Design-time considerations

Note

Try to make your robot assembly mostly based on sub pre-assembled components (avoid to have a lot of constraints that are not relevant for the export). In this main assembly, do not use features such as sub-assemblies network.

Specifying degrees of freedom

• Degree of freedoms should be slider, cylindrical or revolute mate connectors named dof_something, where something will be used to name the joint in the final document

  • If the mate connector is cylindrical or revolute, a revolute joint will be issued by default

    • To make a continuous joint, add continuous or wheel in the name of the joint. For instance, a revolute mate named dof_front_left_wheel will result in a continuous joint named front_left_wheel in the resulting URDF.

  • If the mate connector is a slider, a prismatic joint will be issued

  • If the mate connector is fastened, a fixed joint will be issued

• When doing this connection, click the children joint first. This will be used to find the trunk of the robot (part with children but no parent)

Inverting axis orientation

It is possible to invert the axis for convenience by adding _inv at the end of the name. For instance dof_head_pitch_inv will result in a joint named head_pitch having the axis inverted with the one from the OnShape assembly.

Naming links

If you create a mate connector and name it link_something, the link corresponding to the part on which it is attached will be named something in the resulting URDF.

Adding custom frames in your model

If you want to track some frames on your robot, you can do the following:

• Connect any part to your robot using mate relations in OnShape

• Name one of these relations frame_something, when something will be the name of the frame (a link) in the resulting sdf or urdf

If you want to give it a try, you can use the onshape-to-robot-bullet in urdf mode, it will output the frames on standard output.

Here is a link of a document that can be used as a frame (note: the center cube is 5mm side, so you might need 2.5mm offset to center it).

Closing frames

If your robot includes closing loop, those can’t be handled in the URDF file directly since the robot has to have a kinematics structure that is a tree.

However, it is possible to add multiple frames, and to make constraints out of them during your simulation.

If you add a relation with closing_something as name, two frames will be added to your URDF (closing_something_1 and closing_something_2) that will be attached to the two parts mated. It’s up to you to handle it further by adding proper constraints.

Joint frames

Joint frames are the ones you see in OnShape when you click on the joint in the tree on the left. Thus, they are always revolving around the z axis, or translating along the z axis, even if the _inv suffix is added.