Determination of modal properties of the coffee fruit-stem system using high speed digital video and digital image processing

摘要

Detachment of coffee fruit is usually accomplished by means of mechanical impacts and vibrations applied to the plant. Modal properties of the coffee frui t-stem system represent important information for efficient and selective harvesting. This study aimed to determine the modal parameters of the coffee fruit-stem system, such as natural frequenc y and damping coefficient, using high speed digital videos. With image processing tech niques, it was obtained the resulting displacement of the system subjected to an impulse. The modal parameters were determined by the logarithmic decrement method, considering the system as underd amped. The use of high-speed video and digital image processing techniques allowed the simple and reliable determinat ion of modal parameters of the coffee fruit-stem system. Natural frequencies for the coffee fruit-stem system were 11.62 and 13.29 Hz; the damping coefficient was 0.0253 and 0.029 N s m -1 , and the equivalent stiffness was 8.61 and 7.09 N m -1 for red and green ripening stages, respectively. It was found over lap of resonance bands, between the ripening stages red and green, hindering the selective mechanical detachment in the first natural frequency range of the coffee fruit-stem system.