DIY Laser levelling using webcam and laser level

[Michael Gilligan]

That’s a tricky question, Joe ... but here’s the best I can do for the moment:

In an ideal world, a perfect laser beam would would be refracted by a perfectly cylindrical lens to produce a perfect line on a cylindrical surface.

In the real world, however: Our ‘laser diode’ is an imperfect source and will be refracted by imperfect optics ... with the inevitable [but practically impossible to estimate] consequence that the line will be imperfect.

That said: Your proposed test seems very appropriate.

MichaelG.
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Edit: This is a little laboured, and is intended for an audience of ophthalmic opticians, but it may be helpful to anyone struggling with the general concept of cylindrical lenses: https://youtu.be/vHugCo2md_o
.
Edit: Note, however, that for the laser line generator we are using a negative cylindrical lens
... See Application 8, here: https://www.newport.com/n/beam-shapi...ndrical-lenses
... and for convenience, here’s a direct link to Application 1: https://www.newport.com/n/focusing-and-collimating

[devmonkey]

Ok, but would the line just be noisy or would/could it be bent? I'm not sure I fully grasp how the source is refracted by the cylinder, specifically is it true that a given arc segment of the cylinder is generating a corresponding section of the line or does the entire arc of the cylinder contribute to every part of the line? If the latter then cylinder imperfections would produce nose whereas if former then the line could be bent.

[Michael Gilligan]

I'm not sure I fully grasp how the source is refracted by the cylinder, […].

Ray-tracing would help you understand ... but be warned ... it’s tedious !!

If I can find an easy and convincing answer to your question, I will post it [but don’t hold your breath]

MichaelG.

[devmonkey]

Don't get me wrong I understand how refraction works, what we need to understand is given the light source is not perfect nor perfectly collimated and cylinder is not perfect can these imperfections add up to a line that is bent, or just a line that is noisy (which is fine for our purposes). Presuming that the ideal model of refraction we get taught is not what actually occurs in the real world.

[Michael Gilligan]

... can these imperfections add up to a line that is bent, or just a line that is noisy

Hopefully this was evident from my last response, but:
For clarity, the short answer to that question would be Yes, all results are possible.
... You will see that the quoted specification addresses that ‘uncertainty’ by stating tolerance bands.

MichaelG.

[Michael Gilligan]

For the present purposes, I’m pretty sure that we can take the line as being straight but fuzzy [i.e. noisy]
... Like so many things though: The closer you look, the more difficult it gets to describe.

This is what I find so impressive about this project ... the software is effectively using a ‘best fit’ definition of the line.

MichaelG.

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P.S. for what it’s worth ... here are some numbers from the specification of a ‘metrology-grade’ unit:

Beam Profile Line
Power Distribution over 16 mm ±10 %
Fan Angle 18 °
Focal Distance (factory preset) 180 (±2) mm
Line Length (@ focal distance) ~ 45 mm
Line Width (@ focal distance) 50 (±10) µm
Alignment of Optical to Mechanical Axis ≤ 0.6 mrad
Line Straightness (max. deviation from ideal line) ±2 µm (over 16 mm)
Depth of field (laser line < 60 µm) min. ± 5 mm

[driftspin]

Hi guys..

Can you point me to a cheap (well eh not too expensive) webcam model that might work well, maybe an hd model that has bigger cmos area?



I have moved my machine and want to setup using this method.

I have bought the bosch Quigo self leveling laser for some jobs around the house.
It is a crosshair type laserpointer.

Grtz Bert.





Verstuurd vanaf mijn SM-A320FL met Tapatalk

[devmonkey]

Hi Bert, I use these, only vga but work really well and super easy to take apart and remount on and solid block.

KKmoon USB 2.0 50.0 M Webcam, HD Camera, Web Cam with Microphone for Computer, PC, Laptop, Black https://www.amazon.co.uk/dp/B01L1XAQ..._Y2raEbA1MMSQJ

[devmonkey]

I took a punt and picked up a cheap (£25) Bosch laser today. Main reason was I don't want to tear apart my Dewalt and remove the lasers from the pendulum which I think would be required for our purposes. The Bosch unit has the laser firmly mounted into an aluminium chassis, it is actually quite a decent unit. The laser has a cylindrical lens albeit made of plastic. The lens produces an asymetric line, much more line to one side than the other, but I think this is by design as without the pendulum you couldn't use it to level across a transverse wall as well as the wall you have fixed it to, I think it is inconsequential for our purposes.

Anyway I ripped it apart for your pleasure see photos below. I have also tested it with my levelling software and it works very nicely, I think the laser line is slightly thicker than the dewalt but the app can still consistently locate the centre. It is sufficiently solid, from the exposed casting right to the laser which is both clamped and glued as is the lens, to clamp it directly to something in the garage, the laser will not move at all relative to the chassis which is ideal.

Over the next few days I'm going to attempt to map the surface of my new machine frame I'm building on my other thread using this system, will report back with the findings.



As a bonus it has a dot laser in the other end which I can maybe use for straightening the master rail.

[devmonkey]

This morning I had an empty house so I grabbed the opportunity to do some testing. I used the dewalt laser and my mostly completed gantry, the sensor was clamped to the rear of the lower Y bearing plate and both laser and gantry placed on a 1inch granite slab (kitchen island). Initially I raised the gantry to clear the Z axis on parts of the spindle mount but it turned out these are not machined with parallel faces on each end and I could rock the gantry a few thou, so swapped them out for two hardwood beams.



The test involved two passes in opposite directions moving the Z axis one hiwin bolt hole (approximately by eye), these are on 60mm centres, and sampling the sensor at each hole, I took 9 samples per pass, so around 500mm of travel for each pass. The residuals from a straight line were calculated by the app for each pass.

The two passes (with second reversed so they can be overlayed) are charted below, together with the difference between the passes.



The two passes were fairly consistent given the eye balled positioning I used, and that I hadn't clamped the laser nor the gantry to the granite. Worst case absolute error between the passes was 8 microns (0.31 thou). The sensitivity is ridiculous as I could flex the granite plate more than this by pressing on it.

However it can be seen that the either the gantry or the laser or both are not perfectly straight with a max variance of +/- 40 um (1.57 thou). The results are satisfactory however we return to the problem of not knowing if the laser line is straight, question is do we care about a couple of thou?

Unfortunately I ran out of time to repeat the test with the Bosch laser, I will need to setup this experiment on the concrete garage floor. It is a waste of time trying to do any measuring if people are moving about in the house as you can see every footstep within 5m move the sensor reading.

I also noticed that the laser form wasn't a very good gaussian and near the edges of the dynamic range, i.e. where the laser was starting to drift off the top or bottom of the sensor, the beam centre looked quite arbitrary to me, I think I may add a new detection mode that finds the centre based on the maximum local average reading.

A couple of other points of note,
1. ~2mm dynamic range of the vga sensor is not very much, it would be easier with a sensor with large area.
2. dust on the sensor has an impact on the readings if the particle is large enough.