How to Calculate the contact orces and Individual Forces Experienced by Bodies with Parent-Child Relationships #2168

gaoning0109 · 2024-10-23T02:38:13Z

gaoning0109
Oct 23, 2024

Intro

Hi!

I am a researcher, I use MuJoCo for my research on contact forces.

My setup

mujoco 3.20
API (C )

My question

Hello, I'm attempting to use d->cfrc_ext to analyze external contact forces. My understanding is that this is the resultant external force acting on the geom within a body. However, if I embed another body within this body, the contact forces experienced by the new body will not be reflected in the d->cfrc_ext of its parent body. It seems that embedding a geom within a body works fine. I would like to know if there's any way to calculate the resultant forces experienced by bodies with a parent-child relationship and their individual forces.

Minimal model and/or code that explain my question

Model:
I'm not sure if I should add sensors to each child body and obtain the contact forces via d->cfrc_ext for the child bodies.

		<body name="ship1" pos="0 0 1">
			<site name="force_sensor1" group="1"/>
			<freejoint/>
			<light name="ship1top light" pos="0 0 2" mode="trackcom" diffuse=".4 .4 .4"/>
			<light name="ship1front light" pos=".1 0 .02" dir="2 0 -1" diffuse="1 1 1"/>
			<geom type="box" pos="0.6 -.0 0" euler="0 45 0" size=".01315 .045 .5" rgba="0 0 0.8 1" solref="0.1 1"/>
			<body name="child" pos="0 0 4">
				<geom type="box" pos="0.6 -.0 0" euler="0 45 0" size=".01315 .045 .5" rgba="0 0 0.8 1" solref="0.1 1"/>
			</body>
		</body>
		<body name="ship2" pos="0 0 4">
			<site name="force_sensor2" group="1"/>
			<freejoint/>
			<light name="ship2top light" pos="0 0 2" mode="trackcom" diffuse=".4 .4 .4"/>
			<light name="ship2 light" pos=".1 0 .02" dir="2 0 -1" diffuse="1 1 1"/>
			<geom type="box" pos="0.6 -.0 0" euler="0 45 0" size=".01315 .045 .5" rgba="0 0 0.8 1" solref="0.1 1"/>
		</body>
	</worldbody>
	<sensor>
		<force site="force_sensor1"/>
	</sensor>
	<sensor>
		<force site="force_sensor2"/>
	</sensor>

Code:

            auto extForces = std::make_unique<externalforce[]>(m->nbody);

            const mjtNum* external_force = d->cfrc_ext;

            extForces[*index].forceX = std::make_unique<double>(0.0);
            extForces[*index].forceY = std::make_unique<double>(0.0);
            extForces[*index].forceZ = std::make_unique<double>(0.0);
            extForces[*index].torqueX = std::make_unique<double>(0.0);
            extForces[*index].torqueY = std::make_unique<double>(0.0);
            extForces[*index].torqueZ = std::make_unique<double>(0.0);
            *extForces[*index].forceX = external_force[3 + 6 * (*index + 1)];
            *extForces[*index].forceY = external_force[4 + 6 * (*index + 1)];
            *extForces[*index].forceZ = external_force[5 + 6 * (*index + 1)];
            *extForces[*index].torqueX = external_force[0 + 6 * (*index + 1)];
            *extForces[*index].torqueY = external_force[1 + 6 * (*index + 1)];
            *extForces[*index].torqueZ = external_force[2 + 6 * (*index + 1)];

The index is the index of the body, and since I only have two bodies, they are indexed as 0 and 1.

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 · 2024-10-23T10:37:18Z

yuvaltassa
Oct 23, 2024
Maintainer

cfrc_ext are external forces acting on the body (e.g. contacts), cfrc_int are the forces inside the kinematic tree. Perhaps it would help to read this and this.

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How to Calculate the contact orces and Individual Forces Experienced by Bodies with Parent-Child Relationships #2168

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How to Calculate the contact orces and Individual Forces Experienced by Bodies with Parent-Child Relationships #2168

gaoning0109 Oct 23, 2024

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

yuvaltassa Oct 23, 2024 Maintainer

gaoning0109
Oct 23, 2024

yuvaltassa
Oct 23, 2024
Maintainer