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Recent studies involving selective coffee harvesting by shaking, indicates the necessity to apply to fruit -- stem system significantly high excitement frequencies in comparison with those that are habitually applied at the international practice. To reach the predicted frequency levels, in the order of 150-200 Hz, by mechanical, hydraulic or pneumatic means, is very difficult due to the presence of friction couples in these types of mechanisms that would suffer premature waste by the relative high speeds. It's for this reason that the development of electromagnetic vibrators in which the inertial mass, when moving inside a magnetic field, avoid the direct friction among kinematical couples, is a current task. In this paper, the mathematical modeling of a vibratory system of two degrees of freedom is made, using the method of the mechanical impedance, in order to obtain the frequency response of the system, and to determine the design parameters of a shaking device able to achieve certain levels of amplitude of the vibrations at the high required frequencies. From the study also emanates the conclusion that in such shaking device, the natural frequencies of the vibratory system should be contained in the range of the exciting force frequencies. [ABSTRACT FROM AUTHOR]