Hello everyone ! I'm new in using Ros and Gazebo , and am having a problem with Gazebo . I am using ros kinetic on ubuntu 16.04 with gazebo 9 . When i launch the gazebo.launch the robot's joints rotates randomly and i don't know why ! I tried to add controllers and the problem remains.

here is a video of the simulation : https://drive.google.com/file/d/1LHv4miu5e90Jc_ocfpDuILIPT1i2Ep1v/view?usp=sharing

Is there any solution to this problem ? Thank you !