There are actually several VR devices that permit locomotion in Virtual Environments. The most basic being a treadmill type device. Military researchers have also developed small "two-way" treadmills that permit locomotion in both longitudinal and lateral directions. The University of Utah developed a locomotion interface called the Treadport that provides longitudinal motion, but also provides haptic (force) feedback to the user. The person walking on the treadmill is attached to a translational robot arm that serves the purpose of pushing and pulling to simulate the force of gravity on sloped surfaces. Inertial forces are also simulated with this device.
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For more info: The Utah Treadport