This physical limit in standard linear behavior is probably the main reason why VEh are not widespread today. Fortunately, solutions to increase VEh frequency bandwidth are currently being investigated.
In fact, two main ways exist in the state of the art to widen VEh frequency bandwidth: use of non-linear effects (passive) and tuning of frequencies (active). We present here, as examples, some solutions currently being studied in our labs.
Non-linear effects are present in all mechanical structures but have only begun to be exploited to increase VEh output. Non-linear effects do not have to be activated by a control circuit – they are passive phenomena. They are added to the VEh structure during manufacturing and appear as soon as springs leave the linear domain (for high amplitude displacements). We have already proven that thanks to non-linear behaviors, VEh output power can be increased by 50 percent compared to standard linear behaviors in some cases (car engines, motors).
Even though non-linear effects have proven attractive to increase VEh output power and reliability, ”tuning of frequencies” is the most promising way to increase VEh frequency bandwidth. Its objective is to change VEh natural frequency by modifying spring stiffness. These changes are controlled by an active circuit aimed at searching optimal parameters to maximize output power. Two main ways are currently under study in our labs on piezoelectric VEh.
The first one consists in using three-layered beams made of two piezoelectric layers and a silicon beam (Figure 7a). Piezo 1 (Figure 7a) is linked to a control circuit that applies a voltage able to modify piezo 1 stiffness and therefore beam resonant frequency. Energy is harvested on piezo 2 (Figure 7a).
The second method is based on electrical load adjustment. When piezoelectric layers have strong coupling coefficients, it is possible to modify layer stiffness (and therefore resonant frequency) by adapting the load (Figure 7b).
Figure 7: Piezoelectric VEh applying tuning of frequency a) by applying an electric field and b) by changing the load
(Click on image to enlarge)
Thanks to these methods, VEh resonant frequency can be tuned over a range representing up to 20 percent of their main resonant frequency.