Structural Health Monitoring
The science community has determined that visualization of the first generations of stars will require a successor to Hubble Space Telescope (HST). The James Webb Space Telescope (JWST) with its large light-gathering mirror and superb resolution will be capable of detecting faint signals from the first billion years, the period when galaxies formed. The CSULA's Precision Segmented Reflector project, sponsored by NASA under the IRA and URC programs (SPACE laboratory), serves as a first step in the development and validation of the enabling technologies that ultimately will be used by the JWST mission in 2013.
Figure 1: The SPACE Testbed
The proposed research will build and expand on the existing URC program currently under way. Advanced technologies for thermal analysis of the testbed structure, figure maintenance, fault identification, precision pointing, and reconfigurable control will be developed and experimentally validated on the SPACE testbed. Due to the nature of the structure, the research will employ decentralization techniques for the development of control laws to accomplish figure maintenance of the primary mirror to within 1 micron RMS distortion with respect to a nominal shape of the primary mirror, and pointing accuracy of 2 arc seconds. Design of control laws will be based on various approaches including robust control and neural networks. A system identification task will be responsible for tuning the dynamic models used for controller design.
The James Webb Telescope employs a broad range of materials, including ceramics that must be kept below a temperature of 50 K during the telescopeÂ’s operation so that its own thermal signature does not affect the performance of the sensors. The sensors themselves must be maintained at a temperature of 7 K in order to detect faint infrared emissions from distant objects, which means that they, along with other sensitive components within the instrument module, must be cooled cryogenically. These conditions vary drastically when compared to the temperatures the telescope undergoes during the deployment process where the sunshield and mirror assembly are first fully exposed to solar radiation. Due to this large variation in material properties and thermal conditions, a heat transfer analysis must be conducted in order to ensure that all components of the telescope can endure the dramatic changes in conditions during their deployment and operation.
To verify stable operation for a wide range of scenarios, characterization of heat transfer among components in addition to analysis of individual components will be explored. Coupled with the effect of solar radiation, conductive heat transfer may result in the expansion or contraction of materials to the extent that the proper deployment of telescope mirrors and sunshield may be compromised. Additionally, inadequate consideration of thermal cycling may result in component failure over the long term. This calls for an experimental and computational study of the telescope materials, which will entail the construction of simplified models of telescope subsystems, evaluation of the resulting substrates under high heat and cryogenic conditions by measuring thermal expansion and material strength limits, and determination of failure modes. Experimental results will be supported by thermal models, which can then be applied to predict material behavior beyond what can be tested in a laboratory setting.
Building a larger-aperture telescope by fabricating the optical components in sections, rather than by attempting to polish the continuous surface from a single piece of glass, is an especially attractive concept. Although multiple mirror designs have many advantages, a number of major difficulties are associated with the technique. Because multiple-mirror telescopes are independent devices, combining their images at one focal plane is a difficult task. Not only do the images have to be precisely "stacked" on one another, but the individual focal planes must be coplanar as well.
A reflector built from segments relies on its support structure for stiffness and rigidity and on an active control system for precision alignment of the optical surface. Therefore, an active control system is required to maintain the alignment of the segmented reflectors. This control system will also be necessary to achieve the high level design recruitments. Several technical challenges in the areas of sensing, actuation, modeling, and controls arise in the design and implementation of such a control system.
Specifically, control of the optics associated with the JWST mission requires development of technologies for precision pointing, vibration attenuation, fault identification, controller reconfiguration, robust control, decentralized control, neuro-fuzzy control, and system identification. Figure 1 shows the major features of the SPACE testbed including the primary and secondary mirrors, the supporting structure, and the isolation platform. The testbed is designed to emulate a Cassegrain telescope of 2.4-meter focal length with performance comparable to an actual space-borne system. The system's top-level requirements figure maintenance of the primary mirror to within 1 micron RMS distortion with respect to a nominal shape of the primary mirror, pointing accuracy of 2 arc seconds, a high level of disturbance rejection (100:1), and attenuation of vibration due to gravity, thermal and seismic effects, and control structure interaction.
The primary mirror is composed of a ring of six actively controlled hexagonal panels arranged around a central panel. The central panel is fixed and serves as a point of reference to the moving panels. The testbed is a control-oriented experimental system and due to the difficulty and added expense of actual optical quality segments made from glass, the panels are made of aluminum honeycomb plates. The required paraboloid surface is thus maintained by positioning the flat panels as tangents to the surface.
The primary mirror is supported by a specially designed lightweight truss structure whose structural dynamic characteristics are representative of a large, flexible space-borne system. The entire testbed is supported on a triangular isolation platform made of aluminum honeycomb core with stainless steel top and bottom skin. The SPACE testbed active secondary mirror is attached to the primary by a tripod. This critical feature has been added to the testbed to make its performance and functionality more comparable to an actual space-borne telescope. It has been designed to provide three-axis active control, with control system hardware that consists of a number of reluctance actuators and position sensors that move and control the secondary mirror.
The SPACE testbed is fitted with an optical scoring system consisting of the secondary mirror, a laser source placed in the center of the primary, and an array of optical sensors that will detect any deviation from a reference position. The testbed's control system is composed of a data acquisition system, computers, and actuators. The control system consists of an ensemble of 24 edge sensors mounted on the peripheries of the six actively controlled segments; 18 high-bandwidth, high-force-output, voice-coil linear actuators mounted on the truss, three per each moving panel affecting three degree of freedom motion. These actuators are fitted with collocated position sensors. The sensor signal is processed through a pre-amplifier for signal conditioning and then to an A/D converter. A digital signal processor, a Pentek 4285 with four Texas Instruments TMS320C40 processors, is used to process the data, implement control algorithms, and send commands to a D/A converter and a Glentek GA4555P linear amplifier, which in turn move the system actuators to implement control. To perform experiments related to pointing, a slewing mechanism, already designed, will be fabricated and added to the structure.
The segmented reflector telescope under consideration consists of a large number of structural components (reflector panels, supporting and peripheral structures), as well as sensors and actuators leading to mathematical models that involve hundreds of states. Consequently, the design of control laws based on conventional methodologies becomes exceedingly difficult. Decentralized control appears to be a viable approach in circumventing the difficulties related to the dimensionality problem. Because of the nature of the structure, the research will employ decentralization techniques for the development of various control laws to accomplish precision pointing and vibration control. The design of the control laws will demonstrate that: a) the flexible structure may consist of a considerable number of components that are weakly interconnected in terms of dynamics and actuation; and b) failure in one or more components should be accommodated by the overall structure and should not degrade the overall performance.
In the next five years, the SPACE Laboratory research team` will perform a thermal analysis of the testbed, and will develop an accurate finite-element model. Using this model, a simulation model for control performance evaluation will be developed. A least-squares identification technique for tuning the dynamic models will be used for the controller design.
An actively controlled structure such as a segmented reflector telescope involves a large number of sensors and actuators, increasing significantly the likelihood of a sensor/actuator failure. To continue addressing the substantial performance degradation resulting from such a failure, a task will be included to study suitable methodologies for failure detection, fault location and isolation, and reconfiguration of the control system for performance recovery. Several control algorithms will be developed in this research and they will be experimentally validated on the SPACE testbed.
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