Study On The Compliant Control System Of A Manipulator Handling A Flexible Payload

Robotic manipulator has been widely used in many fields, such as shipbuilding, air craft and car, with the development of the robot technique. Recently, there are many scholars who have done much work in manipulating rigid load, while in many fields, flexible material has been used largely. So far, there are few scholars who have done such work in which flexible material is manipulated by rigid manipulator. The system composed of robot manipulators and flexible payload is a complex system which has characteristics as following: exceeding nonlinearity, strong coupling and infinite degree of freedom. That is why it is important to study the problem of this sort. There are lots of challenging problems which need to be solved in the current conditions domestic and abroad. The way to establish the mathematical model is various, and it is hard to decide which is the best one because of the characteristics of their own. In the process of handling flexible payload, two actions will happen that are rigid motion and vibration, and it is extremely hard to achieve the goal in which the two actions are controlled.In this paper, the background and significance of studying manipulator handling flexible payload are introduced, the modeling of flexible beam and vibration suppressing are introduced respectively, and the achievements of studying manipulator handling flexible payload are recalled simultaneously. Their advantages and disadvantages are analyzed through comparison. The kinetics of the system which consists of a rigid manipulator and a flexible payload moving in the horizontal plane are particularly analyzed before the studying of dynamics based on the known achievement. The dynamic model of the flexible payload is achieved using finite element method. The interaction before flexible beam and manipulator is studied and the coordinates of a rigid manipulator are changed into the workspace, then, the whole system model is achieved by combining the model of flexible payload and the rigid manipulator in the workspace.Considering the error between parameters in ideal model and actual device in the industrial manufactures, when the control scheme is designed, some important matrices could not be used in the controller because not all the parameters in the system are known in fact. In the actual manufactures, the parameters of manipulator's end-effector are sometimes imprecise because of adding special device, such as force/torque sensors, or the maintenance of electronical parts. For which, the error between parameters in ideal model and actual device is not always so little that it could be ignored. To solve these problems, the unknown parameters are estimated by the estimated law based on the adaptive theory, so, the performance would be good enough even under the condition that the actual value of estimated parameters are not known precisely in the controller. In this paper, the controlling objectives are trajectory tracking and vibration suppressing. A lot of well-known control schemes can not be used directly in the model studying in this paper because the control inputs are less than the degrees of freedom of the whole system. So, the adaptive controller in this paper is different from the traditional adaptive controller. A special control part which is used to suppress the vibration is designed, and the flexible vibration can be controlled through the coupling between the directly controllable coordinates and the indirectly controllable coordinates. The uncertainty contains not only the constant parameters but also the effect of unknown force which exerted on the left end of the flexible beam. It is easy to see that the error brought by the unknown force can not be avoided by changing method of modeling or being more precise. Because of this, the unknown force which is obtained from the information sensed by the wrist-mounted force/torque sensor are used in the controller, but this is not good enough because of the measurement error during sensing, so, a compensating part is designed to eliminate the effect of measurement error for the reason that the measurement error is in a limited boundary.The performance of system is nice under this control scheme, which is verified through simulations. However, it is achieved under the assumption that all the states are measurable, which is accessible for manipulator but not easy for flexible payload, though part of the flexible coordinates could be identified using wrist-mounted force/torque sensor, it requires more complicated equipment. So, the advantages of adaptive control scheme and rigid control are combined based on the results of rigid control scheme achieved by others. The new adaptive rigid controller is effective for trajectory tracking and vibration suppressing considering the uncertainty of system. In the simulations which are made in the absence of damping and in the presence of damping, the characteristics of the two controllers are studied, and it is verified that the vibration of flexible beam could be suppressed under the condition in which the nature damping and structure damping exist.Some basic work has been done for manipulator handling flexible payload in this paper, due to its complexity, the method of modeling and the control scheme of trajectory tracking and vibration suppressing are both demanding more research, while the uncertainty is still a challenge to scholars. This paper solved the problem of modeling and designed two controllers which could track the desired trajectory and suppress the vibration while compensating the effect of unknown force, but there are lots of work remained needed to be solved.