A real-time micro-PIV system using frame-straddling high-speed vision

In this study, we introduce a novel concept of real-time microscopic particle image velocimetry (micro-PIV) for high-speed microchannel flows in a lab-on-a-chip using a frame-straddling high-speed vision system with two camera inputs; it can synchronize two camera inputs with the same view field with a time delay on a submicrosecond time scale. To improve the measurable range of velocity in microchannel flow observation, we propose a variable-frame-straddling optical flow (VFS-OF) algorithm that can simultaneously estimate the microchannel flow distribution as gradient-based optical flows using frame-straddled images from the two camera inputs; their frame-straddling time is determined by the amplitude of the estimated optical flow to avoid large image displacements between frames that often generate serious errors in optical flow estimation. We built a real-time micro-PIV system by software-implementing the VFS-OF algorithm in a high-speed vision system with two frame-straddled cameras; it can execute real-time video processing and recording of 512×512-pixel images at 2000 frames per second for the two cameras and control their frame-straddling time from 0 to 0.5 ms with 9.9-ns steps. Our micro-PIV system can estimate the velocity distribution of high-speed microchannel flows at 1 m/s or more in real time by controlling the frame-straddling time. Experimental results were performed for microfluidic flows on microchannels with widths of hundreds of micrometers to verify the performance of our micro-PIV system based on the VFS-OF algorithm.