A mechanical amplifier has been devised to multiply the stroke of a piezoelectric transducer (PZT) intended for use at liquid helium temperatures. Interferometry holds the key to high angular resolution imaging and astrometry in space. Future space missions that will detect planets around other solar systems and perform detailed studies of the evolution of stars and galaxies will use new interferometers that observe at mid- and far-infrared wavelengths. Phase-measurement interferometry is key to many aspects of astronomical interferometry, and PZTs are ideal modulators for most methods of phase measurement, but primarily at visible wavelengths. At far infrared wavelengths of 150 to 300 µm, background noise is a severe problem and all optics must be cooled to about 4 K. Under these conditions, piezos are ill-suited as modulators, because their throw is reduced by as much as a factor of 2, and even a wavelength or two of modulation is beyond their capability. The largest commercially available piezo stacks are about 5 in. (12.7 cm) long and have a throw of about 180 µm at room temperature and only 90 μm at 4 K. It would seem difficult or impossible to use PZTs for phase measurements in the far infrared were it not for the new mechanical amplifier that was designed and built.

A Four-Bar Linkage provides stroke amplification and momentum compensation for the PZT mounted inside it.
To compensate for the loss of travel at cryogenic temperatures, the PZT is mounted in a novel mechanical amplifier that supports one of the mirrors of the interferometer. The mechanical amplifier, shown in the figure, was designed based on an original concept at JPL dating from 1993. The mechanical amplifier resembles an elongated parallelogram with pairs of parallel flexures along each side. The PZT is compressed along the axis of the long diagonal of the parallelogram by support flexures at each end. The expansion of the PZT along the long diagonal causes the ends of the short diagonal to move towards each with a motion amplified by a factor of 3 or 4. The parallel flexures are used to eliminate unwanted twisting and vibration modes such that a small mirror will not tilt when translated by the amplifier. The support flexures that hold the PZT allow a symmetrical expansion of the piezo within the amplifier. The amplifier is designed to be completely symmetric and balanced such that inertia forces are nulled. This provides mechanical stability that allows rapid (100-Hz) sampling without inducing vibrations. Optical interferometers normally obtain the mechanical stability and momentum compensation by using an additional piezo stack mounted back-to-back with the first piezo so that the second one has motions that are equal but opposite in direction. By mounting the stack symmetrically with the support flexures the stack expands equally about its center, does not induce vibrations, and does not require momentum compensation.

This new mechanical amplifier provides both a longer stroke for standard piezo stacks and the necessary mechanical stability through an ingenious mounting arrangement. The device is made of titanium and machined using a wire EDM (electrical-discharge machining) process so as to be as strong and lightweight as possible. It is compact using only a single piezo stack, making it ideally suited for phase-measurement in a cryogenic environment.

This work was done by James Moore, Mark Swain, Peter Lawson, and Robert Calvet 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. NPO-30289.

Topics:
Amplifiers Data management Electronic equipment Exteriors Imaging and visualization Mechanical Components Mechanics Metals Mirrors Noise Optics Sun and solar Technical review Titanium Vibration
This Brief includes a Technical Support Package (TSP).
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Mechanical Amplifier for a Piezoelectric Transducer

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NASA Tech Briefs Magazine

This article first appeared in the February, 2003 issue of NASA Tech Briefs Magazine (Vol. 27 No. 2).

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Overview

The document is a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing a novel mechanical amplifier designed for piezoelectric transducers (PZTs) intended for use in cryogenic environments, specifically at liquid helium temperatures. The invention addresses significant challenges faced in astronomical interferometry, particularly in high angular resolution imaging and astrometry, which are crucial for future space missions aimed at detecting exoplanets and studying stellar evolution.

PZTs are typically effective modulators for phase measurement at visible wavelengths; however, their performance diminishes at far-infrared wavelengths (150 to 300 μm) due to increased background noise and reduced stroke capability at low temperatures. The document outlines the limitations of conventional solutions, which often involve using large piezo stacks that complicate the design and introduce unwanted vibrations into optical systems.

To overcome these challenges, the inventors—Robert J. Calvet, Peter R. Lawson, James D. Moore, and Mark R. Swain—developed a mechanical linkage system that amplifies the stroke of the piezo actuator. This innovative design allows for a path modulation of approximately 100 microns while ensuring momentum compensation, thus preventing vibrations in the optical delay line. The mechanical amplifier employs a four-bar linkage configuration, which effectively recovers the losses in throw associated with temperature drops.

The piezo actuator is strategically mounted within the linkage, allowing it to expand from its midpoint outward, thereby eliminating the need for fixed mounting points that could induce momentum issues. The device is constructed from titanium to ensure strength and durability, making it suitable for the demanding conditions of space applications.

The document emphasizes the novelty of this mechanical arrangement, asserting that it represents a significant improvement over prior art and existing commercial components, which are inadequate for the intended applications. The work was conducted under NASA contract NAS 7-918, and the findings are intended to support future advancements in space exploration technologies.

In summary, this technical support package presents a significant advancement in the use of piezoelectric transducers for space applications, providing a solution that enhances performance while addressing the unique challenges posed by cryogenic environments and far-infrared observations.