A piezoelectric energy harvesting device capable of autonomously adjusting its natural frequency

Abstract

The present invention discloses a piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency. By pre-storing the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft in the controller, and setting a circular turntable and a servo motor, when the cantilever beam undergoes forced vibration under environmental vibration excitation and outputs a piezoelectric alternating voltage to the controller, on the one hand, the controller rectifies the piezoelectric alternating voltage and charges it to the rechargeable battery, on the other hand, calculates the current environmental vibration frequency, and then takes the current environmental vibration frequency as the natural frequency of the cantilever beam. According to the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft, the corresponding rotation angle of the servo motor shaft is obtained, and this rotation angle of the servo motor shaft is the target value for adjusting the natural frequency of the cantilever beam. According to this target value, the servo motor is controlled to rotate to the corresponding angle to change the position of the counterweight mass block. The advantages are that it has low hardware costs, a wide application range and high reliability.

Description

Technical Field

[0001] The present invention relates to a piezoelectric energy harvesting device, and more particularly to a piezoelectric energy harvesting device capable of autonomously adjusting its natural frequency. Background Art

[0002] Piezoelectric energy harvesting technology can capture the vibration energy in the environment and convert it into electrical energy to power low-power microelectronic devices. Compared with other energy harvesting methods such as magnetoelectric and electrostatic methods, piezoelectric energy harvesting has advantages such as high energy density, high electromechanical conversion efficiency, and easy microelectromechanical integration, and thus has received extensive attention. Among them, the cantilever beam type piezoelectric energy harvester is a resonant mechanical structure designed and manufactured based on a rectangular cantilever beam. In the cantilever beam type piezoelectric energy harvester, one end of the cantilever beam is fixed, which is the fixed end of the cantilever beam, and the other end is suspended, which is the free end of the cantilever beam. The piezoelectric sheet is pasted on the cantilever beam near its fixed end, and at the same time, a weight is added to the free end of the cantilever beam to increase the amplitude of the cantilever beam. Because of its simple structural configuration, it is widely used. The cantilever beam type piezoelectric energy harvester has a specific natural frequency. When the environmental vibration frequency is consistent with this natural frequency, the cantilever beam type piezoelectric energy harvester will enter the mechanical resonance state and can achieve the best energy harvesting effect; when the environmental vibration frequency deviates from this natural frequency, the cantilever beam type piezoelectric energy harvester will exit the mechanical resonance state, and the energy harvesting effect will rapidly decay as the deviation degree of the environmental vibration frequency relative to its natural frequency increases.

[0003] It can be seen that adjusting the natural frequency of the cantilever beam type piezoelectric energy harvester to approach the environmental vibration frequency is an important means to improve the performance of the cantilever beam type piezoelectric energy harvester. For example, a Chinese invention patent application with the application number CN202111288785.3 discloses a self-tuning piezoelectric energy harvesting device. Based on a comprehensive optimization design method of reasonable materials, structures, mechanics, etc., this self-tuning piezoelectric energy harvesting device can change its natural frequency to match the environmental vibration frequency under preset external excitation conditions to improve the piezoelectric energy harvesting effect. The method of adjusting the natural frequency of this environmental vibration frequency is passive and does not require an additional active adjustment mechanism or consume external energy, so the hardware cost of this environmental vibration frequency is relatively low; however, the passive adjustment method needs to be activated under preset external excitation conditions, so the application range of this environmental vibration frequency is limited and the reliability is relatively low.

[0004] Compared with the passive regulation method of the natural frequency, the piezoelectric energy harvesting device adopting the active regulation method of the natural frequency has a wider application range and higher reliability. For example, a piezoelectric energy harvesting device capable of autonomously adapting to frequency conversion is disclosed in the Chinese invention patent with the application number CN201710169222.X. This piezoelectric energy harvesting device uses a dedicated vibration sensor to detect the environmental vibration frequency, and then controls a voice coil motor to drive a clamping device installed on the cantilever beam to move along the length direction of the cantilever beam, changing the effective length of the cantilever beam to adjust the natural frequency of the piezoelectric energy harvesting device to approach the environmental vibration frequency. Although the application range of this piezoelectric energy harvesting device is wide and the reliability is high, it requires additional vibration sensors and expensive voice coil motors, resulting in high hardware costs.

[0005] Another example is that a piezoelectric energy harvesting device capable of automatically adjusting the resonance frequency and bandwidth is disclosed in the Chinese patent with the application number CN201811490567.6. This piezoelectric energy harvesting device judges the working state of the cantilever beam by detecting the open-circuit voltage and short-circuit current of the piezoelectric sheet, and uses the forward and reverse rotation of the motor to drive the lead screw to make the mass block do linear displacement movement on the cantilever beam to adjust the natural frequency, thereby improving the piezoelectric energy harvesting effect. Since the short-circuit current of the piezoelectric sheet is usually only a few hundred microamperes and is in the form of alternating current, the requirements for the accuracy and signal-to-noise ratio of the current detection circuit for detecting the short-circuit current of the piezoelectric sheet are extremely high. Implementing the current detection circuit with high-performance hardware circuits can improve the reliability, but this will greatly increase the hardware cost of the piezoelectric energy harvesting device; implementing the current detection circuit with low-performance hardware circuits can reduce the hardware cost, but at the same time, it also reduces the reliability of the piezoelectric energy harvesting device. Therefore, overall, although the application range of this piezoelectric energy harvesting device is wide, there is a great contradiction between its reliability and hardware cost. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency, which has a wide application range and high reliability while having a low hardware cost.

[0007] The technical solution adopted by the present invention to solve the above technical problems is as follows: A piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency, including a cantilever beam with a fixed end and a free end, a piezoelectric sheet, and a counterweight mass block. The direction from the fixed end to the free end of the cantilever beam is the length direction of the cantilever beam. The piezoelectric sheet is pasted on the upper surface of the cantilever beam and close to the fixed end of the cantilever beam. The piezoelectric energy harvesting device further includes a controller, a circular turntable, and a servo motor. A mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft is pre-stored in the controller. The servo motor is installed at the free end of the cantilever beam, and the shaft of the servo motor vertically passes through the cantilever beam from bottom to top. The circular turntable is located above the cantilever beam and is coaxially installed on the shaft of the servo motor. The counterweight mass block is located above the circular turntable and is installed on the circular turntable. The installation position of the counterweight mass block is close to the outer circumference of the circular turntable. The dimension of the counterweight mass block along the radial direction of the circular turntable is smaller than the radius of the circular turntable. The piezoelectric sheet and the servo motor are respectively connected to the controller. When the cantilever beam undergoes forced vibration under environmental vibration excitation, it will cause the piezoelectric sheet to undergo periodic deformation, thereby outputting a piezoelectric alternating voltage to the controller. On the one hand, the controller rectifies the input piezoelectric alternating voltage and charges it into a rechargeable battery for electrical energy storage. On the other hand, the controller calculates the current environmental vibration frequency based on the input piezoelectric alternating voltage, then takes the current environmental vibration frequency as the natural frequency of the cantilever beam, and according to the pre-stored mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft in it, obtains the corresponding rotation angle of the servo motor shaft. This rotation angle of the servo motor shaft is the target value for adjusting the natural frequency of the cantilever beam. According to this target value, the controller controls the servo motor to rotate to the corresponding angle, changes the position of the counterweight mass block, thereby adjusting the natural frequency of the cantilever beam to approach the environmental vibration frequency and improving the piezoelectric energy harvesting effect.

[0008] The target value for adjusting the natural frequency of the cantilever beam is denoted as θ, and its value range is 0 degrees to 180 degrees. When the rotation angle of the servo motor shaft is 0 degrees, the counterweight mass block is located at the outermost end in the length direction of the cantilever beam. At this time, the cantilever beam has the minimum natural frequency, which is denoted as f min ; when the rotation angle of the servo motor shaft is 180 degrees, the counterweight mass block is located at the innermost end in the length direction of the cantilever beam. At this time, the cantilever beam has the maximum natural frequency, which is denoted as f max ; during the process of the rotation angle of the servo motor shaft increasing from 0 degrees to 180 degrees, the counterweight mass block moves from the outermost end in the length direction of the cantilever beam to the innermost end, so that the natural frequency of the cantilever beam increases from f min to fmax ; Denote the current environmental vibration frequency as f e , when f e is less than or equal to f min , at this time, the target value θ for adjusting the natural frequency of the cantilever beam is 0 degrees, and the controller controls the steering gear shaft to rotate to the 0-degree position to adjust the natural frequency of the cantilever beam to f min ; when f e is greater than or equal to f max , at this time, the target value θ for adjusting the natural frequency of the cantilever beam is 180 degrees, and the controller controls the steering gear shaft to rotate to the 180-degree position to adjust the natural frequency of the cantilever beam to f max ; when f e is between f min and f max , the controller determines the target value of the rotation angle of the steering gear shaft according to the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the steering gear shaft pre-stored therein, and finally drives the steering gear to rotate by the corresponding target value angle, so as to adjust the natural frequency of the cantilever beam to f e .

[0009] The controller outputs a square wave signal with an amplitude of 3.3V, a period of 20ms, and a pulse width of (θ / 90 + 0.5)ms to the steering gear. Under the action of the square wave signal input into it, the steering gear drives the circular turntable and the counterweight mass block to rotate to the position corresponding to the target value θ together.

[0010] The piezoelectric sheet has a positive output terminal and a negative output terminal. The positive output terminal and the negative output terminal of the piezoelectric sheet respectively generate an AC voltage signal output. The AC voltage signal output from the positive output terminal of the piezoelectric sheet is called the positive terminal AC voltage signal, and the AC voltage signal output from the negative output terminal of the piezoelectric sheet is called the negative terminal AC voltage signal. The piezoelectric AC voltage includes the positive terminal AC voltage signal and the negative terminal AC voltage signal; the controller includes a synchronous charge extraction circuit, a voltage stabilization circuit, a same-frequency square wave generation circuit, and a single-chip microcomputer. The synchronous charge extraction circuit is used to extract the piezoelectric AC voltage output by the piezoelectric sheet, rectify the piezoelectric AC voltage and then output it to the rechargeable battery to charge the rechargeable battery to increase its power; the same-frequency square wave generation circuit is used to obtain the positive terminal AC voltage signal, compare and process the positive terminal AC voltage signal, and output a same-frequency square wave signal to the single-chip microcomputer. The single-chip microcomputer calculates the frequency of the same-frequency square wave signal by counting the rising edge of the same-frequency square wave signal input into it, and this frequency is the current environmental vibration frequency; the single-chip microcomputer determines the target value for adjusting the natural frequency of the cantilever beam according to the calculated current environmental vibration frequency.

[0011] The size of the counterweight mass block along the radial direction of the circular turntable is less than 1 / 10 of the radius of the circular turntable.

[0012] The fixed end of the cantilever beam is fixed by a fixed base and a cover plate. The fixed base is located below the fixed end of the cantilever beam, and the cover plate is located above the fixed end of the cantilever beam.

[0013] Compared with the prior art, the advantages of the present invention are as follows: by setting a controller, a circular turntable and a servo motor, a mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft is pre-stored in the controller. The servo motor is installed at the free end of the cantilever beam, and the shaft of the servo motor vertically penetrates the cantilever beam from bottom to top. The circular turntable is located above the cantilever beam and is coaxially installed on the shaft of the servo motor. The counterweight mass block is located above the circular turntable and is installed on the circular turntable. The installation position of the counterweight mass block is close to the outer circumference of the circular turntable. The size of the counterweight mass block along the radial direction of the circular turntable is less than the radius of the circular turntable. The piezoelectric sheet and the servo motor are respectively connected to the controller. When the cantilever beam undergoes forced vibration under environmental vibration excitation, it will cause the piezoelectric sheet to undergo periodic deformation, thereby outputting a piezoelectric alternating voltage to the controller. On the one hand, the controller rectifies the input piezoelectric alternating voltage and charges it into a rechargeable battery for electrical energy storage. On the other hand, the controller calculates the current environmental vibration frequency based on the input piezoelectric alternating voltage, and then takes the current environmental vibration frequency as the natural frequency of the cantilever beam. According to the pre-stored mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft in it, the corresponding rotation angle of the servo motor shaft is obtained. This rotation angle of the servo motor shaft is the target value for adjusting the natural frequency of the cantilever beam. According to this target value, the servo motor is controlled to rotate to the corresponding angle, changing the position of the counterweight mass block, thereby adjusting the natural frequency of the cantilever beam to approach the environmental vibration frequency and improving the piezoelectric energy harvesting effect. The present invention can autonomously sense the environmental vibration frequency, adaptively adjust the natural frequency, has a relatively high frequency measurement accuracy, and does not require an additional vibration sensor during the measurement process, nor does it require a high-performance current detection circuit, reducing the hardware cost. Therefore, while having a low hardware cost, it has a wide application range and high reliability. Description of the Drawings

[0014] Figure 1 It is a schematic structural diagram of a piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to the present invention;

[0015] Figure 2 It is a schematic diagram of the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft of a piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to the present invention;

[0016] Figure 3Circuit system block diagram of the controller of the piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to the present invention;

[0017] Figure 4 Waveform diagram of the vibration velocity, working voltage of the piezoelectric sheet, and output signal of the same-frequency square wave generation circuit module of the piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to the present invention. Detailed implementation manners

[0018] The present invention will be further described in detail below in conjunction with the embodiments with reference to the drawings.

[0019] Embodiment: As Figure 1 shown, a piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency includes a fixed base P1, a cover plate P2, a cantilever beam P5 having a fixed end and a free end, a piezoelectric sheet P4, and a counterweight mass P7. The direction from the fixed end to the free end of the cantilever beam P5 is the length direction of the cantilever beam P5. The fixed end of the cantilever beam P5 is fixed through the fixed base P1 and the cover plate P2. The fixed base P1 is located below the fixed end of the cantilever beam P5, and the cover plate P2 is located above the fixed end of the cantilever beam P5. The piezoelectric sheet P4 is pasted on the upper surface of the cantilever beam P5 and close to the fixed end of the cantilever beam P5. The piezoelectric energy harvesting device further includes a controller P3, a circular turntable P6, and a servo motor P8. A mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft is pre-stored in the controller P3. The servo motor P8 is installed at the free end of the cantilever beam P5. The shaft of the servo motor P8 vertically penetrates the cantilever beam P5 from bottom to top. The circular turntable P6 is located above the cantilever beam P5 and is coaxially installed on the shaft of the servo motor P8. The counterweight mass P7 is located above the circular turntable P6 and is installed on the circular turntable P6. The installation position of the counterweight mass P7 is close to the outer circumference of the circular turntable P6. The dimension of the counterweight mass P7 along the radial direction of the circular turntable P6 is smaller than the radius of the circular turntable P6. The piezoelectric sheet P4 and the servo motor P8 are respectively connected to the controller P3. When the cantilever beam P5 undergoes forced vibration under environmental vibration excitation, it will cause the piezoelectric sheet P4 to undergo periodic deformation and output a piezoelectric alternating voltage to the controller P3. On the one hand, the controller P3 rectifies the input piezoelectric alternating voltage and charges it into the rechargeable battery A2 for electrical energy storage. On the other hand, the controller P3 calculates the current environmental vibration frequency based on the input piezoelectric alternating voltage, and then takes the current environmental vibration frequency as the natural frequency of the cantilever beam. According to the pre-stored mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft in it, the corresponding rotation angle of the servo motor shaft is obtained. This rotation angle of the servo motor shaft is the target value for adjusting the natural frequency of the cantilever beam P5. According to this target value, the servo motor P8 is controlled to rotate to the corresponding angle, changing the position of the counterweight mass P7, thereby adjusting the natural frequency of the cantilever beam P5 to approach the current environmental vibration frequency and improving the piezoelectric energy harvesting effect.

[0020] Example 2: This example is basically the same as Example 1, with the difference being that:

[0021] As Figure 2 shown, in this example, the target value of the natural frequency adjustment of the cantilever beam P5 is denoted as θ, and its value range is 0 degrees to 180 degrees. When the rotation angle of the servo shaft is 0 degrees, the counterweight mass block P7 is located at the outermost end in the length direction of the cantilever beam P5. At this time, the cantilever beam P5 has the minimum natural frequency, which is denoted as f min ; when the rotation angle of the servo shaft is 180 degrees, the counterweight mass block P7 is located at the innermost end in the length direction of the cantilever beam P5. At this time, the cantilever beam P5 has the maximum natural frequency, which is denoted as f max ; during the process of the rotation angle of the servo shaft increasing from 0 degrees to 180 degrees, the counterweight mass block P7 moves from the outermost end in the length direction of the cantilever beam P5 to the innermost end, causing the natural frequency of the cantilever beam P5 to increase from f min to f max ; the current ambient vibration frequency is denoted as f e , when f e is less than or equal to f min , at this time, the target value θ of the natural frequency adjustment of the cantilever beam P5 is 0 degrees, and the controller P3 controls the servo P8 shaft to rotate to the 0-degree position to adjust the natural frequency of the cantilever beam P5 to f min ; when f e is greater than or equal to f max , at this time, the target value θ of the natural frequency adjustment of the cantilever beam P5 is 180 degrees, and the controller P3 controls the servo P8 shaft to rotate to the 180-degree position to adjust the natural frequency of the cantilever beam P5 to f max ; when f e is between f min and f max , the controller P3 determines the target value of the rotation angle of the servo shaft according to the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo shaft pre-stored therein, and finally drives the servo P8 to rotate by the corresponding target value angle, thereby adjusting the natural frequency of the cantilever beam P5 to f e ; the frequency range [f min , f max is the adjustable range of the natural frequency of the piezoelectric energy harvesting device. The minimum value f min , the maximum value f max of the adjustable range of the natural frequency of the piezoelectric energy harvesting device and the frequency range width are jointly determined by the geometric dimensions and mass distribution relationships of the cantilever beam P5, the circular turntable P6, and the counterweight mass block P7 in the piezoelectric energy harvesting device, and are customized according to design requirements; the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo shaft is calibrated through the currently well-known resonance experiment test and data fitting.

[0022] In this embodiment, the controller P3 outputs a square wave signal with an amplitude of 3.3V, a period of 20ms, and a pulse width of (θ / 90 + 0.5)ms to the servo P8. Under the action of the square wave signal input into it, the servo P8 drives the circular turntable P6 and the counterweight mass P7 to rotate together to the position corresponding to the target value θ.

[0023] As Figure 3 shown, in this embodiment, the piezoelectric sheet P4 has a positive output terminal and a negative output terminal. The positive output terminal and the negative output terminal of the piezoelectric sheet P4 respectively generate an AC voltage signal output. The AC voltage signal output from the positive output terminal of the piezoelectric sheet P4 is called the positive terminal AC voltage signal Vp, and the AC voltage signal output from the negative output terminal of the piezoelectric sheet P4 is called the negative terminal AC voltage signal Vn. The piezoelectric AC voltage includes the positive terminal AC voltage signal Vp and the negative terminal AC voltage signal Vn. The controller P3 includes a synchronous charge extraction circuit A1, a voltage stabilizing circuit A3, a same-frequency square wave generating circuit A4, and a single-chip microcomputer A5. The synchronous charge extraction circuit A1 is used to extract the piezoelectric AC voltage output by the piezoelectric sheet P4, rectify the piezoelectric AC voltage, and then output the voltage Vr to the rechargeable battery A2 to charge the rechargeable battery A2 to increase its power. The same-frequency square wave generating circuit A4 is used to obtain the positive terminal AC voltage signal Vp, perform comparison processing on the positive terminal AC voltage signal Vp, and output a same-frequency square wave signal V frep to the single-chip microcomputer A5. The single-chip microcomputer A5 calculates the frequency of the same-frequency square wave signal V frep by counting the rising edges of the input same-frequency square wave signal V frep . This frequency is the current environmental vibration frequency. The single-chip microcomputer A5 determines the target value of the natural frequency adjustment of the cantilever beam P5 according to the calculated current environmental vibration frequency. The rechargeable battery A2 supplies power to the servo P8 and the single-chip microcomputer A5 through the voltage stabilizing circuit A3. The synchronous charge extraction circuit A1, the voltage stabilizing circuit A3, the same-frequency square wave generating circuit A4, and the single-chip microcomputer A5 all adopt mature products in their technical fields.

[0024] In this embodiment, the dimension of the counterweight mass P7 along the radial direction of the circular turntable P6 is less than 1 / 10 of the radius of the circular turntable P6.

[0025] In this embodiment, the specific implementation steps of the frequency adjustment of the piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency are as follows:

[0026] Step S100: When the single-chip microcomputer A5 is powered on, its internal timer is started, and its timing period is recorded as T1, and its value range is 20ms to 100ms. The single-chip microcomputer A5 counts the rising edges of the input same-frequency square wave signal V freq through the internal timer.

[0027] Step S110: After the timing reaches a timing period T1, turn off the timer and stop timing;

[0028] Step S120: Read the count value of the timing period T1, denote it as N, and set the environmental vibration frequency as f e , then calculate the environmental vibration frequency f e = N / T1, the currently calculated f e is the adjustment value of the natural frequency of the cantilever beam;

[0029] Step S130: According to the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo shaft pre-stored in the single-chip microcomputer A5, calculate the target value of the rotation angle of the servo shaft to be rotated, and denote it as θ;

[0030] Step S140: According to the target value θ of the rotation angle of the servo shaft to be rotated, calculate the pulse width value of the square wave signal for driving the rotation of the servo, and its value is (θ / 90 + 0.5) ms;

[0031] Step S150: Generate a square wave signal with an amplitude of 3.3V, a period of 20ms, and a pulse width of (θ / 90 + 0.5) ms and output it to the servo P8 to drive the servo P8 to rotate to the position corresponding to the θ angle, changing the position of the counterweight mass block P7 on the cantilever beam P5, thereby adjusting the natural frequency of the cantilever beam to f e ;

[0032] Step S160: Start the internal timer of the single-chip microcomputer A5 again, denote its timing period as T2, and its value range is 60s - 300s, and enter the low-power sleep state;

[0033] Step S170: After the timing reaches a timing period T2, turn off the timer and stop timing, and exit the low-power sleep state;

[0034] So far, the environmental frequency perception, servo shaft angle adjustment, cantilever beam natural frequency adjustment, and single-chip microcomputer low-power sleep of a timing period (T1 + T2) are completed, and then enter the next timing period, repeating in a cycle.

[0035] To verify the environmental vibration frequency perception ability of the piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency of the present invention, circuit simulation is performed on the synchronous charge extraction circuit A1 and the same-frequency square wave generation circuit A4 in the controller P3, where the simulation parameters are configured as follows: the vibration frequency is 50Hz, the internal capacitance of the piezoelectric sheet P4 is 200nF, the open-circuit output voltage of the piezoelectric sheet P4 is 3.6V, and the comparison threshold of the comparator inside the same-frequency square wave generation circuit A4 is 0.2V; the input vibration speed of the piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency of the present invention, the output voltage of the piezoelectric sheet P4 (V p -Vn ) and the same frequency square wave generating circuit A4 output signal V freq The simulation waveform is as follows Figure 4 As shown; analysis Figure 4 It can be seen that the synchronous charge extraction circuit A1 can perform synchronous charge extraction near the peak point of the output voltage of the piezoelectric piece P4, extract the charge in the internal capacitor of the piezoelectric piece P4 and store it in the rechargeable battery A2, so that the voltage of the piezoelectric piece P4 is quickly reduced to zero, and then the charge accumulation starts again, over and over again. At the same time, the same-frequency square wave generating circuit A4 can output a high-level signal when the output voltage of the piezoelectric piece P4 is greater than or equal to 0.2V, and output a low-level signal when the output voltage of the piezoelectric piece P4 is less than 0.2V, so that the output square wave signal has the same frequency as the output voltage of the piezoelectric piece P4 and the vibration speed signal, and the frequency of the same-frequency square wave signal can be measured by the internal counter of the single-chip microcomputer A5; this shows that the piezoelectric energy capture device capable of autonomously adjusting the natural frequency of the present invention can autonomously sense the environmental vibration frequency, the frequency measurement accuracy is high, and the measurement process does not require the aid of additional vibration sensors, reducing hardware costs.

Claims

1. A piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency, comprising a cantilever beam with a fixed end and a free end, a piezoelectric sheet, and a counterweight mass block. The direction from the fixed end to the free end of the cantilever beam is the length direction of the cantilever beam. The piezoelectric sheet is adhered to the upper surface of the cantilever beam and close to the fixed end of the cantilever beam, and is characterized in that The described piezoelectric energy harvesting device further includes a controller, a circular turntable, and a servo motor. The controller pre-stores a mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft. The servo motor is installed at the free end of the cantilever beam, and the shaft of the servo motor vertically penetrates the cantilever beam from bottom to top. The circular turntable is located above the cantilever beam and is coaxially installed on the shaft of the servo motor. The counterweight mass block is located above the circular turntable and is installed on the circular turntable. The installation position of the counterweight mass block is close to the outer circumference of the circular turntable, and the dimension of the counterweight mass block along the radial direction of the circular turntable is smaller than the radius of the circular turntable. The piezoelectric sheet and the servo motor are respectively connected to the controller. When the cantilever beam undergoes forced vibration under environmental vibration excitation, it will cause the piezoelectric sheet to undergo periodic deformation, thereby outputting a piezoelectric AC voltage to the controller. On the one hand, the controller rectifies the input piezoelectric AC voltage and charges it into a rechargeable battery for electrical energy storage. On the other hand, the controller calculates the current environmental vibration frequency based on the input piezoelectric AC voltage, then takes the current environmental vibration frequency as the natural frequency of the cantilever beam, and according to the pre-stored mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo motor shaft in it, obtains the corresponding rotation angle of the servo motor shaft. This rotation angle of the servo motor shaft is the target value for adjusting the natural frequency of the cantilever beam. According to this target value, the controller controls the servo motor to rotate to the corresponding angle, changes the position of the counterweight mass block, thereby adjusting the natural frequency of the cantilever beam to approach the environmental vibration frequency and improving the piezoelectric energy harvesting effect.

2. The piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to claim 1, wherein The target value of the natural frequency adjustment of the cantilever beam is denoted as θ, and its value range is 0 degrees to 180 degrees. When the rotation angle of the servo shaft is 0 degrees, the counterweight mass block is located at the outermost end of the cantilever beam in the length direction. At this time, the cantilever beam has the minimum natural frequency, which is denoted as f min ; when the rotation angle of the servo shaft is 180 degrees, the counterweight mass block is located at the innermost end of the cantilever beam in the length direction. At this time, the cantilever beam has the maximum natural frequency, which is denoted as f max ; during the process of the rotation angle of the servo shaft increasing from 0 degrees to 180 degrees, the counterweight mass block moves from the outermost end to the innermost end of the cantilever beam in the length direction, so that the natural frequency of the cantilever beam increases from f min to f max ; the current environmental vibration frequency is denoted as f e , when f e is less than or equal to f min , at this time, the target value θ of the natural frequency adjustment of the cantilever beam is 0 degrees, and the controller controls the servo shaft to rotate to the 0-degree position to adjust the natural frequency of the cantilever beam to f min ; when f e is greater than or equal to f max , at this time, the target value θ of the natural frequency adjustment of the cantilever beam is 180 degrees, and the controller controls the servo shaft to rotate to the 180-degree position to adjust the natural frequency of the cantilever beam to f max ; when f e is between f min and f max , the controller determines the target value of the rotation angle of the servo shaft according to the mathematical mapping relationship between the natural frequency of the cantilever beam and the rotation angle of the servo shaft pre-stored therein, and finally drives the servo to rotate by the corresponding target value angle, so as to adjust the natural frequency of the cantilever beam to f e .

3. A piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to claim 2, characterized in that The controller outputs a square wave signal with an amplitude of 3.3V, a period of 20ms, and a pulse width of (θ / 90 + 0.5)ms to the servo motor. The servo motor drives the circular turntable and the counterweight mass block to rotate to the position corresponding to the target value θ under the action of the input square wave signal.

4. A piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to claim 2, characterized in that The piezoelectric sheet has a positive output terminal and a negative output terminal. An AC voltage signal output is generated at the positive output terminal and the negative output terminal of the piezoelectric sheet respectively. The AC voltage signal output from the positive output terminal of the piezoelectric sheet is called the positive terminal AC voltage signal, and the AC voltage signal output from the negative output terminal of the piezoelectric sheet is called the negative terminal AC voltage signal. The piezoelectric AC voltage includes the positive terminal AC voltage signal and the negative terminal AC voltage signal. The controller includes a synchronous charge extraction circuit, a voltage stabilizing circuit, a same-frequency square wave generating circuit and a single-chip microcomputer. The synchronous charge extraction circuit is used to extract the piezoelectric AC voltage output by the piezoelectric sheet, rectify the piezoelectric AC voltage and then output it to the rechargeable battery to charge the rechargeable battery to increase its power. The same-frequency square wave generating circuit is used to obtain the positive terminal AC voltage signal, perform comparison processing on the positive terminal AC voltage signal, and output a same-frequency square wave signal to the single-chip microcomputer. The single-chip microcomputer calculates the frequency of the same-frequency square wave signal by counting the rising edges of the pulses of the same-frequency square wave signal input into it, and this frequency is the current environmental vibration frequency. The single-chip microcomputer determines the target value of the natural frequency adjustment of the cantilever beam according to the calculated current environmental vibration frequency.

5. A piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to claim 2, characterized in that The dimension of the counterweight mass block along the radius of the circular turntable is less than 1 / 10 of the radius of the circular turntable.

6. The piezoelectric energy harvesting device capable of autonomously adjusting the natural frequency according to claim 2, characterized in that The fixed end of the cantilever beam is fixed by a fixed base and a cover plate. The fixed base is located below the fixed end of the cantilever beam, and the cover plate is located above the fixed end of the cantilever beam.

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