# Short Glossary

## Animation Path

Found in the Create > Animation > Animation Path. This feature creates "path animations" where rigid body objects are forced to follow spline paths along which objects accelerate, decelerate, or undergo uniform motion according to the user's prescribed input values. This is different from simulation, in which an object's motion is determined time-step by time-step, by using Newton's laws.

## Auto-ees

A data container found in the project menu in the left-side control panel. The auto-ees object controls the input parameters for simulated collisions using the Kudlich-Slibar model. Under its "defaults" menu, the user can specify the depth of penetration, coefficient-of-friction, coefficient-of-restitution, and pitch for Kudlich-Slibar collisions. Under the "object 1" and "object 2" menus, the user can access the corresponding Delta-V, pre- and post-impact speeds, pre- and post-impact yaw rates, and PDOF angles. Under the "selection" menu, the user can scroll through the various impulses that are exchanged during a simulation. Using "create user contact", the user can create a single instance of an ees object in order to modify the parameters for an individual impulse exchange. Doing this allows the user to move the impulse centroid position and enable deformation.

See:

http://www.vcrashusa.com/guide-chapter10

http://www.vcrashusa.com/guide-chapter12

## Axles

Found in the "properties" menu in the left-side control panel. In this menu the user can add and remove axles, set the vehicle overhang, cg to front axle, wheelbase, track widths, specify tire sizes, chose between constant (default), linear, and tm-easy tire force models, set steering axles, set drive wheels, and specify suspension properties.

Found in the “joint” menu, this is the damping coefficient for joint coupling. Every joint type, including the multibody ragdoll joint, uses a damping coefficient which helps enforce joint integrity.

Note, the damping coefficient value used internally by the simulation engine is of the form:

$$b_{internal} = {m_{1}+m_{2} \over 1~kg} \cdot b_{user~input}$$

where $$m_{1}$$ and $$m_{2}$$ are the masses of the connected objects. This conversion helps ensure simulations remain stable for increases in linked object masses.

Note, if one of the connected objects is infinite in mass (using Physics > Make Unyielding / Terrain from Selection), then the internally used damping coefficient is:

$$b_{internal} = {m \over 1~kg} \cdot b_{user~input}$$

where $$m$$ is the mass of the finite mass rigid body object.

## Damping (in “fix orientation” menu)

Found in the “fix orientation” menu, this is the damping constant that controls the damping torque response versus articulation angle rate for a given joint.

Note, for a joint angle of articulation about the joint’s x, y, or z axis, the magnitude of the torque response is given by about the given axis is:

$$|\tau_{damping}| = (1~m) \cdot b_{internal} \cdot (1~m) \cdot |\dot{\theta}|$$

where $$\dot{\theta}$$ is articulation angle rate about the axis. An effective 1 meter lever-arm is used for internal unit conversion. Thus, we can rewrite the torque response by:

$$|\tau_{damping}| = \tilde{b} \cdot |\dot{\theta}|$$

where $$\tilde{b}$$ describes the torque response per unit change in articulation angle rate, and is given by:

$$\tilde{b} = (1~m)^{2} \cdot b_{internal}$$

Note, the damping coefficient value used internally by the simulation engine is of the form:

$$b_{internal} = {m_{1}+m_{2} \over 1~kg} \cdot b_{user~input}$$

where $$m_{1}$$ and $$m_{2}$$ are the masses of the connected objects. This conversion helps ensure simulations remain stable for increases in linked object masses.

Note, if one of the connected objects is infinite in mass (using Physics > Make Unyielding / Terrain from Selection), then the internally used damping coefficient is:

$$b_{internal} = {m \over 1~kg} \cdot b_{user~input}$$

where $$m$$ is the mass of the finite mass rigid body object.

Found in the “limits” menu, this is the spring constant that controls the torque response versus articulation angle rate, when the articulation angle is beyond the input min/max limit value for a given joint.

Note, for a joint angle of articulation about the joint’s x, y, or z axis, the magnitude of the torque response is given by about the given axis is:

$$|\tau_{damping}| = (1~m) \cdot b_{internal} \cdot (1~m) \cdot |\dot{\theta}|$$

if $$\theta_{final} > \theta_{max}$$ or $$\theta_{final}<\theta_{min}$$ and 0 otherwise. Here $$\dot{\theta}$$ is articulation angle rate about the axis. An effective 1 meter lever-arm is used for internal unit conversion. Thus, we can rewrite the torque response by:

$$|\tau_{damping}| = \tilde{b} \cdot |\dot{\theta}|$$

where $$\tilde{b}$$ describes the torque response per unit change in articulation angle rate, and is given by:

$$\tilde{b} = (1~m)^{2} \cdot b_{internal}$$

Note, the damping coefficient value used internally by the simulation engine is of the form:

$$b_{internal} = {m_{1}+m_{2} \over 1~kg} \cdot b_{user~input}$$

where $$m_{1}$$ and $$m_{2}$$ are the masses of the connected objects. This conversion helps ensure simulations remain stable for increases in linked object masses.

Note, if one of the connected objects is infinite in mass (using Physics > Make Unyielding / Terrain from Selection), then the internally used damping coefficient is:

$$b_{internal} = {m \over 1~kg} \cdot b_{user~input}$$

where $$m$$ is the mass of the finite mass rigid body object.

## EES

The equivalent energy speed. See Chapter 10 of the User's Guide for the mathematical development  of EES.

## Exclude overlapping

Vehicle menu found in the left-side control panel.  The exclude overlapping parameters control the fraction of vertices to be excluded from the geometrical size calculations. This is useful, for example, for excluding side view mirrors from the vehicle width calculation.

## Friction-body

Found in a rigid-body object's "contact" menu in the left side control panel (note, a rigid body is any object given physics attributes via "Create > Physics > Make Rigid Body From Selection". This includes vehicles). This parameter sets the upper-limit friction threshold for rigid-body object versus rigid-body object contacts.  It is assumed that interacting objects will come to a common velocity at a given point-of-contact in both the normal direction, as well as the tangent direction, so long as the friction threshold value is large enough to reduce the tangent component of the closing-velocity vector to zero. If this value is too low, the system's separation velocity vector will have a non-zero tangent component at the given point-of-contact. Rigid-body objects (including multibodies) versus other rigid-body contacts use the multi-point contact model ("default-auto") to estimate the distributed impulses.

• For a multibody object impacted by another rigid-body object such as a vehicle, only the multibody's friction-body value will be used by the collision model.

• For two rigid-body objects (not multibodies) undergoing collision under the “default-auto” model, the friction value used for the collision will be the minimum value of the friction-body values specified for either object.

• In Virtual CRASH 4, if the user transforms a multibody using the convert > "to rigid body" feature, the friction-body value used for the collision with any other object will be minimum value of the friction-body values specified for either object.

• Note: by default the Kudlich-Slibar impulse-momentum model is used for vehicle versus vehicle impacts. The Kudlich-Slibar model is only enabled if both objects have "Kudlich-Slibar" selected for the preferred model in the "contact" menu.

## Friction-ground

Found in a rigid-body object's "contact" menu in the left side control panel (note, a rigid body is any object given physics attributes via "Create > Physics > Make Rigid Body From Selection". This includes vehicles). This parameter sets the upper-limit friction threshold for rigid-body object versus terrain (ground) contacts.  It is assumed that interacting objects will come to a common velocity at a given point-of-contact in both the normal direction, as well as the tangent direction, so long as the friction threshold value is large enough to reduce the tangent component of the closing-velocity vector to zero. If this value is too low, the system's separation velocity vector will have a non-zero tangent component at the given point-of-contact. Rigid-body objects (including multibodies) versus terrain contacts use the multi-point contact model ("default-auto") to estimate the impulses distributed over the rigid-body objects as a result of direct contact with the terrain mesh.

## GEV

GEV = |Delta-V|/EES. GEV is interpreted as indicating whether two vehicles will continue relative sliding motion (along the tangent plane direction) post-impact.

0.9 < GEV < 1.2 is interpreted as no post-impact sliding

0.75 < GEV < 0.9 is the start of post-impact sliding

GEV < 0.75 with post-impact sliding (like in a sideswipe)

## Point array

Found in Create > Extended Primitives 3D > Point Array. This is a data container object primarily used for displaying total station data. Points can also be entered into a point array by hand using the "create" button. The Total Station Surface builder can be enabled by expanding the "triangulation" menu, and left-clicking on "triangulate".

## Restitution-body

Found in a rigid-body object's "contact" menu in the left side control panel  (note, a rigid body is any object given physics attributes via "Create > Physics > Make Rigid Body From Selection". This includes vehicles). This parameter sets the negative ratio of the normal axis projection of the separation to closing-velocity vectors for rigid-body object versus rigid-body object contacts.  It is assumed that interacting objects will come to a common velocity at a given point-of-contact in the normal direction. Rigid-body objects (including multibodies) versus other rigid-body contacts use the multi-point contact model ("default-auto") to estimate the distributed impulses.

• For a multibody object impacted by another rigid-body object such as a vehicle, only the multibody's restitution-body value will be used by the collision model.

• For two rigid-body objects (not multibodies) undergoing collision under the “default-auto” model, the restitution value used for the collision will be the maximum value of the restitution-body values specified for either object.

• In Virtual CRASH 4, if the user transforms a multibody using the convert > "to rigid body" feature, the restitution-body value used for the collision with any other object will be maximum value of the restitution-body values specified for either object.

• Note: by default the Kudlich-Slibar impulse-momentum model is used for vehicle versus vehicle impacts. The Kudlich-Slibar model is only enabled if both objects have "Kudlich-Slibar" selected for the preferred model in the "contact" menu.

## Restitution-ground

Found in a rigid-body object's "contact" menu in the left side control panel  (note, a rigid body is any object given physics attributes via "Create > Physics > Make Rigid Body From Selection". This includes vehicles). This parameter sets the negative ratio of the normal axis projection of the separation to closing-velocity vectors for object versus terrain (ground) contacts.  It is assumed that interacting objects will come to a common velocity at a given point-of-contact in the normal direction. Rigid-body objects (including multibodies) versus terrain contacts use the multi-point contact model ("default-auto") to estimate the impulses distributed over the rigid-body objects as a result of direct contact with the terrain mesh.

Found in the “joint” menu, this is the spring constant for joint coupling. Every joint type, including the multibody ragdoll joint, uses a spring constant which enforces the joint integrity.

Note, the spring constant value used internally by the simulation engine is of the form:

$$k_{internal} = {m_{1}+m_{2} \over 1~kg} \cdot k_{user~input}$$

where $$m_{1}$$ and $$m_{2}$$ are the masses of the connected objects. This conversion helps ensure simulations remain stable for increases in linked object masses.

Note, if one of the connected objects is infinite in mass (using Physics > Make Unyielding / Terrain from Selection), then the internally used spring constant is:

$$k_{internal} = {m \over 1~kg} \cdot k_{user~input}$$

where $$m$$ is the mass of the finite mass rigid body object.

## Spring (in “fix orientation” menu)

Found in the “fix orientation” menu, this is the spring constant that controls the torque response versus articulation angle beyond the initial joint articulation angle for a given joint.

Note, for a joint angle of articulation about the joint’s x, y, or z axis, the magnitude of the torque response is given by about the given axis is:

$$|\tau_{spring}| = (1~m) \cdot k_{internal} \cdot (1~m) \cdot |(\theta_{final} - \theta_{initial})|$$

where $$\theta_{initial}$$ is the initial articulation angle about the axis and $$\theta_{final}$$ is the final articulation angle. An effective 1 meter lever-arm is used for internal unit conversion. Thus, we can rewrite the torque response by:

$$|\tau_{spring}| = \tilde{k} \cdot |(\theta_{final} - \theta_{initial})|$$

where $$\tilde{k}$$ describes the torque response per unit change in articulation angle, and is given by:

$$\tilde{k} = (1~m)^{2} \cdot k_{internal}$$

Note, the spring constant value used internally by the simulation engine is of the form:

$$k_{internal} = {m_{1}+m_{2} \over 1~kg} \cdot k_{user~input}$$

Where $$m_{1}$$ and $$m_{2}$$ are the masses of the connected objects. This conversion helps ensure simulations remain stable for increases in linked object masses.

Note, if one of the connected objects is infinite in mass (using Physics > Make Unyielding / Terrain from Selection), then the internally used spring constant is:

$$k_{internal} = {m \over 1~kg} \cdot k_{user~input}$$

where $$m$$ is the mass of the finite mass rigid body object.

Found in the “limits” menu, this is the spring constant that controls the torque response versus articulation angle beyond the input min/max limit value for a given joint.

Note, for a joint angle of articulation about the joint’s x, y, or z axis, the magnitude of the torque response is given by about the given axis is:

$$|\tau_{spring}| = (1~m) \cdot k_{internal} \cdot (1~m) \cdot |(\theta_{final} - \theta_{limit})|$$

if $$\theta_{final} > \theta_{max}$$ or $$\theta_{final}<\theta_{min}$$ and 0 otherwise. Here $$\theta_{limit}$$ is the minimum or maximum joint articulation angle limit input by the user, and $$\theta_{final}$$ is the final articulation angle. An effective 1 meter lever-arm is used for internal unit conversion. Thus, we can rewrite the torque response by:

$$|\tau_{spring}| = \tilde{k} \cdot |(\theta_{final} - \theta_{limit})|$$

where $$\tilde{k}$$ describes the torque response per unit change in articulation angle, and is given by:

$$\tilde{k} = (1~m)^{2} \cdot k_{internal}$$

Note, the spring constant value used internally by the simulation engine is of the form:

$$k_{internal} = {m_{1}+m_{2} \over 1~kg} \cdot k_{user~input}$$

where $$m_{1}$$ and $$m_{2}$$ are the masses of the connected objects. This conversion helps ensure simulations remain stable for increases in linked object masses.

Note, if one of the connected objects is infinite in mass (using Physics > Make Unyielding / Terrain from Selection), then the internally used spring constant is:

$$k_{internal} = {m \over 1~kg} \cdot k_{user~input}$$

where $$m$$ is the mass of the finite mass rigid body object.