My robot collides with objects in Gazebo. How to solve it?

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I have a UR5 model with ROBOTIQ gripper on it. When I try to approach the table surface in a GAZEBO world, the gripper collides with the table surface, and it never stops. I tried to set the "min_depth" of the table and the robot, the gripper. set them to 0.01. But it will never stop. I think there should be a solution for this, otherwise the simulation is not realistic. Could anybody help me? Thank you very much in advance.