Monday, August 2, 2010

Cap Kart Summer Rebuild 4

The Cap Kart runs again, field driver curse notwithstanding.

First, the mechanics. We just needed to mount the freshly-cut rear axle to the outputs of the brand new differential. Except how do you connect a 34mm I.D. hollow axle to a 1-1/4" (31.75mm) keyed shaft?

Step 1: Fill the gap with a flimsy-looking aluminum sleeve.

Step 2: Fill the keyway with flimsly looking 8-32 small pattern hex nuts.

Step 3: Fill the nuts with flimsy-looking 8-32 brass-tipped set screws.

The only good thing I can say about this is that it doesn't do what you think it does. The 8-32 screws are not in shear; they're in compression, pushing up on the nut and down on the shaft, such that at least some of the torque is transmitted through friction interfaces and there isn't any backlash. I don't know if that makes me feel any better about it, since couplers for this size shaft tend to be much beefier looking. But this costs nothing, weighs nothing, and might work. My horrible standard for judging this is that the traction-limited rear wheel torque is something like 100ft-lbf. And I would totally hang off a 1-foot bar attached to this...

...Moving on to software.

Is that a game?

We finally got to load the code. Costas wrote the State Machine of Doom to handle the multi-mode electronic transmission, as well as a whole new set of fault codes that I wish we had back in 2008 when we were regularly blowing up motor controllers. The kart transmits 13 bytes of diagnostics back to a waiting laptop at 20Hz, including one byte of potential faults ranging from "hard faults" such as loss of accelerator signal to "soft faults" like battery power limiting. If anything goes terribly wrong with the controller, the data logger is like the black box.

Even if things go right, the data is still useful for performance analysis. For example, testing the effective resistance of our brand new 40Ah Thunder Sky batteries.

What you see there is actual kart data recorded during a bench test. Battery current is the same as motor current, in this case, because it was run up to 100% PWM and full throttle before engaging the load (i.e. the brakes). Since the motor controller measures motor current and battery voltage, it was a simple matter to cross-plot them to find the effective resistance of the pack. A 6V sag at 150A gives a pack resistance of about 40mΩ, or a per-cell resistance of about 3.3mΩ, right on target. That means an effective peak power of about 7kW from the 12S battery pack (taking 2.5V per cell as a soft limit), the majority of which goes to the ground (we hope). Add in an extra 3kW of on-demand capacitor boost and you get reasonable power out of just 50lbs of energy storage.

That all seems very easy, but if Summer 2008 was any indication, there is no way it all just works. And if one part were to break, I know exactly which one it would be. So continues the curse of the field controller.

Now, there's no real technical challenge to the field controller. It's not like the monster 300A synchronous half-bridge that drives the armature. It's a 10-15A drive into a load that looks like a 1.3Ω resistor and a giant inductor. The current in sets the field strength of the SepEx motor, which in turn sets the torque-speed curve.

But the problem isn't ever a technical one. Curses don't work that way. For example, one time, the reversing switch welded itself shut, taking out the rest of the controller when it was switched back to forward, shorting the battery through the field-drive FETs. This time, the curse first manifested itself in the form of some really shady-looking gate drive waveforms:

That turned out to be a simple problem to fix; it was only on the high-side drive which led me to the conclusion that it just needed a bit more bootstrap capacitor and a bit less pull-down resistor. But it still made this annoying crackling sound at high loads. In fact, it's very similar to the crackling sound I've heard before and always attributed to the switching regulators on my controllers. Except this controller doesn't have any switching regulators... I decided to investigate more. And by investigate, I mean destroy the entire field controller.

This is what happens when you use two of the same color alligator clips. Inevitably, you clip the two load leads together and the two outputs together. This short-circuit immediately vaporized a trace on the circuit board...

...which in some cases would be fine. But this time the resulting voltage spike took out 3 out of 4 MOSFETs and both gate drivers. Are you starting to see what I mean by the curse?

Anyway, with a new set of MOSFETs and gate drivers, plus a little help from real electrical engineers, I finally tracked down the source of the crackling to over-aggressive gate drive. The turn-on waveform would occasionally ring or go unstable. Turns out in some instances it actually helps to slow down the gate drive a tiny bit to regain stability. Kinda defies my whole understanding of gate drive, which to this point had been "go as fast and as hard as possible." It makes me wonder if I can go back to my old controllers and fix the crackling the same way...

But for now, everything could finally go back together for a spin-up of the brand new rear axle:

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