This paper discusses the current status of self supporting precision membrane optical shell technology (MOST) apertures based on thin (25 to 125 um thick) polyimide and polyester films primary shell. Optically relevant doubly curved reflective apertures are realized by inducing permanent curvature into thin substrates that can then be coated. The initial thin nature provides both very low areal density (20 to 200 grams/m2) and compatibility with compact roll stowage. The induced curvature/depth provides the ability to support the shell around the periphery at discrete locations and considerable structural and dynamic stiffness. The discrete mounts also provide an excellent location with which to improve the surface figure and to reject environmental and host structure induced errors. Material microroughness on the leading substrate/coating combination has been measured to down to 3 nm rms over small (100x100um's) sample sizes with white light interferometry. A variety of reflective coated substrates have also been shown to have sub micron rms surface roughness over up to 100mm diameter test apertures using interferometric measurements. Best materials currently have 20nm rms surface roughness noise floors at these sizes. The ability to fabricate shells over a range of prescriptions (R/0.9 to R/2.2) and a range of sizes (0.1 to 0.75m diameter) has been demonstrated. Global surface figure accuracies of 2 to 4 microns rms have been demonstrated at the 0.2m size, and further improvements are anticipated through ongoing improved fabrication techniques (preliminary results indicate sub-micron rms values). The ability of discrete boundary control to improve the shape and maintain it in the face of disturbances (gravity for example) is demonstrated as is the ability to implement high amplitude (multi-wave) Zernike mode surface figure control. Results extending boundary control to interferometric optical level are also presented.
Increasingly, scientific and military missions require the use of space-based optical systems. For example, new capabilities are required for imaging terrestrial like planets, for surveillance, and for directed energy applications. Given the difficulties in producing and launching large optics, it is doubtful that refinements of conventional technology will meet future needs, particularly in a cost-effective manner. To meet this need, recent research has been investigating the feasibility of a new class of ultra-lightweight think-skin optical elements that combine recent advances in lightweight thermally formed materials, active materials, and novel sensing and control architectures. If successful, the approach may lead to an order of magnitude reduction in space optics areal density, improved large scale manufacturing capability, and dramatic reductions in manufacturing and launch costs. In a recent effort, a one meter thin-film mirror like structure was fabricated. This paper provides an overview of tools used to model and simulate this structure as well as results from structural dynamic testing. In addition, progress in the area of non-contact global shape control using smart materials is presented.
KEYWORDS: Actuators, Control systems, Human-machine interfaces, Synthetic aperture radar, Computer programming, Signal processing, Motion controllers, Sensors, Digital signal processing, Sensing systems
A hexapod capable of precision positioning is described. The differences between serial and parallel motion control are presented, and the potential advantages of parallel systems realized as hexapods are highlighted. Actuation options for positioning hexapods are considered in light of a requirement for a high ratio of range to resolution and a need for zero power hold. For positioning of smaller payloads, piezoelectric-based step-and-repeat actuation becomes attractive. The merits of existing and new piezoelectric step-and-repeat actuators are evaluated. A point-and-hold hexapod designated PH1, and its performance, is described, along with several areas identified for possible design improvement. This motivates the development of advanced struts using similar actuation technology. Test results are presented, and a new hexapod, the PH2, is described. This system includes encoder-based feedback control of leg lengths, and a complete software-based user interface and control system. Hexapod test results and performance measurements are presented, and planned future enhancements are described.
Focused research in the area of Multi-Particle Impact Damping (MPID) has resulted in new methods of characterization and prediction. An analytical method has been developed, based on the particle dynamics method, that uses characterized particle damping data to predict damping in structural systems. A methodology to design particle damping for dynamic structures will be discussed. The complete design methodology has been validated in proof-of-methodology testing on a structural component in the laboratory.
KEYWORDS: Actuators, Cryocoolers, Digital signal processing, Control systems, Signal processing, Electronics, Semiconducting wafers, Electronic filtering, Sensors, Amplifiers
Cryocoolers are well known sources of harmonic disturbance forces. In this paper two miniaturized, add-on, vacuum compatible, active vibration control systems for cryocoolers are discussed. The first, called VIS6, is an active/passive isolation hexapod and has control authority in all six degrees of freedom. This capability is desirable when reduction of all cryocooler disturbance loads, including the radial loads, is required. Each of the six identical hexapod struts consists of a miniature moving coil electromagnetic proof mass actuator, custom piezoelectric wafer load cell, viscoelastic passive isolation stage, and axial end flexures. The first five disturbance tones are reduced over a bandwidth of 250 Hz using a filtered-x least mean square algorithm. Load reductions of 30 - 40 dB were measured both axially and radially. The second system, called VRS1, is a pure active control system designed to reduce axial expander head disturbance loads. It works on the basis of a counter-force developed from an electromagnetic proof mass actuator. Error signals are provided from a commercial accelerometer to a standalone digital signal processor, on which a filtered-x least means square control algorithm is implemented. Over the 500 Hz control bandwidth, the 11 disturbance tones were reduced on between 14 to 40 dB.
In this paper, recent results of ongoing studies into the effectiveness and predictability of particle damping are discussed. Efforts have concentrated on characterizing and predicting the behavior of a wide range of potential particle materials, shapes, and sizes in the laboratory environment, as well as at elevated temperature. Methodologies used to generate data and extract the characteristics of the nonlinear damping phenomena are illustrated with interesting test results. Experimental results are compared to predictions from analytical simulations performed with an explicit code, based on the particle dynamics method, that has been developed in support of this work.
The need to be able to predict the self induced thermal heat rise of piezoelectric and electrostrictive stack actuators under AC dynamic operation motivated the research presented in this paper. First, an equation for the electrical admittance of an stack actuator that explicitly includes the effects of having the stack actuator in a host structure is provided. This equation is then shown to be critical when determining the apparent, reactive and dissipative power used by an actuator. With the theoretical predictions of the electrical admittance available, it is possible to calculate the contributions of the individual loss components to the total dissipative power. Using a simple heat transfer analysis, the internal heat rise of the actuator is predicted given the dissipative power input. A case study is used to illustrate how to apply the developed theories. This research provides a first step toward the ability to predict the temperature state of active materials in stack configurations. This would allow accurate prediction of actuator parameters and hence electro-dynamic behavior. Additionally, knowledge of the physics behind self induced heat rise can be used to avoid exceeding the active materials Curie temperature during operation and allow proper design of active isolation elements for temperature sensitive equipment.
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