Powered exoskeleton rehabilitation is an effective way to help stroke patients recover their motor abilities. Bionic structures and human-like control strategies can be used to enhance both the safety and efficacy of exoskeletons. However, the motion characteristics of the shoulder complex are not sufficiently considered. In this paper, we designed a 7-degrees-of-freedom (DOF) upper limb rehabilitation exoskeleton, FREE (functional rehabilitation exoskeleton). The mechanical structures of the shoulder and forearm of FREE are in accordance with human anatomy, and can be used to perform a wide range of synergistic motion of multiple joints while keeping a safe distance from the patient’s head. A multiple-input-multiple-output (MIMO) shoulder girdle motion prediction model was developed to satisfy the synergy between humans and exoskeletons. Moreover, a constrained task priority and projected gradient-based inverse kinematics algorithm (CTPPG-IK) was proposed to achieve assistance with scapulohumeral rhythm. A motion capture system was used to collect different activities of daily life (ADL) motion data to validate the proposed algorithm. The experimental results show that the accuracy of the prediction model is higher than that of existing models, and the inverse kinematics algorithm can handle the end-effector task and joint space with a maximum angle error of $3.04\times {10}^{-3}$ rad.

Active patient participation in the rehabilitation process after stroke has been shown to accelerate neural remodeling. The control framework of rehabilitation robots should provide appropriate assistive forces to users. An assist-as-needed (AAN) control method is proposed to help users to move upper limbs in the workspace freely, and to control the exoskeleton to provide assistance. The method is based on zero moment control (ZMC), helping the user achieve robotic traction with minimal interaction force. Based on the posture of the upper arm and forearm, an AAN controller can modify assistive forces at two human–robot-interaction (HRI) points along the direction opposite to gravity. A shoulder motion prediction model is proposed to enable the exoskeleton to mimic the user’s upper limb natural movements. In order to improve the transparency during rehabilitation training, a nonlinear numerical friction model based on the Stribeck friction model is developed. A healthy adult male was recruited to perform various activities of daily living (ADL) tests to assess the effectiveness of the controllers. The experimental results show that the proposed ZMC controller has high HRI transparency and can control the exoskeleton to complete a wide range of upper limb movements, and the maximum interaction force and torque can be captured within −7.76 N and 4.58 Nm, respectively. The AAN controller can provide appropriate assistance in the desired direction, and the exoskeleton maintains kinematic synchronization with the user’s shoulder during shoulder girdle movement.

Shoulder repetitive training is of paramount importance for rehabilitation of stroke patients with hemiplegia. This paper investigated kinematic structural optimization of an upper limb rehabilitation exoskeleton's shoulder structure, aiming to cover the range of motion (ROM) of human shoulder, achieve sufficient dexterity, obtain a compact structure, and avoid collisions with the user within the workspace. Based on the concept of configuration manifold, configuration parameters and joint angle parameters were fused, and parameter optimization was transformed into a searching problem in high-dimensional configuration space. Geometric constraints between human and exoskeleton were described parametrically. Upper limb movements were mapped to the exoskeleton's configuration space to calculate spatial vectors of joints, and determine whether vectors satisfy constraints. The formulated multi-objective optimisation problem was computed by multi-objective particle swarm optimization (MOPSO) algorithm to determine the shoulder configuration parameters. Experimental results demonstrate that functional rehabilitation exoskeleton (FREE) exhibits a wide ROM, excellent dexterity, and can assist users in completing most activities of daily life (ADLs). The design framework proposed in this paper can help designers determining optimal exoskeleton configurations through formulated constraints.