Experience With 6 Cim Drive train

[BJC]

Here is a tip for everyone in this thread who are popping their main breakers or looking to run 6 CIMs and gear fast:

1. Use lower CoF wheels.

2. Build lighter

Put down the Roughtop tread in favor of something like the VersaWheel DT or a Colson wheel. Consider constraining yourself to 100lbs instead of 120lbs. There is a reason that a team like 1625 could gear a single speed 12:64 (~17.3 fps free for those of you keeping track at home) and still T-bone like a monster this year. Lower CoF wheels and lighter weight mean pulling less current when accelerating and at stall.

Cheers, Bryan

[Oblarg]

Originally Posted by BJC

Lower CoF wheels and lighter weight mean pulling less current when accelerating and at stall.

This isn't strictly correct; a stalled motor will pull the same current regardless of the CoF of the wheels. Rather, lower CoF wheels make it harder to stall the motor in the first place.

[Thad House]

Originally Posted by BJC

Here is a tip for everyone in this thread who are popping their main breakers or looking to run 6 CIMs and gear fast:

1. Use lower CoF wheels.

2. Build lighter

Put down the Roughtop tread in favor of something like the VersaWheel DT or a Colson wheel. Consider constraining yourself to 100lbs instead of 120lbs. There is a reason that a team like 1625 could gear a single speed 12:64 (~17.3 fps free for those of you keeping track at home) and still T-bone like a monster this year. Lower CoF wheels and lighter weight mean pulling less current when accelerating and at stall.

Cheers, Bryan

We were 100 lbs and VersaWheel DT's at IRI, yet we still popped. If you are geared high enough to not be traction limited, Stall is Stall, so weight and CoF don't make much of a difference, other then change where traction limited is. We popped because we pushed, not during accelerating.

[BrennanB]

Originally Posted by Thad House

We were 100 lbs and VersaWheel DT's at IRI, yet we still popped. If you are geared high enough to not be traction limited, Stall is Stall, so weight and CoF don't make much of a difference, other then change where traction limited is. We popped because we pushed, not during accelerating.

I think the idea is it's harder to stall with lower CoF on the wheels. However if you do stall anyways, you have the same problem.

4476 ran a 6 CIM at a 13-14 ft/s theoretical. We never popped a breaker, and I suspect it was a combo of lower friction wheels, and we put a couple of breakers on the PD board for the drive CIMS at 30A instead of 40A. The idea being that the PD breaker would trip before the main breaker would.

I'm not really sure which made a difference as both were implemented prior to testing to make sure we never blew the main breaker, but it makes sense to me that both would have had a significant factor in preventing main breaker trips.

[BJC]

Originally Posted by Oblarg

This isn't strictly correct; a stalled motor will pull the same current regardless of the CoF of the wheels. Rather, lower CoF wheels make it harder to stall the motor in the first place.

Well yes. A stalled motor is a stalled motor. I was referring to stalling your drivetrain. I don't claim to be any sort of expert in this area and the below is oversimplified but the frictional force between the wheels and the ground = CoF x mass x gravity. If this force is greater than the force exerted by your motor on the wheel [torque at wheel / radius of wheel = force exerted] then the wheel will not turn and your drive motors will stall.

When I recommend decreasing wheel CoF and weight it is because this combination is decreasing the normal force on the robot's wheels. When you decrease the force resisting the wheel rotation such that it is less than the force attempting to turn the wheel at drivetrain stall then you are what people refer to as traction limited, meaning the drivetrain's wheels will lose traction and spin in place rather than stopping while the motors continue to try to turn.

Originally Posted by Thad House

We were 100 lbs and VersaWheel DT's at IRI, yet we still popped. If you are geared high enough to not be traction limited, Stall is Stall, so weight and CoF don't make much of a difference, other then change where traction limited is. We popped because we pushed, not during accelerating.

You are exactly correct. The tips I shared are not magical, they are only directionally correct. There are other factors to consider which will effect these equations: efficiency, gear ratio, bumpers, etc. For example, when pushing another robot sometimes bumpers deform such that their weight is shifted onto your robot increasing your robot's effective mass. Sometimes the robot rocks back onto two wheels splitting the weight of the robot over only 2 wheels rather than 4. These types of things can be difficult to anticipate. I don't know your drivetrain situation (or if you are normally traction limited) so I can't comment on what might have contributed to your breaker tripping.

I do want to clarify something though. Popping the main breaker is temperature based and the cumulative result of robot actions over the course of the match. Quickly accelerating from standstill and forward / reverse slams are normal robot actions that pull large amounts of current and cumulatively heat the main breaker. This is where lower CoF wheels can provide small cumulative temperature gains (the wheel CoF determines how easily the wheel spins in place during take-off acting as a mechanical dampener mitigating the otherwise extremely high current spikes.) Using something like Omni wheels to minimize frictional losses when turning can also make a big difference. An inefficient collector or generous use of motors for auxiliary non-drivetrain actions can also be a huge factor that is often overlooked. Of course, stalling 6 CIMs will heat the breakers faster than any other individual action, and is generally the last thing that was happening before the breaker trips, and so is most frequently looked at. Pushing can often be the last large rock on the camel's back, but you can't forget all the straw you heaped up their earlier.

Hopefully this clarifies some of my thoughts on the subject.
Cheers, Bryan