Car airbag system: Principles, components and functions
sistema de airbag de coches

Passive security techniques are answerable for guaranteeing that passengers are protected to the best extent doable within the occasion of an accident. All kinds of airbags can be found in the present day, and now’s commonplace in all car courses. 

CAR airbag SRS: Primary Ideas

This part focuses on the airbag system. We are going to clarify the person parts, what they do, how the airbags and the steps we are able to take to troubleshoot deployed. Current years have seen fast advances in airbag know-how because it was first launched airbag techniques. As such, solely parts and processes are described typically phrases.

For extra detailed info on particular car techniques put in, at all times check with the car producer’s specs. Upkeep and diagnostics need to be carried out solely by skilled professionals.

Make certain all authorized rules and tips are adopted when finishing up this work. The preliminary concept for an airbag system dates again to the 1960s At the moment, a significant impediment was the time out there for the airbag inflates. Makes an attempt to resolve the issue with the compressed air is made, however, this answer was not profitable. The primary successes have been recorded originally of the 1970s with the use of pyrotechnic propellant cost to inflate the airbag within the specified time.

The primary airbag techniques have been put in within the automobiles above the vary, the mid-1970s and the start of the 1980s The tip of the 1980s noticed the introduction of passenger airbag, and this steadily it adopted by different variants as the top airbag and the facet airbag. As we speak, the airbag techniques are supplied as commonplace in automobiles.


Airbag Management Unit

The management unit is the center of the airbag system and put in within the middle of the car. Normally you might be discovered within the space of the dashboard, on the central tunnel.

It’s answerable for the next:

  • Function detection.
  • Detecting indicators despatched by the sensors in a well-timed method.
  • Activate the set off circuitry wanted on the proper time.
  • Offering energy to the ignition circuit via the capacitor, whatever the car battery.
  • Operating a self-test on the entire system.
  • Storage failures within the fault reminiscence.
  • Activation of the airbag lamp within the occasion of system failure.
  • Connection to different management items via the CAN bus.

The knowledge obtained from a collection of crash exams is saved in fashionable management items. This info permits an accident are categorized in response to the “crash severity”.

On this context, a distinction is made between the next:

  • Zero crash severity accident = minor; no air luggage deployed
  • 1 crash severity accident = reasonable; air luggage might be deployed within the first stage
  • 2 crash severity accident = extreme; air luggage are deployed within the first stage
  • Accident Severity 3 = extreme accident; air luggage are deployed in the first and second phases

Subsequent to the crash severity, the management unit additionally takes into consideration the tackle info of the accident (the applying of pressure), for instance, 0 ° or 30 °, and the kind of accident to be able to decide the technique deployment. As well as, it’s thought-about whether or not occupants are sporting their seat belts.

Influence sensors

Relying on the airbag system and the variety of air luggage put in, the impression sensors or acceleration are put in both straight within the management unit or as a satellite tv for pc of the entrance of the car or on the facet of the car.

entrance sensors are at all times supplied in duplicate. These sensors typically function in response to the spring-mass system. With this technique, the sensor has a heavy curler which is crammed with the normalized weights. A spring hinge brass is wound across the weighted curler and its ends are hooked up to the heavy curler and the sensor housing.

In consequence, the weighted curler can solely transfer if the pressure is utilized from one course. If a pressure is utilized, the curler wheel weighed towards the pressure of spring bronze and closes the circuit to the management unit by a contact. The sensor additionally has an excessive ohmic resistance to be able to carry out a self-test.

Another choice enlargement movement sensors are the usage of a mass of silicon. If a pressure is utilized, the mass of silicon within the sensor strikes. Due to the way in which wherein the mass on the sensor is suspended, this adjustments capacitance, which serves as info for the management unit.

Because of the velocity at which may file info, these sensors are used to offer info to the management unit as rapidly as doable within the occasion of a facet impression.

Stress sensors are additionally used. They’re put in on the doorways and reply to adjustments in strain contained in the gates in case of an accident. When engaged on automobiles utilizing these strain sensors, it’s important to re-positioning of sheets seal the door after disposal. If the sealing foil door improperly put in and this ends in strain loss throughout an accident, the operate of the strain sensors could also be affected.

When assembling the impression sensors, at all times observe the set up course, which is indicated by an arrow on the sensor. The deployment threshold is an acceleration of approx. Three to five g. For safety causes, to be able to stop the airbag (s) they displayed inadvertently two sensors that function independently of one another should at all times ship the data to deploy the airbag (s). The protection sensor is used as a security sensor.

safing sensor safety

The protection sensor is answerable for stopping the airbags are deployed unintentionally.

It’s related in a collection with the entrance sensors. The protection sensor is built-in into the airbag management unit. It consists of a reed contact in a tube crammed with resin and a hoop – formed magnet. Reed open contact is positioned in a tube crammed with resin on which the magnet is positioned in a hoop. The magnet is held by a spring on the finish of the housing. If a pressure is utilized, the magnet slides over the tube crammed with resin towards the spring pressure and closes the reed contact. This closes the contact to set off airbags.


The airbag steering wheel comprising an airbag with a quantity of approx. 67 l, the airbag holder, the generator within the generator help and the airbag cowl (flywheel cowl). Within the occasion of an accident, the management unit prompts the generator. Within the course of, a present set off a skinny wire that triggers the firing pellet is heated.

As the method continues no explosion – as an alternative, the propellant burns. This engine is sodium azide. The product fuel whereas the propellant burns expands and reacts with the oxidant (an oxygen-releasing substance similar to copper oxide or iron oxide) to type virtually pure nitrogen which fills the airbag. As a result of toxicity of sodium azide, different free stable azide fuels are additionally used as propellant.

These not solely nitrogen type when reacted, but additionally carbon dioxide (approx. 20%) and steam (approx. 25%). The propellant is mostly supplied within the type of tablets, packed in a hermetic combustion chamber.

What propellant is used it is dependent upon the scale of the airbag and the opening velocity required. The chemical response that takes place after firing temperatures of 700 ° C happen within the combustion chamber. The ensuing fuel flows via a filter display at a strain of approx. 120 bar. Within the course of, it cooled to scale back the temperature on the outlet to lower than 80 ° C to be able to defend the occupants.

The noise is just like a firearm. It takes about 30 ms for the airbag to inflate absolutely. Newer techniques use fuel mills two phases. Relying on the severity of the accident, the management unit triggers taking pictures two tablets, one after one other. The shorter the interval between pictures, the quicker the airbag inflates. In any case, each fuel mills at all times fired to soundly rescue car occupants.

Hybrid mills are used for the passenger airbag or facet airbag. These kinds of mills additionally use a second fuel supply along with combustion gases. A strain vessel containing a gaseous combination of 96% argon and 4% helium at a strain of approx. 220 bar.

The strain vessel is sealed by a diaphragm. If the airbag deploys, the propellant a piston pierces the membrane strikes and permits fuel to circulation out. The fuel produced when the burning propellant is combined with the fuel within the strain vessel. The exit temperature is about 56 ° C in this case. The entrance passenger airbag has a quantity of about 140 l absolutely inflates in about 35 ms.

The method is comparable for facet airbags (thorax airbags). Nevertheless, as there isn’t a deformation space (deformation space) by the impression, it’s essential to show the fuel mills and inflate airbags a lot quicker. Within the occasion of a facet impression at a velocity of about 50 km / h, mills should fireplace after approx. 7 ms and the airbag have to be utterly inflated after 22 ms. The facet airbags are put in within the door panel or the seat again.

Relating to the top airbags, a distinction between the inflatable tubular buildings and inflatable curtains are made. The inflatable tubular construction was the primary design for the top airbag. It seemed like a sausage that develops from the roof lining above the entrance doorways. The inflatable curtain extends all through the car facet on the high. It’s put in within the roof body above the doorways of the car.


The airbag is manufactured from an extremely sturdy cloth resistant polyamide growing older. It has a low coefficient of friction to make sure that unfolds with ease and makes tender contact with the pores and skin. The airbag is dusted with talc to guard and forestall sticking. When the airbag is deployed, this mud might be seen as a white cloud. There are fastening bands inside sustaining the form of the airbag when being inflated. There are outlet openings on the rear which permit the fuel to flee.

There are 2 other ways wherein the air luggage can cross: The usual folding and folding star-shaped. With star-shaped folded, the airbag expands much less in direction of the driving force. This folding is helpful if the occupants are usually not sitting within the appropriate place ( “out of place”).

Volute spring

The volute spring establishes the connection between the column course and inflexible wheel in movement. It additionally ensures the connection between the management unit and the airbag fuel generator when the wheel rotates. The conductive foil is wound in order that it could observe the rotation for two.5 revolutions in every course.

Take particular care when eradicating and putting in the volute spring. You need to be sure that administration is concentrated and the wheels are within the straight-ahead place. Don’t twist the spiral spring as soon as it has been eliminated.

Detecting seat occupancy

Seat occupancy detection is used to regulate airbag deployment extra precisely and keep away from air luggage being deployed unnecessarily. There are other ways to examine if the seat is occupied. mats sensor comprising strain sensors and digital analysis unit are used. Sensor mats can solely be built-in into the entrance passenger seat.

Essentially the most techniques of prior artwork can, nevertheless, even be built-in into the seats of the rear seat and the driving force. Each infrared and ultrasonic sensors can be utilized. Are put in within the inside / rear view mirror monitor mild and never provided that the seat is occupied, but additionally how the entrance passenger sits. Thus, the system detects whether or not the occupant is “out of place”, which might be problematic.

Info from the seat occupancy detection system impacts the deployment of airbags and activation of the seatbelt tensioners and lively headrests. The airbag system detects if particular person seats are usually not occupied and the corresponding safety techniques are usually not activated in case of an accident.

Airbag cables

Connectors airbag are vivid yellow to be able to extra simply determine the cables and connectors airbag.

Contained in the connector is a bridge that stops the airbag being deployed inadvertently if the work is carried out within the airbag system. This might happen on account of static cost, for instance.

The bridge is a contact connecting the 2 contacts contained in the connector when the plug connection, to be able to remove any potential is disconnected.

Seat belt tensioner

The aim of the belt tensioner is to remove the slack within the seat belt in case of an accident. This clearance can come up on account of clothes massive, free or a seating place “relaxed”. The tensioner belt might be built-in into the belt buckle or curler belt.

If the belt tensioner has been put in within the belt buckle, will comprise a retractor tube, cable, piston fuel generator, and firing pellet, for instance. Within the occasion of an accident, the fuel generator is triggered as within the case of an airbag.

The fuel expands and the piston strikes within the retractor tube. The cable connection between the buckle and the strap piston pulls the belt buckle down and removes the slack within the belt. If the belt tensioner is built-in into the curler belt, the slack is eliminated by way of a retractor.

Additionally, in this case, a generator is triggered if the tensioner is activated. This generator gives numerous balls in movement. Balls turn out to be a related to the curler belt spool. This rotation belt over a distance precisely outlined retracts. The beads then fall into a delegated container to forestall injury.

Another choice is to make use of the precept “Wankel rotary engine”. With this technique, when the tensioner is activated the propellant drives a rotor, which eliminates slack as a consequence of rotation. To scale back the load on the chest, within the occasion of an accident, a pressure limiter belt is put in within the entrance passenger seat and driver.

Belt pressure limiter

Belt pressure limiter is adaptive belt machines that use a fuel generator and an airbag, for switching between an excessive and low stage of pressure.

Because of the optimum coordination between the tensioner seat belts and airbags, the kinetic power of the occupants slowly dissipates in the course of the impression, decreasing the hundreds.

Battery Disconnect

To keep away from the hazard of quick circuits and fires ensuing automobiles, the battery of the car electrical system in case of an accident is disconnected.

That is achieved by reducing a relay or a fuel generator. Sign battery disconnect is distributed by the airbag management unit. The fuel generator capabilities equally to tensioner belt right here means. If the shutdown operates off the battery is activated, the connection between the battery and the cable is disconnected contained in the terminal.


As a fundamental precept, work on the airbag system ought to solely be carried out by specialists correctly skilled professionals.

All authorized and manufacturer-specific laws have to be adopted. The identical applies to the disposal of previous or airbags deployed. It’s advisable to coach all workers within the workshop, if doable, as a result of many duties that aren’t essentially straight associated to the airbag nonetheless require the airbag or belt tensioner to be eliminated. An instance of this may work within the instrument cluster.

As is the case for analysis and troubleshooting in different techniques, step one is to carry out a visible inspection. Within the course of, examine all seen parts of the airbag system for injury and to make sure that the plug contacts are related accurately. One explanation for failure is a nasty widespread plug connection tensioners belt or facet airbags within the entrance seats.

Because the seats are moved ahead and backward plug connections turn out to be free, inflicting the contact resistance. The coil spring can be an explanation for failure. The spring could fail as a result of it’s positioned beneath load each time the steering wheel is turned. Enough diagnostic unit is at all times required. If the visible inspection reveals a faulty plug connection, clear the fault reminiscence utilizing the diagnostic unit.

If visible inspection doesn’t reveal any defect, error reminiscence learns with the diagnostic unit. Failures which have occurred within the system is mostly recognized by the self-diagnosis and saved within the fault reminiscence. If the fault “faulty Sign”, “Sign too low” or “too excessive sign” is saved within the error reminiscence, a doable trigger could also be a faulty cable, for instance.

In this case, a multimeter can be utilized to examine the cable connection between the sensor unit and management for continuity and shorted to the border. Required info and particular producer diagrams to determine the sensors and plug connections and pin project within the management unit. Within the course of, at all times make sure that the car battery is disconnected and the sensors and the management unit is disconnected from the harness.

Don’t use “dwelling” check adapters (clips which were bent open) for connecting the check traces to the connectors. They may injury delicate plug contacts and may trigger new faults go unnoticed. It makes extra sense to make use of particular probes that match into the plug contacts and guarantee it’s established that appropriate contact.