DIY Laser levelling using webcam and laser level

[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.

[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

[Michael Gilligan]

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 […]

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

An excellent idea, John

Incidentally: More years ago than I care to remember, I did some environmental tests on a scanner for reading bar-codes at the supermarket check-out [the type that generates a criss-cross of lines] : The interesting thing about this one was that they had dispensed with most of the real hardware and optics ... They were spinning a hologram !!

I don’t know if it ever went into full production.

MichaelG.
.

Edit: ... Just found supporting evidence: https://books.google.co.uk/books?id=...canner&f=false

[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?

[John McNamara]

Hi Joe

The mirrored face prism has 6 sides

It is driven by small three phase board mounted disk motor and a Toshiba TC9242P Motor driver chip.

For a quick test I tried using a small laser pointer and spinning it by hand got a nice bright red line. Very sharp if the target was a short distance away however as suspected on first observation the beam focusing lenses as used by the HP printer were set for a short distance. The dot increased in size quickly as you moved the target away.

However the rotating mirror prism without the lenses worked well, I just spun it by hand for a quick test of the concept.

I will have to do a proper test once I sort out the interface. The laser diode I used was a cheap giveaway pointer!
I removed the invisible light diode as fitted by HP for safety, just two screws and it leaves a nice mounting face for the replacement visible red diode. For this test I just held it by hand....

Just read your post 161

The line is generated by the changing face angle, with 6 faces I am guessing the line generated will be about 60 degrees assuming 360/60 is the correct formula? The HP laser unit itself has a coverage of about 60 degrees.

Mechanical errors?

As I see it the rotating prism mirror presents every side to the laser in turn. The line generated should be straight if the laser is located perpendicular to the axis of rotation. if not on axis a slightly curved line would be generated.

The bearings of the rotating spindle may not be perfect, this may cause the edge of the optical plane generated to wobble up and down for each rotation turn? However your software makes multiple measurements and averages them, This should help cancel out this error.

Regards
John

[devmonkey]

Hi Joe

The line generated should be straight if the laser is located perpendicular to the axis of rotation. if not on axis a slightly curved line would be generated.

The bearings of the rotating spindle may not be perfect, this may cause the edge of the optical plane generated to wobble up and down for each rotation turn? However your software makes multiple measurements and averages them, This should help cancel out this error.

Regards
John

Hi John,

A penta-prism doesn't suffer this problem, it has two faces at 90 degrees, then two other faces at 90+22.5 to the first two, the fifth face is not relevant, some might not even have it or maybe have multiple. Good picture on wikipedia.

[John McNamara]

Hi Joe
Edited above post