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2.2 THE SUPPLEMENTAL RESTRAINT SYSTEMS (SRS SYSTEM)

2.2.1 Airbags

  • An airbag is a vehicle occupant restraint system using a bag designed to inflate extremely rapidly then quickly deflate during a collision. It consists of the airbag cushion, a flexible fabric bag, inflation module and impact sensor. The purpose of the airbag is to provide the occupants a soft cushioning and restraint during a crash event. It can reduce injuries between the flailing occupant and the interior of the vehicle.
  • The airbag provides an energy absorbing surface between the vehicle's occupants and a steering wheel, instrument panel, as well as the body pillars, headliner, and windshield. Modern vehicles may contain multiple airbag modules in various configurations including, driver, passenger, side curtain, seat-mounted side impact, knee bolster, inflatable seat-belt, front right and left side sensors and pedestrian airbag modules.
  • During a crash, the vehicle's crash sensors provide crucial information to the airbag electronic controller unit (ECU), including collision type, angle and severity of impact. Using this information, the airbag electronic controller unit's crash algorithm determines if the crash event meets the criteria for deployment and triggers various firing circuits to deploy one or more airbag modules within the vehicle. Working as a supplemental restraint system to the vehicle's seat-belt systems, airbag module deployments are triggered through a pyrotechnic process that is designed to be used once. Newer side-impact airbag modules consist of compressed air cylinders that are triggered in the event of a side on vehicle impact.[1]

2.2.2 Seatbelt

  • A seat belt (also known as a seatbelt or safety belt) is a vehicle safety device designed to secure the occupant of a vehicle against harmful movement that may result during a collision or a sudden stop. A seat belt functions to reduce the likelihood of death or serious injury in a traffic collision by reducing the force of secondary impacts with interior strike hazards, by keeping occupants positioned correctly for maximum effectiveness of the airbag (if equipped) and by preventing occupants being ejected from the vehicle in a crash or if the vehicle rolls over.

  • When in motion, the driver and passengers are travelling at the same speed as the car. If the driver makes the car suddenly stop or crashes it, the driver and passengers continue at the same speed the car was going before it stopped. A seatbelt applies an opposing force to the driver and passengers to prevent them from falling out or making contact with the interior of the car. Seatbelts are considered Primary Restraint Systems (PRS), because of their vital role in occupant safety.[2]

2.2.3 Vehicle Safety Systems

What are some of the modern safety systems?

Backup camera
Typically projected onto the infotainment screen on your dash, a backup camera provides you with a clear view of everything behind you. Some systems will even display lines that curve as you turn the steering wheel.

Blind spot detection
This system warns you when there’s a vehicle in your blind spot so that you don’t have to turn your head and check for yourself (even though you still should).

Lane departure warning
When you let your vehicle drift over lane markings, this system will alert you.

Lane keeping assist
When you don’t pay attention to that lane departure warning, lane keeping assist will automatically steer your car back into your lane.

Lane centering assist
If you tend to wander back and forth in your lane, this system provides continual steering input to help keep your vehicle centered in between the lines.

Automatic braking
This system applies the brakes for you if it detects that a forward collision is imminent.

Rear automatic braking
When backing up, this system applies the brakes for you if it detects an oncoming vehicle to prevent a rear collision.

Pedestrian automatic emergency braking
If a person is about to walk in front of your vehicle, this system alerts you and applies the brakes.

Rear cross-traffic alert
This system lets you know whenever an oncoming vehicle or object is approaching outside of your backup camera’s view.

Forward collision warning
Even though you should be paying attention what’s in front of you anyway, this system detects and warns you of potential forward collisions.

Adaptive cruise control
After setting a following distance, this system accelerates and decelerates your vehicle. If the car in front of you slows down, your car will, too.

Traffic jam assist
When you’re stuck in heavy traffic, this system will control your speed and brakes to match the flow with traffic. It also centers your vehicle in the lane, even through curves.

Highway pilot
After determining a following distance, this system accelerates and brakes your vehicle and maintains your lane position.

Adaptive lighting
This system momentarily switches your high-beams to low-beams when another vehicle is approaching, then changes them back after it’s passed.[3]


2.2.4 The Principle of Crash Sensors Operation

  • Crash sensors need to detect a collision and convert it to usable signals within milliseconds. The accelerating forces acting on the sensors after a collision can be as high as 100g (100 times the earth's gravitational force). When a car is stopped abruptly by an impact, all bodies or objects that are not firmly fixed to the car will continue to move at the impact speed. The sensors measure this acceleration and relay it to the control unit as usable data.

  • Many of our cars are fitted with ultra-fast pressure sensors in the front doors. These sensors detect a side collision that pushes the outer door panel inwards, creating excess pressure. Acceleration sensors are also fitted near the C-pillars so that collisions from the side, which don't cause deformation of the front doors, can also be detected in time.[4]


2.2.5 Seat Belt Pre-Tensioners

  • Seatbelts in many newer vehicles are also equipped with "pretensioners". Pretensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash. Mercedes-Benz first introduced pretensioners on the 1981 S-Class. In the event of a crash, a pretensioner will tighten the belt almost instantaneously. This reduces the motion of the occupant in a violent crash. Like airbags, pretensioners are triggered by sensors in the car's body, and many pretensioners have used explosively expanding gas to drive a piston that retracts the belt. Pretensioners also lower the risk of "submarining", which occurs when a passenger slides forward under a loosely fitted seat belt.

  • Some systems also pre-emptively tighten the belt during fast accelerations and strong decelerations, even if no crash has happened. This has the advantage that it may help prevent the driver from sliding out of position during violent evasive maneuvers, which could cause loss of control of the vehicle. These pre-emptive safety systems may prevent some collisions from happening, as well as reducing injury in the event an actual collision occurs. Pre-emptive systems generally use electric pretensioners which can operate repeatedly and for a sustained period, rather than pyrotechnic pretensioners, which can only operate a single time.[5]



2.2.6 Tire Pressure Monitoring Systems

A tire-pressure monitoring system (TPMS) is an electronic system designed to monitor the air pressure inside the pneumatic tires on various types of vehicles. TPMS report real-time tire-pressure information to the driver of the vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light. TPMS can be divided into two different types – direct (dTPMS) and indirect (iTPMS). TPMS are provided both at an OEM (factory) level as well as an aftermarket solution. The target of a TPMS is avoiding traffic accidents, poor fuel economy, and increased tire wear due to under-inflated tires through early recognition of a hazardous state of the tires.[6]

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