Sunday, May 24, 2015

Mango Scooter

Ah, the boringness of working at minimum wage at a convenience store while, at the same time, wrapping up my job as a FIRST Robotics team manager! The nice part about it is all the money it brings!

Now, since I have absolutely no self control when it comes to buying making related stuff, especially when stuck with a boring job for a while, I ordered a bunch of stuff off Hobby King. After lurking Charles Guan's blog for years, I knew I wanted to make a silly electric powered vehicle. A kick-scooter looked like the safest (yeah, right) and cheapest way to make an exploratory project about electric vehicles, that is, a project where I start with the main part (in this case, the motor and speed controller) and make it run, instead of starting, as every engineering projects should: with a problem description and a requirement table.

For this purpose, I chose the following ICBM: Turnigy Aerodrive SK-3 6354; which I believe, if I understand right, is a close relative of Charles' Melon Motor. Since it's only a relative, and it's name clearly also has to start with M, I will now christen this motor; the Mango Motor, and as such, Mango Scooter.

Kind of looks like a Mango, right?

This motor has the following specs:

  • kv: 260 RIP'EM (RPM) per volt
  • Max Voltage: 37 Volts
  • Max Power: 2360 W (About 3.16 HP) I doubt it can actually output that much mechanical power, I don't doubt it can eat that electrical power though
  • Max Loading: 70 Amp
  • Internal Resistance: Supposedly 0.023 ohms, I kind of doubt it.

To go with it, I got a "100A" Red brick ESC, why 100A while I only got a 70 amp motor you say? Because:
  1. Safety Factor
  2. It's Cheap Chinese Stuff so it's probably gonna explode as soon as I put current through it.
  3. It's entirely made of MOSFETs (Whatever those are) And (almost) no heat dissipation.
Literally a bick of MOSFETs

...Thinking about it, I should probably have went for 150 or something.

Let's see how fast that could go...suppose a 10cm diameter wheel and direct drive. 10 cm is nice and round.

At 35 Volt, because a 37 volt battery would be at 37 volt only for the first few minutes anyway, the motor spins at 9100 RPM. Thanks to the 10 cm diameter wheel, that's 285 740 cm/min or 171.44 km/hour.

Hey, that's nice! I don't need to go that fast though!...and the motor can't even start if I leave it like that anyway. The red brick controller is a sensor-less controller, as such, it can't know where the motor poles actually are, and which poles needs to be switched. What it means is that the motor just won't work if the inertia it sees is too big, or if the motor is going really slowly. In both cases, we need reduction, so that high speed is actually good.

Here's some interesting reduction cases:

  • a 9x reduction in 2 3x stages reduction gives a nice 19km/h
  • a 12x reduction with a 3x and a 4x stage gives 14.25 km/h
  • 16x reduction with 2 4x stages is 11 km/h
Now, the question is, which is the lowest reduction I can get that will still let the motor start. I will need to test in order to answer this...which will require me to get a bigger iron, because my little 50 watt sparkfun hakko-copy iron just doesn't have the thermal weight to solder those beefy connectors.

Now, obviously I didn't just buy a mango off Hobby king. I always wanted to make a quadcopter, and try my hand at writing the control code for it myself, so, I bought 5 itty bitty motors too! (along with their ESC, obviously)

Here's one of them next to their big brother Mango:

Thinkpad for scale?

So that will be fun. Next week, I will be introducing one more project i've been working on.

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