by being placed on top of a 25.4 X 25.4 X 3.17 mm magnet. All the magnets are positioned to have their south poles pointed downwards. The base magnet is a 25.4 X 25.4 X 3.17 mm rare earth magnet with its south pole oriented upwards to repel the tip magnets. The strong magnetic force attaches the base magnet to a steel block, used to position the base magnet. A rail mechanism allows positioning of the base magnet and the two electromagnetic coils on its sides. The height of the vertical support connected to the column of the energy harvester can be adjusted. By adjusting the elevation of the beam we can vary the distance between the tip and base magnets and achieve different vibration scenarios. Fig. 3: Connection of the hybrid harvester to the shaker The velocity of the tip magnet is measured using a Polytec OFV 303 laser vibrometer. The base acceleration (the acceleration of the frame of the harvester) is measured by a tear drop accelerometer. The voltage across the resistive load connected to the piezoelectric patches, and the voltage across the electromagnetic load are also measured. Siglab data acquisition interface from Spectral Dynamics is used for data collection. Mono-stable piezoelectric harvester The mono-stable nonlinear vibrations occur when the distance between the base and the tip magnets is larger than a certain threshold. In that working condition the passive magnetic forces reduce the natural frequency of the harvester and also make it nonlinear. The zero deflection equilibrium however remains stable. The following tests examine the vibration characteristics and the power harvesting 467
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