Henri P. Gavin

Electrorheological Dampers, Part I: Analysis and Design

Electrorheological (ER) materials are suspensions of specialized, micron-sized particles in nonconducting oils. When electric fields are applied to ER materials, they exhibit dramatic changes (within milli-seconds) in material properties. Pre-yield, yielding, and post-yield mechanisms are all influenced by the electric field. Namely, an applied electric field dramatically increases the stiffness and energy dissipation properties of these materials. A previously known cubic equation which describes the flow of fluids with a yield stress through a rectangular duct can be applied to annular flow, provided that certain conditions on the material properties are satisfied. An analytic solution and a uniform approximation to the solution, for the rectangular duct Poiseuille flow case is presented. A numerical method is required to solve the flow in annular geometries. The approximation for rectangular ducts is extended to deal with the annular duct case.

Electrorheological Dampers, Part II: Testing and Modeling

Electrorheological (ER) materials develop yield stresses on the order of 5-10 kPa in the presence of strong electric fields. Viscoelastic and yielding material properties can be modulated within milli-seconds. An analysis of flowing ER materials in the limiting case of fully developed steady flow results in simple approximations for use in design. Small-scale experiments show that these design equations can be applied to designing devices in which the flow is unsteady. More exact models of ER device behavior can be determined using curve-fitting techniques in multiple dimensions. A previously known curve-fitting technique is extended to deal with variable electric fields. Experiments are described which illustrate the potential for ER devices in large-scale damping applications and the accuracy of the modeling technique.

Control of seismically excited vibration using electrorheological materials and Lyapunov methods

Describes the use of electrorheological (ER) materials in a vibration control system designed to suppress earthquake-excited vibrations. A state-variable model for the ER device is based on micromechanical simulations and rheometric measurements of ER materials. A decentralized bang-bang control law is synthesized using Lyapunovs direct method and is implemented using analog circuitry. The behavior of the closed-loop system is illustrated on a small scale (1:15) building model driven by ground accelerations. The combined effects and relative importance of the controllable stiffness and damping properties of ER materials are examined.

Design and experimental characterization of an electromagnetic transducer for large-scale vibratory energy harvesting applications

This article reports on the design and experimental characterization of an electromagnetic transducer for energy harvesting from large structures (e.g., multistory buildings and bridges), for which the power levels can be above 100 W and disturbance frequencies below 1 Hz. The transducer consists of a back-driven ballscrew coupled to a permanent-magnet synchronous machine with power harvesting regulated via control of a four-quadrant power electronic drive. Design considerations between various subsystems are illustrated and recommendations in terms of minimal values are made for each design metric. Developing control algorithms to take full advantage of the unique features of this type of transducer requires a mechanical model that can adequately characterize the device’s intrinsic nonlinear behavior. A new model is proposed that can effectively capture this behavior. Comparison with experimental results verifies that the model is accurate over a wide range of operating conditions. As such, the model can be used to assess the viability of the technology and to correctly design controllers to maximize power generation. To demonstrate the device’s energy harvesting capability, impedance matching theory is used to optimize the power generated from a base-excited tuned mass damper. Both theoretical and experimental investigations are compared and the results are shown to match closely.

Drift-free integrators

Bias errors introduced by systems designed to measure low-frequency transients negate zero-mean assumptions on the measurement noise. On-line signal processing methods that require accurate low-frequency information can be adversely affected by bias errors. On-line integration of dynamic signals is a classical example of a process that is unstable in the presence of bias errors. Accurately integrated quantities (like velocity and displacement), from easily measured quantities (like acceleration), can inform control systems and reduce on-line computational burdens. This article introduces a feedback stabilization method for a hybrid digital-analog integrator. The analytical performance of this integrator is compared to a filtered analog integrator in the time and frequency domains. For wide-band random signals, the analog circuit performs well with respect to linearity and hysteresis, but does less well for long-period signals. A stabilized hybrid analog-digital integrator has exceptional accuracy when int...

Multi-Duct ER Dampers:

This paper describes the design, construction, testing and modeling of controllable damping devices utilizing electro-rheological (ER) materials. The rheological properties of ER materials (yield stress and viscoelasticity) are extremely sensitive to electric fields. Modulations of the electric field in an electrorheological damper results in a corresponding change in device forces. The key feature of the ER devices described in this paper is a set of multiple concentric annular ducts through which the ER material flows. The annular ducts are formed by a set of concentric metallic tubes, which may be electrically charged with a high voltage potential, or electrically grounded. Three designs targeting different force levels (2-6 kN), are designed, tested and modeled. The device analyses used in the design incorporate a simplified closed form relation for the flow behavior of ER materials. Evolutionary and algebraic device models are fit to the measured force response of the devices over a range of harmonic frequencies and at zero and high electric field magnitudes. The evolutionary model is motivated by observations and simulations of ER materials in simple shear, and the algebraic model is convenient for control and feedback linearization applications. The algebraic model is evaluated with data collected with random excitation and with switching electric fields. The three devices display different levels of pre-yield viscoelasticity. Friction, due to seals in the device, was not accounted for in the initial design, and affected the behavior of the device as tested.

Electrorheological dampers and semi-active structural control

Electrorheological (ER) fluids have a number of properties that make them attractive for use in fluid-filled dampers. Our interest is in the use of such dampers for semi-active structural control. This paper provides an introduction to this subject, with emphasis given to control of large structural systems such as buildings and bridges when excited by ground motions or wind forces. It is assumed that multiple electrorheological dampers are embedded as a part of the structural system. A key aspect in the effective use of these electrorheological dampers for active control is the recognition that they are inherently nonlinear devices, the inputs being (essentially) the electric field levels applied to the electrorheological fluids in the dampers. A decentralized bang-bang control strategy is derived to minimize the rate at which energy from the disturbance is transferred to the structure. It requires feedback of the ER damper deformation rates and feedforward of a disturbance signal. The control strategy is simple to implement and has a number of desirable closed loop properties.<<ETX>>

Parametric Statistical Generalization of Uniform-Hazard Earthquake Ground Motions

Sets of ground-motion records used for seismic hazard analyses typically have intensity measures corresponding to a particular hazard level for a site (perhaps conditioned on a particular intensity value and hazard). In many cases the number of available ground motions that match required spectral ordinates and other criteria (such as duration, fault rupture characteristics, and epicentral distance) may not be sufficient for high-resolution seismic hazard analysis. In such cases it is advantageous to generate additional ground motions using a parameterized statistical model calibrated to records of the smaller data set. This study presents a statistical parametric analysis of ground-motion data sets that are classified according to a seismic hazard level and a geographic region and that have been used extensively for structural response and seismic hazard analyses. Parameters represent near-fault effects such as pulse velocity and pulse period, far-field effects such as velocity amplitude and power-spectr...

Study of airfoil gust response alleviation using an electro-magnetic dry friction damper. Part 1: Theory

In Part 1 of this work, a theoretical simulation study of the non-linear gust response of a three degree-of-freedom typical airfoil section with a control surface using an electro-magnetic dry friction damper is presented. For validation of this theoretical model, an electro-magnetic dry friction damper has been designed and an experimental investigation of the gust response has been carried out in a wind tunnel. Results for both periodic and linear frequency sweep gust excitations have been computed and measured. The fair to good quantitative agreement between theory and experiment verifies that the present electro-magnetic dry friction damper can be used to alleviate the gust response, especially for the plunge and pitch responses. It also shows that the present theoretical method can be successfully applied to determine the non-linear gust response when an electro-magnetic dry friction damper is used in the linear aeroelastic system.

Annular Poiseuille flow of electrorheological and magnetorheological materials

Electro- (ER) and magnetorheological (MR) materials exhibit substantial increases in yield stress when subjected to strong electric or magnetic fields. The applied electric/magnetic field satisfies Laplace’s equations in the ER/MR material. In annular geometries, the resulting spatial variation of the electric/magnetic field results in a yield stress inhomogeneity. The analysis of annular Poiseuille flow of ER/MR materials herein assumes a power-law relationship between the field and the yield stress, and includes the effects of the inhomogeneous yield stresses and hyperbolic shear stress distributions. The approximation of the solution to this problem by flow through an appropriately defined rectangular duct (in which the yield stress is constant and the shear stresses are linearly distributed) can be remarkably accurate, even for nonslender ducts.