Research On Three-Dimensional Force Sensing Of Robot Wrist Joints Based On Fiber Bragg Grating

Since the twenty-first century,robotics research has made great progress and has been widely used in various industries,which has an extremely far-reaching impact on today’s society.As one of the most important sensors in robot systems,3D force sensors are often loaded at the wrist joints of robots to achieve interactive force measurement and feedback,helping robots to accurately complete highly flexible automation tasks in complex assembly environments and improve work accuracy and efficiency.For the 3D force sensor is difficult to resist electromagnetic interference,the wiring is complicated,the coupling between mechanical structure is serious,and the decoupling algorithm is not effective in decoupling.In this thesis,we adopt theoretical analysis,simulation study and experimental study as the main research line of "FBG-based robot 3D force sensing theory and method research,FBG-based spatial hierarchical perception structure 3D force sensor design and experimental study,and sensor inter-dimensional coupling decoupling method research".The main research works are as follows:(1)Based on the FBG sensing theory,the fiber Bragg grating strain sensing model and temperature sensing model are constructed to reveal the mapping relationship between FBG wavelength drift and strain and temperature.Different temperature compensation methods are compared,and the optimal method is selected to achieve temperature compensation.Based on the principle of material mechanics,a mechanical model of the sensor elastomer structure is constructed to reveal the mapping relationship between the 3D force and strain on the sensor,and the mapping relationship between the FBG wavelength drift and the 3D force on the sensor elastomer structure is deduced.(2)For the spatial 3D force measurement of the robot wrist joint,an FBG-based3 D force sensor for the robot wrist joint is proposed.The unique spatial layered structure and assembly structure of the sensor are designed,the deployment method of the sensitive element FBG is studied,the coupling interference of force components between dimensions is reduced,and the temperature compensation is realized based on the reference grating method.The finite element simulation study was carried out to study the load response characteristics,stress distribution characteristics and structural coupling characteristics of the sensor based on the static simulation analysis,and the accurate pasting position of the sensitive element FBG was determined based on the line strain simulation analysis.Based on the modal analysis,the dynamic characteristics of the sensor were investigated.(3)The experimental study of the 3D force sensor of the robot wrist joint was carried out.The mechanical calibration experimental system was constructed and the static performance parameters of the 3D force sensor were studied.The sensitivity of the sensor in x,y and z directions were 3.09pm/N,3.08pm/N and 1.89pm/N,respectively.and 1.62%,and the maximum hysteresis errors were 2.35%,3.71%,and1.12%,respectively.A temperature test experimental system was constructed to study the temperature compensation characteristics,and the average temperature compensation errors were 1.23%,1.16% and 1.56%,respectively.Based on the principle of FBG 3D force sensing,the structural coupling characteristics of the sensor were analyzed,and the maximum coupling interference values of the sensor in x,y and z directions were 5.97 N,5.22 N and 5.38 N.Finally,based on the method of mean calibration matrix,the data calibration was carried out,and the maximum coupling interference values of the sensor in x,y and z directions were 9.45 N,3.79 N and 3.18 N after calibration.(4)To address the problem of poor decoupling effect of sensor decoupling algorithm,linear decoupling research was carried out,and linear decoupling feature analysis was performed based on the least squares decoupling algorithm.In order to eliminate the nonlinear coupling error,the nonlinear decoupling research was carried out,and based on the ELM algorithm,the ELM neural network nonlinear decoupling model was established,the sensor inter-dimensional coupling relationship was constructed,and the nonlinear decoupling feature analysis was carried out.nonlinear decoupling feature analysis.The decoupling effects of the above algorithms were compared,and error analysis was performed.The maximum Ⅰ errors of the SSA-ELM algorithm in each direction were 1.58%,0.99%,and 0.98%,and the maximum Ⅱ errors in each direction were 0.64%,0.47%,and 0.93%,respectively.The average Ⅰ errors in each direction were 0.72%,0.60%,and 0.49%,and the average Ⅱ errors in each direction were 0.36%,0.29%,0.26%,meeting the precision measurement requirements of the robot wrist joint sensor with an average error of less than 1%.In summary,for the problems related to force sensing measurement and fiber grating temperature-strain sensing of 3D force sensor of robot wrist joint,this paper systematically investigates the principle,method and related technology of 3D force measurement based on FBG sensing principle.Based on finite element simulation analysis,an FBG-based 3D force sensor for robot wrist joint is proposed,which has a unique spatial sensing structure and provides a new idea for the design of weakly coupled 3D force sensors.The algorithm has a good nonlinear decoupling capability,which is an important reference value for the optimization and improvement of the nonlinear decoupling algorithm.The average type Ⅰ and Ⅱ errors of the sensor in each direction after decoupling are less than 1%,which meets the precision measurement requirements of the 3D force sensor of the robot wrist joint.