Development of a fast omni-directional treadmill platform for immersive locomotion interface

Abstract

To realize an immersive locomotion interface (LI) of a user in virtual reality (VR), a stationary-type device should be allowed to perform various locomotion of human being (walking, running, sideway moving etc.) in any direction and intentional speed. Thus, a platform providing an immersive LI should maintain a user in a reference position even when moving at a free speed and any direction. To implement the mentioned functions above, the following technologies are required; i) An omnidirectional ground generation mechanism capable of realizing a 2-dimensional ground like the real ground, and ii) A control method that adapts the user's locomotion velocity by estimating the intended walking speed (IWS) through detecting user location and action. Currently, developed omnidirectional treadmills (ODTs) are representative devices that can infinitely generate the ground in all directions. Therefore, an ODT can work as an immersive walking interface platform by applying LI controller realizing the functionality of a user-driven treadmill (UDT) to follow the estimated IWS for the adaptation of a user locomotion speed and providing spatial sensation with visual feedback via VR. However, existing ODTs are heavy and complex, and operate at low speeds. This limits fast user motion and prevents natural interactions in real applications such as military training programs and interactive games. Moreover, in terms of LI controller, robustness of the user position must be ensured by sensitively estimating and accurately converging to IWS to achieve safe and immersive interface. The existing IWS estimation using a linear observer with the cart model (1st order dynamics) can exponentially converge to the true IWS. However, when the estimation sensitivity is increased by increasing the gain, this method causes severe postural instability due to the generation of excessive anomalous forces. Thus, the existing method has an implicit limitation with regards to increasing the position robustness because of the postural instability issues. To develop an ODT platform that increases LI immersion for a user, the reserch presents a method to solve the problems of existing ODTs and developed LI controllers, and is classified into three sections as follows: An ODT based on rack and pinion mechanism concept, and design of cart model-based 2-dimensional locomotion interface: It introduces a novel locomotion interface device with running capability, which uses an ODT with a new power transmission mechanism and a locomotion controller that enables the user to make fast movements. As a result of the improved power transmission performance due to the simple and relatively lightweight structure, the proposed two-dimensional treadmill can generate a maximum speed of 3 m/s, with an acceleration of 3 m/s2. Moreover, through a pilot test with the proposed locomotion interface device, we verified that the fast directional changes during walking and running with the designed speed adaptation controller do not exceed the acceleration performance of the proposed system. Design of inverted pendulum gait model-based locomotion interface and its experiment of conducting forward and sideway locomotion: To simultaneously achieve sensitive and accurate IWS estimation while reducing postural instability on a UDT, in addition to the cart model, we have also utilized the inverted pendulum-based gait model (IPGM) as a 2nd order dynamic to estimate the intentional walking acceleration (IWA) generated by the ankle torque. Thus, the proposed IWS prediction method uses the cart model for accurate convergence to IWS and the IPGM to follow sensitively the change in the IWS. In the proposed method, the internal states of the existing observers applied to the 1st and 2nd order dynamics are shared recursively to estimate the ankle torque acting as a disturbance for the IPGM and to sensitively predict the change in the IWS. Preliminary development of a compact omnidirectional treadmill based on a novel helical gear transmission and its pilot test: A mechanical design of a novel actuation system based on helical transmission for an ODT is presented. Conventional ODTs can realize omnidirectional motion; however, they have issue such as limited motion performance due to low transmission efficiency, excessive thickness because of increasing actuation space, and loud power transmission noise. The proposed design solves this drawback by means of 2-layer meshes consist of helical pulleys and helical gears. The omnidirectional motion is driven by the first layer as the helical pulley mesh, and a motor power for first layer actuation is transmitted by synchronized actuation of the second layer as helical gear mesh. Compared to the ODT developed by the author previously based on spur transmission, the helical transmission mechanism only increases the length in the direction of the rotation axis instead of increasing the diameter to increase the gear tooth coupling, the proposed transmission can realize compact and super-thin actuation space. Moreover, it can potentially reduce the noise compared to spur transmission (Rack and pinion) because of utilizing the helical transmission. Due to the wide range of movement speeds and acceleration capabilities of the rack and pinion based ODT, it can enhance the immersive experience in various VR environments due to increasing motion performance. Experiment of the designed IPGM based IWS estimation show that the proposed method can significantly facilitate the users in following a profile of desired walking speeds more accurately than the existing IWS estimation method under the same position robustness setup. The pilot experiment of the preliminary developed ODT with the helical actuation mechanism to show how it works, has been conducted to check the kinematic feasibility when performing omnidirectional motion.