Flex sensor mat to aid smart airbags

Troy, MI -Current airbag systems are mostly designed for a 50th percentile male seat occupant. To improve safety for children and small adults, vehicle manufacturers and the National Highway and Traffic Safety Administration (NHTSA) are looking at adaptive passenger restraint systems that would determine the occupant's size, weight, and position and automatically tailor airbag deployment or suppression. By September 1, 2006, all "light" vehicles produced must be so equipped.

Design engineers have several challenges ahead because the system will have to automatically disable the airbag or operate in a depowered mode as needed (see figure).

Essential to any system will be direct on-seat measurement of occupant weight (hence size estimate) as well as indication of general position (weight distribution). Bruce Wrenbeck, director of Safety Electronics and Restraint Systems for Siemens Automotive, notes one promising system already in use in some European cars to detect passenger weight. Within the seat bottom is a Mylar mat laminated with layers of conductive ink and graphite to form a pattern of pressure sensitive resistors (see figure). The mat itself is made by IEE (International Electronics and Engineering; Detroit, MI), which adapted it from keyboard flexible-circuit switches. Siemens supplies the control electronics and logic.

Wrenbeck says Siemens is developing electronics and algorithms to use such a mat's pressure profiling for smart airbags. A Siemens/IEE occupant classification system uses a database of human-form statistics to determine weight and general orientation of the passenger. He notes one critical issue in using the mat system is variability of seat foams, in density for instance. "The electronics must work, even when seat comfort is gone [due to aging and wear] and after repairs," he adds.

While some occupant position sensors are being used, such as Jaguar's ultrasound system, most are still in development. All will require integration into a viable system. Other candidate technologies are lasers and 3D imaging cameras, including infrared.

Wrenbeck says an integrated approach will see seat-based sensors for reliably determining weight distribution and occupant position-determining systems for bitmapping what is in the seat and where it is located.