How Pre-Collision Systems Work

There are generally two kinds of safety systems in automobiles -- passive and active.

A passive safety system is anything in a car or truck that, for the most part, sits idle and operates only when necessary. A good example of this is a common seat belt. Once a passenger buckles a seat belt, the belt won't automatically lock into position until the car makes a sudden stop. Some might call airbag systems passive safety, too. However, you could argue that because they rely on impact sensors that determine the severity of an accident, and use that information to determine how quickly they inflate and how long they should stay inflated, airbags could fall into the active safety category.

An active safety system is very different from a passive safety system, especially when you're talking about pre-collision systems. Active systems operate based on signals and information gathered, and they typically either alert the driver to a dangerous situation or assist in important maneuvers like steering while braking. These systems actively seek out information in regards to the vehicle's current state.

Although early collision detection units used various technologies like infrared waves to detect objects, most pre-collision systems today work with the help of radar. Anything that's a wave, like a sound wave, can bounce or echo. You may have experienced this by shouting down into a well or over a deep canyon, only to hear the sound of your voice bounce back and reverberate. Instead of sound, however, radar systems use radio waves. Radio waves are invisible and they can travel much farther than sound.

Pre-collision systems place small radar detectors up near the front of the car, usually within the grill, where they constantly send out quick bursts of high-frequency radar waves. These waves will bounce off the nearest objects and return to the sensor, where a separate unit connected to the sensor calculates how long it took for the signal to leave and bounce back. With this information, a PCS unit can determine another car's position, distance, speed and relative velocity almost immediately, and if any sudden changes in those factors could potentially cause a collision, the system can provide information or assist the driver in avoiding a potential accident.

So, now that we know that if a pre-collision system recognizes a potential car crash, it can't just sit there and let chaos ensue. What do pre-collision systems actually do to help out drivers, and what kinds of systems are available in vehicles right now? Read on to find out.

Some systems sound an alarm to notify drivers that a collision may be imminent -- a sound simply to alert the driver and get him or her ready to take evasive action. Other systems actually take control of certain aspects of the car. There are pre-crash brake systems, which apply additional pressure to the car's brakes to assist the driver in slowing the car down as fast as possible and potentially reducing damage caused by an accident. Some systems also connect the PCS unit to a pre-crash seat belt system, which can automatically tense passengers' seat belts before a crash. These are often referred to as seatbelt pretensioners. The development of these types of systems needs to be fine-tuned and highly accurate, since any malfunction could disrupt a drivers' attention and potentially cause an accident. Obviously, designers and manufacturers put pre-collision systems through rigorous testing to ensure that this doesn't happen.

One of the earliest uses of accident detection was the Mercedes-Benz Pre-Safe system in the 2003 S-class sedan, which the company touted as the "world's first production car equipped with an astonishing new system that can sense a possible collision a few seconds in advance and take pre-crash protective measures." The system used sensors to measure the car's steering angle and acceleration, but not its surrounding environment -- actions such as pre-tensing of the seat belts, automatic sunroof closing and raising of reclined seats were triggered during any emergency maneuver [source: Mercedes-Benz Canada].

More recent accident detection technologies use radar systems, like Toyota's Pre-Collision System. The company introduced its PCS in 2003 on a vehicle sold in Japan called the Harrier. In 2010, the system will be available on the Toyota Prius. The system uses millimeter-wave radar to determine when additional braking assistance is required as well as when to apply tension to the seat belts. Toyota has also added pre-crash seatback preparation for rear seat occupants. If a crash situation is imminent, reclined rear seats are automatically brought to an upright position.

Ford has also announced its own radar system, called "Collision Warning with Brake Support," for its latest models of the Ford Taurus, the Lincoln MKS sedan and the Lincoln MKT crossover. And Honda and Nissan also offer lane-deviation prevention and front collision avoidance systems on many of their domestic models.

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