Voltage Limit #soccer-rules-2024

Hi RoboCup forum,

due the changes in the Soccer Open League, the technology and price per robot has rapidly increased since the new diameter. Especially the motors handicap planning the robot, building a strong kicker or as wished a chipper without dangerous or/and rule breaking StepUp converters, besides losing other more important electronical or mechanical parts.

Because of the similarities in the hardware challenges, we looked at the solutions of the different RoboCup soccer major small size league teams: Their solution, so called “EC Flat motors”. Our first idea was to use them as well in the soccer open league, the problem we encountered is the difficulty to find motors with high torque at the voltage limit of 15V (so practically 16.8V because of LiPo’s). The high torque is needed because of the many collisions between individual robots in a soccer open game, using low torque motors as the SSL league does is no consideration.

Our proposal, increasing the voltage limit to a nominal voltage of 22.2 (practically 25.2V). This change would have besides the simplified design many other improvements in the soccer open league:

  • As already mentioned, the simplified mechanical design, because of the massive space increasement. This would encourage teams to design more advanced kickers, or even chippers in the future. It would also help in developing the needed electronics for the now advanced camera systems used to detect the small ball. Especially with the drop of the camera limitation.

  • EC-Flat (or in general brushless motors) have built in encoders, this increases the overall smoothness of the gameplay and would create more precise movements. It would increase the overall gameplay especially from smaller schools without much experience in workarounds.

  • But the most important advantage, EC-Flat motors are in comparison to other brushless motors or commonly used DC-Motors cheap. Especially teams without huge financial support would be encouraged to build soccer open robots again and being able to compete successfully at the RoboCup.

This rule change would be a massive step to open more space for technological advantages. The soccer open league profits by being more affordable and accessible. Since the downsizing of the robots a lot has changed for good, but without considering further adjustments and improvements it would just decrease the participants in the soccer open category.

We hope we could arouse some interest and consider a rule change for the next year.

Robotics Club of the Lessing Gymnasium Neu-Ulm [Germany]

Hi Elias,
Hello everybody,

we are considering going one step further than you suggested: Retain the “no more than 48V anywhere” rule (meaning that even fully charged batteries may not exceed that voltage - this is because safety regulations in most countries set 50V as the “safe for unqualified personnel” limit that also applies to student-built robots) but just remove the specific limit about battery voltage. If teams can safely handle 48V for the kicker they can also safely handle 48V for the motors. What do you all think?

  • Keep 12V/15V limits in place
  • Raise limit as suggested in the post above: 22.2V/24V
  • Remove limit, just keep 48V overall limit
  • Something else

0 voters

Please provide reasons for your opinions in this thread so we can consider all arguments

Best Regards

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PDF of this draft version is available here: https://robocup-junior.github.io/soccer-rules/dbscoach/raise-voltage-limit/rules.pdf

GitHub PR here: Change voltage limitation to 48V maximum anywhere any time by dbscoach · Pull Request #106 · robocup-junior/soccer-rules · GitHub

As far as I can see the voting looks like 2/3 of the voters want to keep the existing limits.
Another approach could be to change to only one voltage regulation.
Having the main difference between LWL and Open to bee the ball and discussing to increase the weight limit for lightweight, we could also increase the voltage limit to 15 (4s Lipo) to simplify rules and regulations.
Having 15V for both 2vs2 leagues we could then also reduce the voltage limit to 9V for 1vs1 to prevent using 2vs2 lightweight robots in 1vs1 lightweight league.
Just an idea. Ignore it if reasons apply…

I’d love to hear the reasons for that. Given the well thought out argument for allowing voltages I’d love to hear what negative effects people are fearing.

So if you voted for 12/15V: Why? Please post below.

Just a thought towards the 9V, as previously discussed, switching any league to 9V is very difficult, as there are no 9V batteries available at a reasonable wight.

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Elias, that is true for Lipo.
They would have to use 2s with max 8V
But if we want to allow Lego with 6x1.5V Batteries, we have to allow 9V.

Hi David,
I think 12V or 16V is the voltage “toys” are made of. And so are affordable motors for robots.
You would not find “pololu-like” motors with more than 12V.
48V Motors would be professional Maxxon motors or similar stuff.

And somewhere below 20V most of the cheap motor drivers have their max voltage as well.
So opening the voltage to higher level (and 5S with 20V already exceeds most pololu drivers)
would increase the “pay to win” gap, were some teams could pay for high efficient 48V brush-less motors with professional drivers and other teams could not.
So for new teams it will be more expensive to build a “great” robot, older teams will not recycle their bunch of used motors and drivers and only “rich” teams will use the new opportunity an by new motors, new drivers and new batteries (as you need more but smaller ones).

If at all we should only allow higher voltages to 2vs2 open as these few teams usually pay much more than teams building 1vs1 bots anyway.
And then, I would say just open it to 48V.

But I do not see the benefits. I think especially in 2vs2 open the matches are not won by motor power at the moment.

@stiebel What do you mean with cheap drives/drivers?
Comparing the prices between brushed and brushless motors, brushless is far less expensive and self build driver boards, inspired by the ssl teams, aren‘t that expensive either (the raw cost of the chips, resistors and capacitor combined).
If you want to, we could share part of our under development brushless driver.

@stiebel what are the technical reasons against raising the battery voltage limits just for OpenLeague to 6s lipos? In my opinion it would just raise the technical level of basically all drivetrains and also let the teams more freedom in deciding what type of motors they could use (combined with custom self made gears and custom self designed motordriver pcb, inspired by SSL Robots for example). Of course this would increase the cost on some robot parts, but I think weighted with the experience all interested teams would gain in order to keep up with this technological freedom, it is really worth it. And I am sure other teams who want to use 4s or 3s lipos arent forced to change that, because teams do not get more used power by that. (As we clearly saw in Bordeaux, not power is the bottleneck but precission).

combined with Pololu - DRV8874 Single Brushed DC Motor Driver Carrier
makes a 35$ drive system with driver, motor and gearbox.
If you ask, pololu gives up to 50% discount for robocup Teams.
so you can say this drive system easily comes at 20$.

I have never seen a brushless motor, driver and gear system for that price.

However, you are right that a large diameter brushless motor combined with a high voltage offers a gearless direct drive that is indeed cheaper than a high quality dc-motor/gear/driver system.

So for open league I agree that 6S (24V) would offer a wider spread of technologies making robots much more different again and this sounds interesting to me as well.

And again, as much as we want to increase the technical level at 2vs2 open as low should we keep the level at entry league to give newcomers a chance to take part.

@david : if you split the votings for open and entry, I would vote for “up to 48V” in open and “stick to 12 or lower to 9” in entry.

For the entry Leagues I fully agree with the 3s maximum limit! For Open in my Opinion, the best compromise is, to limit the battery power to 6s/22.2V and kicker power to 48V as handeled this year. however for LWL, I don´t really think it is necessary to raise the limit (maybe to 4s/14.8V).
What do you think about that? I hope this will change drivebusses and develop them to a better level!

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The issue with pololu motors is that they dont have enough torque or even if they have, they are incredibly slow. We used this year the same motors that @stiebel recommended but our degrees that we could move in were really limited(since the motors are not strong enough at the slow speeds and stop rotating completely) and also we couldn’t drive at slow speed so the task of getting the ball to our ball capturing zone became much more difficult. Compared to the teams that are reusing their old joinmax motors which are really hard do get a hold of we didnt stand a chance in pushing “battles” or races to the ball after lack of progress. So yes you can make a mediocre robot that will work even at the world competition but there are no hopes of winning the world robocup with pololu motors at least thats what i think.

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@Elias I have similar concerns as @stiebel in the sense that I worry about how new LWL leagues would be expected to compete with a higher voltage limit given that these motors seem much more expensive and complex (it is difficult enough to get new students to learn about motor drivers let alone speed controllers!).

For a thought experiment - if a voltage limit was increased for LWL - how could a team with a practical budget of $500 / robot compete against a team that has $1000 / robot for just the motors! Would you have any example of a drive train that would would work with the higher voltage and a practical budget that could benefit rookie teams?

I could concede the added complexity of students understanding brushless control if there is a good practical solution.


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As mentioned earlier, brushless drives are much more affordable (for example: our brushed motors cost over 300€ a piece and brushless motors are starting at under 100€ on the maxon website, minus the discount for schools). With some simple and cheap plastic gears you can easily build strong drive systems and for the controllers, many of the ssl ones are open source and not that expensive either. Drives systems are parts, that will be used for many more years and not a for a single season of rcj. Spending more money on drive systems is an investment for the upcoming years.
In my first post I opened the discussion for changes in the open league, so if a higher voltage in lwl is a deal breaker I see no problem with just changing the voltage in the open league. As we all know, there is already a difference in voltage in both leagues.

Thanks @Elias for the info. I do see how the EC-flat maxon motors can be an investment.

Just for clarification as I haven’t done much with brushless motors - you’re only considering the 24V or potentially the 36V EC-flat brushless motors correct? In other words, any EC-flat motor rated for less than 24V and under $100 wouldn’t be practical?

If the above is true, it does seem to warrant raising the voltage limit. However, the concern everyone here seems to have is that we don’t want to raise the bar for or discourage rookie teams. Combined with the confusion/debate on what sub-league (or entry) is best to start out with, it becomes difficult to figure out how to advise new teams. For example, I gave a rookie school the suggestion of a $500 budget per robot for a total of $5000 for five teams and ten robots to start with Open this season. Upgrading to brushless motors would almost double that budget!

Of course rookie teams have to manage their expectations. However, going on what @kovact mentioned, would any teams starting in open with the current voltage limit have a shot of performing somewhat well up against a field of higher V brushless drives?


I am against increasing the battery voltage limit because it would help teams with financial possibilities at the expense of those built with less expensive hardware.


Teams with more money already have an advantage, regardless of the voltage rule. Allowing higher voltages would allow teams to reinvent the drive system and as previously mentioned also teams with less money to find a way to compete against „rich“ teams again.

I was wondering where the sentiment that more voltage equals more power was coming from? Almost all DC motors come in multiple voltage / current configurations with different internal windings to suit. Because of the simple winding change, there is rarely any price difference between same output power motors of 12V, 24V & 48V. A 12V, 30 Watt motor is typically the same size as a 24V, 30 Watt motor - only it pulls half the current to accomplish this. If a larger amount of power is desired by teams, they would need to increase the size and weight of their motors which is something they can already do. Both Pololu and Maxon motor catalogs almost always follow the trend that size equals Wattage (power), being the product of Voltage and Current. When voltage goes up, the current goes down and the motors is still only able to output the same amount of power.

Another key point was that beginner teams will get more power out of their beginner components, such as the L298N motor driver that most teams learn on. The module is typically rated to ~36V (and the IC itself at 48V). At just 24V it can now deliver ~48 Watts of power to a motor, rather than 24 Watts at 12V, due to the 2A current limit. Stepping up the voltage for teams means that they will not need to opt for expensive, high current, custom motor driver ICs and can get solid performance from standard parts.