Registering a New Custom SDF

[vmstavens]

Hi MuJoCo community,

I'm a research assistant and I'm trying to use MuJoCo for assembly tasks. Specifically, I want to create non-convex collision meshes to facilitate inserting let's say a USB cable into its female connector. My current understanding is that this can be done using SDF plugins in MuJoCo. While and read and I believe understood the current documentation I seem to be encountering a small road block in registering my custom plugin.

My current progress is as follows:

1. I am unable to locate the sdf/ plugin directory on my local machine when MuJoCo was installed through pip I therefore

   • Cloned the MuJoCo repo form GitHub

   • Navigated to the sdf folder

   • added new .h and .cc files called test.h and test.cc. Both of these hold the same code as torus.cc and torus.h, except all instances of Torus was replaced with Test.

   • I navigated to register.cc and added my plugin

     Details

     modified `register.cc`

     ``` c

            #include "bolt.h"
            #include "bowl.h"
            #include "gear.h"
            #include "nut.h"
            #include "torus.h"
            #include "sdflib.h"
            #include "test.h"
            
            namespace mujoco::plugin::sdf {
            
            mjPLUGIN_LIB_INIT {
              Bolt::RegisterPlugin();
              Bowl::RegisterPlugin();
              Gear::RegisterPlugin();
              Nut::RegisterPlugin();
              Torus::RegisterPlugin();
              SdfLib::RegisterPlugin();
              // I added this line
              Test::RegisterPlugin();
            }
            
            }  // namespace mujoco::plugin::sdf
         ``` 
        </details>
     

   • and added the new files to the CMakeLists.txt file

     Details

     modified `CMakeLists.txt`

     ``` CMakeLists

                    ...
                    set(MUJOCO_SDF_SRCS
                        sdf.cc
                        sdf.h
                        bolt.cc
                        ...
                        sdflib.h
                        torus.cc
                        torus.h
                        # I added these lines
                        test.cc
                        test.h
                    )
                    ...
         ``` 
        </details>
     

   • Having done so, I navigate to the root directory of the repo and build and compile i.e. cmake -Bbuild && cd build && make. When compiling I get output like so:

     Details

     compile output

     ``` bash

                   ...
                   [ 30%] Building CXX object plugin/sdf/CMakeFiles/sdf.dir/nut.cc.o
                   [ 30%] Building CXX object plugin/sdf/CMakeFiles/sdf.dir/sdflib.cc.o
                   [ 30%] Building CXX object plugin/sdf/CMakeFiles/sdf.dir/torus.cc.o
                   [ 30%] Building CXX object plugin/sdf/CMakeFiles/sdf.dir/test.cc.o
                   [ 30%] Linking CXX shared library ../../lib/libsdf.so
                   [ 30%] Built target sdf
                   [ 33%] Built target glfw
                   [ 34%] Built target platform_ui_adapter
                   ...
        ``` 
       </details> 
     

   • which seems like it includes my newly added plugin

2. I now create a new mujoco project and where which include the following

MuJoCo Python code

``` python

    import time
    import mujoco
    import mujoco.viewer
    
    m = mujoco.MjModel.from_xml_path('/path/to/torus.xml')
    d = mujoco.MjData(m)
    
    with mujoco.viewer.launch_passive(m, d) as viewer:
      start = time.time()
      while viewer.is_running() and time.time():
        step_start = time.time()
        mujoco.mj_step(m, d)
    
        # Pick up changes to the physics state, apply perturbations, update options from GUI.
        viewer.sync()
    
        # Rudimentary time keeping, will drift relative to wall clock.
        time_until_next_step = m.opt.timestep - (time.time() - step_start)
        if time_until_next_step > 0:
          time.sleep(time_until_next_step)
``` 

3. With the `XML` file being shown below. Notice here that I am referencing the `mujoco.sdf.test` instead of `mujoco.sdf.torus`

`XML` file from the MuJoCo repo, but modified

``` XML

<mujoco>
  <extension>
    <!-- <plugin plugin="mujoco.sdf.torus"> -->
    <plugin plugin="mujoco.sdf.test">
      <instance name="torus">
        <config key="radius1" value="0.35"/>
        <config key="radius2" value="0.15"/>
      </instance>
    </plugin>
  </extension>

  <asset>
    <mesh name="torus">
      <plugin instance="torus"/>
    </mesh>
  </asset>

  <option sdf_iterations="10" sdf_initpoints="40"/>

  <include file="scene.xml"/>

  <default>
    <geom solref="0.01 1" solimp=".95 .99 .0001" friction="0.1"/>
  </default>

  <worldbody>
    <body pos="-1 0 3.8">
      <freejoint/>
      <geom type="sdf" mesh="torus" rgba=".2 .2 .8 1">
        <plugin instance="torus"/>
      </geom>
    </body>
    <body pos="-1 0 3.4">
      <freejoint/>
      <geom type="sdf" mesh="torus" rgba=".2 .8 .2 1">
        <plugin instance="torus"/>
      </geom>
    </body>
    <body pos="-1 0 3">
      <freejoint/>
      <geom type="sdf" mesh="torus" rgba=".8 .2 .2 1">
        <plugin instance="torus"/>
      </geom>
    </body>
    <body pos="0 0 2">
      <geom type="cylinder" size=".1" fromto="1 0 -1 -1 0 1"/>
    </body>
    <body pos="0 0 2">
      <geom type="cylinder" size=".5" fromto="1.2 0 -1.2 1 0 -1"/>
    </body>
    <light name="left" pos="-2 0 7" cutoff="80"/>
    <light name="right" pos="2 0 7" cutoff="80"/>
  </worldbody>
</mujoco>
``` 

4. When running this simulation I however get the following error:

Traceback (most recent call last):
  File "main.py", line 10, in <module>
    m = mujoco.MjModel.from_xml_path('/path/to/torus.xml')
ValueError: XML Error: unknown plugin 'mujoco.sdf.test'
Element 'plugin', line 5

All feedback and/or ideas for how I could progress/solve this problem of mine are very much appreciated!

[quagla]

Is MUJOCO_PATH pointing to the right mujoco, i.e. the one you built?

[tunewings97]

Hello, I am planning to use MuJoCo for reinforcement learning of robotic assembly tasks.

Have you successfully performed assembly tasks with MuJoCo? I am currently investigating whether MuJoCo is suitable enough for assembly applications.

I would greatly appreciate it if you could share your experience regarding the accuracy and stability of contact modeling in MuJoCo.