Macro-Fiber Composites for Sensing, Actuation and Power Generation
The research presented in this thesis uses the macro-fiber composite (MFC) actuator that was recently developed at the NASA Langley Research Center for two major themes, sensing and actuation for vibration control, and power harvesting. The MFC is constructed using piezofibers embedded in an epoxy matrix and coated with Kapton skin. The construction process of the MFC affords it vast advantages over the traditionally used piezoceramic material. The MFC is extremely flexible, allowing it to be bonded to structures that have curved surface without fear of accidental breakage or additional surface treatment as is the case with monolithic piezoceramic materials. Additionally the MFC uses interdigitated electrodes that capitalize on the higher d33 piezoelectric coupling coefficient that allow it to produce higher forces and strain than typical monolithic piezoceramic materials. The research presented in this thesis investigates some potential applications for the MFC as well as topics in power harvesting.
This first study performed was to determine if the MFC is capable of being used as a sensor for structural vibration. The MFC was incorporated into a self-sensing circuit and used to provide collocated control of an aluminum beam. It was found that the MFC makes a very accurate sensor and was able to provide the beam with over 80% vibration suppression at its second resonant frequency. Following this work, the MFC was used as both a sensor and actuator to apply multiple-input-multiple-output vibration control of an inflated satellite component. The control system used a positive position feedback (PPF) controller and two pairs of sensors and actuators in order to provide global vibration suppression of an inflated torus. The experiments found that the MFC and control system was very effective at attenuating the vibration of the first mode but ineffective at higher modes. It was found the positioning of the sensors and actuators on the structure contributed heavily to the controller's performance at higher modes. A discussion of the reasons for the controller's ineffectiveness is supply and a solution using self-sensing techniques for collocated vibration suppression was investigated.
Subsequent to the research in vibration sensing and control, the ability to use piezoelectric materials to convert ambient vibration into usable electrical energy was tested and quantified. First, a model of a power harvesting beam is developed using variational methods and is validated on a composite structure containing four separate piezoelectric wafers. It is shown that the model can accurately predict the power generated from the vibration of a cantilever beam regardless of the load resistance or excitation frequency. The damping effects of power harvesting on a structure are also demonstrated and discussed using the model. Next, the ability of the piezoelectric material to recharge a battery and a quantification of the power generated are investigated. After determining that the rechargeable battery is compatible with the power generated through the piezoelectric effect, the MFC was compared with the traditional monolithic PZT for use as a power harvesting material. It was found that the MFC produces a very low current, making it less efficient than the PZT material and unable to charge batteries because of their need for relatively large current. Due to the MFC being incapable of charging batteries, only the PZT was used to charge batteries and the charge times for several nickel metal hydride batteries ranging from 40 to 1000mAh are supplied.