Tuesday, July 17, 2007

How-To: Nitro-to-Brushless Conversion: Part 4

Part 3 can be found here.

After a couple-month-long hiatus, during which I pursued some of my other hobbies, I recently returned to working on the brushless conversion of my Losi 8IGHT 1/8th-scale buggy. And to posting about it on Industrial-Strength Science. This part of my how-to concerns selection of the electronics and associated parts for the vehicle.

The motive force for the car, and indeed the centerpiece of the conversion, will be a NeuMotors 1512/2.5D. All Neu's 1512 motors are rated at 1200 W of continuous power and 2500 W for 30 seconds. This particular motor has a Kv rating of 2000 RPM/V, which means that, with the nominal 14.8-V potential difference supplied by my batteries, it should spin up to 29,600 RPM. That should be fine, because the 1512s aren't meant to spin above 60,000 RPM. The manufacture claims that this motor is "86+" percent efficient. I'm not sure how they measure that---it seems to me the efficiency will vary strongly with voltage/current/speed---but the point is this motor is much more efficient than any brushed motor. Here's a photo of the Neu:

As I think I said before, a brushless motor running off a lithium-polymer battery is truly 2 great tastes that taste great together. The high efficiency of the BL motor combines with the high current capacity and low load-induced voltage drop of the LiPos to deliver lots of power to the wheels, propeller, or what have you. For now, I'll be running a MaxAmps pack consisting of 4 LiPo cells in series. Each cell has a nominal voltage of 3.7 V and a capacity of 5000 mA-hr, so this so-called 4s pack has nominal voltage of 14.8 V and a capacity of 5000 mA-hr; the capacities do not add. This pack is claimed to be capable of putting out constant currents of 20 C---that's 100 amps-for a 5-A-hr battery---and burst currents of 50 C---250 amps for 5-A-hr. The downside to this pack is that, even though LiPo cells have much higher specific capacity than nickel-cadmium or nickel-metal-hydride cells, this battery is just plane huge. You can't determine the size from this photo, but it will be apparent when I show it in place in the vehicle:

Because brushless motors have electronic, rather than mechanical comutation, a brushless-specific electronic speed control is required. I've chosen a Quark 125-B Monster Pro, which is claimed to be capable of sustained currents of 125 A and 10-second bursts of 180 A. Here's a pic:

The Quark ESC has a built-in "battery-eliminator circuit," which is simply a voltage regulator that allows you to pick off 5 or 6 V from the supplied higher voltage to run the receiver and servo, rather than including a small "receiver pack" battery to provide that lower voltage. This BEC is resistive and thus inefficient. In order to reduce power consumption, I'll be using a transitor-base switching regulator, which is more efficient. Here's a shot of this external BEC:

To disable the ESC's BEC, I simply cut the positive power lead coming off of it; since that BEC is resistive, I believe that mod should prevent it from sucking up current. I should point out that Nick taught me that resistive power supplies are less noisy than switching types, but, since I'm not trying to build a fiber laser for optical metrology---at least not today---I'm more worried about efficiency than noise.

The only part I bought that was specificly---more or less---made for this project is a motor/differential mount I purchased from Mike Cronin at RC Monster. This mount replaces one of the diff mounts and allows the motor and its pinion gear to mesh with the spur gear on the diff. I said "more-or-less" because the part was actually made for converting the Losi 8IGHT-T, the truggy1 version of my buggy. Since the buggy body is "skin-tight," while the truggy's encloses a lot of unused volume, I found that the mount rubbed on my beautifully painted body. So, I had to grind off the upper outboard edge of the mount. Here's a photo of the unmodified motor/diff mount; the modified version will be visible on the completed car:

To hold that ginormous battery in place, I also purchased a machined plastic battery tray from RC Monster. I modified this part as well, which I'll show you later.

I also custom made a mount for the ESC that places the speed control near the centerline of the vehicle. I don't have any photos of that mount, although I'm Brian with the overhead projector about it.

I mentioned before that I'll be using a Nomadio Sensor radio, which has several nice features, including telemetry and noise insensitivity. Here's a photo of the transmitter:

Part 5 can be found here.

1 A truggy, at least in the world of RC, is a "truck" that is really a modified buggy. Typically, a truggy has these features:
  • The chassis is a slightly lengthened version of a buggy's pan chassis.
  • The suspension ams linking the wheel to the chassis are lengthened.
  • The wheels arnd tires are truck components.
  • The body is similar to the stylized "stadium truck" body found at the 1/10th scale.

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