Development of Precision Surface Profile
Measurement Capability, 10-9197

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Principal Investigators
Ernest A. Franke
Michael J. Magee
Joseph N. Mitchell
Michael P. Rigney

Inclusive Dates: 07/01/2000 - 07/01/2001

Background - The accurate measurement of surface profile is an important requirement in automated inspection of manufactured parts. Current systems using coordinate-measuring machines, laser-displacement gauges, or similar methods measure only a single point at a time. There is a need for an optical, noncontact method capable of measuring an array of points on a surface simultaneously. Structured light methods have been used, but patterns can be difficult to interpret and surface discontinuities can result in ambiguous measurements. Several methods of encoding the projected pattern to eliminate these problems have been proposed, but none provides the speed, resolution and reliability required for industrial inspection systems.

Approach - This project developed an extension of the structured light technique in which a projected pattern of parallel lines is rotated over the surface. A sequence of images is captured, and the surface elevation at any location can then be determined from measurements of the temporal pattern, at that point, without considering any other points on the surface. This method replaces the complex two-dimensional pattern analyses with a large number of independent one-dimensional analysis problems and also removes the ambiguity at discontinuities. The project included development of design procedures to meet specified measurement goals, laboratory investigations to refine the procedures, and tests to verify the performance.

Accomplishments - Methods for system calibration and surface measurement were refined using the concept of projected quadric shells. A design spreadsheet was developed to evaluate measurement resolution for design alternatives, and a laboratory prototype was constructed and tested. Algorithms were developed for image and signal analysis, and computer programs were written to calibrate the system and to calculate three-dimensional (3-D) coordinates of points on a measured surface. A prototype of the Dynamic Structured Light (DSL) 3-D imaging system was assembled in the laboratory to measure parts provided by several prospective clients.

The design spreadsheet procedure was verified and used to implement several different configurations with different measurement volumes and different accuracy. A small-parts measurement accuracy of 32 micrometers (0.0012 inch) RMS was verified by measuring the surface of a precision-machined plane using a 2-inch square field of view. Large aircraft control surfaces were measured with a prototype setup that provided a 4-foot by 8-foot field of view. Measurement times are approximately one minute for 800 points (30 seconds to acquire images and 40 milliseconds per point for computation).

The laboratory setup consists of a rotating grating and optics to project the pattern of lines on test parts. The illustration on the right shows a typical camera view of the lines projected on a complex surface.

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