| Magnetorheological (MR) fluids possess the unique ability to undergo dramatic and nearly completely reversible changes in their rheological properties under the application of a magnetic field. These controllable fluids can serve as quiet, rapid interfaces between electronic controls and mechanical systems. A systematic engineering methodology for designing MR devices is found to be lacking in the literature, which can be attributed to the lack of understanding of the fluid behavior and existence of models to simulate MR device performance for long time scale applications. This dissertation specifically investigates the dynamics of MR fluids during long periods of time, and develops new models for predicting the output torque in MR torque transfer devices, such as clutches and brakes, as a function of magnetic field, rotational speed, geometry and time. A MR clutch was then designed and built so that the output torque could be measured and compared to the theoretical predictions.; The linear Bingham plastic model, which assumes a constant viscosity, has been used to describe the behavior of MR fluids. In this research, the shear thinning behavior exhibited by some MR fluids was incorporated by using the Herschel-Bulkley model, which was modified to take into account changes in fluid properties with time. In order to determine the parameters for this proposed modification of the Herschel-Bulkley model, a MR fluid was tested in a rheometer at constant magnetic fields and shear rates for a duration of two hours. A new test fixture was designed and built so that a magnetic field could be applied to the fluid during the rheometer testing. New insights were gained into the MR fluid behavior, as the fluid experienced separation and a decreased effective shearing area in the parallel plates configuration under which it was tested.; The modified Herschel-Bulkley model was used to develop equations to more reliably predict the output torque for various configurations of MR torque transfer devices. A parallel disk type MR clutch was then successfully designed, built and operated. The clutch utilized a stationary electromagnetic coil to generate the required magnetic field, which was uniform over the active portion of the clutch, easily controllable by adjusting the current passing through the coil, and provided a large range of field strength values. The measured output torque was generally in good agreement with the predicted values, after using a decreased effective radius to take into account the observations made from the rheometer experiments of a decreased shearing area. Although the parameters of the modified Herschel-Bulkley model could not be determined from the rheometer experiments, they were estimated from a curve-fitting of the experimental clutch data, and the values indicated that not only are they functions of time, as originally proposed, but they appear to be functions of magnetic field as well. |
