John T. Spanos

Active vibration isolation on an experimental flexible structure

The paper describes experimental research in the area of active vibration isolation. The objective of the research is to quantitatively assess the performance gained by augmenting a passive isolator with an active stage. Vibration isolation experiments were carried out on a flexible structure utilizing a proof-mass shaker as the disturbance source and a newly developed active member as the isolator. Broad band force feedback control demonstrated more than 20 dB reduction in force transmissibility over passive isolation alone. The broad band controller was augmented with notch filters which resulted in reducing force transmissibility by 40 dB over the passive stage in select narrow bands.

Closed-loop performance of a vibration isolation and suppression system

This paper describes the initial closed-loop isolation testing of the vibration isolation and suppression system (VISS) which can be used to isolate a precision payload from spacecraft borne disturbances. VISS utilizes six hybrid isolation struts in a hexapod configuration. The VISS experiment has recently been tested at the Phillips Laboratory and significant closed-loop vibration isolation was achieved. The control approach, the performance testbed, and the initial test results are described.

Narrow-band control experiments in active vibration isolation

This paper presents experimental results in active vibration isolation using a new version of the adaptive Least Mean Squares control algorithm. The algorithm is implemented digitally as a tracking notch filter in an experiment where the actuator and error sensor are collocated. A pair of all-pass filters are utilized to generate the sine and cosine signals needed by the adaptive filter and troublesome modes of the plant are identified off-line and cancelled by direct inversion. The closed loop system shows two orders of magnitude reduction in force transmissibility over open loop and over a decade of frequencies.

A result on exponential tracking error convergence and persistent excitation

Conditions are investigated for exponential convergence of the tracking error in feedforward adaptive systems having insufficient excitation. Particular attention is paid to the adaptive gradient algorithm with periodic excitation in the overparameterized case. A main result is that for a bounded periodic regressor, the tracking error converges exponentially without regard to parameter convergence or to the degree of overparameterization. These results weaken the persistent excitation (PE) conditions and parameter convergence conditions generally considered necessary to ensure exponential tracking convergence in this class of systems.

Optical pathlength control in the nanometer regime on the JPL phase-B interferometer testbed

This paper describes experimental research being performed at the Jet Propulsion Laboratory to develop and validate control concepts arising out of NASAs Control Structure Interaction program. The facility is meant to be a ground testbed with relevance to a broad class of future precision optical space systems. The objective of the experimental program is to investigate a multi-layer control approach to the maintenance of nanometer level optical pathlength control in the presence of external disturbances and multiple structural resonances. A brief overview of the experimental facility is presented. The control design methodology is discussed, and several experimental results are presented.

System Identification of a Nonlinear Mode for the Shuttle Radar Topography Mission

A study is presented to identify a nonlinear bending mode for a 60-m space structure. This study was done in support of the Shuttle Radar Topography Mission (SRTM) and postflight height reconstruction efforts. For this purpose, one linear model and three nonlinear models of the structural mode were considered and evaluated. The best model was determined based on in-flight data collected during the mission and was implemented as part of the final ground software that was used for reconstructing relative radar antenna motion for the SRTM interferometer payload. High accuracy estimates of the relative states were essential for supporting the motion compensation algorithm used in the radar interferometry processor for calculating the desired topographic maps. The improvement resulting fromidentifying nonlinear modal behavior contributed to meeting mission performance requirements.

Multiaxis vibration isolation, suppression, and steering system for space observational applications

THe paper describes an actively controlled six-axis orthogonal hexapod mount that provides vibration isolation, suppression and steering capabilities for space observational systems. Experimental results for vibration isolation is presented. The results show that though the passive-only isolation reduces disturbance propagation significantly, the combination of active and passive isolation provides the best option. The active stage reduces the propagation over the passive-only case by additional 35 dB in the mid frequency range of 10-100Hz.

Optical pathlength control experiment on JPL phase B testbed

An experimental implementation of a nanometer level optical pathlength control for large baseline space interferometry is presented. The pathlength compensation system is installed on a large flexible experimental truss, thus structural motions play a dominant role in the control system design. The associated control structure interaction problem is addressed to maintain the optical pathlength within the prescribed variation of 10-15 nanometer rms. By a successful blend of a structural control for damping augmentation and a direct pathlength control for the pathlength compensation, the optical pathlength variation has been maintained with 6 nanometer rms under the laboratory ambient disturbance and within 9 nanometer rms under a severe forced resonant disturbance.

Multi-tone adaptive vibration isolation of engineering structures

We present a multi-tone narrow band tracking control algorithm to isotate periodic disturbances from engineering structures. Conditions for fast convergence of control parameters are discussed. It is shown that the choice of appropriate tap delay time and number of taps is important in achieving good convergence. We implemented the algorithm in a digitat computer to attenuate a two-tone periodic disturbance from a flexible aluminum truss structure. Experimental results showed that disturbance rejection of over 25 dB was achieved.

Optical pathlength control experiment on a precision truss structure

The experimental research at the Jet Propulsion Laboratory (JPL) aimed at developing and validating new design methodology arising out of NASAs Control Structure Interaction and Orbital Stellar Interferometer programs is presented. Structural and direct optical pathlength controls are combined to maintain the pathlength variation below 10 nanometer rms. The bandwidth of the controller is 500 Hz with the disturbance rejection of over 70 dB at frequencies below 10 Hz and over 20 dB at frequencies near 100 Hz.