Hello,
I am currently working on a project that combines Chrono’s SMC and DEM
modules within the same simulation. I initially developed my robot model
using SMC alone, following SI units consistently (meters, kilograms,
seconds). In that setup, the robot’s geometry, inertias, and masses
appeared correct and behaved as expected.
However, when I began integrating the SMC-based robot into a DEM
environment, I noticed a severe scale mismatch: the robot appeared
extremely small—on the order of a single DEM particle, or even smaller.
Visually and physically, it looked almost like a “bug” relative to the
granular terrain.
My DEM setup was inspired by the official DEM demos (DEM_ballCosim &
DEM_fixedTerrain), and I am using the following JSON parameter file
(excerpted below for clarity):
{
"sphere_radius": 1.0,
"sphere_density": 2.5,
"box_X": 200,
"box_Y": 200,
"box_Z": 60,
"time_integrator": "centered_difference",
"step_size": 5e-5,
"time_end": 5.0,
"grav_X": 0,
"grav_Y": 0,
"grav_Z": -980,
"normalStiffS2S": 1e8,
"normalStiffS2W": 1e8,
"normalStiffS2M": 1e6,
"normalDampS2S": 10000,
"normalDampS2W": 10000,
"normalDampS2M": 1000,
"tangentStiffS2S": 1e8,
"tangentStiffS2W": 1e8,
"tangentStiffS2M": 5e5,
"tangentDampS2S": 2000,
"tangentDampS2W": 2000,
"tangentDampS2M": 500,
"static_friction_coeffS2S": 0.5,
"static_friction_coeffS2W": 0.5,
"static_friction_coeffS2M": 0.3,
"friction_mode": "multi_step",
"rolling_mode": "schwartz",
"rolling_friction_coeffS2S": 0.1,
"rolling_friction_coeffS2W": 0.1,
"rolling_friction_coeffS2M": 0.05,
"cohesion_ratio": 0,
"adhesion_ratio_s2w": 0,
"adhesion_ratio_s2m": 0,
"verbose": 0,
"psi_T": 32,
"psi_L": 16,
"write_mode": "csv"
}
As an experiment, I scaled my entire SMC system from SI to CGS units. After
doing so, the robot appeared to be correctly sized relative to the DEM
terrain, and interactions looked correct.
That being said, this approach was quite messy and time-consuming, and it
feels like I may be compensating for something incorrectly rather than
using the system as intended.
The reason I am reaching out is that I suspect I may be missing a
fundamental assumption or requirement regarding unit conventions or scaling
when coupling SMC with DEM. Specifically:
-
Is DEM implicitly *always* assuming CGS units (as suggested by gravity ≈
−980)?
-
Is there a recommended or canonical way to keep SMC and DEM consistent
without fully rescaling one system?
Any guidance or clarification would be greatly appreciated. Thank you very
much for your time and support.
Best regards,
Leen Said
--
You received this message because you are subscribed to the Google Groups
"ProjectChrono" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion visit
https://groups.google.com/d/msgid/projectchrono/861b63c0-cef7-4c4c-a3d3-ee75f1a001f1n%40googlegroups.com.
