4. Load Limiter Level Sweep vs. Chest Compression

Why Sweep the Load Limiter Alone?

In a restraint system, the pretensioner and the load limiter act on the same occupant through different mechanisms and at different times. The pretensioner removes slack and couples the occupant to the belt early in the event; the load limiter then controls how much belt force is allowed to build as the occupant loads the system. If you change both at once, you cannot attribute a change in chest compression to either component. This case isolates the load limiter: the pretensioner profile is frozen, and only the load limiter force level is swept. The goal is to find the regime where chest compression and head excursion are dominated by the load limiter plateau rather than by pretensioner behavior.

The sweep answers a practical calibration question: at what plateau force does the load limiter become the controlling variable for chest deflection? Below that regime, the pretensioner and belt geometry dominate; above it, the plateau dominates and the response becomes predictable and monotonic. Identifying the boundary lets you tune the load limiter with confidence and avoid chasing pretensioner changes that no longer matter.

Fixed Pretensioner Profile and Swept Parameter

Hold the pretensioner constant so it cannot confound the result. A representative fixed profile for this exercise:

• Pretensioner peak force: 2.8 kN, reached at 12 ms after trigger.
• Pretensioner rise time: 12 ms (linear ramp from 0 to peak).
• Pretensioner stroke/payout: 60 mm of belt retraction before the force decays.
• Belt geometry and anchor positions: unchanged across all runs.

The swept parameter is the load limiter plateau force F_LL. Use a geometric or evenly spaced set that brackets the expected transition, for example:

F_LL = [2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0]  # kN

Each run uses the same occupant model, the same initial conditions (e.g., 50 km/h frontal pulse, 50th-percentile male Hybrid III), and the same belt routing. Only F_LL changes. This is the definition of a controlled sweep: one independent variable, everything else pinned.

Running the Sweep and Capturing Outputs

For each F_LL, run the simulation or sled test and record the two response metrics that matter for this trade-off:

• Chest compression — peak chest deflection (mm) and the time of peak.
• Head excursion — peak forward head travel (mm) relative to the seat reference point.

Also log the belt force time history so you can confirm the plateau is actually being reached. A run where the belt force never reaches F_LL is not in the load-limiter-dominated regime — the pretensioner and geometry are still controlling. That distinction is the whole point of the sweep.

Reading the Sweep: Where the Load Limiter Takes Over

Plot chest compression and head excursion against F_LL. In the low-force region, chest compression is relatively insensitive to F_LL because the pretensioner and belt stretch absorb the early energy and the plateau is reached late. As F_LL rises, the plateau is reached earlier and the belt force tracks it, so chest compression increases with a steeper, more consistent slope. The knee in that curve — where the slope stabilizes — marks the onset of the load-limiter-dominated regime.

The knee in the chest-compression curve is the calibration target. To the left of it, changing F_LL produces small, inconsistent changes in chest compression because the pretensioner phase and belt geometry still dominate the peak. To the right, the plateau is reached before peak compression, and the response becomes monotonic and predictable. Head excursion follows a similar but flatter trend, which is expected: the load limiter primarily governs chest loading, while head excursion is more sensitive to belt geometry and pretensioner coupling.

Confirming the Regime with Belt Force Timing

Do not rely on the curve shape alone. For each run, check the belt force time history against the chest compression peak:

The transition is where the plateau-reached time approaches and then precedes the peak-compression time. Once the plateau is reached well before peak compression, the load limiter is controlling the peak, and further increases in F_LL produce consistent, monotonic changes in chest compression. That is the operational definition of the load-limiter-dominated regime for this configuration.

Deliverable: Sweep Summary Table

Record the sweep in a compact table so the regime boundary is auditable and reproducible:

Fill the response columns from the simulation or test output, then mark each run's regime using the plateau-timing criterion above. The first run where the plateau is reached before peak compression, and where the chest-compression slope stabilizes, defines the lower bound of the load-limiter-dominated regime. That boundary is the actionable result of this case: it tells the calibration engineer the minimum plateau force at which the load limiter becomes the controlling variable, so subsequent tuning of F_LL can proceed without confounding from the pretensioner.