Modeling and motion control of a pick and place machine with air bearings

Abstract: To minimize friction related heat generation Micronic Mydata is interested in the option of using air bearings in future surface mounting machines, such as the successors of today's pick and place machine. The axes of interest is the linear motions in the horizontal plane whose objective is to place a component in the correct position on a PCB. The purpose of this thesis is to investigate the characteristics of air bearings and evaluate the achievable motion control performance for a new design with such bearings. For simplicity the smallest axis, the Y-axis which positions the PCB, has been used for testing. A background study has been done researching possible benefits and disadvantages of these bearings compared to the linear guide ball bearings used today. The properties of other parts of the system has also been studied, such as the permanent magnet linear motor used to actuate the motion, in order to design a good controller for the system. For the same reason similar motion control applications have been examined to see what approaches have been used to achieve as good positioning and trajectory following as possible. The information from the background study was used to identify system parameters such as friction and motor ripple which were used when designing a controller. The final version of the controller had a state feedback with integral action, a dual observer consisting of a state observer combined with a perturbation observer and feedforward of friction and motor ripple. The velocity estimation turned out to be problematic and even though it could be proved that a better velocity estimation would give better tracking performance no sufficient replacement for the current observer was found. Since the air bearings almost entirely lack friction disturbances such as erroneous velocity estimates have a larger effect than on the system with ball bearings which is well damped by friction. It has therefore not been possible to reach as good tracking performance with the air bearing design but there is reason to believe that sufficient performance can be achieved with a better velocity estimate.