Airbags
What are 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
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.
Airbag Deployment
• Frontal crash scenario: Car crashes into an
obstacle (wall) at 20+ mph
• Sensors detect the deceleration and inflator
unit 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
•
•
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
Additional Features
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•
•
•
•
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
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:
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–
–
–
Normal
Collision avoidable
Collision imminent
Post-crash
• Sensor types:
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–
–
–
–
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