Hi there, great to see you on here! I know exactly the problem - I've had it lots of times before. The finer the pen, the more likely you are to notice it.

It is to do with the gondola pivoting around on the tip of the pen - basically the tip not being moved equally in all directions by the movement of the gondola.

If you notice, the corners of your waves aren't very sharp, or square - one corner is sharp, but the trailing corner is degraded. This is a symptom of the same thing.

There's two things you can do if you don't love the effect - first is to reduce the distance that your pen tip sticks out of the gondola, so there's less "slack" to take up during direction changes, before the tip follows the movement. If you move it too far in, then the tip ends up lifting off the page entirely, at certain points on the page, levered off by the stabiliser ring. Like almost everything with this machine, there's a sweet spot to be found.

The second way is to add a bit of extra weight to your gondola so it all hangs a bit more tautly. I used to do this a lot, but got out the habit actually. Just a bag of nuts or bolts taped to the pen lift servo motor (or some other likely bit). This just encourages the physical machine more closely to match the mathematical model. Problem with this is when you start adding weight to one bit, you've got to start adding it other places too (the counterweights), and then slowing things down so it doesn't drop steps, and so on. Again, there's a sweet spot.

Hope that's helpful!
sn

It isn't so much that one motor behaves differently to the other, it's more thatthe pen tip behaves differently when it's being pulled by the motor, than when it's being pulled by gravity. So the sharp corners are where the cord is being pulled actively, upwards, and the rounded corners are where you're relying on gravily pulling as sharply downwards.

Pulling upwards requires a lighter touch, because on a sloped surface, you have less pen-tip friction to overcome, whereas gravity pulling the gondola downwards is also pressing it more firmly into the paper. I think that's why downward strokes will always be a bit more "sticky" than upward strokes.

Regarding your non-flush gondola, I guess you must have a lightweight pen? You should maybe check the gondola is assembled with the cords inline with the middle bearing. It is possible to assemble it a few different ways - there isn't really a right or a wrong way, only a best way for a particular weight of pen. The further the control cords are from the pen tip, the more wobble you'll get.

Let us know how you get on!

cheers!
sn

Looking good!

sn