775 vs CIM

I’m surprised by this comment. What was the gearing? Any active cooling? Our 6 miniCIM drive heated up quick. Never heated enough to make an impact in competition, but we were definitely limited by temperature in our practise sessions.

The VEX Victor BB controller is controlled by PWM but has completely inbuilt current limiting, and allows the max current output to be set.

A better example of this the is RageBridge which is dual channel controller, mainly used in Combat Robots. The controller has an output current capacity similar to that of the controllers we use in FRC but is capable of driving motors capable of drawing many times more current( a 3hp(@24v) short mag with a stall current of over 400 amps ) than it is capable of supplying.

Similiar, easy to use functionality like this could be very effective in preventing high current draw( and thus brownout or breaker trip) and preventing motor damage.

3 CIM Ball shifter with 11T motor pinion, 2.65 spread (54:30 low gear option), 3rd stage 54:30 with 4" omni wheels. This led to 5.56:1 in high and 14.73:1 in low gear.

We were 119.5 lbs at champs plus batter and bumper.

Talon SRX already has current limiting.

It appears some folks are missing the point of my speculative post. I should have stated it explicitly.

My post was about “smart” current limiting, not a fixed (constant) current limit.

And the main thrust of “smart” current limiting would be to coax max performance from 775 Pro, while still protecting them from releasing their magic smoke.

Yep, surely you would also want to have it to utilise a measurement of total current draw from the battery( and resistance of the battery+lead+PDP) if possible) to protect from tripping the breaker and from brownout.

Keep in mind that you will get the maximum mechanical power for a fixed amount of amperage when you have a large number of 775pros being under-run than a smaller number of 775pros or CIMs being run at full power. So coaxing the maximum performance may not actually be the best option when having to power it from the saggy FRC SLA.

Maybe the drivetrain encoder could be used as an easy method of determining the speed that the 775pros/ their fans are running at.

The talon does not have access to that information, unless your program sends it via CAN messages.

I was limiting my speculation to information the Talon has available for its 1KHz control loop: motor current, motor voltage, motor speed.


Keep in mind that you will get the maximum mechanical power for a fixed amount of amperage when you have a large number of 775pros being under-run than a smaller number of 775pros or CIMs being run at full power.

If I posted something that you interpreted as saying otherwise, please let me know so I can re-word it to make it clearer.

coaxing the maximum performance may not actually be the best option when having to power it from the saggy FRC SLA.

Not sure what point you are trying to make here. So I’ll try to be even more explicit: My speculative “smart” current-limiting post was about providing better performance than a fixed voltage limit or a fixed current limit, even given a “saggy” battery.

If you ever get around to conducting these tests, I think many people would be interested to see how the performance of 4 CIMs + 2 miniCIMs compare to the rest as well.

Hm. We ran the same gearbox and number of reductions, but free speeds of 10 & 22 fps rather than 7 & 18. We were ~20 lb lighter, which balances out the gearing somewhat.

It probably comes down to how much you were in low vs high gear. We always started in low and transitioned to high once we got going. Any sign of defense in the close quarters we would be in low.

I wonder how much actual additional performance there is to be gained over a flat current limit, though. That is, I wonder, with a 8x 775pro drive, if motor burnout is much more of a performance limit than control system brownout - I suspect (though, of course, I could be totally wrong and won’t state this confidently until I test it) that motors current-limited to prevent brownout will not have much problem with overheating.

Of course, a flat per-motor current limit isn’t perfectly optimal for avoiding brownout, either (there’s variability in the internal resistance of the battery, as well as in the amount of current being drawn by the rest of the robot) - but it’s reasonably close.

Moreover, even if my intuitions are correct for an 8x 775pro drive, such “smart current limiting” could still be very useful for making a 6x 775pro drive feasible.

Theoryhammer aside - have you done any experimentation to find if you can actually pull 400A? From talks with John Novak ~250A is the max he was able to actually get out of the battery to the motors.

My team this year at worlds had that issue. We are geared roughly 5.6:1 with 4" wheels, so in the end we were about 13 FPS. Due to the back to back matches of division playoffs and Einsteins, the motors were too hot to touch. Although it didn’t help as much as it would with air cooled motors, we started spraying the motors with compressed air just to try to draw some heat off.

That’s a pretty aggressive gearing for a 4-CIM single-speed.

What configuration and type of wheels did you use? 4/6/8 wheels? What was the drop center distance?

6 traction wheels on a squarish wheelbase, 1/8" drop (palpable rock). I don’t think turning scrub was our biggest current problem (the tread didn’t wear any faster than expected), but then I’m not sure spending too much time in high gear was it either (we ran auto-shifting code). Could’ve been either, I guess, because it had to have been something.

Forgive me for asking, but why exactly does peak power output matter?

Never be sorry for asking questions. :smiley:

Acceleration. The more power you have, the faster you can accelerate, which can justify a higher top speed. It all comes down to moving faster.

With datalogs on our 6CIM drive we saw spikes of 80A/motor, or about 480A total output from the battery, just before brown-outs when using healthy batteries.

No, limiting the battery voltage was always the plan. No magic smoke when you stay within the limitations of the motors, which we have been educating the community on. Whether they listen is up to them. We were planning on running 10X 775Pros at 60% voltage each so they could stall for an entire match, theoretically. We ended up using 8x 775Pros and limiting them to 80% voltage which was more than safe in our experience. This setup gave us similar power to a 6 CIM drive train with a substantial weight savings that allowed us to do others things on the robot.

There were a great many teams that jumped on the 6-CIM band wagon a few years back and tripped breakers also, some teams new to that idea still are. From a post back in 2014, it looks like you guys had the same issue of tripping main breakers. So your comments are confusing to me.

I’d love to see the results of the test for this, if you guys decide to perform it.