This study reports the effects of the parameters of a vibration-based impact mode piezoelectric power generator. First, an evaluation of the effects of the impact parameters, the mass, and the impact velocity is presented. It is found that the output voltage of the piezoelectric device in impact mode is directly proportional to the velocity, whereas the output power is equal to a quadratic function of the same variable. For the same impact momentum, the effect of the velocity in generating a higher peak output is dominant compared with the mass. Second, the vibration-based impact mode piezoelectric power generator is discussed. The experimental results show that a wider operating frequency bandwidth of the output power can be achieved with the preloading configuration. However, regarding magnitude, due to the high velocity of impact, the configuration with a gap between the tip and the piezoelectric device produces a higher output.
Research advancements in mechanical vibration energy harvesting have been widely reported for decades. Energy from these harvesting systems is expected to be used to power low-power devices, such as LEDs, tire pressure monitoring systems, and many others. The objectives of the study are to propose a new design and to evaluate the factors that affect the output power generation. In general, mechanical vibration is converted to electrical energy using three types of devices: piezoelectric, electrostatic, and electromagnetic devices. Both
In the case of vibration energy harvesting using piezoelectric devices, for linear vibration motion, the basic operation of power generation can be divided into two modes: bending mode and impact mode. In bending mode for power generation using a piezoelectric cantilever beam, one end of the device is attached to the vibration sources and the other end freely vibrates with the sources of the vibration. To improve the output power of the piezoelectric power generator in bending mode, the shape of the device is critical, as it has been shown that devices of certain shapes are more effective than others [
Another important factor for the optimum output is impedance matching [
In the case of vibration-based impact mode power generation, piezoelectric devices do not deform due the vibration. The deformation is due to the impact. As reported in [
Other studies have reported the effect of the dimensions of piezoelectric ceramics [
Piezoelectric devices generate electricity through deformation of its structure. The cause of deformation can be vibration or direct impact on the structure. The characteristics of impact mode power generation of piezoelectric devices for various impact parameters can be studied by performing weight drop experiments. Theoretically, when an object with a weight of
The weight drop experimental setup is shown in Figure
Specifications of the round piezoelectric device.
Parameter | Value |
---|---|
Diameter of metal plate | 35.0 ± 0.1 mm |
Diameter of ceramic element | 25.0 ± 0.4 mm |
Thickness of ceramic element | 0.21 ± 0.05 mm |
Young’s modulus | 5.6 × 1010 N/m2 |
Piezoelectric strain constant, |
420 × 10−12 m/V |
Piezoelectric stress constant, |
23.3 × 10−3 Vm/N |
Capacitance, |
35 nF |
Experimental configuration: (a) flat base, (b) base with a hole, (c) a round piezoelectric device.
The experimental method consisted of dropping a steel ball in free fall from a predetermined height such that it hits the piezoelectric device. The output of the piezoelectric power generator was connected to load resistors of 1 kΩ, 10 kΩ, and 20 kΩ. The voltage is recorded to a data logger at a sampling time of 10
The experiment was conducted by varying the height of the steel ball from 10 mm to 70 mm. When the steel ball was dropped, the piezoelectric device produced voltage pulses, as shown in Figure
Example of the pulse signal.
Figure
Instantaneous peak output voltage versus velocity.
Meanwhile, output power of the load resistor
Average output power (2 ms) versus velocity.
Next, among the three load resistors, due to impedance matching, the output power of the 10 kΩ resistor corresponded to the highest average output power compared with the other load resistors.
For comparison and to observe the effect of the weight of the impact object on the output power of the piezoelectric power generator, the 4 g steel ball was replaced with an 8 g steel ball. As shown in Figure
Average output power (2 ms) versus velocity plots for 4 g and 8 g steel balls.
Average output power (2 ms) versus momentum plots for 4 g and 8 g steel balls.
Therefore, for equal momentum, a higher average output power is obtained when using an object with a higher impact velocity rather than a heavier object.
Next, we evaluate the relationship between the output power and the stiffness of the piezoelectric device. The previous experiments were conducted using a piezoelectric device set on a flat iron base, which indirectly increased the stiffness of the device structure as a whole. In turn, incremental increases in the stiffness degraded the strain that developed on the piezoelectric device, resulting in lower output power. Therefore, to optimize the output power of the device, a base with a round hole was used to substitute the flat iron base. Figure
Iron base with hole.
The same weight drop experiments were conducted, varying the height and weight of the steel ball. The output power comparison is shown in Figure
Average output power (2 ms) versus momentum plots for 4 g and 8 g steel balls comparing the flat base and the base with a hole.
This section discusses the design and analyses of the vibration-based impact mode piezoelectric power generator. The configuration of the proposed power generator consists of three main structures: the base beam, an adjustable spacer, and the vibrating beam with the proof mass attached to the free end of the beam. Another important component attached to the free end of the vibrating beam is a metal tip. A schematic of the power generator is shown in Figure
Specifications of the structures of the power generator.
Structure | Value |
---|---|
Base beam (aluminum) | 130 × 50 × 10 mm |
Vibrating beam with hole (aluminum) | 100 × 20 × 1 mm |
Adjustable spacer (aluminum) | 26 × 20 × 1 mm |
Proof mass (aluminum) | 26 and 40 g |
Round shape of tip (iron) | Height: 3 mm |
Construction of the power generator and schematics of the structures.
The operating principle of the power generator is to generate electricity from impact on the piezoelectric device. Every impact on the piezoelectric device produces a voltage pulse across the load resistor. The piezoelectric device is bonded to the base beam with epoxy and is placed on the hole of the beam so that as discussed in the previous section, the power generation can be optimized by maintaining the original stiffness of the piezoelectric device. The proof mass served as a deflection booster for the vibrating beam. Adjusting the weight of the proof mass is also performed for the resonant frequency variation. In addition, at the clamped area of the configuration, adjustable spacers were used to separate the vibrating beam and the base beam. The total thickness of the piezoelectric device is 0.41 mm, and the height of the tip is 3 mm. Based on a simple calculation, a 3 mm thickness spacer creates no gap between the tip and the piezoelectric device, thus allowing for a preloading condition on the piezoelectric device.
Increasing the thickness of the spacer to 4 mm should produce a gap between the tip and the piezoelectric device. However, the proof mass pressurizes the tip of the vibrating beam, which eventually results in the same situation as the 3 mm configuration. Although both configurations lead to a preloading condition on the piezoelectric device, in terms of the strength of the loads, the 3 mm configuration is expected to be higher. Thus, in comparing these two configurations, the 3 mm configuration requires a relatively higher velocity and frequency of vibration before the tip can vibrate and hit the piezoelectric device. Our analyses show that evaluation of the preloading condition on power generation from vibrations of frequencies below 100 Hz can be realized using a spacer thickness of 4 mm. An additional 1 mm of spacer thickness creates a configuration with a gap between the tip and the piezoelectric device. Actual measurements show that the gap is approximately 0.604 mm. Importantly, all of the above analyses were performed using a proof mass of 26 g.
The amount of output power that can be generated is highly dependent on the number of impacts made by the tip. Generally, number of voltage pulses is directly proportional to the number of impacts. Thus, increasing the frequency of the vibrator will increase the number of impacts in one second. However, the impact velocity is dominant regarding the magnitude of the pulse. As discussed in the previous section, the peak of the output power is proportional to the square of the velocity. Thus, the output power generation for a period of time is expected to be dependent on the frequency of the vibrations and the velocity of the impact.
An evaluation of the motion of the vibrating beam is presented below using a piecewise linear model of the power generator as shown in Figure
Piecewise linear model of the forced vibration-based impact mode piezoelectric power generator.
The experimental setup is shown in Figure
Experimental setup.
Based on the weight drop experiment results in the previous section, the optimum output power was generated with a load resistor of 10 kΩ. Therefore, in this experimental evaluation, the same resistor was used as the load. To evaluate the effect of the weight of the proof mass on the output, two proof masses were used: 26 g and 40 g. Based on the system identification experimental results when a proof mass of 26 g was used, the resonant frequency of the power generator without the base beam was 24 Hz. When a proof mass of 40 g was used, the frequency decreased to 19 Hz. Power generators with 4 mm and 5 mm spacer thicknesses were constructed and examined. The differences between the two configurations are described in the previous section.
As the vibrating beam vibrates and the tip hits the piezoelectric device, a voltage pulse signal is produced across the load resistor. The number of pulses per second is dependent on the frequency of the vibration; that is, for a 35 Hz vibration, 35 voltage pulses are generated. A sample of the output voltage is shown in Figure
The output signals for a 35 Hz vibration, a spacer thickness of 4 mm and load resistor of 10 kΩ. (a) The output voltage; (b) the output power.
Figure
The frequency response of the power generator is shown in Figure
Plot of average output power (5 ms) versus frequency.
For a power generator with a gap between the tip and the device, the operating frequency bandwidth decreases to approximately 25% of that of the configuration under the preloading condition. For a gap 0.604 mm between the tip and the device, the motion of the tip of the vibrating beam must be greater than that for power generation. Considering the vibration of the vibrator, increasing the vibration frequency beyond the resonant frequency of the power generator decreases the acceleration of the tip. Therefore, as the frequency reaches 33 Hz or 27 Hz for the 26 g and 40 g configurations, respectively, the motion of the tip becomes less than 0.604 mm. From the same plot, operating frequency of the configuration with the lighter proof mass is slightly higher than the configuration with the heavier proof mass. This characteristic differs from the configuration in the preloading condition, as discussed earlier.
Furthermore, the results show that output power of the power generator for both configurations is very unstable. Our earlier assumption was that by setting the base beam to 10 times thicker than the vibrating beam, the occurrences of antiresonances of the response will reduce. However, as observed in the frequency response, the resonances are separated by antiresonances, especially for the configuration under the preloading condition. To explain this phenomenon, we must analyze the motion of the structures of the power generator.
As discussed previously, the basic structure of the proposed power generator consists of a vibrating beam and a base beam, which are coupled at one end. When a vibration is applied to the structures, the base beam is ideally assumed to vibrate without deflection. In addition, a vibrating beam with a thickness of 1 mm and a proof mass attached will vibrate and deflect at the free end. Variations in the frequency will vary the amplitude of the free end of both beams. Therefore, a small change in the frequency subsequently changes the timing of the tip hitting the piezoelectric device on the base beam. The velocity of the impact severely varies with these changes, which contributes to the unstable output power over the operation frequencies. The resonance output is expected to be generated when the vibrating beam hits the piezoelectric device at its peak velocity as it crosses the equilibrium point. In addition, for the antiresonance output, the impact occurs at a low velocity of the vibrating beam.
Analytical and experimental studies of the effects of the mechanical impact parameters on piezoelectric power generation in impact mode are presented. The experimental data given in the first part of this study show that the velocity of impact affects the forces and subsequently the output power and the energy to a greater degree than the mass. Moreover, it was experimentally demonstrated that, the optimum output power can be achieved if the stiffness of the device is maintained at its original value so that the efficiency does not decrease. In the second part of this study, analyses of the output power of the vibration-based impact mode power generator were presented. Two configurations were analyzed. Although the operating frequency of the preload configuration is wider than the configuration with the gap, due to the relative motion of the two beams, the output power suffers the resonance and antiresonance phenomena. Further evaluations of the structures of power generators should be performed in the future so that a stable output power and wider operating frequency bandwidths can be achieved.
The authors declare that there is no conflict of interests regarding the publication of this paper.