sw2urdf - modeling parallelogram?

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Unfortunately, the URDF specification itself does not allow for kinematic structures with cycles, like a parallelagram. It does support the integration of a "mimic" joint, where one joint can mimic another and I have used that successfully for a Luxo lamp style paralellagram mechanism. (https://www.luxo-lighting.com/).

To add a mimic joint, add the two non-driven joints as children of the base link of your parallelagram, and then after clicking the configure and export, select those joints from the joint configuration and page and setup their mimic properties. You can add an offset and a scale if they don't move exactly 1-to-1 with the joint they are mimicking.