A laser speckle based position sensing technique

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (p. 129-131).This thesis presents the design and development of a novel laser-speckle-based position sensing technique. In our prototype implementation, a He-Ne laser beam is directed at the surface of an air-bearing spindle. An imaging system is set up to capture speckle patterns scattered from the spindle surface. These patterns are highly correlated over small angular displacements of the spindle. We use correlation-based image-processing algorithms to measure offsets between the speckle patterns. These offsets are calibrated against the counts of a commercial incremental optical encoder. A custom-built bicell photointerrupter unit is used as a reference sensor for the incremental optical encoder. To test for the control performance of this speckle-based sensor, we have constructed a transmission drive to run the air-bearing spindle. Our speckle-based metrology system is able to run at update rates of 10 Hz with a measured closed loop -3 dB bandwidth of about 2 Hz. Using a real-time processor interfaced with a desktop PC, we have implemented a novel algorithm that interpolates position estimates with respect to two pre-stored global images. We predict that this technique can potentially achieve resolutions of 0.1 [mu]m for translational and 5 [mu]rad for rotational motion. The limitation of our current implementation is the low update rates resulting from the time-intensive nature of correlation-based methods. Possible methods to overcome this limitation are addressed and ideas for follow-on work are presented.by Vijay Shilpiekandula.S.M

Flexure-based nanopositioning systems : integrated design and control

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 209-219).This thesis deals with the design and control of flexure-based mechanisms for applications requiring multi-degree-of-freedom positioning and alignment. Example applications include positioning a probe or sample in atomic force microscopy, alignment of tool and sample in stamping processes, and fine-positioning of wafer steppers in semiconductor manufacturing. Such applications necessitate nanopositioning systems that satisfy critical functional requirements, such as load-capacity, bandwidth, resolution, and range. Therefore, a systematic approach for design and control is an important tool for research and development for flexure-based nanopositioning systems. In this thesis, a novel methodology is presented for generating flexure-based topologies that can meet performance requirements, such as those dictating structural strength or dynamical behavior. We present performance metrics that allow for the generation of topologies that are tuned for a desired level of structural strength or modal separation. The topology generation is aimed as a valuable addition to the design toolkit, facilitating novel designs that could not have been conceived otherwise. The parameters within any particular topology could be adjusted at a subsequent phase through a detailed shape and size optimization. The thesis also proposes a controller generation approach. Unlike existing controller parameterizations, a novel parameterization formulated in this thesis allows for directly tuning the sensitivity transfer function of the closed-loop system. Tuning sensitivity is critical in mitigating the effects of disturbances affecting the system, as well as those arising from cross-coupling and parasitic error motions. Further, an integrated methodology for design and control is presented. This methodology uses the design topology generation approach and controller generation approach proposed in the thesis. The key distinction of our design for control approach is that the design is iterated over topologies and not just parameters within a selected topology. A simple one-degree-of-freedom positioning system example is worked out to detail the steps of the proposed integrated design and control methodology. A novel design topology that is ideally suited for achieving a desired design and control performance is derived using this methodology. Finally, the hardware design and control of a novel flexure-based nanopositioner implementation for scanning probe microscopy are presented to illustrate the effectiveness of the approaches discussed in this thesis.by Vijay Shilpiekandula.Ph.D

White-light scanning interferometer for absolute nano-scale gap thickness measurement

A special configuration of white-light scanning interferometer is described for measuring the absolute air gap thickness between two planar plates brought into close proximity. The measured gap is not located in any interference arm of the interferometer, but acts as an amplitude-and-phase modulator of the light source. Compared with the common white-light interferometer our approach avoids the influence of the chromatic dispersion of the planar plates on the gap thickness quantification. It covers a large measurement range of from approximate contact to tens of microns with a high resolution of 0.1 nm. Detailed analytical models are presented and signal-processing algorithms based on convolution and correlation techniques are developed. Practical measurements are carried out and the experimental results match well with the analysis and simulation. Short-time and long-time repeatabilities are both tested to prove the high performance of our method.Singapore-MIT Allianc