Improved Kinematic Modeling & Motion Control of the Tiara Hexapod Parallel Kinematic Machine Tool

Abstract

A Hexapod is a novel generation machine tool based on parallel kinematic structures. Due to its potential capabilities for high dimensional accuracy, high structural rigidity and high-speed capacity, it is regarded as the most radical change in the design of machine tools in the last century. However, due to its short development history and lack of wide industrial application, it poses both theoretical and technical challenges. Solutions to overcome its obvious weakness are not yet available. Design tools, commercial motion controllers capable of dealing with non-linear dynamics and coupling effects, and maintenance topics related to measurement, calibration and accuracy evaluation are still open issues. A hexapod, termed the Tiara hexapod, is being developed at the McMaster Manufacturing Research Institute (MMRI) at the McMaster University. The Tiara hexapod has fixed length struts and movable base joints. It is designed to be a milling machine. This thesis presents improvements of the kinematic model and servo control system, the development and implementation of the motion interpolator, as well as preliminary measurements of this machine. Following a comprehensive literature review, the kinematic modeling process of the completely constructed machine, based on homogenous transformation matrices, the machine structures and the assembling sequences is presented. The derived kinematic model provides nominal parameters for the motion interpolator and has the flexibility for deriving the quasi-static error model for future kinematic calibration and compensation. Next, the original servo control system of the machine was extensively tested. Problems related to the hardware and software were found and solved through reconfiguring and rewiring hardware, modifying and developing new software functions. The main motion functions of the servo control system, improved and developed both on a single axis test system and the practical machine, make the machine work safely and effectively. The six axis motion interpolator for the Tiara hexapod was designed based on linear blended move motion. It uses the inverse kinematics model and the identical motion time to coordinate the motions of the six parallel axes to generate the desired tool tip motion with specified feedrate. Based on two user-defined parameters, the peak jerk and the peak acceleration, the interpolator generates a smooth pure, or partial, S-curve velocity profile and cubic position profiles. After being integrated with the servo control system, the interpolator is successfully tested for the single move and blended move motions of the machine. Finally, the performance of the improved servo control system and online interpolation motion were experimentally evaluated for the un-calibrated machine. Both single axis motions and linear interpolated motions were realized with the correct motion directions and shapes, as well as within the specified time period. The positional repeatability of axis homing motion and linear interpolated motion was measured to be less than O.OOlin (25.4microns). Due to errors in the nominal parameter, the linear interpolated motion straightness accuracy was found to be very low with a maximum error of O.108in (2.74mm).