Airbags

• Supplementary Restraint System for driver and/or passenger safety in case of a crash.

• Basic Mechanism: A thin nylon bag in the steering wheel / above glove compartment inflates in the event of an impact and prevents the driver/passenger from hitting the steering wheel/dashboard.

• 3 Main Components: 1) Airbag module

2) Diagnostic Unit

3) Crash sensors

What are Airbags?

Airbag Module• Contains both inflator unit and light-weight fabric airbag and is

located either inside: 1) Steering wheel hub 2) Above glove compartment 3) Near side compartment (as separate/combined head/side/window-curtain airbag)

• Airbag: Thin nylon fabric bag folded neatly into steering wheel that inflates to the size of a large beach ball on impact.

• Inflator unit: Contains a number of sodium azide pellets which are electrically ignited to produce N2 that then fills the airbag. This is preferred to storing compressed gas in the unit (space, durability)

• Both airbag and inflator unit are for single deployment only – ie have to be replaced after a crash

Diagnostic Unit• Enables inflator unit and sensors when vehicle is turned on, performs

self check.• Constantly monitors airbag readiness and indicates malfunctioning

through an indicator on dashboard• Usually stores electricity to activate airbag in the event that a crash

damages the battery / link to battery

Sensors• Several crash sensors located in the front of vehicle and in the

passenger compartment

• Each senses the sudden deceleration or impact in the event of a crash and flips a mechanical switch to indicate a crash.

• Frontal crash scenario: Car crashes into anobstacle (wall) at 20+ mph

• Sensors detect the deceleration and inflatorunit activated

• Deployment sensitivity: To guard against accidental inflation on hard braking, sensors detect collisions into a solid barrier at speeds greater than 8-14 mph only as impacts

• An electric current is used to heat a filament wire that ignites the NaN3 capsules, producing N2:

2NaN3 2Na + 3N2

10Na + 2KNO3 K2O + 5Na2O+ N2

K2O + Na2O SiO2 alkaline glass (safe, unignitable)

130 g of NaN3 produces 67 ltrs of Na

Airbag Deployment

Airbag Deployment• The airbag then inflates fully at speeds > 320mph within 0.05s of crash.

For maximum safety, occupant must have seat belt on and sit with chest 10” from steering wheel

• Immediately after full inflation, the airbag deflates through tiny pores on the surface within 0.3s

Accelerometer

• An on/off switch • Combination with seat-belt pre-tensioners and other safety

systems• Inflation in the event of fire (high temp.) to prevent

explosion of solid compound• Depowering and differential powering • Small rapid deployment airbags for side impact at roof-rail

or door or seat back. eg: 1) Beltline Head/Torso Side Airbag 2) Inflatable Tubular Structure

Additional Features

Electromechanical Crash Sensors

Fluid pressure based weight sensors

Smart Restraint System• Is one that adapts its geometry,

performance or behavior to suit varying impact types and/or occupants & occ. posns.

• Must be able to distinguish between:– RFIS & child seat– Child– Adult– Empty– Subsets of possible seating

posns. for above (< / >10”)– Belted / non-belted– Crash severity– Crash direction

• ie, a smart restraint system must be able to update itself on the following:– Occ. characterisation– Occ. location

• Accordingly decides:– Which airbags to deploy when– Full blown / supressed– Seat belt pre-tensioning,

retraction/collapse of parts– Direction of deployment– Sequencing & Timing– Post deployment action

Smart Restraints• Detection types:

– mechanical– spatial– other

• Means of detection:– Weight & distribution (3)– Seat belt (webbing , rotation

ctr & buckle)– Active Infra-red (OOP sense)– Ultrasonic– Radar/Microwave– Capacitive– tags for RFIS & smart keys – Height sensors in seat / belt

• Advance meth. (complex, high computing power reqd.)– Passive Infra-red – Video systems– Biometric sensing

• Systems must:– Sequence & time appropriately– Extremely reliable– Work within varying auto

interior atmosphere and lighting

– Differentiate camouflage– Low cost

Smart Restraints• Components

– Side/Variable/dual-stage airbags

– Seat-belt pre-tensioners– Side/Central/Satellite/Safing

crash sensors– Occupant sensors– Central ECU– Pre-crash sensors

• Driving states:– Normal– Collision avoidable– Collision imminent– Post-crash

• Sensor types:– Electromechanical– Accelerometers– Pressure– Stress-wave– Pre-crash

• Pre-crash sensing– More details reqd from sensors– Advantage: Enables early

decision and pre-tensioning– Disadv: imprecise object

classification & cost– Same sensors as those for ACC,

CW/CA