3. Occupant packaging and initial conditions

Before any restraint system can be optimized, the occupant must be placed in the vehicle environment in a way that is both physically plausible and numerically stable. In Human Body Model (HBM) simulations, this step is called occupant packaging. It defines where the body sits, how the belt wraps around it, how far it is from the airbag, and how all of these are expressed as parameters that the solver can vary. Poor packaging produces results that look reasonable but are physically meaningless — a belt that floats off the chest, a knee that intersects the IP, or an airbag that deploys into a gap that does not exist in reality.

Seating Posture as a Parameterized State

A seated HBM posture is not a single pose but a set of joint angles and translations. The most influential parameters are seat back angle, seat pan angle, H-point location, pelvis rotation, torso recline, head rotation, knee flexion, and ankle angle. In a typical frontal optimization study, these are not free variables; they are constrained by the vehicle package (pedal position, steering wheel location, roof height) and by occupant anthropometry (5th percentile female through 95th percentile male).

For example, a baseline 50th percentile male HBM might be positioned with a seat back angle of 23°, a seat pan angle of 5°, and an H-point 20 mm forward of the design H-point. The torso is then rotated about the H-point until the chest contacts the seat back. The head is adjusted so the Frankfort plane is approximately horizontal. These values are initial conditions, not optimization targets — but they strongly influence belt fit and airbag distance, so they must be documented and held consistent across a parametric sweep.

Belt Routing and Fit

Belt routing determines how restraint forces enter the body. The lap belt should cross the pelvis between the ASIS and the greater trochanter, not the abdomen. The shoulder belt should pass over the mid-clavicle, across the sternum, and engage the outboard shoulder. In HBM terms, this is controlled by the belt anchor coordinates, the D-ring position, the buckle location, and the belt stiffness and friction properties.

A practical parameterization uses the following inputs:

• Lap belt anchor: outboard and inboard anchor points in vehicle coordinates.
• Shoulder belt anchor: D-ring or height adjuster position.
• Buckle stalk: length and angle.
• Belt width and thickness: typically 48 mm wide, 1.2 mm thick for a standard 3-point belt.
• Pretensioner and load limiter: force–travel curve or force–time profile.

In the simulation, the belt is usually modeled as a series of 1D seatbelt elements with membrane properties. The key check is belt-to-body gap: the distance between the belt and the occupant surface at the chest, pelvis, and shoulder. A gap larger than about 10–15 mm at the chest means the belt will not engage early enough; a negative gap means initial penetration, which causes an artificial spike in the contact force at t = 0.

Airbag Proximity and Out-of-Position Considerations

Airbag proximity is defined by the distance from the occupant's chest and head to the airbag module and to the deployed bag surface. In a driver-side frontal model, the relevant measures are:

• Chest-to-steering-wheel: typically 250–350 mm for a 50th percentile male in a nominal posture.
• Head-to-airbag: measured along the deployment axis, often 300–400 mm.
• Knee-to-IP: 100–150 mm, which affects lower extremity loading.

These distances are not fixed; they change as the occupant moves forward during the crash. The optimization problem is therefore to find restraint parameters (belt force limit, airbag vent size, tether length, deployment timing) that minimize injury metrics across a range of occupant sizes and postures, while respecting the packaging constraints above.

Out-of-position (OOP) scenarios are a special case where the occupant is closer to the airbag than nominal. Here, the initial condition itself becomes a design variable. A common approach is to define a set of OOP postures — for example, chest 150 mm from the wheel, head tilted forward 30° — and evaluate the restraint system against them.

Parameterization in HBM Simulations

In practice, occupant packaging is implemented through a combination of morphing, positioning, and belt-fitting tools. The workflow typically follows this sequence:

1. Morph the HBM to the target anthropometry (stature, sitting height, chest depth).
2. Position the H-point and set seat back and pan angles.
3. Adjust joint angles to achieve the target posture (torso, head, arms, legs).
4. Fit the belt using a belt-fitting algorithm that projects the belt onto the body surface with a specified gap.
5. Define airbag geometry and initial distance to the occupant.
6. Export the parameter set as a design matrix for DOE or optimization.

The following example shows a parameter block for a 50th percentile male in a driver-side frontal model. These values are illustrative and would be replaced by actual vehicle measurements in a real study.

# Occupant packaging parameters (example)
occupant = 50th_male
seat_back_angle = 23 deg
seat_pan_angle = 5 deg
H_point_offset = [20, 0, 0] mm  # relative to design H-point
pelvis_rotation = 0 deg
torso_recline = 0 deg
head_rotation = 0 deg
knee_flexion = 90 deg
ankle_angle = 90 deg

# Belt routing
lap_anchor_outboard = [1200, 400, 200] mm
lap_anchor_inboard = [1200, -400, 200] mm
shoulder_anchor = [800, 400, 900] mm
buckle_stalk_length = 150 mm
belt_width = 48 mm
belt_thickness = 1.2 mm
belt_gap_chest = 10 mm
belt_gap_pelvis = 5 mm

# Airbag proximity
chest_to_wheel = 300 mm
head_to_airbag = 350 mm
knee_to_IP = 120 mm

Once these parameters are defined, the simulation can be run and the results inspected. The key outputs to check are belt forces, chest deflection, head acceleration, and the timing of airbag contact. If the belt force rises too early or too late, or if the head contacts the airbag before full deployment, the packaging parameters need adjustment.

Visualizing the Packaging and Initial Conditions

The SVG below summarizes the spatial relationships between the occupant, belt, airbag, and vehicle interior. It shows the key distances and angles that must be parameterized, and how they map to the simulation inputs.

Practical Checks and Common Failure Modes

After packaging, run a short settling simulation (typically 50–100 ms) with gravity and belt pretension only. Check the following:

• Belt penetration: If the belt penetrates the body surface at t = 0, reduce the initial belt gap or adjust the belt-fitting algorithm.
• Occupant motion: The occupant should settle into the seat without large rigid-body motion. Excessive motion indicates a mismatch between the H-point and the seat geometry.
• Contact forces: Initial contact forces should be near zero. A spike suggests interference between the HBM and the vehicle interior.
• Airbag distance: Verify that the measured chest-to-wheel and head-to-airbag distances match the intended values. Small errors here can shift injury metrics by 10–20%.

Summary

Occupant packaging and initial conditions are the foundation of any HBM-based restraint optimization. Seating posture, belt routing, and airbag proximity must be parameterized explicitly, documented consistently, and validated before the optimization loop begins. The parameters shown here — seat angles, H-point offset, belt anchor coordinates, belt gap, and proximity distances — form the design space that the optimizer will explore. Getting them right is not glamorous, but it is the difference between a simulation that predicts real-world performance and one that merely produces numbers.