How Occupant Classification Systems Work

By: Patrick E. George

Delphi's PODS is a good example of the way an occupant classification system works. See more car safety pictures.
Delphi's PODS is a good example of the way an occupant classification system works. See more car safety pictures.
Courtesy of Delphi

When airbags became required equipment on all U.S. automobiles in the 1990s, they represented a big leap ahead for vehicle safety. Prior to that, drivers and passengers had little more than their safety belts to restrain and protect them in the event of a collision. Today, airbags have reduced driver fatalities by 29 percent and passenger fatalities by 32 percent -- and are the reason more than 27,000 people have survived car crashes [source: Insurance Institute for Highway Safety].

While airbags save lives, they aren't perfect. The National Highway Traffic Safety Administration says nearly 300 people have died as result of airbag impacts since 1990. The majority of those were in cars equipped with early, overly sensitive airbag systems, and most of the deaths were children and infants.


For children and people of small stature, the impact of an airbag inflating at more than 200 mph can cause permanent or deadly head and spinal injuries. This is the reason that drivers are told to put smaller passengers in the back seat. It's a law in some states. But what if you are carrying several passengers in the backseat and have to put a child up front? What if you drive a two-seater car?

For that reason, car companies are now required to equip certain vehicles with Occupant Classification Systems(OCS) -- a system of sensors that detect who's sitting in the passenger seat. OCS eliminates the need for an on/off switch for airbags in most cases because it uses sophisticated computer technology to identify whether an adult or a child is in the seat.

Before we take a look at Occupant Classification Systems, let's review how the airbag itself works. In the event of a crash, a sensor triggers a nylon bag inside the steering wheel, door or dashboard to instantly fill with compressed nitrogen gas. The goal of this is to cushion the occupant from a powerful impact.

In this article, we'll explore the development and use of Occupant Classification Systems, and look at why airbags are still no replacement for safety belts.

OCS Sensors

Dual stage airbags like these can deploy at different speeds, depending of the severity of the crash.
Dual stage airbags like these can deploy at different speeds, depending of the severity of the crash.
John Eder/Riser/Getty Images

Imagine this scenario: you and your family are moving into a new house. The trunk and back seat of your car are stuffed with your belongings, but you have to take your seven-year-old daughter along for the ride. You have no choice but to put her in the front passenger seat. But will she be safe up there? What happens if there's a wreck? What will the airbags do to her?

It's never a good idea to put your kids in the front seat, but sometimes reality forces drivers to do otherwise. If you're driving an American car or truck made after 2006, you're probably okay, since they all contain some method of airbag suppression -- most likely, an OCS.


Let's take a look at one popular OCS made by Delphi. Inside the seat, you will find a pressure sensor, a silicone-filled "bladder", and an electronic control unit (ECU). When someone sits on the seat, the pressure sensor signals the occupant's weight to the ECU. The ECU then sends that data to the airbag, which has its own control unit. Based on that information, the vehicle's computer turns the passenger airbag on or off.

The OCS doesn't just detect weight. It reads the passenger's seating position and determines if they're wearing a seat belt. It also has a seat belt tension sensor that allows the OCS to interpret the pressure created when a child seat is fitted. In other words, the system is designed to tell whether a child safety seat is occupying that seat or whether you're just carrying some heavy object there. A light or sign on the instrument panel tells the driver whether the passenger airbag is on or off.

Once the onboard computer knows the passenger's size and weight, the car's dual stage airbags come into play. Based on the occupants' size, these types of airbags can deploy at full speed, partial speed, or not at all. An airbag deploying at full speed can badly injure or even kill a child or small adult. In addition, dual stage airbags also can deploy at lesser speeds when the car is involved in a minor collision.

Weight sensors have become the most common way of detecting an occupant; however, carmakers are experimenting with new and possibly more effective ways of sensing who or what is riding shotgun. Some experimental systems take optical images of passengers to determine whether a child or adult is sitting in the seat and use that information to turn the airbags on or off accordingly. Others go so far as to detect physiological factors like respiration and heartbeats in sitting passengers to tell the airbags what to do [source: Delphi]. Remember -- even on cars equipped with an OCS, it's always safest to keep small children in the back seat.

In the next section, we'll take a look at the history of airbag sensor systems, and why they're necessary in the first place.

History of the OCS

Automakers can now use female and child-sized crash dummies to test cars.
Automakers can now use female and child-sized crash dummies to test cars.
Romilly Lockyer/The Image Bank/­Getty Images

Drivers demand a lot from modern automobiles in terms of safety features. Airbags were placed in cars to reduce fatalities, but early on they experienced problems of their own.

Part of the problem with early airbags was that consumers expected them to replace seatbelts, said Rae Tyson, a spokesman for the National Highway Traffic Administration. "This could not be further from the truth," Tyson said. "The airbag was never meant to be a primary safety device, but a supplement."


Early airbags were overly aggressive to protect these kinds of motorists, Tyson said, which led them to be too forceful and deploy too easily. As such, new systems -- like the OCS -- had to be developed to keep injuries and deaths down. In 2000, the U.S. Transportation Secretary issued a rule requiring that airbags on cars made after 2006 create less risk of serious injuries, particularly for women and children of smaller size. This rule encouraged automakers to use technologies like weight sensors and dual-stage airbags to get the job done.

The rule also required crash tests on vehicles to include dummies of small women and children in addition to average sized men, which were previously the only dummies required. In addition, the law required a way for "low risk" airbags to deploy if a child is sitting in the passenger seat during a crash to keep them from being injured. The mandate also called for suppression of the airbag for rear-facing infant seats and forward-facing child seats, while enabling the airbag for passengers as small as a five feet tall and 110 pounds.[source: National Highway Traffic Safety Administration].

Today, as part of the mandate, systems like these can be found on every new car made in America after 2006. Every system doesn't have to be the same, Tyson said. It's up to car manufacturers to figure out a way to meet the standard. They can do this with several technologies. Modern airbags have advanced a great deal, and carmakers aren't seeing the same problems they saw in the past. Educating motorists to keep kids in the back and to sit away from the airbag deployment zone has kept injuries down as well, Tyson said.

Next, we'll look at the advantages and disadvantages associated with Occupant Classification Systems.

OCS Advantages and Disadvantages

Despite many advances in airbag technology, children are still safest in the backseat of your car.
Despite many advances in airbag technology, children are still safest in the backseat of your car.
Patti McConville/Photographer's Choice/Getty Images

There are lots of benefits to having an OCS and dual-stage airbags in your car. They pose less of a threat to children and small adults, and they allow drivers more options for seating their passengers. It also means that airbags will not damage inanimate objects placed in the passenger seat.

That doesn't mean that early versions didn't have their problems. Sometimes the OCS didn't properly sense the passenger, which led to the airbag being turned on or off when the opposite should have happened. Also, some smaller female passengers noticed that the airbag would turn on or off depending on how they sat in the seat, as if the airbag was unable to decide whether an adult or child was sitting there. In other instances, the OCS was overly sensitive -- placing a newspaper in the passenger seat would trigger the airbag to switch on, for example. Hyundai, Jaguar, Jeep, Lexus, Nissan and Toyota vehicles all experienced similar problems, which led to complaints to the NHTSA [source:].


With time, these problems were largely fixed, and airbags are now safer than ever before. Occupant Classification Systems have made airbags safer and cut down on airbag-related deaths, the NHTSA's Tyson said. "It's worked fantastically. We've gone from a safety system with unintended consequences to one with [no consequences]. We've had no airbag fatalities in recent years."

For more information on OCS systems and other advances in airbag suppression technology, please see the links on the following page.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • Delphi. "Delphi Passive Occupant Detection System B." (9/28/2008)
  • Delphi. "Monitoring Driver Physiological Parameters for Improved Safety." (9/28/2008)
  • Insurance Institute for Highway Safety. "Q & A: Airbags." (9/27/2008)
  • Mello, Tara Belkus. "The Evolution of Front Airbags." (9/28/2008)
  • National Highway Traffic Safety Administration. "U.S. Transportation Secretary Slater Announces Advanced Air Bag Regulation that Improve Benefits and Reduce Risks." (9/26/2008)
  • 7bee37fb302f6d7c121046108a0c/?javax.portlet.tpst=1e51531b2220b0f8ea14201046108a0c_ws_MX&javax.portlet.prp_1e51531b2220b0f8ea14201046108a0c_viewID=detail_view&itemID=a5407409ce2bff00VgnVCM1000002c567798RCRD&pressReleaseYearSelect=2000
  • Tyson, Rae. NHTSA spokesman. Phone interview. (9/29/2008)
  • SaferCar. "Advanced Frontal Airbags." (9/28/2008)