Design, implementation and control of an overground gait and balance trainer with an active pelvis-hip exoskeleton

Human locomotion is crucial for performing activities of daily living and any disability in gait causes a significant decrease in the quality of life. Gait rehabilitation therapy is imperative to improve adverse effects caused by such disabilities. Gait therapies are known to be more effective when they are intense, repetitive, and allow for active involvement of patients. Robotic devices excel in performing repetitive gait rehabilitation therapies as they can eliminate the physical burden of the therapist, enable safe and versatile training with increased intensity, while allowing quantitative measurements of patient progress. Gait therapies need to be applied to specific joints of patients such that the joints work in a coordinated and repetitious sequence to generate a natural gait pattern. Six determinants of gait pattern have been identified that lead to efficient locomotion and any irregularities in these determinants result in pathological gaits. Three of these six basic gait determinants include movements of the pelvic joint; therefore, an effective gait rehabilitation robot is expected to be capable of controlling the movements of the human pelvis. We present the design, implementation, control, and experimental verification of AssistOn-Gait, a robot-assisted trainer, for restoration and improvement of gait and balance of patients with disabilities affecting their lower extremities. In addition to overground gait and balance training, AssistOn-Gait can deliver pelvis-hip exercises aimed to correct compensatory movements arising from abnormal gait patterns, extending the type of therapies that can be administered using lower extremity exoskeletons. AssistOn-Gait features a modular design, consisting of an impedance controlled, self-aligning pelvis-hip exoskeleton, supported by a motion controlled holonomic mobile platform and a series-elastic body weight support system. The pelvis-hip exoskeleton possesses 7 active degrees of freedom to independently control the rotation of the each hip in the sagittal plane along with the pelvic rotation, the pelvic tilt, lateral pelvic displacement, and the pelvic displacements in the sagittal plane. The series elastic body weight support system can provide dynamic unloading to support a percentage of a patient's weight, while also compensating for the inertial forces caused by the vertical movements of the body. The holonomic mobile base can track the movements of patients on flat surfaces, allowing patients to walk naturally, start/stop motion, vary their speed, sidestep to maintain balance, and turn to change their walking direction. Each of these modules can be used independently or in combination with each other, to provide different configurations for overground and treadmill based training with and without dynamic body weight support. The pelvis-hip exoskeleton of AssistOn-Gait is constructed using two passively backdrivable planar parallel mechanisms connected to the patient with a custom harness, to enable both passive movements and independent active impedance control of the pelvis-hip complex. Furthermore, the exoskeleton is self-aligning; it can automatically adjust the center of rotation of its joint axes, enabling an ideal match between patient's hip rotation axes and the device axes in the sagittal plane. This feature not only guarantees ergonomy and comfort throughout the therapy, but also extends the usable range of motion for the hip joint. Moreover, this feature significantly shortens the setup time required to attach the patient to the exoskeleton. The exoskeleton can also be used to implement virtual constraints to ensure coordination and synchronization between various degrees of freedom of the pelvis-hip complex and to assist patients as-needed for natural gait cycles. The overall kinematics of AssistOn-Gait is redundant, as the exoskeleton module spans all the degrees of freedom covered by the mobile platform. Furthermore, the device features dual layer actuation, since the exoskeleton module is designed for force control with good transparency, while the mobile base is designed for motion control to carry the weight of the patient and the exoskeleton. The kinematically redundant dual layer actuation enables the mobile base of the system to be controlled using workspace centering control strategy without the need for any additional sensors, since the patient movements are readily measured by the exoskeleton module. The workspace centering controller ensures that the workspace limits of the exoskeleton module are not reached, decoupling the dynamics of the mobile base from the dynamics of the exoskeleton. Consequently, AssistOn-Gait possesses virtually unlimited workspace, while featuring the same output impedance and force rendering performance as its exoskeleton module. The mobile platform can also be used to generate virtual fixtures to guide patient movements. The ergonomy and useability of AssistOn-Gait have been tested with several human subject experiments. The experimental results verify that AssistOn-Gait can achieve the desired level of ergonomy and passive backdrivability, as the gait patterns with the device in zero impedance mode are shown not to significantly deviate from the natural gait of the subjects. Furthermore, virtual constraints and force-feedback assistance provided by AssistOn-Gait have been shown to be adequate to ensure repeatability of desired corrective gait patterns