TRW's active head restraint shortens response time

  • 05-Mar-2009 10:12 EST
TRW’s active head restraint triggers in less than 10 ms.

­Head restraints began as one of the first passive safety systems in 1969, but they’re now becoming a tool for automakers that employ active safety systems. TRW Automotive is entering this growing market with a head-restraint system said to be faster and less complicated than competing systems.

Conventional head restraints have reduced the incidence of whi­plash, but they’re often not adjusted properly to reduce injury during rear-end collisions. In recent years, a number of companies have begun offering restraints that move forward during accidents to better stabilize the passenger’s head.

This movement lets seat designers put the restraint far enough away from the driver’s or passenger’s head to be comfortable while still minimizing the amount of head movement. Automakers are increasingly deploying them, partially in response to mandates from agencies including NHTSA, the Insurance Institute for Highway Safety, and Europe’s New Car Assessment Program.

TRW Vehicle Safety Systems Inc. is the latest entrant, unveiling an electromechanical system that it claims is faster, lighter, and simpler than units now on the market. TRW predicts that electromechanical systems will see more use than electronic, largely because electronic systems use small pyrotechnics to move the restraint so they must be taken to dealers after deployment.

Most electromechanical systems use solenoids to trigger springs that move the restraint. These springs can easily be reset, usually by car owners. As more systems hit the market, response time will be a key differentiator.

“Our system is five to 10 milliseconds faster, which is a lot given that the whole event is only 50 to 60 milliseconds,” said Charlie Steffens, Safety Systems Technology Director at TRW Occupant Safety Systems. The full time period is comparable to airbag deployment, he noted.

That sequence begins when crash sensors are activated and send a signal, which typically takes 10 to 15 ms. Then the solenoid is triggered, which generally takes 10 to 20 ms, depending on the system.

“We trigger in seven to eight milliseconds,” Steffens said. The actual movement of the head restraint consumes the remaining 30 to 40 ms.

TRW’s proprietary design incorporates techniques used in the basic seatbelt latches. That mechanism is employed after the solenoid is triggered and a spring extends the mod­ule.

“­When the head hits the module and tries to push it back, the mechanism locks into position, much like locking a seatbelt in place,” Steffens said.

Once the accident is over, drivers can reset the unit so they don’t have to lean forward during the drive home, Steffens mused. Drivers can click a lever and then squeeze the components back into place without using much muscle, he added.

TRW’s system also has fewer components than competing offerings. TRW’s unit has 16 parts including fasteners, while most competitors have around 25, Steffens said. It weighs around nine pounds less than competitive seat-integrated, passive head-restraint systems.

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