3. Sensor cable construction and selection

A crash test is a measurement event that lasts 80–150 ms and produces accelerations from a few hundred g on the vehicle body to several thousand g on structural members and dummy components. The cable that carries those signals is not a passive accessory; it is part of the measurement chain, and its construction determines whether the data recorded by the data acquisition system (DAS) reflects the sensor output or the cable's own electromechanical response. This section examines the four construction layers that matter most — conductor, insulation, shield, and jacket — and translates them into selection criteria for high-shock routing.

Conductor: material, stranding, and the fatigue problem

Nearly all crash-test sensor cables use copper conductors, but the form of the copper matters far more than the metal itself. Solid conductors have the lowest resistance and best signal fidelity per unit cross-section, yet they work-harden and fracture after a small number of bending cycles at a sharp radius. In a crash test the cable is typically taped or clipped to a structure that deforms by tens of millimetres in a few milliseconds, so the conductor experiences a violent, one-shot bend. A solid 0.2 mm² conductor may survive that single event, but the same cable reused across a test series will fail unpredictably.

Stranded conductors solve the fatigue problem by distributing strain across many fine wires. The trade-off is that strand count and lay length affect both flexibility and high-frequency behaviour. For piezoelectric accelerometers with charge output, conductor movement relative to the dielectric generates triboelectric charge — a low-frequency artefact that appears as a large, slowly decaying baseline shift immediately after impact. Tightly stranded, compacted conductors with a short lay length reduce this effect. For IEPE sensors, the same movement modulates cable capacitance and can produce noise at the moment of peak acceleration.

Practical selection guidance for a typical 3-axis accelerometer channel:

• Conductor size: 0.14–0.25 mm² (approximately 26–24 AWG) balances mechanical robustness against the mass that the cable imposes on a small sensor.
• Stranding: 19-strand or finer (e.g., 19/0.10 mm) for any cable that will be bent during installation or reused.
• Material: tinned copper is preferred over bare copper because the tin layer limits oxidation at crimped or soldered terminations, which is a common source of intermittent high-resistance joints after repeated handling.

Insulation and dielectric: keeping the charge where it belongs

The dielectric separates the conductor from the shield and sets the cable capacitance. For charge-mode piezoelectric sensors, cable capacitance is in parallel with the sensor's own capacitance and directly attenuates the signal: a sensor with 1000 pF driving 3 m of 100 pF/m cable loses roughly 23% of its charge sensitivity. This is a calculable, predictable loss, and it should be accounted for in the channel scaling rather than discovered during data review.

Materials commonly encountered are PTFE (low loss, wide temperature range, but stiff and prone to cold flow), FEP (similar electrical performance with better mechanical toughness), and polypropylene or PE (lower cost, adequate for short runs at room temperature). PTFE's stiffness is a genuine liability in high-shock routing: it resists tight bends and transmits force to the termination. Where flexibility is the dominant requirement, FEP or a composite insulation is usually the better compromise.

Shielding: topology, coverage, and grounding

Crash tests are electrically hostile. The DAS, firing circuits, and nearby pyrotechnic events couple common-mode noise onto sensor lines. Shielding addresses this, but the shield's effectiveness depends on coverage, topology, and — critically — where it is terminated.

Three shield constructions dominate:

For high-shock routing, braided shields are generally preferred because foil shields crease permanently at a tight bend and can crack, creating a slot antenna at exactly the wrong moment. The shield should be terminated at the DAS end and left floating at the sensor end for charge-mode piezoelectric channels, which avoids ground loops through the test article. For IEPE sensors, follow the sensor manufacturer's grounding scheme; many IEPE accelerometers are case-isolated, and the shield is then tied to the DAS ground only.

Jacketing: abrasion, cut-through, and the routing interface

The jacket is the layer that survives contact with the vehicle. In a crash test, cables are taped to structural members, pass through deformed sheet metal, and are often crushed between the dummy and the seat. Jacket selection is therefore a mechanical decision first and an environmental one second.

Polyurethane (PUR) jackets offer the best combination of abrasion resistance, cut-through resistance, and flexibility at low temperature, and they resist the hydraulic fluids and cleaning agents used in test facilities. PVC is cheaper and adequate for protected runs but stiffens at low temperature and has poorer cut-through performance. Silicone jackets remain flexible across a very wide temperature range but are easily cut and have low tensile strength — they suit high-temperature engine-bay runs, not crush zones.

Jacket thickness is a trade-off: a thicker jacket improves cut-through resistance but increases stiffness and bend radius, which raises the force transmitted to the sensor connector. For a 3 mm outer-diameter cable, a 0.5–0.7 mm PUR jacket is a reasonable balance for body-mounted accelerometers.

Flexibility requirements and the bend-radius rule

Flexibility is not a single property; it is the sum of conductor stranding, insulation modulus, shield construction, and jacket stiffness. The practical specification is the minimum bend radius, usually expressed as a multiple of the cable's outer diameter (OD). For dynamic, repeated flexing, a common rule is 10× OD; for a one-time installation bend, 5× OD is often acceptable. In crash-test routing, the cable should be given a service loop at the sensor so that the deformation of the structure is absorbed by the loop rather than by the connector.

The following SVG summarises how the four construction layers interact and where each selection decision is made.

Putting the selection together: a worked example

Consider a seat-mounted 3-axis accelerometer on the dummy pelvis, with a 2.5 m run to the DAS. The sensor is IEPE, 100 mV/g, with a case-isolated body. The cable will be taped along the seat frame, cross one hinge line, and terminate at the DAS patch panel.

A defensible selection is a 4-conductor, 26 AWG stranded tinned-copper cable with FEP insulation, a braided copper shield at 90% coverage, and a PUR jacket giving approximately 3.2 mm OD. The shield is tied to the DAS ground at the patch panel and left open at the sensor, consistent with the case-isolated sensor. The hinge-line crossing is given a 40 mm service loop, and the minimum bend radius is held to 32 mm (10× OD). Cable capacitance is measured or taken from the datasheet — say 95 pF/m, giving 238 pF for the 2.5 m run — and recorded on the channel sheet so that any IEPE high-pass behaviour can be checked against the expected corner frequency.

Before the test, verify the assembly with three checks: conductor-to-conductor resistance (should be a few ohms, stable when the cable is flexed by hand), shield-to-conductor isolation (open circuit at 500 V DC), and shield continuity from sensor end to DAS end (a few ohms). A resistance that jumps when the cable is flexed indicates a broken strand or a cold crimp — both are common failure modes that produce intermittent channels during the impact event itself, when diagnosis is impossible.

The construction choices above are not independent. A stiff jacket forces a larger bend radius, which increases the service loop's mass and its susceptibility to whipping during impact; a foil shield saves space but fails at the bend that the stiff jacket forces. Selection is therefore an iterative compromise anchored on two non-negotiables: the conductor must survive the deformation without fracturing, and the shield must remain continuous and correctly terminated through the event. Everything else — capacitance, jacket colour, connector style — is tuned around those two requirements.