Steve Gibson

An unwindowed multichannel lattice filter with orthogonal channels

An unwindowed (or covariance) multichannel lattice filter for recursive least-squares estimation is derived. The channels are orthogonalized to eliminate the need for matrix inversion. The channel-orthogonalization process leads to forward-propagating and backward-propagating blocks in both the lattice filter and the model-parameter construction algorithm. These blocks are particularly suitable for array processing, as illustrated by arrays presented in the paper.

Adaptive control of a MEMS steering mirror for suppression of laser beam jitter

This paper presents an adaptive control scheme for laser-beam steering by a two-axis MEMS tilt mirror. Disturbances in the laser beam are rejected by a /spl mu/-synthesis feedback controller augmented by the adaptive control loop, which determines control gains that are optimal for the current disturbance acting on the laser beam. The adaptive loop is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results are presented to demonstrate that the adaptive controller significantly extends the disturbance-rejection bandwidth achieved by the feedback controller alone.

Damage detection in structures from vibration and wave propagation data

Development of efficient tools to successfully localize and characterize hidden damage in critical structural components is an important task in the design and construction of structural health monitoring systems in aging as well as new structures. In this paper two methodologies for damage identification and localization will be presented. The first is an automatic numerical scheme using a state space system identification approach and the second is based on certain damage correlation indices associated with changes in the frequency response of the structure in presence of flaws. In each case, the structure is to be instrumented with an array of sensors to record its dynamic response including vibration and wave propagation effects. To determine the type and location of an unknown defect, the sensor data detected is used to identify a new system, which then is compared to a database of state-space models to find the nearest match. The second method deals with the definition of a set of damage correlation indices obtained from the frequency response analysis of the structure. Two types of indices have been considered. The first uses the correlation between the responses of the defect free and damaged structure at the same point, and the second uses correlation at two different points. The potential application of the general approach in developing health monitoring systems in defects-critical structures is discussed.

Adaptive Control of MEMS Mirrors for Beam Steering

This paper presents the design and experimental implementation of an adaptive inverse control system for a two axis MEMS tilting mirror used for optical beam steering. The theoretical issues and practical design considerations involved in this task are discussed in detail. The first topic addressed is the system identification of input-output and state-space models of the MEMS mirror. Consistency among the following two system identification methods is verified: identification of a parameterized transfer function and identification of a state-space model by a subspace method. Next, a stabilizing feedback controller and an adaptive inverse control scheme based on an adaptive inverse QR recursive least-squares filter are developed. Finally, the experimental implementation of the control loops is described and the performance of the beam steering system is analyzed.

Adaptive control of jitter in a laser beam pointing system

This paper presents an adaptive control scheme for suppressing jitter in laser beams. The variable-order adaptive controller is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. The multi-channel adaptive control structure is exploited to allow both feedback of the beam position error and feedforward of an accelerometer measurement. In the experiments presented here, the high-order adaptive controller controlled jitter over a broad spectrum of frequencies, almost up to the Nyquist frequency

Input-output dynamics of the JPL microgyroscope

This paper discusses the identification of multi-input/multi-output models of the Jet Propulsion Laboratory (JPL) micromachined vibratory rate sensor (JPL microgyro). The sensor dynamics are accurately identified in the neighborhood of lightly damped degrees of freedom. Interesting features of the gyro are reported, including the presence of right-half-plane zeros. The results indicate that a multichannel lattice filter is suitable for adaptive sensor identification using test inputs of short duration to excite the sensor dynamics.

Adaptive control in an adaptive optics experiment.

This paper presents results from an adaptive optics experiment in which an adaptive control loop augments a classical adaptive optics feedback loop. Closed-loop wavefront errors measured by a self-referencing interferometer are fed back to the control loops, which drive a membrane deformable mirror to correct the wavefront. The paper introduces new frequency-weighted deformable mirror modes used as the control channels and new wavefront sensor modes for analyzing the performance of the control loops. The corrected laser beam also is imaged by a diagnostic target camera. The experimental results show reduced closed-loop wavefront errors and correspondingly sharper diagnostic target images produced by the adaptive control loop as compared with the classical AO loop.

Optimal and adaptive control of aero-optical wavefronts for adaptive optics

This paper compares two control methods to predict and correct aero-optical wavefronts derived from recent flight-test data. The first is an optimal linear time-invariant controller constructed from an identified state-space model of the turbulence flow. The second control method is an adaptive controller based on a recursive least-squares lattice filter. The performance of these control schemes versus classical integrator methods is investigated in an adaptive optics experiment that reproduces the aberrations from in-flight measurements of aero-optical turbulence. Experimental results show the improvement in wavefront correction achieved by both prediction methods. Altering the flow characteristics of the disturbance wavefront during the control process illustrates the ability of the adaptive controller to track changes in the aberration statistics.

Adaptive control of a MEMS steering mirror for free-space laser communications

This paper presents an adaptive control scheme for laser-beam steering by a two-axis MEMS tilt mirror used in current free-space optical communications systems. In the control scheme presented here, disturbances in the laser beam are rejected by a high-performance linear time-invariant feedback controller augmented by the adaptive control loop, which determines control gains that are optimal for the current disturbance acting on the laser beam. The variable-order adaptive control loop is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results are presented to demonstrate the effectiveness of the adaptive controller for rejecting multi-bandwidth jitter. These results demonstrate that the adaptive loop significantly extends the jitter-rejection bandwidth achieved by the feedback controller alone.

Adaptive control of jitter in laser beam pointing and tracking

This paper presents new results on adaptive control of jitter in laser beams. Experimental results illustrate the capability of a recently developed method for variable-order adaptive control reduce jitter in bandwidths well beyond the bandwidth of linear time invariant control systems. The adaptive control loop is based on recursive least squares lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. The adaptive controller achieves both fast adaptation and true minimum variance steady state performance. The main innovation in this paper is frequency weighting in the adaptive control loop to emphasize the relative importance of jitter in particular bandwidths and mitigate the effects of high-frequency sensor noise. Results from an experiment with a MEMS fast steering mirror used in current free space optical communications systems illustrate suppression of jitter with simultaneous multiple bandwidths produced by multiple jitter sources.