Non-contact apparatus and method for measuring surface profile

Abstract

Embodiments of the invention provide a non-contact method for measuring the surface profile of an object that can include generating a point-type optical signal and projecting it on a rotatable precision optical grating, generating a rotating pattern of light and dark lines onto the object, recording a series of images of the rotating pattern moving across the object with an image receiving device and calculating the surface profile of the object. Other embodiments can include a method to calibrate the system and a non-contact apparatus that generally includes a point-type light source, a rotatably mounted optical grating being configured to project a moving grating image on the object, a processor in communication with the image capturing device and configured to receive image input from the image capturing device and generate a surface profile representation of the object therefrom.

1 . A method for calibrating a dynamic structured light measuring system, comprising: 
 determining a focal length of an imaging device;    determining a transformation from an imaging device coordinate system to an absolute coordinate system;    determining absolute coordinates of a point in a plane, wherein a Z coordinate of the plane is know; and    determining at least one equation for at least one quadric surface containing projected grid line tracks.    
     
     
         2 . The method of  claim 1 , wherein determining the focal length comprises: 
 acquiring a first image of a calibration object;    calculating a first calibration parameter for the calibration object from the first image;    adjusting the position of the object a first know distance;    acquiring a second image of the calibration object;    calculating a second calibration parameter of the calibration object from the second image; and    determining the focal length from the first and second calibration parameters.    
     
     
         3 . The method of  claim 2 , wherein the calibration object comprises at least 3 circular markings having collinear centers such that a line joining the collinear centers such that a line joining the collinear centers is parallel with X axis of the imaging device.  
     
     
         4 . The method of  claim 3 , wherein the first and second calibration parameters comprise a distance between outer centers of the at least three circular markings.  
     
     
         5 . The method of  claim 2 , further comprising: 
 adjusting the position of the object a second known distance;    acquiring a third image of the calibration object;    calculating a third calibration parameter for the calibration object from the third image; and    determining the focal length from the first, second, and third calibration parameters.    
     
     
         6 . The method of  claim 1 , wherein determining a transformation from an imaging device coordinate system to an absolute coordinate system comprises: 
 positioning a calibration object such that a plane containing the calibration object is coplanar with an absolute coordinate system and contains an origin of the absolute coordinate system, and such that a line joining circles of the calibration object is collinear with an X axis of the absolute coordinate system; and    determining a distance between an optical center of the imaging device and the plane containing the calibration object using the focal length.    
     
     
         7 . The method of  claim 1 , wherein each individual quadric surface of the at least one quadric surface is derived through curve fitting to the projected gridline tracks in three or more reference planes.  
     
     
         8 . The method of  claim 1 , wherein each individual quadric surface of the at least one quadric surface is derived through a least squares fit method.  
     
     
         9 . The method of  claim 8 , wherein the least squares fit method further comprises solving the equation C=U M T  (M M T ) −1 , wherein C represents a nine element row vector of unknown coefficients, M represents a matrix having nine rows and a number of columns that corresponds to a number of projected points, and U represents a row vector of ones having a number of vector elements that corresponds to the number of projected points.  
     
     
         10 . The method of  claim 9 , further comprising determining a set of coefficients associated with each projected grid line.  
     
     
         11 . The method of  claim 1 , wherein determining at least one equation for at least one quadric surface containing projected grid line tracks further comprises independently determining an equation for each individual quadric surface.  
     
     
         12 . A method for calibrating a dynamic structured light measuring system, comprising: 
 determining a focal length of an optical imaging device positioned above a reference measurement surface;    calculating a transformation from a camera coordinate system to an absolute coordinate system;    determining absolute coordinates of a point in a first plane above the reference measurement surface using the transformation, wherein a distance from the reference measurement surface to the first plane is known; and    determining at least one representative equation for at least one quadric surface containing projected grid line tracks.    
     
     
         13 . The method of  claim 12 , wherein determining the focal length comprises: 
 orienting the optical imaging device;    positioning a planar calibration object having at least three markings having collinear centers above the reference measurement surface a first distance in a manner such that a plane of the calibration object is perpendicular to an optical axis of the optical imaging device and so that a line joining the centers is parallel with an X-axis of the optical imaging device;    calculating a first image plane distance between centroids of outer markings of the at least three markings of the first distance;    positioning the calibration object a second distance above the reference measurement surface, where the second distance is not equal to the first distance;    calculating a second image plane distance between centroids of outer markings of the at least three markings for the second distance; and    calculating the focal length from the first and second image plane distances.    
     
     
         14 . The method of  claim 12 , wherein calculating a transformation from a camera coordinate system to an absolute coordinate system comprises: 
 positioning calibration markings such that a plane containing the calibration markings is coplanar with an absolute system, the plane containing the calibration markings contains an origin of the absolute coordinate system, and such that a line joining the calibration markings is collinear with an X-axis of the absolute coordinate system;    determining the distance between an optical center of the optical imaging device and a plane of a calibration object from the equation Z 0 =f(D/d 0 ), where Z 0  is the distance between an optical center of the optical imaging device and the plane of a calibration object, f is a focal length of the optical imaging device, D is a distance between centers of calibration markings, and d 0  is the distance between projections of the centers of the calibration markings; and    calculating the transformation from the optical imaging device coordinate system to the absolute coordinate system from the equation Z camera =A bsolute +Z 0 .    
     
     
         15 . The method of  claim 12 , wherein determining at least one representative equation for at least one surface containing projected grid line tracks comprises: 
 providing a point light source;    rotating a grid about a central point at a location between the point light source and the reference measurement surface such that the a line joining the point light source and a point on each grid line that is closest to the central point traces out a quadric cone; and    independently determining an equation for the quadric surface.    
     
     
         16 . The method of  claim 12 , wherein each of the at least one quadric surfaces is through curve fitting to the projected gridline tracks in three or more reference planes.  
     
     
         17 . The method of  claim 12 , wherein each of the at least one quadric surfaces is derived through a least squares fit method.  
     
     
         18 . The method of  claim 17  wherein the least squares fit method further comprises solving the equation C=U M T (M M T ) −1 , wherein, C represents a nine element row vector of unknown coefficients, M represents a matrix having nine rows and a number of columns that corresponds to a number of projected points, and U represents a row vector of ones having a number of vector elements that corresponds to the number of projected points.  
     
     
         19 . The method of  claim 18 , further comprising determining a set of coefficients associated with each projected grid line.  
     
     
         20 .- 55 . (canceled)