Modellering och reglering av mjölkningsrobot
In this master thesis a robot arm belonging to an automatic milking machine from DeLaval is investigated. The robot arm is controlled by a pneumatic cylinder and a valve. The goal of the master thesis is to investigate instability problems and to find out if it is possible to solve them by using gain scheduling. Gain scheduling is a type of controller where the parameters of the controller are changed during operation, for example depending on the working point. First a nonlinear mathematical model of the system was made. Then a test rig was built with a robot arm and sensors, identical to the ones mounted on the real machine from DeLaval. The different parameters of the nonlinear model were then tuned and validated against data from the test rig to make sure that the model is correct. After that the nonlinear model was linearized for a closer analysis of the parameters that seemed to have an apparent effect on the system behaviour. The result of the analysis showed that the probable cause of instability is a decrease of the viscous friction, which gives worse damping characteristics. It was more suitable to base the controller parameters on the valve opening instead of the working point for the robot arm, since the gain in the valve increases with increasing valve opening. To avoid deterioration in phase margin because of the increased gain, the controller is adjusted so that the total gain is the same regardless of valve opening. The result is a system with larger bandwidth, less sensitive to changes in the viscous friction.