Robotic System Makes Precision Surface Measurements

Researchers at the Technical University of Vienna (TU Wien) have developed a lightweight optical system for 3D inspection of surfaces with micron-scale precision. The measurement tool could greatly enhance quality control inspection for high-tech products, including semiconductor chips, solar panels, and consumer electronics such as flat-panel televisions.

To create a system capable of operating in the vibration-prone environment of an industrial manufacturing plant, the team, headed Georg Schitter, combined a compact 2D fast-steering mirror with a high-precision 1D confocal chromatic sensor. Precision measurements typically must be taken with large equipment in a lab. To bring this capability to the production floor, the team developed a system based on a 1D confocal chromatic distance sensor developed by Micro-Epsilon, a partner in the research project. Confocal chromatic sensors can precisely measure displacement, distance, and thickness using the same principles as confocal microscopes but in a much smaller package.

Vibrations make it difficult to capture precision 3D measurements on the production line, and, as a result, samples are periodically taken for analysis in a lab. However, any defective products made while waiting for results must be discarded.

The new system during a calibration process that involves a CMOS camera. The light spot where measurements are acquired and the fast-steering mirror (FSM) and confocal chromatic sensor (CCS) can be seen. Courtesy of Daniel Wertjanz, Christian Doppler Laboratory for Precision Engineering for Automated In-Line Metrology.

“Robot-based inline inspection and measurement systems such as what we developed can enable 100% quality control in industrial production, replacing current sample-based methods,” said Ernst Csencsics, who co-led the research team with Daniel Wertjanz. “This creates a production process that is more efficient because it saves energy and resources.”

The system is designed to be mounted on a tracking platform placed on a robotic arm for contactless 3D measurements of arbitrary shapes and surfaces. It weighs 300 g and measures 75 × 63 × 55 mm, which the designing researchers said is about the size of an espresso cup.

“Our system can measure 3D surface topographies with unprecedented combination of flexibility, precision, and speed,” said Wertjanz, a Ph.D. student at TU Wien. “This creates less waste because manufacturing problems can be identified in real time and processes can be quickly adapted and optimized.”

The researchers combined the confocal sensor with a highly integrated fast-steering mirror that measured just 32 mm in diameter. They also developed a reconstruction process that uses the measurement data to create a 3D image of the sample’s surface topography. The 3D measurement system is compact enough to fit on a metrology platform, which serves as a connection to a robotic arm and compensates for vibrations between the sample and measurement system through active feedback control.

“By manipulating the optical path of the sensor with the fast-steering mirror, the measurement spot is scanned quickly and precisely across the surface area of interest,” Wertjanz said. “Because only the small mirror needs to be moved, the scan can be performed at high speeds without compromising precision.”

To test the system, the researchers used various calibration standards featuring structures with defined lateral sizes and heights. These experiments demonstrated that the system can acquire measurements with a lateral of 2.5 µm and axial resolution of 76 nm.

“This system could eventually bring a variety of benefits to high-tech manufacturing,” Wertjanz said. “In-line measurements could enable zero-failure production processes, which are especially useful for low-volume fabrication. The information could also be used to optimize the manufacturing process and machine tools settings, which can increase overall throughput.”

The researchers are now working to implement the system on the metrology platform and incorporate it with a robotic arm. This will enable them to test the feasibility of robot-based precision 3D measurements on freeform surfaces in vibration-prone environments such as an industrial production line.

The research was published in Applied Optics (www.doi.org/10.1364/AO.428374).