A three degree of freedom alignment stage has been designed and built for use in a cryogenic dilatometer that is used to measure thermal strains. The alignment stage enables precise adjustments of the positions and orientations of optical components to be used in the measurements and, once adjustments have been completed, keeps the components precisely aligned during cryogenic dilatometer operations that can last as long as several days.
The alignment stage (see figure) includes a case, a circular tilt/tip platform, and a variety of flexural couplings between the case and the platform, all machined from a single block of the low thermal expansion iron/nickel alloy Invar, in order to minimize effects of temperature gradients and to obtain couplings that are free of stiction and friction. There are three sets of flexural couplings clocked at equal angles of 120° around the platform, constituting a three-point kinematic support system.
Associated with the three sets of flexural couplings are three sets of two actuators each, also clocked at equal angles, which serve to deflect the couplings to adjust the platform in tilt, tip, and/or piston.
By use of actuator/flexural coupling sets that include tangent bar flexures and Invar screws and nuts, one can make coarse adjustments of axial displacement over a range of 1.79 mm with a resolution of 1 mm (corresponding to a tilt/tip angle range of 23.5 milliradians with resolution of about 13 microradians or perhaps somewhat less).
Fine adjustments are made by use of piezoelectric actuators in combination with three different types of flexures that apply the proper axial preload and transmit the piezoelectric displacements to the platform while preventing the coupling of shear and bending loads, which could damage the piezoelectric actuators. The fine adjustments are characterized by axial displacement over a range of 15 µm with a resolution of 10 pm (corresponding to a tilt/tip angle range of 222.85 microradians with a resolution of about 0.15 nanoradian or perhaps somewhat less). The piezoelectric actuators are driven by circuits that are parts of a computer-based feedback tilt/tip/piston control system.
By virtue of the low thermal expansion of the monolithic Invar body and the negative thermal expansion of the piezoelectric actuators, the alignment stage is athermalized to within about 7 picometers of axial displacement and 0.1 nanoradian of tip and/or tilt in the presence of an axial temperature gradient of 0.1 K across its structure. Inasmuch as all adjustments are symmetric about the center and are kinematic, any tip or tilt adjustment of the stage is made about its center, with minimal cross-coupling.
This work was done by Matthew Dudik and Donald Moore of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Mechanics category.
The software used in this innovation is available for commercial licensing. Please contact Karina Edmonds of the California Institute of Technology at (818) 393-2827. Refer to NPO-40390
This Brief includes a Technical Support Package (TSP).
Alignment Stage for a Cryogenic Dilatometer
(reference NPO-40390) is currently available for download from the TSP library.
Don't have an account?
Overview
The document presents a Technical Support Package for the development of a high precision, ultra-stable 3 degree-of-freedom (3 DOF) alignment stage designed for use in a cryogenic dilatometer at NASA's Jet Propulsion Laboratory (JPL). The primary goal of this alignment stage is to achieve precise tip, tilt, and piston adjustments of optics, which are critical for accurate thermal strain measurements over extended periods.
The alignment stage is engineered to maintain picometer-level piston stability (10 picometers) and nanoradian-level angular stability (0.3 nanoradians) across a piston adjustment range of 1.79 millimeters and an angular adjustment range of 23.5 milliradians. This level of precision is necessary for the dilatometer, which measures the coefficient of thermal expansion of various materials, ensuring that the sample remains aligned with the interferometer throughout the measurement process.
The document outlines the motivation behind the development of this alignment stage, highlighting the inadequacy of existing commercial solutions that fail to meet the required precision and stability. To address this challenge, the design incorporates a fully kinematic, piezoelectric-driven, athermal, monolithic, flexure-coupled mechanism. This design minimizes cross-coupling and enhances position stability while being insensitive to temperature variations.
The construction of the alignment stage features a monolithic invar flexure case, which houses three piezoelectric actuators and alignment screws. The use of invar, a material known for its low thermal expansion properties, is crucial for maintaining the integrity of the flexural couplings. The design also includes tangent bar flexures that facilitate rough adjustments, ensuring that the system can be finely tuned for optimal performance.
Figures included in the document provide visual representations of the alignment stage, showcasing its schematic and physical layout. The development cost for this project is noted to be $20,000, with no additional funding anticipated.
Overall, this document highlights a significant advancement in optomechanical design, offering a solution that meets the stringent requirements for precision in cryogenic applications, thereby contributing to the broader field of aerospace technology and materials testing.
