In recent years, the number of patients with lower-limb motion dysfunction caused by aging, diseases, athletic sports, car accidents, natural disasters and daily exercise has been increasing. The lower-limb exoskeleton for assistance walking and rehabilitation training can improve patients exercise capacity, ensure their life quality, reduce their economic pressure and alleviate the shortage of medical resources, which has wide application prospects. Some results on the study of the lower-limb exoskeleton has been achieved by the domestic and foreign advanced scientific research institutions, but research limitations in lightweight structure, low energy consumption, driving technology, sensor technology, coordination control and effect evaluation have delayed the engineering application of the exoskeleton. Based on the above research background and combined the advantages of human lower-limb movement mechanism with the passive bipedal walking theory, a quasi-passive power-assisted lower-limb exoskeleton driven by tendon-sheath actuation system is developed. The main research contents of this paper are as follows:
Based on the studies in human anatomy, kinematics and measurement, the characteristics and relationships of the lower-limb joint motion, walking cycle and the foot movement are analyzed. The whole walking cycle includes five different walking patterns is proposed and provides the theoretical basis for the subsequent analysis and simplification of dynamic models. Models of the rimless wheel and the simplest bipedal walking which indicate the passive bipedal walking theory are analyzed. The Poincare regression mapping method is adopted to analyze the existence and the influence of motion limit cycles, the passive bipedal walking theory is illustrated in perspectives of the model parameter configuration, action of gravity and energy compensation type, which brings inspiration and guidance to design a power-assisted lower-limb exoskeleton which is more humanoid in the DOF configuration and joint driving style with low energy consumption.
A quasi-passive power-assisted lower-limb exoskeleton prototype driven by tendon-sheath actuation system is established and the design of the main structure and control system is following principles of its own dynamics characteristics, reducing the active driving DOFs, increasing the passive compliant driving DOFs and reducing the quantity of sensors. The active hip joint is driven by tendon-sheath actuation system and the walking assistance is achieved by controlling the joint torque output; The passive compliant joint design is implemented by mechanical spring components to adjust the COM of the exoskeleton and comply the movement trend with the energy storage and release, aims to reduce the energy consumption and improve the mechanism performance; The impulse module along the thigh lever is used as the trigger signal for the man-machine initiative movement as pedal, plantar flexion, knee flexion and the thigh flexion to aid the accomplishment of the gait conversion and achievement of the lower limb strength training; Model analysis and experiments about force and displacement transmission characteristics of the compliant single-tendon-sheath and double-tendon-sheath actuation system are accomplished, the effects of pretension, total curvature,the radius of pully and the friction coefficient on the friction torque and the transmission efficiency are analyzed, moreover, the parameter identification experiment of the system internal impedance torque is conducted to compensate the friction torque; The sensor system includes the orthogonal encoder, the plantar force sensor, the human-machine interaction force sensor and the sEMG sensor is installed for the motion identification and the assistance effect evaluation.
The positive kinematics equation of the single exoskeleton leg model is built, and the operation space and workspace analysis of joint variables are carried out, the workspace is solved by geometry method and Monte Carlo method respectively. The inverse kinematics solutions of forward kinematics equation, the Jacobin matrix, kinematic redundancy and motion singularity are analyzed. The dynamics model analysis of early single stance phase and late double stance phase of the lower-limb exoskeleton has been conducted, the Lagrange dynamics equations is given and the torque of each joint in the walking process is calculated. The man-machine exoskeleton system dynamics model is analyzed and the influence factors of the joint drive torque are determined, which provides the basic idea to make the assistance control strategy.
The control strategy of man-machine exoskeleton system is analyzed and the hierarchical intelligent control method based on the model and the calculated torques is adopted to power-assisted motion control of the exoskeleton active joint. First of all, judge the exoskeleton joint motion and experimenter’s walking intention through the perceptual layer; Secondly, deliver the perceptual layer information to the decision-making layer, based on that, the exoskeleton joint ideal output torque, human-machine control torque, single leg gravity compensation torque and system internal impedance torque is calculated, the impulse module release state is determined, and finally the ideal motor output torque is confirmed; Finally, the value of ideal motor output torque is delivered to the executive layer, through the servo motor to realize the active control of exoskeleton joint. In order to validate the effectiveness of exoskeleton control strategy, the wearing walking tests and exoskeleton assisted effect evaluation are conducted.