Steve Gibson, Tsu-Chin Tsao, Dan Herrick, Christopher Beairsto, Ronnie Grimes, Todd Harper, Jeff Radtke, Benito Roybal, Jay Spray, Stephen Squires, Dave Tellez, Michael Thurston
KEYWORDS: Adaptive control, Digital signal processing, Telescopes, Digital filtering, Electroluminescence, Cameras, Line of sight stabilization, Missiles, Feedback control, Optical filters
A field test experiment on a range tracking telescope at the U. S. Army's White Sands Missile Range is exploring
the use of recently developed adaptive control methods to minimize track loop jitter. Gimbal and platform
vibration are the main sources of jitter in the experiments, although atmospheric turbulence also is a factor. In
initial experiments, the adaptive controller reduced the track loop jitter significantly in frequency ranges beyond
the bandwidth of the existing track loop. This paper presents some of the initial experimental results along with
analysis of the performance of the adaptive control loop. The paper also describes the adaptive control scheme,
its implementation on the WSMR telescope and the system identification required for adaptive control.
KEYWORDS: Sensors, Adaptive control, Mirrors, Digital filtering, Control systems, Microelectromechanical systems, Computing systems, Actuators, Error control coding, Linear filtering
In recent experimental research on adaptive control of jitter in laser beams, sufficiently high levels of high
frequency sensor noise have produced high frequency spikes in the output errors. The phenomenon has been
observed for both adaptive and high bandwidth linear-time-invariant (LTI) control loops. Recently, the source
of the problem has been discovered to be saturation associated with the MEMS fast steering mirror used as
the control actuator. Results in this paper demonstrate that the spikes in the output error are eliminated by a
recently developed frequency-weighting method for the tuning signal used to determine adaptive control gains.
The method places more weight on jitter in frequency ranges where large sensor noise otherwise produces the
unwanted response. The frequency-weighted adaptive control loop is based on a 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. 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.
KEYWORDS: Feedback control, Sensors, Control systems, Motion controllers, Actuators, Linear filtering, Electronic filtering, Digital filtering, Analog electronics, Ferroelectric materials
This paper presents digital motion control algorithms, real-time
implementation, and experimental results for dynamic motion
control. A digital control system that includes robust feedback,
previewed feedforward, and repetitive control action is
implemented on a piezoelectric actuator driven mechanical stage
and is demonstrated to achieve nanometer level dynamic precision.
This paper presents a new approach to closed-loop control of optical jitter with a new liquid crystal beam steering
device. In contrast to conventional fast steering mirrors, where the laser beam is reflected of the controlled mirror
surface, the transmissive liquid crystal beam steering device optically redirects the laser beam. The new device
has no moving parts and requires low operating power. This research suggest the new device can replace the fast
steering mirrors in a variety of electro-optic systems. The functionality of the transmissive liquid crystal beam
steering device along with the analysis of real-time adaptive control experiments are described in this paper. The
experimental results show that the new liquid crystal beam steering device can reject disturbances with an LTI
feedback controller, and that the disturbance rejection capability can be improved significantly with feedforward
adaptive control.
We present an adaptive control scheme for laser-beam steering by a two-axis microelectromechanical systems (MEMS) fast steering mirror. Disturbances in the laser beam are rejected by a µ-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 variable-order adaptive controller is based on an adaptive lattice filter that implicitly identifies the disturbance statistics from real-time sensor data. Experimental results demonstrate that the adaptive controller significantly extends the disturbance-rejection bandwidth achieved by the feedback controller alone. The experimental results also illustrate the value of the variable-order capability of the adaptive controller.
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.
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.
In this paper, the effects of using semi-active control strategy (such as MR dampers) in vehicle suspensions on the coupled vibrations of a vehicle traversing a bridge are examined in order to develop various designs of smart suspension systems for bridge-friendly vehicles. The bridge-vehicle coupled system is modeled as a simply supported beam traversed by a two-degree-of-freedom quarter-car model. The surface unevenness on the bridge deck is modeled as a deterministic profile of a sinusoidal wave. As the vehicle travels along the bridge, the system is excited as a result of the surface unevenness and this excitation is characterized by a frequency defined by the speed of travel and the wavelength of the profile. The dynamic interactions between the bridge and the vehicle due to surface deck irregularities are obtained by solving the coupled equations of motion. Numerical results of a passive control strategy show that, when the lower natural frequency of the vehicle matches with a natural frequency (usually the first frequency) of the bridge and the excitation frequency, the maximum response of the bridge is large while the response of the vehicle is relatively smaller, meaning that the bridge behaves like a vibration absorber. This is undesirable from a bridge design viewpoint. Comparative studies of passive and semi-active controls for the vehicle suspension are performed. It is demonstrated that skyhook control can significantly mitigate the response of the bridge, while ground-hook control reduces the tire force impacted onto the bridge.
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