Applying a pure force #2384

shogan50 · 2025-01-25T18:09:21Z

shogan50
Jan 25, 2025

Intro

Hi!

I'm a principal mechanical engineer working on a wind energy project.

My setup

Mujoco / Python / Windows 11

My question

What is the right way to apply a pure force to a model? I've been trying to get a general actuator to apply a pure force, but it seems to be velocity dependent - decaying with increased velocity.
For more context, I calculate lift and drag in my control function and apply it to a general actuator on a site that is always oriented along the apparent wind vector.

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 · 2025-01-25T19:12:42Z

yuvaltassa
Jan 25, 2025
Maintainer

Your question is unclear. What is a "pure force"? Do you mean in the global frame? Motor actuators attached to sites apply force/torque in the local frame. They are not velocity-dependent. Probably you are not taking something into account.

0 replies

shogan50 · 2025-01-25T20:28:15Z

shogan50
Jan 25, 2025
Author

Below illustrates what is going on. The red arm body is being pushed tangentially on the blue box site via the actuator called arm. In the real world, the arm would accelerate at a constant rate until something broke. In the simulation, it quickly reaches a low steady state velocity. The only two ways this could happen is if there exists significant damping in the system somewhere, or the output of the motor is a function, presumably of velocity, rather than a constant.

I'm commanding the motor with
data.actuator('arm').ctrl = 10.0

`

<worldbody>
    <geom type="plane" size="10 10 .1" rgba="1 1 1 1"/>
    <light pos="0 0 20"/>
    <body name="mast" pos="0 0 0">
        <joint type="hinge" axis="0 0 1"/>
        <geom name="mast_geom" type="cylinder" size=".1" fromto=".0 0 0 0 0 3.0" rgba="1 0 0 1"/>
        <body name="arm" pos="0 0 3">
            <geom name="arm_geom" type="cylinder" fromto="0 0 0 1 0 0" size=".05" rgba="1 0 0 1"/>
            <site name="arm_site" pos="1 0 0" rgba="0 0 1 1" size=".1 .1 .2" type="box"/>
        </body>
    </body>
</worldbody>

<actuator>
    <motor name="arm" site="arm_site" gear="0 1 0 0 0 0"/>
</actuator>

<sensor>
    <framelinvel name="arm_tangential" objtype="site" objname="arm_site"/>
</sensor>

`

1 reply

yuvaltassa Feb 13, 2025
Maintainer

mast_geom has a frictional contact with the floor. Hit 'C' in simulate to see this. Also read this. Change the third number in mast_geom's fromto to any positive number to get the behavior you expect. Visualising contacts and contact forces is one of the most important features of simulate. Watch the video.

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shogan50 Jan 25, 2025

Intro

My setup

My question

Confirmations

Replies: 2 comments · 1 reply

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yuvaltassa Jan 25, 2025 Maintainer

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shogan50 Jan 25, 2025 Author

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yuvaltassa Feb 13, 2025 Maintainer

shogan50
Jan 25, 2025

Replies: 2 comments 1 reply

yuvaltassa
Jan 25, 2025
Maintainer

shogan50
Jan 25, 2025
Author

yuvaltassa Feb 13, 2025
Maintainer