The Giant Magellan Telescope (GMT) is a planned large terrestrial telescope with a segmented primary mirror with a
24.5 meter overall diameter. Like most terrestrial telescopes, the GMT resides within an enclosure designed to protect
the telescope from the elements and to reduce the effects of wind on the optical performance of the telescope. Wind
impingement on the telescope causes static deformation and vibration in the telescope structure that affects the alignment
and image jitter performance of the telescope. Actively controlled primary mirror segments and a secondary mirror can
correct for the static and low frequency portions of the wind effects, but typically the actuators do not have the
bandwidth to address higher frequency components of the wind environment. Preliminary analyses on the GMT indicate
that the image jitter associated with wind effects meets budgeted allowances but without much margin. Preliminary
models show that the bulk of the residual jitter arises from excitation of a small number of modes in the 9 to 12 Hz
range. Therefore, as a risk mitigation effort to increase the margin on the wind induced jitter, passive and active
vibration mitigation approaches have been examined for the GMT, which will be the focus of this paper. Using a finite
element model of the GMT along with wind loading load cases, several passive and active vibration mitigation
approaches were analyzed. These approaches include passive approaches such as tuned mass dampers targeting the
worst offending modes, and constrained layer damping targeting all of the modes within the troublesome frequency
range. Active approaches evaluated include two active damping approaches, one using several reaction mass actuators
and the other using active strut type actuators. The results of the study show that although all approaches are successful
in reducing the jitter, the active damping approach using reaction mass actuators offers the lightest weight, least
implementation impact, and most adaptability of any of the approaches.
Vibration is becoming a more important element in design of telescope structures as these structures become larger and more compliant and include higher bandwidth actuation systems. This paper describes vibration damping methods available for current and future implementation and compares their effectiveness for a model of the Large Synoptic Survey Telescope (LSST), a structure that is actually stiffer than most large telescopes. Although facility and mount design, structural stiffening and occasionally vibration isolation have been adequate in telescopes built to date, vibration damping offers a mass-efficient means of reducing vibration response, whether the vibration results from external wind disturbances, telescope slewing, or other internal disturbances from translating or rotating components. The paper presents several damping techniques including constrained layer viscoelastics, viscous and magnetorheological (MR) fluid devices, passive and active piezoelectric dampers, tuned mass dampers (vibration absorbers) and active resonant dampers. Basic architectures and practical implementation considerations are discussed and expected performance is assessed using a finite element model of the LSST. With a goal of reducing settling time during the telescope's surveys, and considering practicalities of integration with the telescope structure, two damping methods were identified as most appropriate: passive tuned mass dampers and active electromagnetic resonant dampers.
This paper describes the use of adaptive filtering to control vibration and optical jitter. Adaptive filtering is a class of
signal processing techniques developed over the last several decades and applied since to applications ranging from
communications to image processing. Basic concepts in adaptive filtering and feedforward control are reviewed. A
series of examples in vibration, motion and jitter control, including cryocoolers, ground-based active optics systems,
flight motion simulators, wind turbines and airborne optical beam control systems, illustrates the effectiveness of the
adaptive methods. These applications make use of information and signals that originate from system disturbances and
minimize the correlations between disturbance information and error and performance measures. The examples
incorporate a variety of disturbance types including periodic, multi-tonal, broadband stationary and non-stationary.
Control effectiveness with slowly-varying narrowband disturbances originating from cryocoolers can be extraordinary,
reaching 60 dB of reduction or rejection. In other cases, performance improvements are only 30-50%, but such
reductions effectively complement feedback servo performance in many applications.
The Image Stabilization Testbed (ISTAT) is a high-bandwidth angular motion system for the simulation of missile dynamics with capability beyond that of current flight motion simulators (FMS). This paper describes the development and initial laboratory integration of the ISTAT. The intention is to mount a missile seeker and any associated inertial measurement sensors, and then allow ISTAT to replicate the dynamic boundary conditions at the base of the seeker resulting from both airframe vibrations (flexible body motion) as well as rigid body motion resulting from vehicle control forces or the flight environment. ISTAT will be driven by the output of deterministic simulations and will replicate the time history of the command signals. It can be used in a standalone mode or possibly in conjunction with a conventional large motion lower bandwidth FMS. ISTAT makes use of high bandwidth hydraulic actuation and advanced feedback and feedforward control algorithms to deliver two- and three-axis motion control at frequencies from DC to greater than 500 Hz. The largest motions, achieved at lower frequencies, are about two degrees. The paper describes the motivation, the servohydraulic, mechanical, and electronic subsystems, control software and algorithms, and the software user interface for the testbed. An initial report on the system integration is also provided.
The Stratospheric Observatory For Infrared Astronomy, SOFIA is being developed by NASA and the German space agency, Deutschen Zentrum fur Luft- und Raumfahrt (DLR), with an international contractor team. The 2.5-meter reflecting telescope of SOFIA will be the world's largest airborne telescope. Flying in an open cavity on a modified 747 aircraft, SOFIA will perform infrared astronomy while cruising at 41,000 feet and while being buffeted by a 550- mile-per-hour slipstream. A primary system requirement of SOFIA is tracking stability of 0.2 arc-seconds, and a 3-axis pointing control model has been used to evaluate the feasibility of achieving this kind of stability. The pointing control model shows that increased levels of damping in certain elastic modes of the telescope assembly will help achieve the tracking stability goal and also expand the bandwidth of the attitude controller. This paper describes the preliminary work that has been done to approximate the reduction in image motion yielded by various structure configurations that use reaction masses to attenuate the flexible motions of the telescope structure. Three approaches are considered: passive tuned-mass dampers, active-mass dampers, and attitude control with reaction-mass actuators. Expected performance improvements for each approach, and practical advantages and disadvantages associated with each are presented.
The Airborne Laser (ABL) system has extremely tight jitter requirements. Acoustic disturbances, such as those caused by the pressure recovery system of the high power laser, are a significant jitter source. Several technologies may be appropriate for reducing the acoustically induced jitter. The first choice for mitigation will be passive approaches, such as acoustic blankets. There is, however, some uncertainty whether these approaches will provide sufficient attenuation and there is concern about the weight of these approaches. A testbed that captured the fundamental physics of the ABL acoustically induced optical jitter problem was developed. This testbed consists of a flexure-mounted mirror exposed to an acoustic field that is generated outside a beam tube and then propagates within the tube. Both feedback and adaptive feedforward control topologies were implemented on the testbed using either of two actuators (a fast steering mirror and a secondary acoustic speaker located near the precision mirror), and a variety of sensors (microphones measuring the acoustic disturbance, accelerometers and microphones mounted on the precision optic, and an optical position sensing detector). This paper summarizes the results from these control topologies for reducing the acoustically induced jitter with some control topologies achieving in excess of 40 dB jitter reduction at a single frequency. This work was performed under an SBIR Phase I funded by the Air Force Research Laboratory Space Vehicles Directorate.
Spacecraft carry instruments and sensors that gather information from distant points, for example, from the Earth's surface several hundred kilometers away. Small vibrations on the spacecraft can reduce instrument effectiveness significantly. Vibration isolation system are one means of minimizing the jitter of sensitive instruments. This paper describes one such system, the Satellite Ultraquiet Isolation Technology Experiment (SUITE). SUITE is a piezoelectric-based technology demonstration scheduled to fly in 2000 on PICOSat, a microsatellite fabricated by Surrey Satellite Technology, Ltd. Control from the ground station is planned for the first year after launch. SUITE draws on technology from previous research programs as well as a commercial piezoelectric vibration isolation system. The paper details the features of SUITE, with particular emphasis on the active hexapod assembly. A description of the PICOSat spacecraft and the other considerations preceding the development of the flight hardware begins the paper. Experimental goals are listed. The mechanical and electromechanical construction of the SUITE hexapod assembly is described, including the piezoelectric actuators, motion sensors, and electromagnetic actuators. The data control system is also described briefly, including the digital signal processor and spacecraft communication. The main features of the software used for real-time control and the supporting Matlab software used for control system development and data processing are summarized.
Launch loads, both mechanical and acoustic, are the prime driver of spacecraft structural design. Passive approaches for acoustic attenuation are limited in their low frequency effectiveness by constraints on total fairing mass and payload volume constraints. Active control offers an attractive approach for low frequency acoustic noise attenuation inside the payload fairing. Smart materials such as piezoceramics can be exploited as actuators for structural-acoustic control. In one active approach, structural actuators are attached to the walls of the fairing and measurements from structural sensors and/or acoustic sensors are fed back to the actuators to reduce the transmission of acoustic energy into the inside of the payload fairing. In this paper, structural-acoustic modeling and test results for a full scale composite launch vehicle payload fairing are presented. These analytical and experimental results fall into three categories: structural modal analysis, acoustic modal analysis, and coupled structural-acoustic transmission analysis. The purpose of these analysis and experimental efforts is to provide data and validated models that will be used for active acoustic control of the payload fairing. In the second part of the paper, this closed-loop acoustic transmission reduction is implemented and measured on a full-scale composite payload fairing.