damage detection algorithm  100  utilizes step  110 , light returns can be evaluated on a rotation-to-rotation basis. For instance, the average forward scatter, side scatter, and back scatter light return for a given rotation can be compared to one or more subsequent rotations. Average light scatter values deviating from a previous average value by more than a threshold may indicate damage to one or more blades  16 . In other embodiments, peaks and or troughs of a given rotation can be used to associate subregions of the light return profile with individual blades. In this instance, changes in forward scatter, back scatter, and or side scatter returns associated may indicate damage to a particular blade  16  of rotor  14 . 
     If controller  24  determines that collected light returns satisfy any of the foregoing damage criteria, controller  24  may output an indication of blade damage in step  116 . For instance, controller  24  may output a signal to an electric engine controller (EEC), a full authority digital engine controller (FADEC), or another peripheral system representative of damage to one or more blades  16  and, in some instance, damage to a particular blade  16 . In some cases, a damage indication can be output to a display viewable by an operator. In aircraft applications, an indication of blade damage can be output to an avionics display in the cockpit of the aircraft. 
     Steps  102 ,  104 ,  106 ,  114 ,  116  and if included, steps  108 ,  110 , and  112  repeat at a predetermined sampling rate. Accordingly, damage detection algorithm  100  inspects blades  16  of rotor  14  in situ and, outputs an indication of damage in real time. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A system for detecting damage of a plurality of blades rotatable within a turbomachine includes, among other possible things, an emitter, a first receiver, and a controller. The emitter is orientated to emit a plurality of light pulses along an emission axis intersecting the plurality of blades. The first receiver has a first field of view intersecting the emission axis to define a first interrogation volume through which the plurality of blades rotates during operation of the turbomachine. The controller includes a processor and computer-readable memory in communication with the emitter and receiver. The computer-readable memory is encoded with instructions that, when executed by the processor, cause the system to emit the plurality of light pulses from the emitter toward the plurality of blades as the blades rotate within the turbomachine. The system receives a plurality of first light returns at the first receiver that have been scattered by the plurality of blades within the first interrogation volume. The system determines a first amplitude change of the plurality of first light returns and outputs an indication of damage to the blades based on the firs amplitude change of the light returns. 
     The system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components 
     A further embodiment of the foregoing system, wherein the first receiver can be collocated with the emitter to receive back scatter light returns. 
     A further embodiment of any of the foregoing system, wherein the controller can output the indication of damage based on a change in back scatter light return amplitude that exceeds a threshold light amplitude change. 
     A further embodiment of any of the foregoing systems, wherein the controller can output the indication of damage based on an increase in back scatter light return amplitude. 
     A further embodiment of any of the foregoing systems, wherein the controller can output the indication of damage based on a decrease in back scatter light return amplitude. 
     A further embodiment of any of the foregoing systems, wherein the first receiver can be spaced from the emitter to receive forward light scatter. 
     A further embodiment of any of the foregoing systems, wherein the controller can output an indication of damage based on an increase in forward scatter light return amplitude. 
     A further embodiment of any of the foregoing systems, wherein the controller can output an indication of damage based on a decrease in forward scatter light return amplitude. 
     A further embodiment of any of the foregoing system, wherein the controller can output the indication of damage based on a change in forward scatter light return amplitude that exceeds a threshold light amplitude change. 
     A further embodiment of any of the foregoing systems, wherein the first receiver can be spaced from the emitter to receive side scatter light returns. 
     A further embodiment of any of the foregoing systems, wherein the controller can output the indication of damage based on an increase in side scatter light return amplitude. 
     A further embodiment of any of the foregoing systems, wherein the controller can output the indication of damage based on a decrease in side scatter light return amplitude. 
     A further embodiment of any of the foregoing systems, wherein the controller can output the indication of damage based on a change of side scatter light amplitude that exceeds a threshold light amplitude change. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to determine, by the controller, a first subset of light returns corresponding to a first rotation of the plurality of blades. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to determine, by the controller, a second subset of light returns corresponding to a second rotation of the plurality of blades. 
     A further embodiment of any of the foregoing systems, wherein the indication of damage can be output by the controller based on a change in light return amplitude between the first rotation and the second rotation. 
     A further embodiment of any of the foregoing systems can include a second receiver having a second field of view intersecting the emission axis to define a second interrogation volume through which the plurality of blades rotates during operation of the turbomachine. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to receive, at the second receiver, a plurality of second light returns scattered by the plurality of blades within the second interrogation volume. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to determine, by the controller, a second amplitude change of the second light returns. 
     A further embodiment of any of the foregoing systems, wherein the indication of damage is output based on the first amplitude change and the second amplitude change. 
     A further embodiment of any of the foregoing systems, wherein the first receiver can be collocated with the emitter to receiver back scatter light returns. 
     A further embodiment of any of the foregoing systems, wherein the second receiver can be spaced from the emitter to receive forward scatter light returns. 
     A further embodiment of any of the foregoing systems, wherein the first amplitude change can indicate increased light amplitude of the plurality of first light returns. 
     A further embodiment of any of the foregoing systems, wherein the second amplitude change can indicate decreased light amplitude of the plurality of second light returns. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to determine a rotational speed of the plurality of blades based on the plurality of first light returns. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to receive, at the first receiver, an ambient light level. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to determine, by the controller, at least one of a first amplitude change of the first light returns based on an amplitude of first light returns and the ambident light level. 
     A further embodiment of any of the foregoing systems, wherein the first emitter can include a lens. 
     A further embodiment of any of the foregoing systems, wherein the lens can include a rounded inlet surface that diverges outward along the emission axis and terminates at an outlet surface normal to the mission axis, and wherein each light pulse of the plurality of light pulses refracts to form a light line upon passing through the lens. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to determine, by the controller, a first time of flight of the plurality of first light pulses. 
     A further embodiment of any of the foregoing systems, wherein the computer-readable memory can be encoded with instructions that, when executed by the processor, cause the system to identify, by the controller, a subset of the plurality of first light pulses based on a time-of-flight range. 
     A further embodiment of any of the foregoing systems, wherein the controller can determine the first amplitude change based on the subset of first light returns. 
     A further embodiment of any of the foregoing systems, wherein a frequency of the plurality of first light pulses can be at least three times a rotational frequency of the plurality of blades times a number of blades. 
     A method of detecting damage of a plurality of blades rotatable within a turbomachine includes, among other possible steps, emitting a plurality of light pulses from an emitter toward the plurality of blades as the blades rotate within the turbomachine. The method includes receiving, at a first receiver, a plurality of first light returns scattered by the plurality of blades and determining, by the controller, a first amplitude change of the first light returns. The method includes outputting, by a controller, an indication of damage based on the first amplitude change of the light returns. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, additional components, and/or steps. 
     A further embodiment of the foregoing method can include determining a first subset of light returns corresponding to a first rotation of the plurality of blades. 
     A further embodiment of any of the foregoing methods can include determining a second subset of light returns corresponding to a second rotation of the plurality of blades. 
     A further embodiment of any of the foregoing methods, wherein the indication of damage can be output based on a change in light return amplitude between the first rotation and the second rotation. 
     A further embodiment of any of the foregoing methods can include receiving a plurality of second light returns scattered by the plurality of blades. 
     A further embodiment of any of the foregoing methods can include determining at least one of a second amplitude change of the second light returns. 
     A further embodiment of any of the foregoing methods, wherein the indication of damage can be output based on the first amplitude change and a second amplitude change. 
     A further embodiment of any of the foregoing methods, wherein the first light returns can be back scatter light returns, and wherein the second light returns can be forward scatter light return. 
     A further embodiment of any of the foregoing methods, wherein the first amplitude change can indicate increased light amplitude of the plurality of first light returns and the second amplitude change can indicate decreased light amplitude of the plurality of second light returns. 
     A further embodiment of any of the foregoing methods can include receiving, at the first receiver, an ambient light level. 
     A further embodiment of any of the foregoing methods can include determining at least one of a first amplitude change of the first light returns based on an amplitude of first light returns and the ambient light level. 
     A further embodiment of any of the foregoing methods can include determining a first time of flight of the plurality of first light pulses. 
     A further embodiment of any of the foregoing methods can include identifying a subset of the plurality of first light pulses based on a time-of-flight range. 
     A further embodiment of any of the foregoing methods can include determining the first amplitude change based on the subset of first light returns. 
     While the invention has been described with reference to an exemplary embodiment(s), 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 invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.