Benchmarking acceleration

Hey, this year a whole lot of teams ran similar drive trains, including us, but we have lingering concerns about the speed and acceleration we’re able to produce. We’re running WCP belt drive with the “fast” (5.5) gears and Falcons, which produces a top speed that seems about what we would expect, and about the same as everyone else’s, somewhere between 5 and 6 meters per second. But our acceleration feels low, and eyeballing the log says that when we saturate the motors at a current limit of 40 amps, we get an acceleration of something like 3 m/s/s, which seems lower than other robots, and a lot lower than the ~7 m/s/s that a naive F=ma calculation would imply, yet our robot was quite light, ~80 lb exclusive of battery/bumpers.

Have you measured acceleration? What are you finding?

My students have asked about adding another motor to the WCP modules with a lower gear to improve holeshot, and it seems like a pretty easy summer project, but before we go there, I’d like to hear what other teams have achieved.

thanks!

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We current limit at 90 amps on the drive train motors for two seconds, then to 50.

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are you sure it’s not your code that’s limiting acceleration

pretty sure, yeah. we log the motor effort and we’re pretty sure we’re saturating it, which means we’re on the current limit.

Why did you choose 40 amps as the current limit?We definitely notice performance when we set a limit at 40.

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ooo, that seems high! 90 amps per motor so 360 total? our measured battery resistance of about 17 mOhm means that would yield about 6 volts of sag which would brown out, wouldn’t it?

honestly we didn’t experiment much. a 40 amp limit (160 amps total) would drive the battery below 10v which seems like a reasonable sag. what limit do you use?

I would recommend testing with no limit or the default limit which on most motors is more than 40A.

We have indeed done that, in a casual way, which is how we ended up with 40A. :slight_smile: What I’m wondering is where other teams came out, so I can direct our efforts. The real issue might have nothing to do with current limiting per se, and our choice of limit might be an echo of some other problem. For example, maybe we have a batch of high-resistance batteries. Or it could be that we tend to swap batteries in practice sessions less often than other teams, or use older batteries, so brown-outs were more annoying to us. Or maybe we just have really high gearing compared to other teams, so the motors stay “near stalled” for longer. We’re using the new-fangled radio POE injector, maybe it’s more sensitive to brown-outs. I dunno.

Is this 40A stator limit or supply limit? 40A stator limit is very low.

115 dip to around 8V when accelerating.

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With that limit, and the L3 gear set, we rarely see the drive motors pull more than 60 amps each. We have never browned out this year.

both, actually, though i think that’s probably a mistake, and we meant to limit supply.

do you limit at all, or do you get 8V sag without limiting?

sounds like 90A for 2s followed by 50A might work for us. thanks!

60 Amps has worked well for us the past two seasons. It’s made us “look” faster than other robots despite running the same hardware (Mk4i L2 w/ Falcons). That amount usually gets us “close” to brownout when the driver hits the gas, but never reaches it. A good battery won’t brown out with 240A current until it reaches below 12V.

cool, thanks! have you measured acceleration, like by logging the encoder rate?

I haven’t measured it specifically but we have an acceleration limit of 20 m/s^2 (full speed in 0.25 sec) and it appears to be pushing that limit. Keep in mind this is swerve with 4" wheels, so a WCD with 6" wheels will have different physics.

And to add some extra info: 60 is our supply limit, 80 is our stator limit.

Something like 65 or 70 I think. It doesn’t make sense to me given that you have a 120 amp breaker and we have been able to brown a full battery when we don’t use any limits. We set our limits based on ensuring no brown in a max current limit scenario. We found this to be a full power wide arc while maintaining attitude. This year the current limit to not allow a brown for 2 min of this was not extremely noticeable to the driver.Even with the current limit we see dips to 8.5 or 9 I think, but it doesn’t inhibit function at all, so it is workable for us. In higher power draw years, I anticipate more of a conversation about how much do we risk a brown for better nominal performance.

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