Is it possible to add velocity limits for an actuator of motor type in Mujoco XML file? #2367

Jingwen-Zhang-Aaron · 2025-01-17T09:20:41Z

Jingwen-Zhang-Aaron
Jan 17, 2025

Intro

Hi!

I use MuJoCo for my humanoid robot simulation.

My setup

In XML, we define an actuator of motor type for each active joint of a humanoid robot. The control limits is for the torque I can apply for that actuator. Is it possible to set velocity limits of the actuator behaviors? I mean if the velocity of the actuator exceed a limit, will Mujoco be able to clip it at runtime?

My question

No response

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:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
    <body> ...
        <joint ... />
    </body> 
  </worldbody>

  <actuator>
    <motor gear="1" joint="joint_leg_l1" name="joint_leg_l1_motor" ctrllimited="true"  ctrlrange='-150 150'/>
    <motor gear="1" joint="joint_leg_l2" name="joint_leg_l2_motor" ctrllimited="true"  ctrlrange='-150 150'/>
    <motor gear="1" joint="joint_leg_l3" name="joint_leg_l3_motor" ctrllimited="true"  ctrlrange='-200 200'/>
    <motor gear="1" joint="joint_leg_l4" name="joint_leg_l4_motor" ctrllimited="true"  ctrlrange='-200 200'/>
    <motor gear="1" joint="joint_leg_l5" name="joint_leg_l5_motor" ctrllimited="true"  ctrlrange='-90 90'/>
    <motor gear="1" joint="joint_leg_l6" name="joint_leg_l6_motor" ctrllimited="true"  ctrlrange='-90 90'/>
    <motor gear="1" joint="joint_leg_r1" name="joint_leg_r1_motor" ctrllimited="true"  ctrlrange='-150 150'/>
    <motor gear="1" joint="joint_leg_r2" name="joint_leg_r2_motor" ctrllimited="true"  ctrlrange='-150 150'/>
    <motor gear="1" joint="joint_leg_r3" name="joint_leg_r3_motor" ctrllimited="true"  ctrlrange='-200 200'/>
    <motor gear="1" joint="joint_leg_r4" name="joint_leg_r4_motor" ctrllimited="true"  ctrlrange='-200 200'/>
    <motor gear="1" joint="joint_leg_r5" name="joint_leg_r5_motor" ctrllimited="true"  ctrlrange='-90 90'/>
    <motor gear="1" joint="joint_leg_r6" name="joint_leg_r6_motor" ctrllimited="true"  ctrlrange='-90 90'/>
  </actuator>

</mujoco>

Confirmations

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

yuvaltassa · 2025-01-17T20:41:01Z

yuvaltassa
Jan 17, 2025
Maintainer

No.

What you are asking for is not physically realistic. If I drop the robot from a great height, the velocity on impact will be arbitrarily large, right? Velocity clipping is never the right thing to do.

If you want actuator-generated velocity to never exceed some limit then you're actually talking about the effect of back-EMF, which is equivalent to adding damping.

2 replies

Nate711 Feb 27, 2025

FYi I think back-emf is only equivalent to damping if you're using voltage control, which is quite rare. Back-emf will typically cause a motor's torque-speed curve to be flat for a while and then decrease linearly to 0

Nate711 Feb 27, 2025

It'd be useful to add a specify this relationship in the actuator xml definition

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Is it possible to add velocity limits for an actuator of motor type in Mujoco XML file? #2367

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Is it possible to add velocity limits for an actuator of motor type in Mujoco XML file? #2367

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Jingwen-Zhang-Aaron Jan 17, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment · 2 replies

Uh oh!

yuvaltassa Jan 17, 2025 Maintainer

Uh oh!

Nate711 Feb 27, 2025

Uh oh!

Nate711 Feb 27, 2025

Jingwen-Zhang-Aaron
Jan 17, 2025

Replies: 1 comment 2 replies

yuvaltassa
Jan 17, 2025
Maintainer