The use of true-membrane reflectors holds the promise of increasing the size of space-based apertures by decreasing payload mass and reducing launch volumes, but figure acquisition and maintenance of the thin, deployed structure present significant control challenges. The ability to manage both the static and dynamic aberrations defines the utility of these compliant mirrors for resolving quality images. The scope of the current study consists of characterizing the non-linear dynamic behavior of membrane reflectors to visible-optics quality under realistic support and loading scenarios. The basis for quality in the finite element model (FEM) deformed shape predictions is established both by comparing FEM and analytical solutions for linear static problems and by studying the convergence of eigen solutions. Most of the results are shown, too, to be within a previously determined range of optically-accurate solutions. The topographical difference between linear and non-linear dynamic solutions is characterized and correlated to support and loading regimes for eventual inclusion in closed-loop-control schemes. The objective of this paper is thus to study the non-linear characteristics of the dynamic behavior of membrane optics as the basis for future work in system identification and figure control.
Ionic polymers are a class of electromechanically coupled materials that can be used as flexible transducers. When set up in the cantilever configuration, the actuators exhibit a large bending deflection when an electric field is applied across their thickness. Being a relatively new research topic, the governing physical and chemical mechanisms are not yet fully understood. Experimental results have demonstrated nonlinear dynamic behavior. The nonlinear dynamics can be seen in the response of current, displacement, and velocity of the actuator. This work presents results for the nonlinear identification of ionic polymer actuator systems driven at a specific frequency. Identification results using a 5th-degree Volterra expansion show that the nonlinear distortion can be accurately modeled. Using such a high power in the series expansion is necessary to capture the most dominant harmonics, as evidenced when examining the power spectral density of the response. An investigation of how nonlinearities enter into the response is also performed. By analyzing both the actuation current and tip velocity, results show that both the voltage to current and current to velocity stages influence the nonlinear response, but the voltage to current stage is more dominantly nonlinear.
This paper presents the development of the control system architecture for vibration mitigation and autonomous phasing of sub-apertures on a sparse-array test bed. The paper begins with a brief description of the telescope system under consideration, including the actuation system providing 3 degree-of-freedom rigid body correction to each sub-aperture, and the metrology system, comprised of a white-light-based low-bandwidth absolute position sensing system and a high-bandwidth, laser-based relative position sensing system. The control problem posed by the telescope is described, including a discussion of the performance requirements the control system must meet, which include asymptotic set point tracking, broadband and tonal disturbance rejection, and tracking of non-stationary objectives. The use of system identification techniques in development of an accurate model of the input-output dynamics of the system is presented. The overall control system architecture including discussions on aspects such as tolerance of sensor dropouts, and the design of these control systems based on the identified model is presented. The paper presents the results of the application of this control system approach to the experimental system, demonstrating performance of the controlled system.
This paper describes a novel real-time algorithm for optically phasing sub-apertures of a sparse-array telescope system based on recursive estimation of the sub-aperture placement that maximizes a fringe-contrast metric. The sub-apertures are phased in pairs using broad spectral band flood-illumination of the sparse-array, while blocking reflections from all but two sub-apertures. The resulting Young’s geometry at the pupil produces an interference pattern that is characterized to determine spatial-frequency filters that are utilized to generate a contrast metric from the fringe patterns. This contrast metric is shown to generate a near-Gaussian variation as a function of optical path-length difference (OPD), with the maximum contrast occurring at zero OPD. The functional relationship between fringe contrast and sub-aperture position based on a common-path, laser-based relative-piston measurement system is developed into an estimator for maximization of fringe contrast (and therefore phasing of sub-apertures). The recursive algorithm produces real-time estimates of the zero OPD value of the relative position that improves as additional data is acquired.
This paper presents an overview of the development and capabilities of a space-traceable testbed developed for investigation of research issues related to deployable space telescopes. The Air Force Research Laboratory (AFRL) is developing the Deployable Optical Telescope (DOT), which upon completion will be a fully-deployable, sub-scale, space-traceable ground testbed for development and demonstration of critical technologies for the next-generation of space-optics systems. The paper begins with an overview of the DOT project’s technology goals, including the specific performance objectives of the various technologies that are being incorporated into the DOT testbed. The paper presents an overview of the DOT design, including the central integrating structure, deployable primary mirror petals, deployable secondary tower, deployment mechanisms, lightweight mirror segments, metrology, and control systems. The paper concludes with a report on the current status of DOT activities as well as a view of the future research that is planned for the project.
The deployable optical telescope, the second project of the Air Force Research Laboratory's Integrated Ground Demonstration Laboratory, will demonstrate critical integration technologies associated with the next generation of beam expanders for space-based laser systems and large apertures for tactical surveillance systems. AFRL's development will be carried in cooperation with the contractor community and have direct ties to the future program offices that will utilize the DOT technologies. A flow down of total wavefront error acceptable for future operational systems has been used to derive DOT experiment requirements. The sub-scale DOT will demonstrate the initial deployment of a segmented primary and secondary tower in a 1-g laboratory environment.
This paper presents experimental results relating to the Air Force Research Laboratory Precision Deployable Optics System (PDOS) ground demonstration. The PDOS experiment represents a sub-scale experimental test-bed for the demonstration of science and technology related to a large-aperture deployable space-based telescope systems. A description of the experimental test-bed is included. A description of microdynamic phenomena, referred to as `events' or `microlurches', observed during the test phase of the ground demonstration is presented. The performance of a three input, three output, high bandwidth structural controller operating in the presence of these events is presented and compared to the performance of the uncontrolled system.
KEYWORDS: Control systems, Vibration isolation, Optical isolators, Fourier transforms, Complex systems, Active isolation, Signal attenuation, Space operations, Prototyping, Actuators
Payloads are attached to launch vehicles by structural adapters known as Payload Adapter Fittings (PAF). The adapters are substantially rigid; therefore, the launch vehicle vibration is transmitted without isolation to the payloads. The payload cost includes a substantial component for provision to withstand the severe launch vibration environment. In this paper, a novel three degree-of-freedom isolation system built in the footprint and within the existing PAFs is presented. The vibration isolator consists of a mechanical constraining mechanism that prevents the rotation of the payload relative to the launch vehicle in both rocking rotation and rotation about the long axis of the launch vehicle. The above rotational motion restraining mechanism would therefore only allow axial and lateral vibration of the payload relative to the launch vehicle. The second major component of the system consists of isolation components that are used to reduce vibration in the latter to directions, i.e., in the axial and the lateral directions. The proposed isolating payload adapter fitting also has a stand-alone thrust-support preload adjustment component that adjusts for the varying quasi-static acceleration seen in different flight regimes in a passive- adaptive control mode. The advocated isolation system is structurally modular and parameterizable such that a range of launch vehicle platforms could be accommodated. Through the utilization of the advocated isolating payload adapter fitting, a fifty percent reduction in vibration transmission is achieved at 10 Hz in the axial direction and 20 Hz in the lateral directions. A 40 db reduction is achieved at frequencies above 100 Hz. Active actuating elements may easily be included in the prosed design to attain an active isolation unit.
This paper presents a review of the Air Force Research Laboratory advanced controls technology experiment program. Representing the first space-demonstration of smart structures technology, the ACTEX-I program has met or exceeded all program goals at each stage, beginning with the program initiation in 1991 through launch in 1996 to the conclusion of the Guest Investigator program and program conclusion in 1999. This paper will provide a summary of the ACTEX-I program from the AFRL perspective, focusing on lessons learned from the program both positive and negative.
KEYWORDS: Satellites, Vibration isolation, Control systems, Digital signal processing, Passive isolation, Active isolation, Actuators, Electronics, Smart structures, Space operations
The Air Force Research Laboratory is currently conducting a number of space flight experiments with the goal of demonstrating and transitioning smart structures technology to the operational user. Three of these experiments have focused on approaches for providing high-performance on- orbit isolation to precision spacecraft payloads. This paper will describe the design and performance of two systems that are slated for a 2000 launch; the vibration isolation, suppression, and steering experiments and the satellite ultra-quiet isolation technology experiment. Additionally, this paper will provide an overview of a third program initiated in 1999, the miniature vibration isolation system.
Eric Anderson, Michael Evert, Roger Glaese, James Goodding, Scott Pendleton, Donald Camp, John Fumo, Marty Jessen, Richard Cobb, R. Scott Erwin, Jonathon Jensen
KEYWORDS: Satellites, Sensors, Actuators, Space operations, Vibration isolation, Digital signal processing, Magnetism, Control systems, Signal processing, Passive isolation
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.
KEYWORDS: Sensors, Control systems, Interferometers, Actuators, Space telescopes, Error analysis, Digital filtering, Active optics, Digital signal processing, Active vibration control
Experimental results are presented for active vibration control of the Air Force Research Laboratory's UltraLITE Precision Deployable Optical Structure (PDOS), a ground based model of a sparse array, large aperture, deployable optical space telescope. The primary vibration suppression technique employs spatio-temporal filtering, in which a small number of sensors are used to produce modal coordinates for the structural modes to be controlled. The spatio-temporal filtering technique is well suited for the control of complex, real-world structures because it requires little model information, automatically adapts to sensor and actuator failures, is computationally efficient, and can be easily configured to account for time-varying system dynamics. While controller development for PDOS continues, the results obtained thus far indicate the need for an integrated optical/structural control system.
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