Friday, March 19, 2010

Pirate Radio Control

I have a new favorite thing ever:

The string of unbelievably nice weather (following four days of hostile rain) motivated me to take the pile of parts that had accumulated on my desk all winter and finish turning it into a brushless-powered RC car. Now the original car (a Team Associated TC4 RTR) was already pretty good, and a lot of fun to drive. But I wanted to add some of my own style to it. Here's the outline:
  • Switch the brushed, 7.2V motor out for a brushless 14.8V motor.
  • Add a 19.8V LiFePO4 battery.
  • Add my own brushless controller and 2.4GHz RC system.
Like many of my projects, it starts with the motor. The original (brushed) motor was actually not that bad. (I characterized it in this post.) But I just had to put a brushless motor in it. I just didn't want to spend the $50-$100 that seems to be the going rate for even sensorless brushless RC car motors. So I went to one of my new favorite sites, Hobby King, and found the cheapest motor I could find. For $16, minus my $3 store credit, I got a "High Performance" BL540ST sensored motor with 13.5 turns, 3150Kv. Sensored. For $13.

I love this motor. Sure, it doesn't quite fit in the motor mount that came with the car, but that's what we have machine tools for:

Operation 1: Carefully turn down bearing cap and...oh crap it broke off.

Operation B: Luckily the bearing itself is 0.500" so it fits
 in the largest bore you could possibly make in the motor mounting
block anyway. Mounting block becomes new bearing cap. Win.

Machining disaster aside, the motor fits nicely into the existing mounting structure and, when combined with comically-oversized wire, looks pretty intimidating in-system. In addition to having a much lower resistance than the brushed motor, this motor is rated for 4S LiPo (14.8V) operation. So I'll go ahead and run it with 6S A123 (19.8V). What? It'll be fine.

The new battery pack, which you can see in the image at the top of this post, is rated for 70A continuous discharge. (That's 1,386W available...) Since I don't expect to ever need that much, I have a 30A fuse on the whole thing right now. I chose to run 19.8V instead of 13.2V or 16.5V for a couple of reasons. One, my controller won't even turn on with anything less than 18V. Two, I think the motor Kv is actually lower than it says (which I argue is a good thing). I'll also be running sinusoidal control. These both mean less speed per Volt of battery, so to make up for it I'm using a higher voltage.

The controller is the 3ph Duo, except with just one side built (so, 3ph Uno?). But it's still got all the bells and whistles of the Duo, including field-oriented sinusoidal control, current (torque) limiting, and built-in two-way 2.4GHz digital radio communication, courtesy of XBees. The Duo was designed for a 33V/20A motor (two, actually). This system is lower voltage, but potentially higher current. So, I made some modifications:

Specifically, the traces got some reinforcement, the capacitors have more capacitance, and the current sensors have been bypassed by an equivalent resistor to effectively halve their gain. I also used a different IXYS six-FET module, this one rated at 40V/180A instead of 100V/90A as in the original design. As it turns out, none of this was probably necessary because the car spins its wheels at 20A anyway. But...overkill isn't necessarily a bad thing.

Just a few more modifications to make! An antenna mount:

Can't screw that up.

And finally I need to make a heat sink / controller mount. Oh wait, I found this random block of aluminum that happens to already by exactly the right size. It even has a relief for the sensor wires in exactly the right place:

See how well it fits?
This never happens.

A bit of programming later, and I have an RC car again. Time to take it for a test drive at my favorite RC test drive site: the top of the MIT North Garage:

It's absurdly nice out and there's something about an empty parking garage roof that makes me happy inside. However, the North Garage also offers one of the most shocking opportunities for epic RC fail:

See the ledge there? Yeah, the one with no railing at the ground?

 This is what's on the other side.

But luckily the RC system works reliably and the XBee radios get plenty of range with the nice antennas. Using the built-in data acquisition system, I was able to record a top speed of 35mph on the garage. I'd like to say that was limited by my fear of driving off the side, but based on the data I think that is very close to the top speed with this setup. (I would guess 37-40mph.) It's not power-limited, though, so I can tweak either the gear ratio or the motor timing to go even faster. As for the acceleration, it goes 0-30mph in 3 seconds:

The motor current is set conservatively to 20A now. (It's current/torque controlled, so it has a very nice TCS/launch control feel to it.) But for utter minimum 0-whatever time, I could definitely afford to up the current a bit. After 15 minutes of driving with the current settings, I would call the motor "warm" and the controller "not even a little warm." I am very pleased with all the components and how well they work together. Things aren't supposed to work this well.

(Edit: With the motor current set to 27A, it now does 0-30mph in 2.2 seconds...)

Lastly, some video! The garage video is a bit boring so I clipped in some messing around at street level as well. If it looks like is.


  1. Would love to see more - I just picked up one of these motors and am waiting for my speed controller to arrive.

    Are any parts of the video showing wide-open throttle? I'm curious if that motor winds up any faster than shown. Also, have you thought about giving it more current, or would it be pretty pointless with the rubber tires and pavement? How about carpet tracks with foam tires and traction compound?

  2. Check out the May 17 post, which has some wide-open throttle video. I now run at 30-40A peak currents and use field-weakening control to get up to just over 45,000RPM (~44mph) at 19.8V. The acceleration is about 0.75g, which leads me to believe that there is still a little more to be had from increasing the current further. The motor does not overheat yet.

  3. The car looks amazing, like a miniature version of something from the fast and the furious movie. I wonder just how expansive is your hobby :)