INSPECT Issue 2, April 2010
Line by Line Stereo
Metrological 3D-Stereo with Line Scan Cameras
Determining the shape or spatial position of a work piece in 3D is increasingly important in quality control, robotics und type identification of products. At the same time metrological tasks are no longer carried in a mere sample check, but 100% control is requested with integrated inline inspection systems. Scanning stereo systems based on line scan cameras are well suited for the task.
Apart from some highly specialized and rather complex non-optical technologies like computer tomography and ultrasonic measurements, mainly optical methods are used today for 3D metrology. The most frequently used optical inline measuring principles today are laser scanning and (mostly phase-shifting) projected fringes techniques. In practice, however, there are several constraints for these technologies often not adressed initially. Thus the measuring distances to the examinated objects are too much restricted and the viewing angles have to be well-controlled. Therefore the necessary handling and positioning systems for the sensor systems may become very complex. If the angles between the incident light rays and the surface normals exceed certain thresholds the measurement errors are rapidly increasing. Moreover, the reflectance properties of the surfaces under inspection substantially influence the measurement accuracy. In any case, the outcome of these techniques is a point cloud of sampled surface points. Thus, the final task of fitting the intended CAD-geometry into the scattered measurement data in order to get resulting form deviations has yet to be done.
Stereo Scanning Mode
Genuine stereo reconstruction, which is extensively treated in scientific literature, is seldomly encountered in industrial applications. The method seems to be not applicable for scanning mode. For broader viewing angles or larger parts the same restrictions are assumed as with the classical methods. In addition, for practical use multi-camera systems would be needed in multi-stereo operation.
The scene changes when taking line scanning cameras into account: not just two, but an extendable number arranged in a line, according to the application demands, with every adjacent pair of cameras forming a partial stereo pair. Today, scanning systems of this type have been developed and tested in applications with high flexibility demands concerning measuring distances, surface properties and new strategies of model-based stereo techniques to overcome the lack or inaccuracy of significant points, edges and frontier curves of visibility. Moreover, these systems allow for free integration of light projection methods whenever necessary, without a complete system redesign.
The illumination can be well-controlled on the scan line, thus nasty reflectance properties are often easier to handle than with laser scanning systems. Non-standard reflectance problems on technical surfaces can be compensated with quick changes of illumination directions (interlaced between lines).
According to the application either the line scan stereo bridge is moved linearly over the object to be inspected, or the stereo scanner remains stationary while the object is moved. The metrological 3D surface data are generated line by line in full cross-sections of the part, thus supporting a direct comparison with the CAD-data.
Manifold Application Areas
3D line scanning stereo systems are easily extendable up to almost arbitrarily big scan line cross sections. However, there are several challenges when designing these systems. Thus, for instance, the calibration techniques for line scan stereo with high accuracy are rather sophisticated. In addition, one of the main tasks is the design and implementation of real time constraint- and model-based reconstruction algorithms suitable for metrological line scan stereo. On the other hand, integration into a production line is often convenient in practice.
Typical fields of application for 3D inspection based on line scan cameras are cutting or sawing processes in big tranches, quick type identifications based on object shape or position detection for subsequent handling. If metrological tasks have to be solved in combination with the detection of surface defects, the technology is optimally suited.