Large light-weight telescopes in space are considered key elements enabling future earth observation and space science. The first large space telescope, “Hubble”, uses a monolithic aspheric primary mirror of 2.4 m diameter. The Hubble Space Telescope primary mirror has an area density of about 180 kg/m2. This monolithic approach cannot be used for much larger telescopes due to mass and volume limitations imposed by today’s launch capabilities. Thus the current generation space telescope, the James Webb Space Telescope now under development, makes use of a segmented aspheric primary mirror of 6.5 m diameter. The area density will be below 20 kg/m2.
KEYWORDS: Sensors, Composites, Data acquisition, Systems modeling, Sensor networks, Error analysis, Model-based design, Data modeling, System identification, Aerospace engineering
For aerospace composite materials and structures, damage due to impact events may not be visible to surface inspection but still can cause significant loss of structural integrity. Therefore, an investigation was performed to develop a real-time health monitoring system for the identification and prediction of the location and force history of foreign object impact on composite panel structures with distributed built-in piezoceramic sensors. The smart health monitoring system is composed of two main subsystems: a measurement subsystem and an identification subsystem. The measurement subsystem with distributed built-in sensor network was used to collect and preprocess sensor data, and then the identification subsystem was implemented to reconstruct the force history and determine impact location with the acquired prefiltered sensor data. Thereupon, the identification subsystem consists of a structure system model, an inverse model operator (IMO) and a response comparator. The identification subsystem was created to identify the impact location and reconstruct the force history on composite structures without the need for the information about actual mechanical properties, geometries and boundary conditions of a structure, and without building a specific neural network with exhaustive training such as neural-network techniques, also without the need of constructing a full-scale accurate structural model. Consequently, a novel dynamic mechanical model based time-series model structure approach is used into the identification subsystem, where the entire impact identification procedure is much faster than that of the traditional model-based techniques. The smart health monitoring system was tested with various impact situations, for all of the cases considered, which verified the accuracy of impact load and position predictions, and the estimation errors fell well within the prespecified limit.
Aerospace structures are often made out of (Carbon) fiber reinforced polymers. In order to enhance their
physical properties especially on thermal and electrical conductivity, the polymer matrix materials are to be
enriched with different types of fillers such as CNTs or Carbon black. Results from different related
investigations show benefits on the one side but also some problem areas which are to be observed. Shape
memory polymers (SMP) are applied to the structures to compensate possible shape deviations due to curing
stresses, where fillers are to be used to reduce if not avoid creep effects in these SMP.
Thermal strain measurements by fiber Bragg grating (FBG) sensors mounted onto different host materials are
demonstrated for low coefficients of thermal expansion (CTE). Such low CTEs are typically found in carbon
fiber reinforced plastics (CFRP). This work has application potential for FBG sensor networks in the highprecision
control of thermal deformations in structures or in curing monitoring. For this purpose, a thermal
error model of the FBG sensor, which accounts for the thermo-optic coefficient and the thermal expansion of
the FBG, was characterized experimentally. The error-model characterization method is based on reference
measurements of FBGs bonded to ZERODUR ceramics. Using this error model, thermal strain can be measured
by surface-mounted FBGs on any given host structure using an external temperature reference and the FBG's
wavelength shift. This method is demonstrated successfully for unidirectional layers of CFRP with a CTE of
-0.4 · 10-6 1/K in fiber direction and for steel (316 Ti), which is commonly used in cryogenic applications.
Measurements are performed for temperatures from 100K to 320K and the results are verified by high-precision
dilatometer measurements. Accuracy limits of the FBG-based thermal strain measurements are discussed, as
well as the minimization of errors induced by the FBG's structural interface. Further, the reduction of errors in the adhesive bonding is discussed. This work expands the understanding of the separation of thermal and mechanical effects in the signals obtained by FBGs.
Satellite mechanical performance is to be further enhanced e.g. by active launch vibration attenuation, and even more so
by in-orbit micro-vibration and shape control and possibly also significant shape morphing. This puts stringent
requirements on the actuators and their materials, such as high resolution of possibly large strokes, or a very broad
operational temperature range going down to -150°C or even lower. The discussion also shows the need to consider the
host material and structure together with the actuator as a highly interacting system. This holds to a considerable extent
also for integrated fiber optic sensors used for strain and temperature monitoring.
Large satellites are equipped with hundreds of sensors for temperature measurement. The large amount of
sensors is expensive in terms of integration effort and mass in the case conventional sensors are used. In this
article an integrated fiber optic temperature sensor network for the hot spot detection on satellite sandwich
panels is introduced. The developed sensor system is integrated with only negligible mechanical impact. It is
electro-magnetic immune and decoupled from mechanical loads. In addition to monitoring hotspots, the number
and aerial density allows a reliable reconstruction of temperature and displacement fields.
This paper investigates the use of filtering techniques such as the Low-pass and Kalman filter in combination
with the quasi-static strain-displacement transformation in order to estimate dynamic structural displacement
based on noisy strain measurements. Numerical simulations and vibration experiments were performed on simple
beam structures under various dynamic loading cases to determine the sensitivity of estimation procedures against
model errors and noise disturbance. In the experimental setup the multiplexing ability of fiber Bragg grating
(FBG) sensors was used to obtain strain data with high accuracy and low noise level. Estimated displacements in
the experiment were verified against laser displacement readings. Depending on the load case the noise-sensitive,
quasi-static shape estimation results could be highly improved by applying recursive filtering methods which
allow an application of the simple approach even for complex, vibrating structures.
Fiber optic sensors are of interest because of their robustness against environmental disturbances, low drift, and ease of
integration. The relatively high population of measurement points also favors the estimation of displacement and
temperature from discrete data. This together with techniques for integration into structural materials is discussed in the
context of satellite structures.
Very thin shells, shell-membrane and membrane structures offer high actuator authority for controlling their behaviour.
On the other side, special care has to be taken for proper material and structural characterisation and its interaction with
the actuators. In the paper different types of actuators are discussed in the context of precision shell-membrane space
reflectors. For actuation, emphasis is given on piezo-ceramics and electro-active polymers. Special integration,
modelling and testing techniques for smart fibre reinforced shell-membranes and membranes are addressed.
A possible approach to meet the increasing performance requirements of lightweight structures in various engineering
fields is the application of smart structures. One of the functions, which are required, is the observation of the structures'
shape. During operation, however, the monitoring of displacement fields is difficult. This paper discusses the
displacement field estimation of a dynamically excited plate using fiber Bragg grating strain sensors. Using a modal
approach, it is possible to derive a transformation matrix to estimate the displacement field using only a few strain
measurements. To reduce systematic estimation errors due to residual modes, a parameter study was performed and the
sensor location optimized using the condition number of the transformation matrix as an objective function. An
experiment with an optimized sensor configuration including 16 fiber Bragg grating strain sensors was performed to
verify the method and the simulation results.
Fiber optic Bragg grating (FBG) sensors show promising capabilities in the measurement of strain and temperatures in structures at many locations. In this work, the potential of FBG sensors for high-precision deformation control in opto-mechanical applications is investigated. This requires a strain resolution of < 1 um/m. A test rig with a simply supported steel beam was developed which should represent the geometry of a lightweight optical mirror with a ribbed support structure. The deformation of this beam is controlled by a piezo actuator. The reference deformation measurement is done
using six capacitive displacement sensors with a resolution < 0.5 nm. It is being investigated to what level of accuracy FBG sensors can be used to reconstruct the displacement information. Different
methods to increase the accuracy are discussed: decreasing the sensor noise by oversampling and increasing the number of sensors. Tests were performed using different diffraction-based interrogation techniques for the wavelength detection: a CCD-based FBG sensor system and a PSD (Position Sensitive Detector)-based high-speed FBG sensor system which - to our knowledge - has not been used for an application of this kind yet. A comparison of both systems discussing the weaknesses and strengths is given for the recording of mechanical strain < 1 um/m. The results showed that a resolution of < 0.3 um/m for the strain measurement using FBG sensors can be achieved. This study shows an interesting application potential for FBG sensors in structural deformation control for various fields such as optics or high-precision machine tools.
Several approaches for design optimization in control-structure interaction problems for precision reflectors and structure are discussed. Applications for shape control and active vibration damping show the need to treat this interaction simultaneously or to use properly defined iterative decomposition.
In a joint effort, the European Southern Observatory (ESO), the Institute for Lightweight Structures, Technical University of Munich, and EADS Astrium, Germany, have developed a set of software tools for integrated modeling of astronomical telescopes and interferometers. Integrated modeling aims at time-dependent system analysis combining different technical disciplines (optics, mechanical structure, control system with sensors and actuators, environmental disturbances). As example for the application of this modeling technique, we present an integrated model of the Very Large Telescope Interferometer (VLTI). It can be regarded as a "precursor" model for future telescope or interferometer projects. Besides its demonstrator role, it also serves for practical applications. An example is prediction of the dynamic VLTI output performance at interface-level to future scientific instruments, such as GENIE. The basic output of the integrated model is a complete description of the time-dependent electromagnetic field within a broad spectral range and for each interferometer arm.
Alternatively, a more elaborated output can be created, such as a fringe pattern resulting from a superposition of several beams after modal filtering by single-mode fibers. The paper shows the architecture of the integrated model with its components such as telescope structures, optics, control loops and disturbance models for wind load, seismic ground acceleration and atmospheric turbulence.
Results illustrating the capability of the model approach are presented.
Within the scope of the Very Large Telescope Interferometer (VLTI) project, ESO has developed a software package for integrated modeling of single- and multi-aperture optical telescopes. Integrated modeling is aiming at time-dependent system analysis combining different technical disciplines (optics, mechanical structure, control system with sensors and actuators, environmental disturbances). This allows multi-disciplinary analysis and gives information about cross-coupling effects for system engineering of complex stellar interferometers and telescopes. At the moment the main components of the Integrated Modeling Toolbox are BeamWarrior, a numerical tool for optical analysis of single- and multi-aperture telescopes, and the Structural Modeling Interface, which allows to generate Simulink blocks with reduced size from Finite Element Models of a telescope structure. Based on these tools, models of the various subsystems (e.g. telescope, delay line, beam combiner, atmosphere) can be created in the appropriate disciplines (e.g. optics, structure, disturbance). All subsystem models are integrated into the Matlab/Simulink environment for dynamic control system simulations. The basic output of the model is a complete description of the time-dependent electromagnetic field in each interferometer arm. Alternatively, a more elaborated output can be created, such as an interference fringe pattern at the focus of a beam combining instrument. The concern of this paper is the application of the modeling concept to large complex telescope systems. The concept of the Simulink-based integrated model with the main components telescope structure, optics and control loops is presented. The models for wind loads and atmospheric turbulence are explained. Especially the extension of the modeling approach to a 50 - 100 m class telescope is discussed.
Within the scope of the Very Large Telescope Interferometer (VLTI) project, a set of software tools for integrated modeling of ground- and space-based stellar interferometers has been developed. Integrated modeling aims at time-dependent system analysis combining different technical disciplines (optics, mechanical structure, control system with sensors and actuators, environmental disturbances). The main components of the software are
BeamWarrior, a tool for creation of dynamic optical models, and SMI (Structural Modeling Interface), which generates linear state-space models from finite element models of a mechanical structure. Based on these tools, models of the various subsystems (e.g. telescope, delay line, beam combiner) can be created in the relevant technical disciplines (e.g. optics, structure). All subsystem models are integrated into the Matlab/Simulink
environment for dynamic control system simulations. The output of the dynamic model is a complete description of the time-dependent electromagnetic field in each interferometer arm. This output serves as input to an instrument model simulating the creation of interference fringes.
This paper shows the application of the integrated modeling concept to the VLTI. The architecture of a Simulink-based integrated model with its main components, telescope structures, optics and control loops, is presented. Disturbance models for wind load, seismic ground excitation and atmospheric turbulence are included. Beam combination is performed using a simplified model of the VINCI instrument. Results of closed-loop dynamic simulations are presented.
In cooperation with the European Southern Observatory (ESO), the Institute of Lightweight Structures (LLB), Technische Universtitaet Muenchen, has developed the Structural Modeling Interface Toolbox (SMI), a Matlab based software package for creation of a dynamical model of a telescope structure. It is called Interface, since it uses the modal data of a finite element (FE) analysis and creates a dynamic model to be used within a time-dependent control loop simulation in the Matlab/Simulink environment. SMI is part of the Integrated Modeling Toolbox (IMT) developed in a joint effort by ESO, Astrium GmbH and LLB.
Since SMI can read modal data in a general format, it is not depending on the FE-software. In addition to that, an interface to the FE-package ANSYS has been developed. It allows the variation of parameters and some settings for the FE-analysis directly within SMI.
Both, force excitation like windloads and base excitation like micro seismic perturbations can be included. Several tools for model reduction are provided. Some of them are modal based, like effective modal masses, others are general model reduction procedures from control engineering like balanced truncation.
For the evaluation of the reduced models, transfer functions of different models can be displayed in a Bode-plot. Time characteristics like step response or impulse response are also available. Moreover, for a typical excitation PSD the response PSD can be computed. This response can either be compared to the response of an exact model or to measured data and the rms-error can be calculated.
The final result is a linear statespace model of the structure and a Simulink block, which can be included into a Simulink model.
This article presents a software package for “integrated modeling” of single- and multi-aperture optical telescopes. Integrated modeling is aiming at time-dependent system analysis combining different technical disciplines such as optics, mechanical structure, control system with sensors and actuators. Various, environmental and internal disturbances can be taken into account. Software design and development is done in a joint effort by the European Southern Observatory (ESO), Astrium GmbH and the Institute of Lightweight Structures (LLB), Technical University of Munich. The architectures of the two most advanced modules generating dynamic models of the mechanical structure and the optical system are described. A “real-life” example related to the Very Large Telescope Interferometer (VLTI) illustrates the application in practice.
Precise large astronomical telescopes require proper control and minimization of static and dynamic structural distortions caused by different loads, e.g. from gravity, wind, temperature and equipment noise. This equally holds for the main structure as well as for the component level. Examples for the first are the alignment of secondary and primary mirror, and for the latter the shape error minimization of the reflecting surfaces. In this paper, essential concepts for passive and active distortion control are presented and discussed. Some of these concepts are demonstrated for the case of a big representative truss structure and the influence of modal reduction is pointed out. The tools developed to simulate and optimize active damping systems on truss structures are flexible and extendable and could be applied to big telescope structures like for example the OWL structure.
Adaptive structures optimal design problems and solution methods are presented and discussed for the cases of shape control and active damping. A general method to solve simultaneously optimal placement and control problems for active damping applications is outlined. This method is based on a finite element structural model and on the calculation of the closed-loop system poles. Numerical results are given to illustrate this method for the case of a beam with collocated PZT sensor/actuator pair and a direct velocity feedback controller.
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