It's about time I blog about ACE (Ambulatory Canine Emulator). This project got neglected while I focused on completing Atronach and getting started with hydraulics, but I've been poking at it every now and then. My most recent conclusion is that I do, in fact, need to replace the PF35T-48 motors. Those bargain-basement unipolar steppers are just not good enough.
My last major overhaul of ACE was - gulp - three years ago. The quick summary is that I solved enough rigidity and tolerance problems that the skeleton could be posed standing at its full height, and repositioned the motors so that none of their weight had to be carried on the legs. Since then, I've been doing motion tests and refining the design of the ankle/hock joint.
Once I started trying to make parts of ACE *move*, it quickly became obvious that, out of the box, the PF35Ts didn't have enough torque to accomplish much. It was difficult for one of them to even swing the upper leg back and forth when it was hanging free - a relatively easy motion, since the only load was the weight of the leg itself. So I detoured into designing gearboxes, taking my motor cradle as a starting point and adding mounting tubes for additional drive shafts. A single gear designed to mesh with the little one that comes attached to the motor was good enough to enable upper leg motion (see the first video, above). But there was still no way that was going to be enough to operate the hock joints. My efforts to *really* gear these motors down led to the creation of the little beauty in the video below, with three gear pairs. I'm still quite proud of it.
I also fiddled with the hock joint design and some of the tendon routing, trying to optimize the mechanical advantage and reduce the strain on the motors as much as possible. In this new version, the tendons are more exposed, but I think I get better leverage out of the deal. So I tried the new joint prototype with the new gearbox, and no gravity loading the leg - it's just moving in a horizontal plane. How did it go? Not well.
It only managed to do as well as it did for that demo because I was over-volting the motor (I typically operate these at 5-6V.) Careful study of this test setup convinced me that the main problem was with the tendons binding against various corners they have to go around. But they're made of nylon fishing line, which presents a very low-friction surface, so they shouldn't be binding that *hard*. I think the latest joint design is about as good as it can get, and the gearbox is doing fine as well - I saw no evidence of the gears jamming or anything like that.
So with no weight on the leg, and the motor geared down so much that it's moving miserably slow, it's still stymied by a little friction. My guess is that I'm not getting the higher torque I expected because losses in the gearbox are consuming it; the amount available was so low to start with that I might not be able to make the situation much better, no matter how many gear pairs I add. Although there are some small adjustments I could make to reduce how tightly the tendons are bent around corners, I'm taking this test result as a sign that the motors are just not right for this project.
Fortunately, there are options with a much better torque-to-weight ratio out there. They do cost a little more, but I'm not budgeting as tightly as I was back in my college days. I've already selected some new models to try, and redesigned the motor cradles to hold them. (It's so nice to have a somewhat modular design.) Here's hoping for somewhat more progress when I get a chance to try them.
Until the next cycle,
Jenny
