SAP Error in MuJoCo Simulation with FlexComp #2320

Guezguezayoub · 2025-01-01T19:31:19Z

Guezguezayoub
Jan 1, 2025

Intro

Hi everyone,

I am a student at Leibniz University in Germany, currently working on simulating a digit sensor in MuJoCo. The sensor consists of two parts: a rigid base and a flexible component (modeled in green). I’ve developed the model and shared the code below for reference.

My setup

Mujoco 3.2.6
Python 3.12.7

My question

The goal of my simulation is to have a ball fall onto the sensor, causing deformation in the flexible part. However, when I attempt to run the simulation, it fails to start and throws the following error:
mujoco.FatalError: mj_collideFlexSAP: SAP failed.

I suspect the issue might be related to the FlexComp configuration—perhaps I’m missing a parameter or have initialized a value incorrectly. As I’m relatively new to MuJoCo, I’ve been learning a lot recently, but I’m unsure how to debug or resolve this issue effectively.

I’d greatly appreciate any suggestions or ideas on how to address this problem.

<mujoco>
	<option solver="CG"  gravity = "0 0 -9.81" timestep="0.002" />
	<size memory="8G"/>
	<asset>
        <mesh name="digitcore" file="core/digit_core_z.obj" scale="10 10 10" />
        </asset>
	<worldbody>
        <light diffuse=".5 .5 .5" pos="0 0 10" dir="0 0 -1"/>
		<geom type="plane" size="5 5 0.1" rgba=".9 .9 .9 1"/>
		<geom pos = "3 0 0.5" type="cylinder" size="0.1 0.5" rgba="0 1 0 1" mass="1"/>
		<body pos="0 0 3.1">
			<joint type="free"/>
			<geom name ="ball" type="sphere" size=".1" rgba=".9 0 0 1" mass="1"/>
		</body>
		<body pos = "3 0 1.1">
			<joint type="free"/>
			<geom type="box" size=".1 .1 .1" rgba="0.95 0.95 0 1" mass="0.1"/>
		</body>
		<body name="digit_body" pos="0 0 0.1">
                <geom name="core" type="mesh" mesh="digitcore" euler="-180 0 0" density="1e3" contype="1" pos="0 0 0" conaffinity="1" 
                   rgba="0.75 0.75 0.75 1.0"  />
			 <flexcomp
				 name="gel"
				 type="mesh"
				 file="gel/digit_gel_lq_z.obj"
		                 rgba="0 0.8 0.2 0.3"
				 rigid = "false"
				 scale="10 10 10"
				 euler="-180 0 0">
                        <elasticity young="5.39e5" poisson="0.499" thickness="0.002" />
                        <edge damping="1"/>
                        <contact solref="0.01 1" solimp="0.9 0.95 0.0001"/>
                        <pin id="0 19 380 399"/>
		 </flexcomp>
		</body>
	</worldbody>
</mujoco>

Best regards,
Salem

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 · 2025-01-01T20:08:19Z

yuvaltassa
Jan 1, 2025
Maintainer

cc @quagla

0 replies

quagla · 2025-01-02T10:10:20Z

quagla
Jan 2, 2025
Collaborator

Could you please try with selfcollide="none" internal="false" in contact inside flexcomp?

4 replies

Guezguezayoub Jan 3, 2025
Author

thanks for your answer that's right i don't get the error message anymore but the ball doesn't fall down when i write these parameters like this i actually wanted the ball to fall down on the flexcomp body

quagla Jan 3, 2025
Collaborator

Can you please send a picture or video to explain the issue?

Guezguezayoub Jan 4, 2025
Author

In the photo, you can see the sensor has two parts: the core, which is gray, and the gel, which is the green part of the flex component. I want the red ball to fall onto the gel. This worked before, but after some changes, the ball no longer moves. I want the ball to interact with the gel again.

quagla Jan 8, 2025
Collaborator

Are you saying that by just adding selfcollide="none" internal="false" the ball stopped moving?

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SAP Error in MuJoCo Simulation with FlexComp #2320

Guezguezayoub Jan 1, 2025

Intro

My setup

My question

Confirmations

Replies: 2 comments · 4 replies

yuvaltassa Jan 1, 2025 Maintainer

quagla Jan 2, 2025 Collaborator

Guezguezayoub Jan 3, 2025 Author

quagla Jan 3, 2025 Collaborator

Guezguezayoub Jan 4, 2025 Author

quagla Jan 8, 2025 Collaborator

Guezguezayoub
Jan 1, 2025

Replies: 2 comments 4 replies

yuvaltassa
Jan 1, 2025
Maintainer

quagla
Jan 2, 2025
Collaborator

Guezguezayoub Jan 3, 2025
Author

quagla Jan 3, 2025
Collaborator

Guezguezayoub Jan 4, 2025
Author

quagla Jan 8, 2025
Collaborator