A video is worth 10,000 words, so let’s start there!
Progress slowed a bit during the summer months, but I spent a lot more time working than writing about it, so you’re looking at a lot of changes.
First, I finished the basic mechanical design all the way through the gripper. While the arm in the video is a bit of a hodgepodge, it’s now possible to build one where all six joints share almost every part. This became a key goal to drive costs down. I only need to make a few more parts to have everything ready to test all 6 axes pretty soon.
Second, while the basic principle of having each joint controlled by its own Arduino has stayed, the implementation went through two big changes. I dumped the Uno-based boards for the Teensy 3.1 (final version will probably use the LC), because it’s capable of running Accelstepper at far higher pulse rates (10-20x, at least). It’s far more power for a lot less money, and it’s effectively as easy to use, so it’s pretty much a big basket of #win.
Next, I switched from the Pololu 8825-based boards to the Panucatt Devices version, and couldn’t be happier. They come pre-soldered with a heat sink for maybe $2 more, and are also much better designed. Setting the current limit on the Pololu boards required the hands of a neurosurgeon, and I killed three of them when my probe tip slipped a millimeter and shorted something. That gets old fast.
The Panucatt boards put most of the parts on the bottom and the pot and (larger) test point on the top, which makes them far more user-friendly. The only remaining question is whether they can run the “elbow” motor at ~1.8A without forced cooling. I’m still using an old Gecko drive for the “shoulder” motor which uses 2.6A. The Panucatt drives theoretically go up to 2.5A which would be enough, but I think they need a much larger heatsink to survive. It may just be easier to run that one joint with a $20 Chinese driver box.
There’s a lot left to do, but it’s starting to feel like I might have something like a final configuration. I’ll need to design and fab up the boards, and I’m still figuring out how to handle homing (which is necessary) and position feedback (which would enable “teach” mode and some other powerful things), but I have an idea in the mail which I should be able to try soon. I also have to make the gripper, but that looks to be easy enough. With a little luck, we’ll be picking things up before Halloween!