Design and control of a parallelogram-type manipulator for lapping of large work surface

Abstract

This thesis presents a manipulator and a control method for automatic lapping process applied to a large work surface. Most surface finishing operations have been regarded as manned work that takes a long time and has high cost. The automation of surface finishing has attracted attention globally because of problems such as a shortage of high-skilled workers and the adverse impacts of industrial diseases. The lapping machine consists of a newly designed parallelogram-type 5-axis manipulator and a conventional 3-axis gantry machine. The gantry machine is used for precision positioning over a large work area, and the manipulator is controlled by a separate controller for lapping operation in a local surface area, which is provided by the CNC controller of the gantry machine. The lapping process is needed as a first step to remove milling tool marks without damaging the original shape and then to correct the shape error. The wheel compliance was changed accordance with the purpose, such as tool mark removal and shape correction. Both processes require high stiffness of the manipulator. Newly designed passive joints are used to secure the stiffness of the joints in both the normal and lateral directions. Active and passive joints were designed through multibody dynamics simulations. In the control process, constrained motion is applied for a feed-forward torque model. The deadweight of the manipulator including the wheel motor and feed-forward torque control were used to suppress the vibration caused by abruptly dynamic cutting forces. The lapping operation was successful in wiping the curved contour without structural vibration. The feed-forward torque control doubled the machining efficiency and achieved 30 percent higher surface quality in comparison with a previous compliance control method. An experiment was done to investigate the effects of the abrasive wheel compliance and the dynamic cutting force. A smooth surface was achieved within a satisfactory quality roughness level of 0.3 μm for the tool mark removal process. A shape correction process was done by position control, and a 100 μm offset error was machined clearly. The results show that the proposed manipulator and control method have great potential to be applied in unmanned lapping systems for large work surfaces. The proposed lapping machine also has a great advantage in that both tool mark removal and shape correction can be performed with a single machine.