Robust control of a magnetic bearing spindle for milling tool path error minimization.

摘要

Various milling applications, such as thin rib machining of electronic components or airframe structures, can benefit from high speed machining for improved surface finish, accuracy, and increased metal removal rate. Utilization of a magnetic bearing spindle can not only provide benefits of high speed machining, but can also improve part accuracy. Error compensation, exploiting the ability of a magnetically suspended spindle to translate and tilt, within air gap limitations, provides perturbational corrective motions. This hierarchical error compensation structure was implemented using a microprocessor based error minimization controller providing real-time error compensation based on a pre-calibrated error characterization driven by on-line process monitoring.; A magnetic bearing spindle facility was established including the mechanical, operational, and control system interfacing required to retrofit an S2M B25/500 magnetic spindle system to a Matsuura MC500V vertical milling machine. Ongoing error metrology, along with levitation system identification and modelling, provided the basis for the control system synthesis. An error compensation methodology was derived which provides the ability to correct a general class of cutting force independent, as well as, cutting force dependent, dimensional errors. The cutting process is viewed as an ordered sequence of tool path trajectories. Sharing of numerical control part program codes, augmented by handshaking functions, enables coordination of computer numerical control and error compensation functions along the tool path trajectory. Using feedforward compensation, active magnetic bearing spindle error compensation of several sample error sources was experimentally evaluated. Cutting process dynamic spindle system stiffness and stability, involving magnetic bearings, rotor dynamics, tooling, and the metal removal process require improvement to achieve greater part accuracy and chatter resistance.; Review of metal removal technology innovations reveals a recent emphasis on improved control systems. Concurrently, much theoretical progress in the development of robust, multivariable control system synthesis methods has been reported. Application of these control system structures to the magnetic spindle levitation problem is investigated.