How does frictionloss work #2856

Alesel-9 · 2025-09-17T11:13:00Z

Alesel-9
Sep 17, 2025

Intro

Hi!

My setup

I work in mujoco python and do not have experiance in C

My question

How does Mujoco apply the frictionloss attribute? I understand that it works like dry friction in that it applies a force pre-movement of the joint it is applied on, and that the joint needs to be affected by a force greater than the frictionloss value to initiate movement, that much have I gatherd from the documentation.
However, how does it work after the slip. is there a constant 'frictionloss value' force that continues to resist the force that enables movement, and is there a way of logging that in python binds? Or does the effect of the constraint simply vanish?
For context: I know about the qfrc_[passive, smooth and constraint] data loggers but they all include the total amount of constraining forces that affects the joint. So, say that i have a stiffness, damper and the frictionloss attribute on the joint, is there a way of isolationg the force that the frictionloss attribute applies to the joint.

Below you'll find a model that'll help with visualization. how do i find the exclusiv frictionloss component that acts on the 'J1' joint?

Minimal model and/or code that explain my question

Model:

minimal XML

<mujoco model ="testbody">
<compiler autolimits ="true"/>
<default>
    <default class ="geom arm">
        <geom type="capsule" size="0.05 .1925" /> 
    </default>
    <default class ="joint arm right">
        <joint type="hinge" pos="0 0 0.1925" axis="0 1 0" springref="15" stiffness="40000" frictionloss="20000"/> 
    </default>
    <default class ="joint arm left">
        <joint type="hinge" pos="0 0 0.1925" axis="0 1 0" springref="-15" stiffness="40000" frictionloss="20000"/>   
    </default>    
    <default class ="geom wheel">
        <geom type="cylinder" size=".315 .1325" rgba=".1 .1 .1 1" /> 
    </default>
    <default class ="joint wheel">
         <joint type="hinge" pos="0 0 0.1925" axis="0 0 -1"  frictionloss="10" /> 
    </default>
</default>

<asset>
    <texture name="sky" type="skybox" builtin="gradient" rgb1="1 1 1" rgb2=".1 .2 .4"  width ="500" height="500" />
    <texture name="checker" type="2d" builtin="checker" rgb1="0.5 0.5 0.5" rgb2=".8 .8 .8"  width ="500" height="500" />
    <material name="ground" texture="checker" texrepeat="1 1" texuniform="true" reflectance=".2"/>
</asset>

<visual>
    <rgba haze="0.7 .7 .8 .2" rangefinder="1 0 0 1"/>
    <scale  com=".1" jointlength="0.5" jointwidth=".06" actuatorlength=".15" actuatorwidth=".18"/>
    <global realtime =".1"/>
</visual>

<worldbody>
    <geom type="plane" material="ground" size="1000 1000 1" pos="0 0 -0.01"/>
    <light directional ="true" diffuse =".5 .5 .5" specular =" 0.1 0.1 0.1 " pos ="0 0 5.0 " dir ="0 0 -1" castshadow ="false"/>
    <light directional ="true" diffuse =".6 .6 .6" specular =" 0.2 0.2 0.2 " pos ="0 0 4" dir ="0 0 -1"/>

    <body pos="0 0 4">
        <geom type="box" size=".5 .3 .3"  />
        <joint type="slide" pos="0 0 0" axis="1 0 0"/>
        <joint type="slide" pos="0 0 0" axis="0 0 1"/>
        <inertial mass="2643" pos="0 0 0" diaginertia="500 1800 2100" /> 
        <camera name = "Main close" mode ="trackcom" pos ="0 8 0" euler ="-90 0 180"/>
            <body pos="0 .4 -0.3" euler="0 -45 0">
                <geom class="geom arm" name="P1"/>
                <joint class="joint arm right" name="J1" />
                <inertial mass="147" diaginertia=".3 2.34 2.55"  euler="0 90 0"/>
                
                <body pos=" 0 0.25 -0.2" euler="90 0 0">
                    <geom class="geom wheel" name="W1"/>
                    <joint class="joint wheel" name="A1"/>
                    <inertial mass="66" pos="0 0 0" diaginertia="2.53 2.53 4.5" />         
                </body>
            </body>
    </body>

</worldbody>
</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.

Alesel-9 · 2025-09-22T07:15:15Z

Alesel-9
Sep 22, 2025
Author

Found the answer through tests

0 replies

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How does frictionloss work #2856

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How does frictionloss work #2856

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Alesel-9 Sep 17, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

Uh oh!

Alesel-9 Sep 22, 2025 Author

Alesel-9
Sep 17, 2025

Alesel-9
Sep 22, 2025
Author