3. Side impact sensing architecture

Side impact crashes are fundamentally different from frontal events. The crush zone between the striking object and the occupant is often only 200–300 mm, and the time available to decide and deploy a side airbag or curtain can be as short as 10–20 ms. This compressed timeline forces the restraint control module (RCM) to rely on a distributed sensing architecture rather than a single central accelerometer. Understanding how accelerometers, pressure sensors, and door-mounted satellite sensors work together — and where they are physically placed — is the foundation of any calibration task.

Why a Single Central Sensor Is Insufficient

A central tunnel-mounted accelerometer measures the vehicle body's response after the impact energy has already propagated through the structure. In a pole or barrier side impact, the door intrudes before the sill or tunnel experiences meaningful deceleration. By the time a central sensor crosses its arming threshold, the occupant may already be in contact with the intruding door trim. The solution is to place sensing elements close to the expected impact zone so that the crash signal is captured at the earliest possible moment.

This leads to a layered architecture: front satellite sensors for frontal events, side satellite accelerometers in the B-pillar or rocker, door pressure sensors inside the door cavity, and the central RCM accelerometer as a corroborating and safing channel. Each sensor type responds to a different physical quantity, and the fusion of these signals is what enables both fast and robust deployment decisions.

Sensor Types and Their Physical Principles

Satellite accelerometers are MEMS capacitive devices, typically ±250 g to ±500 g full-scale for side applications. They are mounted rigidly to the body structure — commonly on the B-pillar inner panel or the rocker/sill — and measure the lateral deceleration of that structural node. Because they are close to the impact, they produce a large, early signal. A typical B-pillar satellite might see 80–150 g within 5–8 ms of a 26 km/h pole impact.

Door pressure sensors are not accelerometers. They are differential pressure transducers mounted inside the door cavity, usually on the inner door module. When the outer door skin deforms inward, the cavity volume decreases rapidly, producing a pressure rise. This pressure signal is extremely fast — it can rise within 2–4 ms of skin contact — and is largely independent of vehicle body motion. Pressure sensors are therefore excellent early indicators of door intrusion, but they are sensitive to temperature, altitude, and door sealing integrity.

Door-mounted accelerometers (sometimes called door satellites) are an alternative or complement to pressure sensors. Mounted on the door beam or latch area, they measure the door's own acceleration. They are less sensitive to environmental factors than pressure sensors but may be slower to respond because the door must first be set in motion.

The central RCM accelerometer serves as the safing sensor. It confirms that a genuine crash is occurring and helps reject misuse events such as door slams, hammer blows, or off-road impacts. Its signal is typically lower amplitude and later than satellite signals, which is exactly why it is used for validation rather than primary triggering.

Placement Principles and Signal Timing

Placement is driven by three constraints: proximity to the likely impact zone, structural rigidity at the mounting point, and signal-to-noise ratio. A satellite sensor mounted on a flexible outer panel will produce a noisy, delayed signal. Mounting on a stiff structural node — the B-pillar reinforcement, the rocker inner panel, or the door beam — ensures that the sensor sees the crash pulse with minimal mechanical filtering.

For a typical sedan, the recommended placement is:

• B-pillar satellite accelerometer: mounted at the lower B-pillar, approximately 300–500 mm above the sill, oriented to measure lateral (Y-axis) acceleration.
• Door pressure sensor: mounted on the inner door module, centered vertically and horizontally within the cavity, with the pressure port facing the outer skin.
• Rocker/sill satellite accelerometer: mounted on the rocker inner panel, near the B-pillar junction, for lower-severity or offset side impacts.
• Central RCM: mounted on the tunnel or floor pan, oriented to measure lateral and longitudinal acceleration.

The timing relationship between these sensors is critical. In a 26 km/h side pole impact, a typical timeline might show: pressure sensor threshold crossing at 3–5 ms, B-pillar satellite at 5–8 ms, rocker satellite at 7–10 ms, and central RCM at 10–15 ms. The calibration engineer must set thresholds and arming criteria such that the fastest reliable sensor (usually pressure) initiates the deployment decision, while the slower sensors provide safing confirmation.

Signal Flow and Decision Logic

The diagram below illustrates the sensing architecture and the signal flow from impact to deployment decision. It shows how the pressure sensor, B-pillar satellite, rocker satellite, and central RCM feed into the RCM's deployment logic, with the safing channel providing a validity check.

Practical Calibration Considerations

When calibrating this architecture, the engineer must define several parameters for each sensor channel:

These values are illustrative and must be tuned for the specific vehicle structure, sensor mounting stiffness, and regulatory test matrix (FMVSS 214, Euro NCAP side pole, etc.). A common failure mode is setting the pressure threshold too low, which causes deployment during door slams or when the vehicle is struck by a low-mass object that does not threaten the occupant. Conversely, setting it too high delays deployment in low-severity pole impacts where the pressure rise is slower.

Satellite Placement and Structural Coupling

The physical mounting of a satellite sensor determines how faithfully it reproduces the structural crash pulse. A sensor bolted to a stiff B-pillar reinforcement will see a sharp, high-amplitude signal. The same sensor mounted on a flexible bracket or with a soft gasket will see a filtered, attenuated signal — potentially delaying threshold crossing by several milliseconds. Mounting torque, bracket stiffness, and surface flatness are therefore calibration-relevant variables, not just manufacturing details.

For door pressure sensors, the critical placement factors are cavity volume and sealing. A larger door cavity produces a smaller pressure rise for the same intrusion depth. The sensor port must face the outer skin and be positioned away from speakers, wiring harnesses, and drain holes that could vent pressure. Any air leak in the door seal will reduce the pressure signal and degrade detection capability.

In summary, the side impact sensing architecture is a coordinated system of complementary sensors. Accelerometers provide robust structural deceleration signals; pressure sensors provide the earliest indication of door intrusion; and the central RCM provides safing validation. Calibration success depends on understanding the timing, placement, and failure modes of each element, and on tuning thresholds and validation windows so that deployment is both fast enough to protect the occupant and robust enough to avoid false activations.