The joints rotate faster than their limits in Gazebo

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Hi there. I write a model, consisting of several links and joints. The joints are revolute and I set the limit of one joint as:

<limit effort="1000.0" lower="-1.57" upper="1.57" velocity="0.5"/>

The "lower" and "upper" work well in Gazebo, however, the angular velocity sometimes is larger than 0.5 rad/s. To explain this, I set the rate as 20 Hz, than in each step the change of angle should be less than or equal to 0.5/20=0.025 rad/s. But sometimes it is larger than 0.5/20 in one step.

I appreciate your help. Thanks.