Analysis

There are four types of simulations or analyses run that LifeMOD/BodySIM™ can run: equilibrium, passive, inverse dynamics (or kinematics) and forward dynamics.

Sections:

• Performing Simulations
• Equilibrium Simulation
• Passive Simulation
• Inverse Dynamics Simulation
• Forward Dynamics Simulation
• Adjusting the Body Velocity Initial Conditions

Performing Simulations

Each type of simulation runs from the panel displayed in Figure 1. This panel allows for the specification of the gravity vector, the start and end time of the simulations and the integrator settings. Four types of integrator settings are available in LifeMOD/BodySIM and described in Table 1:

• Default
• Contacts optimized
• Robust
• User_defined

When simulations are completed, the results may be saved and renamed using the Save Analysis button. By doing so, successive simulation results may be compared and overlaid (see Results).

Figure 1: Panel used to run the dynamics simulation

Equilibrium Simulation

Figure 2 displays the analysis panel used for the equilibrium simulation. This type of simulation is performed when the motion capture data is read in and the motion agents are created on the model. This analysis minimizes the energy in the motion agent springs or similarly reduces the offset error between the motion marker locations and the locations on the human model.

During this process, the user may wish to alter the motion agent weighting parameters to change the way the model equilibrates. This is done in the Parameters panel.

Figure 2: Panel used to run the equilibrium simulation.

The "Freeze Motion Agents for Equilibrium Analysis" selection holds the motion agents to the initial position throughout the entire analysis in order to equilibrate the model with the data. Figure 3 displays the model before and after the equilibrium analysis. After the model is equilibrated, the model posture and location is updated using the "Update Model Posture with Equilibrium Results" button. This action changes the actual position of the model and adjusts the joints based on the new equilibrium configuration.

Following this, the positions of the motion agents attachments on the model are synchronized with the data locations (see Figure 4).

Figure 3: Model before equilibration (left) and after (right)

Figure 4: Motion agents before synchronization (top) and after (bottom)

Passive Simulation

Passive simulations analyze how a human model reacts to the environment. The environment inflicts forces on the segments and the segments respond. The joints created for this type of analysis are passive or Hybrid III crash dummy joints. (see Figure 5) They are kinematic joints which include spring-damper torques at each degree-of-freedom for each joint (see Joints Chapter).

This mode of analysis typically simulates any situation where forces are inflicted on the body during injury-producing events. (see Fall and Car Crash tutorials).

For some analyses, only a portion of the body needs to be passive, while the remainder of the body is active. In this case, the human model is constructed with both passive and active joint elements (see Bicycle Rider tutorial).

Figure 5: Three success frames of a passive simulation for a fall

Inverse Dynamics Simulation

Inverse dynamics simulations are performed for models which are manipulated using motion agents (see Motion). They may be viewed as small "fingers" manipulating the model.

The model is bound to the physics of the environment and itself during this manipulation, therefore the feet will not go through the floor, the joints will not be bent beyond their capacity, etc.

Data used to drive the motion agents may be entered directly into a data spline (see Cervical Spine tutorial), or by importing marker set trajectory data generated from a motion capture system.

This type of simulation is usually precedes the forward dynamics simulation. It is used to "train" the PD-servo joints and muscles by capturing the joint angulation and the shortening/lengthening patterns. The accumulated data is then used in the joint torque formulations or the muscle contractile elements to re-create the motion generated by the motion agents.

Figure 6: Animation sequence for the inverse-dynamics analysis, and close-up view of motion agent activity (right).

Forward Dynamics Simulation

After performing an inverse dynamics simulation, the motion agents are stripped from the model and the recorded joint angulations and shortening/lengthening patterns are used in torque and force functions at the joints. The angulations are used as target figures in the PD-servo controller elements automatically created during the joint editing function (see Joints), and the shortening/lengthening patterns are used in PD-servo linear actuators (see Soft Tissues).

These elements then drive the model in the manner developed in the inverse dynamics simulation. During the forward dynamics simulation, the model is guided by the internal forces (joint torques and/or muscle forces) and influenced by the external forces (gravity, contact, etc.).

Figure 7: Example of a forward dynamics simulation, with trained muscles (based on shortening/lengthening patterns from the inverse dynamics simulation) as the force drivers in the simulation

Adjusting Body Velocity Initial Conditions

Analyze -> Body Velocity Initial Conditions
Figure 5 displays the panel to set the body velocity initial conditions. Both translation and angular velocities may be set for the model. When this panel is used the values will apply to every body segment (see Fall Tutorial).

Figure 5: Panel used to set the body velocity initial conditions