How to get started with an obstacle avoidance robot using sharp IR sensors?

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If you just use IR sensors and an Arduino, you´ll probably be fine without using ROS for your system. If you want to exchange data with a ROS system you can directly follow the rosserial arduino tutorials, like the Hello World tutorial that shows how to publish from Arduino to the ROS network. If you have any specific questions regarding this, you should edit your question with more details on your problem.