Design and control of spherical gimbal for laser tracking system

To assess and improve the accuracy of an intelligent machine such as a precision robot and a computer numerically controlled machine tool, it is exceedingly desirable to have a high performance Coordinate Measuring Machine (CMM). Among various coordinate measuring devices, a laser tracking CMM has the advantages of noninvasiveness and extremely high precision over a large workspace.; In this dissertation, we concentrate on the design and control of a new type of spherical gimbal for laser tracking system, whose motion is constrained by two spherical surfaces and whose axes are motorized. By this design, principal errors of a conventional tracking gimbal are reduced. To be able to integrate the laser tracking unit into an intelligent machine, a compact optical head is also designed. This laser tracking system is thus capable of being either a stand-alone or an on-line measuring device.; An important issue in developing a laser tracking CMM is control. An intelligent control scheme is reported in this dissertation. The controller has the following elements: The entire tracking process of the system is classified into three modes: normal tracking, motionless and change of directions. An artificial neural network is designed to classify on-line which mode the system is in. A Fuzzy Logic Controller (FLC) suitable for the particular tracking mode is then activated to control the system. To deal with the situation in which a target suddenly changes its direction, a feed-forward compensation component is designed. Decoupling units are also added to the control scheme, by which the entire process of tracking controller design can be greatly simplified.; To further improve the system performance, various structures of FLCs are analyzed in the dissertation. It is discovered that there is a constraint in the cascade proportional-integral-derivative (PID)-type FLC. Whenever this constraint is violated, the design of the controller will not be optimal. To solve this problem, a parallel PID-type FLC is proposed.; Yet another important issue in the system control is parameter tuning. To this end, a {dollar}mu{dollar}-law tuning method, which tune both scaling gain and surface of a fuzzy look-up table, is proposed. A new parameter tuning strategy, which combines {dollar}mu{dollar}-law with either a Genetic Algorithm (GA) or a downhill simplex algorithm, is introduced. The GA based {dollar}mu{dollar}-law tuning of FLCs can automatically tune parameters of the FLCs, while the Simplex-{dollar}mu{dollar}-law tuning scheme can reach near optimal results rapidly.; To assess the effectiveness of the concepts proposed in this dissertation, a prototype spherical laser tracking gimbal is constructed at the FAU Robotics Center. The control strategy proposed in this dissertation is tested extensively by simulation and experimentation on the prototype system.