Why doesn't dcmotor support current input mode?

[keunjun92]

Intro

Hi!

I am a researcher.

My setup

MuJoCo 3.7.0
Pyhton
Mac

My question

I see that dcmotor supports stateful current dynamics and also provides position and velocity input modes via built-in controllers.

Is there a specific reason why input="current" is not supported?
Forexample, something like v_cmd = PID(i_desired, i_actual).

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.

[cookesan]

dcmotor models an electrical plant driven by voltage. Current is an internal state when inductance is enabled, not a built-in control input mode.

The XML reference lists input as voltage, position, or velocity. In voltage mode, ctrl is the applied motor voltage. In position/velocity mode, the built-in controller treats ctrl as a reference and produces voltage. Current and torque saturation can limit the result, but that does not make current the commanded variable.

For current control, use an outer controller that reads the motor current state/error and writes a voltage command each step, or implement the actuator as a custom general actuator/plugin if you need current input to be a first-class actuator model.

[keunjun92]

Thank you for your answer.

My question is that I am curious about the underlying technical rationale behind this.

I understand that position and velocity modes in dcmotor act as built-in controllers that generate voltage commands. Since MuJoCo already tracks current as an internal state, I was wondering if there is a specific design reason why a similar built-in loop for current input wasn't included alongside the others? I want to model my actuator as dcmotor to account for electrical dynamics (like inductance), but I specifically need to use it with a current control interface. I would appreciate any insight into why this wasn't implemented as a part of the dcmotor.