Patent Publication Number: US-7595894-B2

Title: Profilometry apparatus and method of operation

Description:
BACKGROUND 
   The invention relates generally to a profilometry apparatus, and more particularly, to a profilometry apparatus for providing real-time measurement of parameters of an object in a machining process. 
   Various types of machining processes are known and are in use for manufacturing and repairing parts. For example, laser consolidation systems are used to form functional components that are built layer by layer from a computer-aided design (CAD) without using any molds or dies. Typically, such systems employ a laser beam to melt a controlled amount of injected powder onto a base plate to deposit the first layer and then create subsequent layers by melting powder onto previously deposited layers. Unfortunately, due to process complexity of such systems it is very difficult to obtain the height of accumulated layers and get an instantaneous three-dimensional (3D) measurement of the volume of the weld-pool. 
   Certain systems employ a two-dimensional (2D) viewing system for monitoring the borders of the weld-pool while the system is in operation. However, such viewing systems provide a rough estimate of the weld area and do not provide a measurement of the weld-pool volume and the height of the accumulated layers. Certain other systems employ off-machine measurement methods to measure the 3D volume of the weld-pool. Such measurement technique requires the machining process to be stopped and to remove the part from the system for measuring the volume of the weld-pool. Furthermore, certain systems employ sensors for measuring the height of the accumulated layers. However, such sensors do not have the required measurement resolution, accuracy or the measurement range to provide a reliable measurement. 
   Accordingly, there is a need for a profilometry apparatus that provides an accurate measurement of the 3D weld-pool volume and height of the accumulated layers of a part formed by a laser consolidation process. Furthermore, it would be desirable to provide a profilometry apparatus that can provide an on-line measurement of the parameters of an object formed by a machining process that can be used to control the process parameters of the machining process. 
   BRIEF DESCRIPTION 
   Briefly, according to one embodiment a profilometry apparatus is provided. The profilometry apparatus includes a fringe projection device configured to project a fringe pattern on an object and an optical unit configured to capture an image of a distorted fringe pattern modulated by the object. The profilometry apparatus also includes a signal processing unit configured to process the captured image from the optical unit to filter noise from the image and to obtain real-time estimation of parameters associated with manufacture or repair of the object. 
   In another embodiment, a manufacturing assembly is provided. The manufacturing assembly includes a machining system having process parameters and configured to manufacture or repair an object and a profilometry apparatus configured to provide a real-time estimation of parameters associated with the manufacture or repair of the object from a single image generated from the profilometry apparatus. The profilometry apparatus includes a fringe projection device configured to project a fringe pattern on the object, an optical unit configured to capture an image of a distorted fringe pattern modulated by the object and a signal processing unit configured to process the captured image from the optical unit to filter noise from the image and to obtain real-time estimation of the parameters associated with the manufacture or repair of the object. The manufacturing assembly also includes a control system configured to adjust the process parameters of the machining system based upon the estimated parameters from the profilometry apparatus. 
   In another embodiment, a laser consolidation system is provided. The laser consolidation system includes a laser consolidation nozzle configured to form an object by providing a powder material in a laser generated melt pool and a fringe projection arm coupled to the laser consolidation nozzle and configured to generate a fringe pattern on a top surface of the object. The laser consolidation system also includes an optical unit configured to capture an instantaneous image of a distorted fringe pattern corresponding to the object and a signal processing unit coupled to the optical unit and configured to process the instantaneous image from the optical unit to filter noise from the image and to estimate parameters associated with the manufacture or repair of the object through Fourier Transform analysis. 
   In another embodiment, a method of controlling a process for manufacturing an object is provided. The method includes projecting a fringe pattern on the object and capturing an instantaneous image of a distorted fringe pattern corresponding to the object. The method also includes processing the captured image to filter noise image and to estimate parameters associated with the manufacture or repair of the object through Fourier Transform analysis and controlling process parameters for the manufacturing process in response to the estimated parameters associated with the manufacture or repair of the object. 

   
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  is a diagrammatical illustration of a laser consolidation system having a profilometry apparatus in accordance with aspects of the present technique. 
       FIG. 2  is an exemplary configuration  30  of the laser consolidation nozzle  14  of  FIG. 1  having the profilomeotry apparatus  12  in accordance with aspects of the present technique. 
       FIG. 3  is a diagrammatical illustration of an exemplary configuration of the profilometry apparatus of  FIG. 2  in accordance with aspects of the present technique. 
       FIG. 4  is a diagrammatical illustration of an exemplary configuration for generating a structured light pattern in the profilometry apparatus of  FIG. 3  in accordance with aspects of the present technique. 
       FIG. 5  is a diagrammatical illustration of another exemplary configuration for generating a structured light pattern in the profilometry apparatus of  FIG. 3  in accordance with aspects of the present technique. 
       FIG. 6  is a diagrammatical illustration of an exemplary configuration of a full field interferometer for generating a fringe pattern in the profilometry apparatus of  FIG. 3  in accordance with aspects of the present technique. 
       FIG. 7  is a diagrammatical illustration of another exemplary configuration of a full field interferometer for generating a fringe pattern in the profilometry apparatus of  FIG. 3  in accordance with aspects of the present technique. 
       FIG. 8  is a diagrammatical illustration of another exemplary configuration of a full field interferometer for generating a fringe pattern in the profilometry apparatus of  FIG. 3  in accordance with aspects of the present technique. 
       FIG. 9  is a diagrammatical illustration of another exemplary configuration of the profilometry apparatus of  FIG. 1  in accordance with aspects of the present technique. 
   

   DETAILED DESCRIPTION 
   As discussed in detail below, embodiments of the present technique function to provide a real-time measurement of parameters associated with a manufacturing or repair operation of an object by a machining process. In particular, the present technique employs a pattern spacing analysis for estimating the parameters from a fringe pattern corresponding to the object. The real-time measurement of these parameters is further utilized for controlling process parameters of the machining process. Referring now to the drawings,  FIG. 1  illustrates a machining system such as a laser consolidation system  10  having a profilometry apparatus  12  coupled to a laser consolidation nozzle  14 . The laser consolidation nozzle  14  includes a laser source  16  configured to generate a melt pool  17  on a substrate  18 . Further, the laser consolidation system  10  includes nozzle  20  configured to form an object  22  by providing a powder material  24  in the laser generated melt pool  17 . In particular, the laser consolidation system  10  employs a laser beam to melt a controlled amount of injected powder  24  onto the substrate  18  to deposit a first layer  26  and then create subsequent layers (not shown) by melting powder  24  onto previously deposited layers to form the object  22 . 
   In the illustrated embodiment, the profilometry apparatus  12  is coupled to or physically attached to the laser consolidation nozzle  14  and is configured to obtain the parameters associated with manufacture or repair of the object  22 . In particular, the profilometry apparatus  12  is configured to obtain the parameters associated with the weld pool  17  that may be further utilized for process control of the machining process. Examples of such parameters include volume of the melt pool  17 , height of accumulated layer  26 , thickness of accumulated layer  26  and so forth. As explained in detail below, the profilometry apparatus  12  employs a profilometry method such as Fourier Transform analysis for measuring such parameters without interfering with the machining or repair process. 
     FIG. 2  is an exemplary configuration  30  of the laser consolidation nozzle  14  of  FIG. 1  having the profilometry apparatus  12 . In the illustrated embodiment, the laser consolidation nozzle  14  includes two arms  32  and  34  having optical components for fringe projection and image capture from the object  22  (see  FIG. 1 ). The two arms  32  and  34  are disposed on either side of the high processing laser  16 . In the illustrated embodiment, the arm  32  is configured to project a fringe pattern on the object  22  and the arm  34  is configured to capture the image of a distorted fringe pattern from the object  22 . As will be appreciated by one skilled in the art different types of pattern may be projected on the object  22  via the arm  32 . For example, in one embodiment, the fringe pattern includes a straight-line pattern. In one exemplary embodiment, the fringe projection arm  32  has substantially large cross-section to cover a targeted area whereas the laser  16  is focused to a point on the object  22  to provide high power density to melt the powder. The optical components of the two arms  32  and  34  for fringe projection and image capture will be described in detail below. 
     FIG. 3  is a diagrammatical illustration of an exemplary configuration  40  of the profilometry apparatus  12  of  FIG. 2 . The profilometry apparatus  40  includes a fringe projection device  42  configured to project a fringe pattern on an object  44  being formed or repaired via a machining system. The fringe projection device  42  projects a continuous sinusoidal fringe pattern onto the object surface. In an embodiment, the fringe projection device  42  projects the fringe pattern through a digital projector such as a Liquid Crystal Display (LCD), Digital Micromirror Device (DMD) or Liquid Crystal on Silicon (LCOS) projectors. In an alternate embodiment, the fringe projection device  42  projects the fringe pattern through a light source such as a laser, Light Emitting Diode (LED), or a lamp combined with diffraction components such as gratings and holographic components. In certain other embodiments, the fringe projection device  42  projects the fringe pattern through an optical interferometer layout. 
   In the illustrated embodiment, the fringe projection device  32  includes a light source such as a lamp  46  or a LED  48  and an optical head  50  coupled to the light source via an optical fiber  52  for light projection on the object  44 . In addition, the profilometry apparatus  40  includes an optical unit  54  configured to capture an image of a distorted fringe pattern modulated by the object  44 . In this exemplary embodiment, the optical unit  54  includes a high pass filter  56  and a camera  58  for capturing the image of the fringe pattern that is further transmitted to a signal processing unit  60  via a cable  62 . In certain embodiments, the optical unit  54  includes a plurality of lens configured to capture the image of the distorted fringe pattern. In one embodiment, the optical unit  54  includes a borescope. 
   The signal processing unit  60  is configured to process the captured image from the optical unit  54  to filter noise from the captured image and to obtain real-time estimation of the parameters associated with the manufacture or repair of the object. Examples of such parameters include volume of the melt pool, height of accumulated layer, thickness of accumulated layer and so forth. It should be noted that the signal processing unit  60  may include a general purpose computer with appropriate programming for estimating the parameters and to facilitate the control of the process based upon the estimated parameters. In certain embodiments, the signal processing unit  60  may include a microcontroller. In an exemplary embodiment, the profilometry apparatus  40  employs Computer Numerical Control (CNC) to estimate the built height of the object  44  thereby eliminating the need of additional height sensors in the system  40 . In operation, the signal processing unit  60  employs a pattern spacing analysis to filter the noise from the captured image from the optical unit  54 . In this exemplary embodiment, the pattern spacing analysis includes Fourier Transform analysis. However, other types of pattern spacing analysis may be envisaged. More specifically, the signal processing unit  60  extracts a phase map of the distorted fringe pattern and estimates the parameters from this phase map. The extraction of phase map from the fringe pattern using Fourier Transform and estimation of parameters from the phase map is explained below. 
   In this exemplary embodiment, the image of the fringe pattern captured by the optical unit  54  is represented by the following equation:
 
 I   k ( i,j )= I   0 ( i,j )[1+γ( i,j )cos(φ( i,j )+δ k )],  k= 1,2,3 . . .  K   (1)
 
   Where: k is the index number of images used in the phase measurement method;
         I is the intensity at pixel (i,j);   I 0  is the background illumination;   γ is the fringe modulation representing image contrast;   δ k  is the initial phase for each individual image k; and   K is the total number of images.       

   For the image represented by equation (1) the two dimensional Fourier transform may be obtained as represented by the following equation:
 
 M ( u,v ) =A ( u,v )+ C ( u,v )+ C *( u,v )  (2)
 
Further, after applying a band-pass filter F(u,v), only C(u,v) is left that is represented by the following equation:
 
 C ( u,v )= M ( u,v ) F ( u,v )  (3)
 
After inverse Fourier transforming, c(i,j) can be obtained as:
 
   
     
       
         
           
             
               
                 
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   Where: I and J are dimensions of pixel index. 
   Further, the phase value at each pixel (i,j) can be calculated as: 
   
     
       
         
           
             
               
                 
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   Where: I m  and R e  stands for imaginary and real parts of complex number c(i,j). 
   Further, the profile of the weld pool may be obtained from the phase map and is represented by the following equation:
 
( x,y,z )= f   x,y,z  ( i,j,φ ( i,j ))  (6)
 
Thus, the profile of the weld pool including the parameters associated with the weld pool may be obtained from a single instantaneous image via the Fourier transform analysis as described above.
 
   In certain embodiments, the signal processing unit  60  is configured to split the instantaneous image from the optical unit  54  into a plurality of images and the pattern of each image is shifted relative to other images. Further, the signal processing unit  60  is configured to generate a phase map from the plurality of images for estimating the parameters. It should be noted that the combination of light fringe projection along with the Fourier transform enables relatively easy filtering to remove the noise such as generated from the powder and background illumination. The phase information generated from the fringe pattern has a substantially high resolution and accuracy. In addition, the Fourier transform profilometry enables generation of the phase map from a single image thereby resulting in relatively less time for image processing and estimation of parameters of the weld pool. 
   The estimated parameters associated with the manufacture or repair of the object  44  may be utilized for process control of the machining system such as the laser consolidation system  10  described above with reference to  FIG. 1 . In particular, process parameters of the laser consolidation system  10  may be adjusted in response to the estimated parameters associated with the manufacture or repair of the object  44 . Exemplary process parameters include laser power, powder flow rate, focus location, laser translation speed, slot size and combinations thereof. In certain embodiments, a control system (not shown) may be coupled to the machining system  10  for achieving a closed loop control of the system  10  based upon the estimated parameters. Beneficially, the estimation of phase map from a single instantaneous image through Fourier transform profilometry enables instant process control based upon the estimated parameters. 
   The generation of a structured light pattern in the profilometry apparatus  40  described above may be achieved through a plurality of configurations such as described below with reference to  FIGS. 4-8 . In particular, such exemplary configurations may be employed for the laser consolidation nozzle  30  (see  FIG. 2 ) for generating a structured light pattern on the object  22  (see  FIG. 2 ). 
     FIG. 4  is a diagrammatical illustration of an exemplary configuration  70  for generating a structured light pattern in the profilometry apparatus  40  of  FIG. 3 . In the illustrated embodiment, a laser  72  is projected to form a spot at the surface of an object  74  to be measured. Further, an imaging lens  76  disposed at an angle to the laser beam  72  forms an image or picture of the laser spot that is captured through a camera  78 . A change in surface height (D)  80  of the object  74  causes the imaged spot to shift laterally on this image plane by a distance (d)  82  which is used to estimate the change in surface height  80  by the triangle formed by the laser  72 , laser spot and the camera  78 . 
     FIG. 5  is a diagrammatical illustration of another exemplary configuration  100  for generating a structured light pattern in the profilometry apparatus  40  of  FIG. 3 . As described above with reference to  FIG. 4 , this exemplary configuration  70  includes the laser  72 , imaging lens  76  and camera  78 . Further, in the illustrated embodiment, the fringe pattern is projected through the laser  72  and diffraction components  102 . Examples of diffraction components include gratings and holographic components. 
   As described above, the fringe projection device  42  (see  FIG. 3 ) of the profilometry apparatus  40  may project the fringe pattern through an optical interferometer layout that projects fringes.  FIGS. 6-9  illustrate exemplary system configurations for optical interferometer layout for projecting the fringe pattern. 
     FIG. 6  is a diagrammatical illustration of an exemplary configuration  120  of a full field interferometer for generating a fringe pattern in the profilometry apparatus  40  of  FIG. 3 . In the illustrated embodiment, the interferometer includes a Michelson Interferometer. In operation, a beam emitted from a light source such as a laser  122  with a beam expander  124  is split into two beams of nearly equal intensity by a beam splitter  126 . One of these beams is directed onto a reference mirror  128  while the other beam is directed onto an object surface  130 . Further, the light produced by reflection of these two beams is made to interfere. When observed from a viewing port such as camera  132 , interference occurs between the image of the mirror  128  and the image of the object surface  130 . Since the light waves reflected by the object surface  130  and the mirror  128  originate from the splitting of the beam emitted by the same light source  122 , these waves are mutually coherent, and consequently a two-beam interference pattern is generated. Further, the interferometric phase recovery may be achieved via phase shifting by a piezoelectric transducer (PZT)  134  phase-stepping. However, other known techniques may be employed to generate the phase map. 
     FIG. 7  is a diagrammatical illustration of another exemplary configuration  150  of a full field interferometer for generating a fringe pattern in the profilometry apparatus  40  of  FIG. 3 . In this exemplary embodiment, the interferometer  150  includes a digital holography interferometer that generates the fringe pattern through interference between the wave reflected or transmitted from the object to be imaged and a reference wave. As with the configuration illustrated in  FIG. 6 , the digital holography interferometer  150  includes the light source  122  with the beam expander  124  for generating a fringe pattern on the object  130 . In addition, the interferometer  150  includes a mirrors  152  and  154  and beam splitters  156  and  158  for generating the object beam and the reference beam that are combined to generate the fringe pattern. 
     FIG. 8  is a diagrammatical illustration of another exemplary configuration  170  of a full field interferometer for generating a fringe pattern in the profilometry apparatus  40  of  FIG. 3 . In the illustrated embodiment, the interferometer  170  includes a shearing interferometer. The shearing interferometer  170  includes the light source  122  with the beam expander  124  for generating the fringe pattern on the object  130 . In addition, the shearing interferometer  170  includes a shearing plate  172 . The wavefronts from the object  130  are incident on the shearing plate at an angle of about 45 degrees and the reflected wavefronts from the shearing plate  172  are laterally sheared because of a finite thickness of the plate. Further, interference of the reflected wavefronts results in generation of the fringe pattern. 
   As will be appreciated by one skilled in the art, depending upon a desired resolution for an application, any of the above-described techniques may be employed for generating the fringe pattern on the object  44  via the fringe projection device  42  of  FIG. 3 . Further, an instantaneous image of the distorted fringe pattern corresponding to the object  44  is captured via the optical unit  54  that is processed via the signal processing unit  60  to estimate the parameters associated with the manufacture or repair of the object  44 . 
     FIG. 9  is a diagrammatical illustration of another exemplary configuration  190  of the profilometry apparatus  12  of  FIG. 1 . The profilometry apparatus  190  includes the fringe projection device  42  configured to project a fringe pattern on the object  44 . In the illustrated embodiment, the fringe projection device  42  includes a light source  192  coupled to a grating  194  and lens  196  through an optical fiber  198 . In one exemplary embodiment, the grating  194  comprises a  250  PLI grating and the lens  196  comprises double convex lens. In addition, the profilometry apparatus  190  includes the optical unit  54  for capturing the image of the distorted fringe pattern modulated by the object  44 . In this exemplary embodiment, the optical unit  54  includes a borescope  200  and a camera  202  that are coupled to the signal processing unit  60  via the cable  62 . As described earlier, the captured image from the optical unit  54  is processed via the signal processing unit  60 . The signal processing unit  60  extracts the phase map of the instantaneous image and estimates parameters associated with the machining operation of the object  44  without interfering with the machining process. In certain embodiments, a typical frame rate and processing may provide an update to the system at about 10 times per second that is substantially fast for feedback and control operations. Further, specialized image processing equipment optimized for this application along with high frame rate cameras may provide an update of about 100 times per second. 
   The various aspects of the method described hereinabove have utility in different machining applications. The technique illustrated above may be used for providing a real-time measurement of parameters associated with a manufacturing or repair operation of an object via a machining process. The technique may also be used for a closed loop control of the machining process based upon the estimated parameters to achieve a desired output. As noted above, even more generally, the method described herein employs a Fourier transform profilometry for estimating the parameters from a single instantaneous image by filtering noise from the system. Further, the technique is particularly advantageous to provide a profilometry apparatus with good resolution and accuracy and is cost effective that may be used for a wide range of machining applications. 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.