Help with laser projector problem with scanlines

[jollymolly]

I am trying to build a laser projector similar to this (scrolldown) .

My issue: scan lines are spaced unequally with massive gap. First 4 lines are largely spaced and the other 4 lines are really close even overlapping at times.

Creating the polygon mirror base: I designed the base using sketch-up.
8 sides, with each face titled by a calculated angle. The design was printed via makebot.
I attached mirrors to the faces and attached it to a motor.
When rotated and laser pointed at it several horizontal lines of laser light are projected.

Calculating angles:
display distance circa 30cm
Height of projection screen = 3.2cm
Scan angle of octagon = 90 degrees
Followingpangolin
If D = 30cm and A = 90 then W = tan(A/2)*D*2 = 60cm (good enough)

scan angle of mirrors
Height (W) = 3.2cm [8*diamterOfbeam]
Required angle?
A = tan-1(W / (D * 2))*2
A = 6.1 degrees say 7 degrees

Between the 8 faces there is 7 angles.
7 degrees / 7 = 1 degree.

To achieve angles in sketch-up, I first created a standard octagon shape. The height is about 15mm.
Using trig, as it's a right angled triangle, tan(A) = Opposite/adjacent.
adjacent = height of octagon, we also know angle.
Thus opposite = adjacent * tan(A)
lengths required to create the specified angles were calculated.

I used these lengths to join edges to make the angled face on the polygon in sketch-up.

here is the values:


here is sketch up design:

[mixedgas]

Your very close on the math.

Get a used or library copy of Laser Beam Scanning, G.F. Marshall, Editor, Marcek Dekkar, New York, New York.
Has a few good chapters on optical polygon design, including tiled polygons for mutiline scanning. It also covers the surface error problems.

It also tells you how to test your polygon.

As for the Makerbot,

They are claiming 100 micron resolution for the new one. In reality the error is about double that because they really do not compensate for backlash by using a true absolute encoder. There are also issues with how the software chooses its print direction. So place a +/- .1 mm bump into your geometry and calculate the error. I think you might see what is going on.

Steve

[jollymolly]

My library does not that this book, really curious what it says.

Is it the maths or is it the makerbot struggling to print small differences.
Still I don't understand how it can change results so dramatically, looking at the table you can see difference between opposite side length of consecutive faces is circa 0.26mm.
So I expect the results to be more or less the same. Very puzzling

[mixedgas]

Makerbot has high position resolution but poor repeatability as it travels. Then the polymer "gives" and "stresses" as it cools through its Tg, the Glass transistion temperature.
You've hit the limits of the material and possibly the machine.

Your math looks fine.

Steve

[swamidog]

meridith instruments (mi-lasers.com) used to sell polygon mirror scanners that are pretty much exactly what you need. you might give them a buzz and see if they have any in stock.

i think an interesting approach might be to use non-angled mirrors and 8 laser diodes.