Hybrid polynomial-based trajectory planning for lower limb exoskeleton robots

To ensure smooth, continuous, and precise motion of lower limb exoskeleton robots, this paper proposes a 6-5-6 polynomial trajectory planning method (6-5-6 PTPM) that combines 5^th and 6^th polynomials. By imposing corresponding kinematic constraints, this method achieves the trajectory plan of seven sub-phases of the gait cycle on the kinematic model of the lower limb exoskeleton robot. The simulation is conducted and compared to the 4-3-4 polynomial trajectory planning method (4-3-4 PTPM). The comparison results indicate that the 6-5-6 PTPM is shown to have a lower error between the planned trajectory and the actual trajectory. Additionally, it avoids spikes of the acceleration curve that existed in the 4-3-4 polynomial method. The proposed method enables the acceleration curve smoother and mechanical impact lessened, which demonstrates the feasibility and stability of the proposed method.

I. Introduction

Lower limb exoskeleton robots are interactive robotic systems worn externally on the human lower limbs that mimic and assist human motion to improve and restore gait. Trajectory planning is vital for exoskeleton robots, providing better assistance to the elderly and individuals with disabilities [1]. The unsmooth trajectory generates a large velocity and acceleration, leading to mechanical impacts when experiencing abrupt changes or discontinuities. This unsmooth decreases the robot's tracking accuracy, which has negative impacts on the safety, health, and comfort of the wearer.