State Feedback Control of Electrorheological Fluids

Electrorheological (ER) fluids have electrically controllable stiffness, viscosity, and heat transfer properties. Since the 1940s researchers have attempted to model the properties of ER fluids and have proposed applications which attempt to utilize their special characteristics in the operation of hydraulic valves, soft clutches, and active suspension systems. Early attempts to make these applications commercially successful were hampered by the relatively slow, nonlinear response of ER fluids under on-off control of high electric fields. Successful applications will require fast, precise control of the response of ER fluids, independent of application at low field strengths. This study presents a new approach to the control of ER fluids that overcomes the problems of imprecise, slow, nonlinear response and high electric fields. An optical sensor was used to indicate the ER fluid state in a layered composite window. Feedback control of ER fluid state was developed and compared to conventionally actuated ER fluids. Feedback control employs the state sensor and high initial electric field strength to speed ER state response, then lowers the field strength to the minimum level required to achieve the desired ER fluid state. Predicted responses were compared to experimentally measured responses and showed excellent agreement. Laboratory measurements showed that a proportional state feedback control system yielded an electrorheological fluid which responded 35 times faster and 21 times more accurately than possible with a conventional openloop fluid control system. Although the use of ER fluids in feedback control systems have been proposed in the past, this work is the first application of feedback control to the fluid itself. INTRODUCTION As early as the 19th century (Duff 1896; Quinke 1897), scientists began studying electrorheological (ER) response, although it was not until research by Winslow (1947) that electroviscous phenomena gained prominent attention. He introduced the concept of controlling the viscosity of an electro-viscous fluid by use of an electric field (Winslow 1947, 1949). Flow resistance of these fluids increased with field strength when exposed to AC electric fields on the order of 4kV/mm. He observed a “fibrous” structure composed of particle chains generally aligned with the applied electric field. Winslow hypothesized that these field induced particle chains increased the viscosity of the fluid. An ER fluid consists of fine polarizable particles suspended in a fluid of lower dielectric constant. Typically such fluids are assembled with a continuous hydrophobic liquid phase (e.g. silicone oil) containing hydrophilic particles (e.g. Zeolite). The density of the particles is matched as closely as possible with that of the oil to ensure good dispersion upon mixing of the ER fluid (Stangroom 1978, 1983). An applied electric field aligns the dipoles of water molecules trapped in particles, thus polarizing the particles. Particle polarization changes their organization in the fluid and causes changes in fluid rheological properties (Fig. 1). When particle chains are subjected to fluid shearing forces, the particles still attract even though they may be pulled away from each other (Duclos et al 1988). Higher electric field strength increases polarization and causes particle chains to pull together tighter and to lengthen the chains through the addition of more particles (Klingenberg et al, 1989). These longer, stronger particle chains result in higher fluid viscosity and stiffness. At