Oct 30, 2014 - technologies exist for this task: stereo ... Figure 1: the automotive technology. Centre of Galicia ... lock machine vision software from Teledyne.
i n dustrial automat ion
Triangulation-based 3D system measures axle diameters A structured light-based system allows axle diameters to be calculated to an accuracy of 30μm. Dr. Angel Dacal-Nieto Laser
JAI CV-A2 To develop a system to inspect car axles, an is faster, more objecinternational automotive supplier in Spain tive, revisable, repethas teamed with the Automotive Technol- itive and often less ogy Centre of Galicia (CTAG) to check these expensive. Since conComm. + power parts for their length and size. The system tact measurement is has been designed to inspect axles mounted slow and may harm the in PSA Peugeot Citroën vehicles but, in the part, machine vision future, is expected to be deployed to inspect was chosen to develop the inspection system. other types of axles. SW Axles that carry a wheel but without power Among the measureto drive it are known as dead axles and are geo- ments made, the diamSherlock metrically complex parts that provide stabil- eter of the interior PC VA-31 ity and hold the weight of the car. Although cylinder is the most be measured figure with 1 the production process used to produce these important and each must1409VSDindAuto axles is repetitive, the length and diameter an accuracy of within 30μm. may vary depending on the make and model of each automobile. In the past, measurement Imaging techniques tasks were performed manually using a ran- Measuring the diameter of a cylinder is a task domly selected subset that can be performed of production batches. using 3D machine Dr. Angel Dacal-Nieto, ProjThis sampling and furvision. Three types of ect Manager and Senior Researchther feedback from the technologies exist for customer were the only er, Victor Alonso-Ramos, this task: stereo vision, methods used to obtain time-of-flight and laser Head of Department and David information about the triangulation. While Gomez, Senior Researcher, Auaccuracy of the diamstereo vision is a pastomotive Technology Centre of eter of each part. sive technique that Galicia (CTAG; Porriño Pontevedra, Compared with integrates at least two manual measurement, images obtained from Spain; www.ctag.com) automatic inspection two sensors from two www.vision-s y stems .com
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Vision Systems Design
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Figure 1: The Automotive Technology Centre of Galicia (CTAG) has developed an automated system that uses off-the shelf imaging components to inspect the diameters of car axles.
different perspectives, time-of-flight imaging sends and receives light pulses over the part. Measuring the time between emission and reception, the distance to the object can be calculated and its shape calculated. Laser triangulation is based on projecting a known light pattern on the part, with the pattern being deformed by its geometry. Scanning a large set of profiles, the 3D shape of the piece can then be reconstructed. Stereo vision needs at least two cameras and the calibration required is usually less Oc tober 2014
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industrial automation
robust than other options. Changing light conditions can also produce undesired results. Time-of-fight techniques, on the other hand, do not obtain the best results on specular surfaces and their accuracy is limited.
Structured lighting Thus, in applications where a 3D profile needs to be measured, structured laser light is the method most often used. This is the method used by CTAG to measure the diameter of the cylindical axels. After an IRB 6600 6-axis robot from ABB (Zurich, Switzerland; www.abb.com) places the axle in a known location, structured laser light is projected onto the axle. Due to the cylindrical shape of the part, it was necessary to employ two structured laser lights to fully illuminate the part. To digitize images from the structured laser light, a monochrome JAI CV-A2 analog camera from JAI (San Jose, CA; USA; www. jai.com) was fitted with a 35mm lens from
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Figure 2: Although the system is fully automated, a graphical user interface is used to show the operator the parts as they are illuminated with structured light.
Goyo Optical (Saitama, Japan; www.goyooptical.com) and a band pass 655 nm filter from Schneider Kreuznach (Bad Kreuznach, Germany; www.schneiderkreuznach.com/industrialoptics) to enhance the reflected laser light. Images from the camera were then digitized and analyzed using a VA-31 vision appliance, an industrial computer from Teledyne DALSA (Waterloo, ON, Canada; www. teledynedalsa.com) that is designed to per-
form machine vision routines and I/O control. The VA-31 vision unit includes Sherlock machine vision software from Teledyne DALSA that provides machine vision functions and access to hardware devices such as cameras and PLCs. Before any image processing can be performed, however, the system must be calibrated. This is performed using the Tsai method, a two-stage technique for 3D
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camera calibration that is used to compute the external position and orientation relative to object reference coordinate system (see “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-shelf TV Cameras and Lenses”, Roger Tsai, IEEE Journal of Robotics and Automation, No. 4, August 1987; http://bit.ly/1nshGwb). This is accomplished by first projecting a dotted pattern onto the part. Then, by capturing the reflected image, the 3D orientation, x-axis and y-axis translation is computed and the focal length, distortion coefficients and the z-axis translation determined. This calibration data is then used to correct any distortion in the image and convert 2D points to 3D points. After the system has been calibrated, the diameter of the axle can be measured. Using the Sherlock image processing software, CTAG has developed image processing routines and additional custom code using Microsoft’s .NET framework. When the struc-
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tured laser light image is captured, any distortion is eliminated using the calibration data. The image is then thresholded using Sherlock to highlight the structured laser in the cylinder and the 2D points in this image are computed. After these 2D points are detected, they are converted from 2D to 3D space using custom code and the circumference is interpolated using a least squares algorithm. The diameter of the axle is then computed. For administration and monitoring, a simple GUI has been developed (Figure 2). Figure 2 (left) shows the original image and Figure 2 (right) the resultant image, where the projected laser is detected and marked with red dots. Then, the dot set is surrounded with a green rectangle (region of interest) that shows whether the image processing algorithm has correctly detected the projected laser. If the measure lies outside the expected range, or it seems erroneous, the operator must confirm that this result is not due to a bad image acquisition. Since the system has
Vision Systems Design
Companies mentioned Automotive Technology Centre of Galicia (CTAG) Porriño Pontevedra, Spain www.ctag.com
ABB Zurich, Switzerland www.abb.com
Goyo Optical Saitama, Japan www.goyooptical.com
JAI San Jose, CA; USA www.jai.com
Schneider Kreuznach Bad Kreuznach, Germany www.schneiderkreuznach.com/ industrialoptics
Teledyne DALSA Waterloo, ON, Canada www.teledynedalsa.com
been implemented, production quality has improved with axle diameters being measured at the required 30μm accuracy.
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