Research On Six-axis Force Torque Sensor And Its Online Calibration For Space Manipulator

Space manipulator is an important tool in the task of space station construction, maintenance, management and extravehicular activity. As the urgent demand of the Chinese Space Station(CSS), it is focusing on the key force/torque sensing technology which is the necessary for space manipulator to carry out fine and complicated operation task, force control and motion compliant control in this paper. The purpose of this paper is to develop a novel six-axis force/torque sensor(F/T sensor) with good isotropy, high reliability, high accuracy, high stiffness, high sensitivity, good integration, zero drift and temperature drift compensation function and overload protection capability. Furthermore, it will pay attention to optimal design, sensor static performance, temperature drift compensation and online calibration of the F/T sensor. This dissertation will enhance the development effectively of China, and provide the necessary condition for the project CSS implemented successfully.The traditional F/T sensor is often developed by trial and error which causes it mainly depend on the designer’s experience and capability. Moreover, it is often unable to obtain the optimal performance F/T sensor. This paper proposes a method to optimize the F/T sensor by response surface methodology(RSM), and take the optimization of space manipulator F/T sensor elastomer as an example. The method uses RSM and requires only a small amount of refined finite element calculation to obtain a sufficiently accurate approximation mathematical model of the F/T sensor by arranging reasonable design of experiment. Take the fitting mathematical model as the basis, the minimum condition number of strain compliance matrix of F/T sensor as optimization goal, the structural parameters of elastomer as design variable; and the sensor sensitivity and stiffness as the constraints, then it can obtain the optimal sensor model quickly.To develop a novel F/T sensor for space manipulator, and refer the foreign references, the developed sensor uses the principle of resistance strain. Considering the requirements high stiffness and integration of space manipulator, a new elastomer structure is designed. The elastomer has through holes in the form of a parallel plate beam structure, so as to improve the sensor sensitivity effectively under the condition keep stiffness. To protect the sensor from damage in the harsh operating conditions, and the mechanical overload protection is designed by ladder pin and through-hole. To fulfill zero drift and temperature drift compensation of the F/T sensor, the D/A compensation and temperature acquisition circuit are designed. In order to facilitate the integration with space manipulator, the sensor has force/torque signals decoupled and temperature information processed capabilities by itself, and communicate to the manipulator joint controller via RS-422.To obtain the static properties of F/T sensor, a calibration system is designed based on the pulley-weight, and the calibration system error is analyzed. Using the least squares method to obtain the F/T sensor calibration matrix, and to obtain static performance and coupling error by load-unload test. Furthermore, we found the sensor temperature drift is very serious by experiment, so the temperature drift is compensated by particle swarm optimization least squares support vector machine. The experimental results demonstrated that the method has effective effect on compensation the sensor temperature drift. In addition, the stiffness properties, overload performance and mechanical vibration characteristics of the sensor are studied by experiment.To solve online calibration problem of the F/T sensor applied in space environment, a method to identify the F/T sensor by space manipulator joint torque sensor is proposed. A number of different forces are obtained by applying control commands to the manipulator joint so that the internal forces in the manipulator equilibrium, when the space manipulator end effector grasps a fixed point and ensure the joint posture unchanged. By the force Jacobian matrix, the torque information detected by joint torque sensor was mapped onto F/T sensor as a standard load, acquire the actual output voltage signal of F/T sensor simultaneous, thereby establishing an equation among standard input loads, the actual output voltage signal and calibration matrix. In order to obtain a more accurate calibration matrix, it can change the manipulator posture, and repeat the above operation to gain numerous experimental data. Finally, to solve problem of F/T sensor online calibration, the least squares method is applied to process the experimental data, the experimental results shown that the method is rapid and effective.