DIY Laser levelling using webcam and laser level

[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.

[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.

[Neale]

Forgive me jumping in (and apologies if I've missed something) but is the geometry of the measuring setup ideal? As mentioned, the measurements are dependent both on the straightness of the rail and of the beam itself and it's not really possible to separate out the two. If you turned the sensor through 90deg so that it looked along a line parallel to the gantry, and placed the line source just off the end of the gantry but with the laser line intersecting the sensor, you would then be moving the target down the length of the beam (straight apart from gravitational bending!) rather than relying on the optical quality of the cylindrical lens generating an accurately straight line. However, you would need to get the laser line horizontal to reasonably close limits both on the roll and pitch axes (if that makes sense).

When I first started following this thread I had a picture in my mind of a point laser shooting a beam parallel to the rail being measured and striking the sensor in the middle. What I have described above is really only the same thing but using a line source to make lateral alignment less critical.

Or I might have missed the point but I do find this whole exercise very interesting to follow.

[devmonkey]

Forgive me jumping in (and apologies if I've missed something) but is the geometry of the measuring setup ideal? As mentioned, the measurements are dependent both on the straightness of the rail and of the beam itself and it's not really possible to separate out the two. If you turned the sensor through 90deg so that it looked along a line parallel to the gantry, and placed the line source just off the end of the gantry but with the laser line intersecting the sensor, you would then be moving the target down the length of the beam (straight apart from gravitational bending!) rather than relying on the optical quality of the cylindrical lens generating an accurately straight line. However, you would need to get the laser line horizontal to reasonably close limits both on the roll and pitch axes (if that makes sense).

When I first started following this thread I had a picture in my mind of a point laser shooting a beam parallel to the rail being measured and striking the sensor in the middle. What I have described above is really only the same thing but using a line source to make lateral alignment less critical.

Or I might have missed the point but I do find this whole exercise very interesting to follow.

Hi Neale,

That works fine for checking something is straight, what I'm trying to do here is build a system that can check things are planar, e.g. for bringing the two X rails into plane, hence the line laser.

One idea to separate the gantry error in this test from the laser error is to run the test twice with different sections of the laser line.

I've just tried a much better detection algo, what it does is find the simple maxima of a moving average window of image rows, it then fits a gaussian to this window only, ideally it would just fit a parabola but I haven't coded that. Readings are now much more stable across the entire dynamic range and it works better with wobbly beam cross sections. Just need to migrate to the cold garage to test it.

I think a reasonable measure for success would be for the system to meet din 876 grade 1, which for a surface plate 600mm long means 16um variation in height. The cost of such a plate for my machine is £5500 !
https://shop.mitutoyo.eu/web/mitutoy...08/index.xhtml

[John McNamara]

Hi Joe (Devmonkey)

Firstly... Happy New Year! I hope 2020 is a good year for all.

That new algo sounds interesting.

I wonder if it will also improve the resolution for stretched wire as well?

While you are at it it would help if there was some sort of adjustment to compensate for the image brightness and or contrast. I found this was quite sensitive when backlighting the stretched wire. some sort of adjustment would be helpful.

Anyway looking forward to testing the new algo and comparing the results to my old tests as disclosed here in this thread using the previous algo when you have the time to upload the new one.

Regards
John

[devmonkey]

Hi John,

The algo changes are committed so you can re-download and test. In settings the there is a new 'model' field, this takes a number 0->4 inclusive.

0: Old global gaussian fit,
1: Simple global energy maxima
2. New global parabolic fit
3: Local energy maxima then gaussian fit around this maxima
4: Local energy maxima then parabolic fit around this maxima

For laser the best is 4, please let me know for the wire, you may find that since the wire shadow is much less noisy and narrower than the laser that the original model works best still. Also note I have increased the default smoothing (settings->smoothingFactor from 3 to 20), you may want to reduce this for the wire shadow as it is narrower.

[devmonkey]

So I ran another set of tests directly on my granite island without the gantry, all tests were with the cheap Bosch laser. I ran 3 tests, each measuring 10 stations over 500mm with the laser position moved between each test. Each test consisted of 4 runs, 2 forwards, 2 reverse.

The laser was positioned as follows for the 3 tests

1. Planar test with laser orthogonal to ruler taped to island from a distance of 1.5m so the laser line spanned the ruler.
2. Planar test with laser in same position as (1) but rotated 45 degrees to test a different section of the laser line.
3. Straightness test with laser inline with the rule, i.e. shining along the rulers length.

The idea was that 1 and 2 would test different parts of the laser line arc (say 45 degrees of arc per test), whilst test 3 would use only a tiny part of laser arc, less than 1 degree. The test surface is a granite island that most likely isn't flat but is stable, flatness doesn't matter here as we are testing the laser not the island.

Consistency between the runs of each test was very good, with an peak-peak repeatability error of a few microns, likely due to me eyeballing the station locations against the ruler markings.

The chart below shows the results, overlayed on each other. You can see at a glance the 4 runs for each test are highly internally consistent but that the results vary considerably between tests 1-3. All are parabolic suggesting the granite island has a parabolic surface error of around 100 microns. The jagged line group is test 1, the slightly lower group is test 2 and the lowest group is the straightness test 3.



So this is telling me that the laser line generator is not very straight and has a parabolic nature to it with more noise in the test 1 arc than the test 2 arc.

Since test 3 used a tiny part of the laser arc and light travels in a straight line I think we can assume the test 3 line accurately reflects the true surface straightness of the granite island.

Separated out this is the chart for test 3.



So (although I cannot be 100% sure without a calibrated reference surface) I think this system is suitable for verifying something is straight. This means it can substitute for the usual straightness test of using a precision level to straighten a rail in the horizontal plane or a tight wire to straighten a rail in the vertical plane. However it needs a better laser or a different approach for checking two things are planar.

I have done a bit of research on how this is done in the optical metrology industry and although they use exactly the same approach of measuring height deviations from the test piece to a planar sheet of laser light they also without exception use a rotating laser rather than a line generating lens. Infact they use something called a penta-prism which bends a laser beam 90 degrees irrespective of the the angle of incidence of the beam to the prism. This means so long as the laser is fixed solidly and somewhat vertical as the prism is rotated above the beam in a solid bearing you will cast a perfect plane irrespective of bearing runout or laser prism miss-alignment.

Unfortunately penta-prisms with a few arc-seconds of accuracy are rather expensive. If you use a cheap less accurate one then you could well be casting a cone rather than a plane as you rotate the prism.