Droplet Image Feedback Control System In Space Experiment Based On DSP

The heat transfer efficiency of droplet evaporation is much higher than that of air flow; and heat transfer problems generally exist in industries and civil fields, for example, the micro electronics. Therefore, the research on droplet evaporation is of great application value. It is an important method to study liquid evaporation by measuring the parameters of droplet. However, due to the evaporation of droplet, we must continually inject liquid to maintain it during the experiment(around 10 hours); and because of its fluidity, contact methods could not be used to acquire the physical parameters such as volume, surface area, surface tension, contact angle,etc. In view of this, it is a valuable subject to develop a specific system for the evaporation and fluid interface effect experiment in space environment.Droplet image feedback control system based on DSP is designed specially for monitoring the evaporation and fluid interface effect on retrievable scientific experimental satellite. The system has three main functions: to capture and store droplet images during the experiment; to calculate droplet geometrical and physical parameters such as volume, surface area, surface tension, evaporation velocity and contact angle at a high-precision level; to keep the droplet volume unchanged.To realize these functions, the system consists of three parts: image acquisition, image processing and feedback control. First, the system captures images through CCD camera and outputs them to DSP; the images are compressed in DSP and are outputted to the host computer. Then, image preprocessing technology is used to extract the contour of droplet. Based on droplet's axisymmetry, volume and surface area are calculated by analyzing the contour. Surface tension and contact angle are calculated by contour approaching process on the basis of the liquid Laplace's equation. Finally, with PID control, The DSP sends signals to injection controller to inject liquid to keep the volume unchanged; the evaporation velocity of droplet could be calculated by measuring the injected volume during the evaporation.The development of this system will enable us to acquire droplet parameters in a 7 second cycle or even shorter. According to experimental results, the system could operate steadily for more than 12 hours, while the rate of measurement error is within 1% and the relative fluctuation rate of volume is within 5%. The system will not only provide the experimental platform for the evaporation and fluid interface effect experiment in space, but also provide a practical reference for other studies that developing space experimental device for observing experimental processes.