Occupant Detection Sensors

June 1, 2005

4 Min Read
Occupant Detection Sensors

Electronics suppliers are gearing up for the growing popularity of smart air bags, and they’re going to need a variety of sensors to bring the technology to fruition. The new breed of air bags, developed in response to fatalities suffered by children and small adults struck by full-power bags, calls for electronic systems to recognize the weight (and possibly the position) of seated individuals.In the near term, most automakers are focusing on front passenger seat applications, but are aware that the technology could one-day spread to rear seats, as well.

“The front passenger seat is the obvious application today, particularly in pick-up trucks and sports cars, where there’s not an option to put a child in the back,” notes Brad Stewart, principal applications in the Sensor and Analog Products Division at Freescale Semiconductor, an occupant detection sensor supplier.

To date, there’s no universal method for distinguishing between children and adults.Tier-one suppliers – including Delphi Corp., Bosch Automotive, Siemens VDO Automotive, and TRW Automotive – are attacking the problem with a variety of techniques. Those solutions include strain gages, capacitance measurements, infrared sensors and pressure sensors. Each technology offers a specific set of advantages, ranging from higher accuracy to better temperature resistance to greater tolerance to impact.

Following are three solutions to the weight sensing problems, each using different technologies:

Delphi’s weight and position detection

Delphi, a tier-one automotive supplier, makes two types of sensors for its own detection systems. The company’s first-generation system, designed for weight detection, uses piezoelectric sensors and Hall Effect sensors in conjunction with fluid-filled bladders located in the car seat. Together, the sensors provide input to a software algorithm in a microcontroller beneath the seat. The second generation of Delphi’s Occupant Position and Recognition System employs infrared (IR) sensors. By filling the passenger compartment with invisible measurement coordinates, the IR sensors enable the system to recognize if the occupant is out of position (say, leaning too close to the air bag module), or if a rear-facing child seat is present.
For more information on Delphi’s detection technology, go to http://www.delphi.com/news/solutions/monthly/ms11788-05012002.

Freescale’s e-field imaging

Freescale’s MC33794 integrated circuit uses electric field (e-field) imaging to detect the presence of a car seat, and therefore stop an airbag from deploying on a child. The SeatSentry system generates a low-level e-field from multiple antennas mounted in the seat back cushion. If a conductive body enters the e-field, the system detects it through a drop in AC voltage. By multiplexing the output of up to nine sensors, the system can detect the height of a passenger, as well as his or her proximity to the air bag.
For more information on Freescale’s e-field technology, go to http://www.freescale.com/files/analog/doc/reports_presentations/RMVID3DIMAGING.html.

Texas Instruments’ occupant weight sensors

TI’s force-based occupant weight detectors employ piezo-resistive strain gages, which are placed in a carefully-designed steel structure within the seat. When small strains (either tensile or compressive) are produced by a body in the seat, the electrical resistance of the gages change, thus producing a voltage signal that is proportional to the weight of the body on each sensor. Using input from four such sensors, an electronic controller sums the information and calculates body weight. TI engineers say that the technology’s biggest advantage is its survivability in minor crashes. “If you get into a minor accident, even one that doesn’t involve air bag deployment, you need the sensing system to survive,” notes Steve Beringhause, a TI business manager. For more information on TI strain gages, go to http://www.ti.com/snc/products/sensors/auto-msg.htm.

Web Resources

Check out these companies, which also offer occupant detection sensors or sensing systems:

CTS Corp.

TRW Automotive
Siemens VDO

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