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A prestressed bolt connection is one of the crucial connection types in timber structures. The daily checking and maintenance of bolt connections have to be carried out in order to avoid the collapse of timber structures due to bolt looseness. Real-time health monitoring of bolt connections can not only reduce the daily maintenance cost of timber structures, but it can also avoid property loss and casualties by giving early warning if the bolt connection is loosened in timber structures. This paper proposes a method of prestress monitoring of bolt joints in timber structures by pasting lead zirconate titanate (PZT) patches on the surface of timber structures, and the time-reversal method is applied to denote the connection status of bolts in timber structures. The prestress loss index of timber structural bolts based on wavelet analysis is designed to quantify the bolt looseness of the timber structure. The experimental timber specimen was fabricated consisting of two timber panels, one bolt, and two PZT patches. One of the PZT patches acted as an actuator to emit the stress waves, and another one acted as a sensor to receive the stress wave propagating through the connection interface. The experimental results showed that the amplitude of the focused signal increases significantly with the increase of the prestress value of the bolts, which verify that the proposed method can be utilized to monitor the looseness of bolts in timber structures. The analysis results of the focused signal is proof that the prestress loss index of timber structural bolts designed based on wavelet analysis can reflect the looseness of timber structural bolts.

SUMMARYThis paper deals with the motion prediction and control of the macro–micro parallel manipulator system for a 500-m-aperture spherical radio telescope (FAST). Firstly, based on principles of parallel mechanism, a decoupled tracking and prediction algorithm to predict the position and orientation of the movable macro parallel manipulator is presented in this paper. Then, taken as the upper layer supervisory controller in the joint space of the micro parallel manipulator, the adaptive interaction PID controller utilizing the adaptive interaction algorithm to adjust the parameters of a canonical PID controller is discussed. In addition, the digital servo filters with feedforward are employed in the linear actuators as the lower layer controllers. Experimental results of a one-tenth scale FAST field model validate the effectiveness of the supervisory controller and the motion prediction algorithm.

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Nonlinear tri-stable piezoelectric energy harvesters with magnetic field coupling have attracted many researchers’ interest in ambient vibration energy harvesting and conversion. In such a tri-stable piezoelectric energy harvester, the nonlinear magnetic force generated by the external magnetic field is mainly calculated by the equivalent magnetic dipole method. However, this method will give highly erroneous results when the magnetic separation distance is small. This study presents a modified magnetic force model to precisely calculate the nonlinear magnetic force exerted on the tip of the cantilever beam. Unlike the equivalent magnetic dipole method regarding the magnet as a point dipole at its body center, this model only considers the surface magnetization current of the left and right surfaces of the magnets and replaced them by the one of the center point dipoles on these associated surfaces. With this model, the nonlinear magnetic force, the potential energy, and the transition mechanism of the tri-stable piezoelectric energy harvester are numerically investigated. Experimental validations are consequently performed to testify the numerical results. Compared with the equivalent magnetic dipole method, the modified magnetic force model has a much higher accuracy to be more applicable for different magnetic separation distances, especially when the magnetic separation distance is small.

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