The team’s early challenges included designing the sensors and finding a way to insert them into composite material without affecting its structural integrity. The sensors had to be wireless because running exterior wires could attract lightning strikes in the field, and since they were embedded, had to be passive and able to operate without battery power.
The team designed a passive, wireless sensor with a specific resonance frequency. When an electronic device called an interrogator sends out an energy wave, the sensor reflects it and the interrogator reads it. Changes in the resonance frequency or amplitude shifts in the reflected energy wave provide an indication of what’s going on inside the composite material.
“There are a couple of things that go into making the decision about what frequency bands to use,” says Stoffel. The team wanted the sensors to operate in ISM radio bands, which are reserved internationally for industrial, scientific and industrial purposes. To control costs, it decided to use the same interrogation devices used by automotive radar detectors that operate in the 25 GHz range. “This isn’t a large application where it makes sense for you to have a custom chip,” Stoffel explains.
“One of the challenges that we have is that we are burying [the sensor] inside the composite, which will modify it somewhat,” she continues. Embedding sensors into the composite also makes it more difficult for the interrogator to read. The team wanted the reading distance to be at least 6.5 feet. This would allow the interrogator to be suspended above the blade mold in the factory, so it could read data from the sensors without interfering with work on the production floor.
An additive printing process is used to manufacture the sensors. An aerosol jet printer sprays conductive metal inks onto a substrate such as a polyimide film. The printed circuits made through this process are fairly complex and include an array of small antennas (called the Van Atta array) to provide a good reading distance, according to Stoffel.
Minute changes in the spacing of the arrays will alter the response of the sensors, so they can be tuned for various properties. That made it possible to use the same sensors for both the GE production monitoring part of the project, including the extent of curing and the real-time temperature of the composite, as well as the longer-term, in-the-field strain monitoring that Sikorsky is testing.