Stretchable Sensors Work in Machine Safety

Monday, May 16, 2011 @ 06:05 PM gHale

An airbag in an automobile is a wonderful safety device, but if a car’s occupants are not in the proper position, the safety device could also cause harm.

There are now sensors under development that can integrate into the car seat, where it detects not only if someone is in the seat, but the position of the occupant as well. Is the person leaning over or sitting back in the seat? Is it a child or an adult?

You can customize and apply dielectric elastomer sensors in a variety of ways.

You can customize and apply dielectric elastomer sensors in a variety of ways.

“The sensor films can measure stretch, as well as pressure,” said Dr. Holger Böse, scientific and technical manager of the Center for Smart Materials at the Fraunhofer Institute for Silicate Research ISC in Würzburg, Germany. “They are made of a highly stretchable elastomeric film, coated on both sides with flexible electrodes. Whenever the sensor is stretched by changes in the shape of the seat, the sensor’s thickness and, as a result, its electrical capacitance also changes, which we can measure.”

In contrast to conventional, rather inelastic strain gauge strips, the new dielectric elastomeric sensors can stretch by up to 100 percent in extreme cases – in other words, they can draw out to twice their size.

Depending on the field, it might be necessary to coat the elastomer film with multiple electrode pairs. This is the case, when measuring the distribution of body pressure to determine a person’s posture in a seat. Each pair of electrodes serves, in effect, as an independent sensor, measuring the local strain. “This is how we can say precisely where and to what degree the pressure has changed,” Böse said.

In making the sensors, researchers choose the material that best meets the specific requirements of each application. The elastomer film consists of a polymer in which the individual molecules chemically bond with one another. The better the network of molecules; the sturdier the material. That is similar to how a fine-mesh fishing net is stronger than one with a larger mesh. Scientists can control the degree of bonding in the polymer.

“If the sensor is being used to measure high pressures, we produce a sturdier elastomer film as substrate; for measuring lower pressures, we use more pliant films,” Böse said.

These sensors have numerous applications. They can measure the pressure of gases. To do this, the elastomer film stretches like a membrane across a ring. If gas exerts pressure on the sensor membrane, it deforms – which the sensor then detects.

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 intelligent materials could even integrate into clothing: Here, they could analyze sequences of movement, thereby helping athletes to optimize their training. Because they are so flexible, sensors that are part of clothing are hardly noticeable at all.

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