cassie-mujoco-sim

A simulation library for Agility Robotics’ Cassie robot using MuJoCo.

Developed and tested on Ubuntu 18.04. Also compiles for Windows using mingw-w64.

To build the shared library:

1. Install libglfw3-dev
2. Clone this repository
3. Download mjpro210 from roboti.us here
4. Put mjpro210 in a .mujoco directory in your home directory
5. make

To cross-compile for Windows, use make PLATFORM=WIN. (NOT TESTED)

A directory named release will be created containing the files needed to use the library with a C interface. A Python interface can be generated with make ctypes, which requires ctypeslib2 to be installed. Note that this is not needed as the ctypes Python wrapper is already provided in this repo.

This library also includes functions that can be used for communicating with Cassie over UDP, and MuJoCo is not required if only these functions are called.

To build and run the examples:

1. make test
2. Run the examples in the test directory

The examples include cassiesim, which simulates a physical Cassie robot controlled over UDP, and cassiectrl, a null controller operating over UDP. The file cassietest.c is a minimal example of running the simulation, and cassietest.py demonstrates controlling the simulated robot in Python.

Documentation for the simulation functions is included in include/cassiemujoco.h. The file header/udp.h declares convenience functions for creating a UDP socket and obtaining the most recent packet. The remaining header files include functions for packing and unpacking structures used to communicate with Cassie into UDP packets and for simulating Cassie’s internal safety mechanisms and state estimation processes.

Changes:

• Updated libraries to use mujoco 2.10, MuJoCo activation key is no longer required
• To make things cleaner when loading MuJoCo and GLFW libraries, we assume the mujoco210 folder is in the ~/.mujoco folder. This allows there to be only one copy of mujoco on your machine, rather than having to copy the mujoco folder to every place where libcassiemujoco.so is.

New Visualization Features

The F1 key will display a help screen that lists the most useful of the following functions.

• Mouse Interactivity:
  • The camera can be rotated by holding left mouse button and can be moved by holding the right mouse button.
  • Zoom the camera in and out using the scroll wheel.
  • To have the camera track a certain body, double control right-click on the desired body. To go back to free camera mode press the esc key.
  • Double left-click to select a body to apply a force on.
  • Once a body is selected, control right-click and drag to apply a force in a direction. The magnitude of the force is proportional to the distance from the cursor the body.
  • Control left-click allows you to apply a rotational force.
• Key Commands:
  • The cassie_vis_t struct now has a paused flag that can be toggled using the spacebar key. This allows to simulation to be paused and restarted. For an example of this, see cassietest.c.
  • The backspace key resets the model to the initial qpos.
  • When paused, the model can be simulated step by step using the right arrow key. To step forward in increments of 100, use the down arrow key.
  • The equal (‘=’) and minus (‘-’) keys can be used to increase and decrease the font size of the info screen respectively.
  • Ctrl+a will recenter the camera to the center of the platform
  • Ctrl+p will print the current qpos to the terminal
  • Ctrl+q will close the visualization window.
  • Toggle visualization flags:
    • ‘j’: Highlight the joint locations
    • ‘u’: Highlight the actuator locations
    • ‘n’: Highlight the constraint locations
    • ‘c’: Highlight points of contact
    • ‘f’: Show contact force arrows
    • ‘t’: Make the model transparent
    • ‘m’: Show the center of mass of the entire model
  • F4 will bring up and overlayed graph of the current ground reaction forces (z foot contact forces).
• Additional functions and changes:
  • The function cassie_sim_apply_force now takes in a char array of the body name to perturb rather than the body id for user convenience. So the user only has to input the string of body name (i.e. “cassie-pelvis”) rather than having to find out what integer body id corresponds to the desired body. The body names correspond to the body names defined in the .xml model file. These changes have been reflected in cassiemujoco.py and cassiemujoco_ctypes.py as well
  • There is a new cassie_vis_apply_force function that will apply and visualize a force. Note that cassie_sim_apply_force will still apply the force in the simulation, but when rendering the arrow showing the direction and magnitude of the force will not be shown. To visualize this arrow, you should use cassie_vis_apply_force. The force will be cleared out after the next call to cassie_vis_draw. So when using this function in python and dealing with varying execution and rendering frequencies, you can call cassie_vis_apply_force and then call cassie_sim_step_pd multiple times, all of which will be taken with the applied force. However, once cassie_vis_draw is called, subsequent simulations forward will not have the force applied (since the xfrc_applied field is set to zero in cassie_vis_draw). To continue applying the force, you must call cassie_vis_apply_force again.

Features to be added:

• Slowmotion functionality? (requires changes to cassiemujoco.py and using GLFW in python when executing policies)
• Add full mujoco 2.00 ui functionality?

Notes:

• Note that there is some small unavoidable memory leak when calling cassie_mujoco_init. I am not sure what causes this, it seems to come from the mj_loadXML when it makes the underlying C model (when mujoco internally calls mjCModel). This is for sure a problem that we cause as normal MuJoCo does not have any memory leaks, perhaps with the weird passing around of strings and structs that we do in our wrapper messes things up. I do not know how to fix it (though perhaps with MuJoCo becoming open source, looking at the source code once it becomes available might help). Regardless, this is mostly a non-issue as the size of the leak is small (2560 bytes) and it is a one time leak, as cassie_mujoco_init will only ever be called once unless you are manually initializing and then cleaning up MuJoCo multiple times for some reason. **UPDATE:**Updating to mujoco210 seems to have somehow fixed this lmao.

Citation

To cite this software package use the following format:
Agility Robotics. cassie-mujoco-sim. (2018) [Online]. Available: https://github.com/osudrl/cassie-mujoco-sim