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In this study it is designed and implemented a robust control technique to suppress excessive vibration and also perform health monitoring of a structure using a single piece of PZT material. The main idea is to use sliding mode controller to achieve the vibration suppression, impedance-based structural health monitoring technique, and to implement sliding mode observer to prevent any undesirable interaction between the health monitoring system and the control system, when they are implemented together. The usage of smart materials to perform nondestructive assessment of structures is of great interest to civil engineering and other engineering fields, since the integrity of the structure is not compromised. In addition, the modern each day more flexible structures bring to researchers in the field the need to find alternatives in vibration suppression. Implementing health monitoring together with vibration control is therefore the combination of these two need in a practical and economic manner, in the sense that uses the smart materials and control devices/techniques.
Smart materials have the ability of changing their properties when subjected to certain external conditions, such as electric or magnetic fields, for example. In this study the smart material used is in the form of piezoelectric patches. The health monitoring technique used in this research is the impedance-based health monitoring, which is based in changes in the structural impedance. The fundamental idea of this approach is to screen the changes in structural mechanical impedance brought on by damage. An experimental set up was performed in a free-free aluminum bar in order to check the applicability of this method. In this experiment, five pairs of PZT patches had been attached to the bar and an electrical impedance analyzer measured the electrical impedance. Damage was simulated by attaching two ten millimeters bolts between the third and the fourth PZT patch. As far as the control technique goes, it is implemented a sliding mode controller, which is designed to attain a robust vibration control performance in the presence of the uncertainties and disturbances.
Sliding mode control is a nonlinear control strategy that adjusts the dynamics of a nonlinear system by forcing the system to slide alongside a surface of the system’s regular behavior. One problem this study has to deal with is the health monitoring signal that enters the feedback control loop, since the same PZT patch is used to simultaneously perform health monitoring and to sense the control system in the feedback control loop. In order to try to solve this issue, a sliding mode observer is designed. In order to evaluate the feasibility of the method proposed, both experimental and numerical studies were performed to a single cantilever beam. The controller and observer were designed based on the four lower vibration modes of the beam.
The experimental set-up performed to assess the impedance-based health monitoring technique showed that a great change in impedance happens in the region where damage occurs. The experimental and numerical results showed the sliding mode observer was capable of filter the high frequency content that comes from the health monitoring, and the sliding mode control was able to suppress successfully the excessive vibration even after disturbances were introduced. The results from both numerical study and experimental set-up show that this integrated approach can provide significant vibration suppression, while simultaneously detecting damage.
The research is important in the sense that brings together the concepts of health monitoring and structural control, and confirms the feasibility of this using sliding mode control/observer and impedance-based health monitoring. However, the study has that are some weaknesses, for example, not much detail is providing regarding the health monitoring of the experimental set-up of the cantilever beam, only vibration suppression results. Also, is not very clear if the results shown are from experimental or analytical evaluation. Only impulse response was verified in this study, and it would be interesting to see how the control behaves in the presence of different types of more complex loads, such as seismic. It is not practical or economically possible to use 4 to 5 PZT patches to every beam a complex civil engineering structure, so the study fails to address how to choose the location of the patches so they can be enough to suppress vibration and detect damage to a more complex structure, such as a building.
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