Seite - 197 - in Proceedings - OAGM & ARW Joint Workshop 2016 on "Computer Vision and Robotics“

5. RealCar ATamiyaRC-racecar in thescale1:10 isusedasbase framefor theackermannrobot, seeFigure3a. The vehicle is powered by a BLDC (Brushless Direct Current) motor, and a servo motor is used for steering. Since thepositioncanbederived frominternalhall sensorswithin theBLDCmotor there is noneed foradditional encoders. AnArduinoUnomicrocontroller isusedbecauseofitsreal timecapabilityanditsspecialhardwarefor suchlowlevelactorsandsensors. Serialmessagesfromtheinterfaceare themeansofcommunication between the Arduino Uno and the Raspberry Pi. In this project, Raspbian is the operating system for the Raspberry Pi because it is based on Debian, which supports ROS. A W-LAN stick is installed on theRaspberryPi togrant access fromotherworkstations. The Sensor Level CPU which is represented by the Arduino Uno is responsible for controlling the car, reading sensors and presenting the data in a useful way. A motion controller [5] is implemented for the BLDC motor. Three signals similar to sinus waves generated with pulse width modulation on the Arduino Uno are applied to the motor. The calculation of the pose and its covariance also takes place on the Arduino Uno based on the velocity motion model mentioned before. The calculation frequency is about 100Hzwhich results in an update rate of 0.01s. The controlling structure of the vehicle is shown inFigure4. RaspberryPi ROS ArduinoUno Controller BLDC Steering ServoSerial USB Figure4: Thecontrolhierarchy fromROSto thecars actuators. Since thesteeringappears tobe theprimarysourceofuncertainty, two improvementsareconsidered. The first is to replace the unsteady steering with a more stable one. The second is to upgrade the car withanencoder for the steering. 6. SimulatedCar Validation of systems and algorithms is an important task in mobile robotics. Thus, Gazebo is used for visualisation and physical simulation of the robot. The simulation contains the parts which are vital for the robots motion. They are imported to Gazebo with a URDF (Unified Robot Description Format) file, see Figure 3b. In the first attempt to simulate the ackermann drive robot, a link was created for each part of the steering and they were connected with joints. The parent-child structure of joints inURDFmakes it impossible tocreate suchaclosed loop, soaworkaroundwasneeded. To get anackermannsteering likebehavior, aROSplug-in isused tocontrol thekingpins, seeFigure 2. The plug-in calculates the angles for both front wheels and adjusts the kingpins accordingly. For these calculations, the knowledge of the wheelbase and the track is required. The curve radius of the imaginary third front wheel has to be calculated. It has to be considered that the radii of the left and the right frontwheeldifferbyahalf trackwidth fromthepreviouslycalculated radius. Basedon this, the steeringanglesϕL andϕR canbecalculated,using the trigonometric functions. 197