Vibration energy harvesting is an attractive technique for potential powering of wireless sensors and other low power
micro devices. In order for the device to have maximum power output, it is necessary to match electrical and mechanical
damping. In this work a coupled piezoelectric and electromagnetic energy harvesting device is evaluated for its
efficiency and compared with optimized standalone piezoelectric and electromagnetic techniques. A piezoelectric
cantilever beam with a cylindrical magnet as its tip mass and a resonance frequency of 19 Hz is used, with a coil winding
vertically aligned with the magnet such that the magnetic tip would pass through the coil. The total power output from
the coupled energy harvesting technique is monitored which produced a power output of ~340 μW compared to 301 μW
from an optimized standalone piezoelectric energy harvesting and 120 μW from an standalone electromagnetic energy
harvesting device. The total damping in the system is determined to be 0.054 compared to 0.046 and 0.04 for
piezoelectric and electromagnetic techniques.
Wireless sensors are becoming extremely popular for their ability to be employed in hostile and inaccessible locations to
monitor various parameters of importance, and vibration energy harvesting shows great potential in powering these
sensor networks. For efficient operation the device should operate in resonance at the environmental excitation
frequency and hence requires a frequency tuning mechanism. Recently efforts have been attempted to broaden the
frequency range of energy harvesting devices, but in terms of power density an efficient design methodology is lacking.
In this work, a tunable energy harvesting device with high efficiency and power density is presented. The technique
involves two single DOF's cantilever beams which are coupled in a novel fashion by means of magnetic force for
resonance frequency tuning. Here the magnetic force acts as a variable stiffness coupling the two cantilever beams,
allowing one to alter the corresponding resonance frequencies of the cantilever beams. Magnetic force of attraction and
repulsion can be used to achieve the magnetic coupling and can increase the overall stiffness of either of the cantilever
beams while decreasing the others. The total power output of the device is found to be between 180 μW to 320 μW.