The primary purpose of this paper is to provide a comprehensive review on the response time of magnetorheological (MR) dampers. Rapid response time is desired for all real-time control applications. In reviewing the literature, a detailed description of the response time of semi-active dampers is seldom given. Furthermore, the methods of computing the response time are not discussed in detail. The authors intend to develop a method for the definition and the experimental determination of the response time of MR dampers. Furthermore, parameters affecting the response time of MR dampers are investigated. Specifically, the effect of operating current, piston velocity, and system compliance are addressed. Because the response time is often limited, not by the response of the fluid itself, but by the limitations of the driving electronics and the inductance of the electromagnet, the response time of the driving electronics is considered as well. The authors define the response time as the time required to transition from the initial state to 95% of the final state. Using a triangle wave to maintain constant velocity across the damper, various operating currents ranging from 0.5 to 2 A were applied and the resulting force was recorded. The results show that, for a given velocity, the response time decreases as the operating current increases. Results for the driving electronics show the opposite trend: as current increases, response time increases. To evaluate the effect of piston velocity on response time, velocities ranging from 0.1 to 4 in s −1 were tested. The results show that the response time decreases exponentially as the velocity increases, converging on some final value. Further analysis revealed that this result is an artifact of the compliance in the system. To confirm this, a series of tests was conducted in which the compliance of the system was artificially altered. The results of the compliance study indicate that compliance has a significant effect on the response time of the damper.

No abstract

The objective of this work is to investigate the magnetorheological (MR) effect at high flow velocities. A slit-flow rheometer has been built which allows for high speed testing of MR fluid under varying field strengths. The gap size of the rheometer was chosen to achieve high fluid velocity and high shear rates. With a 1 mm gap size, fluid velocities range from 1 to 37 m s−1 with resulting shear rates ranging from 0.07 × 105 to 2.5 × 105 s−1. In order to evaluate the performance of the fluid, the force required to drive the fluid through the flow channel is measured and force–velocity characteristics are generated. From the force–velocity curves, the apparent viscosity is found. The apparent viscosity is used to calculate the yield stress for several magnetic field strengths. Two MR valve lengths are considered (25.4 and 6.35 mm). At each velocity the yield stress is found using the closed form solution for the non-dimensional yield stress. Fluid dwell time is introduced as the amount of time the fluid spends in the presence of a magnetic field. For the range of velocities considered, fluid dwell times range from 12.4 to 0.18 ms. A reduction in apparent yield stress is observed as dwell time decreases. Results indicate that the MR fluid can achieve 63.2% of the expected yield stress for dwell times greater than 0.6 ms.

No abstract

Various control policies, such as skyhook and groundhook control, have often been considered for semi-active vehicle suspensions. Past studies have shown the performance limitations of these policies, as well as others that have been considered for vehicle applications. This study will provide a look into an alternative control technique called "hybrid control," which attempts to merge the performance benefits of skyhook and groundhook control. The results of this study are based on an experimental evaluation of hybrid control using a quarter-car rig and a magneto-rheological damper. The control policy is employed and evaluated under a steady-state or pure tone input, and a transient or step input. Peak-to-peak displacement and peak-to-peak acceleration are used to evaluate performance. The results indicate that hybrid control can offer benefits to both the sprung mass and the unsprung mass. The steady-state results reveal that hybrid control can be used to reduce the peak-to-peak displacements and accelerations of both bodies. The transient evaluation shows that hybrid control can be effective at reducing the peak-to-peak displacement of the sprung mass.