Fernando D. Goncalves

A comprehensive analysis of the response time of MR dampers

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.

Investigating the magnetorheological effect at high flow velocities

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.

INVESTIGATING THE TIME DEPENDENCE OF THE MR EFFECT

Magnetorheological fluids are known to respond in a matter of milliseconds to the application of a magnetic field. To date, however, very little work has been done to study the time dependence of the MR response. The purpose of this study is to investigate the response time of the fluid. Experiments were conducted on a high shear rate rheometer capable of fluid speeds in excess of 35 m/s. With an MR valve length of 6.35 mm, the resulting dwell times were as low as 0.18 ms. For each of three magnetic field strengths, a reduction in yield stress is observed as dwell time decreases. A model is proposed to represent the time response of the fluid to the application of the magnetic field. The experimental data and the proposed model are used to identify the response time of the fluid for each field strength. Results indicate that as the magnetic field increases, the response time of the MR fluid decreases. For the range of magnetic field strengths considered in this study the response time of the fluid ranged from 0.24 ms to 0.19 ms.

BEHAVIOR OF MR FLUIDS AT HIGH VELOCITIES AND HIGH SHEAR RATES

The objective of this work is to characterize the performance of MR fluid at high shear rates and high velocities. A slit-flow rheometer has been built which allows for high speed testing of MR flu...

HIGH-SHEAR-RATE, VALVE-MODE RHEOMETER FOR MR FLUIDS

We present details of a new, high-shear-rate, valve-mode MR fluid rheometer capable of measurement at rates appropriate to high-speed dampers and shock absorbers. The instrument is modular and allows for choosing from a number of well-defined magnetic valve geometries or a capillary tube. It requires only a modest amount of MR fluid and is easily cleaned to allow rapid testing of different fluid formulations. Of particular importance is the means for applying and accurately determining the magnetic field H applied to the fluid and provision for demagnetizing the system between measurements.

Introducing “Advanced” Tuned Vibration Absorbers in an Undergraduate Vibrations Course

The primary objective of this paper is to bridge the theory of tuned vibration absorbers (TVA) with the practice of implementing TVAs in systems. Often, the practice of implementing TVAs in systems is a far departure from the theory expressed in many textbooks. These departures are often required in practice to account for the less than ideal conditions that the TVAs will be operating under. Many retrofitted TVAs use “smart” or active materials along with various control techniques to improve the performance of the traditional TVA proposed in textbooks. The intent of the current paper is to demonstrate several of these modern methods of implementing retrofitted TVAs to undergraduate students. The first author introduced the methods in a junior level vibrations course, and is developing a laboratory experiment. Teaching these advanced TVAs to undergraduate engineering students will help them understand how theories learned in class are used in real world problems, and motivate them to explore new fields of research. After introducing a “textbook” vibration absorber theory, this paper describes principles and operations of a new class of vibration absorbers. In reviewing conventional TVAs, students are introduced to many of the engineering challenges encountered in the implementation of TVAs. One such challenge is inevitable off-tuning caused by system parameter changes with time. After identifying many of the challenges associated with the implementation of TVAs, the students are introduced to many modern solutions to these problems. Many of these solutions involve the use of smart materials, such as piezoceramics, magnetorheological fluids, magnetorheological elastomers, shape memory alloys, etc. Through this experience, students are introduced to many smart materials and have the opportunity to see how these smart materials can provide solutions to many engineering challenges and improve existing technologies.Copyright