Inspecting Production Parts With a Hologram

Researchers at the Fraunhofer Institute for Physical Measurement Techniques IPM in Freiburg, Germany, have demonstrated the use of digital holography to discover defects in objects as small as a few centimeters. Markus Fratz, Alexander Bertz, and Tobias Beckmann used 3D digital holography by deploying a set of short laser flashes and interferometry to identify defects to a precision of a few microns.

Digital holography is the process of holistically acquiring and processing the magnitude and phase of an incident optical (electric) field with a digital sensor. In classic holography, a coherent light source, typically a laser, is split into two beams; one directly illuminates a recording media (film or CCD) acting as a reference beam, and the other is scattered along its path to a recording device. After the beam illuminates the object, it is collected and joined with its reference wave by a beam splitter to interfere and create a hologram. In other words, the recorded image is numerically reconstructed into a digital object from digitized interferograms enabling a means of measuring optical phase data and thus gathering three-dimensional (3D) surface images. (For more on applications in laser interferometry, see the August 30, 2016 LREP. [client registration required])

Classic holography requires only a single, coherent light source to illuminate the surface of the object measured. The measurements are precise but time-consuming, from a manufacturing standpoint. To make complete measurements, only a few pieces per production line are sampled. Beckmann et. al showed that multi-wavelength holography enables measurement at a pace that allows measurement of every part.

By using narrowband lasers to create a range (in value) of synthetic wavelengths, the measurement spectrum was broadened from (sub)micrometer to millimeters in range. The distance in value between the various synthetic wavelengths and surface properties determined the resolution and reproducibility of the measurements. On optically rough surfaces, the speckle noise produced by the interference of the returning wave at the transducer aperture or, in other words, the laser’s back scatter from hitting the object’s surface, made these measurements impossible. However, by numerically reconstructing the object image at different wavelengths, the speckle noise was rendered insignificant. The result was a phase map at the beat frequency of each individual wavelength, which contained the data for the object’s dimensions.

In the past, defects were identified through visual inspection, but the process was much slower and far less consistent with only a few parts sampled per line. Holography was considered as a solution but was deemed too slow and too sensitive to vibrations. Now, improvements to the holographic measurement process as well as paralleling the computation steps in the evaluation algorithms, elevate its applicability to production environments. Digital holographic 3D technology fills a bottleneck in manufacturing by enabling sub-second, highly precise measurement of small objects. 3-D images are produced in a tenth of a second and resulting images can be brought up a second after. The pace of measurement enables 100 million 3D points per second to be created where errors are detected beyond human capabilities. “We have been able to eliminate all the disadvantages and have therefore, for the first time, developed a system that allows one hundred percent inspection in production,” Tobias said. For their results, the three researchers received the Joseph von Fraunhofer Prize for development of production-ready digital holography.

This system has already proved production-level applicability for diesel injector manufacturer Werner Giessler GmbH. By employing this process, new opportunities opened to the mid-sized business. For example, the company’s regular client, Bosch, had upped its order from the standard 6.5 million pieces to 10 million pieces and a zero defect rate. Using visual inspection, the traditional process, was out of the question for handling an order of that magnitude. Managing Director Thomas Giessler said digital holography was instrumental to the company accepting the assignment, stating, “I’m not enough of a risk-taker to do without this technology. Companies that have not learned to inspect the quality of their parts will soon disappear.” This research is currently being continued in the “HOLOMOTION” project funded by the German Federal Ministry of Education and Research.

Readers considering optimizing production capabilities through digital holography should consider this new technology. Fault tolerance in production environments, such as the automotive sector, are close to reaching a zero defect rate. Until recently, most inspection was done visually by sampling a few parts per line. This system enables detailed mass inspection at the pace of production – a disruptive technology that can plunge production costs and drastically increase fabrication quality. (For more on disruptive manufacturing trends, see the report “Major Developments in Advanced Materials and Manufacturing and Key Trends for 2017.” [client registration required])

By: Tracy Woo