Comments (5)
from mujoco documentation:
Recall that contacts can have up to 5 friction coefficients: two tangential, one torsional, two rolling. Each contact in mjData.contact actually has all 5 of them, even if condim is less than 6 and not all coefficients are used. In contrast, geoms have only 3 friction coefficients: tangential (same for both axes), torsional, rolling (same for both axes). Each of these 3D vectors of friction coefficients is expanded into a 5D vector of friction coefficients by replicating the tangetial and rolling components.
from SDFormat spec:
//collision/surface/friction
parameters
//friction/ode/mu
//friction/ode/mu2
//friction/ode/fdir1
//friction/ode/fdir1/@ignition:expressed_in
//friction/ode/slip1
//friction/ode/slip2
//friction/torsional/coefficient
Most of the parameters in //friction
are hidden in the //ode
element and assume a friction pyramid with 2 friction directions. There is a long-standing issue to move the friction parameters out of that element: gazebosim/sdformat#31.
The torsional coefficient does have a reasonable specification, so it wouldn't need to change, and we could follow its pattern for rolling friction:
//friction/rolling/coefficient
We would eventually want to allow for anisotropic parameters for rolling friction (for example when used with a pyramid friction model), but that is not urgent for purposes of this project because MuJoCo does not support specifying anisotropic friction parameters for geometric shapes.
Another reference is the ODE API and Dartsim API.
from gz-mujoco.
So to summarize, in the short term we could do the following:
- use existing friction parameters:
//friction/ode/mu
//friction/torsional/coefficient
- add a new parameter:
//friction/rolling/coefficient
from gz-mujoco.
I've committed scpeters/sdformat@deccd47 to a branch with two competing parameters for rolling friction (pick one to use):
//surface/friction/rolling/deformation_length
: units of length//surface/friction/rolling/coefficient
: non-dimensional, scaled by the size of the object
from gz-mujoco.
adding python bindings for sdf::Friction
and sdf::ODE
in gazebosim/sdformat#1036
from gz-mujoco.
setting sliding friction in #70
from gz-mujoco.
Related Issues (18)
- Create python packaging files
- [SDFormat->MJCF] Support nested models
- [SDFormat->MJCF] Support `//world/light` and `//link/light` HOT 4
- conda installation HOT 3
- [SDFormat->MJCF] Support `//visual/material` HOT 1
- [SDFormat->MJCF] Identify a small set of commonly used sensors HOT 1
- README questions HOT 4
- Should enable CI for other versions of python HOT 1
- [SDFormat->MJCF] Should add support for revolute2 and universal joints
- [SDFormat->MJCF] Should add gearbox and screw type joints
- [SDFormat->MJCF] Material specular and emissive is a scalar not a color
- Gazebo and Mujoco have incompatible texture mapping for primitive shapes
- sdformat_to_mjcf: specify nconmax and njmax in SDFormat HOT 2
- [SDFormat->MJCF] Support meshes for collisions and visuals HOT 1
- [SDFormat->MJCF] Handle name collision between geoms
- Should detect if OBJ is composite and issue a warning
- [SDFormat->MJCF] Support kinematic loops
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