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Over the past 50 years, it has been known that there are liquids that respond mechanically to electrical stimulation. These liquids, which have attracted a great deal of interest from engineers and scientists, change their viscosity electroactively. These electrorheological fluids (ERF) exhibit a rapid, reversible, and tunable transition from a fluid state to a solid-like state upon the application of an external electric field [Phule and Ginder, 1998]. Some of the advantages of ERFs are their high-yield stress, low-current density, and fast response (less than 1 ms). ERFs can apply very high electrically controlled resistive forces while their size (weight and geometric parameters) can be very small. Their long life and ability to function in a wide temperature range allows for the possibility of their use in distant and extreme environments. ERFs are also nonabrasive, nontoxic, and nonpolluting, meeting health and safety regulations. ERFs can be combined with other actuator types such as electromagnetic, pneumatic, or electrochemical actuators so that novel, hybrid actuators are produced with high-power density and low-energy requirements. The electrically controlled rheological properties of ERFs can be beneficial to a wide range of technologies requiring damping or resistive force generation. Examples of such applications are active vibration suppression and motion control. Several commercial applications have been explored, mostly in the automotive industry, such as ERF-based engine mounts, shock absorbers, clutches and seat dampers. Other applications include variable-resistance exercise equipment, earthquake-resistant tall structures, and positioning devices [Phule and Ginder, 1998]. While ERFs have fascinated scientists, engineers, and inventors for nearly 50 years, and have given inspiration for developing ingenious machines and mechanisms, their applications in real-life problems and the commercialization of ERF-based devices has been very limited. There are several reasons for this. Due to the complexity and nonlinearities of their behavior, their closed-loop control is a difficult problem to solve. In addition, the need for high voltage to control ERF-based devices creates safety concerns for human operators, especially when ERFs are used in devices that will be in contact with humans. Their relatively high cost and the lack of a large variety of commercially available ERFs with different properties to satisfy various design specifications have made the commercialization of ERF-based devices unprofitable. However, research on ERFs continues intensively and new ERF-based devices are being proposed [Tao, 1999]. This gives rise to new technologies that can benefit from ERFs. One such new technological area that will be described in detail here is virtual reality and telepresence, enhanced with haptic (i.e., tactile and force) feedback systems, and for use in medical applications, for example. In this chapter, we first present a review of ERF fundamentals. Then, we discuss the engineering applications of ERFs and, more specifically, their application in haptics. We describe in detail a novel ERF-based haptic system called MEMICA (remote mechanical mirroring using controlled stiffness and actuators) that was recently conceived by researchers at Rutgers University and the Jet Propulsion Laboratory [Bar-Cohen et al., 2000a]. MEMICA is intended to provide human operators an intuitive and interactive feeling of the stiffness and forces in remote or virtual sites in support of space, medical, underwater, virtual reality, military, and field robots performing dexterous manipulation operations. MEMICA is currently being used in a system to perform virtual telesurgeries as shown in Fig. 1 [Bar-Cohen et al., 2000b].
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