Design Specifications for the Second Generation Sarcos Treadport Locomotion Interface

The Sarcos Treadport is a locomotion interface comprised of a large tilting treadmill, an active mechanical tether, and a CAVElike visual display. This paper presents the design specifications for the second-generation Sarcos Treadport. INTRODUCTION There have been various approaches towards the design of locomotion interfaces, including powered pedaling devices (Brogan et al., 1998), programmable foot platforms (Iwata, 2000), walking-in-place arrangments (Templeman et al., 1999), and treadmill-style devices. Of these alternatives, treadmills are particularly attractive because of the relatively natural, unencumbered walking and running that they allow. Some treadmill-style devices employ linear treadmills which have been augmented in various ways, while other devices employ two-dimensional treadmill belt motion. The ATLAS system (Noma et al., 2000) comprises a linear treadmill on a spherical joint, which can act as a turntable but also tilt upwards and sideways. Turning is achieved by swiveling the treadmill in the direction of walking, based upon visual measurements of foot position. The Ground Surface Simulator (GSS) employs an array Address all correspondence to this author. of 6 vertical motion stages underneath a flexible belt that deform the belt to create uneven terrain (Noma et al., 2000). Turning is most naturally accomodated by two-dimensional treadmill designs. The Omnidirectional Treadmill employs orthogonal overlapping belts to create a two-dimensional surface (Darken et al., 1997). The top belt is comprised of rollers with rotation axes oriented parallel to the belt direction, and an orthogonal belt underneath moves the rollers to create motion at right angles to the top belt. Body position is measured by a mechanical tether. The Torus Treadmill employs an array of 12 small treadmills connected side-by-side to form one big belt, which is then rotated (Iwata and Yoshida, 1999). Magnetic trackers are employed to measure foot position and the intended direction of motion. The Sarcos Treadport comprises a large tilting treadmill, an active mechanical tether attached to the user through a body harness, and a CAVE-like visual display. The six-axis mechanical tether measures body position and orientation, for the purpose of active control of treadmill belt speed and turning in the virtual world. Turning is achieved by rate control, indicated either by the amount of user yaw motion or the amount of sidestep, depending on user speed. Hence it is necessary for the user to reindex to center before turning the other way. The most unique aspect of the Treadport is that the tether’s linear axis is motorized to push or pull on the user, thereby simulating unilateral constraints, Figure 1. THE ORIGINAL SARCOS TREADPORT. slope (Tristano et al., 2000), and inertial forces (Christensen et al., 2000). The original Treadport (Figure 1) employed a commercial treadmill with a 4-by-8 foot belt area. Experience based on its use indicated that a larger belt area would be desirable for greater maneuverability, and that the tilt mechanism was too slow. The sensing of belt speed and tilt needed to be improved, as well as the force capability and responsiveness of the mechanical tether. Finally, we wished a larger and higher resolution CAVE display and the possibility of floor projection. The new Sarcos Treadport is shown in Figure 2. The following sections discuss the design specifications for the treadmill belt, platform tilt, and the active mechanical tether. Safety mechanisms are also discussed, and the CAVE layout is briefly presented.