DIY Laser levelling using webcam and laser level

[AndyUK]

To give you some background, I'm a physicist, and at work I use similar lasers on each wall of the room (ceiling, left wall, right wall, and back wall), all pointing to a single point in space at the middle of the room - part of the job is to ensure these lasers are accurate and are planar with one another.

Assuming the laser is meant to self level. I'd mount it at one end of my garage, with the CNC at the other end. I'd then get a long transparent flexible pipe, and mount one end infront of the laser. I'd then take the other end of the pipe to the other end of the garage past the machine.

Then, fill water into the pipe up to the level of the laser; the meniscus will be at the same height in the room on both ends; the laser should pass through both meniscuses. The deviation (or depth of the meniscus as an error measurement) and distance between the two points will tell you how accurate the laser is self levelling. By moving left to right this will show any 'twist' in the horizontal projection of the laser.

The 'straightness' of the laser isn't an issue - light travels in straight lines unless you're bending it with something in the middle ;) The laser will defocus with distance, but as you're looking at the centre of the profile I don't think its a problem.

[Kitwn]

How satisfying to read a 21st century physicist suggesting the use of a technology which may have been used by the Egyptians when building the pyramids!
You don't have to use the latest technology, just use the appropriate technology. I've just remembered have a Black&Decker laser level somewhere, I must dig it out and see what I can do with it to check the accuracy of my own machine.

Kit

[devmonkey]

Thanks Andy. Yes i know the laser light will travel straight, what I meant was the straightness of the line that is generated by the cylindrical lens that is presumably a function of how accurately machined the lens is? Is testing this what you meant by twist in the horizontal projection? How do you measure this at work, do you use a water level?

Before starting this laser project I did something similar with the image sensor focused on a clear tube that was part of a water level, although I could easily repeatably locate the bottom of the meniscus to around 0.01mm resolution I had a problem getting much measurement accuracy due to the meniscus 'sticking' to the glass tube due to capillary action so gave up.I only used a short u-bend setup, maybe this was the problem and having much more volume of water in the system would overcome this problem.

[AndyUK]

How satisfying to read a 21st century physicist suggesting the use of a technology which may have been used by the Egyptians when building the pyramids!

Haha. When it works it works... :) I once had a trainee who looked very thoughtful when we showed them this method, and after about thirty seconds, came up with the question: "But how do we know the water is level?"

Thanks Andy. Yes i know the laser light will travel straight, what I meant was the straightness of the line that is generated by the cylindrical lens that is presumably a function of how accurately machined the lens is? Is testing this what you meant by twist in the horizontal projection? How do you measure this at work, do you use a water level?

Yes, this is what I mean by checking the horizontal by moving left and right. I was covering all bases with the later comment about light travelling in straight lines - you never know who you're talking to... ;)

Measuring absolute level is rarely done at work, because its more important the lasers are planar with one another than they're absolutely level as such, but this is how we do it when needed. The beauty of course is that the accuracy increases with distance, so its best used over distances over a couple of metres.

When we need absolute level in a slightly different situation, we use a large tank of water with a travelling probe. The probe has two wires, one longer than the other, that dip into the water and they measure the resistance between them. The probe moves from air down into water; when the second shorter wire touches the water's surface, the resistance decreases substantially. This is performed at three corners of the tank, from which the angle between the probe's movement mechanism and the water's surface in two directions can be ascertained.

Before starting this laser project I did something similar with the image sensor focused on a clear tube that was part of a water level, although I could easily repeatably locate the bottom of the meniscus to around 0.01mm resolution I had a problem getting much measurement accuracy due to the meniscus 'sticking' to the glass tube due to capillary action so gave up.I only used a short u-bend setup, maybe this was the problem and having much more volume of water in the system would overcome this problem.

Yep - hence my comment about the 'depth' of the meniscus being the error in the measurement. Try to use reasonably large diameter pipe, and a touch of washing up liquid in the water will reduce the surface tension.

[devmonkey]

Ok so just talking about testing whether the line generator generates a straight line. Although the thing is self levelling it definitely wont be level wrt to earth within the resolution of the sensor and we don't really care about our axis being planar wrt earth, rather to each other.I understand you use the water level to check the work lasers are planar to earth but how do you check the projected lines are straight?

Would you suggest sampling the projected line against an earth reference (using a water level and a microscope) at a number of points then calculating a best fit line. Then take the error between points measured and this best fit line to determine straightness? How straight are the laser line generators your work with?


If I had a long precision edge I could just do the same thing with that, unfortunately I don't...


IDEA (EDIT)
Could be done in two stages with the current setup, first setting a rail to be straight wrt to the laser by sending the beam along the rail then using the now straight rail to check the line laser is straight with the laser positioned orthogonal to the rail.



Cheers.