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Build Your Own SLA/SLS • Galvo pincushion distortion?
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Galvo pincushion distortion?

Posted: Tue Feb 24, 2015 6:10 pm
by Turboedgs
I am building a mirror driven "glavo type" printer using two high res steppers and digital drivers.

My early testing has been productive however I seem to have hit a wall with a pincushion distortion error on my X axis (last mirror before surface bed).

Because it is still early I have not done any printing but did do some projections on the wall to verify the laser spot path visually and I noticed a significant amount of deviation/distortion of the spot positioning on my X axis. From my initial digging around on the web I understand that this type of error is called pincushion distortion (axis bows in like this --> )( <--).

Has anyone encountered this type of x y axis path distortion?

Looking at other laser type SLA printers, i.e Form1, they do not seem to have any reports of this type of distortion nor do they use any type of lens to correct for an error like this.

I have gone through the hardware and confirmed correct alignment of the laser and mirror axis locations and made significant adjustments in an attempt to pinpoint the cause. No luck...

With the power off to the scanner and the laser on I can manually move the x axis mirror in the x/- directions and see an arching path of the laser dot.

Any insight on this would be very helpful.


Re: Galvo pincushion distortion?

Posted: Tue Feb 24, 2015 8:21 pm
by PacManFan
What you need is called an 'F-Theta' lens to correct the distortion. I would not recommend using stepper motors for the scanning.

Re: Galvo pincushion distortion?

Posted: Tue Feb 24, 2015 11:05 pm
by Turboedgs
Yeah, I have one and do see "some" correction but it does not take it all out.

How do these other printers like the Form1 not have any lens correction?

Any reason(s) as to not driving the scanner via a stepper? Speeds are slower than a galvo but still fast enought to etch using a CO2. :D

Re: Galvo pincushion distortion?

Posted: Tue Feb 24, 2015 11:24 pm
by Scatterplot
Perhaps you could do some distortion compensation in software?

Re: Galvo pincushion distortion?

Posted: Wed Feb 25, 2015 11:39 am
by Turboedgs
Software way be a solution, but currently beyond my level of experience. I will have to look into that.

Here is a video example of the speeds that I can scan in with steppers. I have had the motors double the speed in the video but the laser doesn't mark due to lack of power at those speeds. Dialing in the CO2 laser power is a little touch and go. The blue laser I have for the SLA build is a bit easier to work with.

https://www.youtube.com/watch?v=EZ86FFy ... e=youtu.be

The marked grid in the video is about a 3" x 3" area. I would like to run larger parts and my f theta lens is a 210mm x 210mm but anything past the 3" cube and the X axis starts to bow significantly, even with the lens.

I see the same arching laser position path on the smaller blue laser and the large beam CO2. With the scan head powered down I can manually move the X axis and watch the blue dot move in an arch, w/ or w/o the lens. All the housing and mirror holder parts are CNC milled and aligned perfectly. I have adjusted the mirror 1 to mirror 2 and the mirror 2 to lens distances to some extreme +/- levels and there is no change in the arch.

At this point I am just assuming that this is a normal error in a two mirror type scanner?

Thanks for the suggestions!

Re: Galvo pincushion distortion?

Posted: Sun Sep 25, 2016 3:34 am
by 3dtechnology
F-Theta lens is position at the very last position from source light. As like this :Light Source --> X mirror --> Y mirror --> F-Theta Lens --> Screen.
Solution is to minimize distance between X mirror & Y mirror. Ideally F-Theta lens has to be positioned between X mirror & Y Mirror not only after Y-mirror to compensate focal error and tangent theta error.