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Combat Robot Preliminary motor calcs

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mech king

New Member
Hi All,

I am a mechanical design engineer by profession but have recently taken an interest in combat robotics, where my electrical expertise is at a novice level at best.

I have started to do some sums based on using two "S28-400 Magmotors" to drive a 4wd robot (One motor per side of the robot). I am slightly confused about the current usage of the motor when stalled. The spec sheet says that this motor has a 26N.m stall torque, and that the stall current can be as much as 570 Amps, which is very high, so i will use a gear reduction of 12:1 to which will give me a maximum tractive force of 3.5 times the normal force of the robot. So hopefully the robot will wheel spin before ever reaching its stall torque and frying my electrics.

In short i just wanted to ask for some advice as to how i can limit this 570 stall current and how i can determine whether the robot will wheel spin before reaching this peak current.

I could fuse the circuit to ensure the speed controller (probably a Vantec RDFR) does not see current levels higher than its max stated 220 amps, but i also dont want to run the risk of blowing the fuse in combat and being a sitting duck for my opponent. |But i also want to have lots of torque so that i can push the other robots about.

I hope that all makes sense?

What would you suggest?

Cheers,

Mech King
 

dknguyen

Well-Known Member
Most Helpful Member
My understanding is you want no fuse-like failsafes during a combat robot competition because anything is better than your robot dying and being torn to pieces by the other robot (though you might be able to end the match early and save yourself some money). The way to limit stall current IS to gear it down so the loads applied never make the motor stall and to pilot the robot intelligently. I think one point you can start from is to select a gear and wheel size combination so that the robot produces a force double to it's own weight at the edge of the wheel. There are weight limits for the classes of combat robots so you know the maximum weight of the other robot and your robot. You just have to choose the tradeoff between excess torque and speed and hope that it will be enough to overcome the torque of the other robot, but still be fast enough to catch it.

As a rule of thumbs for DC motors:

-The point of maximum power output 50% of stall torque or 50% of no-load RPM (they are the same point on the efficiency line on the torque-RPM graph). You generally do not want to apply more torque than this so that the motor slows down more than this for any continuous operation.

-Maximum efficiency is 1/7th of stall-torque or 6/7ths of no-load RPM.
 
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mech king

New Member
Hello dknguyen,

Thanks for your reply, much appreciated.

I will look over my calculations again, bearing in mind your suggestions.

Cheers again.
 

Russ Hensel

New Member
dknguyen makes some good points, but I would say monitor the current with your microcontroller and if current is too high give it some strategy for resolving the problem, reversing direction, rocking, off-rest-on, grab the other robot and set both on fire.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
As you're planning to use a commercial speed controller, check with the manufacturer, presumably a controller suitable for those motors will be able to handle the problem, or current limit accordingly.
 

dknguyen

Well-Known Member
Most Helpful Member
dknguyen makes some good points, but I would say monitor the current with your microcontroller and if current is too high give it some strategy for resolving the problem, reversing direction, rocking, off-rest-on, grab the other robot and set both on fire.
The vast majority of combat robots aren't automated so it's up to the pilot. The only thing worse than 500lbs of kill-bot running around with a crappy AI is 500 lbs of kill-bot running around with a smart AI. :D
 
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Mr RB

Well-Known Member
Actually 500 lbs of kill-bot running around with an EVIL super-smart AI would be the worst of all...
"Exterminate!" ;)

On a more serious note, any good motor controller should have selectable current limiting built in, during stall and start conditions it should just limit the max current that will flow.
 

mech king

New Member
Good points, as far as i know, the vantec speed controller has no current limiting Built into it, but i wanted to use it because its very good for a 145kg bot with differential steering. I will email the manufacturer to confirm this though
 

dknguyen

Well-Known Member
Most Helpful Member
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mech king

New Member
Sorry, the RDFR47E has some current limiting feature, but its expensive - $800.

The Roboteq ESC seem alot more reasonable and appear more robust and user friendly. Going to scan over the manual and see what the crack is with these.

I think my ESC will end up being expensive, no matter which one i go for, because i live in England and we dont really have any good ESC over here for differential steering applications - well none that are simply plug and play type.
 
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mech king

New Member
Just returning to the ESC topic,

My design uses two Magmotors at the moment, but in the future i may use four motors, if i have enough weight allowance left. All of the ESC i have seen can accomodate 2 motors for tank style differential steeing, but can four be used also? Would you have any suggestions as to this arrangment? I gues i will obviously need more batteries or a higher rated Ahr battery?

Thanks
 
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dknguyen

Well-Known Member
Most Helpful Member
Just returning to the ESC topic,

My design uses two Magmotors at the moment, but in the future i may use four motors, if i have enough weight allowance left. All of the ESC i have seen can accomodate 2 motors for tank style differential steeing, but can four be used also? Would you have any suggestions as to this arrangment? I gues i will obviously need more batteries or a higher rated Ahr battery?

Thanks
I don't see real point...you might as well use two larger motors instead of 4 smaller motors (simpler, more reliable, morepower, less weight). The only real reason is if you wanted 4 driven wheels.

But in either case, it does not change the fact you are likely to get slaughtered if you try don't have differential steering. You need that maneuverability.
 
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mech king

New Member
I was only really thinking in terms of torque. I have 26Nm per magmotor at stall, using the best one they supply (S28-400). the two motors gives me 52Nm of torque (in theory), once geared down, i have 628Nm total torque (at stall with a 12:1 ratio). If i have four of these motors, i will have 1256Nm at stall.

I guess im being a bit greedy, but i want stuupid ammounts of torque and a relatively fast travle speed to maximise ramming impacts. If i use four motors (when i have the cash to by two more that is :)) then i could easilly put out the 628Nm of torque (similar to the 2 motor set up) but run the robot at a higher speed as well.

I guess this will demand more juice from the batteries and also require larger ones, thus increasing weight. But if its possible and then i might give it a shot. I dont want to meet a heavy/super heavyweight with more power than me if i cna help it.

I want to use the four motors in a differential steering set up, but dont know quite how to wire this up to a speed controller which would normally do 2 motors, maybe i would need two speed controllers, but i cant find alot of info about it.

On a final note i guess i would need two ESC in order to share out the current of the motors and avoid burn out?

Hope that all makes sense
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Use identical motors, and just parallel them - you obviously need a speedcontroller twice as powerful though.

I would suggest you check out the Robot Wars machines, and see which ones worked, and which ones didn't.


Spin to win! :D
 
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dknguyen

Well-Known Member
Most Helpful Member
THese are just brushed motors so you only need one ESC for all motors spinning in the same direction (ie. on ESC per side as long as it's big enough to handle the current of all motors on that side).
 
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