Time for a big update!
Progress slowed a bit during summer months but a lot of big changes were made. This video shows 4 joints running simultaneously in something a lot closer to a "production" configuration than ever before. There's still plenty of work to be done, but with this, I feel like the end is almost in sight.
As for the changes, there were two big ones. First, I swapped the Arduino Uno-based joint controllers for Teensy 3.1s. The Unos were struggling to run step rates of over 6000/sec, and that just made for painfully slow joints. The Teensy can drive Accelstepper happily at far higher rates than needed, and provides capacity to spare to add more functionality. At $12 each for the LC version, it's also cheap.
Second, I gave up on the Pololu 8825-based stepper drivers and switched to the Panucatt Devices version. It's based on the same TI chip, and fits the same basic footprint, but everything else about is is just far better, for only a few dollars more. It comes with a nice little heatsink, which may or may not be enough without forced cooling for the 1.8A motor that runs the "elbow" joint, but is plenty for the smaller wrist motors.
More importantly, it's laid out so that you don't need the hands of a neurosurgeon to adjust the motor current. Both boards use the same basic idea of a little pot and a spot you probe to set a voltage which determines the motor current. On the Pololu board, the point is tiny and located on the same side as most of the components. I fried at least three boards because my hand slipped a millimeter and shorted something that shouldn't be. On the Panucatt boards, the test point and pot are on the top, and all the other parts are on the bottom, and the test point is much larger, so it's foolproof by comparison.
With this proof of concept basically working, it's time to move forward to complete the remaining joints and gripper, and start working on a real PCB that can be integrated within the joints themselves. This has, at least for now, proven the basic design and firmware are workable. With any luck I will be picking things up and putting them down before Halloween!