DIY Laser levelling using webcam and laser level

[devmonkey]

Hi Joe,

Do you still use it like this?
Or do you also point the laser directly to the sensor now?

Hi Bert,

No I just use the bare sensor in a milled fixture, see:

[Nealieboyee]

Nice work guys. For us laymen, how would this work (once all the problems are worked out) for, say aligning the parallel rails on a Y axis to make them level with each other? Is it just a case of placing the sensor on one rail and the laser on the other and then shimming until the software shows the line dead center of the screen? Forgive my ignorance.

[John McNamara]

Hi Joe

The M'soft webcam I am using appears to have a sensor active area about 2.5mm wide That would help explain why my resolution results are a little more than 3 microns.

Noise. There is a discussion on the MEW site about astronomical photography and the need to reduce noise. Camera sensors for this application are sometimes super cooled to reduce it. No I am not suggesting this level of sophistication for a webcam. However it may partly explain why the measured value particularly the right of the decimal point is ever changing? the first two digits to the right are very important. If they are stable we get a 100 fold increase in accuracy, maybe impossible but worth striving for.

Catenary sag.
For a DIY person setting up say a router with 3 Metre rails roughly what is needed for 2.4 Metre travel, a stretched wire will sag due to gravity. Using your measured value table as a reference and given an input length the calculation could be applied and the true calculated value displayed. The weight of the wire is almost negligible the applied tension can known, say a barbell weight and needs to be input as a constant also. I am not sure how much work this would be to implement in the software, It would be very useful if it can be done.

One of the advantages of stretched wire is that it is very easy to align to high accuracy, 0.0001" positioning is fairly easy to achieve by placing a precision block at each end of the object being measured. By stretching the wire over the blocks you have very stable line parallel to your work piece.

I will run a set of tests tonight at .01mm increments. and check the residual chart. and work on the light source.

Regards
John

[John McNamara]

Hi Joe
Hi All

Below two images of a test of 0.25mm in 0.01mm steps.

The Dial indicator used was a Mitutoyo 0.01mm resolution unit. Ideally I should have used a higher resolution unit. I have one but I would need to make a mounting base for it. This would reduce the Measurement error slightly. Having said that the residuals look pretty reasonable. I guess I am splitting hairs here.

[devmonkey]

Hi John,

Let me just sanity check your methodology. You have the sensor or wire on a precision positioning system. You move the system in steps based based on your dial indicator and click measure at each step? Is that correct? Just wanted to check you aren't moving the system until the measure value increases by one step then clicking measure as this would always produce a straight line, I don't think you are doing this but just want to be 100% sure.

You have the umPerPixel set to 1 therefore the numbers you are recording are in the units of pixels, this means anything to the right of the decimal point is to a sub-pixel accuracy. This ok to an extent (say 1 decimal place) but will soon deteriorate into noise. The gaussian fit will be afffected by a single intensity point change of a single pixel that is why you get jitter at extreme sub-pixel resolution. To remove all jitter you would probably have to perform the test under lab conditions in a vacuum to avoid dust particles and with a lab quality camera with high precision pixel exposure times. The exposure times (and therefore the intensity readouts) of these cheap sensors with rolling shutters are not perfect, when a pixel/row is exposed for slightly longer its intensity values will naturally be higher. Couple this with the onboard image processing pipeline that is probably trying to do all sorts or AGC and white balance adjustment and noise is inevitable.The image is then compressed onboard to a jpeg adding all sorts of artifacts before being shipped to the PC.

All that said your 0.01mm test run shows your residuals are within +/- 0.4 pixels or 0.0016mm (think you sensor is ~4um per pixel) of a straight line, this is pretty good isn't it? Shimming or scraping in the 1 micron range is most likely impossible for us and probably completely overwhelmed by the thermal movement of the frame.

The 0.0016mm could as you say be down to trying to position the sensor using a 0.01mm dial indicator, quite difficult to judge 1/10th of one division...and outside the tolerance of the indicator.

I can add warping for the catenary sag if you have the equations?

[John McNamara]

Hi Joe

The measuring system is just a manually driven XYZ platform

Once set up I only move the webcam The light is at this time fixed and non moving. This is wrong! it should move with the webcam sensor although a 0.25mm movement is pretty small slightly less than 0.010 inches. I am going to work on the light next.

And yes I clicked measure after each movement to the next tick on the dial indicator. The straight line is proof that the system is linear. Subject to my possible manual positioning errors as detailed in my previous post.

And the results we are getting are better and better, once the lighting is sorted I am certain the repeatable accuracy can be improved. Once this is done I really want to try a higher resolution webcam.

Regards
John

[devmonkey]

Nice work guys. For us laymen, how would this work (once all the problems are worked out) for, say aligning the parallel rails on a Y axis to make them level with each other? Is it just a case of placing the sensor on one rail and the laser on the other and then shimming until the software shows the line dead center of the screen? Forgive my ignorance.

That would get the curve of the rails identical but it wont make them straight nor planar.

I was planning on positioning the laser across the room, levelling one rail, the levelling the other rail to the first rail bringing them into plane. For the Y gantry there are some options, you can do it the same as the X and then adjust for perpendicularity when you assemble the machine either by using a square or by using this laser system in tramming mode. Or you can assemble the machine, straighten one Y rail, then use tramming mode to bring the other into plane.

[John McNamara]

Hi Joe

Just noticed your comment re the catenary sag. Adding this feature to your software will make it exceptional.

The formula is widely cited the Wiki in this case is a good place to start.
https://en.wikipedia.org/wiki/Catenary#Equation

Once your measurement table is filled with points and the user has input a length The catenary sag allowance at each point can be added to the table. The weight of the wire and the tension applied to it need to be added, they do not change so can be stored constants.

Regards
John

[John McNamara]

Hi Joe

Correction this line my post above
"
The weight of the wire and the tension applied to it need to be added, they do not change so can be stored constants.
"

Should read
The weight of the wire does change according to its length.

Roslau are are a world leader in music wire manufacture.

This is where I sourced my wire. https://parkepianostrings.com.au/pia...als-and-tools/
It costs about $40 AUD per coil plus post. they will ship internationally.

Half KG coils of #2 .010" Roslau Blue music wire contain 1250 metres of wire.

Regards
John

[John McNamara]

Hi All

Today I received a nice webcam in the mail, a gift from a forum member,Thank you.

It uses a smaller sensor than mine so it will be interesting to see how it performs. I will have to make a new mount for it.
For the tests I will use a one micron Maher milemess indicator. that should assist in getting an even better idea of achievable accuracy.

For any members who live in Australia I will be at the MSME exhibition.
https://www.msmee.org.au/

Our club has been meeting since 1926 in Melbourne new members are always welcome.
Or just pop in at out next meeting as a visitor and have a chat.

Regards
John