Resolving the redundancy of a seven DOF wearable robotic system based on kinematic and dynamic constraint

Abstract

According to the seven degrees of freedom (DOFs) human arm model composed of the shoulder, elbow, and wrist joints, positioning of the wrist in space and orientating the palm is a task requiring only six DOFs. Due to this redundancy, a given task can be completed by multiple arm configurations, and there is no unique mathematical solution to the inverse kinematics. The redundancy of a wearable robotic system (exoskeleton) that interacts with the human is expected to be resolved in the same way as that of the human arm. A unique solution to the system's redundancy was introduced by combining both kinematic and dynamic criteria. The redundancy of the arm is expressed mathematically by defining the swivel angle: the rotation angle of the plane including the upper and lower arm around a virtual axis connecting the shoulder and wrist joints which are fixed in space. Two different swivel angles were generated based on kinematic and dynamic constraints. The kinematic criterion is to maximize the projection of the longest principle axis of the manipulability ellipsoid for the human arm on the vector connecting the wrist and the virtual target on the head region. The dynamic criterion is to minimize the mechanical work done in the joint space for each two consecutive points along the task space trajectory. These two criteria were then combined linearly with different weight factors for estimating the swivel angle. Post processing of experimental data collected with a motion capturing system indicated that by using the proposed synthesis of redundancy resolution criteria, the error between the predicted swivel angle and the actual swivel angle adopted by the motor control system was less then five degrees. This result outperformed the prediction based on a single criteria.

Kinematic Data Analysis for Post-Stroke Patients Following Bilateral Versus Unilateral Rehabilitation With an Upper Limb Wearable Robotic System

“…The philosophy behind this swivel angle estimation is that the swivel angle is selected by the motor control system to efficiently retract the palm to the head region. It implies that during the arm movement toward an actual target, the virtual target point on the head is also set for the potential retraction of the palm to the virtual target [13], [14].…”

“…The seven DOF exoskeleton robot UL-EXO7 [4], [12] is exploited as a core mechanical system that can support 97% of the human arm workspace [12], [4]. The controllers equipped in UL-EXO7 can provide the assistive force to help patients make the natural arm posture based on the work in [13] and [14]. For the objective and fine-scale rehabilitation assessment, we introduce a new assessment metric: an efficiency index that can tell us how close the patients arm movements are to the normal subject's arm movement.…”

Predicting Redundancy of a 7 DOF Upper Limb Exoskeleton Toward Improved Transparency between Human and Robot

“…The radius (R) and center (P c ) of the circle are easily found through geometry: Fig. 1 [14,15,29] where U and L are the lengths of the upper and lower arm segments [ Fig. 1b].…”

“…When P m , P s , P e and P w are on the same plane, the manipulability on the virtual trajectory is maximized. The pictures were referenced from the original work in [14,15,29] …”

“…For this, dependencies among joint angles are analyzed and modelled by various system identification algorithms such as neural networks, ARX(Auto-Regressive Exogenous) and probabilistic models [10,21]. Recently in our preliminary work, by focusing on the functional difference between the robotic system and the human, new kinematic constraints which utilize the head as a hidden target position was proposed [14,18]. In these works, it is shown that natural hand posture for reaching activities is mainly configured to bring the hand efficiently back to the head region.…”