Voltage Limit #soccer-rules-2024

@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.


I mean you should expect around 1000 eur for one robot because even basic robot with pololu motors, pololu drivers, pixy camera and some pcbs will alone get to somewhere around 600 eurs since you still need to consider the pcb cost and a lot of other small components like display, voltage convertor, batteries and compass. Now this is considering that the new team will never do anything wrong and wouldnt fry a single thing and every idea will be the perfect one used for the final robot. What i am trying to say is that production cost is also a huge factor when making a robot. This year our overall spending were almost double the combined cost of our robots

Also we already got 2 categories that can be utilized by new teams(light wieght and entry) i am not saying that you should from the get go dive into your own object detection done by camera and do your own drivers. But i am saying that there should be a room to improve to, since most of the teams that get into robocup stick with it. Also raising the voltage limit can help newer teams since there is a higher chance of finding some better motors lying around, in case the voltage limit would be higher. Also limiting teams from getting too good is just counter productive for the whole competition also i doubt there is a single team here without sponsors and if there is one money were never issue for them to begin with. So the money factor isnt really that big and i believe that acquiring sponsors is also a skill and if you are able to get more sponsors you should be able to utilize it to its potential.

Also the idea of new teams not having enough money is simply false and we cant really judge the economical states of new teams. The thing is that in my opinion new teams never stand a chance against a team doing it for 5 years since they had time to perfect their robots. Maybe it would be even better to raise the limits of what robots can do contradictorary to believe that there should be an upper cap for robots. This could prove to be a good idea since you have more room for improvements and are not that heavily limited which causes to sharpening the robots to the absolute perfection within the given restriction. What i am trying to say is that lets say that wehave a team that is doing soccer open for 5 years in first 2 years they made a robot that gone to the world robocup and were able to use succesfully maxon motors and now for their next 3 years are just polishing their robot to the outmost perfection meaning a little room for improvement and absolute domination with their robot over the newer teams.

To summarize my point I think that right now the starting point for the newer teams is quite okay saying this as a team that this year made our second no lego robot for open and was able to win almost half of our matches in world competition and the only thing unfair about soccer open is that there are teams with triple your experience. Which is quite understandable. And giving teams to make a little better robots just makes the competition more fun for everyone even the older teams.

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@kovact - I see your point about doubling the budget to $1000/robot or more; good for others to read that and realize that things do fry. My suggestion of $500/bot was for very basic robots - no custom PCBs and simple 3D printed chassis from supplies they already have. I’m keeping it on the cheap side in part b/c I’m looking to have an example that will be on par with other robotics competitions that allow for 10 or more students per team and just one robot (i.e. RCJ soccer will be the foundation of this school’s after school robotics program so budget needs to be tight).

I’d like to push back on your conclusions on the money issue; your evidence seems self selecting based on your experience and what you observed from existing rookie teams. My concerns are for the teams that may never exist if RCJ sets the bar too high. My push back on the rule change is to make sure we can still practically encourage new teams while balancing with better engineered robots.

I agree that it is good and right for a new team to consider gaining sponsorship and fund raising. However, I’m coming from the angle of trying to grow new teams. If a school, family, or any organization is going to make an investment into an RCJ team, they’d want to see some return on that investment. Yes, we can focus on the educational value but the sad truth is that the competition does matter too much to some. The more money (and time) invested, the more results are expected. Would you have performed as well as you did and have been as satisfied as you seem to be if those veteran teams you were up against not only had the experience but also motors allowed by this rule change? Would you as a rookie team have felt pressure to put more time into fund raising to get these more advanced drives? Would you be able to acquire the necessary skills to drive those motors effectively?

Since the rule change seems like a lock, perhaps we should now steer this discussion to how do we prepare potential new teams for this advancement. Do we now say “invest in brushless or expect to get demolished!?” Do we just skip over teaching h-bridges and go right into speed controllers?

Thanks for sharing your experience.

The notes regarding this topic from our open discussion Zoom meeting last week:
Question: David: Is there anyone in the committee that has anything against moving to the 48V limit? [referencing thread on discussion so far].

No discussion. No opposition (silence)! Two thumbs up.

Looks like everyone is in favor of the rule change!

Sorry, I missed the first meeting; that was my fault. However, I disagree with the 48V voltage limit. If we want to implement this, we should introduce a power limitation somewhere. Obviously, voltage is not the only power source, but it is easier to obtain more power from motors with higher voltage. Too much power discourages strategy, and there’s a risk to come cack to a ‘brutal force’ game.

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Sorry, I have a question about this. Are you sure that brushless motors (or EC-Flat) are less expensive than brushed DC motors with their drivers? If you are sure, could you please name some of them? I checked a few, and they were a lot more expensive than brushed motors, which made me confused about how they could be affordable.

Please read the entire thread, I think all points are already answered.

I think this topic has been discussed enough. However, my personal opinion is to keep the voltage limit (or create a speed limit instead of a voltage limit). If we don’t put any restrictions in this field, according to other rules, we will face robots that are engaged in destroying the way of playing competitors with only power and force and high speed.
Anyway, I request that whatever decision is taken be finalized soon. Teams need to prepare equipment for their robots. Therefore, I think the final decision should be announced soon.
Thank you all

Just for clarification :
in the draft rules there are 48V as maximum voltage for both 2vs2 leagues.
This was well discussed and agreed.

We should make clear, that we are talking about 48V DC !
Students in most countries are allowed to work with up to 48 or even 50V DV but only 24 or 25V AC.
With battery powered robots we usually do not deal with AC anyway, so there is no need for further discussion to restrict the 48V to DC only, I think.

So please include these two letters “DC” to the (draft) 2vs2 soccer rules.

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