Alignment Stage for a Cryogenic Dilatometer

A low-friction, low-thermal-expansion kinematic design affords stability and precise adjustability.

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