Abstract:
A method and system are provided for inspecting a surface of an object with an optical scanner and a laser vibrometer. The method includes the steps of: (a) mapping at least a portion of the object surface using the optical scanner; (b) projecting a beam of light from the laser vibrometer onto the object surface at a measurement point; (c) locating the measurement point relative to the object surface using the optical scanner; and (d) measuring a position of the object surface using the laser vibrometer to determine, for example, a deflection of the object surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This disclosure relates generally to non-contact scanning and, more particularly, to a method and system for spatially locating a laser vibrometer relative to an object. 
     2. Background Information 
     Machinery components or objects (e.g., gas turbine rotor blades) can be inspected using non-contact scanning. Vibrationally induced deflections and stresses of the component, for example, can be determined using data measured by a laser vibrometer. The deflections and stresses can subsequently be processed to predict or model how the component will respond to vibrations during operation of the component within the machinery (e.g., performance of the rotor blade within the gas turbine engine). In order to produce accurate predictions and models, however, the position of the vibrometer and, thus, the position of where the data is being measured on the component surface must be located relative to the component. The vibrometer can be located, for example, by (i) mechanically aligning the vibrometer and the rotor blade in known positions, or (ii) performing post processing alignment procedures such as, for example, normalizing measurement data to a normal or reference coordinate space. Both methods for locating the vibrometer, however, require certain base assumptions (e.g., that the component is close to a nominal geometric location and shape, that the surface of the component can be represented by a two dimensional planar surface, that non-scalar image data can be used as a reference, etc.) and can increase time and costs associated with performing the inspection. 
     SUMMARY OF THE DISCLOSURE 
     According to a first aspect of the invention, a method is provided for inspecting a surface of an object with an optical scanner and a laser vibrometer. The method includes the steps of: (a) mapping at least a portion of the object surface using the optical scanner; (b) projecting a beam of light from the laser vibrometer onto the object surface at a measurement point; (c) locating the measurement point relative to the object surface using the optical scanner; and (d) measuring a position of the object surface using the laser vibrometer. 
     According to a second aspect of the invention, a system for inspecting a surface of an object includes a laser vibrometer, an optical scanner and a controller. The vibrometer is adapted to project a beam of light onto the object surface at a measurement point, and to measure a position of the object surface. The optical scanner is adapted to map at least a portion of the object surface, and to locate the measurement point relative to the object surface. The controller is adapted to correlate the measured position of the object surface to the location of the measurement point. 
     The foregoing features and the operation of the invention will become more apparent in light of the following description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic illustration of a system for inspecting a surface of an object. 
         FIG. 2  is a diagrammatic illustration of an optical scanner included in the system illustrated in  FIG. 1 . 
         FIG. 3  illustrates a flow diagram of a method for inspecting at least a portion of the object surface illustrated in  FIG. 1 . 
         FIG. 4  is a three-dimensional model of the object surface illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a system  10  for inspecting a surface  12  of an object such as a rotor blade, generally referred to hereinafter as an “object  14 ”. The system includes an optical scanner  16 , a laser vibrometer  18 , an excitation system  20 , a support stand  22  and a controller  23 . 
     Referring to  FIGS. 1 and 2 , the optical scanner  16  is adapted to map at least a portion of the object surface  12 . The term “map” is used herein to describe a process of applying a triangulated mesh of surface points to an object surface. In the embodiment shown in  FIG. 2 , the optical scanner  16  includes a fringe pattern projector  24  and one or more cameras  26 . The projector  24  is adapted to project a point, line and/or pattern of light at one or more projection frequencies. Each camera  26  is adapted to capture an image within a light intensity band and a camera frequency band, which camera frequency band includes the projection frequencies. An example of such a projector and cameras is disclosed in U.S. Patent Application Publication No. 2009/0033947, which is hereby incorporated by reference in its entirety. The position of the projected mesh can be located relative to the object in several ways. 
     The vibrometer  18  (also sometimes referred to as a “laser Doppler vibrometer” or an “interferometer”) is adapted to measure a relative spatial position of the object surface  12 . The vibrometer  18  includes a laser  28  and a light detector  30 . The laser  28  is adapted to project a beam of light  32  at a vibrometer frequency and a vibrometer light intensity. The light detector  30  is adapted to detect a reflection of the beam of light  32  having a reflection frequency. In some embodiments, the vibrometer frequency is selected such that the reflection frequency is outside (e.g., greater than) the camera frequency band. 
     The excitation system  20  is adapted to induce vibrations in at least a portion of the object  14 . In the embodiment shown in  FIG. 1 , the excitation system  20  includes a loudspeaker  34  adapted to transmit an audio excitation signal. 
     The support stand  22  includes a base  36 , an object turntable  38 , an excitation system turntable  40 , an object support  42  and sensor mounting stand  44 . The base  36  includes a lower platform  46  and an upper platform  48 . The upper platform  48  has an aperture  50  that extends along an axis  52 . The object turntable  38  is mounted to the lower platform  46 , and is adapted to rotate about the axis  52 . The excitation system turntable  40  is mounted to the upper platform  48 , and is adapted to rotate about the axis  52 . The excitation system turntable  40  includes a central aperture  54  that extends along the axis  52 . The object support  42  extends along the axis  52  from the object turntable  38 , through the apertures  50  and  54 , to an object support surface  56 . The sensor mounting stand  44  includes a vibrometer mount  58  and a scanner mount  60 . The sensor mounting stand  44  extends from the upper platform  48  to the vibrometer mount  58 . 
     The optical scanner  16  is connected to scanner mount  60 . The vibrometer  18  is connected to the vibrometer mount  58 . The loudspeaker  34  is connected to the excitation system turntable  40 . 
     The controller  23  can be implemented using hardware, software, or a combination thereof. The hardware can include one or more processors, analog and/or digital circuitry, etc. In some embodiments, for example, the controller can include one or more sub-processors (not shown) respectively configured with the optical scanner  16  and the vibrometer  18 . The controller is in signal communication (e.g., hardwired or wirelessly connected) with the optical scanner  16 , the vibrometer  18 , the loudspeaker  34 , the object turntable  38  and the excitation system turntable  40 . 
       FIG. 3  illustrates a flow diagram of a method for inspecting at least a portion of the surface  12  of the object  14  (see  FIG. 1 ). Referring to  FIG. 1 , the object  14  can be configured as, for example, a component of a gas turbine engine such as an integral bladed rotor (also referred to as an “IBR”). For ease of description, the object will be referred to hereinafter as the rotor  14 . The rotor  14  includes a hub  62  and a plurality of blades  64  and  66 . The present invention, however, is not limited to inspecting any particular object configuration and/or composition. 
     Referring to  FIGS. 1-4 , in step  300 , the rotor hub  62  is disposed on the object support surface  56  such that, for example, a rotational axis of the hub  62  is aligned with the rotational axis  52  of the turntable  38 . 
     In step  302 , the controller signals the optical scanner  16  to map the surface  12  of a first one of the rotor blades  64 . The projector  24 , for example, can project a pattern of alternating parallel light (e.g., white, or any other suitable color) and dark (e.g., black) lines onto the blade surface  12 . The lines may be distorted by contours, edges and/or other features (e.g., apertures) of the blade surface  12 . Images of the lines on the blade surface  12  are captured by the cameras  26 , and processed by, for example, one of the controller sub-processors to provide a three-dimensional model of the blade surface  12 . 
     In step  304 , the controller signals the vibrometer  18  to project the beam of light  32  from the laser onto the blade surface  12  at a measurement point  68 . 
     In step  306 , the controller signals the optical scanner  16  to locate the measurement point  68  created by the laser and, thus, a location of where the beam of light  32  reflects against the blade surface  12 . In one embodiment, for example, the cameras  26  capture images of the blade surface  12 , which images are overexposed (e.g., missing image data) around the measurement point  68 . The overexposure can be caused, for example, by the intensity and/or frequency of the reflected light being outside the detection capabilities of the cameras  26 ; e.g., where the intensity of the reflected light is outside the light intensity band of the cameras  26 , and/or where the frequency of the reflected light is outside the frequency band of the cameras  26 . The cameras  26  therefore do not capture image data from the light beam reflection point. The blade surface images can be processed by, for example, one of the controller sub-processors to provide a three-dimensional model of the blade surface  12  as shown in  FIG. 4 . The laser light reflection can be identified on the 3D model via the missing image data  70  (see  FIG. 4 ). The measurement point  68  can subsequently be located using the model, for example, by (i) determining a centroid  72  of the reflection (i.e., the missing image data  70 ), and (ii) determining the spatial location of the centroid  72  relative to the mapped blade surface  12 . Alternatively, the reflection can be identified via its intensity, color, shape, etc. where the reflection frequency is within the camera frequency band; i.e., the cameras  26  can capture image data from the reflection. In other embodiments, photogrammetry can be used to locate the position of where the beam of light  32  reflects against the blade surface  12 . 
     Referring again to  FIGS. 1-3 , in step  308 , the controller signals the loudspeaker  34  to transmit the excitation signal towards an underside of the first rotor blade  64  to induce a vibratory response. 
     In step  310 , the controller signals the vibrometer  18  to measure vibrationally induced blade deflections and associated blade stresses in the first rotor blade  64  at the measurement point  68 . The laser  28 , for example, projects the beam of light  32  onto the blade surface  12  at the vibrometer frequency. The light detector  30  is used to determine the reflection frequency of the reflection. Doppler shifts between the vibrometer and reflection frequencies caused by the deflections can subsequently be determined to provide data indicative of a position of the blade surface  12  (e.g., a distance between the blade surface  12  and the vibrometer  18 ) at a particular point in time. Additional measurements can be performed over a period of time, and processed to measure the deflections and stresses caused by the deflections using known techniques. 
     In step  312 , the controller correlates the measurements taken in step  310  to the location determined in step  306  of the measurement point  68  and, thus, the location of where the beam of light  32  reflects against the blade surface  12 . In this manner, the controller can generate output data indicative of both the measurements and the location of where the measurements were taken relative to the blade surface  12  (e.g., the blade edges) without performing mechanical or post processing alignment procedures. In some embodiments, however, such alignment procedures can be performed, for example, to verify system accuracy. Subsequent processing of the output data by the controller or another computer system, therefore, can be performed to accurately predict or model how the rotor blade  64  will respond to vibrations during operation of an engine. 
     In some embodiments, steps  302  to  306  may be performed together such that the blade surface  12  is mapped and the measurement point  68  is located without performing multiple scans with the optical scanner  16 . 
     In some embodiments, the vibrometer  18  can perform additional measurements at one or more alternate locations on the blade surface  12  by adjusting the position of the object turntable  38  or the vibrometer mount  58  relative to the measurement point  68 . The additional measurements are subsequently correlated to the locations of where the beam of light  32  was focused on the blade surface  12  during those measurements. The locations may be determined by adding a vector indicative of the distance and direction the object turntable or the vibrometer mount were moved to the spatial location of the measurement point  68  determined in step  306 . 
     While various embodiments of the present invention have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents.