Physical Properties

The Dynamics->Physical Properties command in the Actor module allows you to assign and edit an object's (articulated chain, model, or hierarchy of models) physical properties. You must first assign physical properties to compute the object's centre of gravity and inertia tensor for simulations.

Note:

If assigning physical properties to an articulated chain, you must assign physical properties to all joints.

Procedure

1. Select the chain or object to which you want to assign properties or whose properties you want to modify.
2. Choose the Dynamics->Physical Properties command.
   
   The Model Physical Properties dialogue box is displayed.
   
   
3. Set the values for each option.
4. Click Ok All to accept the physical properties for all selected elements at once, or click Ok to accept physical properties for one element at a time.

Parameters

Density

Sets the object density. Zero or negative values are invalid.

Volume

Volume is automatically determined by the object's geometry. For an articulated chain, the geometry of the joint's Joint Model is used only if you assign a density value to it. Otherwise, a default volume is calculated based on a skin with a default radius and the joint's length. Polygon mesh or patch objects (such as a grid) that normally have a zero volume are assigned a default minimal volume.

Mass

Displays the object's computed mass (density multiplied by volume). Infinite and very small values, such as an object with hardly any mass, should not be used because the calculations required may introduce numerical errors into the simulation.

Elasticity

Used for collisions. The value defines the approximate energy lost on impact as follows:

0 means all the energy is lost: the object does not rebound.

1 means all the energy is conserved: the object rebounds with the same amount of energy, but not necessarily the same height.

Roughness (Static) and Roughness (Kinetic)

These sliders allow you to define the interaction between two surfaces by setting the amount of static and kinetic inertia of the object's surface. Static Roughness describes roughness for stationary objects, while Kinetic Roughness describes roughness for objects in motion.

Only the Static Roughness parameter is considered for impact collisions, while both are required for continuous contact collisions. Typical values for these parameters are between 0.0 and 1.0, but you can enter a greater value by typing it directly into the slider's text box.

Typical values are:

On impact:

• Static 0 - perfectly smooth, the object slips on impact.
• Static 1 - completely rough, the object does not slip.

On continuous contact collisions for a bounding sphere:

• Static 0 - the object slips and does not roll.
• Static 1 - the object rolls without slipping as it moves on the obstacle.
• Kinetic 0 - no energy is lost as the object moves relative to the obstacle.
• Kinetic 1 - most of the energy is lost and the object quickly comes to a stop.

On continuous contact for bounding boxes or actual shapes:

• Static - describes the friction that prevents an object from sliding (such as a block on an inclined plane).
• Kinetic - describes the friction that makes an object lose energy while moving.

Node Collision Seen As

These options override the collision detection type set in the dynamics Option Simulation Setup dialogue box (see Dynamics->Option Setup) unless the In Option Setup option is selected. Different types of detections are useful when a hierarchy is made of objects with different shapes.

Restore

Resets all parameters to their default values.

Key

Creates a keyframe at the current frame position for elasticity, static roughness, and kinetic roughness. You can view the resulting function curves by choosing the FcrvSelect->Dynamics command and the appropriate option. Since this dialogue box stays open when you click Key, you can move the time line pointer and continue to set keyframes.

Ok

Accepts the new parameter settings for each valid element selected one at a time. This permits you to assign different values to individual joints in a chain, or nodes in a hierarchy. Each element is highlighted in turn.

Cancel

Cancels the last parameter setting change for a single element.

Ok All

Accepts the new parameter settings and copies them to every valid element currently selected.

Cancel All

Exits the dialogue box.

Last updated 02-apr-1998