Research On The Cable-pulley Underactuated Lower Limb Exoskeleton

Exoskeleton is a power-assisted device,which has comprehensive applications in military and civil fields.The lower limb exoskeleton is aiming at reducing metabolic cost and muscle effort and increasing walking speed generally.For the traditional fully actuated exoskeleton,although it allows the wearer to carry heavy loads easily,it is bulk resulting from many active actuators and the consumption of energy is large.As for the passive exoskeleton,the energy storage strategies are applied by adding elastic energy storage components at the joints,which could improve the metabolic economy.However,it cannot provide active power assistance for climbing stairs and load-carrying walking due to the lack active actuators.For the traditional underactuated exoskeleton,only one joint is actuated generally to reduce the number of actuators,which limits the assisting performance.A new concept of soft suit driven by Bowden cable was developed by many research institutions.The soft suit is worn like clothes,which minimizes the effect on the wearer’s potential natural biomechanics.To achieve both small number of actuators and good assisting performance,this paper proposes a cable-pulley underactuated principle-based lower limb exoskeleton.Firstly,the human biomechanical model was modeled in an accurate way via ADAMS and Life MOD to acquire the kinematics results of human lower limbs.Based on these results,through using synergies of torque and power assistance,we developed an innovative concept of a cable-pulley underactuated principle-based lower limb exoskeleton that a single cable was able to drive the hip joint and the knee joint simultaneously.Then,the relation between the travel distance and the generalized coordinates of the hip and knee joints was analyzed.Following that,finite element analysis was performed on key components to verify their strength.Secondly,the kinematics and dynamics of the designed lower limb exoskeleton was analyzed.Then the kinematics equations were derived to acquire the end motion equations in working space.After that,the Lagrange method was utilized to establish the dynamics equations in the three selected stance phases.Next,the interactive dynamics between human body and exoskeleton was analyzed.The relationship between the cable tension and joint torque was derived to calculate the required tension or position of the cable when the specific torque or position was desired at the joints.Furthermore,the control strategy which could achieve the torque and power synergies was exploited by exerting an assisting torque in the selected time window.In order to generate the desired joint torque trajectory or reference cable tension,the Newton-Euler dynamics method was implemented,which was more efficient for computer calculation.In the next step,the human-exoskeleton simulation was carried out to evaluate the performance of the assistive effects.The final results demonstrate that the proposed exoskeleton could provide effective power assistance and reduce the energy consumption for human load-walking.Finally,the control system and sensor system was integrated to build a complete physical prototype which is only 8.6kg.Considering different velocity working conditions in actual applications,the experiments for three selected velocity were performed.In the data analysis,the information regarding cable tension and joint torque and power,and heart rate was collected and processed to evaluate the assistance performance.Eventually,the simulation results indicate that the proposed lower limb exoskeleton with only two actuators is compatible with human motion and feasible to reduce metabolic cost in load-carrying walking.