Inertial-Amplified Mechanical Resonators for the Mitigation of Ultralow-Frequency Vibrations

Vibration energy or acoustic waves are common phenomena in most mechanical systems. Several strategies have been employed to mitigate vibration in these systems. Among them, vibration absorbers are widely used to reduce or eliminate vibrations in harmonically excited systems. While conventional absorbers generally work well at higher frequencies, they exhibit poor performance at lower frequency ranges due to the presence of large wavelengths. Despite the limitations of the narrow frequency bandwidth, the subwavelength characteristic of resonance-based metamaterials can result in a compact absorber design. However, ultralow frequency absorption still faces constraints and challenges for noise requiring huge attenuation ratios. Addressing such inevitable problems requires effective strategies.

Dampers are widely used structures to suppress general mechanical vibrations and seismic waves. They consist of damped spring-mass-resonators comprising an embedded energy dissipation mechanism. However, since these dissipative components are usually heavy and bulky, a large mass is needed to make the device responsive at low frequencies. This explains the reason why most dampers with operating frequencies less than 10Hz are usually large structures. Nevertheless, more effective design approaches for vibrational energy-absorption devices are still necessary to improve their functionality in practical applications, especially in those instances where only limited space is available.

Herein, Dr. Zhen Dong and Professor Ping Sheng from Hong Kong University of Science and Technology developed a new design for vibrational energy-absorption device based on the inertial amplification concept. This approach utilized the coupling between rotational and translational motions. It also adopted a rather simple mechanical interconnection involving ball-screw structures. A series of experiments were conducted to verify the ultralow frequency responses of the prototype compared to those of analytical models. Their research work is currently published in the peer-reviewed journal, Physical Review Applied.

The research team showed that lightweight and compact resonator devices, with weights below 1kg and dimensions of 10 – 20 cm, can also achieve ultralow-frequency resonance responses at a seismic frequency range from 0.5 to 5 Hz. This was mainly attributed to the effects of the resulting inertial amplification. The fabricated functional prototypes achieved very large inertial-amplification factors (IAF), as high as 137. The meaning of IAF is that the actual resonance frequency corresponds with a resonator with a mass that is IAF-times larger. Moreover, the experimental results agreed well with the analytical modeling of the inertial-amplification mechanism, suggesting the feasibility of the proposed design approach.

Dr. Dong and Professor Sheng proposed a structurally refined device configuration, and they demonstrated its capability to achieve total vibrational absorption through simulations. Upon excitation, it achieved total vibrational energy absorption with a comparable broadband working range in the ultralow frequency range. Such kind of designs proved advantageous for mitigating ultralow frequency vibrations in situations with limited space. Furthermore, it was noted that diverse practical needs could be met by tuning the operational frequency range of these devices. For example, the proposed design could be adapted easily for resonance frequencies up to 100 Hz by miniaturizing its dimensions and reducing the inertial-amplification factor. It is envisioned that strong, low vibrations can be effectively mitigated by attaching multiple number of such miniature resonant-dampers.

In summary, the study revealed the salient features of inertial-amplified mechanical resonators in terms of ultralow frequency response, compactness and comparative broadband total-vibrational-absorption. Generally, resonators capable of responding to ultra-frequency range are usually bulky structures. The inertial-amplified resonator is an unconventional solution to the inherent problem associated with ultralow frequency vibration mitigation, as compact and lightweight devices can fit in limited spaces. In a statement to Advances in Engineering, Professor Ping Sheng explained that the compact design has adequate flexibility and options for different practical applications. Moreover, it also offers energy harvesting possibilities when coupled with piezoelectric elements.