1kg build suggestions

BiSHGoD · Post by **BiSHGoD** » Sat Mar 13, 2010 5:18 am

Hello.
I'm building a 1kg bot (closer to a beetlebot I guess but I thought it the most applicable category) as my first bot. It isn't going to be for combat, more just for fun as a test (possibly wireless surveillance or something fun). I'll do another later for combat if this goes well.

My goal is to keep it anywhere from 500-1000g (1-2lbs), speed is also important, 7mph hopefully (I might be reaching for that goal).

This is my build list:
2 2.5" Lite Flites
2 Pololu Micro HP 50:1 motors with hubs http://www.pololu.com/catalog/product/998
910mAH 11.1v 3s LiPoly http://www.robotmarketplace.com/product ... SJPL2.html
Sabertooth 5 RC ESC
Futaba receiver

Everything combined without chassis is 4.84oz (137g). Also $250 including transmitter and charger.

Using this website for my calcs: http://architeuthis-dux.org/torquecalc.asp.
Running at 11v and 1kg I should get 8.7mph theoretically. It says the motors will stall at 3amps, and it'll only need 2 amps to spin the wheels. Peak is 4amps.

My questions:

I have heard people running these motors higher than the 6v recommended, does it seem reasonable to do so?

Is the Sabertooth 5 the best for the job? It can do 5 amps with a peak of 10, so should be good there. Will anything cheaper/lighter work for running 2 motors?

The batteries is my biggest question. I have been looking for the best for this situation. If this runs at 4 amps and I have even 800mah that only gives me 12 minutes of runtime (I think), I would ideally like about an hour since this isn't for short bursts like combat bots. Since the low side is 2amps maybe it'll be closer to 30min.
Or is this overkill? It's half of my weight at 2.43oz.

Thanks!

alasdair · Post by **alasdair** » Sat Mar 13, 2010 10:53 am

I dont know much about kilobots, but you could ask jack Orr, whos building one at the minute.

Post by **BeligerAnt** » Sat Mar 13, 2010 12:49 pm

Motors:
I think the motors are probably a bit on the small size for a 1kg robot. I know Peter Waller has successfully used them for 500g mini-sumo robots, but I think a 1kg load in combination with 2.5" wheels and an 11.1V supply may be too much for them.

Although the calculator you used gives a stall current of 2.96A at 11.1V, I don't think the motors can necessarily take that current in real life!

The calculator is good as far as it goes, but there are clearly some limits missing!

Just playing around with it, it tells me that I can use 2 Pololu Micro HP 50:1 motors with 2.5" wheels at 24V to run a 6lb robot at 18mph! The chances of these motors really being able to handle 5.75A are nil !

To answer your original question: We regularly run these motors at 7.4V, and I think some people have run them at 11.1V. The big difference is that we use 0.5" to 2" wheels (usually smaller wheels at higher voltages) and only have a load of 150g.

Speed Controller:
The Sabertooth 5A controller is fine for these motors, several people use them. If your normal running current is below 5A you should be OK. The Sabertooth should be able to handle the higher current when starting off too. However, I think you need to look at different motors, which may in turn lead you to a different speed controller.

Batteries:
If the calculator works the way I suspect, then you need about 20 Watts (2A @ 11.1V) to drive your 1kg bot at 9mph. So regardless of motor choice, you will need about the same current. Your current battery is in fact only about 15% of your your total robot weight. If you want to run for an hour you will need a bigger battery!

Hope this helps, and good luck with the project.
Welcome to the forum, but for a 1kg robot you may be better off asking on the FRA forum or on one of the many autonomous robot forums as you are likely to find more people with experience of building at this weight.

BiSHGoD · Post by **BiSHGoD** » Sat Mar 13, 2010 5:29 pm

Thanks very much. Exactly the info I wanted, real-world experience that you can't easily get from the numbers.

Do you have any motor suggestions? If I don't get the Pololu's I'll probably go with the B16's which are slower, more torque, twice the price, and 7 times the weight (2.3oz each vs 0.34oz).

Team Orr · Post by **Team Orr** » Sat Mar 13, 2010 10:51 pm

alasdair wrote:I dont know much about kilobots, but you could ask jack Orr, whos building one at the minute.

actualy im building 3 lol

gary basicly summed it all up, you can use those motors on 7.4v (i do everyday

) and on 11.1v, but im pretty sure they cant shift 1kg at any great speed, try using 4 with big grippy tires if you can fit them in

alternativly robot market place sell a 4wd kit with everything you need (chassis, motors, batterys, rc gear, speed controllers and charging equitment!)

jack

peterwaller · Post by **peterwaller** » Sun Mar 14, 2010 9:50 am

As Gary says I have built one 500gm minisumo with these small motors but they were geared down more and the result was still underpowered and very slow. My other two minisumo's use the Copal 60:1 motors and they are much more powerful and quicker.
http://www.robotcombat.com/products/0-COPAL60.html