[Ask] Computed torque control gain differences between inner MuJoCo and external control ??

[kshfire]

Intro

Hi!

I am a researcher) at a Company, I use MuJoCo for my robot control work.
I need to understand the MuJoCo more deeply, but I'd like to hear MuJoCo user's opinion about a phenomenon which I've experienced. (similar to this issue content : #1475 )

My setup

I'm applying the CTC (Computed torque control) for a 6 DOF robot manipulator.

• control rate : 1 kHz
• C++ MuJoCo
• Robot's actuators are composed of torque motors (gear ratio: all about 100)
• Using mjcb_control callback for control

There are my own process (robot control process) and MuJoCo simulation process. Two processes are communicated with LCM library (almost 1 kHz speed (not sure yet))

I've implemented CTC method in two cases : using MuJoCo APIs and my own logics in the robot control process.

I've applied same robot kinematics and dynamics parameters. Comparing to the inertial matrix, my own case and MuJoCo's mj_fullM showed similar results (Why it is not equal ??? one question point : someday I may need to analyze MuJoCo's RNE logic)

My question

Surprisingly, CTC based on inner MuJoCo is quite robust and shows good position control performance.

• Interesting point : PD gains of CTC are quite large (in my opinion)
  Kp : 100000 Kd : 2 * sqrt(Kp)
  Anyway, it works well. very robust and high performance.

By the way, my own control process (using LCM : receiving actual state of simulated robot and send control torque to the simulator) : when I applied same PD gains and send the control torque to the simulator, the simulated robot became crazy and unstable.
allowable Kp : about 300
(※ inertia matrix is similar (not equal), but gravity torque is almost same..... Why control gains are quite different... 100000 is too big but it works well... @.@)

Although the system structure is the same except for the IPC (inter-process communication : using LCM), it is interesting to me why such a big difference occurs.

I'm still a beginner at the MuJoCo and this is one of my points of interest about the MuJoCo.

I'd appreciate your feedback or opinions on this. (I want to find out what the cause is someday~~)

• Any opinion is Ok (ex. your similar experiences, etc)

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

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