Sumo Bots C

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paleonaps
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Re: Sumo Bots C

Post by paleonaps » October 11th, 2010, 6:21 pm

Wait... I thought it was 2.0 kilograms....
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Re: Sumo Bots C

Post by ichaelm » October 11th, 2010, 6:36 pm

paleonaps wrote:Wait... I thought it was 2.0 kilograms....
You're right. Sorry, that was from the power point. The actual maximum mass this year is 2.0 kg.

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Re: Sumo Bots C

Post by paleonaps » October 16th, 2010, 2:02 pm

I was hoping so, because I don't want to be over the limit, and last years bot is in this years specs.
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Re: Sumo Bots C

Post by old » October 16th, 2010, 5:29 pm

sj wrote:I was wondering what kind of speed controllers i would need if i was using 2 12v gear-motors with a nominal current of 1.5A and a stall current of 148A each. What kind of specs should they have? Right now we are trying 2 12A continuous 45A peak Controllers but that is probably not nearly good enough. any information would be much appreciated
In theory you need a motor controller that can handle the maximum power that the motors can take. However most motor controllers have continuous and surge power ratings, so you don't need a controller that can take 296 amps continuous. The other thing to take into account is the fact that the maximum motor current will decrease as the voltage decreases, so as the voltage drops due to internal resistance of the battery (as well as resistance in the motor controller, wiring, connectors etc., the maximum current of the motor will drop as well. Even assuming that you have a battery that is actually rated to handle 300 amps surge, the voltage of that battery at 300 amps is going to be substantially lower than the rated battery voltage. So for example, if you have a 60C surge 5 amp hour 3 cell series LiPo as your power source (nominal 11.2vdc), it will not be damaged by a 300 amp surge (from 2 stalled motors) but the voltage of the battery will be much less than 11.1 volt when outputting 300 amps. My point is that even with a stalled motor you won't actually see 148 amps current per motor when connected to a reasonable sized battery (reasonable meaning something that can fit in a Sumo Bot).

Your best bet for a motor controller that can handle the stall current of these motors would be an RC car speed controller. I have seen brushed electric motor controllers that are in the several hundred amp range that do not cost a fortune, but brush type motor controllers are becoming much less common as most people are switching to brushless DC motors. Look on line, on Ebay, etc. for RC electric motor controllers. Just make sure that the one you get is rated for a high enough voltage because many RC cars only run on 7.2 or 9.6 or some other relatively low voltage.

Also make sure that your battery pack can handle the full rated current of the motors. A typical recent vintage LiPo can handle anywhere from 10C to as high as 60C surge current. This would require a battery of anywhere from 5 AH (for the 60C pack) to as big as 30AH for a 10C battery. A 5AH battery would be reasonably small and inexpensive but a 30AH battery would cost many hundreds of dollars (maybe as high as a thousand) and would weigh more than 2KG, so that is obviously impractical.

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Re: Sumo Bots C

Post by rocketchicka » October 20th, 2010, 7:42 am

I'm really looking forward to this event. It should be interesting though because I havent done anything with robotics since 7th grade and robot billards and all that. And I agree that a wedge is the way to go.
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Re: Sumo Bots C

Post by tjessberger » October 25th, 2010, 10:15 am

Hey guys,

I have a quick question. I'm somewhat confused about the voltage, and the whole voltage (14.4V)/cell combination. Would a battery such as this http://www.amazon.com/6-Cell-7-2V-3300m ... 816&sr=8-1 be legal?

Thanks,
Tim

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Re: Sumo Bots C

Post by ichaelm » October 25th, 2010, 10:35 am

Every NiMH cell is 1.2 volts nominal. The NiMH battery you are looking at has 6 cells, so it is 7.2 volts. That would be legal by itself. If you had 2 of those batteries in series, you would have 12 cells, which would be 14.4 volts. That would also be legal.

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Re: Sumo Bots C

Post by Flavorflav » October 25th, 2010, 5:44 pm

I believe it says a combined max of 14.4 volts, so it wouldn't matter if they were in series or parallel.

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Re: Sumo Bots C

Post by riley404 » October 26th, 2010, 8:22 pm

Do speed controllers with two motor channels output the same voltage to each channel as the input? For example, if my input voltage is 7.2 volts, does each motor receive 7.2 volts?
Also, do you favor 2 wheel drive or 4 wheel drive?

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Re: Sumo Bots C

Post by old » October 26th, 2010, 10:47 pm

I think I understand what the maximum voltage of the batteries means, except how they are going to determine it. The nominal voltage of a NiMH battery is 1.2 volts per cell but the actually fully charged voltage is significantly higher than that (1.4 to 1.5 volts per cell). THe same phenomenon applies to all types of batteries, LiPo have nominal voltages of 3.6 or 3.7 volts per cell (seems to depend on the exact battery type of manufacturer) but the actual charged voltage can be as high as 4.2 volts per cell. So if an event coordinator decides to use the actual measured voltage of the fully charged battery many teams could be second tiered for excessive battery voltage. In past years the rules had specified that the voltage would be the labeled voltage of the battery, but this year there is no such language in the rules so how are we to know what the actual maximum voltage is? If you use 12 NiMH cells you would get a nominal 14.4volt battery pack, the exact maximum allowed voltage, but if you were to actually measure the voltage of that battery pack when freshly charged and under no load it might read as high as 16 -17 volts. A Lithium ion battery with a nominal 14.4 volts (4 cells of 3.6 volts each) could similarly read 16 - 17 volts when fully charged and unloaded. You would have to drop at least one lithium cell or as many as 3 NiMH or NiCD cells from the nominal in order to guarantee that you would pass a volt meter test.

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