UBC Theses and Dissertations

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UBC Theses and Dissertations

High speed motion control of CNC machince tools Rahaman, Mostafizur


High speed machining is a relatively new technology that has been applied to industries such as aerospace and die and mold making. By maximizing feedrates and cutting speeds of computer numerically controlled (CNC) machine tools, high levels of productivity are achieved. High speed machining requires rigid tool structures, stable spindles with high power and feed drives which can follow the desired tool trajectory. It is important to point out that both velocity and acceleration are physically constrained for any machine tool. However, the paths supplied by Computer Aided Design (CAD) systems do not take machine constraints into account. Thus, they usually provide very conservative (slow) part programs to this CNC machine, and the operator is then required to adjust the feedrates manually. This process is tedious, and attempts at providing improved preprocessors between the CAD system and the CNC tool have not been successful commercially, since there are too many unknown process parameters. As a result, realtime control techniques such as Cross Coupled Control (CCC), Zero Phase Error Tracking Control (ZPETC), or Realtime Feedrate Modulation (FM) have been studied in the literature. This thesis compares the performance of these traditional control techniques in their ability to reduce path error. It is shown that none of these algorithms are universally applicable to both corner tracking and circular interpolation, which are two of the most common challenges in high speed machining. The thesis then presents two new tracking algorithms that improve and combine the benefits of the existing algorithms. First, a new Linearized Cross Coupled Control (LCCC) algorithm is presented that is easier to implement than traditional CCC and improves the corner tracking capabilities. Then an algorithm termed Robust Tracking Control (RTC) is presented that combines LCCC with FM in order to further improve corner tracking performance and robustness towards multiple machining constraints. Experiments on a custom built x-y table show that RTC performs extremely well in both corner tracking and circular interpolation, even when multiple constraints such as current saturation and voltage saturation are present.

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