Hot fingers

This post is part 5 of 5 of  LEGO bagpiper

I found out that everytime I tested Tuxie McPython the two Technic Hubs were getting hot. Very hot.

Also batteries were draining fast. I hate changing batteries on stationary robots/automata. For my MINDSTORMS EV3 I tend to use the Li-Ion battery as a PSU, leaving it connected to a 12V/1.5A wall power adapter.

So I ordered 2x USB Power Box from PV Productions. They fit on the Technic Hub in the place of the 6xAA battery holder and they can withstand 9V/2A so it seamed a good and clean method.

Less than 3 days after they arrived I melted one 🙁

The 4 Large Angular motors were consuming too much current. To proper close the holes I was forcing the motors to strongly press the fingers against the chanter but never thought that would require 2A (probably even more).

So I changed the code to reduce the fingers strength and opted for other fingertips also made with protective foam but a different one, apparently less “foamy” and a bit thinner. I glued over the EVA foam fingertips I already tried before and cut them a bit larger so the hole would get more covered (no fotos but can be seen used on last video).

Even so, rechargeable NiMH batteries were still draining too fast (and the hubs were still getting hot). So I picked some alligator cables and 2.1 mm barrel adapters and used 2 of my wall power adapters to power the Hubs. I set them for 9V (measured 9.1 with no load) and connected them – it worked:

but while playing I got a blinking LED and very often one of the hubs just powered off. The adapters are rated 1.5A… so the motors were still demanding too much current.

As I said before… I don’t have other proper motors – the smaller version of this motors, sold with the 51515 Robot Inventor, are not easy to find and it would be very expensive to buy 8 of them on Bricklink. I could use the BOOST external motors but I don’t have 8 of them and I am not so sure they have enough power to properly close the holes. Same for Technic motors (except maybe the larger ones) and none of them have internal zero reference (not a major blocker but its something that helps).

So I have 8 motors that demand too much current and two hubs that get too hot and shutdown… I have no other proper motors… but I do have other two identical hubs.

So last version of Tuxie McPython now uses 4 Technic Hubs, each controlling two fingers. Code is essentially the same, just less “if…then…elif” and a shorter “command vocabulary”:

   G = ['0', '0', '0', '0'] # 00 00 00 00
   A = ['0', '0', '0', '1'] # 00 00 00 01
   B = ['0', '0', '0', '3'] # 00 00 00 11
   C = ['0', '0', '1', '2'] # 00 00 01 10
   D = ['0', '0', '3', '2'] # 00 00 11 10
   E = ['0', '1', '0', '1'] # 00 01 00 01
   F = ['0', '3', '0', '1'] # 00 11 00 01
Hi_G = ['1', '3', '0', '1'] # 01 11 00 01
Hi_A = ['3', '2', '0', '1'] # 11 10 00 01

So I now send just 4 different commands (‘0’, ‘1’, ‘2’ or ‘3’) to each hub instead of previous 16 but from the controller side have to send those messages to 4 hubs instead of just 2. My only concern was that my laptop couldn’t proper handle 4 simultaneous BLE sessions but thankfully it works fine.

With 2 motors per hub I can now use NiMH batteries without blinking LEDs. The hubs still get hot (not so hot as before but still very warm) and the batteries will probably run down after a few songs but for that I already ordered 9V/2A power adapters and more alligator cables.

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