Contacts

Contact forces between the body segments and the environment are created using ellipsoid-plane contact elements or the solid-solid contact algorithm. In LifeMOD™ versions 2005.1 and beyond, there are two major contact algorithms available to model the interaction between the human model and the external environment (see fall tutorial) or internal contact physics such as the articular contact joints in the knee (see Figure 3 and the total knee replacement tutorial).

Various contact parameters may be adjusted. See Parameters to tune the model for simulation. Choosing Model Parameters offers more information on data sources and on how to select the parameters mentioned in this section.

Sections:

• Types of Contact
• Creating Contacts between the Base Segment Set and the Environment
• Creating an Individual Contact Force
• Deleting Contact Forces

Types of Contact

Ellipsoid-Plane Contact Elements

The ellipsoid-plane contact elements are the fastest, easiest method of generating contact interactions. The contact produces a normal force and a transverse friction force based on user-specified parameters. The parameters are consistent with actual flesh contacting a surface. They may be based on the underlying ellipsoidal geometry of each body segment as in Figure 1 or discretized ellipsoids representing geometrically homogeneous portions of the body as in Figure 2.

Figure 1: Ellipsoid-contact surface

Ellipsoid-Ellipsoid Contact Elements

The ellipsoid-ellipsoid contact elements are a variation of the ellipsoid-plane elements and are used to create contact points between simple geometric bodies. They are very rarely used, however: mainly in the case of contact interaction between multiple human models (see Human Punching tutorial).

Additional contact elllipsoids may be created for other contact effects such as the condyler contact in a force-based knee joint (see Total Knee Replacement tutorial), or for portions of an articulating segment like those in a detailed foot (see Figure 2).

Figure 2: Contact ellipsoids for whole body or portions of the foot.

Solid-Solid Contact Algorithm

The solid-solid contact algorithm created with the base model is typically used to create the contact force between complex segments and objects in the environment which may include vehicle interiors, chairs, seats, sports equipment handles, etc. Figure 3 illustrates the complex, irregularly-shaped geometry that requires the solid-solid contact algorithm.

Figure 3: Contact forces may be created for internally-contacting surfaces such as the two articular joints in the knee.

Creating Contacts Between the Base Segment Set and the Environment

Contact -> Create BASE Set
To begin creating a base set of contacts, select either the ellipsoid-plane or the solid-solid contact algorithm.

For each algorithm, the contact properties require eight values:

• Contact stiffness
• Exponent
• Damping coefficient
• Penetration depth when full damping is employed
• Static friction
• Dynamic friction
• Friction transition velocity
• Stiction transition velocity

See the appendix for a visualization of these parameters.

For the ellipsoid-plate contact algorithm (see panel shown in Figure 4) enter the contact surface marker (z-axis located normal to the contact surface) and the surface X and Y lengths (see Figure 1). A segment may contact many such surfaces to create contoured surfaces and objects.

Figure 4: Contact force creation panel for the ellipsoid-plane contact algorithm

For the solid-solid contact algorithm (Figure 5), select either Segment Ellipsoid or Crash Dummy geometry and enter the contact solid in the panel in the same row. A reference marker may be added to specify the contact force results reference frame.

Figure 5: Contact force creation panel for the solid-solid contact algorithm

For both algorithms, check the appropriate boxes next to the sements that will contact other surfaces after entering the parameters.

Creating an Individual Contact Force

Contact -> Create Individual
Figure 6 displays the panel to create a single contact force for the ellipsoid-plate algorithm. This panel is usually used to create contact forces between non-standard segments and the environment or between any two segments (see Total Knee Replacement tutorial). Figure 7 displays the creation panel for a single solid-solid contact force.

Figure 6: Single contact creation panel set up for ellipsoid-plane contact.

Figure 7: Single contact creation panel set up for solid-solid contact.

Deleting Contact Forces

Contact -> Delete
This panel is used to delete the contact elements. Contact elements may be deleted one at a time or the complete set may be deleted.

Figure 8: Panel used to delete contacts