AGXWire¶

AGXWire is a wire model using AGX Dynamics. The agxWire module of AGX Dynamics is used for implementation.

Features of AGXWire
Sample
How to write
Explanation of parameters
Collision settings of the wire
Specification

Features of AGXWire ¶

Hard to be torn

Even if the wire is pulled with a strong force it is hard to be torn. For example, it is useful for hanging a container with a crane wire.
Good performance for long length

A general physics engine expresses cables and wires by connecting rigid bodies with a shape such as a capsule and a cylinder with joints.

Therefore, as the number of rigid bodies increases, the amount of computation increases, and finally the calculation is not completed within real time.

In this model, the number of elements can be automatically increased / decreased during the simulation.

This can be reduce computation amount of the simulation.

Note

AGXWire emphasizes speed. The stability of collision response is not high.
It does not correspond to a tough scene such as a robot stepping on wires or sandwiching wires with a door.
In such a situation wire may oscillate and in the worst case the simulation may stop.

Sample ¶

You can check samples to how to use AGXWire. The samples are below. When loading the sample projects with Choreonoid and running the simulation with AGXSimulator, the wire will be displayed.

Winch, Wire, Wire link
- Project file: chorenoid/sample/AGXDynamics/agxWireSample.cnoid
- Body file: chorenoid/sample/AGXDynamics/agxWireSample.body
Fire hose
- Project file: chorenoid/sample/AGXDynamics/FireHose.cnoid
- Body file: chorenoid/sample/AGXDynamics/FireHose.body

How to write ¶

Link composition of sample model is as below.

links:
  -
    name: Drum
    parent: Root
    translation: [ -1.0, 0.0, -3.0 ]
    jointType: fixed
    elements:
      -
        type: Shape
        geometry: { type: Cylinder, radius: 0.25, height: 0.2 }
        appearance: { material: { diffuseColor: [ 1.0, 0, 0 ] }}
      -
        type: AGXWireDevice
        radius: 0.02
        resolutionPerUnitLength: 5
        collision: true
        selfCollision: true
        materialName: Wire
        #wireYoungsModulusStretch: 6E10
        #wireSpookDampingStretch: 0.075
        #wireYoungsModulusBend: 6E10
        #wireSpookDampingBend: 0.075
        Winch:
            linkName: Drum
            position: [ 0.0, 0.0, 0.27 ]
            normal: [1.0 , 0.0, 0.0 ]
            pulledInLength: 50
            haulForceRange: [ -50, 50 ]
        Nodes:
          -
            type: free
            linkName: Root
            position: [ -0.27, 0.0, 0.27]
          -
            type: free
            linkName: Root
            position: [ 0.27, 0.0, 0.27]
          -
            type: link
            linkName: Sphere
            position: [ 0.0, 0.0, 0.1]
            #twistStiffness: 1.0E10
            #bendStiffness: 1.0E10
            #superBendReplacedWithBend: true
  -
    name: Sphere
    parent: Root
    translation: [ 1.0, 0.0, 0.0 ]
    jointType: free
    mass: 1.0
    elements:
      -
        type: Shape
        geometry: { type: Sphere, radius: 0.1 }
        appearance: { material: { diffuseColor: [ 0.0, 1.0, 0 ] }}
      -
        type: AGXWireDevice
        radius: 0.04
        resolutionPerUnitLength: 5
        collision: true
        selfCollision: true
        materialName: Wire
        Nodes:
          -
            type: link
            linkName: Sphere
            position: [ 0.0, 0.0, -0.1]
          -
            type: fixed
            linkName: Box
            position: [ 0.0, 0.0, 0.1 ]
  -
    name: Box
    parent: Sphere
    translation: [ 0.0, 0.0, -2.0 ]
    jointType: free
    mass: 5
    elements:
      -
        type: Shape
        geometry: { type: Box, size: [ 0.2, 0.2, 0.2] }
        appearance: { material: { diffuseColor: [ 0.0, 1.0, 0 ] }}

Add AGXWireDevice to the elements part of arbitrary link
Set parameters of the wire

Set the radius of the wire to the parameter radius

Set the resolution of the wire to the parameter resolutionPerUnitLength

Set enable or disable collision between wires, links and other environments to collsion

Set enable or disable self collision between the wire and the links which has the wire to selfCollision

Set the material of the wire to materialName. Check the reference Additional parameters for physical material.

It is recommend to use the material file. Though, you can set parameters such as wireYoungsModulusStretch directly into the body file.

In order to pull out the wire from the winch, set the following

Set the link name, which to be used as a winch, to linkName. You can specify any link.

Set the pulled out position of the wire to position relative from the link coordinate of the link

Set the pulled out direction of the wire to normal relative from the link coordinate of the link

Set the length of the wire which stock into the winch to pulledInLength

Set the required force to pull out the wire from the winch or put in to haulForceRange

Next, use the Node hash to route the wire in space

Wires can be routed in three ways. The way of routing is specified as type:

type: free Route the wire to the specified position. Not fixed.

type: fixed Fix the wire at the specified position. The fixed point can rotate freely(balljoint).

type: link Connect the wire to the link. The connecting point has twisting stiffness and bending stiffness.

Set the linkName in which coordinate system to route the wire

When the link exist in which name is the linkName: Link coordinate

When the linkName is empty or not exist link: World coordinate

Set the routing position to position

Explanation of parameters ¶

The parameters are described below.

Wire

parameter	default value	uinit	data type	explanation
type: AGXWireDevice	-	-	string	declaration of using AGXWire
radius	0.02	m	double	radius of the wire
resolutionPerUnitLength	-	m	double	minimum resolution of the wire
collision	-	-	bool	set enable/disable collision between the wire and other objects
selfCollision	-	-	bool	set enable/disalbe self-collision between the wire and the body which has the wire
materialName	-	-	string	name of the material
wireYoungsModulusStretch	6e10	N/m	double	young’s modulus in the stretch direction
wireSpookDampingStretch	0.075	s	double	spook damping in the stretch direction
wireYoungsModulusBend	6e10	N/m	double	young’s modulus in the bending direction
wireSpookDampingBend	0.075	s	double	spook damping in the bending direction
twistStiffness	0	N/m	double	Available at type:link. Twisting stiffness between the wire and the link.
bendStiffness	0	N/m	double	Available at type:link. Bending stiffness between the wire and the link.
superBendReplacedWithBend	false	-	bool	Available at type:link. Make the wire more flexible in the bending direction at the connecting point.

Winch

parameter	default value	uinit	data type	explanation
linkName	-	-	string	link name which to be a winch
position		-	Vec3	pulled out position of the wire
normal		-	Vec3	pulled out direction of the wire
pulledInLength		m	double	length of the wire which store into the winch
haulForceRange		N	Vec2	force required to pull out the wire from or put in the wire to the winch

Node

parameter	default value	uinit	data type	explanation
type	-	-	string	type of routing the wire: set free, fixed, link
linkName	-	-	string	link name in which coordinate system to route the wire
position		-	Vec3	position where locate or fix the wire

Collision settings of the wire ¶

As explained in Features of AGXWire, agxWire does not correspond to the case such as the robot stepping on wires.
However, it is difficult to operate the robot avoiding such a case.
Here are some countermeasures.

Disable collision with the specified link with the wire¶

As shown in the above left figure, disable collision between the wire and the specified link. This can be done by setting excludeLinksWireCollision of the collision setting of Additional parameters for body model.

collisionDetection:
  excludeLinksWireCollision: [ linkQ, linkR, ... ]

Attach a wire guard to the link to avoid collision with the wire¶

Another way is to attach a guard to the link which you want to avoid collision with the wire as shown in the above right figure. This can be realized by describing it in the body file as follows. Add a guard link to the body file. However, since it will cause collision with other links and self collision as it is, set excludeLinksDynamic and disable collision between links. Then the guard link will collide with the wire only.

  -
    name: WIRE_GUARD_L
    parent: TFRAME
    jointType: fixed
    translation: [ -0.02, -0.595, 0.600 ]
    material: default
    elements: &WireGuard
      -
        type: Collision
        elements:
        -
          type: Shape
          geometry: { type: Box, size: [ 2.1, 1.5, 0.5 ] }

collisionDetection:
  excludeLinksDynamic: [ WIRE_GUARD_L, WIRE_GUARD_R ]

Specification ¶

Please note that wire material parameters are overridden in the order of material description < direct description .

AGXWire¶

Features of AGXWire¶

Sample¶

How to write¶

Explanation of parameters¶

Collision settings of the wire¶