The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) has an optical prescription which terminates at two focal plane arrays for each module. The instrument will operate at 37K after experiencing launch loads at 293K. The focal plane array housings (FPAHs), including stray light baffles (SLBs) must accommodate all associated thermal and mechanical stresses. In addition, the stray light baffles must be installed in situ on the previously assembled flight modules. The main purpose of the FPAH SLBs is to effectively attenuate mission limiting stray light on the focal planes. This paper will provide an overview of the NIRCam stray light baffle design, mechanical and optical analysis, hardware implementation and test results.
The Interface Region Imaging Spectrograph (IRIS) is a NASA SMall Explorer (SMEX) mission launched onboard a Pegasus™ booster on June 27, 2013. The spacecraft and instrument were designed and built at the Lockheed Martin Space Systems Company. The primary mission goal is to learn how the solar atmosphere is energized. IRIS will obtain high-resolution UV spectra and images in space (0.4 arcsec) and time (1s), focusing on the chromosphere and transition region of our sun, which is a complex interface region between the photosphere and corona. The IRIS instrument uses a Cassegrain telescope to feed a dual spectrograph and slit-jaw imager, which operate in the 133-141 nm and 278-283 nm wavelengths, respectively. Within the spectrograph there are sixteen optics, each requiring subtle mounting features to meet exacting surface figure and stability requirements. This paper covers the opto-mechanical design for the most challenging optic mounts, which include the Collimator, UV Fold Mirrors, and UV Gratings. Although all mounts are unique in size and shape, the fundamental design remains the same. The mounts are highly kinematic, thermally matched, and independent of friction. Their development will be described in detail, starting with the driving requirements and an explanation of the underlying design philosophy.
The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) has an optical
prescription which employs four triplet lens cells. The instrument will operate at 35K after experiencing launch loads at
approximately 295K and the optic mounts must accommodate all associated thermal and mechanical stresses, plus
maintain an exceptional wavefront during operation.
Lockheed Martin Space Systems Company (LMSSC) was tasked to design and qualify the bonded cryogenic lens
assemblies for room temperature launch, cryogenic operation, and thermal survival (25K) environments. The triplet lens
cell designs incorporated coefficient of thermal expansion (CTE) matched bond pad-to-optic interfaces, in concert with
flexures to minimize bond line stress and induced optical distortion. A companion finite element study determined the
bonded system's sensitivity to bond line thickness, adhesive modulus, and adhesive CTE. The design team used those
results to tailor the bond line parameters, minimizing stress transmitted into the optic.
The challenge for the Margin of Safety (MOS) team was to design and execute a test that verified all bond pad/adhesive/
optic substrate combinations had the required safety factor to generate confidence in a very low probability optic bond
failure during the warm launch and cryogenic survival conditions. Because the survival temperature was specified to be
25K, merely dropping the test temperature to verify margin was not possible. A shear/moment loading device was
conceived that simultaneously loaded the test coupons at 25K to verify margin.
This paper covers the design/fab/SEM measurement/thermal conditioning of the MOS test articles, the thermal/structural
analysis, the test apparatus, and the test execution/results.
The mechanical design of any optic mount requires an understanding of the sensitivities of the optical design. The design of the filter optic mounts used on the James Webb Space Telescope - NIRCam filter
wheel assemblies have been designed to support the optics in a manner that does not compromise optical performance, while coping with several environmental conditions. We will review the design of the NIRCam filter optic assemblies and confirm the merits of the approach chosen to mount the optics, considering thermal, vibration and stress effects.
Embedding solid-state ceramic actuators in a bending style deformable mirror presents unique athermalization
challenges when operated at cryogenic temperatures. Approaches to athermally embed actuators in a substrate are
presented in this study. Each approach is rated according to established design criteria: unmatched displacement, range,
compliance ratio, bondline stress, design, and manufacturability. We show the results of our design that allows a large
thermal range of operation for the actuators.
Lockheed Martin Space Systems Company (LMSSC) has performed a feasibility study for bonded cryogenic optical mounts. That investigation represents a combined effort of design, experiments and analysis with the goal to develop and validate a working cryogenic mount system for refractive lens elements. The mount design incorporates thermal expansion matched bond pads and radial flexures to reduce bondline stress and induced optical distortion. Test coupons were constructed from lens and selected mount materials and bonded with candidate adhesives to simulate the design's
bond pads. Thermal cycling of those coupons to 35K demonstrated both the system's survivability and the bond's structural integrity. Finally, a companion finite element study determined the bonded system's sensitivity to bondline thickness, adhesive modulus and adhesive CTE. The design team used those results to tailor the bondline parameters to minimize stress transmitted into the optic.
The Lockheed Martin/Advanced Technology Center (LM/ATC) developed a lightweight, compact, high-load capable and yet high precision latch for use on deployable optical systems such as the Next Generation Space Telescope (NGST). The design allows precise self-centering and control of the stiffness at the latch interface. It also incorporates unique capabilities to evaluate the effects of gravity loads, latch preload level, creep, and very low vibration loads on the dynamics and microdynamics of the deployed instrument.
The stiffness, nonlinearity and hysteresis characteristics of the latch and its catch flexure assembly were thoroughly tested in 6 axes down to the nanometer level at room temperature using the LM/ATC Compliance Measurement Device. The latch is stiff enough to hold an NGST-size mirror segment cantilevered against gravity allowing only small gravity sag when the primary mirror is horizontal, thus enabling end-to-end performance verification in 1-G in that orientation. The latch hysteresis is less than 1.0 nm/N under mechanical loads less than 25 N, which meets the NGST stability requirements with significant margin (20 nm at the tip of the petal in space environment).
Several of these latches were integrated and demonstrated at the petal assembly level on a Single Petal Test-bed and the experimental results obtained on that test-bed are consistent with the component level results described in this report.
We experimentally demonstrated that the latch engagement performance is not affected by exposure to cryogenic temperatures down to 20K, as required for use of the device on cryogenic infrared optical instruments such as NGST.
A structural model of the latch was developed using Finite Element Analysis. Good correlation was obtained between the linear components of the analytical and of the experimental results: the model can therefore reliably be used in future NGST or other mission design efforts.
This paper includes a brief description of the LM/ATC latch hardware and its principle of operation as well as the results of the modeling and the experimental characterization work performed on that hardware in the NGST Phase I formulation.
Adaptive optics correct light wavefront distortion caused by atmospheric turbulence or internal heating of optical components. This distortion often limits performance in ground-based astronomy, space-based earth observation and high energy laser applications. The heart of the adaptive optics system is the deformable mirror. In this study, an electromechanical model of a deformable mirror was developed as a design tool. The model consisted of a continuous, mirrored face sheet driven with multilayered, electrostrictive actuators. A fully coupled constitutive law simulated the nonlinear, electromechanical behavior of the actuators, while finite element computations determined the mirror's mechanical stiffness observed by the array. Static analysis of the mirror/actuator system related different electrical inputs to the array with the deformation of the mirrored surface. The model also examined the nonlinear influence of internal stresses on the active array's electromechanical performance and quantified crosstalk between neighboring elements. The numerical predictions of the static version of the model agreed well with experimental measurements made on an actual mirror system. The model was also used to simulate the systems level performance of a deformable mirror correcting a thermally bloomed laser beam. The nonlinear analysis determined the commanded actuator voltages required for the phase compensation and the resulting wavefront error.
This paper addresses the development of a temperature- dependent constitutive model for relaxor ferroelectrics which is based on the assumption that the material is comprised of an aggregate of micropolar regions having a range of Curie temperatures. The diffuse transition behavior of the material is due to its chemical heterogeneity, and thermodynamic models for the microregions are developed by considering near neighbor interactions for varying cation ratios.. The result in micropolar model can be used to predict the saturation polarization and distribution of regions as a function of temperature. Hysteresis below the freezing point is incorporated through the quantification of energy required to bend and translate domain walls pinned at inclusions inherent to the material. The resulting ODE model quantities the constitutive non linearities and hysteresis exhibited by the materials through a wide range of temperatures and input drive levels. The predictive capabilities of the model are illustrated through a comparison with PMN-PT-BT data collected at temperatures ranging from 263 degrees K to 313 degrees K.
This paper summarizes a mathematical model for characterizing hysteresis in ferroelectric materials. The model is based on the quantification of energy required to bend and translate domain walls and is developed in two steps. In the first, the underlying anhysteretic polarization is quantified through constitutive equations derived using Boltzmann statistics. Three anhysteretic models are considered including the Langevin and Ising spin relations as well as a third formulation which combines attributes of the other two. Hysteresis is then incorporated through the consideration of domain wall motion and the quantification of energy losses due to inherent inclusions or pinning sites within the material. This yields a model analogous to that developed by Jiles and Atherton for ferromagnetic materials. The viability of the model is illustrated through comparison with experimental data from a PMN-PT-BT actuator operating at a temperature within the ferroelectric regime.
This paper presents a static, temperature dependent constitutive model for polycrystalline relaxor ferroelectrics operating near their diffuse transition temperature. The model assumes that the relaxor material consists of superparaelectric, micro polar regions with a diffuse spectrum of Curie temperatures. A simple Ising model with near neighbor ion interaction represents the thermodynamics of the individual micro polar regions. A random-order dispersion of the B-site ions simulates the distribution of phase transitions. The diffuse micro polar region model predicts two important materials parameters, the saturation polarization and the density of the polar regions, as a function of temperature. A macroscopic model was constructed with these parameters to simulate dielectric and polarization response of the aggregate material. The macroscopic model also accounts for interaction between the micro polar regions. Finally, the predictions made by the model are compared with experimental data obtained by other researchers on lead magnesium niobate (PMN) relaxor ferroelectrics.
A dynamics model of an electrostrictive ceramic actuator is integrated with a linear mechanical system. Electrostrictive ceramics have a non-linear displacement response to applied field, which creates harmonic distortion of actuator's output. Traditionally, nonlinear dynamics problems are solved in the time-domain, then Fast Fourier Transforms are used to convert the solution into the more convenient frequency-domain. In this paper, a new approach is introduced separates the nonlinear, actuator from the linear structure. The structure can be represented as an impedance that constrains the actuator, and the actuator problem is solved in the frequency domain directly. The approach significantly simplifies the nonlinear problem. The actuator-mechanical system model is used to predict actuator output and distortion as a function of signal frequency. DC voltage bias is treated as a model parameter than must be tuned to optimized output while minimizing distortion. Power requirements and energy transfer to the attached structure are also examined. The problem of a flextensional underwater sonar transducer with an electrostrictive driver is examined as an example case.
A time-dependent, constitutive model is proposed for electrostrictive, relaxor ferroelectric materials. The model is based on Ising spin theory, and simulates stress, electric field and temperature dependent phase transformations in a ceramic material. The resulting model is consistent with Devonshire's theory for temperature induced phase transformations, however it captures the non- linear saturation response characteristic of ferroelectrics driven by high fields. Electric hysteresis occurs when bifurcations cause the solution state to jump between stable branches. The model shows that these bifurcations depend on electric field, stress and temperature. This bifurcation approach differs significantly from phenomenological models based on phase switching. A 1D version of the constitutive model is used to predict the induced strain and polarization as a non-linear function of applied field for a Lead Magnesium Niobate-Lead Titanate-Barium Titanate ceramic. The results are compared with experiments at various temperatures.
This paper examines the dynamic response of electrostrictive rod actuators. A non-linear constitutive model for electrostrictors is used to obtain periodic solutions for the actuator's displacement and polarization. The method accounts for both inertial forces and a non-homogeneous stress distribution in the device. Results of the analysis predict displacement distortion and power requirements for the actuators as a function of excitation frequency. Resonance behavior of the actuator and the effect of electro-mechanical coupling are investigated. Voltage control parameters for the actuators are studied to determine the optimum control scheme for minimizing distortion of the displacement output.
A non-linear constitutive model for relaxor ferroelectrics developed by Hom and Shankar is examined and verified with electromechanical experiments. This model links polarization and strain to the electric field and stress in an electrostrictive material. A set of tests were performed to study the quasi-static electrical behavior of PMN-PT-BT materials under prestress. Another set of tests investigate the effect of DC electric field on the elastic modulus of the material. The results show excellent correlation between the predicted behavior of the model and the experiments. Failure models for electrostrictive ceramic materials are presented which address the issues of actuator reliability. The constitutive model of Hom and Shankar is incorporated into a nonlinear finite element code. A new finite element technique for computing the J-Integral for cracks in electromechanical materials is developed. This technique is based on the domain integral method and computes both the mechanical and electrical contributions to the energy release rate. The finite element code and the J-Integral computation are used to study crack growth in multilayered electrostrictive ceramic actuators.
Nonlinear, quasi-static finite element calculations are performed for multilayered, electrostrictive, ceramic actuators. Both a stand-alone device and an array of devices embedded in a 1 - 3 composite are studied. The numerical model is based on a fully coupled constitutive law for electrostriction which uses strain and polarization as independent state variables. This law accounts for the stress dependency of ceramic's dielectric behavior and simulates polarization saturation at high electric fields. Two-dimensional plane strain computations are done for a single actuator constructed from Pb(Mg<SUB>1/3</SUB>Nb<SUB>2/3</SUB>)O<SUB>3</SUB>- PbTiO<SUB>3</SUB>-BaTiO<SUB>3</SUB> (PMN-PT-BT). The stress state near an internal electrode tip is computed and a fracture mechanics analysis is performed to assess the device's reliability. The effect of compressive prestress on the actuator's induced strain response is also predicted. In a second problem, a 1 - 3 composite embedded with an array of PMN-PT-BT multilayered actuators is studied with a plane stress version of the finite element technique. A unit cell model is used to compute the surface displacements of the composite.