Adding or increasing odom drift in Gazebo

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Depends on what you mean with drift. If it is from slip of a mecanum based robot, you could try to model all the rollers of the wheels and define friction coefficients to them. If it is from encoders, you could try to subscribe to the joint state topic and calculate your own odom based on the returned joint states velocity and add a noise in this process. Either way it is best to not use the ros plugin that handles only cmd_vel inputs and odom outputs if you want to create a model that simulates a certain kind of behavior.