DIY Laser levelling using webcam and laser level

[devmonkey]

Ah, so you're worried about the wobble in the horizontal projection. The thing is, because of the way the laser leaves the slit, it's going to be quite smoothed out.

If you just swoop left to right with the pipe and it stays within the meniscus, you're laughing. A 4m projection distance and a 2mm meniscus means that you'd be within 0.05mm (back of the envelope..), which is on par with a ~£100 precision straight edge. To know if it's better than that, just increase the distance further and further - if you can extend it to 10m you're down to <0.03mm for example.

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Thanks for persevering with me Andy. This is what I mean imagine the laser line generator is aimed at a wall and we are looking at that wall:

So given we know that the 'self levelling' laser is going not going to be perfectly level to earth, we want to check the error between the generated line (red) and a true straight line (blue) to determine how straight our generated line is.

These lasers generate lines by firing through a cylinder of glass mounted vertically in front of it, when the light leaves the curved surface it refracts in one dimension causing it to fan out, therefore projecting a horizontal line. The glass cylinder cannot be perfect so there must be some imperfection in this projection. Any idea how much before I bother trying to measure it?

Or am I missing some fundamental understanding and that the physics implies the line generated will always be perfectly straight?

[AndyUK]

The more I think about it, the harder it becomes! I'm not sure there is a good way to measure it without a precision straight edge as reference.

Essentially, I don't think it's worth worrying about - I say this because of how that lens is working - if the laser was a single point source then each bit along the horizontal line comes from a different part of the lens, and you'd be right to worry about lens defects causing wobble - but the laser isn't a point source, it has some width. This means that the contribution to any position along the horizontal is an average of a few positions on the lens, so any minor defects are smoothed out. Furthermore, the signal you detect is an average of the laser speckle, which is interference at the detector from a lot of different path lengths.

Not convinced? Okay, what happens if you move the laser slightly and repeat the measurement? If there isn't any systematic variation in the laser, you should get the same result. So my advice is average a few measurements with your system after moving the laser a few times; the standard deviation will be a combination of all your errors, including the wobble in the profile.

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[devmonkey]

Thanks Andy.

I guess if the lens was canted slightly so the beam was not hitting the cylindrical face at the lens normal you would get a bowed line, but this should be easy to spot and also probably not hugely important for aligning two rails in a plane if you positioned the laser inline with the mid point of the rails.

I will start building a proper mount for the sensor and share the software incase anyone else wants to experiment.

[John McNamara]

Hi All

As some of you may know I am interested in using a stretched very fine .008" piano wire to determine straightness using a microscope.

I posted my thoughts through this post here on this site. http://www.mycncuk.com/threads/12534...-and-reference

Subsequent to those posts I found an excellent thesis written by Boris Borisov

"New optical sensing system applied to taut wire based straightness
measurement". Doctoral thesis, University of Huddersfield.

http://eprints.hud.ac.uk/id/eprint/2...inalthesis.pdf

I was quite surprised that the accuracy of high end industrial lasers was inferior to the optically sensed stretched wire method he developed over longer distances >2m. See the graph on page 28 of the thesis.

Even more surprising was the simple sensor he used, Omron EE-SX1096 and EE-SH3 photomicrosensors Cost about 2 dollars on ebay!

This is definitely something I want to try. The levels of accuracy he has achieved are outstanding; a few um.

Regards
John

[devmonkey]

Hi All

As some of you may know I am interested in using a stretched very fine .008" piano wire to determine straightness using a microscope.

I want to try this too John.

As for the sensors, certainly the one i'm using, the resolution is simply amazing. This sensor that came from a £7 webcam from amazon has an active area ~2mm wide, this is 640 pixels, so ~3um pixel spacing. You can quite easily with the averaging achieve sub pixel accuracy. That thesis is excellent, looks like he averaged a few thousand frames.

I've decided to use this new sensor as it is easier to mount and seems to behave exactly the same with the laser as the one I was previously testing.


It is out of one of these:
https://www.amazon.co.uk/gp/product/B01L1XAQAS/

[John McNamara]

Hi Devmonkey

I have been discussing alignment with a good mate who is planning a very nice servo driven router, he may pop in here. We built an arduino driven device to sense a stretched wire buy mechanical contact. He did the electronics I did the mechanical s. Being driven by the arduino we were able to cycle it thousands of times. the attached dial indicator never varied by more than a couple of tenths 90% of the time. occasionally there was an error probably caused by a dirty contact. or maybe some mechanical stiction.

The software looked for a break of contact. First finding the wire on contact then slowly moving away.
A couple of times per cycle.

The mechanical system used a 10:1 lever, a flexure and a stepper driven with 4:1 reduction M8 (1.25mm pitch) screw drive. It was accurate to better than .0001"

I would like to try using a webcam as you have described instead of the electrical contact and compare the results. Being non contact it should not have the dirty contact issues.

I would love to try the software you have worked on if you are willing to share it? . Is it running on a PC? or a micro?

Regards
John

This photo is a little unclear. I still have the unit I will take some better photos. It was made from scrap material I had to hand as a test of concept only prototype.

[devmonkey]

Just a quick one but surely if you are going to use a image sensor it is easier to reference off the perfectly straight laser? I'm also not sure why the stepper motor part is needed, wouldn't you just read the error straight off the image sensor and convert pixels to um?

In my opinion and how I visualise this is you are only ever measuring errors to a known reference you don't need to be parallel to it whether it is a wire or a laser or a precision straight edge. What you are trying to do is ensure these errors lie on a straight line, or in the case of two rails in the same plane.

If you want to use the cmos sensor as a shadow camera without optics my software would work if I inverted the image intensity as you'd get a guassian shadow from a round wire strongly back lit. Mechanically with no optics you need the wire very close to the sensor which may complicate things both wrt protecting the sensor but also aligning your wire in the first place. I would just rebuild a USB microscope at fixed focus in an aluminium block.Ideally you need to measure in 2D not 1D so you can set it straight in both dimensions at the same time, this can be done with two microscopes setup orthogonally on the same block. With a normal dot laser (if you are worried about twist then use a cross laser) pointed directly at an image sensor you are measuring 2D with a single sensor, with a wire you would need two sensors as you cannot 'look' down the wire. The cheap £15 microscopes on Amazon at mid resolution have a depth of field that could maintain focus on your wire if your setup error is within 0.5mm or so. Just be aware that you have to remount the guts of it in something rigid as the assembly that lets the optics slide in and out and the part that holds the sensor pcb can wobble. The latter reason is why i've dropped the optics all together as it simplifies mounting it.

[devmonkey]

No more blutack!

A few happy hours with the milling machine and I've finished a rigid sensor mount. I surfaced the base plate so it the two faces are planar, then drilled it for the hiwin block. Then milled a true cube of Delrin with 6 sides surfaced in planar pairs, machining a cavity in one face to house the camera. Finally a window was milled. The glass in the window is a microscope slide cover slip, extremely thin, not sure how long it will last.The cube was drilled in two directions so it can be rotated 90 degrees for using the laser as a straight edge rather than a plane.

The cube is loaded onto the base plate with a bolt and a spring so it can still be rotated about the Z axis when in use, this means I can transfer it from one rail to another and pivot it back to look at the laser. Hopefully since I surface milled the contact surfaces it will remain in plane when rotated.

For reference that is a 20mm hiwin block. I managed to get a smudge on the sensor, doesn't really impact performance.

[John McNamara]

Hi Devmonky
Hi all

I agree that you could use the output from the sensor to directly plot position.
Using a simple webcam with a 2mm sensor to centre a .008" piano wire or laser, the effective measurement range would be maybe 1mm. The active area of the sensor needs to be found in some way, unless the wire or beam is positioned within the active area all the time.

As you can see below, I have been thinking of making a device that flattens surfaces by "Pecking" using a small cutter stepped over a work piece. The overlapping pecks will create a flat surface that in the end will look like an engine turned surface. Another way of thinking about it is as if the surface was scraped. As with scraping practice the last step is to lightly stone the surface to remove burrs and correct any high spots.

When the positioning device shown earlier was being designed the range of motion was considered - what sort of materials would be measured and for my purposes flattened? I was thinking rolled 50-100mm square steel sections, weldments etc. I reasoned that maybe a 5mm range of motion would be desirable to flatten these objects. Large bends in thin walled tube would obviously not work if severely bent, and would be cut right through without the addition of some sort of built up face made from Steel, Aluminium or Epoxy, standard practice in industry.

My design intent was to design a simple machine to flatten objects that could be built from very low-cost materials. There are numerous posts on the Web regarding the creation of flat surfaces on DIY built CNC Routers Mills and other machines, various methods are used, some quite costly. Many are not entirely successful.

As I described earlier the test jig uses a lever flexure and screw driven by a stepper motor. I used the same mechanism to position a grinding jig to within a repeatable mechanical accuracy of .0001" 2.5 microns. I know it works. It has a range of motion of 5mm.

The mechanism is able to apply a significant force the 10:1 primary lever and flexure provides enough force to easily position the Z carriage. (Flexures are free of backlash although geometric errors have to be considered).

My plan is to also drive the mechanism with a crude Y motion that guides the sensor head and cutter along the work piece. I imagined using a piece of steel, say a piece of I beam or other available steel section with simple ball bearing rollers guiding a carriage along it, the piece being flattened would be placed upon it. The accuracy of the flattening is determined by the wire and sensor not the beam or mechanism. The only preparation of the support beam needed would be to hand grind and stone the surfaces that the ball bearings run on for smooth running.

Two possible arrangements are:

For a horizontally disposed flat surface generator.
The x axis requires very little travel, maybe 75mm will cover just about any bearing surface I can imagine. I am still thinking on this. Moving the work piece laterally on two small slides would be simple enough for shorter lengths. Longer lengths would be a different matter - they would require mid span support or they will deflect slightly due to cutting forces and gravity. The X axis motion will require high precision, however over a very short distance. The two or more slides will all have to be coplanar.

For a vertically disposed flat surface generator. (This has always been my preferred option.)

This solves a number of the design issues of the horizontal arrangement. The y axis can be the same as previously described (except it now has to be described as the Z axis!)

In this case two alignment wires can be used, one each side of the Z axis support beam, simply suspended with a counterweight (the weights can be placed in a container of thick oil to dampen them).

Using this arrangement there is no catenary sag at all, the wires will be perfectly coplanar at all times.

The Cutting head can be mounted on a small precision X axis slide.
To align the x axis slide to the work piece will require two cameras and two stepper driven positioners as described earlier. The software will need to go through a few iterations to get the two cameras in perfect alignment.

The work piece can be simply clamped to the Y axis base beam.

Cost?
2 web cams
2 wire alignment steppers.
2 timing pulleys and a belt

1 X Axis drive stepper.
1 X axis 100mm slide, bought or built.
1 X Axis screw drive - only low resolution is required.

1 Y axis drive stepper
1 Y axis timing belt drive, this will be a long belt
8 or 10 Y axis low cost ball bearings.

1 Z axis Cutter head drive stepper.
2 Z Axis 20mm slides bought or built.
1 Z Axis screw drive (Must have no backlash)

A computer and interface.

A piece of scrap steel beam or tube as long as needed, assorted scrap steel.
Time

It will depend on what you have in stock?

Regards
John

[mekanik]

Thanks John
Over the years i have been wondering if it would be possible to build an assembly that could grind a reference plane on a machine base but couldn't think how to make it rigid enough to be accurate, this would be much better than the epoxy faff and you could add flatbar to the frame giving you a decent amount of material to tap into. keep up the good work and plenty of pictures when you get it sorted.
Regards
Mike