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