I use gazebo7 with ubuntu16.04 to simulate a four wheel robot. The robot was controlled by a model plugin with PID. The problem is that the wheel velocity (radian per second) oscillates, see the attachment pictureC:\fakepath\wheel velocity.jpg. The sdf file C:\fakepath\model.sdfand plugin C:\fakepath\topymodel_push.cppis also in the attachments.
Thank you in advance.
Based on the image you shared of (I assume) wheel velocity vs time, you should first try tuning your PID gains to get rid of the oscillations.
See:
That's the easiest thing to try before exploring other possible fixes.
josephcoombe: thank you for your answer. I have tried to tuning PID gains. The problem still exists. When the angular velocity is set to 0.1 rad/s, it oscillates between 0.8~1.2 rad/s. When set to 5.0 rad/s, it oscillates between 4.97~5.03 rad/s (sometimes there is a peak 4.8, or 5.2...). Body is about 40kg while four wheels are 1kg separately. I wonder, does this matter? Do I need to set damp of joint, Maxforce or some other parameter?
You could try adding a small joint damping value (and enabling ImplicitSpringDamper). The behavior you are observing appears to be a classic controls problem. You can tune P, I, and D gains to eliminate oscillations, but you also have the added disturbance input of the interactions with ground surface.
After that, your main option is to implement a higher-level controller (e.g. PID) to compensate for drift away from an intended heading/trajectory. In the real world, you are going to observe drift (try driving down a straight road in a car without touching steering wheel). In your car, you are the high-level controller.
josephcoombe: thank you for your reply. When i change the <collision> of each link from my own **.stl to box or cylinder, the oscillation is removed. My **.stl is exported from simple solidworks parts. I don't know why this happen. Another phenomenon is that when I decrease <max_step_size> (0.001 --> 0.0001), the simulation performance will improve a lot. Why?
That's interesting. In that case, the oscillation may be caused b/c the .stl (e.g. for wheels) is not as smooth as cylinder geometric primitive - so it's like riding a car with octagon wheels. Decreasing the max_step_size lets the engine better handles constraint violations in a more numerically stable manner, resulting in (theoretically) closer to ideal behavior.
josephcoombe: thank you for your reply. Recently I found that the oscillation is caused by fixed joint, as I asked here(http://answers.gazebosim.org/question/19359/does-fixed-joint-have-a-bug-in-sdf/). The <collision> we talked before doesn't influence the oscillation.
Yeah that's really strange. You might also open a issue in the Gazebo repo: https://bitbucket.org/osrf/gazebo/src
Asked: 2018-05-01 00:27:10 -0500
Seen: 885 times
Last updated: May 03 '18
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