It is necessary to install the extravehicular experimental load by using the load plate in the space station. However, the extravehicular environment is full of danger, which poses a threat to the health and even safety of astronauts. Using robots to replace astronauts to complete this task can effectively reduce the threat to astronauts.
In a research paper recently published in Space: Science & Technology, Dr. Sun from the School of Mechatronical Engineering, Beijing Institute of Technology, proposed the autonomous assembly method of the load plate to solve the difficulties in computing the inverse kinematics of the redundant manipulator, the challenges of complex environmental lighting, and difficulties of matching multiple groups of holes and pins in the load plate assembly task.
The main work carried out by the author's group is organized as follows:
- A brief introduction of the 3-arm robot is made in the first section, which consists of three uniformly distributed 4-DOF arms. Each arm consists of a 3-DOF wrist joint and a 1-DOF elbow, totally 3×4=12 DOFs. Each end of the arm is equipped with a gripper for movement and operation in the space station. In order to get environment information and ontology state, each arm of the robot is equipped with a monocular vision camera and a 6-dimensional F/T sensor.
- Afterwards, a variable D-H parameters inverse kinematics method for redundant DOF serial manipulator is proposed as a foundation for humanoid dexterous operation planning of the robot.
- Then, the accurate pose estimation and the autonomous operation control are discussed in order to achieve the safe and efficient autonomous operation abilities for space robot. On one hand, triangulate geometric constraint 6DOF pose estimation algorithm (TGCPose6D) is implemented for accurate and robust pose estimation of the target through using monocular vision in the environment of strong range variation of light, chaotic background and occlusion. On the other hand, the author develops an autonomous target positioning and motion planning method based on visual guidance, and then ensures the safety during operating by compliance control.
- Finally, the safety and robustness of the robot in the autonomous assembly of load plate with multi pins and holes are verified by experiments. In the assembling experiment, according to the visual positioning results, the robot plans an end trajectory as the reference trajectory of the admittance controller, and uses the formulated inverse kinematics to solve the joint trajectory. On this basis, the control strategy described before is used to continuously modify the assembling trajectory according to the contact force, and finally realize the safe assembling of the load tray and the load chassis. Experiments show that the 3-Arm robot can accurately estimate the pose of the load chassis and effectively suppress the oscillation in the assembly process, so that the robot can complete the autonomous assembly task of the load plate safely and reliably.