SRS

An SRS, or supplemental restraint system, is the collection of sensors, controllers, and inflators that deploy airbags when a vehicle experiences a collision. It works alongside the primary restraint system, the seatbelt, filling the space between an occupant and the interior surfaces of the cabin in the first 50 to 100 milliseconds after impact. The system is passive, requiring no driver action, and is designed to absorb energy and reduce blunt force trauma during a crash.

Modern SRS architectures vary by vehicle size and cost. A basic system in a compact car might include two front airbags (driver and passenger) plus a control module. A full-featured SRS in a luxury sedan can include front, side-impact, curtain, and knee airbags, along with redundant impact sensors and a dedicated SRS control unit that monitors system health continuously. The control module receives input from accelerometers positioned in the vehicle frame; when deceleration exceeds a threshold, it triggers inflation solenoids that ignite the pyrotechnic charges inside each airbag canister, inflating the bag in under 40 milliseconds.

Sensors and deployment logic

Impact detection relies on mechanical or electronic accelerometers that measure the rate of change in velocity. Front-end crashes typically trigger at deceleration around 8 to 15 G, depending on the threshold set during calibration. Side-impact sensors are more sensitive, deploying at lower thresholds because side crashes offer less crumple distance. Some systems use dual-stage inflators that modulate the speed of inflation based on occupant size, position, and seatbelt usage, detected by weight sensors or cameras, to reduce injury risk from the airbag itself.

A critical failure mode in SRS design is the risk of non-deployment when needed or inadvertent deployment during minor collisions. To prevent the latter, systems use redundancy: most airbags require confirmation from at least two sensors before firing. After deployment, the system must remain safe; spent airbag canisters and powder residue are contained within the steering wheel, dashboard, or seat structures. Any SRS fault, such as a faulty sensor or open circuit, is logged in the control module and typically illuminates a warning light on the instrument cluster, signalling that service is required.

The term 'supplemental' reflects the system's role: it augments seatbelt restraint but cannot replace it. Airbags are most effective when combined with seatbelt tension and pretensioners that lock the belt at the onset of impact. In modern vehicles, the SRS control module communicates with other safety systems, including the electronic stability control and collision avoidance systems, sharing sensor data to optimise the overall protective response during emergencies.