DIY Laser levelling using webcam and laser level

[devmonkey]

This morning I had an empty house so I grabbed the opportunity to do some testing. I used the dewalt laser and my mostly completed gantry, the sensor was clamped to the rear of the lower Y bearing plate and both laser and gantry placed on a 1inch granite slab (kitchen island). Initially I raised the gantry to clear the Z axis on parts of the spindle mount but it turned out these are not machined with parallel faces on each end and I could rock the gantry a few thou, so swapped them out for two hardwood beams.



The test involved two passes in opposite directions moving the Z axis one hiwin bolt hole (approximately by eye), these are on 60mm centres, and sampling the sensor at each hole, I took 9 samples per pass, so around 500mm of travel for each pass. The residuals from a straight line were calculated by the app for each pass.

The two passes (with second reversed so they can be overlayed) are charted below, together with the difference between the passes.



The two passes were fairly consistent given the eye balled positioning I used, and that I hadn't clamped the laser nor the gantry to the granite. Worst case absolute error between the passes was 8 microns (0.31 thou). The sensitivity is ridiculous as I could flex the granite plate more than this by pressing on it.

However it can be seen that the either the gantry or the laser or both are not perfectly straight with a max variance of +/- 40 um (1.57 thou). The results are satisfactory however we return to the problem of not knowing if the laser line is straight, question is do we care about a couple of thou?

Unfortunately I ran out of time to repeat the test with the Bosch laser, I will need to setup this experiment on the concrete garage floor. It is a waste of time trying to do any measuring if people are moving about in the house as you can see every footstep within 5m move the sensor reading.

I also noticed that the laser form wasn't a very good gaussian and near the edges of the dynamic range, i.e. where the laser was starting to drift off the top or bottom of the sensor, the beam centre looked quite arbitrary to me, I think I may add a new detection mode that finds the centre based on the maximum local average reading.

A couple of other points of note,
1. ~2mm dynamic range of the vga sensor is not very much, it would be easier with a sensor with large area.
2. dust on the sensor has an impact on the readings if the particle is large enough.