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There is no easy way to model this in URDF. The problem is that you only have one independent joint here, the one that connects the crank to the fixed frame. The rest of the angles (crank to lever, lever to block) are dependent on the first angle.

There are no set ways to control dependent joints. There are some nodes that support use of a mimic tag, although the mimic tag is not well documented. However, my understanding is that it only supports offsets and multipliers, not trigonometric functions, and thus may not work for you.

If you are only looking to visualize this mechanism, you can manually publish the joint angles yourself. However, if you want to simulate it in Gazebo or do any number of other operations, it can get a bit trickier.