Sensor Systems for Production Measurement Technology

Distance sensors based on the interferometric measuring principle provide extremely precise measurements at high measuring frequencies. The measuring radiation can be guided via optical fi bers or in a free beam, which is why measuring positions that are diffi cult to access can also be reached. While relative measuring laser interferometers use a single wavelength and have a unique measuring range of around one micrometer, absolute measuring interferometers of Fraunhofer ILT evaluate up to 4,000 individual wavelengths and, thus, achieve unique measuring ranges of 20 millimeters.

Fraunhofer ILT‘s absolute measuring interferometric sensors originated from in-house developments in the field of optical coherence tomography (OCT) for medical applications. It mainly focused on developing large measurement ranges with simultaneously very low linearity deviations as well as on continuous data evaluation in real time at measuring frequencies up to 80 kHz. The results of long-term climatic chamber tests prove that the sensors Fraunhofer ILT developed function stably – for example, they are robust against temperature fluctuations ranging from 5°C to 50°C.

In production measurement technology, absolute-measurement interferometric sensors from Fraunhofer ILT have already been validated for 24/7 use, i.e. for measuring single-layer thickness of multilayer plastic films, metal strip thickness in cold rolling mills, and weld penetration depths in laser welding systems.

The opto-mechanical integration of the sensors uses compact, robust measuring heads connected to the interferometric evaluation unit via fiber optic cables up to 50 m in length. The measuring radiation is emitted from the measuring head toward the object surface and from there scattered back in the direction of the measuring head. This bidirectional (bd for short) beam guidance requires very little space for the sensor integration in systems and production lines. In laser processing systems, such sensors are advantageous because their measuring radiation can be coaxially superimposed on the processing laser beam and guided via a focusing optical system already present on the process side. This way, distance measurements can be made directly where the processing occurs.

Geometry features such as step heights, hole depths, weld penetration depths, surface topographies and also spatially resolved layer thickness and 3D tomography images can be generated from 1D distance measurements when the measured object is moved or the measuring beam deflected, e. g., with a scanner mirror. The measurement signal quality is not significantly disturbed by measurements on glowing objects (e. g., glass, metal), by laser processing radiation or plasmas (e. g., in laser welding) or by metal powder (e. g., in Laser Material Deposition).

The Fraunhofer ILT‘s absolute measuring devices can be equipped with independent measuring heads for up to four measuring points. In a time-division multiplexing process, the usable measuring frequency of, for example, 80 kHz (»bd-1«) is divided into two measuring points with 40 kHz each (»bd-2«) or into four measuring points with 20 kHz each (»bd-4«). In addition to the equal splitting of the measuring frequencies, asymmetrical splitting is also possible, such as 90 percent of the measurements at measuring point 1 and 10 percent at measuring point 2. For sensor arrangements requiring several measuring points, this splitting significantly reduces the total costs – by about two thirds.

Available light sources and wavelengths The measuring wavelength most commonly used by Fraunhofer ILT for absolute measurements is 850 nm since the corresponding radiation can be detected with inexpensive CCD or CMOS sensors. In addition to measuring distance to surfaces, this wavelength can also be used for layer thickness and for tomography measurements on transparent objects such as glasses and plastics.

Furthermore, sensors with measuring wavelengths of 1064 nm and 1550 nm have been developed to meet application-specific requirements. For these wavelengths, however, comparatively expensive InGaAs sensors are needed to detect the measuring radiation.

Quantum light sources for the generation of entangled photon pairs are also the subject of research and development at Fraunhofer ILT. Here, photons in the mid-infrared (MIR) are generated to scan objects to be measured, and then transfer their measurement information to entangled photons in the easily detectable visible (VIS) or near-infrared (NIR) wavelength range. This makes the MIR wavelength range from 2 μm to 10 μm accessible to production measurement technology for the first time. Long-wavelength measurement radiation is advantageous, for example, for coating thickness and tomography measurements on objects such as ceramics, colored plastics or foams. These scatter strongly in the VIS and NIR range, but show almost no scattering in the MIR range so that greater optical penetration depths can be achieved.

The developments of Fraunhofer ILT are geared to demanding tasks in production metrology which, in addition to fast, robust opto-mechanical arrangements, include above all reliable and uninterrupted data processing. Using measurement frequencies of 80 kHz and PC-based block-by-block processing, this system has latency times of 20 ms and is primarily used for quality assurance. FPGA-based electronics have been developed for controlling fast machining and manufacturing processes in 24/7 use, enabling measurement data processing at 80 kHz and output of a control signal with a latency of only 150 μs.

In the field of absolute measuring interferometric sensors, Fraunhofer ILT offers its partners studies, projects for prototype development, small series production and the subsequent transfer of manufacturing documents to licensees. New fields of application for the sensors are being developed within the framework of publicly funded R&D projects.