Friday, July 13, 2012

FFv1.1 Ground Firmware Testing / FFv1.2s

Charles was kind enough to volunteer his daily mode of transportation for motor controller testing:

Read: The previous controller melted, or something.
Melonscooter, so named for the "melon-sized" Turnigy C8080-170 brushless motor that drives it, has been through a lot of motor controllers. The C8080-170 no longer exists, but the longer variant, the C80100-130 is now available through Leaders Hobby (what you get if you send "HobbyKing" forward and backward through Google translate a few times) under the EMP brand name. I used to think this was the hardest motor to drive, within the set of small EV traction motors. Used to. Then I met this insane thing:

FreeFly David's custom e-bike motor.
After attempting to drive this two-turn monster e-bike motor with only marginal success, the C8080-170 looks rather tame. Especially since Melonscooter has fixed gearing and no freewheel, so it's entirely possible to kick start it. That, and the low-cogging 12-slot/14-pole configuration make it seem more feasible. The C8080-170's resistance (21mΩ line-to-line) and inductance are very low, but I had already pushed the overcurrent latching fault to 200A to deal with the current ripple on David's e-bike motor, so again this seems relatively manageable.

It looks much more legit with a 3d-printed case.
FFv1.1 is still undersized for the C8080-170 motor. Additionally, since Melonscooter runs a 40V battery pack, I'm using one of the FFv1.1 boards with the relatively wimpy 60V FETs (IPB034N06N3 G). Based on the e-bike testing, I decided the maximum phase current they would be able to handle for acceleration would be about 50A. At 3.4mΩ per FET, the conductive losses at 50A phase current add up to (3/2)*(50A)²*(0.0034Ω) = 12.7W. Add to that the switching losses, current sense resistor dissipation, and other parasitic resistances, and the power stage might be dissipating 20W or so of heat, which is a lot for a board with no heat sinks.

Tuning the sensorless FOC parameters for the C8080-170 took surprisingly little time, partially because I'm getting used to the steps required and partially because I already have good measurements of inductance, resistance, and back-EMF constant from this motor. It also helps that it could be push-start, since adjusting the open-loop ramp start takes a while. After a few iterations of parameter adjustment, it was ready for test driving:

It's way quieter than usual, due to the sinusoidal drive voltage. The current is low for what the C8080-170 can handle, so the acceleration is nothing spectacular. Although, due to a software error the acceleration currents in the video exceeded 50A by a bit. You can see the wireless data-logging software running in the video, but I stupidly overwrote all the test drive data, so I'll have to capture it again later.

The peak flux amplitude was about 5.5mWb. Multiply by 7 pole pairs to get the per-phase back-EMF constant, about 0.042V/(rad/s). That's peak line-to-neutral volts. Invert, divide by two, and convert units to get the approximate Kv: 124rpm/Vdc. This is substantially lower than the 170rpm/Vdc the motor gets with BLDC-style square wave control. Less speed per volt, more torque per amp.

The next step for FFv1.1 ground firmware testing will be upgrading to nicer FETs and improving heat sinking a bit. One FET option is the proven 75V IRFS3107-7, used on Cap Kart's half-bridge and the Victor 883107-7 controller. These have already shown that they can handle 75A acceleration current with no trouble. An even lower resistance option would be the 60V IPB017N06N3 G. It also has half the gate charge of the 3107-7, so it will cut down on the switching losses. I'm not sure I would use it with a 12S LiPo (44.4V nominal, 50.4V fully charged) system, but the current 60V FETs have proven sufficient for 12S A123 (39.6V nominal, 43.2V fully charged).

On to smaller things...

FFv1.1 (and v1.0, really, since it was virtually the same) have been very successful so far. The DRV8301 magic chip has proven its worth, since I have yet to see any noise problems at all. The target application for FFv1.0 and v1.1 was large multirotor motors, specifically for the CineStar 6, and I designed them with 20-30A continuous motor phase currents in mind. With good heat sinking or air cooling, they can probably handle 50A continuous phase current. But now I realize that it might be useful to have a smaller, lighter version for things in the 10-15A continuous phase current range. So it's time for a new revision, FFv1.2s:

The logic and gate drive sections of the board are exactly the same as v1.0 and v1.1. The power section has been entirely redone to use six Power SO-8 FETs. Power SO-8s are increasingly common thin, leadless SMT packages for power MOSFETs. There are a huge variety of lower-voltage FETs in this package including some amazing ones like the Si7192DP at 30V/1.9mΩ. They're also easy to heat sink from the top surface, since they're very thin. So even though the power stage looks disproportionately small, it should be able to handle a decent amount of power.

My target application for these is the smaller multirotor motors, such as the ones on my Talon quad. The FFv1.1 boards have already shown significant increase in efficiency on the Talon quad, but they eat up all their advantage by being much heavier than the Turnigy controllers it normally uses (65g from FFv1.1 vs. 19g for the Turnigy Plush 18A). I don't know if I'll get all the way down to 19g, but FFv1.2s should be close to that. Even though the board area is only 33% smaller, the weight should shrink by a lot more with the thin FETs and smaller capacitors.

Because the logic and gate drive section is the same as FFv1.1, it will still be a full-featured sinusoidal FOC setup with wireless capability through the XBee. (For minimum cost/weight configuration, the XBee will be left off, but it's useful to have the header there for wireless programming/debugging.) As usual, I will post the relevant design files pending testing.

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