At what speed the airbag is triggered: activation thresholds

Operation of the passive safety system SRS occurs when the acceleration (deceleration) of the car body threshold values equivalent to a collision with a fixed obstacle at a speed of 20 to 30 km / h, but the exact figures depend on the severity of the impact and the type of collision. The electronic control unit (ECU) continuously reads the data from the accelerometers installed in the central part of the body, and decides on the supply of voltage to the pyropatrons only if the force vector exceeds the threshold specified by the algorithms. It is important to understand that the airbag response rate is not just the speed of the car before the impact, but the speed of a sharp change in the vector of motion recorded by overload sensors.

If the car is moving at a speed of 60 km / h and smoothly brakes to a complete stop, the cushions will not work, since the deformation of the body and the jerk of inertia are not enough to activate the system. On the contrary, when driving at a speed of 40 km / h and a sharp impact on a concrete wall, when the stop occurs in a fraction of a second, the overload reaches critical values sufficient for the construction of a concrete wall. pyropatron initiation. This is why documentation often includes ranges rather than fixed numbers, as the physics of the process depends on a variety of variables.

The principle of operation of shock sensors and activation algorithms

The basis of the safety system is the shock sensors, which can be mechanical, electromechanical or electronic. Modern cars are dominated by electronic sensors that transmit a signal to the central processing unit. This processor analyzes the force and direction of the impact using complex mathematical models. If the impact force exceeds a specified threshold, usually 2g-3g (where g is the acceleration of free fall), the system classifies the event as an emergency.

The algorithm of the ECU eliminates false positives during everyday operation. For example, when hitting a high curb, hitting a deep hole or when a strong door slams, sensors detect a jump in acceleration, but the pulse duration is too small to activate. gas-generator. The system should record a steady deceleration, characteristic of the collision of massive objects.

• 🚗 Frontal impact sensors are usually located in the front of the body or in the floor tunnel and respond to longitudinal overloads.
• 🚙 The side cushions are activated by separate sensors installed in the door racks that capture the transverse acceleration.
• 🚕 Pedestrian protection systems and knee pads have their sensitivity thresholds, often higher, to avoid injuries from disclosure.

The time from the moment of impact to the full opening of the pillow is only 20-50 milliseconds. During this time, the ECU must process the signal, apply current to the fuse, ignite solid fuel and fill the shell with gas. Any delay or error in the calculations can cost lives, so the reliability of these components is at the highest level.

Speed thresholds for frontal collision

For a classic frontal impact, when the car is faced with a fixed obstacle throughout the front end, the threshold values of the airbag response speed are in the range from 20 to 30 km / h. This value is relevant for most modern passenger cars equipped with a standard system. SRS. At this speed, the deformation of the front of the body is already significant, and seat belts without additional support may not deter the passenger from hitting the steering wheel or torpedo.

But there are nuances. If the impact is not in the center, but is shifted to the left or right, additional sensors for the deformation of spars come into operation. In such cases, the system may operate at slightly lower speeds, as the bodywork is deformed more quickly due to the smaller contact area. Also, the threshold can be lowered if the car is fitted with belt pretensioners that work before the pillows.

It is important to note that at speeds below 20 km/h, the cushion is usually not opened. It is believed that at such speeds, the main protection is provided by seat belts and structural strength of the body. Shooting a pillow at low speed can cause more harm (burns, nose fractures from gas pressure) than the accident itself.

Features of activation in side impacts

Side airbags and curtains have different algorithms of operation from the front. Since the side of the car (door, rack) deforms much faster and stronger on impact than the crushed front end, the trigger threshold is lower here. The system must respond almost instantly, as the distance between the passenger and the door is only a few centimeters.

For side impact, the speed threshold may start from 15-20 km/h. Sensors located directly in the doors or lower parts of the racks, detect a sharp pulse. If the force of the impact is sufficient for the door to begin to press into the cabin, the ECU gives a command to shoot. In this case, the bill goes on for milliseconds, and delay is unacceptable.

⚠️ Attention: With a side impact, the pillow opens harder and faster than the front. Being in an abnormal position (for example, a foot on the armrest of a door) can lead to serious injuries to the limbs even with a life-saving shot of the system.

Modern systems are able to distinguish the impact force and depending on it to activate only certain modules. For example, if the door touches a neighboring car in the parking lot, the system can be limited to locking the doors, but not opening the pillows, so as not to provoke expensive repairs and potential harm to health.

Why the pillows didn't work when hit hard

One of the most common questions of owners of broken cars: why the visual destruction of the front of the pillows remained intact? The answer lies in the physics of the impact and the location of the sensors. If the car did not experience sufficient deceleration after the impact (for example, flew into a snowdrift, bushes, a plastic fence, or overturned without a strong impact on a hard surface), the algorithm does not recognize the situation as critical.

It can also be caused by deactivation of the system. In many cars there is the possibility of turning off the passenger cushion (for example, for the installation of a child seat). If the switch PASSENGER AIRBAG OFF If the system is turned off, the system will not work regardless of the impact force. In addition, a malfunction of the sensor itself, a wiring break or a discharged battery (if there is no backup power) can cause the system to remain silent.

• 🔋 Lack of power: when the battery terminals break at the time of impact, the system may not have time to work if the capacitors are discharged.
• 🔧 Sensor malfunction: Corrosion or mechanical damage to the sensor before the accident blocks signal transmission.
• 🚫 Factory Fault or Software Errors: Rare but possible cases where the control unit misinterprets the data.

In some cases, especially in older cars, the deformation zone can absorb the impact energy so efficiently that less congestion reaches the passenger compartment than is required for activation. It is paradoxical, but good energy efficiency of the body sometimes leads to the fact that the thresholds of operation pyropatrons They're not.

Effect of body type and vehicle weight

The safety settings are calibrated individually for each vehicle model. The heavy SUV and light hatchback have different inertial characteristics. For a heavy vehicle, a 30 km/h impact may be less harsh in terms of overload than for a light vehicle, so thresholds can be adjusted by engineers.

In addition, cars with adaptive safety system take into account the number of passengers and their weight (thanks to the sensors in the seats). If there is no one in the seat or if the child is sitting in a special chair (which the system recognizes as low weight), the front pillow may not work even when speed conditions are met, so as not to cause injury.

In sports cars where the seating is lower and the belts are stiffer, the thresholds can be shifted. It is also important to have additional systems, such as active head restraints or pedestrian protection systems (rising hood), which have their own activation scenarios independent of the cabin airbags.

Diagnostics and verification of the SRS system

Checking the serviceability of the passive safety system is a critical procedure after any accident or purchase of a used car. The indicator on the dashboard should light up when the ignition is turned on and extinguish after a few seconds. If the lamp is constantly burning or flashing, it is a signal of a malfunction in the circuit.

For deep diagnosis, you need to use a specialized scanner capable of working with the module. Airbag. Regular engine error readers may not see faults in this system. When scanning, pay attention to the error codes associated with the resistance of the pyropatrons and the integrity of the plumes.

⚠️ It is strictly forbidden to "name" the chain of the airbag with a conventional multimeter in the resistance measurement mode! The supply of even a small current can provoke a pyropatron shot. Use only diagnostic equipment.

If the car has been in an accident, be sure to check whether the control unit has changed. Once the pillows are triggered, the block is often blocked (records the status of "Crash") and requires replacement or professional reset. Installing a unit from another car without being tied to the VIN code and immobilizer will not allow the system to work correctly.

Frequently Asked Questions (FAQ)