This paper presents a vibration-based electromagnetic energy harvester whose resonance frequency can be adjusted to match that of the excitation. Frequency adjustment is attained by controlling a rotatable arm, with tuning masses, at the tip of a cantilever-type energy harvester, thereby changing the effective mass moment of inertia of the system. The rotatable arm is mounted on a servomotor that is autonomously controlled through a microcontroller and a photo sensor to keep the device at resonance for maximum power generation. A mathematical model is developed to predict the system response for different design parameters and to estimate the generated power. The system is investigated analytically by a distributed-parameter model to study the natural frequency variation and dynamic response. The analytical model is verified experimentally where the frequency is tuned from 8 to 10.25 Hz. A parametric study is performed to study the effect of each parameter on the system behavior.

Total or partial hip replacement procedures have become one of the most frequently performed surgical procedures worldwide. However, a significant and increasing percentage of these patients return for revision surgeries due to stress shielding phenomena and the following implications such as aseptic implant loosening. Various strategies are under investigation to reduce the mechanical properties gap between the implant and the femur tissues. In this study, different development aspects have been merged to improve the design of hip implants and minimise the stress shielding. A low elastic modulus β-Ti TNTZO alloy has been used as the implant material. An actively optimised lattice structure has been embedded into the implant to tune its mechanical properties according to the bone properties of the patient. The study showed that using TNTZO reduced the average stress shielding of the implant by 31 % compared to commercial CoCr implants. Moreover, with two-step TNTZO lattice optimisation, the stress shielding was fully eliminated in all the femur sections, which accelerates the healing process of the bone and extends its long-term life. The study presented a scope for further optimisation to address other objectives, such as implant stiffness and eliminating stress concentration zones.

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.