Metrology - how do you measure from scans?

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Felix_the_Cat
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Re: Metrology - how do you measure from scans?

Post by Felix_the_Cat »

Thank you all for taking the time to reply.

Mr. Andrews hit the nail on the head. I am looking for a little more streamlined way to approach this problem, and it appears that there isn't an "easy button", much as I would like there to be.

Just for anyone else who might be reading this thread, I will throw some more information out there and how I am working through it, in case in might be helpful to someone else. I do not claim that this is an elegant way to get what I'm after, or even the right way, but it gets me where I'm going. I welcome any suggestions that might streamline this process.

I am mainly concerned with planes and holes. I have a CAD model of the structure for nominal values, a laser tracker derived cad model of the hole locations for verification, and of course my scans.

My current approach is to manually delete out portions of the point cloud leaving only the planes and enough points around the holes to define them.

I use Scene to chunk the file into bite sized pieces, but the 3D windows get wacky after only a few zooms and pans, and the near clipping plane gets messed up, so it's a constant process of closing the broken 3D window and reopening a new 3D window until that one goes wonky, rinse, repeat. Also, the zoom all function just plain doesn't work.(Anyone else notice this?)

So I need a better functioning tool to handle the fine deletion of points. Recap seems to do a nice job of this. It also seems to handle decimation and filtering lot better than Scene does.

So once I have the aforementioned file with points representing the planes and holes in question, I take a bifurcated approach. I split the file into planes and holes.

Holes go to Rhino. I have recently gotten the Arena4D plugin from Veesus, and while it was a bit of a struggle to get it up and running it seems to do a fabulous job of handling point cloud manipulation in Rhino. With a little good judgement, fitting circles to the holes is straightforward. I experimented with fitting circles in Spatial Analyzer, but it seems like too much overhead for what I'm after. Maybe as I get more used to scanner data vice tracker data analysis in SA this will change.

Planes go to one of my favorite pieces of software, CloudCompare. Steep learning curve, but man what a great tool. Open source! Woohoo! If you can hang in and learn the clunky interface, it is well worth the effort. Using the RANSAC shape detection plugin I get all the information I need about the planes. Data goes into an excel spreadsheet.

Back to rhino, set up a CP origin on the center of one hole, as that is the reference frame for the measurements requested. Dimension all the holes with ordinate dimensions so they can be checked against the tracker measurements of the same holes for reference. Data into the spreadsheet.

Import nominal CAD model into Rhino. Delete out everything except the planes in question, mesh, export.

Back to CloudCompare. merge the plane scan file with the plane nominal model file. Perform a cloud to mesh distance operation and output the histogram to give a visual representation of the delta. Management types love the pretty colors. *sigh*.

And that's it.

Any and all comments are welcomed.
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Re: Metrology - how do you measure from scans?

Post by max72 »

I really love Rhino and Arena, and used it for as builts, as you experienced the combination is excellent, but if you have to compare to a reference model the maybe an inspection tool is to be considered.
You can also check GOM inspect. It's a free metrological tool. Quite a bit of a learning curve.

The steps are:
Import the cloud
Mesh the pointcloud (or parts of it)
Fix mesh issues
Do fits (cylinders and similar)
Import cad reference
In case orient reference and model
Compare and enjoy color maps

I used it in some case. It takes some time to learn it, but in some cases it's a very nice option.

For as builts Arena and Rhino all the time (check the more recent features, like arbitrary sections tool combined with stored views..)

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ing. Massimo De Marchi - +39 347 32 17 049 - www.studiodemarchi.net
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