Load a mujoco xml file

[raisimtech]

Load a mujoco xml file

[jhwangbo]

I'll put this one on my to-do list.

[ashish-kmr]

+1. Is there an estimate of when will this happen, as in how high up in the list is it? Would be great to have this feature!

[jhwangbo]

I'll finish it by the end of this week

[ashish-kmr]

Following up to see if there is an update on this. Thanks!

[jhwangbo]

I was stuck because there are big differences in how we set up the world. Do you need to create the world or just the robot? I think it will be much easier if I just use the mujoco xml as a robot definition.

[ashishkumar1993]

I just need to load the cassie xml robot file.

[jhwangbo]

is that XML file public?

[ashishkumar1993]

https://github.com/osudrl/cassie-mujoco-sim/blob/master/model/cassie.xml

[jhwangbo]

great, this will help

[jhwangbo]

just an update on this

My first implementation is done. I still have to test it with more models and debug it.

[ashish-kmr]

Thank you!

Although there is a urdf for cassie as well, it does not support passive joints like XML in mujoco does. Joints q5 and q6 of each leg in cassie is passive and not actuated. This is the primary reason which is forcing me to switch to XML.

[jhwangbo]

What is a passive joint? Is that a kinematic loop or unactuated joint?

[ashish-kmr]

Eq. 3 of this paper describes it. pasting it here for convenience:

"The joints (q1, q2, q3, q4, q7) are actuated. The joints (q5, q6) are passive, being formed by the introduction of (rather stiff) springs. When the springs are uncompressed, q5 = 0, and q6 = −q4 + 13°"

In the XML file, these are the <left/right>-knee-spring and the <left/right>-heel-spring in mujoco XML file. Below is an image showing all the joints of cassie and the springs.

[YandongJi]

just an update on this

My first implementation is done. I still have to test it with more models and debug it.

Has this implementation pushed to the repo? I'm trying this. :)

[jhwangbo]

not yet. It can load simple ones now but I need more work

[jhwangbo]

I pushed the feature. It is very first time offering this feature so I probably have to iterate it multiple times. let me know how it works for you

[ashish-kmr]

Thanks! Im trying it out right now and will let you know. One issue I noticed --

The latest push contains absolute paths here, here and here. This is causing cmake to fail. It works once these I fixed these on my end.

I'm also unable to use raisimUnityOpenGL (raisimUnity works fine though, but it is a bit slow on my machine).

[ashish-kmr]

The mjcf examples load the xml file as an argument while constructing the raisim::World. I'm presently using raisimGymTorch, in which the Environment.hpp loads an articulated object with
addArticulatedSystem(resourceDir_+"/anymal/urdf/anymal.urdf");. Is there an equivalent call to load an xml file inside the Environment.hpp? I was looking through the api couldn't find it.

[jhwangbo]

hard coded path + raisim unity opengl issues fixed. I moved the world instantiation to the environment class constructor. You can provide the XML file when you create the world.

[ashish-kmr]

Thanks! I tried the updated version and here is where Im stuck -- Cassie has 10 actuated joints whereas gcDim is 35. This is also the dimension that the setPdTarget expects, but I'm not how to move the 10 actuated joints only.

[jhwangbo]

that part is not in the mjcf file. The cassie repo has many c++ files and I guess you can figure out how they simulated cassie there

[ashish-kmr]

I was looking through the cassie xml file and I found that line 230-242 list the actuators, and line 244-267 list the sensors. Although, I'm not sure how easy/hard it is to incorporate this in raisim.

[jhwangbo]

RaiSim does not have a concept of actuators and sensors. You can always set the torques for all joints and sense the positions of all joints. For the cassie model, the spring is not connected in the xml. I am not sure how they handle this.

[ashish-kmr]

Mujoco handles this through constraints written in Lines 223- 228. Instead of an actual spring, they enforce these constraints which mimic a very stiff spring.

[jhwangbo]

Currently, constraints live in World, not in an articulated system. Let me think about how to implement this nicely...

[YandongJi]

Thank you again for your work! But it seems that cassie's model is still not working.

[jhwangbo]

I will push it with the next version. This is not just reading the MJCF file. I need to integrate equality constraints to ArticulatedSystem.

[christsa]

Hi

The mjcf-file reader seems to put all frames except for the root at the same location (see attached image). I've tested it across different files and it occurs with all of them.

[jhwangbo]

@christsa I already fixed this issue and will be included in the next push.

[jhwangbo]

I also added equality constraints for mjcf files. It is not well tested so let me know how it performs.