3. Euro NCAP knee mapping test setup
Knee mapping is the preparatory measurement stage that underpins the entire lower-leg assessment in Euro NCAP frontal impact protocols. Before any sled test or full-scale crash test can be scored, the vehicle's lower instrument panel must be characterized: where the knee of a 50th percentile male occupant would actually strike, how stiff the surface is at that location, and how the geometry varies across the knee-slider travel. The mapping test produces a grid of coordinates and stiffness values that later define the impact points used in the knee impactor test. Getting the setup wrong here propagates directly into mispositioned impact points and invalid scoring downstream.
Impactor and Measurement Hardware
The mapping procedure uses a dedicated knee-mapping impactor, not the certification knee impactor used for scoring. The mapping impactor is a simplified hemispherical or spherically-faced probe mounted on a stiff load cell and displacement transducer assembly. Its purpose is geometric and quasi-static characterization, so it is driven slowly and its force–displacement response is recorded rather than its peak acceleration.
Typical mapping impactor specifications used in Euro NCAP-aligned laboratories are summarized below. These values are representative of common lab practice and should be verified against the current protocol revision before use.
| Parameter | Typical value / requirement | Notes |
|---|---|---|
| Probe geometry | Spherical face, Ø 70–80 mm | Matches the effective knee contact patch; smaller probes over-localize stiffness |
| Probe material | Rigid steel or aluminum, polished face | Must not deform under mapping loads |
| Load cell capacity | ≥ 2 kN, resolution ≤ 5 N | Mapping forces are low; resolution matters more than range |
| Displacement measurement | Linear encoder or LVDT, ≤ 0.1 mm resolution | Used to derive local stiffness |
| Actuation | Motorized linear stage, 5–20 mm/s | Quasi-static; rate must be slow enough to avoid dynamic stiffening |
| Data acquisition | ≥ 1 kHz per channel, synchronized force/displacement | Enables stiffness slope extraction |
The probe face must be clean and free of nicks; a damaged face creates a false local stiffness peak that will later be mistaken for a hard structure behind the panel. Calibrate the load cell and displacement channel on the day of the test and record the calibration factors in the test log, because the stiffness values are derived from the ratio of these two channels.
Mounting the Impactor and Establishing the Reference Frame
Mapping is performed with the vehicle body in a fixed, repeatable condition. The body-in-white or trimmed body is mounted on a rigid fixture or stands so that the vehicle's design position is reproduced: ride height set to the nominal value, doors closed, and the instrument panel fully assembled with all production trim, airbag modules, and knee bolster components in place. Removing trim for access invalidates the result, because the trim and bolster are precisely what the occupant contacts.
The impactor is carried on a multi-axis positioning system — typically a gantry or an articulated arm with linear X, Y, Z adjustment and a rotary tilt axis. The critical step is defining the vehicle coordinate reference. Euro NCAP uses the vehicle manufacturer's coordinate system, with X positive forward, Y positive to the left, and Z positive upward, origin at the vehicle datum. Every mapped point is recorded in this frame, and the positioning system must be zeroed against it before the first point is taken.
A practical zeroing sequence:
- Establish the vehicle datum using the manufacturer's reference holes or a laser tracker against the CAD datum.
- Touch the probe to a known datum surface and set the positioning system's origin to match.
- Verify with a second reference point at least 500 mm away; residual error should be under 1 mm.
- Record the zero offsets in the test log so the run can be reconstructed.
Defining the Target Point Grid
The target points are not chosen arbitrarily. They are derived from the knee position of the occupant reference model at the design seating position, projected onto the lower instrument panel. The protocol defines a mapping region bounded by the lateral extent of the knee travel and the vertical band between the top of the knee bolster and the lower edge of the panel.
A workable grid definition for a driver-side mapping run:
- Lateral span: from the outboard knee centerline to the inboard knee centerline, extended by ±50 mm to capture edge effects.
- Vertical span: from 20 mm above the knee bolster top edge to 20 mm below the panel lower edge.
- Grid pitch: 25 mm in both directions, giving a dense enough field to resolve local hard points such as steering column brackets, fuse box covers, and body-side reinforcements.
- Boundary points: additional points at the panel edges and at any visible fastener or bracket location, because these are the most likely hard points.
At each grid point the probe is advanced normal to the local surface to a defined penetration depth — commonly 15 mm — and the force–displacement curve is recorded. Local stiffness is taken as the slope of the linear portion of the unloading or loading curve, typically between 5 mm and 15 mm penetration. Points whose stiffness exceeds a threshold (for example, 150 N/mm in a representative lab criterion) are flagged as hard points and become mandatory impact locations in the subsequent knee impactor test.
Executing the Mapping Run and Reading the Result
With the frame zeroed and the grid defined, the run proceeds point by point. At each location the operator advances the probe to contact, verifies that the probe axis is normal to the local surface (a tilt error of more than about 10° biases the stiffness reading), and drives to the target penetration. The force–displacement trace is captured and the stiffness slope extracted automatically by the acquisition software.
Two practical checks catch most setup errors before they contaminate the dataset. First, repeat the first grid point at the end of the run; a difference of more than 10% in stiffness indicates drift in the fixture or a loose probe. Second, map a known rigid reference block at the start and end of the session; its stiffness should be effectively infinite and identical both times. If the reference block reads soft, the load cell or displacement channel is misbehaving.
The output of the run is a coordinate–stiffness table in the vehicle frame. From this table the engineer selects the impact points for the knee impactor test: the highest-stiffness points within the knee contact region, plus any flagged hard points, plus representative soft points to establish the baseline response. These selected coordinates are the direct input to the next stage of the assessment, and their accuracy depends entirely on the mounting discipline and grid definition described here.