DIY Laser levelling using webcam and laser level

[devmonkey]

One final test, I repeated the straightness test on my milling machine table, again 10 stations over 500mm, 3 runs again eyeballed off a ruler bolted to the table. The table is pretty straight within +/- 0.01mm over 500mm.



It is certainly much straighter than my kitchen island !

I did this test very quickly and it wasn't performed perfectly, for example the laser was really too close sensor meaning that the sensor was saturated or nearly saturated around the beam, this would lead to a little inaccuracy as the maxima is effectively on a flat peak, it would be better if the laser was further away reducing its brightness.

[devmonkey]

Forgot to add you can now press 'z' to zero and 'm' to take measurements during a run, this is much easier at least for me rather than fiddling with the mouse.

[devmonkey]

So I have 'borrowed' a much higher quality line laser with decent optics, and believe I have verified that line lasers are not straight. Setup was with the sensor on my milling table moving along 10 stations over 500mm against a ruler that was clamped to the table. Note the sensor was moved, the table was stationary to avoid any rocking on the saddle block.

The laser was setup in a fixture about 2m away so that it projected its line across the top of the table hitting the sensor. The fixture allowed the laser to be rotated so I did 4 runs then flipped the laser 180 degrees axially (so the projected line was now inverted wrt the first runs) and did 4 more runs.

I think this suggests that this laser has a parabolic error in its cylindrical lens or the lens alignment, what do you think? Chart shows all 8 runs, you can clearly see two groups of 4 parabolas, one group the inverse of the other, this parabolic error is greater than any error in table straightness.

[Kitwn]

Joe,
I doubt I will ever be measuring to a few microns in my own workshop but I love the way you are making such precision achievable in a DIY environment. The revelation about the non-straight laser is important but presumably the ability to invert it means the error can be quantified and allowed for. Does this mean a cheap line laser mounted in a suitable rotating/inverting housing could be virtually as accurate as the high quality one?

Kit

[Kitwn]

Joe,
Whilst replying on your build thread an idea came to me. I'd be interested in comments on it's practicality:

Could your laser leveling camera and software, possibly using a taught wire as John is doing, be used to measure the height errors as a gantry machine moves along it's imperfect rails and edit the Z axis G-code to correct for them in much the same way as a probe is used to measure and correct flatness errors in PCB cutting?

In this way a machine of modest accuracy could be used to build parts for a better one. I'm thinking specifically of jobs like grinding the top of gantry profile as you have done for your build.

Initially I'm thinking of only flattening a linear cut as above but since your stated aim is making planar measurements is it reasonable to think of correcting over an area in order to turn plain old ally plate into something approaching tooling plate or even turn a piece of granite worktop into a reasonably accurate surface plate?

Kit

[John McNamara]

Hi All

My Recovering from the holliday's brain has been thinking on line generation.

Having disassembled many laser printers over the years I found in many of them the single point laser was scanned across the page by a (single point) laser beam reflected by a rotating polygon prism and a very nicely ground asymmetric lens.

My first thoughts were that the integral laser fitted to these units may be invisible and possibly dangerous.
So the manufacturers laser diode would have to go and be replaced by a known and safe (BUT YOU SHOULD NEVER LOOK INTO THE BEAM) of any visible laser.

The ground asymmetric lens is an unknown? It may not be of the correct focal length for our application?

This leaves the rotating prism with surface coated mirror facets.
It is driven by a nice control circuit that will be easy to interface.

I have a few modules recovered from scrapped laserjet 3 Any machine with a Canon SX series engine is likely to be similar. I have a couple of laserjet 3 modules kept for an idea like this.

Scanner modules are fairly cheap $20 to $50
Or you may already have one similar? All we are after it the rotating prism.

If this concept proves doable we can generate a nice scanned plane.

Remember in this case the laser itself is not rotated the prism simply reflects the beam as it is rotated through an arc that changes as the reflecting mirror angle changes.
The laser and scanning mirror unit will need to be located far enough away for the beam to cover all the needed measurement points.

It should have three leveling feet.


https://www.google.com/search?q=lase...w=1045&bih=547

Regards
John

[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]

Joe,
Whilst replying on your build thread an idea came to me. I'd be interested in comments on it's practicality:

Could your laser leveling camera and software, possibly using a taught wire as John is doing, be used to measure the height errors as a gantry machine moves along it's imperfect rails and edit the Z axis G-code to correct for them in much the same way as a probe is used to measure and correct flatness errors in PCB cutting?

In this way a machine of modest accuracy could be used to build parts for a better one. I'm thinking specifically of jobs like grinding the top of gantry profile as you have done for your build.

Initially I'm thinking of only flattening a linear cut as above but since your stated aim is making planar measurements is it reasonable to think of correcting over an area in order to turn plain old ally plate into something approaching tooling plate or even turn a piece of granite worktop into a reasonably accurate surface plate?

Kit

Yes you could try to calibrate the machine rather than align it, same as you do for pcb milling which I've used very successfully on very inaccurate machines in the past. It would be more complex if you are hoping to improve 3D (PCB being 2D) but still possible to gain some improvement.

[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!