Study On Structure Design And Mechanical Propertes Of Pneumatic Flexible Wrist With Braking Function

According to structure,driving mode and the flexible degree,the robot wrist mainly divided into series-wound wrist,shunt-wound wrist,motor driven wrist,gas driven and hydraulic drive wrist,rigid wrist and flexible wrist.There are many disadvantages of the mechanical wrist,such as poor flexibility of noumenon structure,low safety of man-machine integration,and heavy structure,and so on,which seriously restrict the application and development of service robots.While the robot wrist driven by flexible pneumatic joints has good adaptability and flexibility and pollution-free as well,which result in a wide application prospect in the service robot arm.In this paper,the driving and braking mechanism,static and dynamic performance,control system and motion control strategy of pneumatic flexible wrist are studied,in order to solve the problems such as the great influence of load on the deformation of flexible joints and the difficulty in maintaining posture.Firstly,the driving and braking mechanism of pneumatic flexible wrist were studied.The pneumatic artificial muscle axial driver is used as the wrist driving device to push the upper end cover of the wrist to rotate about the geometric center of the brake to complete the pitching and yaw of robot wrist when the artificial muscle elongates in axial direction.We designed the normally open spherical brake to solve the problem of multi-direction space braking of robot wrist.This brake can control the braking torque and it also can adjust the motion resistance of the ball seat and the cage and modify the stiffness of robot wrist by varying the air pressure of the drum brake air bag.Meanwhile,the theoretical model of the braking performance of the normally open spherical brake is established and verified by experiments.The results show that the theoretical model agrees with the experiment and the brake torque is proportional to the air pressure.Secondly,the annular section axial actuators and the internally guided pneumatic flexible axial actuators are presented.The structure and motion characteristics of these two actuators are analyzed,the theoretical model of driving force and air pressure is established,and the experimental research is carried out.The results show that the theoretical model is consistent with the experimental results,and the driving force increases linearly with the increase of air pressure.Then a functional prototype of pneumatic flexible wrist was developed.The whole wrist is cylindrical,with a height of 100 mm and a diameter of 110 mm.It is composed of 4 drivers and 1 brake and the mass of the whole machine is 0.54 kg.The wrist has 1 degree of freedom and 3 degrees of mobility,each of the two actuators work together can make the wrist end cover to move in different directions.When the working pressure is 0.34 Mpa,the rotation angle of end cover is 20.72°.The relationship between the rotation angle of the end cap and the working pressure of the wrist is obtained by solving the moment balance equation,The correctness of the theoretical model is verified by experiments: the rotation angle of the end cap is positively correlated with the working pressure.The three-dimensional motion capture system are used to complete the wrist dynamics experiment.The time-domain and frequency-domain characteristics of the wrist under different signals are analyzed,and the influence of the working state of the brake on the time-domain and frequency-domain characteristics of the wrist is studied by changing the brake state.Next,the stability of the wrist control system and control strategy is verified by grasping experiment.The motion posture of the wrist is detected by the gyroscope sensor and feedback is formed with the wrist controller to improve the motion precision of the wrist.The upper end cover of the wrist is connected with the pneumatic flexible five-finger manipulator,which can realize the position adjustment of the manipulator,and the control system and control strategy are stable and reliable.The grasping modes can be classified into five-finger grasp,five-finger pinch,four-finger grip and two-finger pinch,according to the weight and shape of the object to be grasped.The experiments show that the robot could grasp the spherical and cylindrical objects stably,and the maximum weight of the grasping object can reach 1.34 kg.In a conclusion,the pneumatic flexible wrist developed in this paper has good flexibility and strong ability to keep posture,and it also has the ability to imitate human wrist,such as pitching and yaw,and so on.The research of this paper lays a foundation for the research and application of flexible wrist of service robot.