Design and control of active magnetic bearings for a high speed machining spindle

Machining metal at high speeds leads to many process benefits and increases productivity. The machining spindle, which delivers the rotary speed and power for cutting, typically relies upon specially designed rolling element bearings for support. Magnetic bearings, however, offer an appealing alternative as they may operate at significantly greater rotational speeds, and additionally, their properties may be actively controlled to accomodate a variety of dynamic conditions that arise during machining. The challenges facing magnetic bearings in high speed machining applications are: (1) a significantly smaller load capacity than rolling element bearings of the same size, and (2) an increase in power losses at high speed.; One focus of this dissertation is design of the magnetic bearing actuator to increase the specific load capacity (load per unit volume) while operating at high speed. An algorithm is presented which integrates thermal, magnetic and mechanical design issues of this task. Three steps in the algorithm are developed in detail: (1) power loss determination at high speed, (2) temperature distribution approximation using finite elements and (3) accurate budgeting of the bearing radial clearances.; A new experimental method for power loss determination in magnetic bearings is presented which solves the inverse heat transfer problem. New power loss results for cobalt iron and silicon iron bearings are presented for surface speeds up to 3.1 million mm{dollar}cdot{dollar}RPM and flux densities approaching saturation. Two dimensional finite element modeling methods for the thermal performance of a magnetic bearing with a rotating journal are presented. Resulting temperature distributions are used to select the optimum current density in the magnet wire such that the bearing does not overheat. Finally, an algorithm for accurate budgeting of the magnetic bearing radial clearance is given. Reduction of this clearance leads to a larger specific load capacity. By utilizing the methods presented, the RMS specific load capacity of the HSM spindle magnetic bearings is increased by 61% over typical magnetic bearing designs.; The second focus of this dissertation is control of the machining spindle through the magnetic bearings in order to enhance the cutting process. A detailed dynamic model of the spindle, cutting tools and magnetic bearing system is developed. A performance evaluation for over 50 candidate controllers for the HSM spindle is presented. Evaluation is based upon cutting performance, machining chatter tendency and control effort. Both proportional-derivative and {dollar}mu{dollar}-synthesized, {dollar}Hsbinfty{dollar}controllers are evaluated using {dollar}mu{dollar}-analysis based test conditions for robust performance. Control coordination is evaluated as the spindle has three magnetic bearing actuators. Recommendations are made as to the appropriate location of system uncertainty and performance weights.