Leslie P. Fowler

Self-contained active damping system for pneumatic isolation tables

High resolution metrology and production systems are pushing the capability of existing pneumatic isolation systems. The need for better isolation at low frequency is growing as mechanical noise further constrains the lower limits of many electro-optical technologies and techniques. Unfortunately, the problem is magnified due to the 1-4 Hz natural frequency of pneumatic isolators. An active damping system, the Activator, uses an electromagnetic actuator to apply a damping force to an isolation table to reduce the transmission of these low amplitude, low frequency vibrations from the floor to the table. The system is low cost and easy to install. The controller, sensor, and electronics are integrated with the actuator. Given floor excitations on the order of submicrons, a 12 dB attenuation in transmissibility is achieved. This level of performance was attained after overcoming many physical hurdles due to the very low frequencies and small measurement signals. This unique, self-contained active damper result in a system more tolerant of existing floor locations and environments and is a simple upgrade, relieving the end user of facility improvements or relocation costs. This paper describes these challenges and shows how the performance goals were met providing a compact, economical system.

Optimal sensing strategy for adaptive control of optical systems

Optical jitter degrades the pointing and imaging performance of precision optical systems. When a correlated measurement of the disturbance is available, improved control performance can be attained. In this research, an adaptive optimal sensing strategy for optical systems is proposed. An array of reference sensors makes it possible to estimate the disturbance and model the disturbance-to-reference paths. The least-square algorithm is applied for the disturbance model estimation. A sensor scoring algorithm is then used to select an optimal disturbance reference from the available reference signals. The optimal disturbance reference is comprised of sensors which are well correlated with the disturbance. This disturbance reference is then fed forward and used in an adaptive generalized predictive control design. This adaptive control approach is advantageous in the presence of time-varying or uncertain disturbances. The proposed technique is applied to an experimental test bed in which an array of accelerometer sensors measures the structural vibration of optical elements. Reduction of the structural vibration of optical components is attained using a fast steering mirror which results in a reduction of the corresponding jitter. Performance using optimally selected disturbance reference is shown to be better than for system in which a disturbance reference signal is chosen to be the sensor with the lowest score.

Real-time optimal sensing strategies for active control of optical systems

The pointing and imaging performance of precision optical systems is degraded by disturbances on the system that create optical jitter. These disturbances can be caused by structural motion of optical components due to vibration sources that (1) originate within the optical system, (2) originate external to the system and are transmitted through the structural path in the environment, and (3) are air-induced vibrations from acoustic noise. Beam control systems can suppress optical jitter, and active control techniques can be used to extend performance by incorporating information from accelerometers, microphones, and other auxiliary sensors. In some applications, offline fixed gain controllers can be used to minimize jitter. However there are many applications in which a real-time adaptive control approach would yield improved optical performance. Often we would like the capability to adapt in real-time to a system which is time-varying or whose disturbances are non-stationary and hard to predict. In the presence of these harsh, ever-changing environments we would like to use every available tool to optimize performance. Improvements in control algorithms are important, but another potentially useful tool is a real-time adaptive control method employing optimal sensing strategies. In this approach, real-time updating of reference sensors is provided to minimize optical jitter. The technique selects an optimal subset of sensors to use as references from an array of possible sensor locations. The optimal, weighted reference sensor set is well correlated with the disturbance and when used with an adaptive control algorithm, results in improved line-of-sight jitter performance with less computational burden compared to a controller which uses multiple reference sensors. The proposed technique is applied to an experimental test bed in which multiple proof-mass actuators generate structural vibrations on a flexible plate. These vibrations are transmitted to an optical mirror mounted on the plate, resulting in optical jitter as measured by a position sensing detector. Accelerometers mounted on the plate are used to form the set of possible optimal reference sensors. Reduction of the structural vibration of optical components is attained using a fast steering mirror which results in a reduction of the corresponding jitter.

Estimating the number of uncorrelated disturbance sources in structural systems

The number of input signals used as reference inputs for feedforward control applications is limited due to cost, computational burden, input processing capability, and installation constraints. Identifying an optimal subset of reference sensors from a larger set capable of conveying the dynamics important in the performance path can result in greater performance with reduced complexity and order in the active control system. However, before determining an appropriate subset of sensors, the number of exogenous disturbance sources must be determined. Principal component analysis can be used to determine the number of uncorrelated disturbances acting on a structural system. Singular value decomposition of a covariance matrix of measured sensor signals is used to determine the number of independent disturbances present in the dynamic system. Limitations imposed by sample data length, path dynamics, and noise can limit the ability to resolve the number of exogenous disturbance sources. To estimate the number ...

Advanced technique to estimate the number of uncorrelated disturbance sources in structural systems

In many control applications the number of input signals for use as reference sensors in feedforward control is restricted due to limitations on cost, computational burden, input processing capability, and cabling and installation issues. Techniques to identify an optimal subset of reference sensors from among a larger set of possible sensor locations is therefore beneficial in attaining improved closed-loop control performance. This optimal subset is a minimum number of sensors that together convey the dynamics important in the performance path. To determine this subset, an approach for estimating the number of exogenous disturbance sources is first required. The proposed technique for estimating the number of uncorrelated disturbance sources acting on a structural system from sampled data is based on the Principal Component Analysis (PCA). This classical statistical method is used in conjunction with Singular Value Decomposition (SVD), and PCA/SVD of a covariance matrix of measured sensor signals is used to determine the number of independent disturbances present in the dynamic system. In practice however, this is difficult to resolve due to limitations on sampled data length, path dynamics, and the influence of noise. The dominant disturbance is evident but secondary disturbances important for the performance path of interest are not readily apparent. In order to better estimate the number of secondary disturbance sources, the addition of a control signal source to minimize the sensor response due to the dominant signal source is proposed. When such a control signal is applied to the system, it is then possible to determine the number of significant secondary signal sources using PCA/SVD. This improvement results in a better determination of the minimum number of reference sensors required for feedforward control of disturbances to structural systems.

Optimal Sensing/Actuation Strategies for Vibration and Acoustic Control of Optical Systems

Optical jitter can result in the beam pointing inaccuracy and poor optical system performance. With a correlated measurement of the disturbance, improved control performance can be achieved. In this research, an adaptive optimal sensing strategy for optical systems is proposed. When an array of reference sensors is available, an optimal set of reference sensors that are coupled to modes of interests can be selected. The weighted reference signal from the optimal sensor set is then used in an adaptive control design algorithm. An adaptive generalized predictive control design algorithm combined with the proposed adaptive optimal sensing strategy achieves better performance than the control system using only one of the reference sensors. The overall algorithm is also advantageous in the presence of time-varying or uncertain disturbances. The proposed technique is applied to an experimental test bed in which multiple accelerometer sensors measure the structural vibration of optical elements. Reduction of the structural vibration of optical components is attained using a fast steering mirror which results in a reduction of the corresponding jitter.

A comparison of active and adaptive passive noise control systems for the DC‐9

The aft interior of the DC‐9 aircraft is generally considered to be uncomfortably loud (100–110 dBC). This noise is primarily periodic and is caused by imbalances in the fan and turbine of the Pratt Whitney JT8D engine. Lord Corporation has recently developed active and adaptive passive systems to combat this problem. The active system uses four force‐generating actuators located on the left and right engine pylons to minimize the response at eight control microphones which are located in the rear of the cabin. The adaptive passive system consists of eight tuned vibration absorbers whose natural frequency is adapted to the engine speed. An overview of each system with theory will be presented as well as experimental results where the tonal noise reduction and engine speed tracking capabilities of each system were evaluated using an identical engine vibration input on a DC‐9 fuselage. These experimental results showed that the active system achieved larger tonal noise reductions and had faster tracking abi...