Owen G. Thorp

Active Control of Axial-flow Fan Noise Using Magnetic Bearings

In this paper we present a novel approach to reducing blade-rate noise of axial-flow fans. By using magnetic bearings as a noise control actuator, it is possible to collocate the anti-noise source with the disturbance noise source. This approach allows for global noise reduction throughout the sound field. A DC motor connected to a fan by a short rigid shaft was used to demonstrate this approach. The shaft was supported in the radial and axial directions by magnetic bearings. The bearings provide position control of the shaft and fan; this position control can be used to vibrate the fan at a desired frequency and amplitude. Controlled vibration of the fan allows its use as a speaker in an active noise control scheme. Noise control was implemented on a dedicated digital signal processor using a least mean square algorithm. The output of the noise control algorithm supplies the position commands for the magnetic bearing controller. Experimental data showed that by actuating the axial thrust bearing, the noise output of a fan could be reduced by 4 dB at the error microphone, and 3 dB at points away from the error microphone.

Experimental verification of an optical beam stabilizing controller

This paper presents experimental verification for a Hf stabilizing controller for an optical beam subject to tonal disturbances. The proposed controller forces the power density at all disturbance frequencies below a prescribed level. Furthermore, the effect of significant input saturation on stability and performance is investigated.

Attenuation and localization of wave propagation in rods with periodic shunted piezoelectric patches

Shunted piezoelectric patches are periodically placed along rods to control the longitudinal wave propagation in these rods. The resulting periodic structure is capable of filtering the propagation of waves over specified frequency bands called stop bands. The location and width of the stop bands can be tuned, using the shunting capabilities of the piezoelectric materials, in response to external excitations and to compensate for any structural uncertainty. A mathematical model is developed to predict the response of a rod with periodic shunted piezoelectric patches and to identify its stop band characteristics. The model accounts for the aperiodicity, introduced by proper tuning of the shunted electrical impedance distribution along the rod. Disorder in the periodicity typically extends the stop-bands into adjacent propagation zones and more importantly, produces the localization of the vibration energy near the excitation source. The conditions for achieving localized vibration are established and the localization factors are evaluated for different levels of disorder on the shunting parameters. The numerical predictions demonstrated the effectiveness and potentials of the proposed treatment that requires no control energy and combines the damping characteristics of shunted piezoelectric films, the attenuation potentials of periodic structures, and the localization capabilities of aperiodic structures. The theoretical investigations presented in this work provide the guidelines for designing tunable periodic structures with high control flexibility where propagating waves can be attenuated and localized.

Cross-coupling estimation for optical beam stabilization

This paper presents a real-time algorithm for estimating the cross-coupling coefficients in a fast-steering mirror that serves as the actuator for an optical beam stabilization system. The algorithm computes the cross-coupling coefficients estimates by comparing the measured beam position with the predicted beam position from the uncoupled system model. This algorithm provides a means for estimating the relative orientation between the beam source platform and target platform as well. The algorithm is verified using experimental data from the United States Naval Academy Directed Energy Laboratory.

Frequency domain modeling and experimental validation of surface vehicle-wave interaction for planar motion control

This paper aims at developing a model based on theoretical first principles involving nonlinear equations for the coupled heave and pitch dynamics of surface vehicles that is suitable for evaluating planar motion control strategies. The model developed uses a novel approach of representing the ship dynamics in the frequency domain that provides greater insight to the surface vehicle’s behavior over a wider operating range than the traditional time domain models. The theoretical model was validated by experimental data that was obtained from tow tank experiments. The theoretical model results compared well with the experimental data.

Parameter estimation for a standard surface water vehicle model

A parameter estimation scheme for a surface water vehicle is presented and is based on a standard surface water vehicle model developed at the Naval Research Laboratory. The feasibility of the proposed scheme is demonstrated using the Popov-Belevitch-Hautus (PBH) test for observability. The proposed scheme is independent of the value of the surface water vehicle parameters and could be used for multiple types of surface water vehicles that fit the given model structure.