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2934 |
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Noise Reduction and Disturbance Rejection at the Sub-nanometer Level |
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Design of Precision Machines and Instruments |
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This paper examines the noise and disturbance sources that bound the positioning resolution on the Sub-Atomic Measuring Machine (SAMM) from an experimental approach. These sources are common to many ultra-precision machines. As we continue to push the envelope for positioning capabilities new limitations arise. The SAMM is uniquely designed to position and measure in the sub-nanometer range over a volume of 25 mm x 25 mm x 0.1 mm.
The custom laser interferometer sensors that monitor the x, y, and theta z degrees-of-freedom on the SAMM have been upgraded from a resolution of 0.1 nm to 0.01 nm [1, 2, 3]. By reducing the sensor resolution by an order of magnitude, noise and disturbance signals that were previously below the sensor lower bound now dominate the feedback signal and limit the positioning resolution. Since the SAMM is a magnetically levitated stage, positioning in 6 degrees-of-freedom requires a closed-loop controller for stability. The position holding capability represents the positioning resolution and is used to quantify the noise and disturbance influence on the measurement. This paper will evaluate the dominant noise and disturbances present in the SAMM positioning data.
This paper will address the challenges associated with collecting useful data at levels required of ultra-precision machines. For instance, one difficulty with measurements in the sub-nanometer range is the availability of sensors. In many cases the quantity to be measured is below an external sensor’s resolution but not below the feedback sensor resolution. Another problem is measuring at low frequencies since many sensors’ exhibit a reduction in sensitivity for frequencies approaching DC levels.
Implementing solutions for noise reduction and disturbance rejection can carry consequences. We will discuss such items as the thermal difficulties that arise due to the addition of passive shielding to reject acoustical disturbances. The custom offset-frequency locked lasers required a brute force approach to reject pressure influences since the control system did not have sufficient disturbance rejection [4]. This is an important lesson for precision machines since passive techniques for disturbance rejection can reduce the control effort and hence reduce the heat generated at the actuators. In summary this paper aims to identify and reduce the sources of noise and disturbances that limit the positioning resolution of the SAMM to levels below 0.1 nm.
1. Jerald L. Overcash, Charles Stroup, and Robert J. Hocken. Updating the Sub-Atomic Measuring Machine. ASPE 2008 Annual Meeting.
2. Jerald L. Overcash, Charles Stroup, and Robert J. Hocken. Resolution Considerations in Interferometer Based Ultra-Precision Machines. 10th euspen Conference, 2008.
3. Jerald L. Overcash, Charles Stroup, and Robert J. Hocken. Recent Developments And Upgrades In-Progress Of A Precision Six Degree-Of-Freedom Magnetically Suspended Measuring Instrument. ASPE 2007 Annual Meeting.
4. Charles Stroup, Jerald L. Overcash, and Robert J. Hocken. The Development of an Offset-Locked Frequency Stabilized Heterodyne Laser Source for Precision Metrology. ASPE 2008 Annual Meeting.
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