Resolve: lowering voltage limits to 9V will significantly reduce robot speed. Show me the data!

[david]

I’ll respond to @stiebel s arguments from Draft rules for RoboCupJunior Soccer 2022 released - #43 by stiebel on a technical level here:

@stiebel

But then I read the line: “For the 2023 rules, the Soccer Committee intends to limit the voltage to 9.0 V
nominal for both leagues.”

The intention is to make perception and planning more important and better motors less important in gameplay.

@stiebel

I know there where discussing about this right here and I definitely want to protest to this decision!
Every single soccer team I know uses LiPo or Li-IO batteries which have 7.4V, 11.1V or 14.8V per definition.
So If you want to reduce power from 15V (where I still do not understand the reason), than you have to consider these voltage steps.
If you set a voltage limit to 9V, you push us to using 6 non-recharchable 1.5V batteries or to reduce to 7.4V with a 2S Lipo setting.

9V is meant to not outlaw Lego bots from new teams with their AAs - we know we’d effectively limit most teams to 2S.

@stiebel

This will have only one effect: all the motors and motor drivers the teams kept buying over the last years will be pushed beside to buy some other motors with higher currents to provide the torque needed to run the robot. This will have no effect to reduce speed.

The thinking behind 2S is the following: For motors to have the same torque they need the same current, to get the same current at lower voltages they need gearboxes (weight penalty) or lower inductance motors (a.k.a. fewer windings / lower kv in DC/BLDC terms). Motors with lower inductance generate more back-emf as rpm increase. So teams would have to make a trade-off between stall torque, max (usable) rpm and weight: to keep the exact same power (i.e. significantly higher currents) heavier motors would become necessary. All this should - and we have as far as I can recall only heard assertions that teams would buy different motors but have not seen data to contradict this reasoning - where I looked at motors (which I admit was just HobbyKing in this instance) the tradeoffs I described broadly hold true - also losses from ohmic resistance increase, heat dissipation in the motor controller increases and so on - lots of things that should add up to a reduction in speeds (we think).

But if you can demonstrate how voltage/current don’t matter here’s another though: Would going the other way with voltage at least help level the playing field if slowing down robots isn’t going to happen (i.e. allow cheap motors to go close to the same speed as expensive ones)? Like if something up to 48V (or 36V or 24V) nominal battery voltage were to be allowed allowed (48V nominal is the maximum before high voltage safety rules apply)?

Cheers David