DIY Laser levelling using webcam and laser level

[John McNamara]

Hi Joe (Devmonkey)
Hi All

Firstly I am assuming profiled rails are being used not round rail for high accuracy, repeatability and negligible backlash.

Lateral adjustment of rails is difficult, particularly when you take into account the small amount of adjustment allowed by the clearance around the mounting screw in the bored hole in the linear rails. for type 25 rails typically a 6mm hole and a 5mm machine screw. This only allows +- 0.5 mm of lateral adjustment, and this is if all the mounting holes are perfectly placed. Note in the description the order of processing to minimise chips creeping into the joining surface and lifting the rail.

To solve this problem I devised method where the rails were positioned laterally first using cams. Once this was done I drilled and tapped the mounting holes using an inexpensively built jig. You can make the cams yourself or get them laser cut. You still have to countersink them as detailed in the link.

It is documented here:
https://www.model-engineer.co.uk/for....asp?th=139042

The method worked very well. Alas I have not completed the machine.
I can't wait to get back to it! Alas work has been getting in my way too.

Regards
John

[devmonkey]

I found a few different self adhesive glazing gaskets to experiment with. The one that works best is a 5x10mm rectangular profile neoprene strip, no idea where it came from. It isn't the very soft foam draft stop stuff, nor is it full rubber, it is designed to be compressed between the rebate and the glass on a fixed window. It is a closed cell structure not the sponge.

I also picked up the aluminium from SPA, huge stocks and fantastic price with cash at the door, perfectly happy with me picking up a couple of lengths.

So the experiment was to check the compression rate of the gasket and also whether I could inject some potting compound. I used around 400mm of 50x6 flat bar and clamped it into two lengths of the gasket stuck to a bit of 4x2. With the clamps at maximum pressure the gasket was compressed to about 2mm, the clamping force from the bolts I'm planning to use will be at least as much as from these hand clamps. Also the gasket is strong enough to just distort the aluminium which is perfect.

A 6mm hole was drilled in the middle for injection and countersunk. I used a normal decorators caulk gun to inject glazing sealant which has about the same consistency as West's 105/205 colloidal silica thickened epoxy which is what I will use on the actual job. The gun was pressed hard against the countersunk hole and sealant injected. I had to use quite a lot of force but it pushed the sealant through easily,.

So not very scientific but all very promising. Next step is to drill up the sub-plates and get them mounted on the machine frame.

[driftspin]

Ok so i found a little time to do a leveling test at the new home of the machine.

I have setup the laser pointing along the long axis crosshair at about the middle of the gantry.

My flexing concrete floor is making things much harder then i expected while measuring adjusting and measuring again.
Walking around influences measurements.

My concrete floor is cast on sand, dont have any more information about it now.


In general all 4 x,y corners are in the same plane now .. within about 5 um.
Trying to get it any better seems silly compared to what walking around does to the measurements.

In the end my goal is checking the machine for general flatness and setting it up level

Both the long axis now look like they are within 10um deviation over 1800 mm of travel, this is far better than i expected.
Thank you epoxy.

Having the laser setup on a solid object other than the cnc table frame is a must.
When connected to the frame things get complicated quickly when 1 corner is adjusted everything changes.

Like mentioned here before, now we can visualize micrometers everything looks like made of jelly.


The gantry though looks like it has a deviation of about 50 um upward in the middle. This is a bit weird to me, the epoxy proces was done the same way and the axis is much shorter. There was no welding after epoxy.


I am going to re test this axis by moving the laser 90deg and in line with the gantry.
This should cancel out some optics quality issues. I guess the projected line could be non straight.

Thank you guys, for this cool solution.


Grtz Bert.



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[devmonkey]

Hi Bert,

Sounds like it is working for you, fantastic. What laser and camera are you using? Are you sure of the units, i.e. did you set the um per pixel?

I'm assuming you moved the machine frame to meet the laser plane to get the corners so close, adjustable feet?

My 6 inch concrete slab also looks like jelly with this system.

Joe

[driftspin]

Hi Bert,

Sounds like it is working for you, fantastic. What laser and camera are you using? Are you sure of the units, i.e. did you set the um per pixel?

I'm assuming you moved the machine frame to meet the laser plane to get the corners so close, adjustable feet?

My 6 inch concrete slab also looks like jelly with this system.

Joe

Today i got to some measuring again.

About my setup.

I have a cheap bosch crosshair laser which I now know is not up to this task.

The webcam is a 1920x1080 6.2 ish by 3mm sensor... so +/- 3um.

Yes i have m12 adjustable bolts in the feet.
10 deg of turning on the feet is clearly visible in the software.

About the units i am sure it is still on defaults so 1um, need to check.
Yes my measurements in the earlier post are off by factor 3.
Not yet that important since i was testing the setup and finding out the sop.

### about The laser.
The selflevelling seems to be spot on.
The optics are not.
I found out today.
Moving towards and from the laser the light is straight (light travels in a straight line) moving along the projected line gives false readings. +/- 60 um / 80 cm at about 3meters distance so only a few degrees

Now i know this, it is okay the way it is.
Just make sure you move in a straight line from and to the lightsource.

I have to put the laser a bit further away to get into the better sensitivity range of the camera. It all helps.

Actionplan:
* Input the correct units in the software
* Use a tripod laserplatform
* Put the tabel diagonals in plum.
* Check the 2 long axis.
* Check the short axis for horizontal variations.
* Check the short axis at lowest point and highest point to check top and bottom rail for allignment.

I have not thought of a way to check the spindle for misalignment by webcam/laser without trusting the optics.

But maybe a conventional plumbline and a light source (is that the right English word?) can do the job.


Grtz Bert.

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[devmonkey]

Really useful details, thanks Bert.

When you see the error from moving along the projected line, is it random noise or is it like the line is bent up or down in the middle, like your gantry measurement ?

I think I have seen the line bent like this. I did a little experiment with a dot laser and a glass cylinder (wine glass stem). When the laser passes through the cylinder at 90 degrees you get a straight line projected, when you angle the glass a lot you get a visibly bent line either up or down depending which way to hold the angle.

Maybe this is a common problem with the line optics, and maybe we could calibrate it out by taking two sets of measurements along a straight edge with the laser flipped 180 axially for each set. Will have to think on this, any thoughts welcome...

Although it would not be easy to flip your laser as it is on a pendulum.

[driftspin]

Really useful details, thanks Bert.

When you see the error from moving along the projected line, is it random noise or is it like the line is bent up or down in the middle, like your gantry measurement ?

Hi devmonkey,

Yes it was like you described with the laser vs wineglass stem.



The noise was within 5 orso units.
when measure is pushed several times in 60 sec... maybe +/- 1 or 2 units.




Grtz Bert.






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[devmonkey]

Hi Bert,

I have had an idea of how to create a tool to check whether the rails are planar that doesn't matter if the laser line is bent.

It is effectively a large scale repeat-o-meter using the laser rather than DTI and thereby avoiding a very long infinitely stiff top bar:


The setup would consist of 4 spherical feet arranged in a square. 3 of these feet would be rigidly mounted on a triangular frame, the fourth foot would float on an arm that can pivot up and down on a hinge at the centre of the square. The camera would be fixed above this forth foot and the laser fixed to the triangular frame diagonally opposite. None of this would need to be high precision, the only requirement is that the triangle be stiff.

To calibrate the tool you could use any reasonably flat surface, it doesn't have to be a perfect plane but shouldn't be pitted.

Lay the tool on the surface and mark where the 4 feet touch as accurately as you can. Take a height reading of the laser on the sensor. Now lift the tool and rotate it 90 degrees such that the feet sit back on the 4 marks made and take another height reading. I think that the middle of the two height readings will be the point where all 4 feet are in plane, so we zero the tool to this mid point.

If this is a new machine then you would level the master rail bed in isolation using the laser projecting along it.

Now place the new calibrated tool on the machine frame such that two feet of the triangle rest on the master rail bed and the other two on the slave rail bed. Now adjust the slave rail bed under the floating foot until you read zero. These four points on the machine are now in plane. You can check the tool is still calibrated by rotating it 90 degrees on the machine frame and checking for zero.

We now have two points on the slave rail bed that are planar with the master so the slave rail bed can now be levelled in isolation to a line passing through these two points.



I think this a similar approach to using two crossed wires between 4 points to check for plane when the wires just touch, but this should be much more accurate.

If you had a machine where the two rail beams are adjustable at the ends then this would be an extremely simple way to bring them into plane.

Cheers, Joe

[driftspin]

Hi Bert,

I have had an idea of how to create a tool to check whether the rails are planar that doesn't matter if the laser line is bent.

It is effectively a large scale repeat-o-meter using the laser rather than DTI and thereby avoiding a very long infinitely stiff top bar:


The setup would consist of 4 spherical feet arranged in a square. 3 of these feet would be rigidly mounted on a triangular frame, the fourth foot would float on an arm that can pivot up and down on a hinge at the centre of the square. The camera would be fixed above this forth foot and the laser fixed to the triangular frame diagonally opposite. None of this would need to be high precision, the only requirement is that the triangle be stiff.

To calibrate the tool you could use any reasonably flat surface, it doesn't have to be a perfect plane but shouldn't be pitted.

Lay the tool on the surface and mark where the 4 feet touch as accurately as you can. Take a height reading of the laser on the sensor. Now lift the tool and rotate it 90 degrees such that the feet sit back on the 4 marks made and take another height reading. I think that the middle of the two height readings will be the point where all 4 feet are in plane, so we zero the tool to this mid point.

If this is a new machine then you would level the master rail bed in isolation using the laser projecting along it.

Now place the new calibrated tool on the machine frame such that two feet of the triangle rest on the master rail bed and the other two on the slave rail bed. Now adjust the slave rail bed under the floating foot until you read zero. These four points on the machine are now in plane. You can check the tool is still calibrated by rotating it 90 degrees on the machine frame and checking for zero.

We now have two points on the slave rail bed that are planar with the master so the slave rail bed can now be levelled in isolation to a line passing through these two points.



I think this a similar approach to using two crossed wires between 4 points to check for plane when the wires just touch, but this should be much more accurate.

If you had a machine where the two rail beams are adjustable at the ends then this would be an extremely simple way to bring them into plane.

Cheers, Joe

Hi Joe,

I am not sure i understand correctly.
Does this device need to be at the exact centre of diagonals?

Doesnt the mentioned reference flat surface have to be in the same paralel plane as the rails then?



Grtz Bert.

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[devmonkey]

Hi Bert,

No nothing needs to be centred on the diagonals, the only restriction is that the device can only check a square not a rectangle, but this doesn't matter as we can always define a square ontop of the rectangle formed by our pair of rail beds.

The reference surface doesn't really exist. What we are doing is defining a square as four points on the device, call them A, B, C, and X:

AB
XC

With [A,B,C] fixed in relation to each other and X able to move up and down. Since [A,B,C] are fixed relatively to each other (in a triangle) they form a plane. The laser is fixed to this plane on the device near B pointing towards X. The camera is fixed to X.

Now define a square the same size on a table top or the machine frame or whatever, label the corners of this square 1, 2, 3, 4:

12
43

Place the device over this square such that A==1, B==2, C==3, X==4. Measure the laser height on the sensor at X. Now rotate the device 90 degrees so that say A==2, B==3, C==4 and X==1 and measure the new laser height at X which will have changed because your surface defined by [1,2,3,4] you drew on your table is not a perfect plane.

Now if you were to move X up or down so that the laser measured height is half way between the two heights measured then I think that X will be planar with A,B,C. So zero the software at the mid height between the two X heights measured and you now have a calibrated device which will read zero when placed on a true plane and a signed error when placed on an untrue plane. In other words [A,B,C,X] will be planar when the newly zero'd X is at zero. You can use this to check a machine is in plane and adjust the machine as required until it reads zero.

I haven't tested this only thought about it, I might be missing something, will draw it in CAD and check the theory is correct.

Cheers, Joe