DIY Laser levelling using webcam and laser level

[devmonkey]

So I ran another set of tests directly on my granite island without the gantry, all tests were with the cheap Bosch laser. I ran 3 tests, each measuring 10 stations over 500mm with the laser position moved between each test. Each test consisted of 4 runs, 2 forwards, 2 reverse.

The laser was positioned as follows for the 3 tests

1. Planar test with laser orthogonal to ruler taped to island from a distance of 1.5m so the laser line spanned the ruler.
2. Planar test with laser in same position as (1) but rotated 45 degrees to test a different section of the laser line.
3. Straightness test with laser inline with the rule, i.e. shining along the rulers length.

The idea was that 1 and 2 would test different parts of the laser line arc (say 45 degrees of arc per test), whilst test 3 would use only a tiny part of laser arc, less than 1 degree. The test surface is a granite island that most likely isn't flat but is stable, flatness doesn't matter here as we are testing the laser not the island.

Consistency between the runs of each test was very good, with an peak-peak repeatability error of a few microns, likely due to me eyeballing the station locations against the ruler markings.

The chart below shows the results, overlayed on each other. You can see at a glance the 4 runs for each test are highly internally consistent but that the results vary considerably between tests 1-3. All are parabolic suggesting the granite island has a parabolic surface error of around 100 microns. The jagged line group is test 1, the slightly lower group is test 2 and the lowest group is the straightness test 3.



So this is telling me that the laser line generator is not very straight and has a parabolic nature to it with more noise in the test 1 arc than the test 2 arc.

Since test 3 used a tiny part of the laser arc and light travels in a straight line I think we can assume the test 3 line accurately reflects the true surface straightness of the granite island.

Separated out this is the chart for test 3.



So (although I cannot be 100% sure without a calibrated reference surface) I think this system is suitable for verifying something is straight. This means it can substitute for the usual straightness test of using a precision level to straighten a rail in the horizontal plane or a tight wire to straighten a rail in the vertical plane. However it needs a better laser or a different approach for checking two things are planar.

I have done a bit of research on how this is done in the optical metrology industry and although they use exactly the same approach of measuring height deviations from the test piece to a planar sheet of laser light they also without exception use a rotating laser rather than a line generating lens. Infact they use something called a penta-prism which bends a laser beam 90 degrees irrespective of the the angle of incidence of the beam to the prism. This means so long as the laser is fixed solidly and somewhat vertical as the prism is rotated above the beam in a solid bearing you will cast a perfect plane irrespective of bearing runout or laser prism miss-alignment.

Unfortunately penta-prisms with a few arc-seconds of accuracy are rather expensive. If you use a cheap less accurate one then you could well be casting a cone rather than a plane as you rotate the prism.