[Solved] DIY Table Theta Question

Welcome to our community! Though an unusual entrance route, we want to support an open sharing of ideas. In answer to your question, in the Sisyphus coordinate system, positive Theta is CW. This is opposite of convention, where Theta = 0 is shown at 3 o'clock, and positive movement is CCW. The reason behind this "choice" was a simple error of mine during early development (where trials of multiple different mechanical and electrical configurations frequently flipped the directionality of the steppers). Since all my tracks were algorithmic and nonrepresentational, it never mattered much to me if they were "backward" with regard to Theta. I was totally focused on getting Rho right, since if you mess up with that axis, it physically hits something (a.k.a. literal crashing). Once I realized my error, we already had so many Sisyphi out in the world and a good number of community tracks written with this quirk already embedded in them.

So it goes.

But really, it's fundamentally an arbitrary choice, and is trivial to compensate for. If you have the skills to write your own control-ware, it should be obvious how and where to flip the sign of Theta values.

We'd all love to see how your DIY project turns out - please keep us posted.

Bruce

Bruce,

Thanks for the welcome.

There is absolutely no need to feel like the way the mechanism works is "wrong" in any way. Since you are the pioneer in this artistic endeavor, whatever you did is just right with me. The fact that the designs start at the 12 o'clock position lends itself nicely to the clockwise functionality.

Even though I have only been thinking about this build for a few days, I feel like I already have my first victory with your confirmation about the direction.

I will start another thread detailing the progress of my project.

Bob

I guess, like most people, I pay most attention to the things that most interest me - in this case, Rho position control is paramount, while Theta is "all relative," As for Sisyphus designs starting at "12 o'clock" - it never occurred to me to see it that way. Designs simply start "where they are" :). The first design after homing starts at Theta = 0 (regardless of the first Theta coordinate value in the *.thr file). What actual direction that is in relation to the physical world didn't concern me (since I create algorithmic tracks). If, however, I was using Sisyphus as a logo display or sign of some kind (not at all my thing), orientation of the table would be extremely important. 

BTW - your 12 o'clock is my 3 o'clock ;).

Bruce

Maybe because it's the order I learned things, but I find both clockwise from 12 and counter-clockwise from 3 polar coordinates natural and equal. Since they are mirrored on the x=y axis, you can just switch the sin/cos to switch which one you're doing. On my own as a youngster programming my TRS-80 I discovered x=sin(t) and y=cos(t) for clockwise from 12 and it was only later in school I learned the "correct" way of doing counter-clockwise from 3. I also work now in an industry where if there are multiple ways of doing something you can be assured they are all in common use (although I'd love to have a talk with the genius who decided bottom-up raster images made good sense after decades of them going the other way). So when programming the Sisyphus I just observed how it acted to the coordinates I sent it and went from there, I never once felt like it was "wrong", it's just how it works. I'm just a visitor here, so I will follow the local custom.