DIY Laser levelling using webcam and laser level

[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.

[Kitwn]

I was wondering about how a taught wire might be used for measurements and very much like the arrangement John describes. I have a box full of Arduinos waiting for something to do.
Joe, One idea to throw into the melting pot... If two wires are stretched one above the other, separated by a distance slightly less than the diameter of the laser beam (wrapped around two suitably chosen drill shanks for example), can your sensor be used to accurately show the relative illumination of the two wires as the beam moves horizontally along them? Quite small variations of height might show up as significant variations of illumination brightness or width on the wires.

Kit

[John McNamara]

Hi Kitwin

I would like to avoid a laser altogether. Instead ideally I would like to use the shadow of the wire over the sensor backlit with a dispersed light source. This would avoid any focussing issues. There would be a shadow formed on the sensor with a normal distribution characteristic darkest in the center. to lightest near the two edges.

Borisovs paper mentioned earlier uses omron sensors in this manner, no optics.

As the camera sensor is only about 2mm square. with .008" wire I can imagine that the workable measurement range would be about 1mm at best. This is where the device I made would help. It has a range of about 5mm, It could be used to center the sensor. Once this is done the current count of the stepper would represent the null point on the sensor.

These counts could be plotted along the wire giving a chart of the flatness of the object being measured.

For longer distances over a metre the catenary sag of the wire will become noticable. This can be computed as the measurements are taken along the wire assuming the drive is stepper or a servo with an encoder.

Alternatively glass optics, I guess however there are many hurdles to a perfect arrangement.

Note
One of the very practical reasons for using wire is that it can be positioned very accurately prior to measurements being taken. Both endpoints can be fixed and very accurately aligned to the surface being measured. and once fixed they stay put.

I also have a Taylor Hobson alinement telescope. It works well but not to .0001" of an inch. (.002" is achievable) Setting it up buy aligning the targets is no easy matter. An hour can easily pass. If a target is knocked you have to start again. pointing a laser will be the same. Using both systems there is no obvious error. It can only be found out by checking.

When I did work on my lathe I used stretched wire mounted at each end on the bed. Here I used a Microscope. A digital camera sensor should be a lot more accurate. Scroll down to near the bottom of the following link.

https://www.model-engineer.co.uk/for....asp?th=136771

Regards
John

[Kitwn]

John,
I had a look at the other thread on this forum where you described the taught wire system on your lathe and also scanned through Borisov's paper, though I must admit to not studying all 170 pages in detail. Cunning use of cheaply available sensors and I can well understand not wanting to involve adjustments to both the wire and the laser. I was thinking more of Joe's desire to check the horizontal flatness of the beam from his commercial laser device when the 2 wire idea came to me.
As someone who plays with a DIY CNC router as just as one of several hobbies I'm always on the lookout for ways of accurately aligning my machine without having to buy several expensive measurement devices that will only ever get used a handful of times. There's also a great deal of satisfaction from devising a method which is inexpensive but effective... which I haven't quite managed yet. I only cut wood, for the present at least, so my current aim is achieve 0.1mm accuracy. So I shan't be worrying about the sag in the middle of a 1200mm taught wire anytime soon

Kit