The fabrication process of designs started by gluing three gluing 3 different material having different properties polymer pvdf, ceramic LiNbO3 and metal shape memory alloy NITINOL 55:45 Ni: Ti with a dimension specified in the table 1. (commercial piezoelectric material pvdf from Piezo Systems Inc, SMA from Nexametals and lithium niobite from Roditi systems Inc). for making a hybrid structure with the all three materials to serve as a single beam, a mixture of silver glue and epoxy is used. To solidify the glue, the resulted tri-layer plate is heated inside an oven at 100 degree Celsius during one hour and then cooled down. After that, a laser machine (Siro Lasertec GmbH, Pforzheim, Germany) is used to cut each design following its CAD model. The resulted prototypes showed in Fig.2(are then connected to wires using the same gluing process mentioned previously. It is worth to notice that a rectangle domain is added on each resulted design in order to clamp the prototypes on the experimental bench support. We have fabricated three cantilevers with different placing of the material, these cantilevers are able to resonate under 280 Hz of frequency range. Their total piezoelectric energy has been simulated, depending on different configuration of the materials used. However, the LSP configuration (Lithium Niobate as the top layer and PVDF as bottom layer) has the highest value of harvested energy of 12Vpp. moreover, its resonance frequency is also second highest among all the three-cantilever configuration at 268.7Hz.
- Experimental Bench
As it is shown in Fig.3, the experimental setup consists of a shaker which is used to produce the mechanical vibration ,The shaker is excited at the natural frequency of each cantilever and driven by a sine wave from a function generator (Agilent Technologies 33250A 80 MHz waveform generator) amplified by a power amplifier (LDS PA100E Power Amplifier). and the output voltage is calculated from the measurement by an oscilloscope 4 inputs is used to measure the voltage produced by each prototype. The design is clamped by a 3D printed support to completely simulate the clamped part in the modelling and COMSOL. The whole setup is placed on an anti-vibration table to avoid the ambient vibration that may excite the system.
Figure 1: COMSOL modeling of cantilever Table 1: parameter of materials
Figure 2: The cantilever with the setup
- Experimental Results
- Piezo part
- Resonant Frequency, Power and Voltage Generated
The excitation frequency of power and voltage is maximized by monitoring the excitation level of resonant frequency. In this experiment, no tip mass is mounted on the cantilever and the load is fixed to be 50 kΩ. Figure 4 shows the generated voltage from the harvesting system without tip mass. It is observed that the resonant frequency of the harvester is 267.8Hz (maximum peak of the extracted power). Under this excitation frequency; the power output and voltage reached to its maximum value equal to 1 μW and 12 Vpp and the RMS voltage is 4.24 V we have tested all the sample at the same amplitude or we can say at a master gain of 2 in the power amplifier. physical system is phenomena in which multiple resonant frequency have flexible structure. The cantilever was mounted on the shaker in horizontal direction, we tested all the 3 different configured cantilevers one by one but taking into consideration only the best one that is LSP. The sinusoidal voltage on the signal generator is set to has 20-volt amplitude with 268Hz frequency. In table2 a comparison is made based on the results obtained in COMSOL and experiments with percentage of error. We have measured the current in the range of 1 Hz till 300Hz with short circuit condition for all the three different configured cantilevers and obtained the RMS current shown in Fig 5. It was observed that the best configuration LSP shows max value at the resonance frequency with 0.23
- Pyro part
For the pyro part we have used the a very specialized thermal machine known as Clima temperature system in fig 6 in which we can choose the heating rate the range of temperature for the machine is -40°C to 300°C So we can work on high temperature as well but as we used PVDF in our system we were bound to work till 150°C because degradation temperature of PVDF is 180°C .
We have designed the heating and cooling cycle in Fig7 with 30°C/m, with starting at 20°C till 140°C and cooling again till 20°C, CID software was used for programming and documentation. We have seen that when the temperature was rising from 20°C to 140°C the output voltage was fluctuating as well but when the cooling cycle started and temperature started decreasing we noted the maximum output voltage in Fig 8 as 3.5 V at 76°C and at the same temperature in short circuit condition we have obtain the voltage value as 1.5 V.