Piezoceramic Sensors/Actuators with Interdigitated Electrode Patterns
Monolithic piezoelectric ceramic devices are well understood and employed in a wide variety of structural actuation and sensing applications. Over the last twenty years piezoceramic fiber composites with interdigitated electrodes (IDE) have fallen into favor. These piezocomposite devices with IDE have been shown to be more conformal, durable and responsive than conventional monolithic devices. One of the more prevalent piezocomposite devices with IDE is the Macro Fiber Composite (MFC) developed at the NASA-Langley Research Center. The MFC shows superior free strain actuation performance, manufacturability and reliability over conventional devices.
While the MFC boasts some improved characteristics over conventional devices, the use of IDE also introduces added complexity. Simple in design, the IDE causes nonuniform electric fields, large electric field gradients and increased hysteresis in the device. Characterization and modeling efforts of the MFC beyond a linear approximation have been limited. The majority of published work relies on experimental quantification and a heavy reliance on linear finite element analysis. The MFC has significant time dependent effects, conduction issues, creep, and other nonlinear effects that have not been explored.
This study serves as an attempt to rectify some of the previously overlooked issues of non-linearity in piezocomposite actuators with IDE. As a baseline of comparison, the capability of a MFC to serve as a strain sensing/actuating rosette was compared to single crystal PMN-PT. It was found that increased hysteresis and creep caused the MFC to perform poorly by comparison. This spurred the process of seeking improved IDE designs. A twenty actuator study was performed using actuators with different electrode line widths and spacings. It was found that the hysteresis of an actuator with IDE could be reduced, but with the sacrifice of some of the free strain actuation. For accurate strain sensing/actuation, devices with large electrode line widths were found to show less hysteresis. For maximal free strain actuation, devices with small electrode line widths and large electrode spacing were found to have larger free strain actuation. The free strain frequency response was explored for the MFC, and custom devices were designed to mitigate some of the frequency dependence. The IDE devices were characterized using a dielectric finite element model and using a nonlinear ferroelectric finite element model. A phase field model was also developed to explore domain formation along electrode edges.