Decoding the intrinsic frequency response behaviors of piezoelectric output current toward advanced sensing and monitoring applications

Piezoelectric materials (PEMs) can convert mechanical energy into electrical energy. Substantial works were performed to optimize PEMs and devices for smart sensing, human monitoring, and medical applications, however, their electromechanical behaviors under such scenarios with different applied frequencies remain unclear. Here, the frequency response behaviors of piezoelectric output current are revealed based on lead-free and lead-based PEMs toward advanced applications. The results show a linear relationship between output current and frequency under short-circuit condition, with the slope varying according to loading force (F) and piezoelectric coefficient (d₃₃). However, when a load resistance (R_L) is applied, a nonlinear relationship is shown: the current increases with frequency, reaches a critical point, and subsequently levels off. The saturation current is influenced by F, R_L, d₃₃, and device capacitance (C), while the critical frequency is related to R_L, C, and action distance. This fundamental behavior of PEMs aligns with Maxwell's displacement current deducing and is well-supported by circuit simulations and physical modeling. The universality of these results is further confirmed through lead-based PEMs. Open-circuit voltage is unaffected by f, while load voltage exhibits the same nonlinear frequency response as load current. Applying these findings by combining machine learning, human posture monitoring and robot motion recognition were achieved. This work gives a deep understanding of intrinsic frequency response behaviors of electromechanical conversion in PEMs and guides the emerging piezoelectric sensing and monitoring applications.