On A Multi-stable Piezoelectric Energy Harvester With Variable Potential Function And Its Characteristics

Piezoelectric energy harvester,which can convert ambient vibration energy into electrical energy,has been attracting amount of attention due to its advantages of high energy conversion efficiency and simple structure.For overcoming the problem that the bistable piezoelectric energy harvester generally has a higher potential barrier,a lot of techniques has been utilized including the multi-stable harvester and the variable potential energy configuration.this thesis focuses on combing multi-stable with variable potential energy for performance enhancement of a harvester.A multi-stable piezoelectric energy harvester model with variable potential function is proposed.The energy harvester consists of a cantilever beam,one end of which is mounted on the vibrator and the other end fixed with a magnet,and three magnets connecting with the vibrator,two of which are fixed directly to the vibrator,and between them is the middle magnet connected to the vibrator with a spring.Under the actions of magnet force and spring force,a multi-stable system with variable potential energy function is formed,and the potential energy function of the system can be symmetric or asymmetric according to the relational position of the three magnets.The main work of this thesis includes: the derivation of dynamics equation of the system with extended Hamilton’s Principle,the potential energy function of the system with different initial magnet distance and that respective to two cases when the two side magnets are positioned symmetrically or asymmetrically to the middle magnet are studied.Following that,the dynamics characteristics and energy harvesting characteristics of the multi-stable harvester with symmetrical and asymmetrical variable potential energy functions under harmonic excitation are respectively analyzed by numerical method.The main conclusions of this thesis are as following:1.Owing to the variable potential energy configuration,as the harvester is in working,the system will be on different potential energy function curve with the variation of the displacement of the middle magnet.Once the system on a potential energy function curve of with lower barrier,the piezoelectric beam will jump the barrier and interwell oscillate with a higher amplitude,so that the energy harvester can also produce more energy even at a lower excitation level.2.When the two side magnets are positioned symmetrically to the middle magnet,the multi-stable piezoelectric energy harvester with symmetric variable potential function may exhibit periodic motion with small amplitude in a well,chaotic motion or periodic motion interwell if the parameters of harvester are different.It is shown with the simulation results that the energy capture bandwidth of the system will gradually increase with the increase of excitation amplitude.Compared with the case when the initial magnet distance is smaller,the larger initial magnet distance will lead to larger the displacement response of the piezoelectric beam,and a larger peak of root-mean-square voltage,a wider of the effective operation frequency bandwidth.Compared with the case when spring stiffness is smaller,the larger spring stiffness will make the system jump between the wells but reduce the peak of root-mean-square voltage.With the increase of the load resistance,the root-mean-square voltage of the system output increases,and the output power increases firstly and then decreases.The optimal load is different for the different frequency,therefore,appropriate load resistance should be determined in the light of the frequency of excitation for performance enhancement.3.When the two side magnets are positioned asymmetrically to the middle magnet,and the distance between the two side magnets is preserved,an offset of the two side magnets against the middle magnet is formed.The offset is one of crucial factors of influence on the dynamic characteristics and energy harvesting characteristics of the multi-stable piezoelectric energy harvester with asymmetric variable potential energy function.The results of simulations show that the harvester with the value of the offset being unequal to zero,compared with that of with the value of the offset being equal to zero,can harvester energy over a wider frequency range,and is readiness to realize large-amplitude interwell motion at a lower excitation frequency,result in a higher root-mean-square voltage and efficiency of harvesting.This merit is more obvious when the initial magnet distance is smaller.It is also shown that the increase of the load resistance has less influence on the variation law of relationships of the output voltage and power with the load resistance under the different amplitude and frequency of excitation for different values of the offset.And the optimal load resistances for different value of offset under different frequency of excitation will be different.