When Occupant Detection Sensors Don't Make Sense?

On December 17, 2011, Hyundai settled, for an undisclosed sum, in a crash that wouldn’t and shouldn’t have caused a fatality but for a defective occupant seat sensor – a problem that may be more common – across many manufacturers – and more potentially deadly than realized.

On January 3, 2010, Donna Lynn Hopkins was a front-seat passenger in a 2008 Hyundai Accent, with her husband, Tom, at the wheel. As they approached an intersection on Highway 181 in Bexar County, Texas, another driver failed to yield the right of way. The Hyundai T-boned the other vehicle with sufficient force to deploy the airbags. But only the driver’s airbag inflated. The occupant seat sensor mat in the front passenger seat determined that, Donna Hopkins, a 165-pound woman, was actually a child, and turned off the airbag. Worse, Hyundai’s sensor strategy also turned off the seat belt pretensioner. Like some other manufacturers, Hyundai’s occupant sensor is designed so that the front passenger seat belt pretensioner fires only if the airbag is deployed. Mrs. Hopkins had none of the advanced safety features needed to adequately protect her in that crash, even though she was belted, and weighed 55 pounds more than the regulated cut-off for smart airbag deployment. Her husband, Tom, walked away from the crash; Donna Hopkins died.

Attorney Stephen Van Gaasbeck, who represented the Hopkins family, says that his research revealed many airbag non-deployment complaints for the Accent and its model twins. In fact, in May 2008, then-Senator Elizabeth Dole (R-NC) wrote to NHTSA on behalf of a constituent who complained about his 2006 Kia Rio. Kia is a Hyundai owned company. In his letter to Dole, the Mint Hill, NC owner wrote:

“There is an airbag sensor on the passenger side that is supposed to keep the passenger airbags disabled when only the driver is in the front seat. There is a dashboard light connected to this sensor that illuminates when this airbag is deactivated. However, very often when my wife, who weighs approximately 220 lbs. is seated in the front passenger seat, the passenger airbag sensor light is still on, indicating that the front passenger seat airbags are not activated. This means that should there be an accident with this vehicle when she is riding as a passenger in it, the airbags on her side may not activate. This is a very dangerous situation…The service department at Folger Automotive would tell me that they had never seen or heard of a similar problem. They said on many occasions that the reason the problem was occurring was probably because of the way my wife was 'sitting in the seat'. They stated to me that they had discussed this situation with KIA technical personnel and that that nobody there had ever heard of a problem like ours. We were made to feel that something we were doing was causing this problem.”

Hyundai occupant sensor system also presents many opportunities for failure, because it does not directly measure weight, rather, it measures the depth of suppression of cells within the mat. Designed by supplier company International Electronic Engineering (IEE), this mechanism can be affected by seat manufacturing differences, moisture, temperature – even body shape.

“The sensor strategy Hyundai uses is so fragile, it’s going to fail somewhere along the line,” Van Gassbeck says. “The degradation can be at any level – even the stitching on the seat cover becoming looser or the fabric becoming looser.”

So-called “smart” airbags came into being in 2000s, when the National Highway Traffic Safety Administration mandated that vehicles include technologies that minimize risk for  children and small adults by either automatically turning off the airbag in the presence of young children or deploying the airbag in a manner much less likely to cause serious or fatal injury to out-of-position occupants. The weight-based cut-off between small-statured adults and children was 110 pounds – the weight of the fifth-percentile female dummy used in the tests.

This rule capped a four-year debate following that recognition that airbags could not only prevent injury, but cause injury and fatality by inflating with too much force. In 1996, the agency opened the discussion by requesting comments about the undesired side effects of the then-current air bag designs. NHTSA pointed to injuries that were occurring during pre‑impact braking as unrestrained occupants moved forward relative to the seat and into close proximity of the deploying air bag. The agency went on to note that certain technological enhancements such as occupant sensing and phased deployment could minimize these side effects.  In 1998, NHTSA issued a notice of proposed rulemaking to require advanced air bag technology. The proposal would have required air bag improvements to protect occupants of different sizes (both belted and unbelted) and require systems that are designed to minimize risks to infants, children, small adults and out-of-position occupants, with air bag deployment suppression strategies or less aggressive deployment strategies. The rule was finalized two years later.

Because of the high potential for misclassification, the supplier must implement a rigorous testing regime to identify sensor performance with a wide range of body shapes – not just weights.  For example, some suppliers will place many occupants of the same weight, but with different body types in the seat to ensure that the sensor is registering the occupant accurately.

Andrew Varga, a former seating engineer who worked on developing sensor systems, says that multiple factors can affect a sensor’s performance. At the unit level, there can be bugs in the analysis algorithms, wide variations in each sensor cell arrayed in the occupant sensing mat and in how the seat is constructed. For example, knit fabric seats have greater compliance than leather seats. The curing of the foam can affect the resilience of the seat, dictating how well the sensors detect occupant weight.

“In the early days, there could be variation as much as 20 to 30 percent,” Varga says. “Improved cells and the use of sensor arrays for better pressure mapping, have reduced errors.”

Once in use, the way people sit in the seats can affect the sensors’ performance. Most mats are designed to measure weight when the occupant is sitting erect in the center of the seat. If, for example, the occupant shifts his weight to accommodate a package on the seat beside him, a lateral shift of two or three inches off-center can place the heaviest load point in a dead-zone, where there are no sensors. Passengers sometimes put their feet on the dashboard, or out of the window. All of these variations can affect sensor performance.

“A good development program with properly executed FMEAs would catch all that.  Still intrinsic variations in those sensors must be taken into consideration” he says.

Nonetheless, this issue of occupant sensor detection reliability is widespread and has led to many manufacturer recalls. For example, in 2002, General Motors recalled some 2000 Chevrolet and GMC C/K models, because the air bag sensing diagnostic module (SDM), contained “an anomaly that could result in the driver and passenger's air bag failing to deploy during certain frontal collisions.”  In February 2010, Hyundai recalled some 2010 Tucson vehicles, because “properly seated adult right front seat passengers weighing over approximately 240 pounds cause the passive occupant detection system (pods) module program to illuminate the air bag warning lamp.”  In 2011, Kia recalled some 2007-2008 Sorento passenger cars because “the occupant sensor might misclassify an adult and turn off the front passenger air bag.” In 2010, Nissan recalled some 2005 through 2007 Infiniti G35 sedans because the wire harness connecting the belt tension sensor and the occupant detection sensor control unit under the front passenger seat “can experience relative movement which can cause wear and oxidization of the terminals and may cause interruption of the signal,” causing a non-deployment in a crash.

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