International Symposium on
Measurement Technology and Intelligent Instruments
28 November - 1 December 2003
On-Line Industrial 3D Measurement
Techniques for Large Volume Objects
S.H. Ye, J.G. Zhu, X.Y. Yang, X.H. Qu, C.J. Liu and B. Wu
State Key Laboratory of Precision Measurement Technology and Instruments, Tianjin University, China
In area of manufacturing, especially, in large scale manufacture and precision engineering, the traditional geometric metrology techniques mostly use off-line mode, e.g., the measured objects (mechanical parts or assembly) were took to the special measurement environment from manufacturing field, then using coordinate measurement machine (CMM) to accomplish measurement task. The offline mode has low efficiency. The status of the measured object is different from that in manufacturing procedure. These seriously weaken the real time and the value of the measurement results, and reduce the measurement effect to ensure the accuracy of the manufacturing procedure. Modern manufacture technique progress rapidly, it demands new corresponding measurement methods with online characteristic. Comparing with offline, the online measurement faces more problems: high real time, strong anti-jamming, simple installation and adjustment in field and reliable accuracy evaluation. In all techniques for online measurement, the visual inspection based on CCD and computer vision shows more appropriate and promising. Now, actual visual inspection technique mainly use the principle of optical triangulation and artificial illumination, called structured lighting, many research works and applied systems were based on it, the feature of structured lighting is that: good contrast and high quality of image for processing; simple structure and triangulation principle for requested accuracy, etc., but it has drawbacks: the measurement model can not be united; calibration is difficult with low accuracy. So the applied visual inspection station (system) usually has different forms for diverse application with high complexity and expensive cost. This report studies a improved structured lighting technique, it combines the structured lighting and stereo vision, comprises two CCD cameras and a projector, the two CCD cameras constitute stereo vision layout, using stereoscopic parallax for measurement, the projector provides for illuminating (marking) object, to produce high quality image with marked features to be detected, making the stereo matching more reliable and robust. The improved structured lighting has united model and structure, it can solve different problems with identical form, to some extend, it is possible to supply a general solution for 3D measurement. Generally, the large scale online 3D measurement problems can be defined into two categories: with dense measured points and with sparse measured points. The paper presents a systematic solution for these questions, the solution comprises two parts: Multi-sensors Visual Inspecting Station (MSVIS) and Digital Close-range Visual Inspecting Station (DCVIS). MSVIS is a inspecting station with many sensors, each sensor is a measuring cell based on improved structured lighting for one measured point, all sensors were installed with different position and orientation in a base frame according to the demand of the specific task, the characteristic of MSVIS is that, for each sensor, the requested space volume is small and relative accuracy need not be very high, with good flexibility, MSVIS can solve measurement problems such as online inspection (measurement) of large parts or assembly. On other hand, in MSVIS, because of having many sensors, the calibration for determining the space relation parameters among the sensors (called global calibration) is complicated, also the installation of each sensor need space volume, all sensors need occupy large space volume, so MSVIS is only appropriate for the applied situation with sparse measured points. DCVIS use high resolution digital camera (such as Kodak Megaplus series) to reach very high relative accuracy (surpassing 1:100,000), in DCVIS, only two digital camera can achieve high absolute accuracy within large scale volume (also based on improved structured lighting principle), appended with one or more projector, DCVIS can meet measurement requirement of the application with dense measured points. The combination of MSVIS and DCVIS can form a general solution for online industrial 3D measurement with large scale volume.
Shenghua Ye is a professor of Tianjin University and the director of State Key Laboratory of Precision Measurement Technology and Instruments. His main research work relates to laser and photo-electricity measuring technology, visual inspection technology and so on. Over 30 projects had been achieved and hundreds of papers had been issued. The rewards coming from government, which include rewards for science and technology development by the nation, exceeds to 16. His main projects include study on the theory of foundations of visual inspection, online measurement of geometrical quantities by optic-electronic principle, study on the visual inspection technology, on-line and on-time visual collimation technology, IVECO car-body visual inspection station, laser auto-measuring station for car-body dimensions based on visual inspection technology, study on ways-free image joint technology and large field on-site inspection, and visual inspection instrument and system.
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09 March 2010