Design and Construction of a Soccer Player Robot ARVAND

Arvand is a robot specially designed and constructed for playing soccer according to RoboCup rules and regulations for the medium size robots This robot consists of three main parts mechanics motion and kicker hardware image acquisition processing and control unit and software image process ing motion control and decision making The motion mechanism is based on a drive unit a steer unit and a castor wheel Robot machine vision system uses a CCD camera and a frame grabber Two microcontroller based boards are specially designed for carrying out the software system decisions and transfer ring them to the robot mechanics The software system can perform real time image processing and object recognition It implements algorithms for playing soccer These algorithms are written in a high level language ArvandLan specially designed for mobilizing these robots By changing the algorithms our robots can play as goal keeper attacker or defender We have constructed such robots and successfully tested them in a soccer eld de ned accoring to RoboCup regulations Mechanics Architecture In the following we describe the steps of design and construction of our special purpose robot for soccer playing According to the motion complexity of a soccer player robot proper design of its mechanics can play a unique role in simplifying the playing algorithms For attaining this goal there is a need to a mechanism that can easily and with the most accuracy provide the robot with its motion demands In this regard after performing several experiments on the motion mechanisms of mobile robots a speci c mechanism was designed and implemented that together with the sensors and control feedbacks to a good extent veri ed our expectance The rst experiment in producing Arvand was making a model car chassis that did not provide us with expected motion capabilities such as dribbling penalty kick intercepting and preserving the ball For this reason the mechanism described below was designed and implemented Motion Mechanism Arvand consists of two motion units in front of the robot and one castor wheel in the rear Each motion unit has a drive unit and a steer unit The functionality of drive unit is moving the robot and that of steer unit is rotating the drive unit round the vertical axis of its wheel The drive unit consists of a wheel which is moved by a DC motor and a gearbox of ratio The steer unit uses a DC motor and a gearbox of ratio Drive unit and steer unit use the same kind of DC motors For controlling the steer unit the optical encoders are mounted on the respective motor shaft and their resolutions are such that one pulse represents degrees of drive unit rotation The castor wheel consists of a spherical ball that roles in an special purpose ball bearing By this structure Arvand can move in any direction freely This mechanism has the following capabilities By rotating the drive unit round its vertical axis the rotation center of the robot changes accordingly and this allows the robot to turn around any point in the plain This point can be selected inside or outside the robot It is necessary to adjust the speed of two drive units according to the following formula v r v r In the above formula v is speed of the left drive motor v is speed of the right drive motor r is the distance of the left drive unit from the rotation center and r is the distance of the right drive unit from the rotation center Therefore the robot rotation center will not depend on the robot gravity center and on the position of drive units in the robot For instance if we consider the center of ball the rotation center the robot is able to turn around this center point in a way that it does not lose its sight of the ball and make the appropriate direction according to the opponent team goal position One of the advantages of our design is the possibility of making the robot rotate around its geometrical center This kind of rotation is done in a minimum amount of area which reduces the chance of accidental bump to wall or other robots In our software system we can set the drive units to be parallel to each other and also have a speci c angle related to robot front This mechanism is useful for taking out the ball when stuck in a wall corner and also dribbling other robots Kicker Mechanism Appropriate use of a kicker in robot plays an important role in team play alogorithms and individual technics Therefore we have designed a kicker with controllable kicking power For instance it can be applied to passing in the team play We tested several kickers using a motor unit and also a solenoid The solenoid was selected because of its e eciency in power usage The kicker consists of a solenoid and a simple crowbar connected to a kicking arm The kicking power is controlled by duration of DC voltage applied to it Hardware Architecture The goal of our hardware architecture is to have a kind of hardware control on the robot that be independent of software system as much as possible and also reduce the robots mechanical errors Arvand hardware system consists of three principal subsystems which are de scribed in the following The Image Acquisition Unit The image acquisition unit output is a digitized color RGB image For the rst ex perimental implementaion of Arvand a Connectix Color QuickCam was utilized It could transfer the digitized captured image via the parallel port Because of its low resolution and low capture speed a faster imaging system was needed Conse quently we chose a PixelView CL GD XP capture card which providesArvand with images having a resolution of x with the frame rate of frames per second The camera which we use is a Topica PAL color CCD camera with a mm lens The PixelView card can be utilized under Linux Windows and Dos The Processing Unit The robot main processing unit consists of an Intel Pentium MMX together with a main board and MB RAM There are two serial ports onboard that are used as communication means with the control unit A oppy disk drive is installed on the robot from which the system boots and runs the programs Because of power supply problems and also hit sensitivity a hard disk drive could not be applied The Control Unit The control unit has been designed such that it can sense the robot can inform the processing unit of the status and also ful ll the processing unit commands Because of reduction in the number of wires which increases the robot robustness communication between the control unit and the processing unit is done via two serial ports with RS standard For more information about the PC serial port speci cations refer to The control unit consists of two similar Intel C microcontroller based boards which we have designed For further information about the Intel C microcontroller scpeci cations you can refer to One board is represented in gure As it is shown in the gure an Intel C microcontroller has been utilized to control the respective motors kicker encoder and limit switches There are three power ampli ers which amplify the pulses which are generated by the microcontroller to control the speed of the motors and to drive the solenoid The PWM pulse frequency is about kHz The ampli er output is applied to the respective motor or kicker As the robot size is limited drive units must not exceed the robot boundary limits Therefore two limit switches have been devised for each steer unit These two limit switches con ne the rotation domain of the drive unit by not allowing the power ampli er to apply any pulses to the steer motor in the respective directions when the drive unit has reached its limit The current drawn by each motor can be sampled by the microcontroller using an A D Convertor This gives the microcontroller the capability of realizing the motors status and informing the processing unit of the robot status As it is mentioned in the mechanics architecture section the rotation center point critically depends on the angle of the drive units For the accuracy of controlling the angle of a drive unit an encoder has been mounted directly on the respective motor shaft One of the microcontroller duties is counting the pulses generated by this encoder Each pulse represents degrees of the drive unit rotation Hence this architecture can provide the robot with the following capabilities Rotating around a point in the plain formula can be achieved by controlling speed of the drive units together with rotating them to an appropriate angle Controllable motor speed allows Arvand to move smoothly in the eld This augments the quality of Arvand individual technics For instance if Arvand can reduce its speed when it approaches a still ball in the eld the chance of gaining the ball increases In addition controllable kicking power can be useful for team play skills such as passing and stopping the ball It has been observed that a mobile robot may bump into barriers in its work eld It is an advantage for a robot to detect when it is stuck Arvand control unit detects when it is stuck and alerts the processing unit to make Power Ampli er PWM Direction