Robert R. Clappier

A High-Performance Force Cancellation Control System for Linear-Drive Split-Cycle Stirling Cryocoolers

Miniaturized linear-drive Stirling-cycle cryocoolers designed with noncontacting parts are ideal for long-life cryogenic cooling onboard a wide range of spacecraft. These cryocoolers consume little power, have an almost indefinite operational life, and require no expensive ground handling equipment or procedures. A major problem in applying these cryocoolers to sensitive focal-plane instruments is the vibration induced by the reciprocating motion of internal components in both the compressor and displacer. Development of a Stirling cryocooler system optimized for minimal residual vibration has been a major goal at the Lockheed Research & Development Division.

Precision Temperature Control of Stirling-Cycle Cryocoolers

This paper describes the analysis, development, and laboratory testing of a closed loop control system that maintains temperature over a range of 50 to 80 K to +/- 0.01 K. The control loop uses position, velocity, and integrated error information to achieve excellent short-term and long-term temperature stability by modulating the length of the compressor stroke. The resulting control system is compatible with adaptive vibration cancellation techniques. The control algorithm is implemented on a personal computer using a 386 processor, but it is simple enough to run as one of many tasks on a less powerful microprocessor.

Design of a prototype primary mirror segment positioning actuator for the Thirty Meter Telescope

The Thirty Meter Telescope (TMT) is a collaborative project between the California Institute of Technology (CIT), the University of California (UC), the Association of Universities for Research in Astronomy (AURA), and the Association of Canadian Universities for Research in Astronomy (ACURA). In order for the Thirty Meter Telescope (TMT) to achieve the required optical performance, each of its 738 primary mirror segments must be positioned relative to adjacent segments with nanometer-level accuracy. Three in plane degrees of freedom are controlled via a passive Segment Support Assembly which is described in another paper presented at this conference (paper 6273-45). The remaining three out of plane degrees of freedom, tip, tilt, and piston, are controlled via three actuators for each segment. Because of its size and the shear number of actuators, TMT will require an actuator design, departing from that used on the Keck telescopes, its successful predecessor. Sensitivity to wind loads and structural vibrations, the large dynamic range, low operating power, and extremely reliable operation, all achieved at an affordable unit cost, are the most demanding design requirements. This paper describes a concept that successfully meets the TMT requirements, along with analysis and performance predictions. The actuator concept is based on a prototype actuator developed for the California Extremely Large Telescope (CELT) project. It relies on techniques that achieve the required accuracy while providing a substantial amount of vibration attenuation and damping. A development plan consisting of a series of prototype actuators is envisioned to verify cost, reliability, and performance before mass production is initiated. The first prototype (P1) of this development plan is now being built and should complete initial testing by the end of 2nd QTR 06.

Design and preliminary test of precision segment positioning actuator for the California Extremely Large Telescope primary mirror

In order for the California Extremely Large Telescope (CELT) to achieve the required optical performance, each of its 1000 primary mirror segments must be positioned relative to adjacent segments with nanometer-level accuracy. This can be accomplished using three actuators for each segment to actively control the segment in tip, tilt, and piston. The Keck telescopes utilize a segmented primary mirror similar to CELT employing a highly successful actuator design. However, because of its size and the shear number of actuators (3000 vs. 108 for Keck), CELT will require a different design. Sensitivity to wind loads and structural vibrations, the large dynamic range, low operating power, and extremely reliable operation, all achieved at an affordable unit cost, are the most demanding design requirements. This paper examines four actuator concepts and presents a trade-off between them. The concept that best met the CELT requirements is described along with an analysis of its performance. The concept is based on techniques that achieve the required accuracy while providing a substantial amount of vibration attenuation and damping. A prototype actuator has been built to validate this concept. Preliminary tests confirm predicted behavior and future tests will establish a sound baseline for final design and production.

Paper: A microprocessor-based position control system for a telescope secondary mirror

The shuttle infrared telescope facility (SIRTF) is being designed as a 0.85-m cryogenically cooled i.r. telescope to be flown as a shuttle-attached payload in the late-1980s. Pointing requirements for SIRTF dictate image stability of 0.25 arcsec. In addition, enhancement of weak source signal-to-noise ratio is accomplished by chopping the incoming beam. The articulated secondary mirror in SIRTFs Cassegrain optical train provides image-motion compensation, in order to achieve the desired stability, as well as simultaneously chopping the beam. This paper describes a unique, microprocessor-based position control system developed to control the SIRTF secondary mirror. The system utilizes a special control law to minimize energy dissipation, a precision capacitive position sensor, and a specially designed power-amplifier/actuator combination to achieve the desired performance. The microprocessor generates the commanded angular position and rate waveforms to maintain a 90%-dwell-time-to-10%-transition-time ratio independent of selected chop frequency or amplitude. Additionally, the microprocessor supervises system start-up and shutdown to eliminate unnecessary transients to the sensor and actuator, and provides for control-system gain scheduling to compensate for nonlinearities as a function of frequency and amplitude. Performance and test results of a prototype system designed for use with a demonstration model of SIRTFs focal plane fine-guidance sensor are presented.

Development and Microdynamics Characterization of a Deployable Petal Assembly at Full Scale

As part of its risk mitigation efforts related to large, future space-based deployable optics such as NGST, Lockheed Martin developed, implemented, and evolved a full-scale, lightweight, deployable petal structure and associated deployment mechanisms for cryogenic and microdynamic stability testing. The test-bed features a single petal assembly for an 8-meter diameter telescope, including a flight-like mirror support structure and full-size hinges and latches. The work completed on this test-bed include: 1) Characterization of the dynamics and microdynamics response of the full-scale petal and its hinge/latch interface to low-level vibration sources down to 0.1 nanometer, 2) Evaluation of petal deployment repeatability, 3) Evaluation of the performance of simple passive damping strategies for petal vibration control at cryogenic temperatures. In all respects, including microdynamics, deployment repeatability and stability, the hardware demonstrated performance well in excess of the NGST requirements. In this paper, we summarize the development and the results of the performance testing completed during the NGST Phase I formulation, including testing of hysteresis and deployment repeatability at room temperature.

Cryogenic tunable near-infrared Fabry Perot interferometer for Next Generation Space Telescope

An engineering test unit Fabry-Perot interferometer has been designed and built to operate in the 1.5-1.7 um regime from room temperature to 30 K°. The Fabry-Perot interferometer is tuned by controlling the gap spacing between the two highly reflecting mirrors. Capacitance sensors are used to control the gap spacing and maintain parallelism of the mirrors. An overview of the optical, mechanical, electrical, and control designs of the instrument are described. Some early results at cryogenic temperature indicative of the performance of the instruments are presented.