The potential of magnetic bearings for high speed machining is impressive. With AMBs, the metal removal rate in milling operations could be doubled resulting in a 25% decrease in machining time, smaller part inventories, and reduced part cost. The National Center for Manufacturing Sciences has estimated that increased productivity resulting from this improved spindle technology could expand the U.S. annual Gross Domestic Product by $12 billion. This application also presents some of the most interesting and difficult challenges in magnetic bearing control, including robust Hinfinity performance specifications, variable spindle dynamics, unknown substructures, and suppression of chatter onset.
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Over the past few years, in cooperation with an industrial sponsor, we have designed, fabricated, and tested a new high speed milling spindle equipped with active magnetic bearings. This spindle has been magnetically levitated first with decentralized PID feedback control and later with multivariable controllers developed via application of m-synthesis.
The spindle has three radial magnetic bearings as well as a thrust bearing for controlling the motion in the axial direction. The spindle’s rotor carries the three corresponding AMB journals, thrust disk, and motor rotor, all shrunk on. Within this main rotor is a smaller drawbar located within an internal bore. The drawbar actuates the cutting tool, allowing for tool changes, as several different tools may be used during a machining operation. The spindle uses differential optical sensors to determine the radial and axial position of the shaft. These low noise sensors, designed at the University of Virginia, are based on the shaft occluding a light beam from an infrared LED to a phototransistor. The spindle is controlled by a parallel processing digital controller designed and built at the University of Virginia. Four Texas Instruments TMS320C40 digital signal processors are employed for control algorithm computations. This hardware platform permits the execution of a 7 input, 7 output, 75th order controller with a throughput rate in excess of 12 Khz.
In the last year we have demonstrated some extremely sophisticated multivariable feedback controllers on the spindle. The results indicated that we can expect to achieve a 30 to 50% improvement in performance (reduction in dynamic tool compliance) over the best that could be achieved by conventional decentralized control methods. This work has greatly advanced the state-of-the-art in magnetic bearing feedback control. The spindle has been operated up to 20,000 rpm.
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