Servo motor shenanigans!

The PolargraphSD’s pen gondola has a servo motor attached that turns a control horn against the paper, and lifts (or drops) the pen tip against the page.

In the new design, the servo is powered by a 5v supply, but the instructions (the signal to say which position to move to) comes from the ESP32, which is a 3.3v device, and so it’s signals are 3.3v. It turns out this was a bad idea and it causes a lot of twitchy moves.

For two dozen machines, it’s not been a problem that concerned me – indeed I didn’t even find it to be a problem, which is why I didn’t do anything about it.

In retrospect, the mixed voltages seems obviously risky. I recall that it was an open question when I was first designing the new Polarshield 3. Because the old machine was 5v-based, the initial design for the new Polarshield used the same scheme to power the motor. I remember being a bit surprised that it worked fine! I don’t think I even tested it on the 3.3v supply.

However, this week I built a couple of machines in a row that were unacceptably twitchy, and couldn’t ignore it because I knew it’d cause problems for people, and be very hard to work around “in the field”.

The fix for this is pretty simple – cut the 5v line to the servo and link in the 3.3v line instead, but it’s a soldering job, not something I could expect anyone to do themselves.

And after that, what a smooth arc!

Another side-effect is that the machine can now sweep the servo when it’s only powered by USB. Previously that would draw too much power and cause a reset! Nice!

Conclusions:

  • Just because it works 24 times, doesn’t mean it’s right
  • Don’t ignore the inklings, you’ll just have to fix it later when it’s harder

The machines that I’ve already sent out since the beginning of September may exhibit this problem and need a fix. I’m really sorry that I didn’t spot this earlier.

I’d be delighted to do the repair if you’d like to send your control box back to me!

If you’d like the fix, please drop me an email and I’ll arrange a collection as quickly as I can.

(You can see a boring video of the actual fix here: https://youtu.be/J56dzZaDV8s .)

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