Research On Joint Control System And Trajectory Planning For Space Large Manipulator

In the wake of developments in space exploration, robotic system has graduallybecome a useful tool of opening up space resources, especially are the space largemanipulators playing an irreplaceable role in the construction and maintenance of theinternational space station. The space large manipulators are characterized by highoutput torque, wide working space, high control accuracy, long design life-span, andhigh reliability; thus the design and control techniques related to the space largemanipulator are extremely challenging. With the development of manned space stationin China, the space large manipulator technique has attracted much attentions of therelevant departments of the government, and been becoming a research hotspot in thedomestic space field. Supported by the project titled Development of Ground PrinciplePrototype for Space Large Manipulator, my doctoral work was devoted to developing ajoint control system (a key component of the space large manipulator system) with highreliability, high precision, and strong fault tolerance. On the basis of it, the workfocusing on the high-precision position control and vibration suppression strategy forthe robotic joint, and the trajectory planning method in the Cartesian space for themanipulator was conducted for a further investigation.On-orbit maintenance for the space large manipulator system is difficult to becarried out, whereas the mission of the space large manipulator system demands longlife-span and high reliability. A reconfigurable joint control system was thereforedeveloped on the basis of FPGA-FPGA to resolve the contradictions. Supported by theembedded microprocessor of FPGA, the design methodology combining hardware andsoftware was adopted to realize some specific functions, such as CAN communicationmanagement, sensor information collection and processing, motor drive, motion controland trajectory planning, etc. The merits of both cold-backup and hot-backup structureswere integrated into the newly proposed joint control system. Moreover, thecompleteness of joint function can be maintained via reorganization of fine resources incase of multiple misalignment faults between the master and slave control sub-systems.Compared to the traditional cold-backup structure of the controller, the reliability of themotor servo unit was raised from92.25%to95.90%. Furthermore, a powermanagement system and a temperature control system were designed with the corresponding anti-radiation measures, aiming to make the joint control system adapt tothe space environment. Thus the reliability of the joint control system can be improvedfrom the view of design as much as possible.To achieve the effective and fault-tolerant control of the servo system of dual-winding PMSM, the anti-windup vector closed-loop control of motor current with thecycle shorter than50μs was realized by means of hardware logic of FPGA using VHDLlanguage, when only single winding of the motor was at work. The response speed andanti-jamming ability of the motor servo system were therefore improved. A fault-tolerant vector control method for dual-winding PMSM was also presented, which wascapable of keeping current balance between the two windings of PMSM andmaintaining continuous operation of the motor even if one of the joint controllers failedto work. As a consequence, the fault-tolerant ability of motor servo system would befurther improved. Meanwhile, the real-time fault detection and the corresponding fault-tolerant solution were performed throughout the joint controller, the central controller ofrobotic arm, and the astronaut. That is, the security of manipulator can be guaranteedfrom the view of operation control.A cascade non-linear global close-loop anti-windup controller, composed of non-linear position control loop with speed feedforward,2-dof anti-windup speed controlloop and anti-windup current loop, was developed to improve the position controlaccuracy of the flexible joint’s output port. The controller exhibited global asymptoticstability. The experimental results indicated that the newly proposed controller was ableto guarantee the position control accuracy of the output port, even when a relativlylarger plastic deformation existed in the joint. In addition, the nominal calculated outputtorque of the joint was used to decompose the torque components, which were causedby the joint vibration, from the feedback information provided by the torque sensor. Thedecomposed torque components would be used for the negative feedback control ofjoint vibration. The experimental results showed that the approach was capable ofsuppressing the vibration of the joint rapidly, exerting little influence on the precision oftrajectory tracking.The terminal position and attitude of the free-floating space manipulator systemcan be planned using the inverse generalized Jacobian matrix method. However, theattitude disturbance of basement arising from the motion of manipulator cannot berestrained, as a result of the dynamic coupling between the space manipulator and its basement. To optimize the path of the space manipulator tip under the condition ofmulti-position restraint in the Cartesian space, the motion trajectory was parameterizedon the basis of revised cubic spline. The disturbance of basement arising from themotion of manipulator was thus expressed by the objective function about theinterpolating point of the spline and the migration time between two adjacent restrainedpositions. The global optimization of the objective function was realized using thegenetic algorithm. The simulation results indicated that the presented approach caneffectively reduce the influence of disturbance on the attitude of basement, which wascaused by the motion of manipulator, under the condition that the restraints wereexerted on multiple objective positions and attitudes of the manipulator tip.