Tianwei Ma

Enhancing mechanical energy harvesting with dynamics escaped from potential well

The potential of utilizing the dynamics outside the potential well of a device for harvesting energy from vibrations is investigated. A pendulum-type system subjected to parametrical excitation is used to demonstrate the concept. When the dynamics of the device is outside the potential well and stays in stable orbits of period-one rotations, the harvested energy is proportional to the energy level of the orbit. It is neither dependent on the natural frequency of the device nor on the intensity of the excitation. For low-level vibrations, the effectiveness of the proposed method is limited in relatively low frequencies.

Panel Flutter Detection and Control Using the Eigenvector Orientation Method and Piezoelectric Layers

A basic eigenvector orientation approach has been used to evaluate the possibility of controlling the onset of panel flutter using a simple flat panel (wide beam) as an illustrative example. The use of the eigenvector orientation method for prediction of the flutter boundary (indicated by a gradual loss of orthogonality between two eigenvectors) was developed in a previous study and can thus provide a lead time for possible flutter control. As a first step, piezoelectric layers are assumed to be bonded to the top and bottom surfaces of the panel in order to provide counterbending moments at joints between elements. The standard linear quadratic control theory is used for controller design and full state feedback is considered for simplicity. The controllers are designed to restabilize the system at the onset of flutter; as a result, flutter occurrence can be offset to a higher flutter speed. To illustrate the applicability and effectiveness of the developed method, several simple wide-beam examples are studied and presented. The effects of control moment locations are studied so as to fulfill the objective of adjusting the flutter speed to be within a desirable range. Potential applications of this basic method may be straightforwardly applied to plate and shell structures of laminated composites using the versatile finite element method.

Improved Decentralized Method for Control of Building Structures under Seismic Excitation

A decentralized control method with improved robustness and design flexibility is proposed for reducing vibrations of seismically excited building structures. In a previous study, a control scheme was developed for multistory building models using nonlinear, decentralized control theory. This control method has now been improved in this study in that less information about material properties and geometrical parameters of the building is needed and the selection of control design parameters is more flexible. The nonlinear behavior of the proposed control system is studied and its stability property is proven mathematically. To evaluate the effectiveness and robustness of the proposed method, three illustrative structural models, i.e., an eight-story elastic shear beam model, a two-story nonlinear elastic shear beam model, and a 20-story elastic benchmark model are studied. The 1940 El Centro and the 1995 Kobe earthquakes are used in these examples. The performance of the current control design, as applied to these examples, has shown to be more effective in reducing structural responses and improving robustness.

A Nonlinear Method for Harvesting Mechanical Energy from Vibrations

A novel pendulum-generator system taking advantage of parametric resonance is proposed as an alternative way of energy harvesting in civil engineering application. A mass or length augmentation coefficient is introduced to consider the inertia effect of the generator. Theoretical analysis of the nonlinear behavior of the device is performed. It is found that there exists an optimal as well as an upper bound of equivalent viscous damping of the electrical load. The average electro-mechanical output power characteristics in terms of excitation frequency and total equivalent damping are developed for the case of parametric excitations. Two typical working modes, i.e. fixed amplitude of either displacement or acceleration of the external excitation source, are considered. Numerical results show that the output power reaches the maximal value when the equivalent damping is between 70.7∼80.9% of the upper bound of the equivalent damping. It is also shown that the proposed device may collect more energy from the source than its linear counterparts at lower frequencies.

Roles of the Excitation in Harvesting Energy from Vibrations

The study investigated the role of excitation in energy harvesting applications. While the energy ultimately comes from the excitation, it was shown that the excitation may not always behave as a source. When the device characteristics do not perfectly match the excitation, the excitation alternately behaves as a source and a sink. The extent to which the excitation behaves as a sink determines the energy harvesting efficiency. Such contradictory roles were shown to be dictated by a generalized phase defined as the instantaneous phase angle between the velocity of the device and the excitation. An inductive prototype device with a diamagnetically levitated seismic mass was proposed to take advantage of the well established phase changing mechanism of vibro-impact to achieve a broader device bandwidth. Results suggest that the vibro-impact can generate an instantaneous, significant phase shift in response velocity that switches the role of the excitation. If introduced properly outside the resonance zone it could dramatically increase the energy harvesting efficiency.

A MEMS vibration sensor based on Mach Zehnder interferometers

Two micro-optomechanical accelerometers based on Multi-Mode Interference (MMI) couplers were designed and evaluated in this study. The optical components were optimized with the Parameter Scan Method. According to the photoelastic effect, the change in refractive index of a waveguide made of crystal materials is related to the mechanical strains in the waveguide. In this study, such change was calculated using the mechanical strains obtained from the Finite Element Analysis (FEA) results. Beam Propagation Method (BPM) was used to study the relationship between the input acceleration and the output optical power and thus the performance of the proposed accelerometers. The results show the two designs are suitable for different acceleration ranges.

Developing hybrid structural health monitoring via integrated global sensing and local infrared imaging

Sensing technology and sensor development have received increased attention in the recent years, and a number of types of sensors have been developed for various applications for materials and structures. In this paper, we will discuss the concept of combining sensing of global vibration and local infrared imaging techniques. The global vibration-based techniques determine the health condition of structures by the changes in their dynamic properties or responses to external disturbs or excitations. Infrared Imaging is introduced here to detect local defects or problems so that to provide more direct and accurate assessment about the severity and extent of the damage. The progress on developing a hybrid structural health monitoring system is presented through the results on both the global sensing algorithm study and local infrared imaging investigation on a steel C channel.

Harvesting energy from low-frequency excitations through alternate contacts between water and two dielectric materials

Recent studies have demonstrated the benefits of water-dielectric interfaces in electrostatic energy harvesting. Most efforts have been focused on extracting the kinetic energy from the motions of water drops on hydrophobic surfaces, and thus, the resulting schemes inherently prefer cases where the water drops move at a high speed, or vibrate at a high frequency. Here we report a method for directly harvesting ambient mechanical energy as electric potential energy through water droplets by making alternate contacts with CYTOP and PTFE thin films. Because CYTOP and PTFE acquire significantly different surface charge densities during contact with water, such a difference can be utilized to effectively generate electricity. We demonstrate this concept using prototype devices fabricated on silicon substrates with a simple procedure. In the experiments conducted, a water drop of 400 μL alone could generate a peak open-circuit voltage of 42 V under a 0.25 Hz vibration. Under a 2.5 Hz vibration, the peak open-circuit voltage reached 115 V under an external bias of 8 V. The demonstrated efficiency is orders of magnitude higher than those of existing devices of similar dimensions.

Recovering bridge deflections from collocated acceleration and strain measurements

In this research, an internal model based method is proposed to estimate the displacement profile of a bridge subjected to a moving traffic load using a combination of acceleration and strain measurements. The structural response is assumed to be within the linear range. The deflection profile is assumed to be dominated by the fundamental mode of the bridge, therefore only requiring knowledge of the first mode. This still holds true under a multiple vehicle loading situation as the high mode shapes don’t impact the over all response of the structure. Using the structural modal parameters and partial knowledge of the moving vehicle load, the internal models of the structure and the moving load can be respectively established, which can be used to form an autonomous state-space representation of the system. The structural displacements, velocities, and accelerations are the states of such a system, and it is fully observable when the measured output contains structural accelerations and strains. Reliable estimates of structural displacements are obtained using the standard Kalman filtering technique. The effectiveness and robustness of the proposed method has been demonstrated and evaluated via numerical simulation of a simply supported single span concrete bridge subjected to a moving traffic load.

Structural damage detection and assessment using acceleration feedback

This paper presents a method for structural health monitoring using acceleration measurements. In a previous study a method for detecting, locating, and quantifying structural damages has been developed by directly using the time domain structural vibration measurements. However, only displacement and velocity measurements were used in that study. In this paper, acceleration measurements are used as feedback. Because it is more practical to measure acceleration using accelerometers, it is preferable to use acceleration rather than displacement and velocity measurements for the purpose of structural damage detection and assessment. However, using acceleration measurements is more difficult since the effects of different damages can not be decoupled completely as in the cases of displacement and velocity measurements. One approach of circumventing this difficulty is presented and it involves increasing the order of time derivatives of the linear system. The effectiveness of the proposed method using acceleration feedback is evaluated with illustrative examples of a three and an eight-story model. Results obtained are found to be comparable with results from simulations using displacement measurements as feedback.