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 a proportional state feedback control system yielded an electrorheological fluid that responded 35 times faster and 21 times more accurately than possible with a conventional open-loop 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.

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