DIY Laser levelling using webcam and laser level

[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

[John McNamara]

Hi Joe
Edited above post

[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.

[devmonkey]

In the pursuit of madness I have just done another test on the milling table with the better quality laser, I did 6 runs each with different parts of the laser line some with the laser inverted, with the angle of incidence to the table random, etc. Again 10 stations over 500mm. Results are very consistent which makes me think there was something wrong with my last test.



I don't think the laser mount is anywhere near stiff enough, it takes a few minutes for it to stop moving around on the screen to start a run and by the end of the run it has drifted by at least half the error of the run. Note that I haven't been testing with the laser on that desk rather out in the garage on a heavy bench over the bench leg.




I think this laser needs to be clamped to a vblock attached to a lump of iron. It also has a focusing ring attached which works well other than it wobbles the laser all over the place. This might need to be locked in place as well.I have now gone from 'borrowing' the laser to owning it so it might well get some locktite on the focus slider.

If anyone else wants one to play with they are sold here, £40.
https://odicforce.com/epages/05c54fb...Products/OFL72

[driftspin]

Hi guys,


I recieved my cheap webcam today.

It is a ASHU H800 1080p cam, about 18 euro incl shipping.

https://a.aliexpress.com/Zos0Tdull


If internals are really 1920x1080 (windows says it is) this might be an ok device.

The chip size is ~6.5mm x ~3.2mm


So this sensor could be 0.003mm ish per pixel.

I wanted a bigger size sensor than the 640 pixel 2.5 mm one. Because of the width of the laser line on my bosch laser


Does the software account for not having the sensor 100% level?
I hope it does.

Or does not having it level actually help achieve sub pixel accuracy .

The lens can be screwed out and put back on so it is only exposed to dust while in use.

I wil drill 2 mounting holes in the housing and bolt it to a piece of 4080 profile.


I will try to install the software and try icw the bosch laser i have.

Will report back on progress.





Verstuurd vanaf mijn SM-A320FL met Tapatalk

[driftspin]

Hi guys,


I recieved my cheap webcam today.

It is a ASHU H800 1080p cam, about 18 euro incl shipping.

https://a.aliexpress.com/Zos0Tdull


If internals are really 1920x1080 (windows says it is) this might be an ok device.

The chip size is ~6.5mm x ~3.2mm


So this sensor could be 0.003mm ish per pixel.

I wanted a bigger size sensor than the 640 pixel 2.5 mm one. Because of the width of the laser line on my bosch laser


Does the software account for not having the sensor 100% level?
I hope it does.

Or does not having it level actually help achieve sub pixel accuracy .

The lens can be screwed out and put back on so it is only exposed to dust while in use.

I wil drill 2 mounting holes in the housing and bolt it to a piece of 4080 profile.


I will try to install the software and try icw the bosch laser i have.

Will report back on progress.





Verstuurd vanaf mijn SM-A320FL met Tapatalk

Ok so i set 3um per pixel and got this result for a stationary setup.

it looks like there is an upward trend... not sure what causes that yet.

maybe the laser is heating up in the bosch auto levelling system moving it over time.


This will be the best setup i can do so far to measure the flatness of the machine so far.

Happy so far.


Grtz. Bert

Verstuurd vanaf mijn SM-A320FL met Tapatalk

[devmonkey]

Hi guys,


I recieved my cheap webcam today.

It is a ASHU H800 1080p cam, about 18 euro incl shipping.

https://a.aliexpress.com/Zos0Tdull


If internals are really 1920x1080 (windows says it is) this might be an ok device.

The chip size is ~6.5mm x ~3.2mm


So this sensor could be 0.003mm ish per pixel.

I wanted a bigger size sensor than the 640 pixel 2.5 mm one. Because of the width of the laser line on my bosch laser


Does the software account for not having the sensor 100% level?
I hope it does.

Or does not having it level actually help achieve sub pixel accuracy .

The lens can be screwed out and put back on so it is only exposed to dust while in use.

I wil drill 2 mounting holes in the housing and bolt it to a piece of 4080 profile.


I will try to install the software and try icw the bosch laser i have.

Will report back on progress.





Verstuurd vanaf mijn SM-A320FL met Tapatalk

The app expects the sensor is in portrait orientation wrt the laser line, it then sums the intensity over each pixel row (sensor column) to produce an intensity curve which it post processes to determine the centre. This means if the laser line is at an angle to the sensor it doesn't matter, it should always find the height of maximum intensity at the centre of the sensor width. You don't want a crazy angle as it will reduce accuracy, a few degrees is fine though.

I'm also going to move to a larger sensor, it takes too long to align the laser to the 2mm sensor over any reasonable distance.

[devmonkey]

So I just tried it with a very nice cheap 1920x1080 camera from Amazon, sensor is perfect for our use, easy to rip apart and about 6mm of vertical resolution. However there is a problem, the library I'm using to connect to the camera only supports YUV format, which is a raw uncompressed image format. This means that over the USB connection higher def cameras cannot deliver the full frame rate of the camera because there is too much data to ship, I can only get 5 fps.

I will have a look about for a different library, this would use MJPEG to compress the frames on the camera so they fit down the USB connection, that may in turn lead to other problems due to compression artifacts reducing the image accuracy, will have to see.

[davebaldwin]

Are you aware of this: https://openmv.io ?

It is a single board computer with camera. You can develop machine vision algorithms in python or in C. They have different sensors but I am not sure they have one with a large enough area for what you desire.

I can see how you would setup a single rail in a straight line with a laser/camera combination but don't understand how you would use a generated laser reference plane to setup a second rail.

Dave.