Research On Force Assisted Assembly Strategies For Robotic Remote Welding In Unstructured Environment

For extreme environment tasks such as maintenance in nuclear power plants, marine engineering undersea, construction of space stations and pipelines repairing underground, it is necessary for operator to implement remote welding at a safe place away from the worksite, monitor the welding process and control the welding equipments with sensor feedback information from a distance. The maintenance scheme based on pipe segment replacement strategy with robotic remote welding illustrates a wide prospect of application for crack repairing task on pipelines in hazardous environment. In this dissertation, the assembly strategies for robotic remote welding in unstructured environment based on force sensing and control have been studied for the realization of tool assembly operation in remote welding scheme with pipe segment replacement, and the research work will be the foundation for futher practical application of robotic remote welding technology in extreme environment pipeline maintenance area.According to the features of tool assembly operation during pipeline maintenance process, a force sensing and control system for robotic remote welding is designed and developed with position-servo open architecture robot, six-dimensional force/torque sensor and force-feedback joystick, which is the research platform for key technologies of force sensing and control, such as shared force control for surface tracking and active compliant control for tool assembling. The core issue for research on robotic remote welding assembly strategy is how to use the key technologies to implement assembly task in unstructured environment.Based on the analysis of remote welding task space and human-machine intelligence distribution mode, the strategies of shared supervisory with passive compliance and local autonomous with global teleoperation are proposed for assembly task in unstructured environment. For shared supervisory strategy with passive compliance, task space calibration technique based on surface tracking with shared force control is adopted to turn the unstructured environment into a structured environment. The shared supervisory control of assembly task is achieved by human-computer interactive task analyzing and planning technology and human-machine shared teleoperation assistance function. The assembly error is compensated by passive compliant device. For local autonomous strategy with global teleoperation, the assembly tool is manipulated in global teleoperation mode to contact with the task environment. The discrete event control of tool assembly process is based on the actual contact force information with tool gravity compensation and contact state model with controlled Petri Net. The deviation of assembly position and orientation is eliminated by active compliant control.Task space calibration technique based on surface tracking with shared force control is studied, while the task environment surface tracking strategy based on spherical force probe and constrained motion plane is adopted. According to the force status analysis of contact point on tracking trajectory, a local autonomous force control algorithm for surface tracking with position-servo robot is proposed, and the shared force control system for surface tracking is established to realize task space calibration. Based on the algorithm of coordinate system transformation, the accurate spatial locations of contact points on tracking trajectory are calculated. A task space calibration algorithm for pipe workpiece, in which the elliptic tracking trajectory on pipe surface are fitted by direct least squares algorithm, is presented to make structured environment model meet the requirements of passive compliant assembly accuracy for pipe maintenance remote welding.Shared supervisory strategy with passive compliance for assembly task is based on the concept of hierarchical intelligence and atomic task. A human-computer interactive task analyzer and planner is designed and implemented to construct the supervisory control command sequence of atomic tasks. For assembly task in restricted motion space, a human-machine shared control strategy based on planar and linear-velocity-force teleoperation assistance function is proposed to improve the adaptability and reliability of task analyzing and planning system. An atomic task compiler is developed to interpret high-level command sequence into intermediate struct-format command sequence. Robot command sequence is generated to achieve tool assembly task with passive compliance.The load parameters calibration algorithm is based on tool gravity calculation and direct least squares solution under non-contact gravity condition. Tool gravity compensation technique is studied to eliminate the gravity disturbance during active compliant tool assembly process, and make the precision of gravity compensation meet the requirements of local autonomous force control for tool assembly task.Local autonomous strategy with global teleoperation for assembly task is based on contact formation description and contact state analysis. A discrete event controlled Petri-net model of tool assembly contact state is constructed. An identification method of assembly contact state transition with state mark checking is proposed. A discrete event controller is designed to realize local autonomous compliant control of tool assembly process which is based on the actual contact force information with tool gravity compensation. The deviation of assembly position and orientation during tool assembly operation is gradually eliminated by active compliant control.Based on pipe segment replacement strategy of crack repairing tasks on pipelines in extreme environment with remote welding, the experiments on assembly operation in unstructured environment are performed with shared supervisory strategy with passive compliance and local autonomous strategy with global teleoperation respectively. The comparative analysis is made with respect to the operation accuracy, execution efficiency, adaptability and reliability. The analysis results show that, the adaptability and reliability of shared supervisory strategy with passive compliance are greater, while the operation accuracy and execution efficiency of local autonomous strategy with global teleoperation is higher. The research work on force assisted assembly strategies for robotic remote welding in unstructured environment provide a practical solution of tool assembly operation during remote welding for pipeline maintenance.