Handling kinematic loops

Some robots have kinematic loops, meaning that the kinematic chain is not a tree but a graph.

Introduction

Here is a 2D planar robot with kinematic loop, we assume the two first joints to be actuated and the others to be passive:

However, robot description are usually trees. To model this type of robot, we break it down to a tree. We attach frames to this tree, and need to enforce run-time constraints.

Specifying closing constraints

While you could manually add frames, onshape-to-robot provides a more convenient way to handle kinematic loops: mate connectors.

To achieve that, add a mate with the name closing_something:

Support for <equality> in MuJoCo

When using the MuJoCo format, onshape-to-robot will add <equality> constraints to enforce the kinematic loop.

For example, the above robot can be exported using the following config.json:

{
    // Document URL, MuJoCo output
    "url": "https://cad.onshape.com/documents/04b05c47de7576f35c0e99b3/w/68041f3f5c827a258b40039c/e/db543f501b01adf8144064e3",
    "output_format": "mujoco",

    // Disable the freejoint to fix the robot
    "freejoint": false,

    // Don't create actuators for passive joints
    "joint_properties": {
        "passive1": {"actuated": false},
        "passive2": {"actuated": false}
    }
}

Here is the result of the export:

Ressources

Here are some ressources on how to handle kinematic loops in software:

• Onshape assembly for the above example robot.

• MuJoCo equality constraints.

• In pyBullet, you can use createConstraint method to add the relevant constraint.

• In the PlaCo solver, you can create a RelativePositionTask. See the kinematics loop documentation section for more details. Some examples created with onshape-to-robot can be found in the example gallery.