My team from UNSW Australia defeated team B-Human from Germany 3-1 last week to claim back-to-back Robocup SPL World Championships. Here’s how we did it.
The competition involves fully autonomous robots, with no remote control, competing against each other in 5-on-5 soccer. Each team uses the same robots, so the competition is focused on software and artificial intelligence (AI) development, not on hardware construction.
The final was a nail-biting match locked at 1-1 with less than three minutes remaining. Both teams had strong strategies and well-tested code, but in the end, our speed proved too fast for the Germans to keep up with.
We went into half time with a 1-0 lead after dominating field position for most of the 10 minute half. Despite playing mainly in the German’s side of the field, we struggled to score many goals against their heavily defensive strategy. At one stage the Germans brought all five robots back to their goal box in an attempt to stop us from scoring.
The second half saw things briefly fall apart for us midway through. We had three robots lose power as a result of heavy falls and were suddenly reduced to only two active robots on the field. The Germans capitalised, and equalised with around three minutes remaining, leading to us calling a time out to revive our injured robots.
After our timeout though, our robots got things back on track. With five robots now on the field, we pulled the momentum of the game back in our favour and scored two late goals to secure our second title in as many years.
Our strategy for winning was focused on exploiting our fast walking speed. Our robots are able to accelerate much faster than most teams’ and they reach a top speed of about 30 centimetres per second (about 1 kmh). Although this is slow compared to humans, in robot soccer it’s really quick!
When the ball is in our half of the field, we don’t bother trying to pass it to a teammate, we just try to boot it up the other end of the field. The idea is that we kick the ball deep into their half, then use our fast walk speed to reach the ball before our opponents have time to clear it away.
This leads to us playing most of the game in our opponent’s half of the field, where it’s hard for them to score goals, but easier for us to score.
Listening for whistles
The competition changes the rules each year to make the games more challenging. This year, all the finals matches were started by a referee’s whistle, instead of the regular Wi-Fi message. This meant that the robots had to listen out for the whistle before each kick off.
We were the only team to reliably start all our robots on the referee’s whistle. A big reason for this was that the team decided whether or not they had heard the whistle, together.
If only one robot heard a whistle, but the other four didn’t, they decide that the one robot must have been wrong, and don’t play. If three out of the five hear a whistle, but the last two don’t, then they decide the two must have missed it and they all start playing.
This majority vote system was crucial in ensuring our team listened to the whistle reliably.
Another of the major reasons we think we were successful is the development and testing practices we have as a team. Each week we run a series of standardised tests to see how fast we can score a goal.
We start the robot in the same set of places each week and time how quickly it gets to the ball and shoots it into the goal. This quickly highlights how effective the past week’s worth of development has been and where we need to improve. It also quickly shows us any major bugs we have introduced with the latest set of changes.
We also play small practice games of 5 vs 0 or 3 vs 3 as we get closer to competition. Firstly, this shows us how well our robots position and play as a team, which is crucially important in a team sport like soccer.
Secondly it shows how well we are able to perform in a changing environment, playing against a live opposition. It’s always much harder to score when you’ve got opponents getting in the way!
Looking to the future
Winning two world championships in a row has put a huge target on our back now. All the other teams will be closely watching our progress and focusing their strategies to beat us next year. We will spend the next 12 months continuing to innovate and ensure that we bring an even better team to the competition next year.
Robocup also announced that it is coming to Sydney in 2019! Although it’s a long way off yet, we are looking forward to having a home ground advantage in a few years time.
Image by Sean Harris/UNSW.