DIY Laser levelling using webcam and laser level

[John McNamara]

Hi All
some homework for the weekend.

A general search using... diy scanner module from laser printer

https://www.google.com/search?ei=pBQ...4dUDCAs&uact=5

Also found this

https://www.ebay.com.au/i/2638943681...SABEgJdBPD_BwE

Among these

https://www.google.com/search?q=swee...99023353302373

And this from Bangood
Looks similar to the first one but costs more.

https://www.banggood.com/SLAMTEC-RPL...r_warehouse=CN

Regards
John

[devmonkey]

Hi John,

Do you have a picture of the 'polygon' prism from the laser printer? Is it a penta-prism? If so then this would be suitable, if it is just a 90 degree mirror then we wouldn't be able to align it accurately enough. I have been looking for a penta prism, we would need an accurate one, most are specified in arcminute accuracy but the super precision ones are around a few arcseconds. I'm guessing this means is that over 1 meter the error in height measurement due to the prism would be:

1 arcminute: 290um (a massive 0.29mm)
1 arcsecond: 5um

So this begs the question, how accurate are these super expensive industrial machine levelling metrology systems that use lasers? What I have now with the cheap laser and camera sensor is within say 50um per meter, given there is a limit on the precision of the prism are the industrial products really much better than this?

Cheers, Joe

[devmonkey]

Hi John,

Do you have a picture of the 'polygon' prism from the laser printer? Is it a penta-prism? If so then this would be suitable, if it is just a 90 degree mirror then we wouldn't be able to align it accurately enough. I have been looking for a penta prism, we would need an accurate one, most are specified in arcminute accuracy but the super precision ones are around a few arcseconds. I'm guessing this means is that over 1 meter the error in height measurement due to the prism would be:

1 arcminute: 290um (a massive 0.29mm)
1 arcsecond: 5um

So this begs the question, how accurate are these super expensive industrial machine levelling metrology systems that use lasers? What I have now with the cheap laser and camera sensor is within say 50um per meter, given there is a limit on the precision of the prism are the industrial products really much better than this?

Cheers, Joe

To answer my own question, this system:
https://www.hamarlaser.com/systems/l...th-plumb-beam/

Has a stated accuracy of 0.5 arc seconds (2.5um per meter):
Laser plane flat to 0.5 arc second (.00003 in/ft or 0.0025 mm/m).

No idea about price but can't imagine it is cheap!

[devmonkey]

I have had an idea, or maybe remembered something previously discussed here. To get around the error in the laser line which is proportional to arc length how about first flattening the master rail in isolation with the laser projecting along the rail, this uses less that 1 degree of laser arc. The master rail is now flat. Now move the laser so it is orthogonal to the master rail and projecting across the frame as to encompass the slave rail we want to bring into plane with the master.

The height deviation from flat caused by the error in the laser line now we are using say 30 degrees of arc can be measured and the true deviation calculated for stations along the known flat master rail and then projected using simple geometry to stations on the slave rail so that it can be also be flattened bringing it into plane.

Sketch:


This should work shouldn't it, if we consider that a ray of light travels straight and that a very small arc of laser line is 'ray like', and if we consider the laser as a point source (which it isn't). Thoughts?