Control Scheme for Sideways Walking on a User-driven Treadmill

For immersive interaction in a virtual reality (VR) environment, an omnidirectional treadmill (ODT) can support performance of various locomotive motions (curved walk, side walk, moving with shooting stance) in any direction. When a user performs lateral locomotive motions on an ODT, a control scheme to achieve immersive and safe interaction with the ODT should satisfy robustness in terms of position error of a user to keep a reference position of the ODT by accurately estimating intentional walking speed (IWS) of the user, and it should guarantee postural stability of the user during the control actions. Existing locomotion interface (LI) control focuses on the reference position tracking performance regarding the position of the user's center of mass (COM) in order to respond to forward locomotion that can move at high speed. However, in sideways walking, the movement of the lower extremities is different from that of forward walking, and when the conventional LI control was directly applied to sideways walking, it was observed that excessive acceleration commands caused postural instability. For appropriate interface of sideways walking, we propose an estimation scheme based on an accurate walking model including the movement of the ankle joint. The proposed observer estimates the acting torque generated by the force of both lower extremities through the position information of COM and ankle joint to more accurately predict the user's intentional walking speed (IWS). In the sideways walking experiment conducted using a 1-dimensional user-driven treadmill (UDT), the proposed method allowed more natural interface of the lateral-side locomotion with better postural stability compared to the conventional estimation method that uses only the COM position information.