Multi robot position_controller problem

Hello everybody, I am trying to spawn several robot which use ros_control. I have a problem with my position controllers : > [INFO] [WallTime: 1454516624.223580] [1.800000] Loading controller: left_arm_position_controller [ERROR] [1454516624.266122670, 1.839000000]: Could not find parameter robot_description on parameter server [ERROR] [1454516624.266369289, 1.839000000]: Failed to parse urdf file [ERROR] [1454516624.266397313, 1.839000000]: Failed to initialize the controller [ERROR] [1454516624.266420609, 1.839000000]: Initializing controller 'left_arm_position_controller' failed [ERROR] [WallTime: 1454516625.272021] [2.753000] Failed to load left_arm_position_controller But I have two velocity controllers (defined in the same files) which are working fine. I have a launch file that include my robot launch file : main.launch The gazebo_env.launch is in charge of the gazebo environment. In the robot launch, I don't use any namespace in the node (they are already launch in a group). There is the spawn node and controllers node in the robot launch file : Is there a bug with the position controllers ? Thanks in advance