将环境中广泛存在的振动能量高效地收集起来，转化为可用的电能具有重要的实际意义，但也一直因为一些频率控制的难题限制了其实际的应用。其中一个主要的原因是环境中的振动通常是低频宽带的低振幅信号，但能量收集器往往在共振时才有较高的输出功率。因此，常规的线性能量收集器工作频带较窄，难以在小体积下实现超低的固有频率，也很难收集低幅值的振动能量。为此，本文针对能量收集器的被动频率控制问题，提出了两种调频方法和一种变频方法，分别实现了拓宽收集器的工作频段、提高对超低频率超低幅值的振动响应和将低频宽带信号自动转变为恒定的输出频率。针对拓宽收集器频带的难题，多模态结构是一种有效的解决途径。为克服传统多模态结构的离散带宽的缺陷，本文提出了多模态结构连续带宽的设计方法，巧妙地利用了双振子之间的相位差来均衡各阶模态的幅值。数值模拟和实验结果表明，用该方法设计的多模态电磁能量收集器在谐波激励和随机激励下均表现出宽带特性。针对超低频超低幅值激励下收集器响应弱的难题，本文设计了直接型准零刚度装置，并实现了一种折梁型结构的能量收集器，该装置具有易于加工制造、无需组装和容易小型化的优点。理论预测与实验结果表明，该装置不但在超低频、超低幅值激励时表现出良好的能量收集性能，而且还可作为小型的低频隔振器。为综合解决宽带和低频的难题，必须打破系统响应对激励频率和结构尺寸的双重依赖。为此，本文提出了一种基于混沌超结构的自动变频方法，首先将任意频率的有序输入转变为很宽频带的混沌运动，再转变为恒定频率的有序输出，从而实现了自动变频的功能。该方法中超结构的作用在于利用非线性基元耦合的特性来提供一种易于产生无序运动的机制，从而在超大参数空间内实现了复杂的混沌运动。采用功率谱熵来定量地评价混沌运动的无序程度，并从理论上给出了超结构产生混沌振动的近似解析判据，明确了进入混沌的参数空间。基于混沌超结构设计了自动变频装置，其中包含了I型功能基元，可在单稳态、双稳态和三稳态之间切换。理论、数值模拟和实验结果都证实该装置可以将较宽频段的输入转换成具有恒定频率的输出。基于该变频方法制作的能量收集器，在激励的全频段都能产生明显的输出电压，从而综合实现了宽带和低频的能量收集。
Vibration energy is widely present in the environment. It is of great practical significance to efficiently harvest this energy and convert it into usable electrical energy, but its practical application has been limited by some frequency control problems. One of the main reasons is that the environmental vibrations are usually low-amplitude signal with low frequency and broad bandwidth, but energy harvesters tend to have higher output power at the resonance. Conventional linear energy harvesters can work well in a narrow frequency band, and they are difficult to achieve ultra-low natural frequencies in a small volume and harvest low-amplitude vibration energy. Therefore, aiming at the passive frequency control problem of energy harvester, this paper proposes two frequency modulation methods and one frequency conversion method, which broaden the frequency band of the energy harvester, improve the vibration response to ultra-low frequency and ultra-low signals, and converting excitation frequies to a constant output frequency, respectively.For the problem of broading frequency band of havester, multimodal structure is an effective solution. In order to overcome the shortcomings of discrete bandwidth of traditional multimodal structures, this paper inovatively uses the phase difference between the two oscillators to equalize the amplitude of each order of modes, and proposes a design method for continuous bandwidth of multimodal structures. Numerical simulation and experimental results show that the multimodal electromagnetic energy harvester designed by this method exhibits broadband characteristics under both harmonic and random excitations.For the problem of weak response of energy harvester under ultra-low frequency and ultra-low amplitude excitation, this paper proposes a design method of direct quasi-zero stiffness harvester, and realizes an energy harvester with folded beam structure, which has the advantages of simple geometry, no assembly and easy miniaturization. Theoretical prediction and experimental results show that the device not only exhibits good energy harvesting performance when excitated at the ultra-low frequency and ultra-low amplitude, but also can be used as a miniaturized low-frequency vibration isolator.In order to comprehensively solve the problems of broadband and low frequency, the dual dependence of the system response on excitation frequency and structure size must be broken. For this reason, this paper proposes a frequency self-conversion method based on chaotic metamstructures. Firstly, the ordered input of any frequency is converted into a chaotic motion with a wide frequency band, and then it is converted into an ordered output with a constant frequency. The role of metamstructures in this method is to use the characteristics of nonlinear unit coupling to provide a mechanism that is easy to generate disordered motion, thereby realizing complex chaotic motion in a large parameter space. The power spectrum entropy is used to quantitatively evaluate the degree of disorder of chaotic motion, and the approximate analytical criterion is given for predicting the excitation parameter space of metamstructures to enter chaos. A frequency self-conversion device is designed based on chaotic metamstructures, which contains I-type functional units, which can switch among monostable, bistable and tristable states. Theoretical, numerical, and experimental results all confirm that the device can convert a wide-band input into an output with a constant frequency. The energy harvester based on this frequency conversion method can generate obvious output voltage in the whole frequency range of the excitation, thus it comprehensively realizes the energy harvesting of broadband and low frequency.
