Contact Force Formulation

Contact forces between the body segments and the environment or objects are created using ellipsoid-plate contact elements or the solid-solid contact algorithm. The segment ellipsoids created with the base model are typically used to create the contact force between each segment and the objects of the environment which may include vehicle interiors, chairs seats, sports equipment handles, etc.

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

Ellipsoid - Plate Contact Algorithm

In LifeMOD™ versions 2005.1 and beyond, there are two contact algorithms available to model the interaction between the human model and the external environment (see fall tutorial) or internal contact physics (see total knee replacement tutorial).

The original ellipsoid-plane algorithm, can calculate the contact physics between any ellipsoid and any plane. The ellipsoids may be individual body segments of the model (see Figure 1) or segments representing single bones (see Figure 2). This algorithm runs efficiently and results in fast contact calculations.

The new solid-solid algorithm generally allows for solid-solid contact interaction between any two solid shapes. These may be bones, ellipsoids, crash dummy segments, and any object in the environment. In addition, the user may import any Parasolids geometry for contact between the object and the human model. The contact calculation is very accurate and creates normal forces based on depth and velocity of penetration and transverse forces of friction (both stiction and dynamic).

The general form of the contact force function for each algorithm is displayed in the next series of figures.

Figure 1: Normal force equation for both ellipsoid-plane and solid-solid algorithms.

Figure 2: Contact damping coefficient relation to penetration.

Figure 3: Relationship of friction coefficient to slip velocity

Generalized Surface Contact Algorithm

The surface contact algorithm used in LifeMOD is the RAPID contact engine avaliable in the ADAMS/Solver, which supports:

Multiple contacts
Dynamic friction
Contact between three-dimensional solid geometries
Contact between two-dimensional geometries

It does not support non-solid three-dimensional geometries, such as shells that do not encompass a volume, and sheets. It also does not support contact between two-dimensional and three-dimensional geometries.

The geometry engine is responsible for detecting contact between two geometries, locating the points of contact, and calculating the normal force at the contact points.

Once the contact kinematics are known, contact forces, which are a function of the contact kinematics, are applied to the intersecting bodies.