and Bejczy, 1985). One of the first applications of force-reflecting hand controllers to VEs was in project GROPE at the University of North Carolina (Brooks et al., 1990). The Argonne Mechanical Arm (ARM) was used successfully for force reflection during interactions with either simulations of molecule docking or with data from a scanning tunneling microscope. Recently, high-performance devices have been specifically designed for interaction with VEs. The MIT Sandpaper is a 3-DOF joystick that is capable of displaying virtual textures (Minsky et al., 1990). In Japan, desktop master manipulators have been built in Tsukuba (Iwata, 1990; Noma and Iwata, 1993). At the University of British Columbia, high-performance hand controllers have been developed by taking advantage of magnetic levitation technology (Salcudean et al., 1992). PER-Force is a 6-DOF hand controller that delivers high performance (Cybernet Systems, 1992). The PHANTOM, built in the MIT Artificial Intelligence Laboratory, is a multilink, low-inertia device that can convey the feel of virtual objects (Massie and Salisbury, 1994).

Sophisticated teleoperation masters have been built that can be used to feel and manipulate virtual objects as well. At harvard, Howe (1992) has developed a teleoperation system with a two-finger master that can be used to execute precision tasks with a pinch grasp between the thumb and the index finger. One of the most complex force-reflecting devices built to date is the Dextrous Teleoperation System Master designed by Sarcos, Inc., in conjunction with the University of Utah's Center for Engineering Design and the Naval Ocean Systems Center (NOSC). Although it is primarily ground-based, by having attachment points at the forearm and upper arm of the user it has the advantages of an exoskeleton, such as a large workspace comparable to that of the human arm. This device utilizes high-performance hydraulic actuators to provide a wide dynamic range of force exertion at relatively high bandwidth on a joint-by-joint basis for 7 DOF. Another high-performance force-reflecting master is a ground-based system built by Hunter et al. (1990) to enable two-handed teleoperation of a microrobot that can meet the dual requirements of wide bandwidth (exceeding 1 kHz) and high accuracy (as low as a few nanometers). Improved versions of these devices have been built for teleoperated eye surgery and represent the state-of-the-art performance that can be achieved using currently available technology (Hunter et al., 1994).

Exoskeletal devices are characterized by the fact that they are designed to fit over and move with the limbs or fingers of the user. Because they are kinematically similar to the arm and hands that they monitor and stimulate, they have the advantage of the widest range of unrestricted user motion. As position-measuring systems, exoskeletal devices (gloves, suits, etc.) are relatively inexpensive and comfortable to



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