Tuesday, October 18, 2011

Okay, tinyKart frightens me now.

So it turns out tinyKart was hiding something...

...and by something I mean about half its full power. I know I've had nothing but good things to say so far about tinyKart, but somehow I think I underestimated it. tinyKart. Kicks. Serious. Ass. We need to make more of these things right away and race them against each other.

tinyKart was the Edgerton Center Summer Engineering Workshop's 2011 project. It's an ultralight electric go-kart, something I've dreamed about ever since Cap Kart. The total weight is 55lbs, it fits in the trunk of a car, and it has a trigger throttle. We took it to Maker Faire NY a few weeks ago, and it was a lot of fun. 

But it seems this entire time it's been operating with at least one powered-back controller. The left side Kelly KBS36101 would give a "frequent reset" error and, at high sustained load, cut out entirely, leaving the kart with half power. The right side would also give errors occasionally, but rarely cut out. Then, I tried the parking garage climb and both sides cut out at full throttle. I was getting ready to ditch everything and put in my own controllers.

I guessed that the problem was bad sensor timing. I've seen it before on KBS controllers. The timing is off enough that the drive voltage and back EMF don't match up, leading to current spikes that trigger the controller's protective features. It's exaggerated by the low inductance of the brushless airplane motors. So, I went to carefully re-position the external Hall-effect sensors...

...or actually the entire sensor mount just disintegrated because I failed to remember that Loctite and plastic parts do not play nicely together. For now, I had no choice but to make a new one and forgo the Loctite, but for the future I am switching over to PCB-mounted sensors:

In addition to fixing the sensors in the correct positions and providing for up to 60º (electrical) of adjustment, the board also has a pair of jumpers which can be used to swap two sensor leads, for easy reversing. Much better than having to dig through the software, anyway. These are the first PCBs I've attempted with internal routing, in addition to the curved outside shape. It was a good opportunity to try out MyRO PCB, since Advanced Circuits does not allow internal routing on their $33 Each or Bare Bones deal. (Advanced Circuits' $33 Each deal does handle complex outside dimensions, though, as evidenced by the so-named 4pcb quadrotor.) I've dealt with MyRO before, and it seems okay. So I ordered six of these boards in a tab-routed panel...

...at least, I think I did.
The process I use for "timing" the motors is pretty simple. I set the throttle limit on the Kelly controllers to 30%, so that I can tune the sensors at a reasonable wheels-up speed without much frame vibration. Then, at max (30%) throttle, I move the sensor mount by hand within its 60º (electrical) of adjustable range until the current going into the controller is at a minimum. If the minimum is at an extreme of the adjustable range, I swap motor leads and/or reverse the direction of commutation and try again. 

Minimum current means the back EMF and drive voltage are as close to in-phase as possible. (At no load / assuming zero inductance, blah blah blah.) With the new acrylic sensor mounts, I did this quickly with my clamp meter, then reset the max throttle to 75% (because the controllers and motors may or may not explode above 40,000 electrical rpm.). 

And...well, a new tinyKart was born, apparently. One that is much more evil than the original one. It will happily give full power on both motors now, right up to the motor current limit (80A per side) or the battery input current limit (60A per side), or the maximum throttle (75%), whichever is lower. It accelerates as fast as Cap Kart and might even hit the same top speed if not for the throttle limit. No more play toy...it recorded a peak input power of 4.1kW on the hill climb. And the motors only eat about 200W each to heating at that power, so it's probably putting out very nearly 5hp now.

With the new, scarier tinyKart, I also took the opportunity to re-balance the two front disk brakes and change the steering ratio to give more travel on the steering wheel, and hence more mechanical advantage over the wheels. As if we had actually planned ahead, this only involved moving some pin joints to pre-drilled adjustment holes:

Drive link gets shorter by moving the ball joints closer to the steering column...

...and follower link gets longer by moving the pin joint away from the kingpin.

The length of the tie rods also changes, but that is adjustable by nuts on a threaded rod. I don't think it could have been any easier.

After that, we took it back to the parking garage (along with some other things) and the video above is the result. With 5hp available, narrow drive wheels, and low-traction parking garage concrete, it would happily lose grip coming out of turns, even while hitting the uphill slopes. That was a little surprising. But what I find amazing is that it actually happily re-finds grip as well. It does what I would expect a go-kart to do when I steer against a drift and adjust the power. This thing that we built with 80/20 and McMaster parts behaves the way I think it should, presumably from some experience driving real go-karts... I like it.


  1. Now all it needs is some fancy carbon fiber bits!!

  2. I lost 10 minutes of my memories and got a $3000 hospital bill (and several days of pain) falling off an electric bike onto concrete...put on a helmet!

  3. > put on a helmet!

    was my first thought as well...

  4. Yeah, don't worry, a Stig helmet has been procured: http://eeanonymous.blogspot.com/2011/10/meche-after-all.html