My team and I pretty much already decided on what kind of robot we’ll be working next year. A robot with a 12 motor drive, with 8 4" wheels, probably 6 colsons and 2 omnis, using 12 mini-CIMs (3 per gearbox per 2 wheels) … This leads us to a few questions.
Is there any record of a FRC robot using 12 motors or more in a competition??
What is the most effective gearbox we can buy off the shelf? We want a very high speed since we’ll already have high torque. Would it just be better to design our own gearbox for this?
Keep in mind you will reach an upper limit in performance not because of motors but you have to get all the power from the same battery. So at some point the collective losses in each of the motor motors will counter any benefit for adding more.
I understand that, having so much motors being used non-stop for 2min30 can be pretty demanding, but this aspect would be the main focus on this robot, but this is actually a concern, would you have any suggestions? Like having a “mode” at half speed and thus half power and half torque? It would still result into the use of 6 full motors wich is not bad at all concerning torque… Also, if that robot is built, would it be the first FRC robot with a 12 motor drive? I didn’t find anything on a 12 motor drive FRC robot yet.
My question, and it sounds like I am being a parent, is what exactly is your main goal. Is it speed, pushing torque or both. Sounds like you want a very fast very strong robot. You don’t see 12 motor drives because there’s is no advantage. With the loses you may actually be disadvantaged. The only reason to have a 12 motor drive is to say you have a 12 motor drive.
Aside from torque and theoretical performance. Assume each motor is 90% efficient. So 10% of the energy used goes to heat. So the more motors you add the mores collective waste you have. This would not be an issue if our power source was unlimited but again you have a limited amount of power. The more motors you add the more of that limited power is wasted and the less power remains to get the full value out of your 12 motor drive. You better off running less motors to their limit and shifting based on if you want speed or torque.
Remember that if you reduce voltage input by 1/2 then both your torque and RPM reduce by 1/2. This means that your 12 motors run at 50% speed will have the same power as 3 motors running at 100%, so basically your 12 minicim drive has the same power output as 2 full CIMs.
If Talon SRX’s are used in Speed PID mode, then this isn’t quite true. Acceleration can still be pretty aggressive, and in our tests pushing wasn’t diminished by much.
With that said, we’re running voltage mode on ours this year (6 CIM). Acceleration in low gear is pretty snappy. Our ‘snail’ mode cuts all outputs to 70% of max, and the drive train is still very responsive.
Nothing like over $1000 cost in Talons, and 26 pounds of motor weight just so you can trip your main breaker faster than any other competitors robots.
At 250 amps, that only about 20 amps maximum per motor (ignoring losses).
At 12 Volts your performance will be limited to.
Torque Load = (Current-Free Current) * Stall Torque/(Stall Current-Free Current) = (20A-3A)*(1.41NM)/(89-3)=0.279 NM
Thus your limited to less than 20% of the Mini-Cim stall torque capability
I would like to point out that, on a field our size with 6 other robots and unknown field obstacles, there’s a point where a robot can become difficult to drive. Too much acceleration, too high of a top speed, and you won’t be able to control it well enough to be effective. That’s why many of the teams that currently do 6-CIM drives do it with shifting gearboxes. Shifting into low speed limits their top speed and acceleration to something manageable so they can actually play the game, while still getting top speed when they need to.
You would be better off looking at those shifting gearboxes than going after more motors. You’ll see better top speed, much more manageable low speed, and likely more torque at low speed.
The goal was indeed to have both high speed and a high torque. Prices aren’t a concern either. I understand that this idea is difficultly viable, but i don’t think that it should just be a direct no. For more info, we would plan to use de 40 amps plugs for 2 motors on each gearbox and 30 amps for the rest. So 8 motors at 40 amps and 4 motors on 30 amps. Actually, we didn’t think this through but I guess this is just a no-brainer. Also, I heard that CIM motors only have their full power at 60 amps and mini-CIMs at 40 amps, making them equal in torque, but not speed at 40 amps. Is this true? 775s aren’t an option for us either.
Is the only issue that too much motors drain too much power? We will be using shifting gearboxes, but in our case, should we design our own or does someone know about a gearbox that would fit us?
Call me unimaginative, but I can’t even remotely fathom a way in which a 12-motor drive is a good solution for an FRC game, unless the typical constraints completely change next year.
I hate to be discouraging but honestly, if you don’t go into the project already knowing how to pick gearboxes and read motor curves, your 12 miniCIM robot will like be a very heavy, very expensive paperweight. Even if you already had plenty of experience designing custom drivetrains and gearboxes, I would expect a project like this to take years in the offseason before it would even be close to field ready. Even then, I’m not sure that it would ever work, or would be found to be better than existing options. I’m not saying that you should just give up and use the KOP chassis, but maybe you should take time to learn about how regular drive trains work and how to prefect a more conservative custom drivetrain before you go for this “innovative” option. I am definitely suggesting that you develop this first in the offseason before you bet your whole season on it working.
I would take the time to answer your question and explain where to find and how to read motor curves, but I’m on mobile now. I’m sure there are other people on here who will be more than happy to explain, or a search for “motor curves” will probably return some helpful threads.
As many issues as 12 motors can cause, there’s your biggest mistake of all, unless you somehow got a sneak peek at the 2018 game. Imagine showing up with that drive train for Recycle Rush or Lunacy.
Your time and money is better spent understanding the physics behind FRC drive trains and DC motors before committing to a 12 motor drive train. The good news is you have 9 months before 2018 kickoff to do the math and understand the realities behind the idea you are pursuing.