This blog post is contributed by 3D3 Solutions’ UK distributor, Simon Stone from Mech Innovation Limited.
3D scanning large objects such as trains poses its own set of challenges. When faced with the task of measuring a train front where the vehicle can’t be transported to a special measuring facility (if one exists) or lifted onto a CMM machine, there’s a real problem. Where simple point to point measurements aren’t good enough to understand how well the vehicle conforms to drawing or what damages may have occurred on impact, white light 3D scanning provides the solution. This is another area where this technology can improve the development process.
PHOTO: 3D scans of a train front measuring 2.6m wide and 2.3m high.
We used 3D scanning solution, FlexScan3D, which is very portable and flexible for this project. An entire train front can be captured in 3D using the exact same piece of equipment that is used to measure a mobile phone, simply by changing the field of view of the 3D scanner.
The scanning head is light enough so it can easily be mounted on a jib for capturing top down shots from 5m in the air. With a wide field of view comes a large depth of focus, which makes detail on either side of the focusing ‘sweet spot’ easy to capture.
TIPS FOR SCANNING LARGE OBJECTS
Use a Highly Accurate Calibration Board
For 3D scanning large scale objects it is worth spending some time making a really accurate calibration board. In this case a 60mm square was used on a 12x9 grid. Large accurate boards are hard to make but if you do a lot of 3D scanning of this scale it's worth it. Also taking 30 to 40 lens calibration images will help reduce distortions in the finished result, thus increasing accuracy and reducing processing time.
Control the Environment to the Best of Your Ability
Controlling the environment as much possible to reduce ambient light levels will really help reduce noise. For very large objects this often means scanning at night, sometimes even outside. Make sure that both the object and 3D scanner are completely still before capturing a 3D scan. This is not always easy, particularly if there is a breeze. It is also important to ensure that the exhaust port of the projector does not blow warm air in front of the cameras as this will result in movement artifacts in the 3D scan data. You can use the zoom view on the live camera view to make sure there's no movement. This method can be used to check that there is no physical movement or heat distortion caused by the projector.
Capture Feature Geometry for Alignment
It's important to make sure there is enough feature geometry within each frame to align it to the next and to really lock them together. If there aren’t enough natural features, you can artificially add reference objects. The alignment feature has to be big enough to do the job. I used styrofoam cereal bowls for scanning the train front. The half bowls and plates were about right size for an object this large.
Having put the effort to scan a large object such as a train front, you can continue to refer back to it, time after time. It's like having it on a CMM on your desktop. Even with objects of this size you can still clearly pick out the detail, including the tape used to hold the markers in place!