Alberto Belloli

Structural Vibration Control via R-L Shunted Active Fiber Composites

This article presents a successful extension of passive R-L shunt damping to piezoelectric ceramic elements working in direct 3-3 mode and a performance comparison to elements working in indirect 3-1 mode. A new circuit topology is implemented to synthesize the very large inductances required by the low inherent piezoelectric device capacitance at relatively low frequencies. This allows for efficient tuning of the R-L circuit to the structure resonance frequency to be damped. The vibration suppression performance of monolithic piezoelectric ceramic actuators and active fiber composites is compared in this study. For this purpose, different actuators are bonded on aluminum cantilever plates. An integrated FE model is implemented for the prediction of structure resonance frequencies, optimum values for electric components, and the resulting vibration suppression performance. The passive structure, bonded active patch, and shunted electrical network are analyzed within the same FE model. Active fiber composite patches working in the direct 3-3 mode show equivalent specific damping performance compared to conventional monolithic 3-1 actuated patches. Issues related to the sensitivity of R-L shunts to variations in environmental and operational conditions are discussed in this study. In short, monolithic actuators operating on the 3-1 piezoelectric effect seem to be the best for use in R-L shunting.

Novel Characterization Procedure for Single Piezoelectric Fibers

The characterization of the ferroelectric properties of piezoelectric ceramic fibers is paramount for optimizing their manufacturing processes, for quality control purposes, and for modeling the response of components and structures. Until now, fibers were generally characterized by measuring the so-called 1-3 composites, fiber arrays embedded in a polymer matrix. The fiber properties can then be extracted, provided the volume fraction and stiffness of each phase, the fiber piezoelectric charge constant as a function of the electrical field strength, and the matrix permittivity are known. This implies a large amount of time and experimental effort. This article presents a comprehensive procedure for the direct characterization of single piezoelectric ceramic fibers in terms of butterfly and polarization loops, as well as their blocking force. The experimental setup is composed of a waveform generator, a high-voltage amplifier, a dynamic mechanical analyzer, a current/charge measuring circuit, and an oscilloscope. The active circuitry used for reliably collecting the charge generated by a single fiber is presented in full detail. The very good repeatability of the measurements showed the proposed procedure to be robust. The comparison between single fiber measurements and the investigation of 1-3 composites revealed both procedures to be equal, at 99.9%, in determining the average strain and polarization properties. In addition, the single fiber measurement provides an estimation of the variation in fiber properties within a single production batch. This information is essential to understand how to optimize processing routes and build robust devices.

Ferroelectric Characterization of Single PZT Fibers

In a previous work, the authors have presented a comprehensive procedure for the direct characterization of single piezoelectric ceramic fibers in terms of butterfly and polarization loops and blocking force. The ability to investigate single fibers is relevant for optimizing their manufacturing processes, for quality control purposes, and for modeling the response of components and structures. In this study the novel testing procedure is used to characterize commercially available fibers distributed by Advanced Cerametrics Inc., CeraNova Corp., and Smart Material Corp., respectively, and to compare their performance with fibers developed at Empa. Their porosity, grain size and phase composition were investigated to correlate the ferroelectric properties with the microstructure. Fibers supplied by Smart Material Corp. exhibited the best ferroelectric performance, in particular the highest saturation and remnant polarization, the lowest coercive field and the highest P—E loop squareness. The said properties result from a low porosity, a sufficiently large grain size and a phase composition near the morphotropic phase boundary. After removing a surface layer dominated by a rhombohedral phase, Empa fibers developed maximum average free-strains 15% larger than any commercially available fiber. Better control of the sintering atmosphere thus promises to be the key to very high performance fibers.

Modeling and characterization of active fiber composites

The scientific community has put significant efforts in the manufacturing of sensors and actuators made of piezoceramic fibers with interdigitated electrodes. These allow for increased conformability and actuation capability at high field regime. The prediction of their coupled field behavior, however, is so far limited to low field applications, where the piezoelectric coupling coefficient is assumed to be constant. An approach, which takes into account the strain driven nonlinearity of a representative work cycle at high field regime is still lacking. This study presents a nonlinear Finite Element Model to simulate the free strain properties of Active Fiber Composites (AFCs) under high electric field conditions. Input data for the fully parametric model are the Representative Volume Element (RVE) geometry and the material properties of its piezoceramic and epoxy resin components. The high field properties of single PZT fibers under free strain conditions were determined using a novel characterization procedure. Free strain properties of the actuators were measured experimentally, and important geometrical parameters (contact angle between the fiber and the electrode, average spacing between the fibers) were measured using micrographical imaging. The results of the simulation show good agreement with the free strain measurements, allowing for prediction of a representative work cycle hysteresis. The influence of important geometrical parameters on the actuator properties such as electrode spacing and electrode-fiber contact angle was investigated both numerically and experimentally.

Vibration control via shunted embedded piezoelectric fibers

The scientific community has put significant efforts into the manufacturing of sensors and actuators made of piezoceramic fibers with interdigitated electrodes. These allow for increased conformability, integrability in laminate structures and offer high coupling factors. They are of particular interest for damping applications. This paper presents a comparison between piezoceramic monolithic actuators and Active Fiber Composites (AFCs) for shunt damping. For this purpose, the different actuators were bonded on aluminum cantilever plates, respectively embedded in a glass fiber composite cantilever plate. The vibration suppression was attained by converting the electric charge by means of the converse piezoelectric effect and dissipated through robust resonant shunt circuits. A new circuit topology was used, which enables efficient damping even with low piezoelectric capacitance. An integrated FE model was implemented for prediction of the natural frequencies, the optimum values for the electric components and the resulting damping performance. Patches working in the direct 3-3 mode show much better specific damping performance than the 3-1 actuated patch. The comparison between monolithic and AFC actuators shows that AFCs fulfill integrability and performance requirements for the planned damping applications.

Optimum placement of piezoelectric ceramic modules for vibration suppression of highly constrained structures

The vibration suppression efficiency of so-called shunted piezoelectric systems is decisively influenced by the number, shape, dimension, and position of the implemented piezoelectric ceramic elements. This paper presents a procedure based on evolutionary algorithms for optimum placement of piezoelectric ceramic modules on real-world, highly constrained lightweight structures. The optimization loop includes the CAD software CATIA V5, the FE package ANSYS and DynOPS, a proprietary software tool able to connect the Evolving Object library with any simulation software that can be started in batch-mode. A user-defined piezoelectric shell element is integrated into ANSYS 8.1. Modal generalized electromechanical coupling coefficients are used as optimization objective and constraints. Position, dimension and shape of commercial, customized and free-form patches are determined for optimum multi-mode vibration suppression of a pinned, quadratic plate. An aircraft fuselage panel with a window cutout is investigated as test object for complex, curved geometries.Copyright