This terahertz measurement system for non-destructive testing measures the thickness of multi-layered plastic films at a rate of 40 times per second. Credit: Fraunhofer IPM |
The
planned arrival time, the request to land or the landing direction—this
is the kind of information pilots discuss via radio with ground staff
in the control tower. The nose of the aircraft, the “radar dome”,
receives incoming radio signals and transmits radio signals sent by the
pilot as well. It is made of a fiberglass composite. But if even tiniest
imperfections arise during production—if, for instance, little foreign
particles, drops of water or air bubbles become enclosed in the
resin—over time they can cause fine cracks through which moisture can
seep. This causes interference in radio traffic through the aircraft
nose, introducing static into the signal.
As
part of the Dotnac project, researchers at the Fraunhofer Institute for
Physical Measurement Techniques IPM in Kaiserslautern are working with
partners in industry and research to develop a new testing system: The
system uses terahertz waves to completely scan the aircraft nose, which
is several centimeters thick, and immediately identify any flaws.
The
frequency of terahertz waves falls between that of microwaves on the
one hand and infrared light on the other. They are completely harmless
to humans. The waves are generated in a rolling cabinet not unlike those
found in many offices: it contains a microwave source and all
electronics to control the system and to collect the data. A frequency
mixer multiplies the frequency of the microwave radiation generated into
the terahertz range. Researchers have connected the actual measurement
module to this container by means of electrical wires. This module emits
the terahertz waves toward the radar dome. The material reflects the
radiation, and the detector integrated in this module analyzes the
reflected terahertz radiation. If there are any air bubbles or little
imperfections embedded in the material, they turn up in the reflected
signal.
The
main challenge facing researchers was to find out which terahertz
frequencies they would have to use to bombard the material to achieve
the most effective results for the various imperfections. Higher
frequencies create better resolution, while lower frequencies have less
difficulty penetrating the material. The researchers select from a range
of different frequencies depending on the errors the researchers are
looking for in the case concerned. The scientists have already developed
a prototype of the testing system. It will be presented at the Control
trade fair, May 8-11, in Stuttgart, Germany. Around a year from now, the
scanner will have advanced to the point that it will scan and analyze
aircraft noses automatically. Thus far, simple scanners for level and
rotations symmetrical objects are available.
Researchers
have come up with another terahertz testing system as well, one that
analyzes the thickness of layers—such as are found on aircraft and cars.
“Our
terahertz measuring system is one of the few robust enough for
industrial use,“ according to Dr. Joachim Jonuscheit, deputy head of
department at Fraunhofer IPM.
Just
like the system that checks aircraft noses, this one also consists of a
rolling cabinet along with a transmitter and a receiver connected to
the container by cables five meters long. This system works with very
short terahertz pulses. Each pulse is partially reflected off of the
interfaces of the layers: the surface of the first layer, the interface
between layer one and two, and so on. The deeper the layer reflecting
the pulses, the longer the pulses take to return to the detector. Using
the time each pulse takes to make its way back to the detector, built-in
software automatically calculates the thickness of the various layers.
The
system‘s great advantage is its robustness. But how did researchers
accomplish this? “First of all, we no longer shoot the laser that
excites the system by open beam as typically used in terahertz systems;
instead, we feed it through optical fibers. And secondly, we have fixed
and arranged the optical elements to make them mechanically robust. We
have also improved the manufacturing processes for the semiconductor
components—the transmitters and detectors—to make the individual
elements more resistant,“ Jonuscheit explains. At the Control trade
fair, the researchers will demonstrate live measurements on
multi-layered plastic films of varying thicknesses.
Source: Fraunhofer Institute
