Additional parameters for physical material

When user uses AGXDynamics Plugin the following physical material (properties) can be used.

Sample

User can find the sample of AGXDynamicsPlugin material from below. The motion will be changed as per value of parameters.

  • choreonoid/samples/AGXDynamics/agxMaterialSample.cnoid

How to set the material

Friction coefficient and restitution coefficient of links in AGXSimulator can be set by below procedure.

  1. Describe Material and ContactMaterial in material file.

  2. Set Material defined in material file in body file.

How to describe in material file

Material file is the list file that describe the properties like friction and restitution coefficient.
In this file user can describe the same or different ContactMaterial for each material.
When user descibes the material name defined in this file in the Body file, the material on the model can be set.
Material file is loaded by setting in the property of world item.
The default setting is choreonoid/share/default/materials.yaml , and automatically loaded.
materials:
  -
    name: Ground
    roughness: 0.5
    viscosity: 0.0
  -
    name: agxMat5
    density: 1.0

contactMaterials:
  -
    materials: [ Ground, agxMat5 ]
    youngsModulus: 1.0e5
    restitution: 0.1
    spookDamping: 0.08
    friction: 0.416667
    surfaceViscosity: 1.0e-8
    adhesionForce: 100
    adhesivOverlap: 0.2
    frictionModel: [ cone, direct ]
    contactReductionMode: reduceGeometry
    contactReductionBinResolution: 3

Explanation of material parameters

Bulk material

parameter

default value

unit

data type

explanation

density

1000

kg/m3

double

density that is used for calculating of mass of link, inertia tensor, and center of mass.

youngsModulus

4.0e8

Pa

double

Young’s modulus that represents the hardness of link(rigid body). Smaller value may cause penetration between links.

viscosity

0.5

-

double

viscous restitution.The pair of viscous restitution becomes restitution coefficient.

spookDamping

0.075

s

double

spook damping. Relax the penetration of links(rigid bodies).

poissonRatio

0.3

-

double

Poisson’s ratio (deprecated since AGX 2.27.0.0)

Surface material

parameter

default value

unit

data type

explanation

roughness

0.5

-

double

Roughness, Corresponds to friction coefficient.

surfaceViscosity

5e-09

-

double

Surface viscosity. The pair of each surface viscosity becomes the surfaceViscosity of ContactMaterial. It defines how “wet” a surface is.

adhesionForce

0.0

N

double

adhesion force that determines attactive force acting between two colliding rigid bodies in the normal direction. Acting like adhesive agent.

adhesivOverlap

0.0

m

double

an absolute distance that defines the contact overlap where the adhesive force is active. It becomes active when penetration of link is larger than adhesiveOverlap

Note

If ContactMaterial is set, it is prioritized. Surface material of Material is not used.

Wire Material

Parameter

Default value

unit

data type

explanation

wireYoungsModulusStretch

6e10

Pa

double

young’s modulus for strech

wireSpookDampingStretch

0.075

s

double

spook damping for stretch

wireYoungsModulusBend

6e10

Pa

double

young’s modulus for bending.

wireSpookDampingBend

0.075

s

double

spook damping for bending

Explanation of ContactMaterial parameters

parameter

default value

unit

data type

explanation

youngsModulus

2.0e8

Pa

double

young’s modulus

restitution

0.0

-

double

restitution coefficient. 0:completely inelastic collision、1:completely elastic collision

spookDamping

0.075

s

double

spook damping

friction

0.5

-

double

friction coefficient

secondaryFriction

-1.0

-

double

secondary friction coeefient. It is activated when friction model is orientedBox and secondaryFriction>=0.

surfaceViscosity

1.0e-8

-

double

surface viscosity coeeficient. Complaiance for friction constraint.

secondarySurfaceViscosity

-1.0

-

double

secondary surface viscosity coefficient. It is activated when friction model is orientedBox and secondaryFriction>=0.

adhesionForce

0.0

N

double

adhesion force

adhesivOverlap

0.0

m

double

adhesive overlap

frictionModel

[ default, default ]

-
string
string
friction model : default(cone), cone, box, scaledBox, orientedBox
solver : default(split), split, direct, iterative, iterativeAndDirect

contactReductionMode

default

-

string

the way of contact reduction: default(reduceGeometry), reduceGeometry, reduceALL, reduceNone

contactReductionBinResolution

0

-

uint8_t

bin resolution(number of bins per dimension) of contact reduction. In case of zero, the parameters of AGXSimulator item are used.

primaryDirection

[ 0, 0, 0 ]

Unit vector

Vec3

primary direction of the vector when orientedBox friction model is used.

referenceBodyName

-

-

string

reference body name when orientedBox friction model is used.

referenceLinkName

-

-

string

reference link name when orientedBox friction model is used.

Note

AGX Dynamics does not distinguish between dynamic friction coefficient and static friction coefficient. Actually, the difference in value is around 10-20%, and most situations do not have to worry about it.

If the ContactMaterial is not defined

It is desirable that the all Material pairs are described in ContactMaterial, but it is difficult.
If the ContactMaterial is not defined, AGX Dynamic compute parameters of ContactMaterial from the parameters of Material as follows.
When paramters of Material are not set, default paramters are used.

  • youngsModulus = (m1.youngsModulus * m2.youngsModulus)/(m1.youngsModulus + m2.youngsModulus)

  • restitution = sqrt((1-m1.viscosity) * (1-m2.viscosity))

  • spookDamping = max(m1.spookDamping, m2.spookDamping)

  • friction = sqrt(m1.roughness * m2.roughness)

  • surfaceViscosity = m1.surfaceViscosity + m2.surfaceViscosity

  • adhesionForce = m1.adhesionForce + m2.adhesionForce

How to describe the material in the body file

This section describes how to set material in the body file.
You can select the types of setting center of gravity, mass and inertia with massType.
If massType is mass, values of center of mass, mass and inertia which described in the body file are directly used.
If massType is density, values of center of mass, mass and inertia are automatically calculated by AGX Dynamics.
The default type is mass.
massType: mass             # Use values of center of mass, mass, inertia which described in the body file
massType: density          # Calculate values of center of mass, mass, inertia automatically
You can set the material with material:.
Default is Default which is defined in the material file choreonoid_dev/share/default/materials.yaml.
material: Default          # Default material
material: Ground           # Ground material defined in choreonoid_dev/share/default/materials.yaml or user defined material file
material: useLinkInfo      # Use parameters of material described in the body file

Below are examples of how to describe.

Note

Currently, you could not get or check the result values of center of mass, mass, inertia from the Choreonoid Links and GUI when using massType: density

Conventional description

  • Conventional description of Choreonoid

  • Use centerOfMass, mass inertia which are described in the body file

  • Parameters of material are set default values except density

  • ContactMaterial will be default vs xxxxxx

links:
  -
    name: box1
    centerOfMass: [ 0, 0, 0 ]
    mass: 1.0
    inertia: [
      0.02, 0,    0,
      0,    0.02, 0,
      0,    0,    0.02 ]