No devices connected.... waiting for devices to be connected

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Hi, I was following the tutorials on learn.turtlebot.com . Everything has been fine until I started the section Create a Map Via Teleoperation ( http://learn.turtlebot.com/2015/02/01... ). I was asked to run: roslaunch turtlebot_bringup minimal.launch, which turned out OK, and run: roslaunch turtlebot_navigation gmapping_demo.launch, which the above error message start popping up: No devices connected.... waiting for devices to be connected. Can anyone help solve this issue? Thanks!