High resolution imaging from space requires very large apertures, such as NASA’s current mission the James Webb Space Telescope (JWST) which uses a deployable 6.5m segmented primary. Future missions requiring even larger apertures (>>10m) will present a great challenge relative to the size, weight and power constraints of launch vehicles as well as the cost and schedule required to fabricate the full aperture. Alternatively, a highly obscured annular primary can be considered. For example, a 93.3% obscured 30m aperture having the same total mirror area (91m2) as a 10.7m unobscured telescope, can achieve ~3X higher limiting resolution performance. Substantial cost and schedule savings can be realized with this approach compared to fully filled apertures of equivalent resolution. A conceptual design for a ring-shaped 30m telescope is presented and the engineering challenges of its various subsystems analyzed. The optical design consists of a 20X annular Mersenne form beam compactor feeding a classical 1.5m TMA telescope. Ray trace analysis indicates the design can achieve near diffraction limited images over a 200μrad FOV. The primary mirror consists of 70 identical rectangular 1.34x1.0m segments with a prescription well within the demonstrated capabilities of the replicated nanolaminate on SiC substrate technology developed by AOA Xinetics. A concept is presented for the deployable structure that supports the primary mirror segments. A wavefront control architecture consisting of an optical metrology subsystem for coarse alignment and an image based fine alignment and phasing subsystem is presented. The metrology subsystem is image based, using the background starfields for distortion and pointing calibration and fiducials on the segments for measurement. The fine wavefront control employs a hill climbing algorithm operating on images from the science camera. The final key technology required is the image restoration algorithm that will compensate for the highly obscured aperture. The results of numerical simulations of this algorithm will be presented and the signal-tonoise requirements for its successful application discussed. It is shown that the fabrication of the 30m telescope and all its supporting subsystems are within the scope of currently demonstrated technologies. It is also shown that the observatory can be brought to geosynchronous orbit, in its entirety, with a standard launch vehicle.
ACCESS (Actively-Corrected Coronagraph for Exoplanet System Studies) was one of four medium-class exoplanet
concepts selected for the NASA Astrophysics Strategic Mission Concept Study (ASMCS) program in 2008/2009 [14,
15]. The ACCESS study evaluated four major coronagraph concepts under a common space observatory. This paper
describes the high precision pointing control system (PCS) baselined for this observatory.
Prediction of optical performance for large, deployable telescopes under environmental conditions and mechanical disturbances is a crucial part of the design verification process of such instruments for all phases of design and operation: ground testing, commissioning, and on-orbit operation. A Structural-Thermal-Optical-Performance (STOP) analysis methodology is often created that integrates the output of one analysis with the input of another. The integration of thermal environment predictions with structural models is relatively well understood, while the integration of structural deformation results into optical analysis/design software is less straightforward. A Matlab toolbox has been created that effectively integrates the predictions of mechanical deformations on optical elements generated by, for example, finite element analysis, and computes optical path differences for the distorted prescription. The engine of the toolbox is the real ray-tracing algorithm that allows the optical surfaces to be defined in a single, global coordinate system thereby allowing automatic alignment of the mechanical coordinate system with the optical coordinate system. Therefore, the physical location of the optical surfaces is identical in the optical prescription and the finite element model. The application of rigid body displacements to optical surfaces, however, is more general than for use solely in STOP analysis, such as the analysis of misalignments during the commissioning process. Furthermore, all the functionality of Matlab is available for optimization and control. Since this is a new tool for use on flight programs, it has been verified against CODE V. The toolbox' functionality, to date, is described, verification results are presented, and, as an example of its utility, results of a thermal distortion analysis are presented using the James Webb Space Telescope (JWST) prescription.
JWST will be used to help understand the shape and chemical composition of the universe, and the evolution of galaxies, stars and planets. With a 6.5 meter primary mirror, the Observatory will observe red shifted light from the early history of the universe, and will see objects 400 times fainter than those seen from large ground-based telescopes or the current generation of space-based infrared telescopes. NASA Goddard Space Flight Center (GSFC) manages JWST with contributions from a number of academic, government, and industrial partners. The contract to build the space-based Observatory for JWST was awarded to the Northrop Grumman Space Technology (NGST)/Ball/Kodak/ATK team.
An approach for achieving a several order of magnitude reduction of vibration in space telescopes is described. The building blocks encompass: active damping in the payload, passive and active vibration isolation in the payload mount, and active steering of the entire payload using feedback from optical sensors to the actuators in the isolator. The layers of control sequentially mitigate vibrations emanating from the disturbance source.
This strategy is especially applicable to large space-based optical systems, such as deployable telescopes. A dynamically scaled test bed has been developed to demonstrate this. The test bed incorporates a full-telescope isolator based on passive isolation technology demonstrated on the Chandra X-ray Observatory. A PZT force sensor and a voice coil actuator have been incorporated to provide additional active isolation at low to mid frequencies. The active isolation is implemented in a stable manner using independent modal space control (IMSC). Residual vibrations in the telescope secondary mirror supports are damped using PZT actuator/sensor patches and local analog feedback. Finally, signals from line-of-sight sensors in the two tilt directions are fed to the tilt states in the IMSC controller to effect payload pointing control.
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.
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.
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.
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.
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.
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 Lab's 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.
An overview of three ongoing TRW projects in the area of active structures is presented. The first project involves the development and validation of active member technology for future use in space based optical systems. The autonomous identification of changes in active structures and updating local compensator parameters for maximum performance is investigated in the second project. The third project is concerned with system level applications of active members to slewing and maneuvering aircraft. Mechanical, structural dynamics, and electronics tests of active members are described. Systems identification, compensation design, and performance simulation in active structure monitoring is reviewed and precision control of agile spacecraft is discussed.
27 February 2006 | San Diego, California, United States
Smart Structures and Integrated Systems
7 March 2005 | San Diego, California, United States
Smart Structures and Integrated Systems
15 March 2004 | San Diego, CA, United States
Smart Structures and Integrated Systems
3 March 2003 | San Diego, California, United States
SC126: Active Structures for Vibration and Shape Control
Smart structures are used in a wide range of products. The Hubble telescope contains smart materials for correcting optical deficiencies; chip fabrication equipment includes PZT actuators to cancel undesirable vibrations, and brassieres use memory metals to retain their shape in the laundry. This course presents fundamental concepts and practical instruction in the use of smart structures for these and other structural applications. Principles are reinforced using the Smart Strut Demo, a self-contained, portable active damping learning aid.