Abstract

Magneto-rheological (MR) fluid dampers (MRFDs) are adaptive semi-active devices that hold great promise for real-time vibration control applications due to their field-dependent damping properties, fail-safe feature, fast response (in milliseconds), and low power requirement. While MRFDs have been successfully developed for on-road vehicles and civil engineering applications, very limited studies have been conducted on the development of MRFDs for off-road and particularly tracked vehicles. Furthermore, MRFDs with combined annular and radial bypass fluid channels have recently shown superior performance compared with those conventional MRFDs with single annular/radial fluid gaps. However, limited studies have been dedicated toward developing such MRFDs with the combined fluid flow path. There are also limited research studies on MRFDs with variable damping and variable stiffness (VSVD), which can significantly enhance the vibration isolation performance as compared with traditional variable damping MRFDs. Many key aspects of VSVD-MRFDs, such as design, physic-based modelling, and experimental characterization, have yet to be addressed. The existing VSVD devices are complicated in design and have a limited capacity to be implemented in practical applications.
To address the above-mentioned knowledge gaps, in this research dissertation, a large-scale bypass MRFD with an annular-radial valve was designed and developed. The developed MRFD possesses VSVD properties and can be implemented in off-road tracked vehicles. For this purpose, first, a quasi-static physic-based model of the proposed MRFD was formulated using the Bingham plastic characteristics of the MR fluids. The magnetic circuit of the MR valve (MRV) was analytically formulated to evaluate the magnetic flux densities in MR fluid gap regions. Magneto-static finite element model of the MR valve has also been conducted to verify the analytical results. A design optimization problem was subsequently formulated to identify the optimal geometrical parameters of the MRV to maximize the damper dynamic range under specific volume, geometrical and magnetic field constraints. The optimized MRV can theoretically generate an on-state damping force and high dynamic range of 7.4 kN and 6.7 under a piston velocity of 12.5 mm/s. The damper also has a large piston stroke of 180 mm that makes it suitable for off-road vehicle applications.
In the next step, a physic-based model was developed to theoretically investigate the non-linear dynamic behaviour of the proposed MRFD. The developed physic-based model, which is also based on MR fluid Bingham behaviour, can consider the unsteady behaviour of the MR fluid. In contrast to widely use phenomenological models, which are experiment based in which their characteristic parameters must be identified from experimental data, the proposed dynamic model depends only on the physics of the problem with no parameters to be identified experimentally. This is of paramount importance for the analysis and design of MRFDs at early design stages. Results from the model suggest that the MRFD experiences non-linear hysteresis behaviour due to the unsteady MR fluid behaviour at high loading conditions (e.g., large deformation and high frequency). The dynamic model was further modified to consider the fluid compressibility effect, which has been mostly neglected in previous studies, despite its significant contribution to the hysteretic response of MRFDs at low-velocity regions.
The proposed MRFD was fabricated and experimentally characterized to validate the design optimization strategy and examine the developed quasi-static, dynamic, and modified dynamic models. Extensive experimental tests were conducted to investigate the dynamic characteristics of the proposed MRFD considering wide ranges of excitation frequency, loading amplitude, and electrical currents. Figure of merits, including equivalent viscous damping and dynamic range obtained from experiential data under varied loading conditions were in good agreement with those obtained theoretically. Results also suggest the proposed modified physic-based dynamic model could provide an accurate description of the non-linear hysteresis behaviour of the MRFDs observed experimentally.
Finally, the developed bypass MRFD has been designed and integrated with a mechanical spring to realize VSVD capability for the MRFD. The proposed bypass VSVD-MRFD with an annular-radial gap was also experimentally characterized. The dynamic characteristics of the VSVD-MRFD were conducted under a wide range of excitation frequency, loading amplitude, and electrical current. The force-displacement, and the force-velocity hysteresis curves were obtained. Both the equivalent stiffness and damping, dynamic range, and their dependency on the loading conditions were investigated. The results revealed that the VSVD-MRFD is capable of adjusting its stiffness and damping properties to a large extent, thereby providing a high damping force and dynamic range.
The developed dynamic physic-based model can provide an essential guidance on development of bypass MRFD with annular-radial gap at early design stages. The proposed novel VSVD-MRFD can also be potentially employed in off-road suspension systems for suppressing the vibration amplitude under unexpected loading conditions.