Materials Engineering: TPE-Sheathed Sensors Improve Pedestrian Safety

Feb. 7, 2008
A thermoplastic elastomer (TPE) is the outer skin of a sensor strip that will help protect pedestrians if they step into the path of a moving car.

The sensor is currently installed in the “active bonnet” of Jaguar XK and Citron C6 models. It is between 1.1 and 1.4 m (3.6 to 4.6 ft) long and sits between the front bumper and radiator. The device was developed by systems supplier Magna Electronics Europe.

The high-performance Hytrel 5556 TPE from DuPont, Wilmington, Del., resists aging and remains highly elastic at temperature extremes. “Its nearness to the radiator and exposure to solar radiation may force it to work at 85°C (185°F); in winter, it may see –40°C (–40°F),” says Ralf Konnerth, component manager at Magna Electronics Europe. “The plastic is robust enough to withstand impacts from road debris and won’t be degraded by water or road-salt solutions.”

Magna Electronics used cascade injection molding to form the two strips of Hytrel that make up the sensor’s outer skin. The process holds tight tolerances, which is key to sensor operation. Adds Konnerth, “Du- Pont gave us guidance for assembling the light guide inside the polyurethane foam and about how to pretreat the surfaces to improve adhesion between the foam and Hytrel skins.”

How the “act ive bonnet” works

The sensor-operated “active bonnet,” one of the latest developments in automotive design, aims to better pedestrian safety. Inside the sensor strip, which installs between the front bumper and the radiator, is a soft polyurethane foam in which a light guide made of polymethylmethacrylate is embedded. A light-emitting diode (LED) launches light impulses to the light guide. The LED is synchronized with a receiver at the other end of the sensor strip.

The receiver registers the amount of light that actually arrives and constantly compares it with the amount of light emitted. An object impacting the sensor strip cuts the amount of light received. Electronic measuring equipment connected to the sensor calculates the force of the impact from the difference between the emitted and received light.

From the progression of the force over time, the electronics conclude things about the nature of the impacting object. If it concludes that the object was a person, it ignites pyrotechnic actuators which within about 40 msec lift the bonnet cover a few centimeters. This distances the impacting body from the hard motor components, thus absorbing more energy. It also changes the angle in such a way that a pedestrian will not head strike the windshield. This device is effective at speeds up to about 45 km/hr (30 mph).

Sensor strip from Magna Electronics Europe, helps reduce the risk of pedestrian injuries resulting from a frontal collision with a car. The strip, between 1.1 and 1.4 m long, features an outer skin made from Hytrel 5556 thermoplastic elastomer.

Sensor strip from Magna Electronics Europe, helps reduce the risk of pedestrian injuries resulting from a frontal collision with a car. The strip, between 1.1 and 1.4 m long, features an outer skin made from Hytrel 5556 thermoplastic elastomer.

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