Allen J. Bronowicki

Active vibration control of large optical space structures

The paper describes techniques for designing and implementing active damping systems for large optical support structures. Each step in the design process is illustrated with results from the Advanced Composites with Embedded Sensors and Actuators (ACESA) vibration control system. The system is installed on a space based laser structural simulator at the Air Force Research Laboratorys Advanced Space Structure Research Experiments facility. The ACESA system consists of three large tubular active members; embedded bad zirconate-titanate (PZTs) wafers in each strut, which allows control of deformation axially and in two bending planes; 400 V drive electronics for each active member; and a nine-channel, digitally programmable, analogue control electronics unit. Design begins with the determination of the critical modes through a gain factor analysis. Actuators are located through modal strain energy analysis. Using piezostructural analysis methods, sensing and actuation functions are included in the open- and closed-loop dynamic simulations. The simulation includes local strain feedthrough effects through a static correction. Damping is applied to all modes in the frequency range up to 100 Hz, with fundamental modes achieving 20% damping, two orders of magnitude greater than the intrinsic damping level. The actuator PZTs used for active damping were also experimentally shunted with resistive elements in an attempt to introduce passive damping, although the effect was barely measurable. This demonstrates that active damping gives three orders of magnitude better performance than a passive resistive shunt in a controlled comparison.

Mechanical validation of smart structures

TRW is developing smart structures technology for vibration control in space and automotive applications. Performance verification of composite structures with piezoceramic sensors and actuators in severe mechanical and thermal environments is a key requirement. Graphite epoxy, graphite polycyanate, and graphite thermoplastic members have been fabricated with thin lead zirconate - titanate (PZT, navy types I and II) actuator and sensor wafers embedded in the composite layup. These members were subjected to tension and compression loading, hundreds of cycles of fatigue loading at levels indicative of launch loads, and thermal cycling tests at temperatures found in the hard vacuum of space. Analytic derivations show that the product of PZT modulus and piezoelectric coefficient is a figure of merit for both actuation and sensing. Test results for embedded PZTs are promising, and show correlation with analytic predictions based on vendor material data. Static actuation performance of the type I and II PZTs was found to remain relatively unchanged following static application of tensile strains of up to 600 and 1500 , respectively. A hundred fatigue loading cycles at 60% of these static strain levels were found to cause no degradation in dynamic actuation/sensing performance, while fatigue loading at the 1500 level did cause a degradation of 13%. Twelve thermal cycles over the range C caused a further 13% average degradation in dynamic performance. Dynamic actuation and sensing characteristics were found to vary within the range of over a single temperature cycle. More testing to quantify the results on a statistical basis is indicated, especially for the temperature cycling effects, which were significant.

Dual-stage passive vibration isolation for optical interferometer missions

Future space-based optical instruments such as the Space Interferometer Mission have vibration-induced error allocations at the levels of a few nano-meters and milli-arc-seconds. A dual stage passive isolation approach has been proposed using isolation first at the vibration-inducing reaction wheels, and a second isolation layer between the bus portion of the space vehicle (the backpack) and the optical payload. The development of the backpack isolator is described, with unit transmissibility results for individual isolator struts. The dual stage isolation approach is demonstrated on a dynamically feature-rich, 7-meter structural testbed (STB3). A new passive suspension that mitigates ground vibrations above 0.4 Hz has been integrated into the testbed. A series of OPD performance predictions have been made using measured transfer functions. These indicate that the 5-nm dynamic OPD allocation is within reach using the dual isolator approach. Demonstrating these low response levels in a noisy air environment has proven to be difficult. We are sequentially executing a plan to mitigate acoustic transmission between backpack and flight structure, as well as developing techniques to mitigate effects of background acoustic noise.

Active vibration suppression using modular elements

A system for active suppression of structural vibrations has been developed. The system consists of piezoelectric ceramic actuator and sensor elements which can be either bonded on to or embedded in structural components. For active damping, these are placed at locations of high modal strain energy. For active isolation, locations of high disturbance transmissibility are chosen. Small analog and digital control electronics units have been developed which include all sensing, processing, and actuator drive electronics. The analog unit is appropriate for active damping using strategies such as positive position and integral force feedback. Damping levels in structures has been increased from 0.1% to 100% using a single analog controller. The digital system is capable of executing any algorithm having two inputs and two outputs. Active damping using feedback and active force cancellation using feedforward have been demonstrated. Block diagrams, specifications, photographs, and test results describing the elements of the modular vibration suppression system are presented.

ACESA active-member damping performance

The performance of the Advanced Composites with Embedded Sensors and Actuators (ACESA) vibration control system is described. The system consists of: three tubular active members sixteen feet long and five inches in diameter, with embedded piezoceramics (PZTs) allowing control of deformation axially and in two bending planes; a 9-channel digitally programmable analog local vibration control electronics unit; and 400 Volt drive electronics for each strut. The system is installed on a space based laser structural simulator at the AF Phillips Labs Advanced Space Structure Research Experiments (ASTREX) facility at Edwards Air Force Base. The system has demonstrated ability to settle vibrations after a thruster induced slew in 0.2 seconds.

Structural design challenges for a shuttle-launched Space Interferometry Mission

The Precision Structure Subsystem (PSS) for the Space Interferometry Mission (SIM) is a large composite structure designed to house the interferometer optics in a structurally and thermally stable environment on orbit. The resulting design requirements of the PSS must be weighed against the demands of the baseline launch vehicle: the Space Shuttle. While a Shuttle launch provides new opportunities for the mission, it also presents new challenges. Many of these chal-lenges are reflected in the design of the PSS, including structural stability for supporting the optics on orbit, launch vehi-cle interface considerations (acoustic and stress loads), minimization of launch mass to provide maximum payload to orbit, thermal control to achieve necessary structural stability and a stable thermal environment for the optics, and isola-tion of the optics mounts from jitter sources and microdynamics effects. Many of these design challenges result in inherently conflicting requirements on the design of the PSS. Drawing on our experience with large composite structures such as the Chandra X-ray Observatory, TRW has created a conceptual design for this structure that addresses these challenging requirements. This paper will describe that conceptual design including trades and analyses that led to the design.

Experiences with active damping and impedance-matching compensators

TRW has been implementing active damping compensators on smart structures for the past five years. Since that time there have been numerous publications on the use of impedance matching techniques for structural damping augmentation. The idea of impedance matching compensators came about by considering the flow of power in a structure undergoing vibration. The goal of these compensators is to electronically dissipate as much of this flowing power as possible. This paper shows the performance of impedance matching compensators used in smart structures to be comparable to that of active damping compensators. Theoretical comparisons between active damping and impedance matching methods are made using PZT actuators and sensors. The effects of these collocated and non-collocated PZT sensors and actuators on the types of signals they sense and actuate are investigated. A method for automatically synthesizing impedance matching compensators is presented. Problems with implementing broad band active damping and impedance matching compensators on standard Digital Signal Processing (DSP) chips are discussed. Simulations and measurements that compare the performance of active damping and impedance matching techniques for a lightly damped cantilevered beam are shown.

Viscoelastic Damping of Structural Joints for Disturbance Isolation and Vibration Attenuation

Dam ping is a n important consideration in state -of -the -art space missions, as it is often us ed to mitigate launch loads and low -amplitude spacecraft disturbances . Although damping treatments are very effective, their implementation may be limited due to mass , cost and schedule constraints , or due to added complexity to the system design and verification efforts . Damped joints are a simple and cost effective damping option which can be easily incorporated into a spacecraft design . This approach can be used at bus panels, equipment interfaces, isolator ball joints and flexures, and other structural joints . In addition to increased damping, the addition of viscoelastic material to structural joints results in improved joint performance due to removal of nonl ine ar effects of friction and gaps . A viscoelastic damped ball joint was developed and demonstrated on three applications: 1) tuned -mass damping , resulting in 30 -50% critical damping to fixed -base damper modes; 2) acoustic test panel supports, resulting in up to a factor of 4 reduction of acoustic loads ; and 3) reaction wheel isolator struts, resulting in 5 -10% critical damping of isolator modes (and higher at strut modes) . An overview of the damped joint design, analysis and testing will be presented in th is paper.

Newcomb: a small astrometric interferometer

Newcomb is a design concept for a low-cost astrometric optical interferometer with nominal single-measurement accuracy of 100 microseconds of arc ((mu) as). In a 30 month mission, it will make scientifically interesting measurements of O-star, RR Lyrae, and Cepheid distances, probe the dark matter in our Galaxy via parallax measurements of K giants in the disk, establish a reference grid with internal consistency better than 50 microsecond(s) , and lay groundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during the next century. With an extended mission life, Newcomb could do a useful preliminary search for other planetary systems.