Autonomous mobile robots are ever increasing their number of different applications, even in ludic applications or in sports. In the last few years, several robotic football competitions have been organised with participating teams from all over the world. This paper describes a team of Autonomous Mobile Robots which play football, developed by
the Group of Automation and Robotics at the Industrial Electronics department (School of
Engineering) of the University of Minho, in Guimarães (Portugal). In these competitions
each team is free to use and/or build all the different electronics, sensory systems, playing
algorithms, etc. as far as they cope with the rules imposed by the organisation. From every team new ideas emerge, sometimes the most incredible ones, but proving in the end that they work. These competitions proved to be very fruitful scientifically as well as very mediatic

Machado, Carlos

Ribeiro, A. Fernando

Sampaio, Sérgio

Universidade do Minho: RepositoriUM

IMAGE PROCESSING APPLIED TO AROBOTIC FOOTBALL TEAMCarlos Machado, Sérgio Sampaio, Fernando RibeiroGrupo de Automação e Robótica, Industrial Electronics Department,University of Minho, Campus de Azurém,4800 Guimarães, PORTUGALfernando@dei.uminho.pthttp://www.robotica.dei.uminho.pt/AbstractAutonomous mobile robots are ever increasing their number of different applications,even in ludic applications or in sports. In the last few years, several robotic footballcompetitions have been organised with participating teams from all over the world. Thispaper describes a team of Autonomous Mobile Robots which play football, developed bythe Group of Automation and Robotics at the Industrial Electronics department (School ofEngineering) of the University of Minho, in Guimarães (Portugal). In these competitionseach team is free to use and/or build all the different electronics, sensory systems, playingalgorithms, etc. as far as they cope with the rules imposed by the organisation. Fromevery team new ideas emerge, sometimes the most incredible ones, but proving in the endthat they work. These competitions proved to be very fruitful scientifically as well as verymediatic.INTRODUCTIONFollowing a participation in other football competition, this team decided to increasereliability of these robots as well as implementing new improvements. The mechanicsneeded rebuilding and the software needed to cope with communication between therobots. Also some rules demanded some changes in the software and game tactics.Basically, the robots are typical ones with two motors/wheels with a differential typesteering control. Each robot has a standard personal computer inside and some electronicsto read the wheels movement (through encoders).Instead of using several different sensors increasing electronics complexity, this teamdecided to use only one major sensor and invest some time on it: a vision system with asmall colour camera. The main feature of this team consists of an innovative imageprocessing system developed on purpose for this robots. In order to see the whole field,this vision system uses a convex mirror placed at the top of the robot looking downwardswith the video camera looking upwards, towards the mirror. With this technique, therobots can see all around themselves with a top view, which means continuous vision ofthe ball, goals and other robots. All the image processing algorithms were developed fromscratch. With this vision system, all the information necessary for the game is readavoiding though other sensors. Collision detection was an important aspect taken intoconsideration and is achieved also with image processing.ROBOTS DESCRIPTIONThe team is made up of four equal robots (with a small difference only to the goal keeperlater described). Even the software running on each robot is the same. This waystandardisation was achieved which means that they should behave the same.The robot base used is made up of very light wood. It consists of a two levels platformwith the two wheel voids. On the bottom level was placed the DC/DC converter (betweenthe two wheels) and the two 12V 7Ah batteries (one at the front side and the other at therear side). At the top level, it was placed the computer mother board and respective boards(video and graphics boards). The hardware consisted of a personal computer mother boardwith a Pentium processor running at 200 MHz (MMX), with 32 Mbytes of memory(although the DOS operating system was used and therefore only 1Mbyte was accessed).The hard disk had 2 Gbytes. A colour video camera was used with a frame grabber typeBt848. The communication hardware and software was not ready in time for theRoboCup99 competition and therefore each robot played on his own. Figure 1 shows onerobot.Figure 1 – One footballer robot with the ballPERCEPTIONThe game is played through the use of certain known colours. The ball is red, the goals areblue and yellow, the field is green with white lines, the surrounding walls are white andthe robots are black. This sort of approach forces the call for a video camera. Thereforethese robots use one only sensor to perceive all these items needed to play a game, whichis a simple colour camera with its frame grabber plugged on a computer slot. But, mostimportant of all is the way the camera sees. The camera is pointing upwards onto a convexmirror (as described in Figure 2), allowing therefore vision all around the robot from thetop. This top view allows the robots to see far even when an opponent robot is coveringthe field of view. The mirror is pointing slightly to the front since it is more important tosee to the front rather than to the back.The image distortion provoked by the convex shaped mirror is not very relevant as can beseen in the example in Figure 3, since what matters is the direction of the entities and nottheir actual position.Figure 2 – One footballer robot with the ball Figure 3 – Image seen by the robotTo perceive all the necessary entities of the game (ball, goals, etc.), these robots grab oneimage every 20 ms, and the software finds the peak of a certain colour (after removingnoise). For example, to track the ball, the software searches a peak of red. Since coloursdepend very much on the light conditions, a calibration is made prior to a game, in orderto inform the software what is the minimum value for a red to be a ball. The softwaretracks down the following  items:• the ball (by its red colour)• the two goals (also by their yellow and blue colours)• all other robots (mainly black coloured)• the field surrounding walls (mainly white coloured)These robots avoid collisions, by perceiving as uncollidable items all the black and/orwhite items. This is the way they avoid walls (mainly white) and other robots (mainlyblack). The white lines on the green field are ignored because what the robot sees is not"mainly white" due to the slim thickness of the lines.WORLD MODELAt any given time, these robots are aware of the ball (knowing its direction) and bothgoals direction. Robots on the field are also seen but not distinguished whether theybelong to own or opponent team. This information is kept in the form of a directionvariable, and when required they move towards it, updating that direction variable at everyframe captured. These robots do not memorise anything else.It is important to point out that these robots do not know precisely where they are on thefield at each moment, but that is irrelevant according to the way they play football. Theyonly need to know the other entities direction (ball, goals, etc.) from their actual position.This way, complex systems of triangulation or similar techniques are avoided (bothhardware and software) simplifying the whole solution.COMMUNICATIONUntil now, this team did not use any communication between the robots. However,wireless network boards are now being implemented in order to achieve better resultsduring a game. The network boards will allow faster programming, debugging and alsoknowledge on the field (variables of the game) which can be shared with the other robotsof the team.Not a lot of information needs to be sent, just the state of a few variables, like instructionsabout actual state (a position, or a decision) or what to do next, and these messages havedifferent levels of importance. Each robot can communicate with one other particularrobot or with all of them at the same time. Being so, a complete confusion could begenerated and therefore different levels of importance are used. This level depends on theowner position, or distance to the ball, goals, opponent robots, etc. The robotscommunicate only when it is needed (not all the time) in order to keep the radioenvironment free for urgent messages to pass through.Since the goal keeper is most of the time still on the field, he can be used to send to allother robots important information the like the position of the opponent robot players orthe direction of the ball, or even if he is alone at the goal and need help (just call all theother robots). Once again, the information have different degrees of importance.SKILLSIn order to drive the ball, these robots use an arc shape controller with a re-entrance of7cm (maximum allowed by the rules). This way, ball control is achieved just by pushingit, although a sudden change of direction might mean loosing the ball. These suddenchanges of direction are avoided by the robots software by following longer and widertrajectories.These robots intercept the ball very easy. When they see the ball, they just go towards it,avoiding collision with the opponent robot, but insisting and never giving up, until theopponent robot looses the ball. Once they have the ball, they move towards the opponentgoal dribbling the opponents (and avoiding collisions). In case they loose the ball,instantaneously start the procedure "following ball" again.When owning the ball near the opponent goal, these robots do not kick the ball. They runinto the goal pushing the ball with their body. This means a disadvantage since mostgoalies are very good and attack the ball sufficiently fast to avoid scoring.This team’s goalie is different from the other players only what concerns the direction ofthe wheels. These are rotated 90 degrees in order to be fast defending the goal rather thanmoving towards the front. The goalie software is very simple and consists of looking andobserving the ball all the time. That is possible with the convex mirror. It then movessideways in order to keep its body always in the ball’s direction no matter how distant thisis. When the ball approaches, the goalie kicks the ball with its arc rotating its body, doinga movement like a tennis player with its racket. This movement is very beautiful andimproves the quality of the game and the emotion when watching. This technique not onlyavoids a goal but also kicks the ball far away from its goal.STRATEGY OF THE GAMESince these robots always have an eye on the ball, their reaction is very simple andefficient. When they don't have the ball, they go towards it and don't give up until they getthe ball. Once they have the ball they go towards the opponent goal in order to score, andavoiding obstacles. If, for some rare reason they don't see the ball, they start moving in aspiral until they see the ball again (avoiding the walls, of course).As the only sensor available for the game, the vision is of extreme importance. The gamestrategy consists of a set of rules and contains the robot’s “personality and behaviour”.The game strategy needs some variables, which are given by the image processing. Fromeach image the following variables are extracted.BD - indicates if the ball is seen by the robotBP - indicates if the ball is near the robotBH - indicates if the ball is in the ball holderBAD - indicates if the opposite goal is seenBAP - indicates if the robot is in front of the opposite goalBND - indicates if the own goal is seenBNP - indicates if the robot is in front of the own goalSome others variables are also extract but not as relevant as these ones. If the maximum ofa pre-determined colour is inside one of the defined squares, that means that variable is on.Five (or more, depending on the configuration used) white rectangles around the cameraare used to detect collision. Should one of the “sensors” be mostly filled with the white orblack colours or pixels (normally these should be green) then it means that an obstacle isin that location. In that case, the obstacle avoidance routines automatically starts off,making it high priority moving the robot away from that place and therefore avoiding theobstacle no matter it is another robot or a wall. The white lines drawn on the green filedwill not be considered an obstacle since they will never fill in one of the rectangles (theyare too narrow). Figure 4 shows an example of an image seen by the robot with all thesquares (or rectangles) superimposed. Arrows point to coloured rectangles or maximum ofsome colours.Figure 4 – Variables captures from the image seen by the robotWith all these variables, the robot strategy simply consists of a Grafcet , which sets up therobot's aim as well as the dynamics moves towards a specific target, such as the ball or thegoal.In all, the control used by this robot has three levels:• Level 3 (Grafcet) ⇒ This is responsible for the robot’s global behaviour. Thislevel sets up the targets and sequences to reach the final aim.• Level 2 (dynamics) ⇒ This level is responsible for the immediate behaviour, i.e.,the control of movements, based on the aims established by level 3.• Level 1 (Controller PI) ⇒ This level serves directly the motors and is responsible toperform the plan established by level 2. At this level it is possible to extend theperformance of the superior levels.Figure 5 – Levels of ControlSPECIAL TEAM FEATURESThe image processing was the most important aspect of this team. It proved to be veryconsistent, very fast and original. All the video drivers were re-written by the team inassembly language in order to take the most out of the video board. This way, 50 frameswere achieved making the rest of the control program an easy task. All the imageprocessing routines were written in assembly language in order to increase speed and itproved to be very necessary. The general control program and strategy was written in Clanguage since it did not need to be extremely fast. Sometimes, in two consecutive cyclesthe same image was analysed (giving the same result as the previous cycle), proving that itwas not necessary to have a so fast machine.The mirror technique also proved to be very efficient since everything can be seen at alltime. With everything on sight, it is much easier to make the flux control program.CONCLUSIONSAs main conclusions it can be said that the image processing developed and used by thisteam is the most important characteristic. It is very reliable, consistent, fast and original.The robot movements are very smooth and acceptably unpredictable unlike a typicalalgorithm with known steps used by many teams.All the hardware in the robots was designed, developed and built by undergraduatestudents at the University of Minho. Only four robots were built due to lack of budget (nospare robots). These 4 robots were designed, built, programmed and tested by threeindustrial electronics students only. By participating on RoboCup’1999 the team learned alot and gained experience in competing. The main improvements needed by these robotsconsist mainly in improving the mechanics reliability.REFERENCES• Sérgio Sampaio, Carlos Machado, Ilídio Costa e António Ribeiro, “PortugueseRobotic Football Team, From Guimarães” included in the book "RoboCup-99: RobotSoccer World Cup III" by Springer.• "The dynamic approach to autonomous robotics", E. Bicho, G. Schoner, P. Mallet,Journe Robotique, CNRS, Marseille, France, June 1997.• "The dynamic approach to autonomous robotics demonstrated on a low-level vehicleplatform", E. Bicho, G. Schoner, Robotics and Automation systems 21(1997) 23-25.

Image processing applied to a robotic football team

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Image processing applied to a robotic football team

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