| Friction exists widely in real systems and there is no exception for robot. Friction is a complicated, nonlinear phenomena, it varies with temperature and age. Due to the strongly nonlinear characteristics of friction at low velocity, control performance deteriorates dramatically when robot is moving slowly or turning around. Friction may lead to large tracking error and limit cycles when velocity reversals in the trajectory are required.Hydraulic turbines of power stations are subjected to destruction due to serious cavitations and wear, so they are needed to weld and polish with Special-Purpose Robot regularly. In this case, we require the robot is easy to operate, has fast response and flexible controller. Joint friction is one of the major limitations in performing high precision manipulation robot tasks. Thus, how to identify and compensate nonlinear friction in the Special-Purpose Robot for Hydraulic Turbine Repair is very significant for enhancing tracking precision and improving economic effect. This paper deals with joint friction compensation techniques for accuracy trajectory tracking control of the Special-Purpose Robot for Hydraulic Turbine Repair.At first, on the basis of physical characteristic of friction, four stages were analyzed when frictional force varies with speed in detail. Several kinds of friction models put forward in recent years have been introduced. Through comparison Stribeck, LuGre and Coulomb+Viscous, three kinds of models were chose as the research object.Having analyzed the robot's various joints function when in welding. Adopting Lagrange method, robot's last two joints dynamics equations is set up, which affect the robot trajectory tracking heavier, and calculated the corresponding parameter. In model-based friction compensation, static exponential model are selected and PD+feed forward friction compensation is carried out, in order to overcome the tracking error when the robot is in welding. Computer simulation results have shown that the method is effective to improve the robot trajectory accuracy.Aimed at the robot's end joint, Reasoning the ideal condition of joint servo control, and dynamic model is established which including transmission system, drive motor and joint friction. (1) LuGre model for friction compensation is converted into simulation model by MATLAB; proper parameters are selected to operate. (2) According to the systems response, the on-line genetic algorithm is presented to studyand optimize the friction coefficient of the robot's joints friction model. The friction compensation of robot is done with the optimize friction model; simulation results indicate that our proposed controller had advantages of rapid response and better performance, eliminated the influence of the friction. |
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