Parallel polarised multimorph Evaluation of Electrical Output

169 When driving a very low resistive load ≈ short circuit, the upper and lower sections of PZT produce electrical current of 55 µA and 44 µA respectively. An optimum output current of about 88 µA was measured for a parallel configuration 1+4; 2+3, while the hybrid configuration generates 44.2 µA of electrical current. This verifies that a parallel configuration can be an effective current source compared to other possible configuration for the multimorph structure. Figure 7-13: Output power as a function of resistive load for a parallel polarised sample. Figure 7-14: Output current-voltage for a parallel polarised sample. Figure 7-15 shows the dependence of open circuit voltage for sample BA3 on acceleration level. Similar to the results shown in Figure 7-12, the increment of the open circuit voltage is consistent for all the configurations at an increased acceleration level. 170 An open-circuit voltage of about 2.5 V was measured for a configuration of 1;3 short 2+4, when the multimorph cantilever was excited to its resonant frequency at an acceleration level of 1.5 g. The configuration is equivalent to a network of individual voltage sources with configuration of 2;3 and 3;4 connected in parallel and linked together in series with configuration 1;2. It is noticed that the central PZT section for sample BA3 is weakly polarised and generates relatively small open-circuit voltage when excited to the resonant frequency of the structure. This is more obvious when the acceleration level increases. An open- circuit voltage of 100 mV was measured from the central PZT section with configuration 2;3 at an acceleration of 1.5 g, as shown in Figure 7-15. Since the central PZT section was weakly polarised, it plays a part in the resultant electrical output. When the multimorph is bent downward as shown in Figure 7-7 b, the polarities of electrodes at the central section are similar to those of the outer layer, e.g. electrode 1 and 2 are at same polarity but with different electric field strength. The difference in electrical potential between them is lower than those of series polarised samples and therefore generates less electrical output. This effect, however, is useful for actuation applications, where a smaller input voltage is required to deflect the cantilever, as a result of converse piezoelectric effect, at the same magnitude as a series polarised samples with higher input voltage. Table 7-4 summarises all the major configurations of the terminal and the equivalent circuit of the connection for a parallel polarised sample. The equivalent circuits were verified experimentally. 171 Figure 7-15: Open circuit voltage as a function of acceleration level of a parallel polarised sample. Table 7-4: Summary of connection configurations for a parallel polarised sample. Connection Configuration Connection Diagram Equivalent circuits 1+4; 2+3 4 , 3 2 , 1 3 2 , 1 4 C C C     4 , 3 2 , 1 4 , 3 2 , 1 3 2 , 1 4 R R R R R     2 4 , 3 2 , 1 3 2 , 1 4 V V V     2; 4        3 , 1 2 , 1 4 , 3 3 , 2 3 , 1 2 , 1 3 , 1 2 , 1 3 , 2 3 , 1 2 , 1 4 , 3 4 , 2 C C C C C C C C C C C C C                      3 , 2 3 , 1 2 , 1 3 , 1 2 , 1 3 , 2 4 , 3 4 , 2 R R R R R R R R           2 3 , 2 3 , 1 2 , 1 4 , 3 4 , 2 V V V V V 172 2; 4 Short 1+3   4 , 3 3 , 2 2 , 1 3 , 2 2 , 1 4 , 3 4 , 2 C C C C C C C S     3 , 2 2 , 1 3 , 2 2 , 1 4 , 3 4 , 2 R R R R R R S    2 3 , 2 2 , 1 4 , 3 4 , 2 V V V V S    1; 3        4 , 3 4 , 2 3 , 2 2 , 1 4 , 3 4 , 2 4 , 3 4 , 2 3 , 2 4 , 3 4 , 2 2 , 1 3 , 1 C C C C C C C C C C C C C                      4 , 3 4 , 2 3 , 2 4 , 3 4 , 2 3 , 2 2 , 1 3 , 1 R R R R R R R R           2 3 , 2 4 , 2 4 , 3 2 , 1 3 , 1 V V V V V 1; 3 Short 2+4   4 , 3 3 , 2 2 , 1 4 , 3 3 , 2 2 , 1 3 , 1 C C C C C C C S     4 , 3 3 , 1 4 , 3 3 , 1 2 , 1 3 , 1 R R R R R R S    2 4 , 3 3 , 2 2 , 1 3 , 1 V V V V S    173

7.5.3 Excitation with Proof Mass

In another experiment, the electrical output of a multimorph cantilever sample was increased by attaching a proof mass at the tip of the cantilever. This experiment was carried out to investigate the practical use of the device in energy harvesting. An output voltage of approximately 300 mV to compensate for the voltage dropped across a rectification diode and an electrical output power of 60 µW are the minimum requirement for a micro-system to function properly, as reported in [65]. Consequently, these values are used as the benchmark for this experiment. Sample BA2, a series polarised multimorph, was used in this experiment. It was connected in an optimum series configuration to draw out as much output power and voltage as possible to meet the minimum requirement. A new resonant frequency was measured at 155 Hz when the multimorph cantilever was attached with a proof mass of 0.38 g. With this proof mass, an output power of 110 µW was generated, which is about a factor of 3 higher compared to the same sample with no proof mass, as shown in Figure 7-16. However this arrangement required a greater resistive load of about a factor of 2.6 compared to excitation without proof mass. The optimum resistive loads for an excitation with and without proof mass for the multimorph structure are 90 kΩ and 35 kΩ respectively as shown in Figure 7-17. If the resistive load is maintained at 35 kΩ, an output power of 75 µW is produced. Although the value is not up to its maximum, it is good enough to meet the minimum requirement at 60 µW. In an experiment where the sample was excited with different acceleration levels, the open circuit voltage increases with acceleration level as expected, from about 1.5 V for an acceleration level of 0.1 g to about 5.8 V for an acceleration of 0.6 g, as shown in Figure 7-18. The increment of acceleration level, however, is limited to the maximum allowed stress of 77.0 MPa for the resonant structure before it suffers fracture. With a proof mass of 0.38 g, the maximum allowed acceleration level, according to equation 7-11, is 0.89 g. 174 Figure 7-16: Comparison of output power between excitation with and without proof mass for the same multimorph sample at connection 1;4 short 2+3. Figure 7-17: Output power as a function of resistive load for the multimorph sample when excited with and without proof mass.