Design, Synthesis and Evaluation of Twin-Tube Valve Mode Magneto-Rheological (MR) Damper for Semi-Active Automotive Suspension System

Abstract

The change in rheological properties of smart materials like magneto-rheological (MR) fluid when brought under the influence of a magnetic field can be utilized to develop MR devices where the output has to be continuously and quickly varied using electronic control interface. A viscous damper which uses this MR fluid as the viscous medium is called as MR damper and the damping force generated by the MR damper can be varied by modulating the current given to the electromagnetic coil in the MR damper. Hence MR dampers have electronically controlled variable damping co-efficient and have a promising future for application in automotive semi-active suspensions. The main aim of the present work is to explore the design and application of twin-tube valve mode MR damper for use in automotive semi-active suspension. A viscous damper of twin tube design working in the passive mode is evaluated on a damper testing machine and its performance is characterized. Its behaviour at different velocities and frequencies of excitation is studied using sinusoidal excitations of fixed amplitude and varying frequencies. The force vs displacement plots show that the size of the loops increase with increase in frequency of excitation. Using the dissipated energy method, the equivalent damping coefficient of the damper is calculated for different frequencies of excitation. Mathematical models are developed for the force vs velocity behaviour of the damper as the damper force is a function of the damper piston velocity. These mathematical models based on the experimental results will be very much useful in designing an alternative or improved suspension system for the vehicle under consideration. A commercial MR damper, RD-8040-1 by Lord Corporation, USA, is experimentally evaluated for its application in a semi-active suspension. The experiments were carried out in damping force testing machine. Sinusoidal displacement input was given to the test damper. The set of experiments were repeated for different levels of current (0A to 1.5A in steps of 0.25A) supplied to the MR damper. Plots of force vs displacement for each frequency of excitation and plots of maximum force vs frequency of excitation show that higher values of current leads to elevated values of MR damper forces. This increase of MR damper force with current supplied is studied and analysed to develop a mathematical model of the MR damper under investigation. The non-linear softening hysteretic behaviour of the MR damper is simulated by using Genetic Algorithm (GA) provided in the optimization toolbox of MATLAB. Calculations on energy dissipation and iv equivalent damping coefficient of the MR damper show that the same damper can make the suspension system behave as an underdamped system, critically damped system or overdamped system depending on the value of current supplied to it. The performance of this MR damper in a spring-mass vibrating system is studied with the help of MATLAB simulations. A commercially available passive damper of a passenger van is tested to find the characteristic damping requirement of the vehicle. With this as reference, a twin tube MR damper working in valve mode is designed and fabricated. The magnetic flux density induced in the fluid flow gap is maximized using Taguchi analysis and Finite Element Method Magnetics (FEMM) software. The FEMM results are validated with results obtained analytically using electromagnetic circuit theory. The MR damper filled with commercially available MR fluid was experimentally tested in damper testing machine. The results demonstrate that the force developed by the MR damper is indeed increasing with the value of the current supplied. At various frequencies of input oscillation, the energy dissipated by the MR damper in a single cycle increases significantly with current supplied. The novelty of this work is that a twin tube MR damper working in valve mode was designed as a replacement for the passive damper used in a passenger van. The MR damper thus developed is capable of producing practical levels of damping force at actual operating frequencies and amplitudes of the passive damper in the passenger van. For further analysis, the behaviour of the MR damper is modelled by using the Bouc-Wen model for hysteretic systems. A Proportional-Integral-Derivative (PID) controller is used to track the desired damping force in time domain to demonstrate the application of the MR damper in a semi-active suspension system. MR fluid was synthesized to be used as a smart fluid in a twin tube MR damper operating in valve mode. The behaviour of the MR fluid is experimentally characterized in a rheometer and mathematically modelled using Herschel-Bulkley (HB) model. The parameters of the HB model are expressed as polynomial functions of strength of magnetic field in order to find the shear stress developed by MR fluid at any given strength of magnetic field applied. This fluid is filled in the MR damper which was designed for application in a passenger van and it is tested in damper testing machine. The performance of the damper at different damper velocities and current supplied is studied. The range of values for the parameters of the experimental testing are chosen to emulate the actual conditions of operation in its intended application. Non-dimensional analysis is performed, which links MR fluid rheological properties and geometrical parameters of MR damper v design with the force developed by damper. Finite Element Method Magnetics (FEMM) is used to find the strength of the magnetic field at the fluid flow gap. Analytical methods are used to calculate the damper force developed due to the field dependent yield stress and compared with experimental force values. The resulting dynamic range of the MR damper is also assessed. A mathematical model of the quarter car suspension is built for numerical simulation to compare the performance of semi-active suspension and passive suspension. Vibrations coming from the wheel due to road roughness and unevenness are given as input displacement to the suspension model. Sinusoidal excitation and random road excitation are used as input displacement. Based on experimental characterization, mathematical models are developed for the hysteresis behaviour of commercial and twin-tube MR damper using a polynomial model of hysteresis. These are used for implementing skyhook control in the semi-active suspension model. The current given to the MR damper is varied in order to achieve the best ride comfort which is demonstrated as a reduction in the sprung mass acceleration of the quarter car suspension. The dynamic behaviour of the MR damper based semi-active suspension is studied using MATLAB Simulink to show that its performance is better than passive suspension. The twin-tube MR damper working in valve mode is further developed for application in a semi-active SUV suspension system. In order to prove the superiority of semi-active suspension, a single degree of freedom quarter car test rig is built and ground excitation is given in the form of displacement input from a hydraulic actuator. Constant current control, Skyhook control and Rakheja-Sankar (RS) control are employed as three different control strategies and compared with passive suspension to study the advantages. Peak acceleration response of the sprung mass is studied for better passenger ride comfort and peak ground force is studied for preventing damage to road surface as well as to vehicle suspension elements. RS control method provides better ride comfort to passengers due to lower peak vertical acceleration when compared to constant current control or Skyhook control method. RS control method also generated much lower peak ground force values when compared to Skyhook control, especially in the high frequency region.