Magnetorheological (MR) damper is one of the most promising smart devices for dissipating seismic energy and reducing structural vibrations. The MR dampers with multiple coils are widely adopted to enhance the output damping forces and seismic performance. In order to study the dynamic characteristics of multicoil MR dampers from a micro to macro viewpoint, a three-coil MR damper was designed and manufactured in this paper. The performance tests of the three-coil MR damper under different excitation currents, displacement amplitudes, frequencies, and coil combinations were conducted.Assuming that the distance between two adjacent ferromagnetic particles meets chi-square distribution, the two-column micromodel for the MR fluid was modified by introducing a distribution parameter. The modified MR fluid micromodel was verified by comparing the damping forces calculated by this model with experimental data. Combining the modified micromodel for the MR fluid and the double-sigmoid model for MR dampers, a mathematical model for the three-coil MR damper was proposed. The microstructure-based sigmoid model considers the effects of yield shear stress of the MR fluid and internal magnetic induction intensity on the damping force. The comparison of the proposed model and the performance test data shows that the microstructure-based sigmoid model can finely describe the dynamic characteristics and magnetic saturation properties of the three-coil MR damper under different excitation currents and loading conditions, which provides a good basis for control strategies and dynamic analysis of structures with multicoil MR dampers. K E Y W O R D S microstructure-based sigmoid model, model verification, modified micromodel, performance tests, three-coil magnetorheological damper 1 | INTRODUCTIONMagnetorheological (MR) fluid is a typical intelligent material, which is a suspension formed by the distribution of magnetic particles in micron or nanometer size in a carrier liquid. It mainly contains magnetic particles, carrier liquid, and various additives. The most significant feature of the MR fluid is its magnetorheological effect; that is, the magnetic particles in the MR fluid can be transformed from a random disordered state to ordered chain or columnar structure in

No abstract

Magnetorheological (MR) damper is a semi-active control device designed by utilizing the instantaneous fluid-solid conversion characteristics of MR fluid, thus the microstructure of MR fluid fundamentally determines the mechanical properties of MR dampers. In order to study the influence of MR fluid microstructure on the macroscopic mechanical properties of MR dampers, a micro-macro mathematical model for MR dampers was proposed to describe the dynamic properties of MR dampers affected by the microstructure of MR fluid. Firstly, the micromodel of MR fluid was brought into classic quasi-static model and the double-Sigmoid model to propose a mathematical model, which considers the MR fluid microstructure by expressing the yield force parameter in the traditional double-Sigmoid model with the microstructure parameters of MR fluid. By analyzing the data of the performance test of a single-coil MR damper, the parameters of the proposed mathematical model were fitted. The proposed micro-macro model for MR dampers was verified by comparing the results calculated by this model with the performance test data. Based on the proposed micro-macro mathematical model, the nonlinear hysteretic curves with different MR fluid microstructure parameters can be numerically analyzed and compared. Finally, the influences of the volume fraction, size, and coating thickness of ferromagnetic particles on the mechanical properties of MR dampers were revealed and discussed. The research can provide guidance for the preparation and formulation optimization of high-performance MR fluid.

The bridge stay cable, one of the most critical components in cable-stayed bridges, is vulnerable to vibrations owing to its low inherent damping capacity. Thus effective vibration control technology for bridge stay cables is extremely critical to safe operations of cable-stayed bridges. Several countermeasures have been presented and/or implemented to mitigate this vibration; however the passive method can only add a small amount of damping to the cables, excessive energy demand of active control devices severely limits its practicality, the semi-active control methods still have the drawbacks of complex state estimation module and a large amount of control algorithm calculation. This paper proposes a practical magnetorheological pseudo negative stiffness (MR-PNS) control system coupled with control strategy for bridge stay cables. The current reference point is introduced in the dynamic modeling of the MR-PNS control system to characterize the current control strategy. This paper investigates the adjustable of MR-PNS control system performance and energy consumption caused by different current strategies. Taking the vibration control of the Nanjing Second Yangtze River Bridge J20 cable as an example, the simulation results highlight the advantages of the MR-PNS control system that the failure area is small, the quasi-optimal area is wide, and it can still keep sort of vibration damping performance in the degenerate area. The model cable vibration control test proves the feasibility and efficiency of the single input MR-PNS bridge stay cables control method.

Magnetorheological damper is a typical semi-active control device. Its output damping force varies with the internal magnetic field, which is a key factor affecting the dynamic performance of the magnetorheological dampers. Existing studies about the magnetic field of magnetorheological dampers are limited to theoretical analysis; thus, this study aims to experimentally explore the complicated magnetic field distribution inside the magnetorheological dampers with multiple coils. First, the magnetic circuit of a three-coil magnetorheological damper was theoretically analyzed and designed, and the finite element model of the three-coil magnetorheological damper was set up to calculate the magnetic induction intensities of the damping gaps in different currents and numbers of coil turns. A three-coil magnetorheological damper embedded with a Hall sensor was then manufactured based on the theoretical and finite element analysis, and internal magnetic field tests under different conditions were carried out to obtain the actual magnetic induction intensities. At last, the magnetic field coupling model of the three-coil magnetorheological damper was proposed by introducing a coupling coefficient to describe the complex magnetic field distribution due to the strong coupling effect of the three coils, and the results calculated by the proposed model agreed well with the finite element analysis and magnetic field test data. The proposed model lays a foundation for the optimal design of the magnetic circuit and the mathematical model of multi-coil magnetorheological dampers.