Quality control is of paramount importance to all manufacturers and for those producing critical parts of high-precision systems, the tolerance limits are even more stringent. Just a fraction of a millimetre in error could render a part unusable. Existing offline and in-process 3D measurement solutions are still leaving much to be desired. That is why Prof. Benny Chi-fai Cheung from the Department of Industrial and Systems Engineering worked with his research team to invent a groundbreaking multisensor in-situ 3D measurement system that is portable, economical and precise, while compatible with almost any tooling machines. In future, it can even communicate with cyber-physical systems wirelessly via Bluetooth to instruct a tooling machine to fix unqualified workpieces without human intervention. Welcome to the era of Industry 4.0 smart factory.
Existing offline and in-process measurement
In the past, a finished workpiece had to be removed from the production line for offline measurement. That comes with an obvious problem – it is time-consuming, and sometimes impossible, to reposition a problematic part that needs further tooling or polishing at the exact same spot on the machine as before. “For ultraprecision tooling processes, the slightest displacement of a workpiece may ruin it. Therefore, on-line or in-process non-contact measurement where the workpiece is not moved or touched at all is preferred,” said Prof. Cheung.
In recent years, certain machine tool manufacturers have produced on-line measurement modules to be attached to their own machines. Yet, they may not be compatible with machines of a different make. They also involve complicated algorithms deriving measurements from coordinate information of the motion axes of machine tools. They may also be fairly bulky and may not be easily moved around a factory. “Although the traditional on-line 3D measurement apparatuses work well on simple geometric forms such as spherical and aspherical surfaces, there are great challenges for measuring freeform surfaces. For instance, the shapes of progressive multifocal lenses that correct presbyopia are freeform shapes that are difficult to be measured by traditional on-line measuring tools,” added Prof. Cheung.
Motion sensor + laser scanners
The multisensor 3D measurement system developed by Prof. Cheung is made up of three components: a sensor module, a control box and a computer software. The sensor module combines a motion sensor and laser scanners. It excels in complicated surface scanning without interfacing with the motion control system of the machine tools. “It works like a GPS. The motion sensor detects the locations of the laser scanners to calibrate the scanned result and reconstruct the scanned shape. For this reason, it works independently from the machine tool.”
Better still, the whole sensor module only measures about 15 cm on the longest side so that it fits on almost all machine tools and can be moved from one machine to another easily. As the sensor module is attached right next to the tool bit, any workpiece that a machine is big enough to process can also be measured by the system. “Apart from its large measurement range, the system is also powerful because of its high resolution. For instance, when measuring a workpiece about 100 mm by 50 mm, the measurement uncertainty is only 10 micrometres, that is 1/10 of the average diameter of a human hair. This is precise enough for most applications.”
The next step of development would be to incorporate Bluetooth functions in the sensor module. It can then be hooked up wirelessly with cyber-physical systems in an Industry 4.0 factory and communicate with all other machines via cloud servers. By then, the scanned results can be compared with the CAD design data right away, and unqualified workpieces can be re-tooled automatically. Prof. Cheung concluded, “The system would make an invaluable in-process quality control unit in a fully automated supply chain.”
In April 2016, Portable Multisensor Apparatus for In-situ High Dynamic Range 3D Measurement won a gold medal in the 44th International Exhibition of Inventions of Geneva, Switzerland.
