Alignment Control System Research Articles

The Development of an Integrated Experiment to Study the Controls/Structures Interaction Problem in Large Optical Systems

The next generation of ground and space-based optical systems is characterized by their large size and high performance requirements. They rely heavily on active control to compensate for inadequate mechanical accuracy caused by manufacturing processes, thermal distortions, and gravitational or dynamic loads. Because of the prohibitive weight penalty incurred by fabricating a monolithic reflector with sufficient stiffness, as well as the problem of packaging a large, fragile optical element for transport to orbit, the use of segmented large optical components is the only practical approach for large space systems. This paper describes an experimental apparatus developed at the Lockheed Palo Alto Research Laboratory called the Advanced Structures/Controls Integrated Experiment (ASCIE). The ASCIE is a laboratory test bed designed to support a number of interdisciplinary studies, especially experiments pertaining to the interaction between structural dynamics and control systems in large segmented optical systems. The test article emulates a Cassegrain telescope with a segmented primary mirror. The segments and the secondary mirror are actively controlled to simultaneously provide the correct alignment as well as image stabilization. The segment alignment control system utilizes a self-referenced ensemble of edge sensors, and an independent optical measurement system provides for initial calibration and scoring. A light-weight truss structure supports both the secondary and primary mirror elements. The whole assembly is tuned to replicate the dynamic behavior of a large system. Details of the segment alignment control system, as well as preliminary experimental results are presented.

Performance analysis of the segment alignment control system for the ten-meter telescope

The W. M. Keck Observatory Ten-Meter Telescope (TMT) will be a high-performance astronomical telescope capable of unprecedented ground-based observations. One of the unique features of the TMT is its primary mirror which is composed of 36 hexagonal segments. A segment alignment control system will be used to achieve the optical quality of a glass monolith in the segmented primary mirror. The problems associated with the accurate pointing and imaging of a large flexible telescope such as the TMT are quite different from those posed by traditional rigid telescopes. These problems arise from two main sources: structural dynamic effects, such as enhanced sensitivity to external disturbances, and interactions between the segment alignment control system and the telescope structure. This study consists of a combined structural and control system analysis based upon mathematical models of the TMT structure, the segment alignment control system, and the aerodynamic loads on the primary mirror that are induced by the wind. Data for the structural model were derived from the most recent design iteration available during the time frame of the study. Actual wind velocities measured at three observatory sites were used to develop a generic model of the power spectrum of forces generated by wind loads. Information about the r.m.s. mirror figure errors and limits on the bandwidth of the control system as it is currently implemented are presented.

Performance Analysis of the Segment Alignment Control System for the Ten Meter Telescope

The W. M. Keck Observatory Ten Meter Telescope (TMT) will be a high peformance astronomical telescope capable of unprecedented ground-based observations. One of the unique freatures of the TMT is its primary mirror which is composed of 36 hexagonal segments. A segment alignment control system will be used to achieve the optical quality of a glass monolith in the segmented primary mirror. The problems associated with the accurate pointing and imaging of a large flexible telescope such as the TMT are quite different from those posed by traditional rigid telescopes. These problems arise from two main sources: structural dynamic effects, such as enhanced sensitivity to external disturbances, and interactions between the segment alignment control system and the telescope structure. This study consists of a combined structural and control system analysis based upon mathematical models of the TMT structure, the segment alignment control system, and the aerodynamic loads on the primary mirror that are induced by the wind. Data for the structural model was derived from the most recent design iteration available during the time frame of the study. Actual wind velocities measured at three observatory sites were used to develop a generic model of the power spectrum of forces generated by wind loads. Information about the rms mirror figure errors and limits on the bandwidth of the control system as it is currently implemented are presented.

Performance Analysis of the Segment Alignment Control System for the Ten Meter Telescope

The W. M. Keck Observatory Ten Meter Telescope (TMT) will be a high peformance astronomical telescope capable of unprecedented ground-based observations. One of the unique freatures of the TMT is its primary mirror which is composed of 36 hexagonal segments. A segment alignment control system will be used to achieve the optical quality of a glass monolith in the segmented primary mirror. The problems associated with the accurate pointing and imaging of a large flexible telescope such as the TMT are quite different from those posed by traditional rigid telescopes. These problems arise from two main sources: structural dynamic effects, such as enhanced sensitivity to external disturbances, and interactions between the segment alignment control system and the telescope structure. This study consists of a combined structural and control system analysis based upon mathematical models of the TMT structure, the segment alignment control system, and the aerodynamic loads on the primary mirror that are induced by the wind. Data for the structural model was derived from the most recent design iteration available during the time frame of the study. Actual wind velocities measured at three observatory sites were used to develop a generic model of the power spectrum of forces generated by wind loads. Information about the rms mirror figure errors and limits on the bandwidth of the control system as it is currently implemented are presented

PAC-MAN, A Precision Alignment Control System for Multiple Laser Beams Self-Adaptive through the Use of Noise

A control system is described that aligns the 96 beams of the Aurora KrF laser system to within a pointing accuracy of 5 {mu}rad within 5 min and maintains the alignment in real time. This performance is made possible through a novel use of random noise. The 96 beams, together with optical benchmarks, are imaged on a single television (TV) camera. The pointing angles of those beams are estimated from the arithmetic means of the pixel coordinates within the beam images. The pixel intensities of each TV frame are mapped into a binary decision array based on whether or not the pixel intensity is above or below a threshold criterion. Existing, or introduced, random noise in the TV signal causes the contents of this array to vary from frame to frame, even when the actual beam is stationary. The beam positions are estimated from the pixel coordinates and their associated elements within this array. Finally, the beam angle estimates are updated from these position estimates, each TV frame, in combination with all previous estimates.

An electron beam image projection system with automatic wafer handling

A new electron beam image projection system has been developed for 0.5 μm lithography. The system features three newly-designed subsystems: a double Helmholtz-type superconducting magnet, an automatic wafer handling system and a mask-wafer alignment and parallelism control system. The system also has a high accelerating voltage operation capability. Practical resolution of 0.5 μm, wafer handling time of 120 seconds and alignment mark detection accuracy better than 0.03 μm in 0.1 seconds were attained.

Whg/Store Flutter with Nonlinear Pylon Stiffness

Recent wind tunnel tests and analytical studies show that a store mounted on a pylon with 'soft' pitch stiffness provides substantial increase in flutter speed of fighter aircraft and reduces dependency of flutter on mass and inertia of the store. This concept, termed the decoupler pylon, utilizes a low-frequency control system to maintain pitch alignment of the store during maneuvers and changing flight conditions. Under rapidly changing transient loads, however, the alignment control system may allow the store to momentarily bottom against a relatively stiff backup structure in which case the pylon stiffness acts as a hardening nonlinear spring. Such structural nonlinearities are known to affect not only the flutter speed but also the basic behavior of the instability. This paper examines the influence of pylon stiffness nonlinearities on the flutter characteristics of wing-mounted external stores.

Control of Microinch Displacements

An inexpensive servo-motor-driven hinge-arm actuator device has been realized with the capability of producing linear displacements on the order of 0.5 μin. The device is applicable to precision control of machine tools and alignment of optical systems. Transfer-function and calibration data for the actuator are given. A closed-loop alignment control system incorporating the actuator and linear differential transformer (LVDT) sensors has a precision of 0.05" with a dynamic range of ±3.5" and a response time of 2 s.

Laser Alignment Techniques in Tunneling

With the advent of the mechanical mining machine or mole, the engineer's role has become one of providing a constant course of direction for the machine rather than establishing and projecting the tunneled path traversed by it. The engineer's primary tool in accomplishing this task is an alignment control system capable, not only of providing guidance, but of detecting and distinguishing the component of motion generated by the mining machine. Consisting of a laser beam instrument, auxiliary control points along the tunnel, and a target system affixed to the machine, such a system provides a graphical guide to the machine operator. The mechanics of setting up such a system are outlined and various types of target systems are categorized. Techniques for adapting such a system to various alignment problems are proposed.