The hardest decision an eCall system makes is whether to call at all. Too sensitive and every pothole or parking bump generates a false emergency call that ties up public safety lines; too conservative and a genuine severe crash is missed. Automatic crash detection is therefore a layered signal-processing problem, combining inertial measurements, restraint-system status and vehicle dynamics with careful debounce and validation logic. This article explains how a vehicle decides that a collision has happened, how the T-Box receives that decision, and how false triggers are suppressed without delaying real ones.
The primary crash sensors belong to the restraint control module (the airbag ECU), which deploys a network of accelerometers — often supplemented by pressure sensors in the doors and peripheral satellite sensors — sampled at kilohertz rates. Signal-processing algorithms evaluate not just peak deceleration but the shape of the pulse: a rigid-barrier impact produces a characteristic high-energy, sustained deceleration signature distinct from a low-speed hammer blow or a curb strike. The airbag controller fuses these channels to classify event severity and direction (frontal, side, rear, rollover) within milliseconds, deciding whether to fire restraints and simultaneously raising a crash-event signal on the vehicle bus or a dedicated hardwired line to the T-Box.
Two paths are commonly used in parallel for redundancy: a discrete hardwired crash line from the restraint module, and a CAN message carrying the crash flag, severity and event type. The hardwired path survives even if bus traffic is disrupted by damage; the CAN path carries richer context. The T-Box also monitors vehicle dynamics — sudden delta-V, airbag deployment status, seatbelt pre-tensioner firing, rollover from gyroscope data — to corroborate the trigger. Requiring agreement between independent channels is the first defence against false calls.
A raw threshold crossing never triggers a call by itself. Production logic applies a sequence of guards:
Rollover detection leans on angular-rate sensors and tilt angle prediction, often over a longer observation window than planar impacts. Side impacts rely heavily on door-cavity pressure sensors, which react faster than deceleration alone. Low-speed events present the hardest tuning problem: standards and OEM policies define a severity floor below which no automatic call is placed, while still allowing the occupant to trigger manually. Manual SOS-button logic is deliberately simple by contrast — a defined long press, with its own short cancellation window to cover accidental presses.
Even well-tuned systems occasionally trigger on severe-but-non-injury events. Regulation therefore provides a short cancellation window after automatic triggering: the IVS signals an impending eCall audibly and allows the occupant to cancel within seconds if no emergency exists, after which the call proceeds automatically. Once placed, calls are not silently aborted — if the occupant hangs up, the module remains reachable for PSAP callback during the standby window. Every trigger, cancellation and call outcome is logged for later analysis, feeding continuous calibration improvements.
Crash-trigger performance is validated with sled tests, barrier crash tests, misuse-event libraries (potholes, curb drops, door slams, off-road vibration) and hardware-in-the-loop simulation of hundreds of pulse recordings. UN R144 and EN 16454 test cases verify that genuine events trigger within mandated time limits while misuse library events do not. The T-Box must also trigger correctly when the main battery is lost at impact — which is precisely when its internal backup pack takes over.
Crash detection ends and the emergency call begins in the same instant the main battery may be destroyed. Weijiang Power manufactures the wide-temperature, high-reliability NiMH and lithium backup cells and custom packs that take over the T-Box rail at impact, sustaining trigger logic, GNSS, modem transmit and voice through the full call profile. Send us your inrush profile and certification targets, and our engineers will design the backup power that makes the trigger decision actionable.
