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Wednesday, August 19, 2009

Drivetrain v3.1 With "Bench" Test VIDEO!

Since now I am working with these new smaller and lighter 12V 17Ah batteries, I decided to change it to 36V from 24V. This will give me a bit more power than I had before, but requires I change the motor gearing, since it will be about 1/3 faster than it had been, at max.

I was already figuring out what chainring to add to the 2-ring set and how to do it, but this allows me to bypass doing that and leave it as-is.

At the moment I have decided to test it with one of the higher-RPM gearboxes, too. Originally 135RPM at 24V, I'll be using it at 36V and so getting a nominal max of 202RPM. Since that's about twice what I actually want, (90-100RPM), then I can use my newly-switched over chainrings more efficiently than I could the other way.

There's 52 teeth on the largest drivetrain input ring (the one with the cut-off crank arm), and 24 teeth on one of the motor rings I have setup, and 28 teeth on the other. Either one can be easily slipped on and bolted down, and the chain run over it. (Have to add or subtract links, though, because I don't have any other way to fix that right now; no tensioner/etc.). That's a 0.46:1 gear down in speed with 24:52 for 93RPM max at the drivetrain input, or 0.54:1 with 28:52, for 108RPM max.

The 28-tooth version would give me the best top speed, but at highest speed would also make me be windmilling my feet again, while the 24-tooth version would be just about perfect at max throttle as long as I didn't need any "overdrive" from the motor for an "emergency" speed boost, without changing gears. I'll test it with each and see how it feels.

This also moves my pedal chain to the smaller 40tooth ring (which required some indenting of the chainstay frame on the left side to prevent it rubbing on the chain), which combined with the front 41tooth ring gives me nearly 1:1 ratio again, same as I had before with the 3-ring set on this side, but now with the three ring set on the other side gives me my low gears back *and* still have high gears, as if the pedals were directly connected to the regular bike drivetrain, essentially. Pretty much what I was after.

For now, I'm going to go with the 24:52 93RPM max ratio, and see what happens.

Now I don't have to add a third small chainring to the outside of that 2-ring set, which was proving problematic due to location of inter-ring-connect bolts and whatnot blocking chain paths.

With chains on both chainrings right next to each other, the clearance is minimal, but there:

Above is a pic from directly over the top of the chainlines where they cross between the motor and the drivetrain input rings.

That one shows from in front, just under the pedal's chainring. The rustier chain is the one on the pedal's ring.

I took the above pic several times, but the only one you can clearly see the chain clearance in is this dark one, with the light-colored carpet showing between the chains. Smack dab in the middle of the pic is the motor's output hub with the 24 tooth chainring bolted to it (using the bolts and nuts from the 4-pole's wheel hub, since they fit better than the ones I'd scrounged up before).

Now, the moment you've all been waiting for: a new VIDEO! It's not exactly exciting, and it's sure noisier on the camera than it is in real life, but here you go.


There's a yellow DMM setup to measure DC motor voltage, and between the sampling rate of the meter and the PWM frequency, it causes it to be quite variable even though it is not, really. The red DMM is setup to measure DC motor current, which as a no-load current should be 3.6A at 24V according to it's label. It is actually much lower at 36V, as you can see in the video, even at full throttle and my highest gear (with the only load being the drivetrain and the wheel, with all their frictions and masses). The meter is only meant to take 10A, as it doesn't appear to be fused inside (just a shunt on the PCB to measure from). So I tried not to push it too long past that limit.

Even so, when I grab the rear wheel tread with my welding gloves to load the system (which sounds like a table saw because of the MTB tread!), it jumps to above 20A, out of range of the meter to even display. It doesn't slow the wheel down a lot, though it's noticeable, especially at the higher gears. In lower gears, naturally, it has less effect on the speed because there's more torque available to overcome my grip. No matter what gear it's in, at max throttle I still can't STOP the wheel even with both hands on the treads trying to grip as hard as I can, until the friction heat even thru the gloves makes me let go. About the same at the 2/3 throttle lowest setting I can go to right now (till I figure out a quirk described below), which is good.

I'm sure my grip is not the same as the load it will have when riding on the road, but it sure seems good enough at the moment. It *should* be better than it was before, because the weight will be less by around 10 pounds, with 3x 12V17Ah vs 2x 12V 31Ah, even though the max current draw from these is less.

Plus, I've removed the lighting battery, around 6-7 pounds or so, and will be running that off the traction pack. For now that will be just directly hooked to one of them. The automotive turn signals on one, the LED and CCFL lighting on another (so it doesn't flicker with the current drain from the turn signals). At some point I'll get a Roman Black SMPS in place for each one, and have the automotive incandescents replaced with LED grids, but I have had no time to do all the wiring yet.

Back on the new drivetrain topic, the noise is actually quite a racket compared to just a bike drivetrain, but this wheelchair motor is only meant to run at 24v and it's gearbox around 135RPM output, and I'm running it at 36V which ends up a lot faster than it was made for, and thus is even noisier than it was before. Unfortunately even without the gearbox engaged (if I flip the clutch lever off) the motor is still pretty noisy and whiny. Another reason I'd like to get the treadmill motor going with a first-stage grooved-belt drive, with chain as second stage. Should be less noisy than the gears in the gearbox are.

Even with the extra noise from the higher speed, it's still quieter than it was with my crazy derailer-hanger chain tensioner. I *will* need to change the little derailer wheel I have on the brake-lever-throttle-arm-to-be to a rubber wheel off a skateboard or rollerblade, though, because that is a big enough source of noise I actually had to tie it up out of the way to be heard clearly on the video! It's not that bad when riding, but in a closed room with a sensitive-mic'd camera, it's bad.

There is a quirk in the controller I have to figure out where if I run it at 36V, it starts out at around 24V motor output average, around 2/3 throttle, even if I have the throttle at zero, and even though I verified that at the throttle input pin to the comparators, it is at 0V, and should have no output at all. (0V throttle = zero pulse width; 4V throttle = 100% pulse width). I don't know what I did wrong yet, but I'll figure it out--it is probably a component I have in there that doesn't go as high as it is supposed to, and so is pulling down the rest of it. Probably one of my zener-kludges, since I didnt' have the exact values for them, and in at least one case had to series stack two to get the right voltage.

It also takes about three seconds to shut down after I switch off (cutting the throttle pot out of circuit). If it's at full throttle when I shut it off, it immediately reverts to that 2/3 startup throttle speed, then after a second of that it speeds way up to full again, then just stops.

It works perfectly at 24V. (and should also at 36V without any adjustments I can remember; then there are component changes for 48V operation since some transistors (as specified, anyway) might not take the fully-charged voltage of a 48V SLA pack (nearly 55V).

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