AI Magazine Volume 21 Number 1 (2000) (© AAAI) Articles The CS Freiburg Team Playing Robotic Soccer Based on an Explicit World Model Jens-Steffen Gutmann, Wolfgang Hatzack, Immanuel Herrmann, Bernhard Nebel, Frank Rittinger, Augustinus Topor, and Thilo Weigel I Robotic soccer is an ideal task to demonstrate new reacting on mostly uninterpreted sensor input techniques and explore new problems. Moreover, as in pure behavior-based (Werger et al. 1998) problems and solutions can easily be communicat- or reinforcement learning approaches (Suzuki ed because soccer is a well-known game. Our et al. 1998), soccer seems to be a game that has intention in building a robotic soccer team and a structure that requires more than just react- participating in RoboCup-98 was, first, to demon- ing on uninterpreted sensor input. Our claim strate the usefulness of the self-localization meth- ods we have developed. Second, we wanted to is justified by the fact that the two winning show that playing soccer based on an explicit teams in the simulation and the small-size world model is much more effective than other league in RoboCup-97 used this approach methods. Third, we intended to explore the prob- (Burkhard, Hannebauer, and Wendler 1998; lem of building and maintaining a global team Veloso et al. 1998). Further evidence for our world model. As has been demonstrated by the claim is the performance of our team at performance of our team, we were successful with RoboCup-98, which won the competition in the first two points. Moreover, robotic soccer gave the middle-size league. us the opportunity to study problems in distrib- Third, we intended to address the problem uted, cooperative sensing. of multirobot sensor integration to build a global world model and exploit it for coopera- obotic soccer is an interesting research tive sensing and acting. In the end, we identi- domain because problems in robotics, AI, fied more problems in this area than we solved. multiagent systems, and real-time rea- However, we believe that it is an interesting R topic for future research. soning have to be solved to create a successful team of robotic soccer players (Kitano et al. Although perception and sensor interpreta- 1997). Furthermore, it is an ideal task to tion were definitely the focus of our research, demonstrate the feasibility of new ideas and it was also necessary to develop basic soccer techniques and explore new problems. skills and forms of multiagent cooperation to We started to design a robotic soccer team show the advantage of our approach. Al- with the intention of participating in though this part certainly needs improvement, RoboCup-98 for three reasons: First, we intend- it was still effective enough to be competitive. ed to demonstrate the advantage of our percep- Furthermore, based on an accurate world mod- tion methods based on laser range finders el, our robots were much more reliable than (Gutmann et al. 1998; Gutmann and Nebel other teams. 1997; Gutmann and Schlegel 1996), which The rest of the article is structured as fol- make explicit world modeling and accurate lows: In the next section, we give a brief sketch and robust self-localization possible. of the robot hardware. We then describe the Second, we believe that soccer is a game, general architecture of our soccer players and where it is advantageous to base deliberation the soccer team. The next section focuses on and action selection on an explicit world mod- our self-localization approach, and then we el, and we intended to demonstrate that such describe our player- and ball-recognition an approach is superior to other approaches. methods that are needed to create the local Although it is possible to play robotic soccer by world model. The integration of these world Copyright © 2000, American Association for Artificial Intelligence. All rights reserved. 0738-4602-2000 / $2.00 SPRING 2000 37 Articles a Toshiba notebook LIBRETTO 70CT running LINUX. The robot is controlled using SAPHIRA (Konolige et al. 1997), which comes with the PIONEER robots. Finally, to enable communica- tion between the robots and an off-field com- puter, we use the WAVELAN radio ethernet. In addition to these components, we added PLS200 laser range finders manufactured by SICK AG to all our robots. These range finders can give depth information for a field of view with an angular resolution of 0.5 degrees, and an accuracy of 5 centimeters to a distance of 30 meters. Handling the ball with the body of the PIO- NEER 1 robot is not an effective way of moving the ball around the field or pushing it into the opponent’s goal. For this reason, we developed a kicking device using parts from the MÄRKLIN METALLBAUKASTEN. Furthermore, to steer the ball, we used flexible flippers that have a length of approximately 35 percent of the diameter of the ball. Although these flippers led to some discussions before the tournament, it was final- ly decided that the use of such flippers does not violate the RoboCup rules. In fact, we believe that taking the idea of embodiment seriously, such a ball-steering mechanism is necessary to play soccer effectively and authentically. In fact, without the flippers, it is almost impossi- ble to retrieve the ball from the wall, which means that the referee must relocate the ball, Figure 1. Three of Our Five Robots: Two Field Players and the Goal Keeper. which is annoying for everyone—in particular, for spectators. Furthermore, without the ball- steering mechanism, the ball is easily lost when models into a global model and the problems running with the ball. we encountered are described in the next sec- tion. We then sketch the behavior-based con- trol of the soccer agents and show how a basic General Architecture form of multiagent cooperation is achieved. Our robots are basically autonomous robotic Finally, in the last section, we describe our soccer players. They have all sensors, effectors, experience participating in RoboCup-98 and and computers on board. Each soccer agent has present our conclusions. a perception module that builds a local world model (figure 2). Based on the observed state of Robot Hardware the world and intentions of other players com- municated by the radio link, the behavior-based Because our group is not specialized in devel- control module decides what behavior is activat- oping robot platforms, we used an off-the-shelf ed. If the behavior involves moving to a partic- robot—the PIONEER 1 robot developed by Kurt ular target point on the field, the path-planning Konolige and manufactured by ActivMedia. In module is invoked, which computes a colli- its basic version, however, the PIONEER 1 robot is sion-free path to the target point. hardly able to play soccer because of its limited To initialize the soccer agents, start and stop sensory and effectory skills. For this reason, we the robots, and monitor the state of all agents, had to add a number of hardware components we use a radio ethernet connection between (figure 1). the on-board computers and an off-field com- On each robot, we mounted a video camera puter (figure 3). connected to the Cognachrome vision system If the radio connection is unusable, we still manufactured by Newton Labs, which is used can operate the team by starting each agent to identify and track the ball. For local infor- manually. A large number of the other teams mation processing, each robot is equipped with in the middle-size league used a similar 38 AI MAGAZINE Articles Communication Behavior- Effectors Perception based Sensors control Path- planning Player Figure 2. Player Architecture. approach (Asada and Kitano 1999). Unlike other teams, we use the off-field com- puter and the radio connection for realizing global sensor integration, leading to a global Global Graphical world model. This world model is sent back to Sensor User all players, and they can use this information to extend their own local view of the world. Integration Interface Thus, the world model our players have is sim- ilar to the world model constructed by an over- head camera, as used in the small-size league by teams such as CMUNITED (Veloso et al. 1998). Commu- nication Self-Localization Off-field Computer We started the development of our soccer team with the hypothesis that it is an obvious advantage if the robotic soccer agents know Radio Ethernet their position and orientation on the field. Based on our experience with different self- localization methods using laser range finders (Gutmann et al. 1998), we decided to use such Goal Player 1 Player 2 Player 3 Player 4 a method as one of the key components in our Keeper soccer agents. A number of different self-localization meth- ods exist based on laser scans (Gutmann and Schlegel 1996; Weiß and von Puttkamer 1995; Figure 3. Team Architecture. Lu and Milios 1994; Cox 1990). However, these methods are only local; that is, they can only more costly from a computational point of be used to correct an already-existing position view. For these reasons, we designed a new self- estimation. Thus, once a robot loses its posi- tion, it will be completely lost. Furthermore, all localization method that trades off generality the methods are computationally demanding, for speed and the possibility of global self-local- needing 100 milliseconds to a few seconds on ization. Our method first extracts line segments a modern computer. Global methods are even from laser range scans and matches them SPRING 2000 39 Articles Scan with extracted line segments Robot Position hypotheses RoboCup field model Figure 4. Scan Matches Lead to Position Hypotheses. against an a priori model of the soccer field.