Design Of Magnetorheological Prosthesis And Research On Trajectory Tracking Control

Lower limb prostheses play an important role in human production and life.On the one hand,it can provide support and maintain balance during the stance phase of the human body.On the other hand,it can provide bending motion during the human body swing phase to make people walk normally.Compared with active and passive prostheses,semi-active prostheses have the advantages of relatively low price,low power consumption,controllable damping and natural gait,and thus they have broad market prospects.Magnetorheological prosthesis(MRP)is a new type of semi-active prosthesis which utilizes a magnetorheological damper(MRD)as an actuator.Its innovation lies in that by controlling the input current of the MRD,the required damping force of the knee joint of prostheses during movement can be continuously adjusted.According to MRD-based control strategy,it can produce consecutively adjustable damping force,so as to achieve the effect like active prostheses.Existing prostheses based on magnetorheological technology usually have commercial MRDs directly mounted on the knee joint,which not only fails to exploit the damping performance of the MRDs as an actuator,but also the structural size of the MRDs often does not meet the design requirements of prosthesis.The control algorithms of MRPs show poor control effects and is complicated due to the nonlinearities and coupling terms of MRP systems;furthermore,existing research overlook system modeling uncertainties and external disturbances in practical applications.In addition,it is of great necessity to consider the influence of the dynamic characteristics of MRDs when devising MRP control strategy.Based on this,this article has launched the following research work.1.A single-axis MRP with MRD as the actuator is proposed,and the structure of the MRP is designed.According to the established rigid body model of the prosthesis,its kinematics model is established;based on the first-order Lagrange method,the dynamic model of the prosthesis is established.Based on the gait cycle of normal people,the swing trajectory equations of hip and knee joints when walking on flat ground are obtained.Combined with the kinematics and dynamics models of MRP,the layout type and installation size of MRD are determined.On this basis,the range of damping force and the stroke of MRD can be determined.2.According to the demand of MRP,a new type of MRD is proposed and designed;the magnetic circuit of the piston head is designed and calculated;the material of each part is selected to determine the structural parameters of MRD.Based on Bingham constitutive relation,the forward and reverse mechnical models of the damping force are derived.The dynamic performance of MRD is tested and analyzed,and the damping force characteristic curves under different loading frequencies,amplitudes and currents are obtained.In order to accurately characterize the dynamic performance of MRD,the forward mechanical model of MRD is established based on the adjustable sigmoid model,and the reverse mechanical model is established by using back propagation(BP)neural network.3.Aiming at solving the nonlinearities and coupling terms of MRP,the computing torque plus proportional differential(CT+PD)control algorithm is presented to track the knee joint trajectory of MRP.Considering the problems of uncertainties and external disturbances in MRP model,the second order sliding mode control(SOSMC)algorithm is proposed to track the reference swing trajectory of knee joint effectively.Based on Lyapunov stability theory,the asymptotic stability of CT+PD control algorithm and SOSMC algorithm is theoretically proved.4.The MRP prototype was processed and the MRP experimental test system was built to measure the swing angle of knee joint of MRP based on fixed damping coefficients,CT+PD control and SOSMC algorithms.The swing trajectory based on fixed damping coefficients,CT+PD control and SOSMC algorithms are compared with reference gait trajectory and the results indicate that the proposed MRP based on SOSMC can realize natural gaits.