Wednesday, September 9, 2009

What's Next: 3ph Duo Controller

Fall '09 is under way with an interesting set of old and new projects on the horizon. The most immediate is yet another motor controller - version 2.0 of the 3-phase brushless series and version 7.2? or something of the modular half-bridge design. A brief history:

Version 5, oddly enough not pictured, is the only one that has never blown up on the go-kart.

Version 1.0 of the 3ph brushless controller, after a bit of troubleshooting, works just fine. It's actually overkill for the application, namely driving a 500W in-wheel motor for the B.W.D. Scooter. The motors can handle a peak current of about 20A. The v1.0 controllers, fans enabled, were tested to handle 75A for 45 seconds on a single phase before the MOSFETs started to desolder themselves. Divide that up between three phases and you get something that would probably handle 75A continuously. Combine that with 48V-compatible MOSFETs (75V rated) and you get a 3.6kW controller...for 500W motors. However, overkill alone would never make me design a new controller. The biggest problem is this:

No Step.

Despite my best efforts to make this more compact than any controller I've designed before, there just isn't enough room for two of them on the scooter. The only solution was to have them stick out through the deck. This is bad for a few reasons: One, it takes up foot space and is going to get has gotten stepped on. Two, it just looks bad. The whole point of putting the motors in the wheels and the batteries under the deck is to make it look as compact and normal as possible. Also, stacking two controllers on top of each other makes one of the fans completely useless. (The fans, along with almost every other component that wasn't soldered down, get beaten up pretty badly due to vibration, too.)

So, the challenge for v2.0:
  1. Run both motors in current/torque control mode with a single controller.
  2. Maintain at least 40A continuous current capability on each motor, at up to 48V. Two 1.9kW controllers on one board will make it actually more powerful than a single v1.0 controller.
  3. Maintain the same x-y footprint as v1.0, if not smaller. This was easy: Eagle doesn't allow you to make boards larger than 80mm x 100mm.
  4. Shrink the maximum height so that everything fits within the scooter deck. This means the entire controller must occupy less than about 1.3" (35mm).
  5. Maintain the existing modular half-bridge driver design. It's optically isolated and I know it works. There are smaller, cheaper ICs that will do half-bridge drive, but now is not the time for radical design change.
  6. Maintain the ability to record and transmit data.
Notice that cost is not an objective? That could be bad... But here are the commercially available alternatives:

The Turnigy Sentilon 100A HV. Ok, I will admit this is impressive. It is one of the most beastly high-voltage hobby RC controllers out there: 100A @ 50V. That's a 5kW controller in a 60mm x 60mm x 11mm package. And it actually wouldn't be that hard or space-consuming to add a current-controlled feedback loop around this. (As-is, it only does voltage/speed control.) But it's not sensor-commutated. Meaning, startup torque will suffer. How much, I don't know. And keep in mind that despite the massive power, it still only runs one motor. Cost for two: $232.

The Kelly KBS48051. This is the cheapest serious-business EV brushless controller you can find for 48V. It can handle 20A continuous, 50A peak for one minute, so about 1-2kW. It is fully current/torque controlled with hallf-effect sensors and regen. You can program it, although getting real-time data off is a different story. The down sides: It's the same size as my v1.0 controller. And still, only one motor. Cost for two: $200.

Okay, down to business. The first step was to find a new MOSFET solution. As much as I love the IRFB3077, there is just no way 12 of them will fit. The key discovery was an inexpensive, integrated three-phase bridge module: The IXYS GWM series, specifically the GWM100-01X1-SL, which is the only one in stock anywhere (Arrow Electronics).


At $21 each, these replace six IRFB3077s at $2.79ea. I consider that a fair trade. Specifications-wise, they have about twice the resistance per FET (7.5mOhm), but also a higher voltage rating (100V). They are rated at 1.3K/W with a proper heat sink, which is about the same as the IRFB3077, except that these share a heat sink tab. I doubt they will match the performance of the 3077s, but 40A continuous per chip should be achievable.

The other key discovery was that I only need one MSP430F2274 microcontroller to generate all six channels. It has four output-compatible timers, but each timer is mulitplexed to two pins, so really there are eight available output pins that can do high-speed PWM. And there are only really two independent PWMs required (one for each motor). It should have no trouble handling the extra hall-effect encoder interrupts, too. The only real worry I have is the 4kB code limit imposed by my free version of IAR Embedded Workbench. Here's why:

Not a lot of space left.

If the code to control one motor costs 3,255 bytes...and I need to control two motors... Well, I tend to just assume that all software problems can be solved. Realistically, most of the code is overhead that doesn't need to be repeated. But let's just hope that they know that 4kB is 4,096 bytes, not 4,000 bytes. I will probably need those extra 96.

Ugh, why am I such an EE? New constaints.... WHAT ABOUT FITTING THE WHOLE THING INTO THE DECK? Which, I believe, was the original point of all this. Here:





The IXYS FET modules get thermal pasted to the aluminum deck, which should make an excellent heat sink. Wires get soldered in directly like in the Turnigy - no more bus bars. The single MSP430F2274 controller board gets a new home dead-center. All the gate-drive circuits have been made surface mount and take up both sides of the board now. The 15V switching regulator and associated inductor are surface mount. The only thing that can't really be made smaller are the current sensors...but they are in the right place now, at least. The result: the board occupies only 1.2" of vertical space.

And the final cost of components? Let's see... $42 for the FETs + $23 for the MSP430 board + $32 for the radio + $73.50!!! for six isolated high-side supplies + $31.62 for twelve optically-isolated gate drivers + $32.74 for two current sensors + $8 for the 15V supply + $25 for misc passive components and connectors. $270. No, I didn't count the cost of circuit boards. But I'm only building one. Extrapolate to high quantities and the total cost here isn't that bad.

I'm sort-of hoping it just works. The only major hardware changes are the MOSFETs and the placement of the current sensors. The most likely failures are actually on the software side, since I changed so many of the pins. I tried my best to think through the pin assignments, but there's always the chance that I missed something that will result in a disappointing, smokeless failure. If so, there will have to be a version 2.1...

I'll post the new schematic after testing.

1 comment:

  1. I also hope that this thing would work out so that it can be released to the public and so that many will be using this invention. By that, the inventor will be gaining money also.