2. Facility and environmental controls
Crash test repeatability is not achieved by the dummy and the data acquisition system alone. The test cell itself is an active part of the measurement chain. A barrier impact that produces 42.0 g at the B-pillar in one run and 44.5 g in the next may reflect a real difference in the vehicle, but it may equally reflect a 3 °C shift in cell temperature, a 15 % change in relative humidity, or a 200 lux change in lighting that altered high-speed camera exposure. This section defines the facility and environmental controls that keep those variables inside a documented band, so that any observed difference can be attributed to the test article rather than the room.
Test cell layout and fixed geometry
The layout of a crash cell is a metrology decision, not a floor-plan convenience. Every fixed element — the tow track, the barrier face, the camera pedestals, the light trusses, and the overhead crane rails — must be surveyed and recorded in a facility datum drawing. The datum is normally the barrier face plane, with the longitudinal axis of the tow track as the X axis, lateral offset as Y, and vertical as Z. All camera positions, target locations, and photogrammetry reference points are then expressed in this single coordinate system, and any change to a fixed element requires a re-survey and a revision of the drawing.
Key layout controls include:
- Barrier face perpendicularity and position: the barrier face should be verified against the datum at defined intervals (for example, quarterly) with a tolerance typically held within ±2 mm in position and ±0.1° in perpendicularity to the tow track.
- Tow track straightness and level: lateral deviation along the run-up should be controlled so that the vehicle arrives at the barrier with the intended yaw and offset. A common working tolerance is ±5 mm over the last 20 m before impact.
- Clear zone: a defined volume around the impact point must remain free of structures, cable runs, and temporary equipment, so that debris trajectory and camera sight lines are not obstructed.
- Camera and light positions: pedestals and trusses should be indexed to the datum with hard stops or pinned mounts, so that a removed camera can be returned to the same position without re-aiming from scratch.
Layout discipline matters most when a test is repeated. If the second run uses a camera that was repositioned by eye, the two image sets are no longer directly comparable, and any optical measurement derived from them inherits an unknown bias.
Lighting: quantity, quality, and consistency
High-speed cameras need far more light than the human eye. At 1,000 fps with a 1/2000 s shutter, the exposure window is extremely short, so the cell must deliver high illuminance with low flicker. Two practical requirements follow. First, illuminance at the impact zone should be measured and logged before each test, typically in the range of 100,000–150,000 lux for high-speed coverage, with uniformity across the field of view controlled to avoid hot spots that clip highlights on the vehicle surface. Second, the light source must be flicker-free at the camera frame rate; this normally means DC-driven or high-frequency ballasted sources rather than conventional AC mains lamps.
Lighting is also a repeatability variable. A lamp that has aged, a truss that has been lowered, or a diffuser that has been removed will change exposure and therefore motion blur. The practical control is to record illuminance at fixed reference points on the floor and on the barrier face before each run, and to compare against the baseline. A deviation beyond a defined band (for example, ±10 %) should trigger lamp replacement or re-aiming before the test proceeds.
Temperature and humidity
Temperature affects the test article, the barrier, the tires, and the instrumentation. Elastomers, foams, and adhesives in the vehicle are viscoelastic: their stiffness and damping change with temperature, which shifts occupant kinematics and structural response. A vehicle soaked at 5 °C will not behave like the same vehicle soaked at 25 °C. For this reason, most programs specify a soak band, commonly 20 ± 3 °C, and require the test article to reach equilibrium within that band before the run. Soak time depends on the vehicle mass and the cell's air circulation; a practical check is to place thermocouples in the cabin, in the engine bay, and on the barrier face, and to confirm that all readings are inside the band and stable.
Humidity is a secondary but real variable. High relative humidity can affect friction at the tire–track interface, promote condensation on camera lenses and windows, and influence the behavior of some foams. A working band of 30–70 % RH is common, with the cell actively conditioned rather than left to ambient. The critical control is to log temperature and humidity at the start and end of the soak period and immediately before impact, so that any excursion is documented and can be considered during data review.
Cleanliness and contamination control
Debris from a previous test is a contamination source. Glass fragments, tire rubber, and barrier material on the track or floor can alter friction, damage tires, and create false targets in camera images. The cell should be cleaned to a defined standard between tests: track swept and inspected, floor cleared of loose debris, camera windows and light diffusers wiped, and the barrier face checked for embedded fragments. A pre-test housekeeping check is a simple but effective control, and it is often the difference between a clean run and a repeat.
Integrated control: from setpoints to a pre-test record
The controls above only work when they are integrated into a single pre-test routine. The figure below shows the relationship between the fixed layout, the environmental parameters, and the go/no-go decision that precedes a run.
The decision logic is deliberately simple: if any parameter is outside its band, the run does not proceed until the deviation is corrected and re-verified. This prevents the most common audit finding in crash laboratories — a test conducted with an undocumented environmental excursion, where the resulting data cannot be defended as repeatable.
Practical pre-test record
A workable pre-test record captures the minimum set of values needed to defend repeatability. The following example is a template, not a set of measured results; the values shown are illustrative placeholders.
| Parameter | Target band | Recorded value (example) | Status |
|---|---|---|---|
| Cell temperature | 20 ± 3 °C | 21.4 °C | Pass |
| Relative humidity | 30–70 % RH | 48 % RH | Pass |
| Illuminance at impact zone | 100,000–150,000 lux | 128,000 lux | Pass |
| Barrier face position | ±2 mm from datum | +0.8 mm | Pass |
| Track lateral deviation (last 20 m) | ±5 mm | 2.1 mm | Pass |
| Housekeeping check | Completed | Signed | Pass |
When a value falls outside the band, the corrective action should be recorded alongside the deviation. For example, if the cell temperature reads 16 °C after a cold night, the vehicle may need additional soak time; if illuminance reads 85,000 lux, a lamp may need replacement. Recording both the deviation and the action creates the traceability that auditors look for, and it gives the test engineer a factual basis for deciding whether a completed run should be repeated.
In summary, facility and environmental control in a crash laboratory rests on four pillars: a surveyed and frozen layout, quantified lighting, conditioned and logged temperature and humidity, and disciplined cleanliness. Each pillar has a measurable band, a verification method, and a record. Together they ensure that when two tests differ, the difference belongs to the test article — not to the room around it.