base local planner with dwa only send negative rotational velocity

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Hi guys, I'm using move_base/base_local_planner with dwa parameter "true", and also I set the parameters "max_vel_theta: 0.1" and "min_vel_theta: -0.1", although the theta velocity range is from positive to negative, my robot still only performed clockwise rotational while navigating. However, the robot performed clockwise and counter-clockwise which is normal when reaching the goal (in-place rotation). I don't know why the controller only sends the negative(clockwise) velocity command while navigating.