Research On Grasping Characteristics And Control Strategies Of An Under-actuated Cable-truss Robotic Hand

With the increase of human space exploration activities and deep space explorations,on-orbit service is need to meet to spacecraft large-scale needs,to ensure spacecraft safe and long-term working.The use of capture mechanisms to achieve on-orbit capture is the basis for on-orbit service,and has become a hot research topic in the aerospace field.Currently,special docking mechanisms and space robotic arms have been successfully applied in the aerospace field.However,special docking capture requires high mechanism matching,and the number of motors in space arms is large,the energy consumption is large,the motor control system is complicated.These two capture methods are mainly used for specific capture tasks,especially for cooperative targets capture.The application of SARAH hand on the ISS makes underactuated mechanisms as an important research direction in space robots.According to the characteristics of the existing underactuated mechanism,a noval underactuated cable-truss(UACT)robotic hand is proposed,which consists of tendonpulley(TP)transmission and parallel four-linkage mechanisms and can be extended to large-scale space robotic hand.According to the virtual work principle,the TP trassmission is transalted to the equivalent joint drive forces,and the relationship among the cable driving force,joint coordinates and contacting forces are established.Based on the indexes of radius ratio length and ratio quality of whole-knuckle envelop grasping,the influences of number of knuckles,knuckle length to width ratio and knuckle surface friction coefficient on grasping space are analyzed.The quasi-static model of the UACT finger is established,and the influences of joint spring and joint friction on the quasi-static motion are analyzed.When a single main drive cable works,the distributions of the generalized contacting forces are analyzed by considing or non-considing the passtive elements.According to the distributions,the grasping process of the UACT finger is divided into four stages.At the pre-bending stage,four grasping strategies and corroding distributions of cable driving forces are further analyzed,and the reason of the motion singularity is explored theoretically.At the grasp-closing stage,the UACT finger has the characteristic of grasping adaptation which is common in underactuated mechanisms.In this thesis,the adaptive grasping force is studied at the grasp-closing stage.Taking 2-DOF finger as the research object,fixed-point grasp model is established.Then the joint workspaces and the relationship between the equilibrium state of each finger and the joint workspaces are analyzed.Thereby,the self-adaptive grasp is explained in principle.In addition,the fixpoint grasp model can be used to analysis muli-knucle UACT fingers.Taking 3-DOF UACT finger as an example,on the basis of a situation where one or two phalanges are in contact with the object,fixed-point grasp models and the joint workspaces are discussed,respectively.The corresponding workspace that is formed when the distal phalange contacts the object at a fixed point is assessed through numerical analysis.All eight self-adaptive grasp processes are discussed,and four self-adaptive grasping results are analyzed.The cable driving force in the UACT finger is distributed on each knuckle unit at the same time,and the deformation of each knuckle unit directly affects the distribution of the equivalent joint driven forces of the rope.Therefore,the UACT finger has strong nonlinearity and motion coupling characteristics.Based on the energy equivalent method and the equivalent method of the TP transmission system,the parallel four-linkage mechanism unit is simplified.On basis of the Lagrange method,the general dynamic equation of the 3-DOF UACT finger is established,and generalled dynamic equation with coupling joint driven forces is translated into the "A-P" type underactuated system.Based on the dynamic model and quasi-static equilibrium position distribution,three kinds of motion control strategies at the pre-bending stage are analyzed.The dynamic reduction model of the UACT finger is established the the finger at grasp-closing stage.Under condition that the distal phalanx contacting object with a constant force,the motion control strategy at grasp-closing stage is analyzed.The prototype of UACT hand is designed and developed,which owns two fingers and each finger has three knuckles.The sensor system,motor drive system and hardware control system are installed and tested.Single knuckle sinusoidal trajectory tracking experiment and robotic enveloping function verification experiment are carried out.The self-adaptive grasping experiments are designed and carried out.five groups of effective self-adaptive grasping experiments verified the correctness of the theoretical analysis of 3-DOF self-adaptive grasps completely,three motion control experiments at the prebending stage and one motion control experiment at the grasp-closing stage verify the effectiveness of the proposed grasping control strategy one by one.The prototype experiment shows that the innovative design and analysis method developed in this thesis is reasonable and effective,which provides a new theoretical basis and technical achievement for the research of underactuated robotic hand.