Abstract
Detachment of coffee fruit is usually accomplished by means of mechanical impacts and vibrations applied to the plant. Modal properties of the coffee fruit-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 frequency and damping coefficient, using high speed digital videos. With image processing techniques, 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 underdamped. The use of high-speed video and digital image processing techniques allowed the simple and reliable determination 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 overlap 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. Keywords: image segmentation, mechanical harvesting, mechanical vibrations. Determination of modal properties of the coffee fruit-stem system using high speed digital video and digital image processing Detachment of coffee fruit is usually accomplished by means of mechanical impacts and vibrations applied to the plant. Modal properties of the coffee fruit-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 frequency and damping coefficient, using high speed digital videos. With image processing techniques, 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 underdamped. The use of high-speed video and digital image processing techniques allowed the simple and reliable determination 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 overlap 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.
Highlights
Brazil is the world’s largest coffee producer with a harvest of 2.7 million tons in 2014
Geometric properties influence the coffee stem stiffness, since it is a function of the moment of inertia of the cross section and its length, and the elasticity modulus of the material (Rao, 2008)
Samples longer with smaller diameters of the stem have lower stiffness and lower values for the natural frequencies. These properties vary according to harvests, variety, ripening stage, location and age of the coffee plant
Summary
Brazil is the world’s largest coffee producer with a harvest of 2.7 million tons in 2014. Minas Gerais has 50.4% of the coffee crops, followed by Espírito Santo, with 28.6%. The two main grown species, Coffea arabica and Coffea canephora, represent 70.8 and 29.2% of total production, respectively (Instituto Brasileiro de Geografia e Estatística [IBGE], 2015). Regarding the coffee production chain, fruit harvest is the most difficult for mechanization. Mechanized harvesting of different fruit is commonly performed by applying impacts and vibrational forces to the plant in order to promote the detachment of fruits (Sessiz & Özcan, 2006; Polat et al, 2007; Pezzi & Caprara, 2009; Torregrosa, Ortí, Martín, Gil, & Ortiz, 2009; Du, Chen, Zhang, Scharf, & Whiting, 2012).
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