Vibration Control in Meta-Structures Using Reinforcement Learning

This chapter considers using reinforcement learning (RL) to adaptively tune frequency response functions of meta-structures. RL algorithm tunes the stiffness of the spring of the lumped multi-DOF system, as the lumped mass is varied. As some of the lumped masses are modified by 10%, the spring’s stiffness is tuned to maintain the original bandgap. A Q-Learning algorithm is used for RL, wherein the Q-value is updated based on Bellman’s equation. The results compare the frequency response functions of the terminal masses of the baseline and varied mass structure

Investigation of Using Log-Spectrum Averaging (Cepstral Averaging) for Blind Reconstruction of an Unknown Impact Input Force

Consider the case of a mechanical structure being impacted at an arbitrary location by an unknown loading profile (i.e., force–time curve). Then, estimating the unknown impact loading profile (ILP) based on response vibrations is a challenging problem. If the impact location is also unknown, traditional inverse problem approaches (i.e., deconvolution) cannot reconstruct the ILP. This problem is particularly complex when inferring someone’s footstep loading profile by monitoring floor vibrations. Therefore, this preliminary study attempts to overcome the missing input location issue by producing a blind estimate (without knowledge of excitation location point) of the unknown ILP. Producing a blind ILP estimate is appealing since there is no need to know the location of the input force. Additionally, knowledge of the ILP can potentially uncover important information about the excitation source, such as, for example, identifying individuals from their footfall-induced floor vibration. The blind input reconstruction is done using log-spectrum averaging of the structural response at several locations. Our approach investigation is done via a MATLAB simulation, utilizing a Timoshenko finite element (FE) beam model as the virtual mechanical structure. Simulation results encourage further refinement of the approach

Self-sensing active artificial hair cells inspired by the cochlear amplifier, Part II: Experimental validation

Mimicking the nonlinear compressive behavior of the mammalian cochlear amplifier that results in the compression of high-intensity sounds and amplification of faint stimuli can lead to transformative improvements in the dynamic range, sharpness of the response, and threshold of sound detection in cochlear implants to aid individuals with hearing loss. Furthermore, it can enhance the dynamic properties of sensors. This research on developing self-sensing artificial hair cells (AHCs) validates the phenomenological control algorithm established in Part I of the paper to achieve a cochlea-like response from the quadmorph AHCs. As the beam is excited, the voltage of the piezoelectric layers is measured and used to generate a control voltage. Consequently, the controller applies cubic damping to the AHC, while reducing linear damping near its first natural frequency to replicate the biological cochlea’s function. Experimental results validate the model built in Part I of the paper and the work is extended to implement a multi-channel AHC. The system works independent of external sensors and offers significant advantages over previous generations of AHCs such as the ability to embed AHCs in a limited space and to combine several AHCs in an array without the need for external feedback measurement devices

Miniature Underwater Robot – An Experimental Case Study

One of the easiest to observe conditions where waves occur in nature is the undulatory motion of aquatic animals and micro-organisms. In these bio-mechanisms, there is oscillatory locomotion which results in propulsion as the motion is accompanied by energy transfer from one end of the specimen or structure to the other end. Recent years have also seen a rise in the replication of the propulsive capabilities of these animals into aquatic robots. The use of smart materials to actuate and mimic the fin and tail characteristics of a fish has been attempted in various ways. Miniature robots actuated by piezoelectric materials have been effectively used for propulsion due to their simplicity and innovative actuating mechanism. These miniature robots find their application in the regime of underwater propulsion because of their size, flexibility, and ability to mimic fish locomotion. In most of these studies, the undulatory motion of these aquatic robots is achieved by discretizing the fin of the robot into multiple segments and synchronizing the oscillatory motion of individual segments to replicate continuous traveling waves. As a part of such endeavors, the present work attempts to use smart materials to actuate and mimic the fin motion characteristics of a fish. The bio-inspired design of the miniature robots consists of two brass shims supported by four piezoelectric bimorphs. The undulatory motion displayed by aquatic animals is mimicked by generating anechoic traveling waves in these brass fins. Anechoic traveling waves propagate in a structure without undergoing reflections at the structural boundaries. Such waves are generated by taking advantage of the structural dynamics of the fin under multi-input excitation

Developing coexisting traveling and standing waves in Euler-Bernoulli beams using a single-point excitation and a spring-damper system

In the inner ear, the Basilar Membrane (BM) and inner hair cells work to transduce acoustic waves into electrical signals; of these, the BM is the structural membrane that carries the acoustic information as traveling structural waves. These waves propagate from the base of the cochlea towards its apex. The helicotrema is the main component of the cochlea\u27s apex, which prevents the waves from traveling back from the apex to the base. Due to this unique characteristic of the BM, humans are able to hear continuous acoustic signals without any reflection and overlap of acoustics. The work herein is inspired by this biological behavior and seeks to understand and replicate such behavior in a passive manner in basic structures. This feature of the inner ear leads us to study the dynamics of a uniform beam connected to a spring-damper system as a passive absorber, in order to understand some of the observed phenomenological behaviors of the basilar membrane. The location of the spring-damper system divides the beam into two dynamic regions: one which exhibits non-reflecting traveling waves and the other with standing waves. In this paper, traveling waves co-existing with standing waves in the two distinct regions of the structure and are realized analytically, numerically, and experimentally. The results of this work can be used in various applications such as vibration energy harvesting, particle transportation, vibration suppression and control, and in exploring possible explanations of the functionality of the helicotrema in the cochlea

A study on steady-state traveling waves in one-dimensional non-dispersive finite media

The focus of the present work is to study the generation of steady-state traveling waves in one-dimensional non-dispersive finite media. Steady-state traveling waves are propagating mechanical waves that move from one end of a structure to the other without appearing to undergo reflections at the boundaries. In this work, non-reflective flexural and longitudinal traveling waves are experimentally generated in strings and bars and used to validate theoretical models. A two-force excitation approach is used to investigate the generation and propagation of such traveling waves at different frequencies. The effect of parameters such as frequency and phase difference on the quality of the waves is studied based on a cost-function approach. Additionally, the feasibility of generating square-traveling waves as a Fourier combination of multiple traveling waves at different frequencies is introduced in this work and evaluated against first-of-its-kind experiments