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11-18-2011

Sensors integrated into the car seat

Sensors that Can Stretch

Sensors integrated into the car seat

A car speeds far too fast toward the tail end of a traffic jam – a crash is unavoidable. The airbag deploys and should protect the car’s occupants. But if the front-seat passenger is leaning too far forward, perhaps looking for something in a bag in the foot space, the force of the airbag can cause injury.

Researchers at the Fraunhofer Institute for Silicate Research ISC in Würzburg, Germany, have now developed sensors that can help prevent such scenarios. These sensors can be integrated into the car seat, for example, where they detect not only if the seat is occupied but the occupant’s posture as well. Is the person leaning forward or sitting back in the seat? Is it a child or an adult? The sensors are capable of measuring both stretch and pressure. They are made of a highly extensible elastomeric film, both sides of which are coated with flexible electrodes. Whenever the sensor is stretched by changes in the shape of the seat, its thickness and consequently its electrical capacitance also change, and can be measured. In contrast to conventional, somewhat inelastic strain gage strips, the new dielectric elastomeric sensors can stretch by as much as 100 % in extreme cases – in other words, they can be stretched to twice their length.

Depending on the field in which these smart materials are applied, multiple electrode pairs might have to be applied to the elastomeric film. This is the case, for example, when the researchers want to measure how the pressure created by a person sitting in a seat is distributed. Each pair of electrodes serves, in effect, as an independent sensor that measures the local strain. This makes it possible to say precisely where and to what degree the pressure has changed.

In making the sensors, the researchers choose the material that best meets the specific requirements of each application. The elastomeric film consists of a polymer in which the individual molecules are chemically bonded with one another. The more extensive the network of molecules, the sturdier is the material – similar to how a fine-mesh fishing net is stronger than one with a larger mesh. The degree of bonding within the polymer can be controlled by the scientists. If the sensor is being used to measure high pressures, a sturdier elastomer film is produced as substrate; for lower pressures, a more pliant film is employed.

These sensors have numerous applications. For instance, they can be used to measure gas pressure. For this, the elastomeric film is stretched like a membrane across a ring. When gas exerts pressure on the sensor membrane, the latter deforms and this deformation is detected by the sensor. Pressure sensors are also useful in safety technology. If someone enters an area too close to a hazardous machine, sensors embedded in the floor can detect this and set off a warning. These smart materials could even be integrated into clothing. Here, they might be used to analyze sequences of movement, thereby helping athletes to optimize their training. As they are so flexible, sensors incorporated into clothing are hardly noticeable at all.

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