JVN Design Calculator Follow Up Questions

Hello all,
My team has recently come across the JVN design calculator and noticed it has the potential to answer a few of our questions. However, how do we apply this information to improve our robot? With our current inputted information, we are getting a reading of 69 amps of current draw per motor…Isn’t that impossible because the fuses would trip (40A fuses for CIMs?). What would be the ideal current draw to not experience battery brownouts? What other main factors can drastically lower the amperage? Let me know if anyone needs more info to help us out.

Thanks,
Matthew Calvo
Brooklyn Blacksmiths #806

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Matthew-- welcome to CD! What great questions!

So yes, your 40A breakers will indeed trip if you expose them to 69A, but not immediately. They work based on internal temperature, and it takes a while for objects to heat up. The time-current curve (spec pdf) for the breaker indicates that at 170% load it will trip after 2-5 seconds, which could be an eternity in a 150-second match.

So the critical thing is: how long do you expect the robot to stay in a high current draw condition? I would expect this design would behave like this:

  • Can you accelerate forward from a dead stop? - Probably yes.
  • Can you engage in a pushing match against something immovable? - Not for very long!
  • Can you turn? - This would be a super interesting thing to look at.

Pausing to let others answer the other questions. Once again, they are good ones.

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Darn, Nate beat me to the easy answers :upside_down_face:

Other factors that can lower the amperage:

  • Robot weight (and remember, this is your weight with batteries and bumpers)
  • Number of motors in the gearbox (at least on the motor level; you still have the main breaker to worry about)
  • How fresh are your motors? Brushed motors wear down with time, even if they haven’t been abused. Demos would normally be considered abuse in the context of competition.
  • How cool are your motors? Hot motors have things expanding inside, which increases friction.
  • Did you grease your gearboxes?
  • Is there binding or misalignment? Do you hear bad noises from the drivetrain? Is the drivetrain tracking roughly straight (without software or sensor help) when you command it to go forward?
  • Are you applying current limiting through code or motor controller settings? (SPARK MAXes make this absurdly easy, easy enough that even I got it. We nerfed our NEOs to 80A and eliminated all but the most abusive brownouts.)
  • Look at your driver station logs. Are the PDP channels corresponding to each side’s motors roughly even from motor to motor?
  • How much turning are you doing?
  • Traction wheels on the corner, or omniwheels?
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This will depend on numerous factors…

  • Number of motors/total current draw (I)
  • Main wiring size (R)
  • Quality of wire terminations (R)
  • Battery condition (R)

Using Ohm’s law of V=IR, you need to keep V > 7v (or whatever it is these days).

I (total current draw) you can control by current limiting in code and/or addressing any mechanical issues or gear ratio choices (@Billfred’s comments).

R, resistance, you can control by ensuring your main wiring is in good shape and upsizing it (we use #2awg main wiring). I have seen so many brownout issues solved by redoing main wiring or installing a fresh battery… A happy, healthy, battery will go a long way to reducing the overall R of your robot.

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