Saturday, October 12, 2013

tinyKart Black: Alien Power

Well, shortly after the last test drive, the remaining Turnigy SK6374-170 shed its entire rotor can:

See the gap between the end cap and the rotor can?
I didn't expect this motor to last very long considering the history of tinyKart with these motors. I even remarked in the very first tinyKart build post that they are not the most well-constructed motors. The interface length between the can and the end cap looks to be no more than 2mm. I don't remember if any of the other original SKs failed exactly this way or if their entire rotor assembly shifted axially, but the end result was the same: the rotor can grinds on the face plate.

By this time, though, I had three new motors from Alien Power System ready and waiting. I decided to only replace the broken SK, since the EMP 6374-200 with the custom shaft on the left side drive still seems to be holding up. The Alien Power System 6374-170 is similar to the SK in construction (no can bearing, no radial screws), but since Alien Power System specializes in electric long boards, I was counting on the motors being somewhat more suitable for EV duty.

One other consequence of being a purpose-built EV motor is that the shaft has a nice 3mm keyway for transmitting torque. (RC airplane motors typically transmit torque to a propeller through a hub mounted to the end cap of the rotor.) This means no milling/grinding set screw flats. In fact, no modification is required at all; the motors become drop-in replacements. Of course, now the pulley needs a matching keyway.

I picked up a 3mm keyway broach and 10mm bushing from McMaster for about $60. (It will probably come in handy for other projects as well, as long as I don't lose it like all my other keyway broaches.) The broaching force is low enough that it can be done on a small 1T arbor press. There was just enough material inside the pulley hub to give a full-depth keyway without cutting into the pulley itself, and the broach was able to make it through both hub sides with no problems.

The key interface length is only about 0.5in, but math doesn't lie so it should be able to handle the full torque. Installation was no problem:

Fits the new color scheme as well...
The keyed motor shaft ends about halfway through the pulley, so I'm using a leftover piece of 10mm shaft to span from the other side of the pulley to the outside bearing plate (and a bit further, since I'm too lazy to cut it.) This might offer more radial offset tolerance than the single machined long-shaft with three bearings (two in the motor and one on the outer bearing plate).

The sensor timing was easy, which means there's plenty of magnetic flux available outside the rotor can. (I wonder how much more torque per amp you could get with a steel flux jacket around the entire rotor...) The no-load current seems very good (lack of a large can bearing helps here) and the motor sounds good. There's definitely can resonance, but it's not as sharp of a hollow ringing resonance sound as the SK. So that might bode well for the structural integrity of the rotor at high speeds.

Time for test driving:

Ryan, Dave, and I are apparently too careful to be good test drivers, so I've taken to handing it off to other coworkers (Zack and other-Ryann) who are less emotionally attached to the machine. Other-Ryann, who did the hot lap, is also an experienced racing kart driver, so that helps. Other-Ryann's test laps were done on the following settings:

Left Side EMP 6374-200: 
Current Limit: 75% (90A, 4.3Nm, 43lbf) 
Voltage Limit: 80% (31.7V, 6340rpm, 33.5mph)

Right Side APS 6374-170:
Current Limit: 65% (78A, 4.4Nm, 44lbf)
Voltage Limit: 93% (36.8V, 6260rpm, 33.1mph)

Lacking any telemetry (one thing I do miss from Cap Kart), I don't know how often, if at all, the limits were hit. But after three or four laps at nearly full speed, things still came back relatively cool. Borrowed the FLIR camera from work to see how cool:

Turns out you can rent them at Home Depot as well for about $50, too. And of course if you just need spot measurements, IR temp guns are dirt cheap. (There must be a way to turn one of these into a crude imager with some optics, right?)

It's not surprising, given the way tinyKart drives, that the rear tires came back hottest, about 30ºC above ambient. You can see the Alien Power System motor stayed relatively cool, only about 20ºC above ambient. (Although the stator is where most of the heat is generated, so looking at the rotor only gives a lagging indication of motor temperature.) The belt was only a bit warm as well. The Kelly controller barely seemed to notice that it was being used.

Since it survived other-Ryann's thrashing, I turned up the current on both sides for the next day of testing (backwardly, the first part of the video, with Dave and Zack driving and a shoe cameo by me):

Left Side EMP 6374-200: 
Current Limit: 85% (102A, 4.9Nm, 48lbf) 
Voltage Limit: 80% (31.7V, 6340rpm, 33.5mph)

Right Side APS 6374-170:
Current Limit: 72% (86A, 4.9Nm, 48lbf)
Voltage Limit: 93% (36.8V, 6260rpm, 33.1mph)

This makes it a bit higher torque than tinyKart's ever seen before. Closing in on 100lbf of traction, which is about 0.5g, depending on driver weight (certainly in Zack's case...). After the Friday evening test session things are still holding together. No apparently movement of the rotor can on the Alien Power System motor and the keyway seems to be working well. The EMP motor shaft is also still hanging in there, apparently.

I couldn't really feel much of a difference with the slightly higher torque settings. It could be that the throttle control program running on the Arduino that is taped to the back of the seat is starting to limit the current. There's a filter built in to it to clean up and smooth out the RC car trigger analog input. I'll have to dig into the code and see what the filter settings are. It would also be useful to get some kind of AC current reading on the motors to know if they are actually hitting the Kelly controller current limits.

So far the Alien Power System motor seems like a win. If (when) the custom shaft on the EMP motor breaks, I will install a second Alien motor. At that point, with matched motors and keyed shafts on both sides, and with the high-speed Kelly firmware, the current and torque should be able to go up to 100%...and then we find the next weakest link.

Friday, October 4, 2013

tinyKart Black: Some Nightkarting and 4WD Thought Experimenting

Now that  tinyKart is back in action, I've rediscovered the fun of running it around the parking garage lot late at night when nobody's around. I feel like the new paint job is more fitting for some night karting as well.

New venue, new test drivers: Zack driving, with Mitch providing chase lighting?

It's still running a mismatched motor pair, with an EMP 6374-200rpm/V motor on the left rear drive and a Turnigy SK6374-170rpm/V motor on the right rear drive. I attempted to balance out the torque and speed ratios using the maximum current and speed (voltage) settings in the Kelly controllers, with the left side at 75% current (90A) and 80% speed (32V) and the right side at 60% current (72A) and 93% speed (37V). Not that I think you would feel the mismatch anyway.

One thing that you do notice when poorly matched are the disk brakes. tinyKart has only front wheel braking, so if the braking forces are not well-balanced, it will pull the steering to one side. It was for this reason that we used this recumbent tricycle dual brake lever, which pulls both cables with equal force. Equal cable tension doesn't exactly equate to equal braking force, though, due to different amounts of travel in the brake calipers themselves and different friction in the cable housings. 

So after a half hour or so of trial-and-error tweaking and almost crashing into curbs, the brakes were balanced. When they are working properly, they are quite good. (The 160mm disks might be a bit overkill.) I do want to get the rear drive regenerative braking working too, though; not for more braking force, primarily, but for taking some traction load off the front wheels so they can do a better job of steering into a turn. tinyKart does have a considerable braking understeer, though it's usually complemented by a tiny amount of power-on oversteer, as demonstated in the video above.

I don't plan to leave the kart in its current mismatched-motor state for long. The left motor shaft could suffer the same fate as the former right motor shaft at any moment and the new right motor is transmitting torque through the feeblest of set screw flats at the moment. As a result I haven't been able to use the new Kelly controllers to their full potential yet. Time for a motor upgrade.

These are a new flavor of grapefruit-class motor from Alien Power System. With Leaders Hobby gone and HobbyKing switched over to the 59mm SK3 63xx motors, this is one of the only places I've found that still carries true 63mm outer diameter motors. And for a reasonable price, although still 50% or so more expensive than Leaders Hobby was. They do not have a can bearing, which means they might very well resonate themselves to death at high speeds like tinyKart's original SKs. But beggars can't be choosers. If I found a dodo bird walking around I wouldn't be like, "Damn, it doesn't fly."

They are well-constructed:

Nice double-sided magnet retaining provisions and seemingly good epoxying.
Clean and dense windings with no loose strands.
Total weight is just under 800g and the line-to-line resistance is 29mΩ. All good specs for a 170rpm/V motor. (I haven't confirmed the Kv just yet, but even if it comes up a little higher I would not be disappointed.) And maybe the most interesting feature:

A keyed 10mm shaft!
These motors are purpose-designed for electric long boards, so they provide an actual torque-transmitting feature on the motor shaft: a 3mm keyway! This makes my adventures in custom long-shaft machining and set screw escalation moot, I think. A quick FEA check on the aluminum pulley hub convinced me that even a short 3mm key should be okay for transmitting the full motor torque (6.7Nm at 120A).

Safety factor of about 4.
My mind has trouble accepting this result so I will probably also use the set screws I spent so much time on. But at least I don't need to machine a custom shaft for these motors. I can go back to using a separate idler shaft sticking out of the other side of the pulley to go to the outside bearing plate.

I should be able to get the new motors on and re-time the sensors this weekend. But in the mean time my mind became occupied with an interesting hypothetical test.

4WD Upgrade Feasibility

A long time ago, before the design for tinyKart was underway, I had designed the rear drive modules. (In fact, all the pictures of tinyKart have a prefix of "dm" for drive module instead of "tk" for tinyKart, since the media folder was inherited from this parent project...) The drive module was pretty simple: a motor (almost any face-mountable motor) mounted to a plate, belt drive, a tensioner, and a cantilevered scooter wheel on a custom 17mm aluminum shaft. But back then I didn't design it as a rear drive...I designed it as a steerable drive module for something like a giant Twitch.

Anyway, the drive modules' design got re-purposed for tinyKart's rear wheel drive. But recently I started thinking about whether it could have worked on the front wheels too. The conclusion I came to was, mostly, no. At least, not with disk brakes. The belt drive itself could work, but the brake disks would move to the outside and that would require significantly more structure to mount the caliper.

So, looking for any way to put the drive and brakes on the same side of the wheel, I sketched this up:

Drive Mod v2.0?

Starting from tinyKart's existing front wheel modules, I made only two very minor changes. First, I added a 75-tooth, Module 1, 8mm face-width gear to the brake disk hub. I originally thought of having it replace the hub entirely, but the hub is threaded for interfacing to the wheel rim and it's easier to simply pocket out the gear and fix it on top of the hub:

Pocketing out the gear saves a lot of weight too.
The diameter of this gear is limited by the brake caliper moutning. A gear ratio of 75:18 would match the rear drive pretty well, although there's no reason why they have to match.

Second, I replaced the normal brake caliper mount with an extended brake/motor mount that also holds an intermediate idler gear for achieving frame clearance for the motor.

Shown with the brake disk hidden.
My mind keeps coming up with reasons to dislike this spur-gear design, and then eliminating them almost as quickly. It's a fun game to play:

Spur gears? Really?
Yes. Belt will not fit in the small gap between the disk and the caliper mounting plate. Any other solution would involve lengthening the spindle shaft, which as all sorts of nasty side effects, or a complete redesign. (Either putting drive and brake on opposite sides, or something something even more drastic like a live shaft design.) Chain would work, but  it would be tough to install. Also, I have an deep-set aversion to chain drives after many years in FIRST robotics. I got some inspiration from this drivetrain for an Eco-marathon car I saw in Singapore:

But they are noisy!
Well so are chains and square-wave-drive BLDC controllers.

But the idler gear is stupid and inefficient. It looks like a 2.007 robot drive train.
The efficiency hit is negligible, compared to direct gear drive. It's certainly no worse than a belt drive. Chain might be more efficient but I kinda doubt it.

Such a thin gear can't handle that much torque.
It can, I did the math. Module 1 gears (shown in CAD) are borderline. But for sure Mod 1.5 or an equivalently beefy inch gear with the same face width would work. That's also assuming full motor torque, same as the rear drive. That's certainly not necessary and probably not even desirable, for other considerations such as battery life. A 40/60 or 33/67 front/rear torque split could work just fine.

The motor shaft isn't doubly-supported. It will break, or deflect so much that the gears will go out of mesh.
It does pain me to leave the shaft unsupported, but the maths don't lie:

Safety factor of 2 at 120A torque. Guess where it would break if it did....
Max deflection is 0.02mm.
Safety factor of 2 on the shaft is based on 1020 steel. I don't actually know what the shaft is made of, and the dynamic loading could in theory go higher than 2x static. But I did not model in the gear hub which will spread some of the bending moment closer in to the root of the shaft. And again the front does not need to run 120A torque, see above.

But the idler gear will for sure need to be doubly-supported. It can't have a 10mm steel shaft.
At first I thought the idler gear was safe because the forces on it would be balanced. But this is only true of the separation forces. The torque-transmitting forces actually add up and put even more bending load on this gear's shaft. But, the idler gear could potentially be on a dead shaft that is screwed directly into the mounting plate. So the moment arm will be much smaller than that of the motor shaft. This method would involve putting bearings in the idler gear itself. The details of this are TBD.

What about the aluminum plate? That could bend enough to un-mesh the gears.
True. I state without proof that it can be designed and engineered not to, even if the current CAD placeholder would.

It's impossible to assemble.
It was. There would be no way to put the motor pinion on from the outside. So instead I made a slot in the mounting plate into which the pre-pinioned motor slides. Then, it may take some specially-modified right angle hex wrenches to secure the motor in place. But it's no longer physically impossible.

So yeah, I started out very skeptical of the spur gear drive mod, but now I'm fairly convinced it would work. The appeal is that it can be tested on tinyKart with very little modification. That doesn't mean I'll actually do it - the cost and weight of two more motors and controllers isn't really justified. Also, the power to run the front wheels has to come from somewhere. The choices are either shorter battery life, more battery weight, or less rear wheel power. So for now it remains a thought experiment.

Though it would be an interesting proof of concept for future vehicle projects.........