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Onshape to URDF Converter

K-Scale's toolkit for converting Onshape files to URDFs.

See the source code for this repository here .

Getting Started

Dependencies

The onshape package requires Python 3.11 or greater.

Installation

onshape can be installed through pip using the following command:

pip install onshape  # To install the vanilla version
pip install 'onshape[all]'  # To install with all dependencies

Verify that the package was installed correctly by checking that the onshape CLI is available:

$ onshape
usage: onshape {run,download,postprocess,pybullet,mujoco}
onshape: error: the following arguments are required: subcommand

Next, you should retrieve an Onshape Access Key and Secret Key from here . Set the required environment variables like so:

export ONSHAPE_ACCESS_KEY='XXXXXXXXXXXXXXXXXXXXXXXX'
export ONSHAPE_SECRET_KEY='YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY'

Usage

The Onshape CLI has the following components:

  • download - takes a link to an Onshape assembly and downloads it as a URDF
  • postprocess - takes a path to a downloaded URDF and applies postprocessing to make it useful
  • run - Combines download and postprocess into a single script
  • pybullet - Opens a Pybullet visualization of a generated URDF file, for debugging purposes
  • mujoco - Opens a Mujoco visualization of a generated MJCF file, for debugging purposes

Download

To download a URDF file, use the following command:

$ onshape download --help
usage: onshape [-h] [-o OUTPUT_DIR] [-f CONFIG_PATH] [-n] document_url

positional arguments:
  document_url          The URL of the OnShape document.

options:
  -h, --help            show this help message and exit
  -o OUTPUT_DIR, --output-dir OUTPUT_DIR
                        The output directory.
  -f CONFIG_PATH, --config-path CONFIG_PATH
                        The path to the config file.
  -n, --no-cache        Disables caching.

You can override configuration options from the command line by passing the values using omegaconf syntax or by providing a configuration file.

Postprocess

To postprocess a downloaded URDF file, use the following command:

$ onshape postprocess --help
usage: onshape [-h] [-f CONFIG_PATH] urdf_path

positional arguments:
  urdf_path             The path to the downloaded URDF.

options:
  -h, --help            show this help message and exit
  -f CONFIG_PATH, --config-path CONFIG_PATH
                        The path to the config file.

You can override the postprocess configuration arguments in the same way.

Config File Reference

To view all the available configuration options, see this file .

Ignore Welding Joints

By default, fixture joints on the same body gets combined into one body. To avoid this you can set the following params:

ignore_merging_fixed_joints:
  - "imu_link"

or

merge_fixed_joints: false

Examples

Full Example with Robot
default_revolute_joint_effort: 17
default_revolute_joint_velocity: 360
suffix_to_joint_effort:
  "_00": 14
  "_02": 17
  "_03": 60
  "_04": 120
suffix_to_joint_velocity:
  "_00": 12.566
  "_02": 12.566
  "_03": 6.283
  "_04": 6.283
convex_collision_meshes: true

ignore_merging_fixed_joints:
  - "imu_link"

rotate_joints:
  imu_link: [-90.0, 0.0, 0.0]

max_mesh_triangles: 10000
max_convex_collision_mesh_triangles: 100

exclude_collision_meshes:
  # Shoulder yoke stops.
  - "YOKE_STOP_INNER"
  - "YOKE_STOP_INNER_2"
  # Hip yoke stops.
  - "KB_D_102L_L_Hip_Yoke_Drive"
  - "KB_D_102R_R_Hip_Yoke_Drive"
  # Upper arms
  - "L_Bicep_Lower_Drive"
  - "R_Bicep_Lower_Drive"
  # Forearms
  - "L_Forearm_Upper_Drive"
  - "R_Forearm_Upper_Structural"
  # Knee
  - "KB_D_301R_R_Femur_Lower_Drive"
  - "KB_D_301L_L_Femur_Lower_Drive"
  # IMU
  - "imu"

shrink_collision_meshes:
  # Leg roll
  "RS03_4": 0.8
  "RS03_3": 0.8
  # Ankle
  "KB_D_401R_R_Shin_Drive": 0.85
  "KB_D_402L_L_Shin_Idle": 0.85
  # Wrist
  "KB_C_501X_Bayonet_Adapter_Hard_Stop": 0.8
  "KB_C_501X_Bayonet_Adapter_Hard_Stop_2": 0.8
  # Base
  "KB_B_102B_TORSO_BOTTOM": 0.8

move_collision_meshes:
  "R_Bicep_Lower_Drive": [0.0, 0.0, -0.01]
  "L_Bicep_Lower_Drive": [0.0, 0.0, -0.01]

joint_separation_distance: 0.001

base_rpy:
  - 0
  - 0
  - 4.7123889803 # So that x is forward in mujoco

flip_joints:
  - "left_knee_04"
  - "left_shoulder_roll_03"
  - "right_shoulder_roll_03"
  - "left_hip_roll_03"
  - "right_hip_roll_03"
  - "left_ankle_02"

mjcf_metadata:
  joint_params:
    - name: "motor_00"
      armature: 0.0005
      frictionloss: 0.1
      actuatorfrc: 14
      suffixes: ["_00"]
    - name: "motor_02"
      armature: 0.002
      frictionloss: 0.1
      actuatorfrc: 17
      suffixes: ["_02"]
    - name: "motor_03"
      armature: 0.005
      frictionloss: 0.3
      actuatorfrc: 60
      suffixes: ["_03"]
    - name: "motor_04"
      armature: 0.007
      frictionloss: 0.1
      actuatorfrc: 120
      suffixes: ["_04"]

  # imus:
  #   - body_name: "imu"
  #     acc_noise: 0.01
  #     gyro_noise: 0.01
  #     mag_noise: 0.05
# This is for debugging purposes.
# use_collision_meshes_as_visual_meshes: true

CAD Considerations

onshape currently only supportsFixture mates and Revolute joints.

  • You must set joint limits for all Revolute joints
    • Pay attention to orientation of the axes and the 0-position
    • Onshape provides a dropdown action "reset" on all mates to return to 0-position
  • Ensure that there are no redundant mates in your assembly, as they will interfere with generating a sensible graph structure for the URDF

Tips and Tricks

We suggest first downloading your URDF file using the download command, then checking to make sure that it looks correct. Once this works, you can then postprocess the resulting URDF to clean up the meshes.

Note that 500 errors or other errors that happen during the initial download phase can sometimes be fixed just by rerunning the command (in our experience, the Onshape API can be a bit flaky).

The Onshape tool includes quite a few checks to make sure that the robot model can be converted to a valid URDF, although sometimes these checks can be overly restrictive to prevent unintended side effects. If you encounter an issue, the error message should be relatively informative - if it is not, please file an issue on our Github page.

FAQ

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Additional Resources

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Updated about 2 months ago