There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Numerical Initial Value Problems in Ordinary Differential Equations

Semiactive control devices have received significant attention in recent years because they offer the adaptability of active control devices without requiring the associated large power sources. Magnetorheological (MR) dampers are semiactive control devices that use MR fluids to produce controllable dampers. They potentially offer highly reliable operation and can be viewed as fail-safe in that they become passive dampers should the control hardware malfunction. To develop control algorithms that take full advantage of the unique features of the MR damper, models must be developed that can adequately characterize the damper's intrinsic nonlinear behavior. Following a review of several idealized mechanical models for controllable fluid dampers, a new model is proposed that can effectively portray the behavior of a typical MR damper. Comparison with experimental results for a prototype damper indicates that the model is accurate over a wide range of operating conditions and is adequate for control design an...

Phenomenological model for magnetorheological dampers

A scheme for understanding the organization of human postural movements is developed in the format of a position paper. The structural characteristics of the body and the geometry of muscular actions are incorporated into a three-dimensional graphical representation of human movement mechanics in the sagittal plane. A series of neural organizational hypotheses limit a theoretically infinite number of combinations of muscle contractions and associated movement trajectories for performing postural corrections: (1) Controls are organized to use the minimum number of muscles; (2) frequently performed movements are organized to require a minimum of neural decision-making.These hypotheses lead to the prediction that postural movements are composed of muscle contractile strategies derived from a limited set of distinct contractile patterns. The imposition of two mechanical constraints related to the configuration of support and to requirements for body stability with respect to gravity predict the conditions under which individual movement strategies will be deployed.A complementary organizational scheme for the senses is developed. We show that organization of postural movements into combinations of distinct strategies simplifies the interpretation of sensory inputs. The fine-tuning of movement strategies can be accomplished by breaking down the complex array of feedback information into a series of scalar quantities related to the parameters of the movement strategies. For example, the magnitude, aim, and curvature of the movement trajectory generated by an individual strategy can be adjusted independently.The second half of the report compares theoretical predictions with a series of actual experimental observations on normal subjects and patients with known sensory and motor disorders. Actual postural movements conform to theoretical predictions about the composition of individual movement strategies and the conditions under which each strategy is used. Observations on patients suggest how breakdowns in individual steps within the logical process of organization can lead to specific movement abnormalities.Discussion focuses on the areas needing further experimentation and on the implications of the proposed organizational scheme. We conclude that although our organizational scheme is not new in demonstrating the need for simplifying the neural control of movement, it is perhaps original in imposing discrete logical control upon a continuous mechanical system. The attraction of the scheme is that it provides a framework compatible with both mechanical and physiological information and amenable to experimental testing.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X00020008

The organization of human postural movements: A formal basis and experimental synthesis

In recent years, considerable attention has been paid to research and development of structural control devices, with particular emphasis on alleviation of wind and seismic response of buildings and bridges. In both areas, serious efforts have been undertaken in the last two decades to develop the structural control concept into a workable technology. Full-scale implementation of active control systems have been accomplished in several structures, mainly in Japan; however, cost effectiveness and reliability considerations have limited their wide spread acceptance. Because of their mechanical simplicity, low power requirements, and large, controllable force capacity, semiactive systems provide an attractive alternative to active and hybrid control systems for structural vibration reduction. In this paper we review the recent and rapid developments in semiactive structural control and its implementation in full-scale structures.

/pdf/state-of-the-art-of-structural-control-20ojs9jpb9.pdf

State of the art of structural control

A combined theoretical and experimental study of electrorheological (ER) fluid dampers is presented here. A moving electrode ER damper was built and tested for its dynamic characteristics for different electric field strengths and varying displacement amplitudes. Based upon the phenomenology observed in the experimental results, an augmented nonlinear model is proposed to describe the dynamic characteristics of the damper. The six model parameters are estimated from the experimental hysteresis data. The force versus displacement and force versus velocity hysteresis cycles are then reconstructed using these estimated parameters. The results show that the model captures the nonlinear damper behavior quite accurately. The importance of the various components in the model is illustrated.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nonlinear modeling and performance prediction of electrorheological fluid dampers

A magnetorheological (MR) fluid-based hydraulic power system is analyzed and experimentally validated by testing a prototype. A set of MR valves is proposed to implement within a Wheatstone bridge hydraulic power circuit to drive a hydraulic actuator using a pump. The MR valves are used in place of conventional mechanical servo valves. The proposed use of MR valves in hydraulic actuator systems has many advantages. First, MR valves have no moving parts, enhancing reliability. Second, the MR valves operate at the same speed as the actuation bandwidth (typically below twenty Hz in our applications). Third, the actuator relies on flow rates for a given pump speed, and avoids, to a large degree fluid compliance. Fourth, if a change in stroke direction is required, the flow through each of the MR valves can be controlled smoothly via changing the applied magnetic field. The performance of the Wheatstone bridge with MR valves is theoretically derived using three different models of the MR fluid behaviors: an idealized model, a Bingham-plastic model and a biviscous model. The analytical system efficiency in each case is compared, and departures from ideal behavior are recognized. The driving force and efficiency will be evaluated in the MR hydraulic power actuator system for both Bingham plastic and biviscous flows. An MR valve is designed using a magnetic finite element analysis. The magnetic flux density developed in the MR valve are verified by analytical and experimental methods. The yield stresses achieved in the MR valve due to the applied current are also measured to validate the design methodology. The overall performance of the MR fluid based hydraulic power system is described using the experimental MR valve performance data.

https://terpconnect.umd.edu/~jhyoo/my_paper/SPIE01.pdf

Design of a MR hydraulic power actuation system

Mechanisms-based Analysis of Elastomeric Lag Damper Behavior under Single and Dual Frequency Excitation Including Temperature Effects

Nanofiber-based magnetorheological (MR) fluids display an increased achievable yield stress and greatly reduced sedimentation as compared to conventional MR fluids which typically consist of micron-sized spherical ferromagnetic particles suspended in a carrier fluid. The maximum achievable yield stress of nanofiber-based MR fluids can be more than double that of MR fluids that contain strictly spherical particles at the same particle loadings. Conventional MR fluids display appreciable settling whereas the nanofiber-based fluids display no discernible settling at volume fractions greater than ∼6 vol%. However, a maximum volume fraction of <10 vol% is achievable in suspensions containing only nanofibers. To achieve the higher loadings required for most commercial applications, dimorphic MR fluids were created in which spherical particles are partially substituted with nanofibers. Dimorphic MR fluid display significantly enhanced particle sedimentation properties with yield stresses equivalent to or greater than those of conventional MR fluids. Nanofibers are well suited as model particles for the systematic experimental and theoretical study of the shape- and composition-dependent properties of MR fluids owing to their well-defined geometry, controllable dimensions and composition. The use of nanofibers has greatly enhanced MR fluid properties and has resulted in a better understanding of their physical behavior.

CHAPTER 2:Magnetorheology of Fe Nanofibers Dispersed in a Carrier Fluid

Energy-absorbing materials have extensive applications in aerospace and automotive applications. Research has shown buckling initiators, or triggers, in energy-absorbing tubular structures increase the energy absorbed by encouraging the side panels to fold when loaded out of plane in compression conditions. Additively manufactured TPE honeycombs were designed in this study to include these buckling initiators, which introduced a slight decrease in initial weight, as well as initial stress concentrations, while improving crashworthiness characteristics. The samples with buckling initiators (1BI) showed an increase in crush efficiency when directly compared to their no buckling initiator (0BI) counterparts. The 1BI samples maintained an increased crush efficiency regardless of the strain rate used. The samples with 1BI were able to better equilibrate the peak stress with the plateau stress. These honeycomb samples were found to maintain their crush efficiency, even after multiple rounds of compression testing. The quasi-static 0BI samples experienced a 23.4% decrease in the peak stress after multiple rounds of compression testing, while the 1BI samples saw approximately a 23.0% decrease. The 1BI samples averaged a decrease in crush efficiency of 0.5%, while the 0BI samples saw a decrease in crush efficiency of 5%. As the strain rate increased, the crush efficiency for the 1BI samples showed an increase in performance, with a smaller degradation in crush efficiency over multiple uses. Visco-elastic honeycomb with buckling initiators has a higher energy absorption than samples with no buckling initiators when exposed to multiple impact cycles. 

Visco-Elastic Honeycomb Structures with Increased Energy Absorption and Shape Recovery Performance Using Buckling Initiators

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