This week has just been a torrent of Deathblades work. I’m (kind of…) proud to say that the skate motors are all finished. Well actually, two of them still need their share of custom arc magnets, which I hope to take care of soon through Supermagnetgeorge again. At the least, they can support my weight and I can skate around unpowered.
In other words, I’ve successfully built a set of roller blades. Hurray.
Yesterday, I was able to terminate and test one motor, so today I repeated the process for all of the remaining ones.
Here they are, sitting on the “drying rack”. Again, I kept C-clamps on the pigtails so they would be pulled down as low as possible. This time, I mixed up some thin laminating epoxy and let it flow across the wires. The 5-minute stuff I used before was sort of gel-like and didn’t really flow well.
To make the thin epoxy set in under a day’s time, I added a few times the recommended amount of hardener. This probably trades off alot of strength, but it’s not really structural anyway.
Know what’s cool? Having two motors.
Know what’s not cool? Not having enough breadboard space to put another DEC rig on. I wasn’t that interested in pursuing the simultaneous control of both motors immediately, so I decided against mocking something up. To test the other motor, I just swapped wires.
What I found very interesting was that the motors were heavily timed – possibly up to 30 degrees difference between the two directions. Both motors registered about 1,600 RPM in the “reverse” direction, where reverse is defined as the direction the frame would fly in if I dropped it on the floor with the motor spinning. They only achieved around 1,300 in “forward”.
Additionally, one of the motors required a cyclical shift of the phase terminations (i.e. motor-A to controller-C, C to B, B to A), but the other one only ran with A and C swapped but B unchanged.
Motors are strange.
The fake motors get the red wheels. I only ordered 2 sets of magnets and haven’t gotten around to ordering the rest, so for now one skate (the left side, as I’m right-dominant) will have dummy motors.
Or, if you think of it another way, motors with really really crappy Kt.
So what did I do with two dummy motors? Put the dummy skate together!
The stock ‘blades used a very unique M6 bolt with a very flat and broad head to attach the wheel frame to the boots. I wanted to keep that bolt since it was unlikely that I could find a similar bolt.
Consquently, I decided to extract the matching nut because there was no way you could get me to dig through all of MITERS for a M6 nut. The stock wheel frames split in two after all hardware is removed, so extracting the nut was simple.
The stock nut was also a very broad kind, which translates to a bit more rigidity. Unfortunately, I can’t remove the boot without also removing the wheels. Oops.
And here is the dummy skate.
I decided to orient the motors such that their cables faced inwards. This is probably a position that’s safer in terms of snagging. The good news is that they all reach to the middle of the frame, so any conceivable control board orientation is achivable.
The bad news is there’s no board to attach them to yet. Sigh.
And here’s both of them!
Clearly I’ll have to take these apart again to install the rest of the components, but in the mean time, publicity shot!
I took a short unpowered run around the hallway to see if they were still useful as.. you know, skates. The LRK winding and magnet arrangement yield very little drag and ripple torque. I suspect my bearings are contributing more loss than the motor drag. Overall, they “skate” well.
So if the motors or batteries ever become dysfunctional, the whole thing doesn’t become instantly useless. Such is the beauty of hub motors.
wrist guard control aka “palm pilot”
I finally devised a method of controlling the skates wirelessly while being unobtrusive. It has the added benefit of being integrated into something that already exists, which scores big points in terms of both convenience and ease of fabrication and duplication.
These are the wrist guards that came with the skates, which I actually pulled out and investigated for the first time today. I was under the impression that they would have individual finger extensions, but they don’t. They are essentially injection-molded plastic plates that are woven into the fabric and leatherwork.
So, overall, this ruins my plans of making a “virtual pistol grip” by detecting finger motion.
This is the plastic plate that is on the bottom side of the glove. Presumably when you take a fall, you stretch your arms out and land on these pads, in lieu of something more ouchy-bleedy. The many gouges and skid marks on the bottom of the curved section tells me it’s been well-used. Good thing I intend to keep them.
The plate also offers a completely new control solution that I haven’t seen elsewhere. When worn, the lip of the plate on the right side rests squarely within my palm, and my wrist is over the broad flat section on the left side.
Combined with the wrist strap compression and stiffness that the top plate offers, I noticed that just by curling my wrist inwards, I could deflect the bottom plate by a small amount, using it like a leaf spring. No finger movement (or even finger muscle tension) is required – it’s a fully wrist force operated thing, and the force is applied essentially perpendicular to the end of the angled section.
Now, the classic solution would be to attach a strain gauge to the curved area. However, my experience with strain gauges tells me that they will be more pain than they’re worth – the deflection is actually quite large in magnitude for a strain gauge, and I suspect I’ll run into attachment and durability problems. The upside is that they’re a dime a dozen and well-studied.
Here’s something else that immediately came to mind:
They’re force-sensing resistors. As long as the applied force is essentially normal to the plate surface, I could use these things to sense the applied pressure. They start with a zero load resistance of really high (megohms), but quickly drop to a lower resistance when a minimum pressure is applied, then the resistance decays until saturation. The curve is nonlinear with respect to load, but some glue circuitry can translate the 1/resistance relationship to linear voltage.
Using force sensing resistors (“force pads”) has a few upsides. The inherent minimum force will act kind of like a deadband near zero, and the saturation at high loads (where high can be whatever, from grams to kilograms) means the upper control limit can easily be detected.
Sporkfun sells some small variants of these, so I’ll get a few to experiment with.
If I felt brave, I could even attach one to the upper plate, which gets roughly the same force applied to its tip when I bend my wrist outwards. This sets up an interesting controls scheme – inward pressure to accelerate, no pressure to coast, and outward pressure to engage dynamic braking.
- Get my damn batteries
- Send the Double DEC’er boards out – this is pending an evaluation of their necessity. PCBs are nice, but they are most definitely not cheap – five Half Bridge boards cost me a clean $150. The wiring is simple enough to make using protoboards and point-to-pointing everything.
- Get my damn batteries
- Order the last set of custom magnets
- Get my damn batteries
- Experiment with the FSRs and devise a way to mount circuitry onto the gloves. I’m thinking Lilypads, because they’re cute.
- Where are my fucking BATTERIES?
- No, seriously.