Terahertz Quality Inspection for Automotive and Aviation Industries
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Nondestructive quality inspection with terahertz waves has become an emerging technology, especially in the automotive and aviation industries. Depending on the specific application, different terahertz systems—either fully electronic or based on optical laser pulses—cover the terahertz frequency region from 0.1 THz up to nearly 10 THz and provide high-speed volume inspections on the one hand and high-resolution thickness determination on the other hand. In this paper, we present different industrial applications, which we have addressed with our terahertz systems within the last couple of years. First, we show three-dimensional imaging of glass fiber–reinforced composites and foam structures, and demonstrate thickness determination of multilayer plastic tube walls. Then, we present the characterization of known and unknown multilayer systems down to some microns and the possibility of measuring the thickness of wet paints. The challenges of system reliability in industrial environments, e.g., under the impact of vibrations, and effective solutions are discussed. This paper gives an overview of state-of-the-art terahertz technology for industrial quality inspection. The presented principles are not limited to the automotive and aviation industries but can also be adapted to many other industrial fields.
KeywordsTerahertz Quality inspection Automotive Aviation Volume inspection Composite material characterization Foam inspection Layer thickness determination Multilayer Wet coatings Vibration compensation
In the recent years, impressive developments from the terahertz community have shown that terahertz technology is a very attractive tool to complement well-established methods in the field of nondestructive testing [1, 2, 3, 4, 5, 6, 7, 8]. The unique characteristics of terahertz radiation, e.g., being nonhazardous and providing high-contrast images when penetrating dielectrics (compared, e.g., with X-ray techniques) combined with a higher resolution than microwaves, have led to many different applications, especially in the automotive and aviation sectors. Following the study of TEMATYS SARL , the market size of nondestructive testing with terahertz radiation in 2012 was comparable with the size of terahertz research at these times. Until 2020, the study predicted a doubling of the industrial market compared with the research volume, while assuming that the overall terahertz market increases with 16% per annum. Most probably, there will not be an abrupt change from existing and reliably working solutions in industrial production lines even when the terahertz solution is able to serve additional values at the same time. Mostly implemented as an additional technology parallel to already-implemented quality inspections systems today, terahertz measurement techniques could gradually lead to fully terahertz-controlled processes. Here, challenges given by rough industrial environments in comparison with research laboratories must be overcome. The largest and most attractive industrial application at the moment is the thickness measurement technology [9, 10]. In cooperation with several industrial partners, we have already solved a number of practical problems, e.g., the vibration of objects during high-precision thickness measurements. Using interferometric-controlled real-time distance information, we were able to eliminate the impact of such vibrations almost completely. Hence, we realized a reliable terahertz-based thickness gauge being able to resolve multilayer systems of, e.g., car bodies in the production line.
Terahertz solutions have proven highly attractive in particular for material characterization of novel and complex materials, where state-of-the-art testing technologies often do not work or at least their employment is highly restricted. In the context of the energy revolution, especially, the automotive and aviation industries are in search for new materials, which provide sufficient structural stability and are lightweight in comparison with, e.g., metal-based constructions. Fiber-reinforced composite materials made of multiple layers of carbon, glass, natural fibers, etc. are on the rise. Here, terahertz technology is able to support the quality inspection of such structures in two ways. On the one hand, volumetric inspection of the materials to identify possible inclusions, cracks, or other defects is of high interest. This is done, e.g., for radomes typically constructed out of glass fiber–reinforced plastics (GFRP) forming a protecting shell for radar antennas. A high degree of transparency of radomes is essential to ensure proper signal transmission. We developed a terahertz inspection system for the radome production industries, which inspects this critical part of the radar transmission link for aircrafts. On the other hand, the thickness of the coatings protecting the composite structures has to be controlled. Well-established methods based on eddy-current or photo-thermic principles often fail for composite substrates, whereas the substrate-independent terahertz technology for thickness measurements meets these demands.
The terahertz technologies presented in this paper can be roughly divided into two categories. In the first part of this paper (section 2), we demonstrate electronic, fast-scanning, frequency-modulated continuous-wave (FMCW) technology, employed mainly for applications focusing on volume inspection of composites and foam structures. In addition, thickness information of layer systems in the millimeter range can be gathered with this technology. In the second part of the paper (section 3), we present optical terahertz systems based on the time-domain spectroscopy (TDS) principle for precise measurement of thin layers down to several microns. The measurement principle itself, the challenges arising with the implementation of the technology under industrial conditions, and the generated solutions are described. The final section—section 4—gives a conclusion and an outlook of terahertz technologies used for quality inspection in the automotive and aviation industries.
2 Volume Inspection
2.1 FMCW Terahertz Measurements
Figure 2b shows a schematic of the implementation of an all-electronic FMCW terahertz measurement unit (here, W-band system with a center frequency of 100 GHz). Linear frequency ramps are generated in the low-frequency K-band with a DAQ-controlled voltage-controlled oscillator (VCO) and converted into the terahertz range by an active frequency multiplier chain (S)—in the illustration example with a multiplication factor of 6. The frequency-modulated terahertz wave is then transmitted (Tx) by a conical horn antenna and the same antenna also receives the reflection signals (Rx) from the sample under test. An attached directional coupler feeds the received signals to a Schottky diode–based receiver (D) where they are mixed with the reference frequency ramps to obtain the beat signals. The latter are sampled by a DAQ for further processing in a computer for the reconstruction of depth information of the sample under test.
The measurement systems developed in our group typically operate at center frequencies of 90, 140, 270, and 410 GHz providing bandwidths of 40, 60, 100, and 180 GHz, respectively. For imaging purposes, our monostatic transceivers are commonly operated in connection with motion systems and with a quasi-optical lens or mirror setup in order to focus the terahertz radiation to a measurement spot and to guide the reflected signals back into the transceiver, as indicated in the scheme in Fig. 2b. The optical configuration defines the focal sport size as well as the depth of focus of the terahertz beam. Hence, depending on the intended application, a careful consideration of the optical setup is required.
As an example for a typical image obtained with a FMCW terahertz measurement system, Fig. 2c shows the C- and B-scans of an inner layer of a 2-cm-thick and 50 × 50 cm2 large GFRP multilayer sample. The yellow line in the B-scan image indicates the (z-)location of the shown C-scan image and vice versa. The composition of different panels within the sample as well as internal defects can well be recognized within both the B- and C-scan images. While the motion of a single-point sensor limits the acquisition time for volumetric images significantly, multistatic array configurations in connection with digital beam forming techniques, as described in section 2.4, can provide faster operation and a proper solution to this limitation.
The example proves that the FMCW approach in connection with terahertz and millimeter waves allows a volumetric inspection and three-dimensional localization of defects and/or material layers within terahertz-transparent samples. The following sections address a number of specific application scenarios of FMCW terahertz measurements.
2.2 Radome Inspection
2.3 Quality Control of Acoustic and Thermal Insulation
The high sensitivity of the terahertz measurement technique allows for the inspection of soft materials such as open-cell foam, which is widely used for thermal and acoustic insulation in boats, trains, automobiles, and aircrafts. Foam materials often contain hidden defects from the production process, such as voids within a larger volume. Standard X-ray techniques usually do not exhibit sufficient contrast for defect detection and application of a coupling medium required for ultrasound measurements is not easily possible. On the other hand, the capability to obtain depth information via the FMCW measurement principle in combination with millimeter spatial resolution of terahertz radiation allows for an accurate three-dimensional localization of voids within such low-density foam materials.
The terahertz measurement shown in Fig. 4 was performed with a single transceiver unit in connection with x-y raster–scanning mechanics. In order to address industrial inline inspection, a fast-scanning axis in connection with a conveyor belt can be used to achieve feed motions of a few millimeters per second. An even faster approach for gathering volumetric images could be the use of sensor array configurations as described within the following section.
2.4 Inline Inspection with Sparse Arrays
A fast switching matrix is used to sequentially operate the emitters for each frequency point, while the receivers simultaneously record the received signals from the illuminated object. This approach allows volumetric imaging at feed motions of several 10 cm/s, depending on the amount of frequency points with an underlying frequency switching time of the system of 300 μs. The implementation of fast image reconstruction algorithms in connection with parallel data processing on a graphics processing unit (GPU) provides real-time image reconstruction . The image in Fig. 5c shows the inner layer of GFRP composite test sample (see photograph in Fig. 5b), which was recorded with a feed motion of the translation axis of 19.2 cm/s. The test sample was the same as was shown in the terahertz image in Fig. 2c. Again, the artificially implemented defects within the structure can be clearly seen in the image acquired with the line array measurement system. In contrast to the image in Fig. 2c, the image recorded with the array exhibits a significant line pattern due to a violation of the Nyquist-Shannon sampling theorem by the extreme sparsity of the array. The spatial sampling along the array axis can be controlled by the array design and can be customized for the requirements of a specific application.
2.5 Mobile Inspections
2.6 FMCW Thickness Measurements
While broadband terahertz systems based on the TDS measurement principle are widely employed for thickness inspections of thin dielectric multilayer structures—e.g., for paint layers, as highlighted in section 3—a strong signal attenuation, particularly in thick samples with several millimeters to centimeters of thickness, can limit the applicability of this technique. In the millimeter wave and lower terahertz regime, FMCW systems provide a high dynamic range of up to 70 dB at measurement rates of several kilohertz in connection with higher signal penetration depth than pulsed terahertz systems. Recently, we demonstrated the suitability of such systems for thickness measurements and proved that layer thicknesses even below the Rayleigh resolution limit can be resolved by taking advantage of a model-based signal processing technique [23, 24]. In a basic approach, modeled versions of the measurement signal are computed a priori based on estimated properties of the multilayer material system under test. The real measurement signal is then correlated with the individual signal models. The layer properties with the best agreement between signal model and measurement represent the final result. In this way, the minimum layer resolution of the measurement system can be improved significantly by proper consideration of the a priori information. Multiple reflections can be taken into account by using the transfer-matrix method for the calculations of the signal models .
3 Submillimeter Layer Characterization
3.1 Thickness Determination of Painted Car Bodies for the Automotive Industries
Nowadays, not all car body parts are based on metal, but also on plastics, especially crash-relevant structures, which should return to their original form after minor crashes (bumpers etc.). As these structures shall look identical to the rest of the car body for design reasons, they are coated with multilayer structures as well. As long as the refractive index contrast is sufficient, terahertz technology is able to determine these layer thicknesses as well, as it does not depend on a metallic substrate. Coatings on composite materials such as GFRP or CFRP (carbon fiber–reinforced plastics) can also be determined [43, 44], which is of growing interest, as modern vehicles often contain these materials.
3.2 Intermediate Devices—Thick and Thin at the Same Time
3.3 Wet Paint
Despite the advantage of nondestructively measuring product quality after the manufacturing and refining process, the inline measurement of these properties is most helpful to the manufacturer. This poses new challenges for the measurement equipment as the need for measurement speed, robustness, and especially for the paint layer thickness measurement new material properties have to be taken into account. In paint processes, it is of highest interest to recognize deviations from the desired process during the process itself. So the measurement of wet paint has to be enabled to provide the manufacturer the most useful information to the earliest possible time. In the wet state, it is often better to add additional paint for thickness correction in comparison to do so after the first curing, not to generate a boundary in between which influences the optical appearance of the product and its mechanical properties. In addition to that, the cycle time for inspection and production is reduced. In general, there are two categories that have to be taken into account, when considering the measurement of wet paint: solvent-based and water-based paint. As the alcohol-based solvents are showing considerably different optical properties in comparison with the strongly absorbing bulk water, these two classes behave differently in the terahertz frequency range. Even though the absorption of bulk water is very high in the terahertz frequency range (about 250 cm−1 at 1 THz), the measurement of water-based paint is possible in general, as the layer thickness of wet paint is often less than a millimeter [33, 34, 48, 49].
3.4 Measuring Safety-Relevant Features of Car Interiors
Due to the small hole size of the perforation, the depth estimation accuracy is limited, which could be enhanced by different optical setups. Further, as the surface and the interior structure of the slush skin are not flat and homogeneous, this application is challenging, as the terahertz waves get scattered. Different improvements, e.g., by using other terahertz optics, are in development to enhance the data quality, but the presented results already prove the feasibility of this application.
3.5 Aspects of Real-world Application in Production Environment
When using terahertz technology and especially the TDS principle in the automotive industry, one has to adapt to the conditions present there. Often, the testing should be performed close to the place of manufacturing, which often implies the influences of a harsh environment. The usability of robotic systems to bring the sensor to the device under test has been proven in the past [3, 50]. The application of robots and the rough environment lead to the need of insensitivity to vibrations [51, 52]. They can lead to severe disturbance of the acquired terahertz signal—especially when applying the most common TDS principle.
Depending on the product of amplitude and frequency of the vibration, a deviation of the extracted layer thickness is induced. This deviation strongly depends on the layer structure and increases with the number of layers. Using the DMI information, the layer thickness determination accuracy can greatly be enhanced, which results in a vanishing deviation. This technique enables the employment of terahertz TDS technology in a rough industrial environment.
4 Conclusions and Outlook
In this contribution, we have shown that terahertz technology is a very useful tool in the field of nondestructive testing. The presented results of measurements on samples from automotive and aviation industries give a proof that quality inspection with this new upcoming technology is very attractive and no longer only working under laboratory conditions. The step out of the lab into industries depending on the specific application is either in progress or has already been done as shown for the vibration compensation method for multilayer paint systems.
Enhancing the scanning frequency of TDS systems is a demand that is very often discussed especially in terms of industrial use to either realize a rapid 2D scanning or to observe fast single processes. The limitations in classical systems to less than 100 Hz of pulse acquisition rate are mainly determined by the mechanical delay line. It has been shown that a smart timing management of two femtosecond lasers—known as asynchronous optical sampling (ASOPS)—could lead to pulse repetition rates in the kilohertz regime. Further improvements basing on this principle are named electronically controlled optical sampling (ECOPS)  and single-laser polarization-controlled optical sampling (SLAPCOPS) . Both methods have the advantage to flexibly change the scanning range to match the region of interest and not to waste measurement time. Here, SLAPCOPS is potentially more cost-effective as only one laser is needed using its different polarization states in the fiber instead of two single-laser systems. It should be noted that the dynamic range is only depending on the measurement acquisition time so fast-scanning delay lines need the same time to achieve the same dynamic range.
Another demand which has been coming up with the birth of the first terahertz systems is the reduction of the systems costs. As already mentioned, the main cost-driving device of a TDS system is the femtosecond laser system. Hence, there are efforts to get rid of the pulsed femtosecond laser sources by introducing multimode diode lasers  or as most recently demonstrated superluminescent diodes . Up to now, these systems supply a lower performance but in principle similar signals compared with standard TDS that might be sufficient for specific applications with moderate demands.
Future electronic terahertz systems profit from the tremendous development of millimeter-wave and terahertz-integrated circuits, which can provide a significant cost reduction and allow for more compact system designs as well as highly integrated multistatic array solutions such as shown for microwave imaging in . Besides stepped-frequency and FMCW radar approaches [56, 57, 58], also broadband pulsed systems can be considered .
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