Browning out

Hello, we are having an issue with drivetrain brown out. Here are some initial specs:
4 cim drivetrain
2 speed
Initial reduction: 11:42
Low gear: 14:60
High Gear: 24:50
We have 6 in colsons
The battery we are using was beaked at 130% and an IR of .015
We did a pushing test and was pulling an average of 50 amps per motor. We brown out when PID testing and it browns out our NavX so we can’t do auto-align testing. We have ran with about 10 different batteries. The issue also only seems to happen when quickly turning.

Any help would be fantastic, thank you.

We are also running at low gear for these tests.

Four colsons? Six? Drop center? No?

Are you browning out trying to drive straight as well, or just when turning?

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We are running a 1/8 in drop center with 6 colsons. Doesn’t brown out when driving strait but there seems to be a significant voltage drop of around 12.5 resting down to an average of 10 volts while straight.

Are you certain that you have sound and secure wire terminations from the battery to the PDP, including the main breaker?
Are you using a pre-built harness, or did you fabricate your own?
When you wiggle the 6AWG conductors, do the terminals follow the wire - or are they fixed at the battery/main breaker/PDP?

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How rigid is your frame? Can you pick up one corner of the frame while the other 3 stay touching the ground?

Frame is fairly rigid. I can pick up one corner and the rest of the bot lifts up

4 CIMs should not be a problem. You mention quick turning, so your PID might be overly aggressive.

Go back to P only and set it low enough that it doesn’t turn far enough. Slowly increase P until it overshoots and wiggles back and forth a little. That wiggle will show you the natural frequency of your system. You won’t turn much faster than that so that is what you want to tune for.

If you can’t get that far, you have other issues. Wiring. Drag.

The electrical captain checked that and it seems everything is in working order.

We even have these when we turn fast as well, would this be a PID issue. Also happens when we move quickly in high gear

What motor controllers are you using?

If you are using TalonSRX’s (or something equally fancy) then you can set your openLoopRampRate to something reasonable (we use .3 seconds).

That will make sure that it takes at least 1/3 of a second to go from zero to full, which is very responsive to the point the drivers don’t notice it, but keeps us out of the brownout conditions.

If you don’t have Talons, and are using the built-in WPI PID controller, you can look at incorporating the SlewRateLimiter class to keep your PIDs from being too aggressive in the voltage changes.

What sort of surface are you driving on?

Wheel scrub can contribute to brownouts. And, that’s devilishly hard to diagnose – a robot that ran fine on the cement floor of your shop may brown out on the field carpeting.

An easy test: switch your front/back wheels to omnis. (Note: That may not be a permanent solution – there was a post here yesterday about omni wheels being chewed up by the steel boundary to the RP.

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We were running both on carpet and like school tile. The issue was initially there on tile then we moved to carpet

Well, that’s not that big a drop for an all-out with four CIMs. Make sure your gear boxes are well lubricated and properly assembled (if you’re using the kitbot or any AndyMark toughbox for your initial reduction, kids love to put the gears in facing the wrong direction, which creates all sorts of friction).

If you’re browning out while turning but not while driving straight, my bet is wheel scrub as your primary culprit. A quick and easy test for that is to put tape (something slippery like masking tape or duct tape, or even packing labels) on the four outside corner wheels to dramatically lower their CoF. If that significantly eases or fixes the problem, then there you go. [Your options at that point are a larger drop on your center (likely tough at this stage in the game), maybe turning down your outer wheels by 1/8" or so (might be a fun lathe project), swapping the outer wheels to omnis (again, use caution with these in the Rendezvous Point), or maybe even just leaving the tape on them and replacing it as it wears (rather less than ideal)].

make sure your battery or any wires aren’t shorting to the frame of your robot. make sure all your connections are solid and check the amount of voltage/amperage your motors are drawing. We found that out that the falcons on our drive train were causing brownouts because they were drawing more than the 40amp breaker could handle. it ended up just being a quick code change and then back to the field.

I’m not sure if this would solve your problem but earlier in the season I experienced a similar issue regarding PID Controllers. When it was happening, I wasn’t normalizing my PID output between [-1, 1] so the output could have been greater than 1 which browned us out. After binding the output between [-1, 1], we no longer experienced any brownouts.

EDIT: We bound the output between -1 and 1 because we were using percent output with talons. The bounds could be different if you are using a different Control Mode.

I highly recommend you use TalonSRX’s or SparkMAX motor controllers for your drive train and implement their current limiting option. We have 6 - 8" pneumatics with 5/16" of center drop and still had wheel scrub that was browning us out. Set the current limit to 40 or 50 amps (can’t remember the exact value) and problem solved.

The sparkMax method is - > setSmartCurrentLimit (int Amps)

TalonSRX Example:

50 amps per motor is a total of 200 amps. You’re pushing up against the limit of what a reasonable battery can handle. I’d suggest limiting your motors using current limiting to the range that your breakers can handle - 40 amps. This should help your brown out problem and insure you don’t pop any breakers.

I find it hard to believe that Talons don’t clamp the output and can use a percentOutput greater then one (ie more voltage then the battery can output)

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Or wrap your front and rear wheels with duct tape to reduce the traction.

How did your electrical captain check the electrical system? The check needs to be thorough, one wire at a time. Check each one as @p_h_argo suggests including the th 6AWG wires as well as all the other wires for each motor and motor controller.