Design And Hysteresis Study Of Four-bar Elbow Joint Driven By Pneumatic Muscles

The pneumatic muscle is compliant,and it driven elbow joint is light and flexibility,with high power weight ratio and perfect bionic character,which is beneficial to improve the safety of human-machine cooperation.However,the pneumatic muscle has strong nonlinearity and hysteresis,which affects the position control accuracy of the elbow joint.On the basis of the self-developed pneumatic muscle,the pneumatic muscle(diameter 40 mm)is prepared and the four link elbow joint driven by the pneumatic muscle is designed.Based on the hysteresis model of the elbow joint,the feed-forward Integral Inverse Compensator(I-I C)is designed to improve the accuracy of position control.Initially,the static model of pneumatic muscle is established.According to the working principle of the pneumatic muscle,the pneumatic muscle of 40 mm diameter is prepared.Building the test platform,test and analysis the pneumatic muscle contraction,output force,stiffness and hysteresis properties.Secondly,with reference to the body arm parameters and the principle of muscle force,the index of elbow joint and the way of driving are determined.Based on the requirements of the project to determine the size of the four link mechanism.The mathematical relationship between the rotation angle,the length of the pneumatic muscle and the size of the upper arm is built,and the Simulink simulation-platform is set up in order to analyze the driving force.In addition,using SolidWorks to design the three dimensional prototype,and the physical prototype of the elbow joint is made.The next step is building the control system and test platform.Then,the angle range and load capacity of the elbow joint are tested and the analysis of hysteresis characteristics are achieved.Then the PI(Prandtl-Ishlinskii)model of the hysteresis characteristics of the elbow is established,and the parameters of the model are identified by the Levenberg-Marquardt method.By comparing and analyzing the modeling results of classical PI(Classical Prandtl-Ishlinskii,CPI)and modified PI(Modified Prandtl-Ishlinskii,MPI)model,we can see that the MPI model has higher fitting degree for asymmetric hysteresis curves.Based on the MPI model,an feed-forward integral inverse compensator is designed and an integral inverse compensation controller(I-I-PID)is formed with PID.The simulation of position control of I-I-PID and PID controller has been completed.The results show that the control effect of I-I-PID is better.When tracking variable amplitude and variable period signals,the compensation effect is reduced.The final step is setting equal amplitude and periodic sinusoidal signals,experimental research on PID and I-I-PID controler.According to the compensation effect influenced by the load and angle dead zone which is explosed in the experiment,the control accuracy is improved by adding the output force variation coefficient in the compensator and the piecewise PID control.Eventually,the collision experiment is designed to verify the tracking effect of the I-I-PID controller for the variable periodic signal.Experiments results show that I-I-PID can improve the accuracy of elbow joint position control,but I-I-PID belongs to static compensation.When the tracking signal frequency is high or the load changes dynamically,the compensation effect is weakened.