Airbag sensors help a vehicle decide in milliseconds whether a crash is severe enough to deploy airbags. This matters because an airbag must inflate before the occupant moves too far forward, but it must not deploy during minor bumps or hard braking. Modern systems combine sensors, wiring, software, and safety checks inside a supplemental restraint system.
The goal is to reduce injury by matching the response to the crash type and severity.
In a frontal collision, accelerometers measure rapid changes in the car's motion and send data to the airbag control module. The module estimates crash severity using acceleration, change in velocity, impact direction, seat belt status, and sometimes occupant detection. If the data passes programmed thresholds and safety conditions, the module sends current to an inflator squib, which starts a chemical reaction or gas release to inflate the airbag.
The whole sensing and deployment decision usually happens in about 10 to 30 ms.
Key Facts
- Acceleration is the rate of change of velocity: a = Δv / Δt.
- Crash severity often depends on change in velocity: Δv = vf - vi.
- Force during a crash is related to deceleration: F = ma.
- Airbag control modules compare sensor signals to programmed deployment thresholds.
- Front airbags typically begin deploying within about 10 to 30 ms after a severe frontal impact is detected.
- Safing sensors and software checks help prevent deployment from minor bumps, potholes, or electrical noise.
Vocabulary
- Accelerometer
- A sensor that measures acceleration, often used to detect the rapid deceleration caused by a crash.
- Airbag Control Module
- The computer that receives crash sensor data and decides whether to trigger airbag deployment.
- Crash Pulse
- The pattern of acceleration or deceleration over time during a collision.
- Squib
- A small electrical igniter that starts the airbag inflator when the control module sends enough current.
- Deployment Threshold
- A programmed limit for crash severity that must be exceeded before an airbag is commanded to inflate.
Common Mistakes to Avoid
- Assuming airbags deploy in every crash is wrong because the system only deploys them when the crash direction and severity meet specific thresholds.
- Confusing speed with crash severity is wrong because a vehicle can be moving fast but experience a small change in velocity, or moving slower but stop very suddenly.
- Thinking one sensor alone decides deployment is wrong because modern systems usually use multiple sensor inputs and confirmation checks to reduce false deployments.
- Ignoring time in deceleration calculations is wrong because the same change in velocity over a shorter time produces a much larger acceleration and force.
Practice Questions
- 1 A car's speed changes from 18 m/s to 2 m/s in 0.08 s during a frontal crash. Calculate the average acceleration in m/s^2 and state whether it is a deceleration.
- 2 An airbag system begins deployment 20 ms after detecting a severe crash. If the occupant is moving forward at 12 m/s before the restraint slows them, how far would the occupant move in 20 ms without restraint effects?
- 3 Explain why an airbag control module should consider both acceleration data and seat belt status when deciding how to deploy the restraint system.