I might be missing something with a 6 cim drive...
 

So I was thinking about main breaker trips with 6 cim drive trains, and I was thinking about how to avoid that.
But because torque is directly proportional to current, and the motors are all cims, for a given current (say, 120a) wouldn't the toruqe output be the same regardless of the number of cims?

Basically, if the amount of current was limited per cim, through the 40a breakers only, then 6 cim drive would have more max power. But if the power of the system is limited to 120a total, then how is it advantageous to use 6 cims instead of 4?

Right now I think it's because the breakers need a few seconds to trip and efficiency gains, but that seems like a small gain to me for the extra space and weight.

Re: I might be missing something with a 6 cim drive...
 

Why do you believe that the system is limited to 120A? The main breaker is guaranteed NOT to trip until the current is ABOVE 120A. Even at 240A that main breaker will not trip in under 45 seconds.

Also, a 6 CIM drivetrain pushing the same as a 4 CIM drivetrain will draw less current per CIM, thereby possibly preventing the 40A snap action breakers from tripping.

The real difference between 6 CIM and 4 CIM drivetrains is acceleration. Assuming both drivetrains are geared for the same max speed, the 6 CIM drivetrain will accelerate faster. If the both drivetrains are stalled, the 6 CIM drivetrain will pop the main breaker faster as well. Such is tradeoffs in engineering.

Re: I might be missing something with a 6 cim drive...
 

The 120a Breaker is capable of instantaneous currents of over 720 amps before tripping (see attachment), and 240 amps for a considerable amount of time (for a FRC match). While not breaking the easier-to-trip 40 amp fuses, you could run much higher currents than a 4 CIM drive, which would be limited to 160 amps, for some seconds.

Re: I might be missing something with a 6 cim drive...
 

What spec are you looking at ?

Re: I might be missing something with a 6 cim drive...
 

This is from a large PDF from the manufacturer. I clipped this image out a while ago for the breaker we use. I will try to find the source


EDIT: Page 34 of this pdf

Re: I might be missing something with a 6 cim drive...
 

That was 'off the top of my head'.
It's corrected now :eek:

Re: I might be missing something with a 6 cim drive...
 

Ah, okay, that makes sense. I thought the breaker would trip faster than that, but great! We're going to be designing for a 6 cim drive then.

Re: I might be missing something with a 6 cim drive...
 

Just a word of caution: If you stall a CIM it pulls ~133A. If you stall a 6 CIM drive, it'll pull somewhere between 400-500% of the rated current for the main breaker (current being limited to the batteries internal resistance, I don't have that number in front of me right now). That will trip the main breaker in 1-6 seconds (as per the chart above). The 40A snap action breakers will be at ~ 250% of rated current and may take longer than the main breaker to pop (again I don't have the spec sheet in front of me) and that's not accounting for any other electrical load you have at that moment.

Basically, ensure your 6 CIM drivetrain is never in stall (make sure it's traction limited, not torque limited).

Re: I might be missing something with a 6 cim drive...
 

If each CIM were drawing 133A, a 6 CIM drive would pull 800 amps.
In addition to frying all 6 motor controllers, the plastic PDB would be molten as well. Your wires would be scalding to the touch, and there would be a large quantity of gaseous substance emitting from the robot. Basically, yes it is a bad thing, but no, it will practically never happen. This is really not something to worry about.

We ran a 6 CIM VexPro ballshifter running 3" wheels, and geared to 5.5fps and 15.5fps. In addition to bulldozing everyone we faced, we never blew a breaker once. That being said, we did have a "ramp up function" in the code. Basically, if you jammed the joystick into full forward, the code would automatically and linearly increase the PWM "throttle" very quickly (read: like 1/2 second) from 0 to 100%. Even when throwing the robot from full forward into full reverse, we never had a breaker-blowing situation. The acceleration time is negligible compared to instant-100% because the ramp up is so quick and you have the comfort of knowing whatever you could possibly do to that joystick will not disable the drivetrain. The ramp up has the advantage of suppressing the instantaneous current draw into a slightly longer duration but lower peak draw.

Re: I might be missing something with a 6 cim drive...
 

When testing this year, we found that a new breaker can take 6 stalled CIMs and a compressor (~250 amps) for about 25 seconds before tripping. Note that this is to a large extent cumulative over the course of a match, because these breakers are thermal and they don't dump heat instantly (or even particularly quickly); that is, if you stall for 20 seconds early on, you're going to be in real trouble if you get into another pushing match 30 seconds later.

Re: I might be missing something with a 6 cim drive...
 

Actually, I heard 254 stores covers breaker in ice packs or something right before the match. Not sure how true that is.

Of course if we go 6 cim we would have a shfting gearbox. That way we would have pushing power as well.

Re: I might be missing something with a 6 cim drive...
 

Quick question, how would you stall a 6 cim drivetrain in normal usage? The wheels would lift at some point, even driving into a wall, right?
Of course, we would still build in current limitations and preferably shift.

Re: I might be missing something with a 6 cim drive...
 

Get into a pushing match with a robot, if the traction of your wheels exceeds the torque created by your robot then the motors will be in stall. (Assuming all wheels are on the ground)

Even with 2 or 4 wheels off the ground, it's possible for your wheel traction to still be greater than the torque of a 6CIM drivebase (However, unlikely)

Keep in mind that in a 6wd configuration, there are only 4 wheels in contact with the ground at any given time. It remains the same for 8wd, with only 4 wheels ever actively touching the ground (assuming center two wheels are dropped)

Re: I might be missing something with a 6 cim drive...
 

Fun fact: 6 CIMs at stall consume a total of roughly 9576 watts of electrical power, which is over 9000!


Seriously though, a robot with 6 CIMs under "normal load" output (excluding friction) a total of 2.6 Horsepower, while a 4 Cim drive does just 1.75. At exactly 40 Amps per motor the numbers are (theoretically) 3.86hp and 2.57hp, respectively.

And as everybody knows, power/weight ratio is the most important figure. :rolleyes:

Re: I might be missing something with a 6 cim drive...
 

That's why we ran 6 CIMs and 2 MiniCIMs this year. More power is always better. :)