A Direct Gait Kinematic Model to Estimate Forward Progression and Spatial Stepping Features in Cartesian Space

Robotic exoskeletons are usually controlled in joint-space, being spatio-temporal step features an uncontrolled consequence. However, clinical practice is focused on the cartesian movement of the foot rather than analytical hip/knee/ankle flexion/extension movements. In order to enable robotic exoskeletons to control the cartesian trajectory of the foot, a kinematic model to translate from joint-space to cartesian-space is needed. Besides, such a model requires to consider the forward progression of the direct (joints-to-foot) kinematic model to calculate the spatio-temporal stepping features. This paper presents a direct gait kinematic model including pelvis, hips, knees and ankles degrees of freedom based on Denavit-Hartenberg method and also proposes a method to iteratively compute the progression of the kinematic chain considering a straight walking over a flat floor. This model has been experimentally validated, comparing its results with the gait of 5 healthy subjects recorded by a Vicon Motion Capture System. Results show RMS errors lower than 1 cm and 5 cm in vertical and horizontal directions respectively.