Mechanism interface mechanics play an important role in the static and dynamic dimensional stability of deployable optical instruments. Friction mechanics in deployment mechanisms has been found to be a source of kinematic indeterminacy allowing elastic energy to be stored throughout the structure. At submicron scales, microslip mechanics allow this behavior to persist well below the classical Coulomb friction limit. This paper presents the design of a cryogenic tribometer for measuring this behavior in candidate mechanism interfaces in both room temperature and cryogenic environments. Room temperature results are presented and compared to a proposed generalized microslip model form. This model form is intended to allow the parametric characterization of microslip behavior caused by smooth nonconforming contact as well as roughness-induced microslip. Spherical ball-on-flat interface geometries were used with two unlubricated material combinations: 440C stainless steel ball on a 440C stainless steel flat and a silicon nitride ball on a 440C stainless steel flat. Consistent parameters were identified for the generalized microslip model from steady cyclic shear responses for both of these interface cases. While these parameters exhibited a measurable sensitivity to normal preload levels, the model form appears to provide the necessary level of robustness. Non-ideal transient shear phenomena including rate dependence were also observed but should play only a secondary role in future modeling efforts.
This paper examines requirements trades involving areal density for large space telescope mirrors. A segmented mirror architecture is used to define a quantitative example that leads to relevant insight about the trades. In this architecture, the mirror consists of segments of non-structural optical elements held in place by a structural truss that rests behind the segments. An analysis is presented of the driving design requirements for typical on-orbit loads and ground-test loads. It is shown that the driving on-orbit load would be the resonance of the lowest mode of the mirror by a reaction wheel static unbalance. The driving ground-test load would be dynamics due to ground-induced random vibration. Two general conclusions are derived from these results. First, the areal density that can be allocated to the segments depends on the depth allocated to the structure. More depth in the structure allows the allocation of more mass to the segments. This, however, leads to large structural depth that might be a significant development challenge. Second, the requirement for ground-test-ability results in an order of magnitude or more depth in the structure than is required by the on-orbit loads. This leads to the proposition that avoiding ground test as a driving requirement should be a fundamental technology on par with the provision of deployable depth. Both are important structural challenges for these future systems.
The use of an elastic memory composite member (EMC) as the active element in deployable optical instruments has tremendous potential. Elastic memory composite mechanisms can remove the need for mechanical latches and remove the post deployed microdynamic instabilities associated with them while providing a low shock, controlled deployment. Additionally, elastic memory composite mechanisms are lightweight, simple, and have a very low coefficient of thermal expansion, which are also desirable properties for deployable optical systems. This paper describes an effort that has been done to explore this possibility. A mechanical latching actuator in an existing precision deployable optical testbed was replaced by an EMC self-locking actuator. Feasibility was assessed through a detailed design and fabrication exercise followed by experimental evaluation of a prototype actuator system in the ground-based deployable optics testbed.
A number of future space based science instruments key to NASA's Origins program require exceptionally large and precise support structures. The scale of these structures and stringency of their dimensional stability will present a number of challenges in the ground verification testing stage of their development and deployment. This paper will discuss a number of the unique challenges involved in developing validation procedures for these structures. It will also describe a novel approach to the development and validation of nonlinear component models of the structural mechanics. This "Component in the Loop" approach offers the ability to directly measure the in situ coupled behavior of a structural component as part of the ex situ component testing process. This testing methodology would allow the coupled system level response of the larger structure to be assessed without the need for assuming particular nonlinear component model forms. The proposed method is not limited to conducting virtual system tests. Feedback functions can be specifically designed to maximize the sensitivity of the output with respect to uncertain parameter(s). Maximum sensitivity is desired to accurately characterize the parameter in question, which is fundamental in model updating procedures.
Decisions about structural architecture for future large space observatories will influence how overall optical stability scales with observatory size. This is examined using basic structural design analyses that relate overall stability requirements to telescope structural modal frequency and damping ratio. In this way, the influence of certain system level architectural choices on the performance can be assessed. In particular, trades between structural depth and optical correction requirements is examined, and compared against other design parameters such as the material specific modulus. For representative configurations and loads, the required optical correction increases with dimension to the fourth power, but reduces with the square of the structural depth and in proportion to the material specific modulus; areal density has no direct affect. This means that, unless the structural architecture improves with dimension, the optical error produced in a 6-meter telescope might increase by a factor of 123:1 for a 20-meter telescope and 77000:1 for a 100-meter telescope. If the structural depth, however, increases in proportion to telescope dimension, these requirements can be reduced by two orders of magnitude. Architectural options for achieving these benefits are discussed, with particular emphasis on considerations of the deployment or assembly scheme.
This paper reports an experiment that demonstrated the functionality of a commercial heterodyne interferometer with an oblique-angle fold mirror in the laser feed path. The key issue investigated is whether the oblique angle fold mirrors introduce unacceptable levels of polarization mixing. The manufacture's specifications recommend that only orthogonal fold mirrors be used to control polarization mixing. However, this constraint limits the use of the interferometer for applications such as the measurement of structural dynamics. In the experiment, an orthogonally-fed interferometer was compared to an oblique angle interferometer at roughly 45°. The result indicated a linear relationship between the two measurements to within 0.5 percent. As a result, no evidence of the λ/2 wavelength sinusoidal bias characteristic of polarization mixing errors was found. A 9 percent error did however exist between the intended and observed scaling between the two measurements. While hardware alignment errors are a likely cause of this disagreement, a third axis of measurement is recommended for future investigations.
The objective of ground based microdynamic testing of deployable space structures is to determine and develop a better understanding of the submicron dynamics that exists between moving contact surfaces. Such measurements often use laser and video metric metrology systems. These measurement however, are corrupted by random environmental perturbations of the metrology optics. This paper represents a statistical method for obtaining the margin of perturbations on the optics. The optics used are those for one of the preliminary flight configurations for the Micron Accuracy Deployment Experiments Space Station laboratory. The approach and method used to determine the extent of the effect of the random environmental perturbations on the measurements is explained and results are obtained. The results indicate that for 100 nanometer total allowable error. The optics need to be stable to within 85 nm of displacement and 10 mArcSec of rotation.