XML format

General layout of Moby's XML files

• Add a driver and set camera, window location using DRIVER tags

<DRIVER>
  <camera position="0 0 10" target="0 0 0" up="0 1 0" />
  <window location="0 0" size="640 480" />
</DRIVER>

• Add Primitives, RigidBodies, Integrators, etc. within MOBY tags

<MOBY>
  <!-- Example Primitive -->
  <Box id="b1" xlen="2" ylen="0.001" zlen="2" density="10.0" />
</MOBY>

Primitives

• Primitives that are available to add to a simulation include Sphere, Box, TriangleMesh, Cylinder, Cone, Heightmap, Plane
• Example use of each primitive, showing specific options:

<MOBY>
  <Sphere id="sphere" radius="0.1" />
  <Box id="box" xlen="1.0" ylen="1.0" zlen="1.0" />

  <!-- the cylinder's axis is aligned with the y-axis -->
  <Cylinder id="cylinder" height="1.0" radius="1.0"  />

  <!-- the plane's equation is y=0 -->
  <Plane id="plane" />

  <!-- the axis of revolution is the z-axis -->
  <Torus id="torus" major-radius=".7" minor-radius=".3" />
</MOBY>

All primitives take the following attributes:

• id [required] the unique identifier for the primitive
• mass the mass of the primitive (behavior undefined if density already specified)
• density the density of the primitive (behavior undefined if mass already specified)
• position the translational offset of the primitive
• rpy the roll-pitch-yaw orientation of the primitive (behavior undefined if aangle or quat already specified)
• quat the unit quaternion orientation (qw,qx,qy,qz) of the primitive (behavior undefined if aangle or rpy already specified)
• aangle the axis-angle orientation of the primitive (behavior undefined if aangle or quat already specified)

GravityForce

• To add a gravity force:

<GravityForce id="" accel="0 -9.81 0" />

RigidBody

• Add a RigidBody to the simulation and link it to a primitive through visualization-id
• Add CollisionGeometry or InertiaFromPrimitive through your primitive's id

<MOBY>
  <Sphere id="s1" radius=".5" density="1.0" />

  <RigidBody id="sphere" enabled="true" position="0 .5000 0" visualization-id="s1" linear-velocity="1 0 0" angular-velocity="1 2 3">
    <InertiaFromPrimitive primitive-id="s1" />
    <CollisionGeometry primitive-id="s1" />
  </RigidBody>
</MOBY>

The RigidBody tag takes the following attributes:

• id [required] the unique identifier for the rigid body
• enabled if "false", then the rigid body is treated as having infinite inertia. If the rigid body is a link within an articulated body defined in reduced coordinates (see RCArticulatedBody below), enabled="false" is only valid if the link is a base link (changes the base from "floating" to "fixed"). The default value is enabled="true")
• mass the mass of the rigid body (behavior undefined if density or one or more InertiaFromPrimitive tags already specified)
• inertia the 3x3 inertia matrix of the rigid body (MATLAB/Octave format, or one or more InertiaFromPrimitive tags already specified)
• position the location of the rigid body's reference frame (not necessarily its center-of-mass, applied after rotation specified by rpy/quat/aangle)
• rpy the roll-pitch-yaw orientation of the rigid body's reference frame (behavior undefined if aangle or quat already specified)
• quat the unit quaternion orientation (qw,qx,qy,qz) of the rigid body's reference frame (behavior undefined if aangle or rpy already specified)
• aangle the axis-angle orientation of the rigid body's reference frame (behavior undefined if aangle or quat already specified)
• inertial-relative-com the offset of the body's inertial frame from its reference frame (applied after rotation specified by inertia-relative-aangle'/inertia-relative-rpy/inertia-relative-quat` if any)
• inertial-relative-rpy the relative roll-pitch-yaw orientation of the rigid body's inertial frame (behavior undefined if inertial-relative-aangle or inertial-relative-quat already specified)
• inertial-relative-quat the unit quaternion describing the relative orientation (qw,qx,qy,qz) of the rigid body's inertial frame (behavior undefined if inertial-relative-aangle or inertial-relative-rpy already specified)
• inertial-relative-aangle the axis-angle orientation of the rigid body's inertial frame (behavior undefined if inertial-relative-aangle or inertial-relative-quat already specified)
• linear-velocity the linear velocity of the rigid body at its center-of-mass
• angular-velocity the angular velocity of the rigid body, defined in the global frame
• visualization-id the identifier of the primitive used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph)
• visualization-filename the filename of the geometric description used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph)

The RigidBody tag accepts the following child nodes:

• Visualization one or more tags used for visualization
  • visualization-id the identifier of the primitive used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph, behavior unspecified if visualization-id also specified)
  • visualization-filename the filename of the geometric description used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph, behavior unspecified if visualization-filename also specified)
  • visualization-rel-origin the translation offset of the geometry used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph)
  • visualization-rel-rpy the relative orientation of the geometry, specified using roll-pitch-yaw, used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph, behavior is undefined if visualization-rel-quat also specified)
  • visualization-rel-quat the relative orientation of the geometry, specified as a unit quaternion (qw qx qy qz) used for visualizing the rigid body (only applicable if Moby is used with OpenSceneGraph, behavior is undefined if visualization-rel-rpy also specified)
• CollisionGeometry one or more tags used for computing geometric intersections and computing points of contact between rigid bodies
  • primitive-id (required) the primitive used for intersection checking
  • relative-origin relative offset of the primitive
  • relative-aangle relative orientation of the primitive, defined in axis-angle orientation (behavior undefined if relative-rpy also specified)
  • relative-rpy relative orientation of the primitive, defined using roll-pitch-yaw (behavior undefined if relative-aangle also specified)
• InertiaFromPrimitive one or more tags used for computing the inertia of a rigid body
  • primitive-id (required) the primitive used to compute the inertia
  • relative-origin relative offset of the primitive used to compute the inertia (applied after rotation specified by relative-aangle'/relative-rpy` if any)
  • relative-aangle relative orientation of the primitive used to compute the inertia, defined in axis-angle orientation (behavior undefined if relative-rpy also specified)
  • relative-rpy relative orientation of the primitive used to compute the inertia, defined using roll-pitch-yaw (behavior undefined if relative-aangle also specified)

RCArticulatedBody

An RCArticulatedBody is an articulated body defined in reduced (minimal or almost minimal) coordinates. The dynamics for such bodies can be computed rapidly if joints admit few degrees-of-freedom (ex. revolute, prismatic, universal). Reduced coordinates also have the advantage of not requiring any constraint stabilization (as long as there are no kinematic loops) and straightforward provision of Jacobian matrices used in robotics applications. Reduced coordinates become less computationally efficient when used with joints with greater degrees-of-freedom (spherical, ball-and-socket).

• Such articulated bodies are constructed by embedding rigid bodies and joints within, as below:

<RCArticulatedBody id="" floating-base="false" fdyn-algorithm="crb" fdyn-algorithm-frame="link" rpy="0.0 0.05 0.0" translate="0.45,0,0" >
  <RigidBody id="BASE" color="0 0 0 0" position="0 0 -0.02" angular-velocity="0 0 0" linear-velocity="0 0 0" visualization-id="base" compliant="false">
  </RigidBody>

  <PrismaticJoint id="shaker" location="0 0 0" inboard-link-id="BASE" outboard-link-id="SLIDE1" axis="1 0 0"/>

  <!-- the body -->
  <RigidBody id="SLIDE1" color="0 0 0 0" position="0 0 0" angular-velocity="0 0 0" linear-velocity="0 0 0" visualization-id="slide1-viz" compliant="false">
    <CollisionGeometry primitive-id="slide1" />
    <InertiaFromPrimitive primitive-id="slide1-viz" />
  </RigidBody>
  <FixedJoint id="fj1" location="0 0 0" inboard-link-id="SLIDE1" outboard-link-id="SLIDE2" />
  
  <RigidBody id="SLIDE2" color="0 0.5 0 1" position="0 0.055 0.01" angular-velocity="0 0 0" linear-velocity="0 0 0" visualization-id="slide2-viz" compliant="false">
    <CollisionGeometry primitive-id="slide2" />
    <InertiaFromPrimitive primitive-id="slide2-viz" />
  </RigidBody>
  <!-- ... -->
  <!-- full example Moby/example/parts-feeder/feeder.xml -->
</RCArticulatedBody>

The RCArticulatedBody tag takes the following attributes:

• id [required] the unique identifier for the articulated body
• urdf-filename the filename to read a URDF from
• fdyn-algorithm the forward dynamics algorithm, valid choices are "crb" (the Composite Rigid Body Algorithm) and "fsab" (the Featherstone Articulated Body Algorithm)
• fdyn-algorithm-frame the frame that forward dynamics is computed in. Different choices have slight advantages for efficiency, as described in Featherstone's "Robot Dynamics Algorithms" book. Valid choices are "global", "link", "linkinertia", "linkcom", and "joint". These values can be experimented with, but the user is unlikely to see substantial practical performance differences. "global" should be avoided, however, because of issues with floating point arithmetic described in Featherstone's book.
• translate the 3D translation vector applied to the articulated body's base (and all links emanating from it, applied after rotation specified by `rpy' if any)
• rpy the roll-pitch-yaw orientation applied to the articulated body's base (and all links emanating from it)

Joints

• The following joint types are available: FixedJoint, PrismaticJoint, RevoluteJoint, SphericalJoint, UniversalJoint, PlanarJoint (cannot be embedded within a RCArticulatedBody), and Gears
• Joints can be explicit (embedded within an RCArticulatedBody) or implicit (embeddable within an RCArticulatedBody or can connect two bodies together)
• Joints are added between two rigid bodies by specifying the joint's inboard-link-id and outboard-link-id

  <!-- taken from the example above -->
  <RigidBody id="SLIDE1" color="0 0 0 0" position="0 0 0" angular-velocity="0 0 0" linear-velocity="0 0 0" visualization-id="slide1-viz" compliant="false">
    <CollisionGeometry primitive-id="slide1" />
    <InertiaFromPrimitive primitive-id="slide1-viz" />
  </RigidBody>
  <FixedJoint id="fj1" location="0 0 0" inboard-link-id="SLIDE1" outboard-link-id="SLIDE2" />
  
  <RigidBody id="SLIDE2" color="0 0.5 0 1" position="0 0.055 0.01" angular-velocity="0 0 0" linear-velocity="0 0 0" visualization-id="slide2-viz" compliant="false">
    <CollisionGeometry primitive-id="slide2" />
    <InertiaFromPrimitive primitive-id="slide2-viz" />
  </RigidBody>

Joints can take the following attributes:

• id [required] the unique identifier for the joint
• visualization-id the identifier of the primitive used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph)
• visualization-filename the filename of the geometric description used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph)
• lower-limits rigid lower limit(s) on the joint (not valid for FixedJoint or Gears)
• upper-limits rigid upper limit(s) on the joint (not valid for FixedJoint or Gears)
• q sets the initial configuration of the joint (moves the attached bodies, not valid for FixedJoint or Gears)
• qd the initial velocity(-ies) of the joint degrees-of-freedom (not valid for FixedJoint or Gears)
• q-tare labels the configuration of the joint degrees-of-freedom (does not move the attached bodies, not valid for FixedJoint or Gears)
• restitution-coeff Poisson-type restitution coefficient in [0,1] applied to joints that collide with their limits
• inboard-link-id (required) the link of the rigid body that is the inboard link of this joint (distinction between inboard/outboard only relevant for RCArticulatedBody types)
• outboard-link-id (required) the link of the rigid body that is the outboard link of this joint (distinction between inboard/outboard only relevant for RCArticulatedBody types)
• location the global location of this joint (only used for initializing the body)

Each joint tag accepts the following child nodes:

• Visualization one or more tags used for visualization
  • visualization-id the identifier of the primitive used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph, behavior unspecified if visualization-id also specified)
  • visualization-filename the filename of the geometric description used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph, behavior unspecified if visualization-filename also specified)
  • visualization-rel-origin the translation offset of the geometry used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph)
  • visualization-rel-rpy the relative orientation of the geometry, specified using roll-pitch-yaw, used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph, behavior is undefined if visualization-rel-quat also specified)
  • visualization-rel-quat the relative orientation of the geometry, specified as a unit quaternion (qw qx qy qz) used for visualizing the joint (only applicable if Moby is used with OpenSceneGraph, behavior is undefined if visualization-rel-rpy also specified)

PlanarJoint

The inboard link of a planar joints must be a disabled rigid body (see attribute enabled of RigidBody)

The planar joint takes one unique attribute:

• normal the global axis of rotation

Gears

Enforces a torque/speed relationship for one degree of freedom joints. Unlike many other popular forms of simulating joints, incorporates gyroscopic forces.

The gear joint takes one unique attribute:

** TODO: fix this **

• gear-ratio the ratio of the movement of the inboard link to the outboard link

Simulators

Two types of simulators can be created: Simulator and TimeSteppingSimulator. The latter permits contact between bodies. The former is provided for speed in robotics applications that can leverage it. The time stepping simulator computes, and steps precisely to, contacting configurations (if any).

Both Simulator and TimeSteppingSimulator accept the following attributes:

• id [required] the unique identifier for the simulator
• current-time the current time of the simulator [0,infinity]
• dissipator-id an optional kinetic energy dissipation mechanism for promoting simulation stability

Both Simulator and TimeSteppingSimulator accept the following child nodes:

• DynamicBody
  • dynamic-body-id the unique identifier of a RigidBody or RCArticulatedBody object
• ImplicitConstraint
  • joint-id the unique identifier of an implicit joint constraining two bodies
• RecurrentForce
  • recurrent-force-id the unique identifier of a force applied in a recurrent manner to bodies (like gravity)

TimeSteppingSimulator accepts the following attributes:

• min-step-size the time stepping simulator will not take steps below this size

TimeSteppingSimulator accepts the following child node:

• ContactParameters
  • object1-id (required)
  • object2-id (required)
  • epsilon the coefficient [0,1] of restitution (Poisson restitution model)
  • mu-coulomb the coefficient of Coulomb friction [0,infinity]
  • mu-viscous the coefficient of viscous friction [0,infinity]
  • friction-cone-edges the number of edges in the polygonal approximation to the friction cone
  • compliance the compliance of the contact [0,infinity], zero is equivalent to a fully rigid contact

Example:

<MOBY>
  <TimeSteppingSimulator id="simulator">
    <DynamicBody dynamic-body-id="sphere" />
    <DynamicBody dynamic-body-id="ground" />
    <RecurrentForce recurrent-force-id="gravity"  />
    <ContactParameters object1-id="ground" object2-id="sphere" epsilon="0" mu-coulomb="10" mu-viscous="0" friction-cone-edges="8" />
  </TimeSteppingSimulator>
</MOBY>

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