Sunday, August 22, 2010

Cap Kart: Shakedown

First, some new test drive video:

As good as that looks, it did not come easy. In the last test drive, the kart suffered a MOSFET failure after just a few runs. For those unfamiliar with power electronics and motor control, the MOSFETs (or IGBTs in higher voltage controllers) are the primary elements that control how much current gets sent to the electric motor. A good analogy in an internal combustion engine might be the throttle body. When they fail, it's usually the end of driving.

IXYS VMM 1500-0075P: Possibly the largest MOSFET you can actually buy.

Tracing through the data from the last test drive shows the MOSFET failure clearly enough, but only offers hints about the cause. My best guess was that the 12V supply was insufficient for the gate drive optocouplers (Avago HCPL-3120) that turn the MOSFETs on and off. They're really designed for 15V supplies, and the combination of a smaller 12V battery and a higher gate charge requirement on the large MOSFETs may have tipped them just over the edge. If they shut down permanently, the kart should just coast, but if they shut down and come back on after a brief interval, the MOSFETs may see a quick burst of current since the controller feedback loop had been broken and it needs to "find itself." Should this destroy the FETs? Probably not. Did it? I don't know.

In any case, the data and some bench experiments confirmed the presence of some current spikes when running the gate drive on 12V. Switching to 15V eliminated these spikes. Since the gate drive runs off its own battery anyway (for noise immunity), it was an easy matter to change to a small 4S lithium polymer battery, at 14.8V nominal. In a moment of pure overkill, the battery of choice is now a 1.8Ah Turnigy Nano-Tech. Yes, a batttery that could potentially supply 45-90A, powering the 0.5A gate drive and logic circuit on the kart. Where did I put those fuses?

Electrical problems out of the way, it was clearly time to move on to some sort of mechanical failure. And the most obvious place for that would be in the brand new rear axle differential.

Why can't it be this easy?!

After the first test drive, we noticed that one of the snap rings holding the gears on their respective shafts in the differential had popped off and was nowhere to be found. The entire case had also shifted a good quarter-inch. Rather than admit the obvious - that some unconstrained axial forces knocked the snap ring off - we chose to assign questionable blame to things like loose set screws and misaligned chain. Well, the following week we made it as far as the elevator before the snap ring popped off again.

The full extent of last week's test drive.

So, we took it back up the elevator, cried for a while, then decided that what was really needed was a better way to constrain the differential axially. After a few hours of trying to avoid taking the whole differential off the kart, we wound up taking the whole differential off the kart. And by that point, the easiest solution was just to add spacers to the shafts that fix the distance between the outer and inner bearings, as shown in this top view:

The outer bearings are already well-constrained by the shelf that we built around the rear axle. The new spacers, in red, fix the differential in place axially. As an added bonus, one side's spacer accurately sets the gap for the brake disk. This fix took us an entire day, and since we can really only test drive on the weekends, it basically meant one fewer test drive this summer. Bummed as we were, the fix was important. There is no acceptable outcome if the gears fall out of the differential.

Back to present day. If you're keeping track, we're due for an electrical failure now.

The goals for this test drive was to abuse/test the rear axle a bit on our slalom course, and to see if the change to a 15V gate drive supply would save the MOSFETs. The first goal was met unquestionably. No snap rings fell off and the differential seemed perfectly happy the entire time. Congats, Max. The slalom, which was pretty damn near impossible in 2008, is now too easy with the lighter kart and differential. The only real mechanical issue left to deal with on the kart is the somewhat squishy brakes.

Back to the controller. One of the untested bits of the new software was the Costas Variable Transmission that does constant-power field weakening on the separately-excited DC motor. I decided I would just go for it:

You can see me flooring it, the armature PWM maxing out, then the field PWM dropping to almost nil as the kart continues to speed up. The armature current holds at a pretty constant 250A and the battery voltage is right at the 30V cutoff. This seems to work exactly as designed. Upon releasing the pedal, the field PWM first increases, then the arm-- ugh not again. Almost as soon as the current goes negative, bad things happen. Zooming in on the one-second interval around the fault:

Here I've highlighted three interesting intervals. In interval 1, the huge negative current spike (-560A) is detected. It would be easy to blame such a spike on bad regen control tuning, but if that were the case, the power would be going back onto the battery and the DC bus voltage would skyrocket. Instead, it does the opposite, dipping to 15V and then even 10V. To me, this suggests that a ton of power is coming out of the motor and out of the batteries at the same time. And there's really only one place for that power to be going...straight into the MOSFETs...

Luckily, the new fault detection software picks up the problem almost instantly and shuts down the drive. (Thus, it has twice prevented further destruction.) The hard faults cause the PWM drivers to shut down immediately, which is why the armature and field commands drop to zero. Interval 2 is the time between the drive turning off and the contactor opening. (Some time is given to allow inductive energy to be dissipated.) Unfortunately, by this point, the MOSFETs are totally dead. It's clear that current is still going into the motor and coming out of the batteries, even with no drive command. This actually shows up as a brief burst of acceleration, since the controller is no longer capable of throttling the current. The contactor opens between interval 2 and 3, less than a half second after the fault, at which point the bus voltage starts draining through whatever horrible new low-resistance paths have been created in the MOSFETs.

All this happened while I was happily driving along. I didn't realize there had even been a fault until I came to a stop and noticed the DC bus voltage was reading 5V. At least the new software is way smarter than I am. At this point, screw electronics. Seriously. I don't know what the failure mechanism was and the data stream does not show anything that would indicate a cause, only effects. The only thing significantly different about this controller than the one that ran the kart in 2008 are the IXYS FETs. From an experimental troubleshooting point of view, replacing these was the next best target.

Enter MetalFET.

After the previous MOSFET failure, I decided to enact a Plan B since we have only a limited number of IXYS MOSFET replacements. Plan B was a custom-designed circuit board that is more copper than not. And I'm not talking ground planes. I'm talking a serious current-carrying copper bus bar and FR4 sandwich, hence the metal part of MetalFET. Oh yeah, and the FET part...tiny MOSFETs. Tiny only in size, though. These are seven-legged IRFS3107 HEXFETs, the most badass MOSFETs in existence as far as I'm concerned. 

Sorry IXYS, stealing your critical dimensions.

The board and associated aluminum/brass hardware adapts these tiny FETs to the same form factor as the IXYS MOSFET brick, facilitating drop-in replacement. The kart formerly ran on the through-hole version of these IR FETs, four in parallel for 300A. But they were mounted to bus bars that doubled as heat sinks. To get a similar thermal performance for these surface mount FETs, I used unmasked boards and screwed 1/4" aluminum heat transfer blocks to the drain planes of both sides:

The truth, though, is that I really did not want to use these. The IXYS FETs are a much nicer, more professional-looking solution. These are a hack I came up with in a few hours of work (plus three days of waiting for circuit boards). The Cap Kart knows just exactly how to push me to the limits of my patience, hence the original post title. So with really very little time left of summer test driving, we swapped to an untested, locally-grown MOSFET module.

Something traumatizing about the first Cap Kart test drives must have still been in the back of my head, because a few weeks ago I went to Radio Shack to replace the 350W inverter I bought back in 2008 and subsequently lost. Pretty much the only reason I would ever need a 350W inverter would be for field soldering off a 12V battery, and pretty much the only reason I would ever need to do that would be to fix the Cap Kart. And as luck (or lack thereof) would have it, I was able to put it to good use, if only once, to move the gate drive wires over to the untested MetalFET.

Yes, the homemade MOSFET module is now zip-tied to the shelf.

Honestly, I would have given this less than 50/50 odds of working, especially since it was completely untested. But, as luck would have it, it works totally fine. All of the test drive video above was run on the MetalFET. At times it was a bit warm...not hot, just a bit warm... So there you have it - a $30 hack that works where the $300 MOSFET modules fail. Go figure.

That nightmare out of the way, time for some more fun. Here's a run that uses the "three speed manual" mode:

You can see the system traverse peak power three times, with the estimated motor output power (P_Vemf) holding steady at around 5-6kW. While the Costas Variable Transmission mode will do this smoothly and automatically, the goal of three speed mode is to actually feel the gear change "kick" as the field voltage is varied in steps.

And here's one more battery I-V curve:

This is the widest one yet, covering -75A to 300A. The battery internal resistance (V/I slope) remains constant over that whole range. They may not be the most powerful batteries, but the Thundersky cells are remarkably consistent.

The Cap Kart: Overinstrumented? Yes. Of questionable power electronic legitimacy? Yes. But a lot of fun to drive.


  1. thank u for sharing all this info

    can u please explain where do you get these standoffs or posts called : aluminum hardware adaptor
    that are used for high current connectors . i donot know what to search for on digikey

    thanks in advance

    1. I think they are 1/4-20 aluminum hex standoffs, from McMaster Carr:
      Although they are available elsewhere in different sizes.

      I think some of the current is going through the brass screws as well. (I'm not sure what the contact resistance across the aluminum's sufrace oxide layer is, but the screw threads give it much more surface area to work and keep the surfaces compressed together.) Anyway, I would definitely not use normal steel screws with this setup.