Monday, November 22, 2010

Pneu Scooter Test Drive Data / 3ph v3.1 / New Project Teaser...

Building your own custom motor controller suuuuuuuuuuuuuucks, but it means I get free wireless data acquisition on my vehicles. I use XBee 2.4GHz transceivers to stream out things like battery voltage, motor current, speed, etc. to a waiting laptop which displays it in real time and/or records it.

An XBee radio on the 3ph HD.

Even though I made an utter mess of the HD in the last post, the data acquisition works fine. And it's not unbearably cold outside yet, so I wanted to do a short test-drive with the patched-up controller before it's too late. Minor problem: The controller sits inside the aluminum deck Faraday cage and the transmitter range with the on-chip antenna was terrible. This is easily solved by using a little external antenna flap thing that I got from Digi-Key, which attaches to the version of the XBee with U.FL connector and comes up through the back of the deck:

Other minor problem: the receiving program needs to be running on a laptop. This is why I need a fanless netbook, but for now I figured it's cold enough that I can run my laptop, closed, in my backpack, fans pointed upward. (I have an abusive relationship with my laptops...

Mobile wireless link established, and awake during daylight hours for pretty much the first time since returning from Singapore, I went for my first on-campus test-drive:

The Nordschleife, not to be confused with the Ostschleife.

Here's some raw current and speed data from the first 1/3 of this run:

What does it mean?

Raw data is raw, but here are some whole-trip statistics:
  • Trip Distance (measured): 2.1 mi
  • Capacity Used: 1.17 - 1.39 Ah
  • Energy Used: 38.6 - 45.9 Wh
  • Efficiency: 18.6-22.0 Wh/mi
  • Peak Power In: 543 W
  • Peak Power Out: 375 W (est.)
  • Top Speed: 14 mph
These are very nice numbers. The variance in energy use estimate is due to slightly different integrals from the DC current sensors and the AC current sensors. I tend to trust the AC current sensors a bit more, and they're giving a worst-case estimate on par with RazEr's 25Wh/mi, so let's go with that. I won't even bother converting that to mpge because the value is absurd. 

These efficiency numbers give the scooter a best-case range of about 6 miles, though I would expect something more like 4-5 miles for a reasonable SOC range. Not bad for two DeWalt drill batteries. The lithium-polymer pack I had in Singapore (two of these) would have extended the range to about 7-8 miles without adding any weight.

I also did a few more controlled tests. First, a no-load test confirms 1,520rpm at 33V. That's a Kv of 46rpm/V, or a Kt of 0.21Nm/A, right on target. This corresponds to a ground speed of 27mph, but friction and drag will always load down the top speed. I would estimate that the loaded top-speed of this scooter is about 20mph, and I will confirm this when I have mechanical brakes. ;)

I also repeated a test from the early days of my field-oriented control adventures: a launch with and without d-axis/phase control. Here's the original explanation: Part 1 and Part 2. Here's Pneu Scooter without d-axis control:

The controller here can't adjust the phase of the sine waves driving the coils, only their amplitude. As speed increases, the motor's inductance causes current to lag onto the d-axis. To keep a steady q-axis current (torque-producing current at right angles to the rotor flux), it must increase the overall magnitude of the current to accommodate for the lag.

Here's the launch again, with d-axis control on:

Now the phase of the driving sine waves is unlocked, and as the speed increases it is feedback-controlled to keep the d-axis current at zero. In this case, about 17º of phase advance is required at 12mph. The total current magnitude stays steady and equal to the q-axis current. Nothing new here, just more demonstration of the modified synchronous current regulator at work.

Next order of business: Go back and make some of the 3ph HD hacks permanent in a new minor version of the controller, the 3ph 3.1. Here's the board:

It looks...pretty much identical to the last one. I moved the DC/DC converter to the bottom of the board, even though the capacitors technically don't clear vertically. They're only off by about 0.4mm, and I can use some gap-filling silicone padding to space out the MOSFET brick a bit more if necessary. That frees up tons of space for moving the analog RC filters right next to the microcontroller pins and adding a crapload of capacitance everywhere, two things that helped get the HD in working order.

The biggest change is the addition of an isolated 5V logic supply, in the form of a DCR021205. These are similar to the isolated supplies I use on the higher-current controllers to feed the gate drivers, but these convert from 12V to a regulated and isolated 5V. The interesting thing is that the logic can't float arbitrarily: the IRS21844 gate drivers allow only a +/-5V level shift between logic and power ground. And the battery voltage measurement needs some kind of reference, since I'm not using analog isolators. But in theory, no current should flow from power ground to logic ground without going through the DCR02. You can see how I dealt with this problem here:

If you see what I did, shut up.

If this quasi-isolated logic supply doesn't fix the problem, then I would be forced to go back to something more drastic like optically-isolated gate drive. But since the HD is working pretty decently now with just the addition of a 10Ω resistor between logic and power ground, I think this will do the trick. Though I would like to be able to turn the peak current up to about 60A if this cleans up the noise significantly. Look for an update in a few weeks when I get the new boards.

And before I forget, a New Project Teaser for you to guess on:



  1. This comment has been removed by the author.

  2. Josh said...
    I do know! ^.^

    By the way, this is a bit late, but thanks for linking back to your previous explanatory posts on Field Oriented Control. I'm gunning for MIT and need all the knowledge I can commit to memory before I apply!

  3. Awesome post as always. I also love the new electric powered snowboard thingy? At least that's what it looks like to me.

  4. only FYI.

    Good work Shane.

  5. You are working on a track snowboard with Charles!

    After seen Charles blog post, I saw a pair of track on the bottom of his post. Then I thought maybe they attach to your drawing.

    Got the inspired by the DTV from Ben?

  6. This one's a little different. But you're right about the snow. ;)