Institute for Systems Research Technical Reports
Permanent URI for this collectionhttp://hdl.handle.net/1903/4376
This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.
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Item Analysis of a high-resolution optical wave-front control system(2002) Justh, Eric W.; Krishnaprasad, Perinkulam S.; ISR; CDCSSWe consider the formulation and analysis of a problem of automaticcontrol: correcting for the distortion induced in an optical wave frontdue to propagation through a turbulent atmosphere. It has recentlybeen demonstrated that high-resolution optical wave-front distortionsuppression can be achieved using feedback systems based on high-resolution spatial light modulators and phase-contrast techniques.We examine the modeling and analysis of such systems for the purposeof refining their design. The approach taken here might also beapplicable to other problems involving feedback controlof physical fields, particularly if the field sensing is performedoptically. (In Proc. Conf. on Information Sciences and Systems, Vol. 2, pp. 718-723, 2001.)Item A Simple Control Law for UAV Formation Flying(2002) Justh, Eric W.; Krishnaprasad, Perinkulam S.; ISRThis paper presents a Lie group setting for the problem of control of formations, as a natural outcome of the analysis of a planar two-vehicle formation control law.The vehicle trajectories are described using planar Frenet-Serret equations of motion, which capture the evolution of both the vehicle position and orientation for unit-speed motion subject to curvature (steering) control. The set of all possible (relative) equilibria for arbitrary G-invariant curvature controls is described (where G = SE(2) is a symmetry group for the control law). A generalization of the control law for n vehicles is presented, and the corresponding (relative) equilibria are characterized. Work is on-going to discover stability and convergence results for the n-vehicle problem.
The practical motivation for this work is the problem of formation control for meter-scale UAVs; therefore, an implementation approach consistent with UAV payload constraints is also discussed.
Item Advanced Phase-Contrast Techniques for Wavefront Sensing and Adaptive Optics(2001) Vorontsov, Mikhail A.; Justh, Eric W.; Beresnev, Leonid A.; ISR; CDCSSHigh-resolution phase-contrast wavefront sensors based on optically addressed phase spatial light modulators and micro-mirror/LC arrays areintroduced. Wavefront sensor efficiency is analyzed for atmosphericturbulence-induced phase distortions described by the Kolmogorov and Andrews models. A nonlinear Zernike filter wavefront sensor based on anoptically addressed liquid crystal phase spatial light modulator isexperimentally demonstrated. The results demonstrate high-resolutionvisualization of dynamically changing phase distortions within the sensortime response of 10 msec.SPIE Proc., High-Resolution Wavefront Control: Methods,Devices, and Applications II, Vol. 4124, pp. 98-109, 2000. Item Nonlinear Analysis of a High-Resolution Optical Wave-Front Control System(2001) Justh, Eric W.; Krishnaprasad, Perinkulam S.; Vorontsov, M.A.; ISR; CDCSSA class of feedback systems for high-resolution optical wave-front control(or adaptive optic wave-front distortion suppression) is modeled andanalyzed. Under certain conditions, the nonlinear dynamical system modelsobtained are shown to be gradient systems, with energy functions that also serve as Lyapunov functions. The approach taken here to a problemof nonlinear control system design and analysis might also be applicableto other problems involving high-resolution control of physical fields,particularly if the field sensing is performed optically.Proc. IEEE Conference on Decision and Control, pp. 3301-3306,2000. Item Adaptive Wavefront Control using a Nonlinear Zernike Filter(2001) Justh, Eric W.; Vorontsov, Mikhail A.; Carhart, Gary W.; Beresnev, Leonid A.; Krishnaprasad, Perinkulam S.; ISR; CDCSSA conventional Zernike filter measures wavefront phase by superimposing the aberrated input beam with a phase-shifted version of its zero-orderspectral component. The Fourier-domain phase-shifting is performed by afixed phase-shifting dot on a glass slide in the focal plane of a Fourier-transforming lens.Using an optically-controlled phase spatiallight modulator (SLM) instead of the fixed phase-shifting dot, we havesimulated and experimentally demonstrated a nonlinear Zernike filterrobust to wavefront tilt misalignments. In the experiments, a liquid-crystal light valve (LCLV) was used as the phase SLM. The terminology "nonlinear" Zernike filter refers to the nonlinear filteringoperation that takes place in the Fourier domain due to the phase changefor field spectral components being proportional to the spectral componentintensities.
Because the Zernike filter output intensity is directlyrelated to input wavefront phase, a parallel, distributed feedback systemcan replace the wavefront reconstruction calculations normally requiredin adaptive-optic phase correction systems. Applications include high-resolution phase distortion suppression for atmospheric turbulence,optical phase microscopy, and compensation of aberrations in optical system components.
A factor of eight improvement in Strehl ratio wasobtained experimentally, and simulation results suggest that even betterperformance could be obtained by replacing the LCLV with a more sophisticated optically-controlled phase SLM.
SPIE Proc., High-Resolution Wavefront Control:Methods, Devices, and Applications II, Vol. 4124, pp. 198-200, 2000. Item Opto-Electronic Zernike Filter for High-Resolution Wavefront Analysis using a Phase-Only Liquid Crystal Spatial Light Modulator(2001) Carhart, Gary W.; Vorontsov, M.A.; Justh, Eric W.; ISR; CDCSSAn opto-electronic technique for high-resolution wave-front phase imagingis presented and demonstrated experimentally. The technique is analogousto the conventional Zernike phase-contrast approach, but uses modernspatial light modulator technology to increase robustness and improveperformance.Because they provide direct measurements of wave-frontphase (rather than wave-front slope measurements, as in Shack-Hartmannsensors), robust phase-contrast sensors have potential applications inhigh-speed, high-resolution adaptive optic systems.
Advantages of theopto-electronic approach over alternative advanced phase-contrasttechniques (such as a related phase-contrast sensor which uses a liquid-crystal light valve exhibiting a Kerr-type optical response toperform Fourier filtering) are discussed. The SLM used for theexperimental results is a 128x128-element pixilated phase-only liquidcrystal spatial light modulator from Boulder Nonlinear Systems, Inc.
SPIE Proc., High-Resolution Wavefront Control: Methods,Devices, and Applications II, Vol. 4124, pp. 138-147, 2000.