Sunday, January 8, 2012

tinyKart: "Winter" Special

I was planning to keep tinyKart inactive for most of the winter to upgrade the power system and move to custom controllers, figuring that it's too cold to do any test driving anyway. But for some reason  it's 50ºF in early-January in Cambridge, MA. That, combined with tinyKart co-designer Max Hill being in town, along with a some other distinguished visitors, was enough reason to put the kart back together for a brief "winter" testing season.

The first things to fix up were a few loose screws in the steering assemblies. Normally, loose screws aren't a problem: take them out, add more Loctite, and re-tighten. But these screws were a little hard to access:

They're the six screws that hold the uprights together, and they were not designed to be accessed without taking the entire front chassis plate of the kart off. By dislocating some of the steering linkages, though, two out of three on each side could just barely be reached with a screwdriver. To make matters worse, they were stainless steel cross-head screws, which are easy to strip. So, after careful removal, they were replaced with alloy steel Torx-head screws which will hopefully never come loose:

Max and I also finished yet another motor swap. tinyKart started on Turnigy SK-6374-170 motors, which were excellent except for the lack of can bearings, which allowed them to tear themselves apart at high speeds. I then installed but never test drove the newer SK3-6364-190s. They have can bearings and nicer windings than the old SKs, but I'm still annoyed that they're not actually 63mm motors. They're 59mm and they have a smaller shaft than the old SKs (8mm instead of 10mm). The smaller diameter messes with the timing of the external Hall effect sensor boards I made, which were specifically designed for 63mm. The resistance of the SK3-6364-190 is also 60% higher than the old SK. So, we removed those and installed the third set of motors tinyKart has seen so far:

These are Turnigy EMP C6374-200s from Leaders Hobby. As far as I can tell, they're identical to the Turnigy C6374-200 that is no longer stocked on Hobby King. It is actually 63mm in diameter and it has a 10mm shaft, like the old SK. But it has a can bearing like the new SK3. The only noticeable shortfall is the messy/loose windings, characteristic of the old SK and old Turnigy motors in general. The SK3s are probably perfectly suitable motors, but right now these are a better deal, especially since Leaders Hobby ships them from the US. It's good to have a few options; I dread the day when the imported fruit-sized motors disappear and all that's left are the economically prohibitive purple-flavored ones.

Because the C6374-200 is actually 63mm, the sensor board fits nicely and the motors are relatively easy to time for forward and reasonable reverse, unlike the SK3s. These motors should have about the same amount of torque as the old SKs. Even though those were rated at 170rpm/V, our data showed something closer to 190rpm/V, almost the same as the new EMPs. These should be able to handle higher speeds, though, since they have a can bearing. At 190rpm/V and 40V, the no-load speed is 40mph. But we will have to solve the controller limits and find more testing space before that can happen.

The Kelly controllers have been okay, but quirky. When they don't like something, they cut power temporarily and give a useless "Frequent Reset" error code. It's dependent on load and motor timing, and seems to trip when the maximum current is set to 80A or above (on a 100A-rated controller). I think it's a hardware current limit being tripped by current spikes. I think the high speed firmware version would handle these motors a lot better, since the switching frequency is higher and the current ripple should be lower. But I'm not sure I want to spend $400 exploring that option when I could implement DirectDrive and, some day, have sensorless field-oriented control. For now, though, we have to live with these Kelly controllers.

We also installed new batteries to replace the 0.33kWh of LiPo high explosives that tinyKart has used up until now. This pack looks shadier, but it's LiFePO4, which is significantly less frightening to work with:

The orange straps make it safer.
The packs are custom 12S3P A123 26650 m1-sorta-B's (39.6V, 6.9Ah, about 30mΩ). They've got a little less energy storage than the LiPos (0.27kWh), so less run time per pack. But the packs are easy to swap now and can be fast-charged in under 30 minutes with Cap Kart's 15A charger. The also have a lower internal resistance than the LiPos or an equivalent pack of A123 M1A cells, which means higher peak power (theoretically up to 8kW).

With the new motors and battery installed, we took tinyKart out for some garage testing:

It's nothing as brutal as the last couple of garage runs, just making sure everything still works. The controllers seemed to be okay, although I was able to make the right side cut out a few times at full throttle. The new motors are equal in torque to the old ones, and the new battery seems fine. The handling is as wonderfully drifty as usual, and a close inspection afterwards revealed no loose parts.

So, while I sort out the details for converting it to custom controllers, we at least have something to play with if it stays warm. If not, we also have a back-up plan involving snow tires...

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