UBC Theses and Dissertations
Rotary-axial spindles for precision machining Paone, Matthew Paul
This thesis presents the design, analysis, fabrication, instrumentation, and control of a new type of machine tool spindle. Its primary contributions include the design and experimental demonstration of: two rotary-axial spindle prototypes, MIMO current control for a 1 kW linear power amplifier, sensorless rotary motion feedback, a novel method for increasing ADC resolution, and loop-shaping motion control systems. Some machining operations, such as face grinding, require rotational and feed motion to remove material. Conventional machine tools accomplish this by attaching a spindle which has thrust and journal support to a feed drive which also has thrust and lateral support systems. In modern machine design, the trend is towards increasing the stiffness of each individual element. However, the inherent serial duplication of support presents a fundamental limitation to stiffness and precision. The rotary-axial spindle architecture alleviates this problem by discarding the feed drive and spindle thrust bearing, replacing them both with a high force electromagnetic actuator. This provides millimeter range stroke for the spindle shaft, resulting in a single inertial element capable of both rotary and axial motion. This topology has several advantages. It allows kHz range bandwidth and hundreds of N/μm dynamic stiffness, improves acceleration, reduces structural bending moments, and eliminates thermal effects of fluid thrust bearings. Two prototypes are developed to demonstrate this technology. The first is a small scale rotary-axial spindle. Driven by a four-channel 1 kW linear power amplifier with decoupled current loops, the magnetic thrust bearing can handle 600 N peak axial loads over a 1 mm stroke. A novel method for increasing ADC resolution achieves sub-5 nm RMS positioning noise. Loop shaping compensation of the position loop results in 100 N/μm minimum dynamic stiffness and 2.6 kHz closed loop bandwidth. To control the spindle speed, a sensorless rotary motion feedback algorithm was developed. It produces results equivalent to a 1000 line rotary encoder. The second prototype is a full size machine tool. It demonstrated 6 kN continuous axial load capacity, 340 N/μm minimum dynamic stiffness, 800 Hz bandwidth, and 7 nm RMS positioning noise over a 1.5 mm stroke.
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