The Frequency-up Conversion Effect And Energy Recovery And Vibration Suppression Of Friction-induced Vibration

With the rapid development and large-scale application of low power consumption microelectronics,the power supply solutions to these components have become a prominent problem.Traditional chemical batteries are limited in lifespan;it is highly costly to regularly replace the expired batteries,which,on the other hand,is also very unfriendly to the environment.Therefore,developing a self-power technology independent of external power sources may be a preferred alternative.Currently,harvesting ambient vibration energy has become a promising research due to its advantages of widespread distribution and easy extraction.The main mechanism of vibration energy conversion is governed by the laws of forced vibration.A relative displacement between the oscillator and the ambient vibration source is generated when the oscillator is excited by the ambient vibration,thereby the vibration energy can be converted into electric form through piezoelectricity or electromagnetic induction.According to the fundamental principles of mechanics we know that only when the natural frequency of the energy harvester matches the ambient vibration frequency can the energy harvester yield a considerable amount of energy,otherwise,the performance of the energy harvester will drastically reduce.Nevertheless,most ambient vibrations are characterized by large amplitude and low frequency,which are stochastic and time-varying and distributed in a wide spectrum range.These properties of ambient vibration inevitably result in obstacles to linear energy harvesting in practical applications.It has been theoretically demonstrated that the power yield of the vibration energy oscillator is positively correlated with the vibration frequency.Consequently,Frequency upconversion(FUC)mechanisms which can bridge the gap between high-frequency resonance and low-frequency excitation have been proposed to allow energy harvesters to operate efficiently at low frequencies and broaden their bandwidth.In essence,most of the reported FUC techniques are realized by the impact between the energy harvester and the flexible stoppers that resonant at the ambient frequency,in which frequency-matching strategy is indispensable.Another common and unique vibration source exists in our surroundings,which is a kind of self-excited vibration induced by the dry friction at the sliding contact between two solid surfaces,namely the friction-induced vibration(FIV).FIV arising from modecoupling or lock-in instability scenarios can lead to high-frequency and high-intensity vibrations,generating sustaining and stable acoustic emissions during the sliding at the contact interfaces.And the energy comes from the friction system itself,no external excitation is needed to maintain its energy emission.Therefore,FIV is a potential vibration source that can overcome frequency mismatching.Although FIV can be exploited to improve the performance of linear piezoelectric energy harvester in low-frequency vibration scenarios.It is,however,a detrimental phenomenon in many real applications which needs to be eliminated,especially that occurs in the brake systems of vehicles.FIV can destabilize mechanical systems and affect the operational safety of vehicles,emitting annoying groan or squeal,which destroy the ride comfort of the passengers.Due to the low energy conversion efficiency of piezoelectric energy harvesting from FIV,it can’t substantially damp the FIV only depending on piezoelectric energy conversion.Thus,we introduced a new composite structure of elastic damping components and piezoelectric elements,in which the piezoelectric elements are sandwiched by two damping layers.The damping components are used to dissipate the unstable FIV and stabilize the friction system,meanwhile,part of the vibration energy is converted into electric energy by the piezoelectric elements.Both vibration reduction and energy harvesting can be achieved by this approach.The main conclusions are summarized as follows:1.A new and novel energy transfer concept from ultra-low frequency vibrations to the secondary high-frequency FIV field using contact friction nonlinearities was proposed for effective harvesting of vibration energy at ultra-low ambient frequencies.Using this approach,the vibration frequency was increased hundreds of times,and the excitation level was enhanced several orders of magnitude;the proposed mechanism is referred to as the frequencyexcitation-up-conversion mechanism.Compared to the conventional impact-driven FUC strategy used in energy harvesting from ultra-low frequencies,the impedance of the piezoelectric generator is lowered dozens of times,and the amplitude of the open-circuit voltage is also increased in the new approach.Thus,the harvested power was increased several times even though a vulnerable excitation acceleration was adopted.2.The experimental results indicate that the frequency of FIV is nearly unaffected by the low-frequency external excitation,and the piezoelectric power generated from the secondary FIV filed is maintained within the lowest primary excitation frequency range,indicating that the operating bandwidth of the energy harvester can be broadened to ultra-low frequencies.The numerical results show that the frictional contact will result in multiple mode coalescences of the friction system,which complicates the frequency spectrum of the FIV and the piezoelectric voltage signal.The friction coefficient is a crucial factor in FIV energy harvesting;a larger friction coefficient results in more vibration energy,which is beneficial for piezoelectric energy harvesting via FIV.3.A new composite structure of elastic damping components and piezoelectric elements is proposed,in which the piezoelectric elements are sandwiched by two damping layers.The experimental results and numerical simulations indicate that both vibration reduction and energy harvesting can be simultaneously realized.Correlation analysis between the voltage and vibration signals confirms that the FIV energy is converted into electric energy.4.The grooved damping components have a better performance for reducing the level of FIV,however,the output voltage is also reduced when the FIV is suppressed.The finite element simulation shows that,compared with the smooth ones,the grooved damping components exhibit a more uniform deformation between the leading edge and the trailing edge,and the amplitude of the deformation is also smaller,which consequently improves the contact state and stabilizes the friction system.However,the deformation of the piezoelectric elements located between the two damping layers is positively correlated with the deformation of the damping components.As a result,there is less deformation in the piezoelectric elements that are sandwiched by the grooved damping components,resulting in a smaller amplitude of output voltage.