On adjusting the rotary inertia of a cantilever-type energy harvester for wideband operation

Abstract

Inertial vibration energy harvesters are effective only when they operate at their natural frequencies. A slight deviation from operation at resonance conditions leads to a substantial reduction in the generated power, which places hurdles for the development of devices that operate across a wide frequency range. To address this issue, we present a vibration energy harvester with a tunable natural frequency. Frequency adjustment in a cantilever-type energy harvester is achieved by varying the rotary inertia of the beam through changing the angular position of a rotatable mass mounted at its tip. The mass is driven by a beam-mounted servomotor which controls its angular position about an axis normal to the beam’s mid-plane, thereby varying the effective mass moment of inertia. In this arrangement, the power required for frequency adjustment is kept to a minimum, since no mechanical straining forces are applied on the beam, and power is only needed when the motor turns to adjust the orientation of the tuning mass. A frequency tuning algorithm is developed to keep the beam operating at resonance. The algorithm relies on information gathered by an infrared sensor/receiver module that detects the prevailing frequency of vibration by a microprocessor and drives the servomotor in accordance with a lookup table that relates natural frequency with angular position of the motor. An analytical model is developed to predict the system dynamics and the variation in natural frequency in terms of the various design parameters. While the proposed design is applicable to various types of vibration energy harvesters, an electromagnetic prototype is presented to verify the concept. Findings of the theoretical model are verified experimentally.