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My suspicion is that the arm_controller is at fault here. I had a similar problem with the robot falling extremely slowly when I used PositionControllers for the two front wheels. In this case the front side of my robot was falling down slowly while the back side was sort of dangling since it was falling much faster. It seems to me that these types of controllers are trying to maintain the joint's absolute position in the world, so when you attempt to move the base, there is a lot of resistance.

Now I'm not exactly sure how the arm_controller works since I could not find a source for it. But if possible you might be better off trying to use a force or velocity controller on your arm.