My robot is moving by itself

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Hello,

I am currently working on a project where I am using some pretty complex equations to move my robots. However, I can see (even when im not applying any velocity, nothing) that my robot pose on rot.z is changing by itself (little values, but I want it to be = 0 if there is no reason not to be).

I don't understand why, my robot is a Pioneer3AT, I used the official robot on wich I added an hokuyo laser at it's top. Where does this problem come from ? I think it's from a setting in the .world file but I don't know what it could be.

Here is my file : (sorry it's very messy) world.world

thank you in advance guys, it's for my internship it's pretty important.