Online droplet monitoring in inkjet 3D printing using catadioptric stereo system

Inkjet 3D printing is becoming one of the most disruptive additive manufacturing technologies, due to its unique capability of precisely depositing micro-droplets of multi-functional materials. It has found widespread industrial applications in aerospace, energy and health areas by processing multi-functional metal-materials, nano-materials, and bio-materials. However, the current inkjet 3D printing system still suffers from a low production quality issue, due to low process reliability caused by the complex and dynamic droplet dispensing behavior. Due to the challenges in terms of efficiency, accuracy, and versatility, robust droplet monitoring and process inspection tools are still largely unavailable. To this end, a novel catadioptric stereo system is proposed for online droplet monitoring in an inkjet 3D printing process. In this system, a regular industrial CCD camera is coupled with a flat mirror and magnification lens system to capture the tiny droplet images to detect the droplet location in 3D space. A mathematical model is formulated to calculate the droplet location in 3D world space from 2D image space. A holistic hardware and software framework is constructed to evaluate the performance of the proposed system in terms of resolution, accuracy, efficiency, and versatility, both theoretically and experimentally. The results show that the proposed catadioptric stereo system can achieve single micron resolution and accuracy, which is one-order-of-magnitude higher than the 3D printing system itself. The proposed droplet location detection algorithm has low time complexity, and the detection efficiency can meet the online monitoring requirement. Multi-facet features including the droplet location and speed can be effectively detected by the presented technique. The proposed catadioptric stereo system is a promising online droplet monitoring tool and has tremendous potential to enable trustworthy quality assurance in inkjet 3D printing.