DIY Laser levelling using webcam and laser level

It could be aliasing from reflections on the lens holder thread, I got much better results with the sensor mounted with nothing close to it in the near field. See picture below, that window started off as a 6mm round hole, it caused aliasing hence I opened it up so no reflected laser light could hit the sensor.
Attachment 26688
I don't know much about dslrs but maybe it could have lens distortion correction built in which would be warping the image non-linearly across the sensor. Have you got a cheap web cam to try with? It is also unlikely that sensor pcb is hard fixed to the body, the ones i've take apart are usually on soft mounts that let the board move around to absorb shocks.

It's cheap Sj4000 clone distortion is massive, if there is correction it does very bad job. I have cheap webcam and 0.08mm steel wire on order. Will try both laser and wire to rule out sensor nonlinearity errors.

Quote:

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Originally Posted by Sasquatch

It's cheap Sj4000 clone distortion is massive, if there is correction it does very bad job. I have cheap webcam and 0.08mm steel wire on order. Will try both laser and wire to rule out sensor nonlinearity errors.

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One other thing that occurred to me, how did you ensure you moved the sensor/vice assembly absolutely in a straight line? Any deviation in this procedure would show up as non-linear errors in your linearity measure since it would have been impossible to get the laser perfectly planar with the granite. This applies to both types of laser dot and line. I think you need a precision straight edge as well as the reference surface to check the accuracy of the system.

Quote:

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Originally Posted by devmonkey

One other thing that occurred to me, how did you ensure you moved the sensor/vice assembly absolutely in a straight line? Any deviation in this procedure would show up as non-linear errors in your linearity measure since it would have been impossible to get the laser perfectly planar with the granite. This applies to both types of laser dot and line. I think you need a precision straight edge as well as the reference surface to check the accuracy of the system.

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I used straight edge clamped to the granite, just as you said.

Cheap webcam arrived, useless with lasers, sensor is smaller than the beam, will test it with wire when it gets here.

Quote:

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Originally Posted by Sasquatch

I used straight edge clamped to the granite, just as you said.

Cheap webcam arrived, useless with lasers, sensor is smaller than the beam, will test it with wire when it gets here.

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Sounds like your laser is no good, can you post a picture of it? Even my dewalt laser level produces a line narrower than the VGA sensor.

Greetings from NorthWest England

I was alerted to this very interesting project by John McNamara [many thanks, John] and have joined the forum to participate in this discussion. ... Hopefully I can be of some help on the optical side of things, but this first post concerns the software:

I have tried running the current version on my 2009 MacBook Pro, under el Capitan, and everything seemed fine except that I could not close the application ... it did not even respond to ‘Force Quit’ and I was obliged to re-boot the machine.

Windows users have a .bat file; so I am wondering if some ‘initialisation’ might be required before Java is happy running the application under OS X

I would be most grateful if anyone can advise.

MichaelG.

Hi Michael,

Welcome! I haven't tried it under OSX, but I'm pretty sure someone in this thread did.

It might well be a driver issue with it not unloading properly, unfortunately I don't have a mac book pro old enough to replicate your system.

Joe

Thanks for the prompt reply, Joe

Hopefully there is someone else out there, limping along with a much-loved but obsolete Mac !

MichaelG.

.

Edit: I’ve just tried a little ‘self-help’ and found this:
https://apple.stackexchange.com/ques...vents-shutdown

Unfortunately it is beyond my comprehension.

Quote:

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Originally Posted by Michael Gilligan

Edit: I’ve just tried a little ‘self-help’ and found this:
https://apple.stackexchange.com/ques...vents-shutdown
Unfortunately it is beyond my comprehension.

---

As far as I can see, that's just talking about the issue and other ways to force quit the application.

I'd try uninstalling and reinstalling java on your Mac.

Failing that, you could try running Linux in a virtual machine, and download Java onto that. I'm a fan of Oracle's Virtualbox.

Thanks, Andy

I will try a clean install of Java this evening
... There have been several versions on my Mac over the years, so perhaps there is something unnecessary lurking.

MichaelG.

.

Edit:
I removed all trace of Java from the machine [following Oracle's instructions], and installed afresh:
____ You have the recommended Java installed (Version 8 Update 231) ____

For whatever reason: The application now closes as it should. ... Thanks again, Andy

Hi Michael :)

I am glad you have popped in here to shed a little light on the optical side of these experiments.

Regards
John

I think it worth sharing this FAQ answer from Edmund Optics:

Quote:

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Frequently Asked Question
Question

What is the difference between refractive and diffractive line generators?

Answer

A refractive line generator uses a cylinder or rod lens to focus a laser in only one axis only (drop-moved placement) in order to create a line of light. A diffractive line generator uses a flat optic with an etched microstructure that breaks apart a laser beam and forms an interference pattern in the shape of a laser line. Refractive optics do not correct for the inherent Gaussian profile of a laser beam and form a line with a "hot spot" in the center and fading edges. Diffractive optics will create a line that is uniform in thickness over its length, but is segmented. Diffractive line generators also cause a small portion of the light to be redirected into different diffractive orders, which causes additional faint lines to appear. Some laser line generators use a unique patented Powell glass lens design in order to achieve a continuous (not segmented) line with an even distribution along the length of the line.

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Here is Powell’s patent:
https://worldwide.espacenet.com/publ...C&locale=en_EP

and a more comprehensible description:
https://www.laserlineoptics.com/powell_primer.html

MichaelG.

Hi Michael

Buying A Powell lens from Laserline, Edmund or other big optical equipment suppliers is pretty expensive starting at around $250 USD. I did a little google search and found a few from Ebay etc. at a more reasonable cost for an amateur experimenter. The question is which one?

I have not yet got my head around what would be the ideal angle for the Powell lens assuming that the l operating range is from 100 to say 3000mm.

The laser driving the unit also is also a consideration I nice clean spot is required.

Regards
John

https://www.google.com/search?q=powe...l%20lens#spd=0

You’re quite right, John ... they are pricey, ‘though It’s encouraging to see better prices on ebay

My post was really just for technical information.

For what you are are doing [with the wire shadow and very small displacements], I don’t think you need worry about laser lines anyway: Unless I am very much mistaken, you could use any collimated light-source that filled the area of the sensor.

As for laser spot quality ... the classic solution is to use ‘Spatial Filtering’ ... but that’s another potentially costly and bulky solution to a problem that might not exist.

My impression is that this project is all about leveraging the precision of mass-market sensors with ingenious software.

MichaelG.

Hi Michael,

Do you have a view on how flat the projected plane of a cylindrical lens laser line is likely to be? This is something I still haven't measured as I have no large enough reference plane.
Along a given ray it should be absolutely straight but will it undulate across 'rays'? By ray I mean a line from laser source to sensor.

My latest thoughts on how to measure this was to place four pillars in a square on a reasonably flat stable surface (concrete floor). Place the laser so that it projects across the tops of the four pillars and use height readings from 3 of them (as any 3 points will be planar) to bring the fourth into plane with some shimming. Now move the laser around the room and have it cast a different area of the beam over the pillars and measure again, check if all 4 pillars are still planar, repeat this a few times.

Thanks, Joe

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.
.

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

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.

Quote:

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Originally Posted by devmonkey

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

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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.

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.

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.

.

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

Quote:

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Originally Posted by devmonkey

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

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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.

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

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

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.

Attachment 26995 Attachment 26996 Attachment 26997 Attachment 26998

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

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.

Attachment 27001 Attachment 27002 Attachment 27003 Attachment 27004

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.

Attachment 27005

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.

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.

Quote:

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Originally Posted by 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.

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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

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

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.

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.

Attachment 27009

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.

Attachment 27010

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.

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.

Attachment 27011

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.

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.

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.

Attachment 27035

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

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

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

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

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

Quote:

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Originally Posted by 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

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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.

Quote:

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Originally Posted by 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

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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!