Vibrations Analysis of the Fruit-Pedicel System of _Coffea arabica var._ Castillo Using Time-Frequency and Wavelets Techniques  
[PERSON]  
1 Experimental and Computational Mechanics Laboratory, Universidad Autonoma de Manizales,  
Antigua Estacion del Ferrocarril, Manizales 170001, Colombia; carlos:[EMAIL_ADDRESS] (C.I.C. ); [EMAIL_ADDRESS] (L.P.-H.); [EMAIL_ADDRESS] (J.L.-G.)  
[PERSON]  
1 Experimental and Computational Mechanics Laboratory, Universidad Autonoma de Manizales,  
Antigua Estacion del Ferrocarril, Manizales 170001, Colombia; carlos:[EMAIL_ADDRESS] (C.I.C. ); [EMAIL_ADDRESS] (L.P.-H.); [EMAIL_ADDRESS] (J.L.-G.)  
[PERSON]  
1 Experimental and Computational Mechanics Laboratory, Universidad Autonoma de Manizales,  
Antigua Estacion del Ferrocarril, Manizales 170001, Colombia; carlos:[EMAIL_ADDRESS] (C.I.C. ); [EMAIL_ADDRESS] (L.P.-H.); [EMAIL_ADDRESS] (J.L.-G.)  
[PERSON]  
1 Experimental and Computational Mechanics Laboratory, Universidad Autonoma de Manizales,  
Antigua Estacion del Ferrocarril, Manizales 170001, Colombia; carlos:[EMAIL_ADDRESS] (C.I.C. ); [EMAIL_ADDRESS] (L.P.-H.); [EMAIL_ADDRESS] (J.L.-G.)  
[PERSON]  
4 Department of Automatics (DAT), Federal University of Lavras (UFLA), P.O. Box 3037,  
Lavras 37200-000, MG, Brazil; [EMAIL_ADDRESS] 4 Correspondence: [EMAIL_ADDRESS]; Tel. : +57-687272724444  
The average labor productivity in the picking process of the Colombian coffee region is 82.6 kg of coffee cherries per workday, and this process accounts for over 75% of the labor requirement [5]. For these reasons, the mechanized harvesting process has gained attention as an imperative strategy to overcome the lack of labor force and to guarantee the Colombian coffee future. Mechanical harvesting has been successfully implemented and used in different crops such as olives [6], grapes [7], strawberries [8], and even coffee [9]. However, at this moment, the introduction of machinery to access the Colombian mountain slopes is seen as unfeasible. Despite some improvement and adaptation of mechanical or semi-mechanical techniques proposed to reach an optimal ripe--unripe coffee fruit ratio [10; 11], these approaches have not been successful implemented. The complexity of selective coffee harvesting tasks also depends on the three characteristics, such as the plant shape, height, length, and width differences [12]. Furthermore, each branch has uneven fruit ripeness [13], where each ripening stage has its own mechanical [14] and geometrical [15; 16] properties. The mechanical harvesting methods generally used in industry are limb shaking, air blasting, canopy shaking, and trunk shaking [17]. These technologies use low-frequency vibrations to harvest different kinds of crops as apples [18], olives [19], citrus [20], and cherry [21], among others. However, some proposals based on portable shakers were studied to accomplish a high ripe-total harvested fruits ratio, but these just reached values between 13 and 70% [11]. Higher detachments in ripe fruits than in unripe fruits were found when a shaker was implemented to excite a coffee branch to estimate the appropriate frequency, amplitude, and vibration time to detach fruits [22]. Shaking the entire coffee tree could conduce to plant death [23], high leaf, and branch detachment [24]. Nevertheless, semi-mechanical harvesting based on vibrations could be a promising approach if the coffee fruits were stimulated at their characteristic modal frequencies that vary at each ripening stage, as studied by [14]. The fruit-peduncle system becomes the main substructure to be analyzed for selective harvesting with the aim to understand the mechanics of the detachment based on vibrations [25; 26]. The fruit-peduncle's vibrational frequencies were experimentally [11; 22; 26] and semi-theoretically [14; 27] screened, producing 20 possible natural frequencies and their associated vibration modes, respectively. Discrete frequency values have been applied for coffee Arabica compared with the dynamic, flexible body model [28].