Rules limited electric kart

[Physics Guy]

For the Power Racing series I am working with a high school robotics team to build a 1440W (fuse limited) go kart based, very loosely, on a Power Wheels body. I have a fair amount of experience building FIRST robots (100+ lbs) but they tend to run flat out for 3 min. This kart will need to at least go around 1.5 hours without burning up the motor. Probably going to have under 150kg combined kart, batteries, and rider (high school students afterall). BTW, there is a looks/style portion also so we are going with 8/9" wheels.

The dull option is to buy a big Unite, MY1020, or Currie motor (something sold for 1000 or 1500W @ 36V) and chain drive the rear axle. However the tinkerer in me wants to go with two motors so that we can use a microprocessor to slow down the inner wheel in turns. A real differential is out of our price range.

So the question is how do the other options compare.

1. Two brushed motors (650/750/800/1000W at either 24V or 36V) with chain drive to each rear wheel.
2. Two brushless 600W BMC motors (built in controllers) again with chain drive to the rear wheels http://www.thesuperkids.com/600wasutocum.html. Since these have a built in controller we'd use 24V. The other options are all 36V.
3. Two direct drive 800W brushless hub motors meant for electric scooters. http://www.aliexpress.com/store/product/9-800W-36V-electric-wheel-hub-motor-electric-bicycle-motor-electric-skateboard-motor/1279208_32438320428.html I'm worried these will not have the torque needed for acceleration. There is no real tech data available on this and I'm not sure how to translate the rpm 400-1500 (on other Sarach hub motors it says it can be set between 400-1500rpms but that's not that much clearer).

 

[dcastillo]

micro processor to slow the inner tire... that sounds pretty intense and complicated...
But hey, if you got the time, knowledge will power, go for it.

I was planning a dual motor on my next build, but connecting them in parrallel with one controller... My thought/hope is, as the outside increases in speed, its current draw would theoretically drop to 0 (possible negative, not sure how to deal with that yet, Diode?) and all the power would transfer to the inner tire. My other hope is that when offroading, if a tire looses traction and free spins, it would immediate drop the current to 0 as well, giving all available power to the tire with traction...

Another option is to wire them in series, meaning the power/current to each is always the same, but as the outside tire increases in speed, its voltage would increase (voltage=speed). but not sure how well that would work off road.

hopefully I will have this tested out in the next 2 months or so.

 

[Physics Guy]

Interesting. I've never really thought about using one bigger controller.

 

[Physics Guy]

micro processor to slow the inner tire... that sounds pretty intense and complicated...
But hey, if you got the time, knowledge will power, go for it.

Oddly enough it seems simple to me as its just taking the throttle and the angle of the steering as inputs and sending different throttle outputs to the two sides. It would take a while to optimize but its not that different from some programing we do on FIRST robots to make sure they move straight in autonomous mode.

 

[itsid]

However the tinkerer in me wants to go with two motors so that we can use a microprocessor to slow down the inner wheel in turns. A real differential is out of our price range.

Sorry, but this is ... uhm... STRANGE to say the least.

a mechanical differential can be had for as little as 25 bucks used (CL , ebay etc...)
a peerless 100 can be found on several small buggies and karts.
that can be had for maybe 100 bucks.

Anyways, a microcontroller is still cheaper (considering all you'd need is an arduino nano for the task and a few dodats)

But still, the amount of code you'll need to achieve the very same thing should be measured in weeks, even MONTHS not days and thus even if you only consider a symbolic 1buck/hr or even per day... you'll quickly
spend more programming the electronic differential than you would paying for a used.. maybe even new peerless 100
(to be had new for as little as $110 or so)

So yeah... I don't get that tbh.

especially if you consider the fact, that in order to dynamically control two motor speeds you'll certainly need two controllers
(in which case you'll at least spend ~20 bucks for the second controller)

true the single controller will need to be bigger and thus more expensive...
but still I don't see much benefit.

'sid

 

[Physics Guy]

I suspect it is my robotics experience. The rules for every robotics competition I know of the regulates such things says one motor per controller (and the reverse). I had never thought of running two motors in parallel. Series is also a no no in the robotics competition world but I had thought of it. However it seems to conflict with running a 36v battery pack; I don't want, and the racing rules will not allow, a 72v pack.

The code seems easy to me. Finding cheap used stuff seems hard to me. CL and ebay haven't been returning a lot of hits.