Why my control function is called more than one time per step?

[Hardwarize]

Intro

Hi!

I am a master student at UNED, I use MuJoCo for my research on generative human motion.

My setup

MuJoCo python

My question

I've found a behaviour, maybe due to my mistake, where the controller function callback is called several times per step. Why is this happening?

If I only load a model, as you can see in the attached script, the function seem to be called 2 times per step.
If I perform actions like setting the qpos or qvel the functions is called 4 times per step.

To identify this situation I print data.time in the controller function, this function is passed as argument to mj.set_mjcb_control an I get the following logs:

...
0.3300000000000001
0.3300000000000001
0.33500000000000013
0.33500000000000013
0.34000000000000014
0.34000000000000014
0.34500000000000014
0.34500000000000014
0.35000000000000014
...

As you can see the same timestep is printed two times.

Note: I've renamed the attached file to .txt to avoid the Github limitation but it is a python script.

doubt.txt

Minimal model and/or code that explain my question

No response

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[shogan50]

I noticed similar behavior when using RK4 integrator.

[Hardwarize]

Thanks for the point about RK @shogan50
Answering to myself from doc :
The control callback is called from within mj_forward and mj_step, just before the controls and applied forces are needed. When using the RK integrator, it will be called 4 times per step. The alternative way of specifying controls and applied forces is to set them before mj_step, or use mj_step1 and mj_step2. The latter approach allows setting the controls after the position and velocity computations have been performed by mj_step1, allowing these results to be utilized in computing the control (similar to using mjcb_control). However, the only way to change the controls between sub-steps of the RK integrator is to define the control callback.

My doubt now is: why is this 4 calls needed?
I've seen that if I set qvel to 0 previously to call mj_step in the first controller call the qvels are 0 but it evolve in the 3 following ones.

I'm implementing a PD controller so I'm not sure which one of these 4 calls should use to apply the torque.

[yuvaltassa]

That is how Runge–Kutta methods work

[Hardwarize]

Thanks for the link!