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UBC Theses and Dissertations

A three-axis virtual computer numerical-controlled (CNC) system Yeung, Chi-Ho

Abstract

The recent trend is to perform the machining operations in a comprehensive simulation environment before physical production. This thesis presents a virtual design and simulation of CNC machine tools with Cartesian drive configurations. The Virtual CNC system accepts reference toolpath generated on CAD/CAM systems in the form of industry standard Cutter-Location (CL) format. The CL file contains the cutter dimensions, toolpath coordinates, and travel speed in machining a particular part on a CNC machine tool. The motion commands are processed by considering the trajectory generation, control law, physical characteristics of feed drives, which describe the mathematical model of the selected CNC system. The Virtual CNC system consists of parametric modules, which can be either selected from the database or defined as a new module by the user. The trajectory generation modules include various feed profiles with low and high smoothness levels along the toolpath. The transfer functions of the individual physical elements in the drives, such as ball screw, gear reduction, inertia and viscous damping of the equivalent drive train, servomotor, motion sensors, and amplifiers are mathematically modeled based on the specifications. The non-linearities such as saturation of actuator, guideway friction, and backlash are also considered. The Virtual CNC system is furbished with experimentally proven various control laws, such as P-PI, PD, PID, Pole Placement Control (PPC), and Sliding Mode Control (SMC). The linear control laws are analytically tuned according to desired performance criteria. The non-linear SMC is tuned by developing a fuzzy logic based auto-tuning algorithm. A toolpath modification technique is also introduced for smoothening out the sharp corners with quintic splines whenever it is required. With this technique, the tracking performance of the machine tool is significantly improved as the feed direction changes continuously without stopping at the corners, resulting in less excitation to the drives. The Virtual CNC system processes the entire toolpath motion, provides a time-domain response of the entire feed drive control system by including the non-linearities, and predicts the tolerance violation locations along the toolpath. The system incorporating with auto-tuning and toolpath modification technique is experimentally verified in a 2-1/2 axis machining center.

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