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

[asid61]

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.

[asid61]

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.

[Dunngeon]

Originally Posted by asid61

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.

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)

[BBray_T1296]

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.

[AllenGregoryIV]

Originally Posted by BBray_T1296

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.

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

[asid61]

So have people stalled a 6 cim drive before then?

[Oblarg]

Originally Posted by asid61

So have people stalled a 6 cim drive before then?

Yes.

With a 125lb robot (including battery and bumpers) geared 6.1:1 with 4'' HiGrip wheels, we stalled 6CIMs when simultaneously running the compressor on a not-quite-full battery, resulting in a current draw of ~250 amps.

With the compressor off and the battery completely full, the same robot spun the wheels and pulled ~180 amps.

[BBray_T1296]

Originally Posted by Oblarg

Yes.

With a 125lb robot (including battery and bumpers) geared 6.1:1 with 4'' HiGrip wheels, we stalled 6CIMs when simultaneously running the compressor on a not-quite-full battery, resulting in a current draw of ~250 amps.

With the compressor off and the battery completely full, the same robot spun the wheels and pulled ~180 amps.

But CIMs stall at 133 amps apiece...? (theoretically as always) their manufacturing specs say so. that would be 800 amps, aka very bad...

[Oblarg]

Originally Posted by BBray_T1296

But CIMs stall at 133 amps apiece...? (theoretically as always) their manufacturing specs say so. that would be 800 amps, aka very bad...

They stall at significantly less than that when you're running them off of a real battery whose effective voltage drops significantly when you start drawing lots of current from it.

It's the same reason that you can stall them at all in the first place; if you naively just calculate from the stated stall torque of the motor you'll find that your robot would have to weigh an ungodly amount to stall them at any gearing you'd see on an FRC bot.

[BBray_T1296]

Originally Posted by Oblarg

They stall at significantly less than that when you're running them off of a real battery whose effective voltage drops significantly when you start drawing lots of current from it.

It's the same reason that you can stall them at all in the first place; if you naively just calculate from the stated stall torque of the motor you'll find that your robot would have to weigh an ungodly amount to stall them at any gearing you'd see on an FRC bot.

True true. I thought about that but did not account for it.

As the resistance outside the battery decreases and approaches near zero (v=ir so 12=133r so r= ~.1ohms) (the motor stalls), the internal resistance inside the battery becomes a significant percentage of the overall resistance, so the potential difference beyond the two terminals is proportionally less than it were when the internal resistance was a small fraction of the overall resistance. Another point for Ampere's law.

[Oblarg]

Originally Posted by BBray_T1296

True true. I thought about that but did not account for it.

I, too, failed to account for it in our design this year, with the result being that we had to swap out our gearkits (which was an excruciating process due to the design of our drive) before worlds.

That's the way you learn best, though.

[asid61]

Ooooookay, we'd better be careful then!

[Ether]

Originally Posted by BBray_T1296

As the resistance outside the battery decreases and approaches near zero (v=ir so 12=133r so r= ~.1ohms) (the motor stalls), the internal resistance inside the battery becomes a significant percentage of the overall resistance, so the potential difference beyond the two terminals is proportionally less than it were when the internal resistance was a small fraction of the overall resistance. Another point for Ampere's law.

How does what you described relate to Ampere's law?

[Stephen.Yanczura]

On our practice bot we drove the robot up to a cement wall and had a mentor apply his weight to the top of the bot upping the traction big time. I had the driver lay on the stick full power into the wall effectively stalling all six CIMs at 100%. Fresh battery, new CIM motors, talon motor controllers.

The main tripped in ~8 seconds and for each successive test it tripped sooner and sooner, I stopped after it was tripping in ~5 seconds.

We are geared 5.95:1 in the box and 1:1 (belts) to the 4" HiGrip wheels. (I did some speed tests using encoder logs and we were seeing 12-13ft/sec max robot speed on slightly worn wheels).

We never tripped a main breaker during a match on the competition bot and to my knowledge never tripped a breaker during an estimated 200 hours of driving the practice bot (same breaker I tortured above).



YMMV

[fox46]

After building and running robots with 6 motor drive for the past few years, I had never experienced an issue with blown main breakers. This year the team had no issues through 3 regional competitions and champs seeding matches until they found themselves in eliminations on Curie. At this level of competition we had the main breaker kick out near the end of four of our elimination matches. Luckly it did not impact the outcomes of the match as they were very close to the end but it just goes to show the stress on the equipment at that level of competition.

Now you've gotta understand our machine is running 6 CIMs (2 speed 18fps/6.5fps) on drive, 2 RS775s on the reloader, an RS550 on intake and the compressor, so it is a very hungry little thing- drivetrain excluded. We would never dream of running a battery that hadn't come right off the charger fully charged. Even a battery that was fully charged and settled overnight was noticeably sluggish in our machine. It was typical for the battery to be warm after our matches as well as all the 6AWG wiring. I wouldn't use this experience to say that 6 CIM draws too much power. There were other inefficiencies which could have been improved to mitigate the breaker trips. First off, you've got to get the power from the battery to the PD board. Solder all your 6AWG terminals (torch, flux, solder, heatshrink)! The mentor in charge of wiring our machine refused to follow my advice and insisted crimped connections were sufficient. During elims on Currie, each of the terminals was a hotspot capable of burning you at the end of the match- any heat generation is a loss of power so here is improvement #1. Secondly, all the 6AWG wiring was warm- In the future it would be prudent to use 4AWG including higher capacity Anderson connectors (not sure if this is legal as per rules though since there is wording specifying a part number for these to be used). Thirdly, the main breaker was cooking hot. We did try swapping it out for a brand new unit but this did not affect the performance. Instead we borrowed some computer duster from the Robonauts #118 (Thank you so much!) and were able to delay the trips by cooling the breaker between matches. I am not sure if the heat buildup was being generated within the breaker or if the poor connections with the terminals were sinking heat into it through the lugs.

Basically, you just have to be very conscious that a 6CIM robot is going to be hungry. You need to design your gearboxes at the mercy of the main breaker and battery. If you are using 6 CIM single speed, make sure the machine breaks traction before stalling the motors. If you are using 2 speed, then make sure you actually use it! Next time around I would insist on some sort of current sensing device combined with the feedback from an encoder on the driveline which would automatically kick the gearboxes to low gear if there existed a current increase past a certain limit for a certain motor speed for a set amount of time. I can't count how many times I had to yell at our driver "LOW GEAR!" when he found himself in a shoving match. Outside of this, make sure your electrical system is robust as the additional strain is hard on these components. Oversize the wiring if you can afford the weight so that you eliminate your losses due to heat.