This paper presents the results of numerous design studies carried out at Perkin-Elmer in support of the design of large diameter controllable mirrors for use in laser beam control, surveillance, and astronomy programs. The results include relationships between actuator location and spacing and the associated degree of correctability attainable for a variety of faceplate configurations subjected to typical disturbance environments. Normalizations and design curves obtained from closed-form equations based on thin shallow shell theory and computer based finite-element analyses are presented for use in preliminary design estimates of actuator count, faceplate structural properties, system performance prediction and weight assessments. The results of the analyses were obtained from a very wide range of mirror configurations, including both continuous and segmented mirror geometries. Typically, the designs consisted of a thin facesheet controlled by point force actuators which in turn were mounted on a structurally efficient base panel, or "reaction structure". The faceplate materials considered were fused silica, ULE fused silica, Zerodur, aluminum and beryllium. Thin solid faceplates as well as rib-reinforced cross-sections were treated, with a wide variation in thickness and/or rib patterns. The magnitude and spatial frequency distribution of the residual or uncorrected errors were related to the input error functions for mirrors of many different diameters and focal ratios. The error functions include simple sphere-to-sphere corrections, "parabolization" of spheres, and higher spatial frequency input error maps ranging from 0.5 to 7.5 cycles per diameter. The parameter which dominates all of the results obtained to date, is a structural descriptor of thin shell behavior called the characteristic length. This parameter is a function of the shell's radius of curvature, thickness, and Poisson's ratio of the material used. The value of this constant, in itself, describes the extent to which the deflection under a point force is localized by the shell's curvature. The deflection shape is typically a near-gaussian "bump" with a zero-crossing at a local radius of approximately 3.5 characteristic lengths. The amplitude is a function of the shells elastic modulus, radius, and thickness, and is linearly proportional to the applied force. This basic shell behavior is well-treated in an excellent set of papers by Eric Reissner entitled "Stresses and Small Displacements of Shallow Spherical Shells".1'2 Building on the insight offered by these papers, we developed our design tools around two derived parameters, the ratio of the mirror's diameter to its characteristic length (D/l), and the ratio of the actuator spacing to the characteristic length (b/l). The D/1 ratio determines the "finiteness" of the shell, or its dependence on edge boundary conditions. For D/1 values greater than 10, the influence of edges is almost totally absent on interior behavior. The b/1 ratio, the basis of all our normalizations is the most universal term in the description of correctability or ratio of residual/input errors. The data presented in the paper, shows that the rms residual error divided by the peak amplitude of the input error function is related to the actuator spacing to characteristic length ratio by the following expression RMS Residual Error b 3.5 k (I) (1) Initial Error Ampl. The value of k ranges from approximately 0.001 for low spatial frequency initial errors up to 0.05 for higher error frequencies (e.g. 5 cycles/diameter). The studies also yielded insight to the forces required to produce typical corrections at both the center and edges of the mirror panels. Additionally, the data lends itself to rapid evaluation of the effects of trading faceplate weight for increased actuator count,
Laser beam directors have grown from centimeter size to near two meters in the last 20 years. As the size increased, they began to adopt the appearance of large astronomical telescopes, while retaining the operational features of high speed tracking mounts. This paper describes the current generation of beam directors and discusses design requirements for larger mounts.
A review of the current practice in wavefront sensor design at Adaptive Optics Associates (AOA) is presented. The principles of Hartmann sensing and matrix reconstruction are reviewed, and sensor performance required for compensation of atmospheric imaging is estimated. The structure of the digital processor based sensors built at AOA is discussed, with two recent systems as examples.
Active mirror systems present one of the most complex control problems encountered in electro-optical systems. Such mirrors not only require the usual attention to servo control performance, but also involve the simultaneous control of many parallel channels which may interact with each other. Moreover, the data rates involved are such that conventional digital processora are overwhelmed, and special purpose hardwired analog or digital control systems are required. Present mirrors have from a few dozen to a few hundred control channels. Future mirrors may have tens of thousands. The purpose of this paper is to describe our experience with a 57-channel active mirror system developed to remove atmospheric aberrations in solar images. I will describe a new control method which greatly increases con-trol system reliability under daytime atmospheric conditions.
Low spatial and temporal frequency correction of optical wavefronts can be achieved with continuously deformable mirrors that are being developed by the Atomic Vapor Laser Isotope Separation (AVLIS) Program at Lawrence Livermore National Laboratory. These devices are simple in design, low in cost, require relatively few actuators, and are capable of submicron deformation control using conventional stepper motors. The technique involves bending a mirror substrate with actuators that apply variable bending moments typically around the perimeter of the mirror. The location and orientation of these actuators and the thickness variation of the substrate determine the particular static shape that will be generated and the optical distortion that it can correct. For constant-thickness substrates the deformation will generally follow a curve that can be described by a quadratic function. However, by contouring the back surface of the substrate, higher-order deformations can be generated. Among the optical aberrations that can be generated by this technique are focus, astigmatism, coma, and spherical aberration. More complex shapes and other applications are being investigated. This method may also be useful in the manufacturing of aspherical optics.
A mirror pointing controller has been designed for a high-speed beam steering mirror which exhibits a highly resonant time-varying system function. Instead of attempting to continuously control the mirror pointing trajectory, the control algorithm constrains the mirror pointing angle only at the discrete strobe occurrences during the pulsed experiment by modifying the input waveform. Adaptation of the controller parameters to values which satisfy a mirror response error criteria is done during the interval between pulses. Pointing errors on the order of 3% of full scale have been measured.
A unique approach to achieving high temporal bandwidth control of large active primary mirrors has been conceived and demonstrated experimentally. This high speed control is accomplished via implementation of a lightweight, reactionless facesheet that overcomes the typical mechanical resonance limitations on bandwidth for large primary mirror assemblies. Laboratory testing of a reactionless facesheet breadboard has:
. Validated the reactionless principles of self-canceling symmetry of mass motion and stiffness forces
. Confirmed the localized influence function of the inherently stiff high density, piezoelectric actuators used to control the mirror surface figure
. Confirmed the feasibility of achieving high bandwidth control of large primary mirrors.
This paper describes the design, test methods, and results of a concept demonstration and compares test data and predicted response.
This paper covers the design and application of a state-of-the-art optoelectronic instrumentation system called the Projectile Follower System (PFS). This system is designed to produce high speed motion picture photographic or video records for investigating dynamic ballistic phenomena of ammunition during external flight. It is designed to be used for developing and testing U.S. Army ammunition and especially for high velocity projectiles fired from direct fire weapons such as tank guns. The fundamental operational concept is that the vertical axis of a rotatable mirror is driven synchronously with the motion of the projectile, and thus the projectile is maintained within the field of view of stationary motion picture or video cameras aimed at the mirror. The PFS incorporates microprocegsor and computer control to achieve angular accelerations of the mirror of up to 2,100 rad/s . The mirror is manufactured from beryllium using a honeycomb construction and has a 300 x 500 mm optical surface. It can also be used to provide auxiliary illumination by reflecting a beam generated from a stationary light source onto the moving projectile. Compared to a conventional high speed motion picture camera system, the PFS covers a far greater portion of the projectile's flight from a single camera station with a relatively larger photographic image size. The resulting photographs display a considerable reduction in blur. This paper describes the PFS optoelectronic engineering system design and application. Included are the mirror and its drive, control and drive electronics, projectile synchronization, muzzle velocity correction, projectile illumination, and cameras and related optics. The range installation, data recording methods, and future expansion capability are also discussed.
Scanning interferometers detect motion through optical fringe modulation. The fringe visibility depends on optical alignment and the modulation purity depends on mechanical stability. Efficient designs employ a mix of active and passive means to achieve high performance and to maintain it over long time and in adverse conditions.
The electro-mechanical design for precision pointing and tracking systems is a major task in the electro-optical system development process. Design tradeoffs must be optimized in selecting the electro-mechanical components and in designing the structure to achieve adequate system level performance. Key design issues and their interaction with the precision control systems are presented.
A line-of-sight (LOS) stabilized platform is an electromechanical subsystem designed to isolate a "payload" from its environment and point the load in a given direction as it operates in its environment. There are numerous mechanical configurations which may be chosen for design of the stabilization system. Mass Stabilization refers to the class of designs in which the entire payload is supported by a gimbal assembly with as low as practical friction in the gimbal bearings; this approach exploits the inherent tendency for a mass to retain its orientation in inertial space. The control loop is then designed to attenuate the effect of inadvertent torque disturbances due to friction, unbalance and/or other disruptions. Mirror Stabilization refers to the class of designs in which a mirror (or optical element which can alter the LOS) is controlled to achieve steady LOS orientation. The mirror arrangement has an inherent two-to-one optical doubling effect which must be accounted for in the design. Other mechanical configurations also are possible; indeed practical designs may use a combination of several. This paper describes the following approaches to the stabilization and tracking tasks: mass stabilization, mirror stabilization, gear-driven gimbals, and momentum wheels. Block diagrams are given for each approach which are then used to discuss the advantages and limitations of each.
The Infrared Imaging Stabilizer (IRIS) subsystem completing development at the Singer Company Kearfott Division (SKD) uses off-the-shelf SKD components within a state-of-the-art mechanization to provide off-boresight, stabilized, infrared imagery in various military environments.
Air combat training has evolved into a highly sophisticated and expensive process. To effectively train fighter pilots in air-to-air combat, interaction between pilots is essential. This interaction can be accomplished using multiple low cost laser image projections of friend and/or foe aircraft controlled by pilots in a multiple dome configuration. A Laser Target Projector (LTP) produces a calligraphically written aircraft model comprised of up to 200 vectors which are updated at a 60 Hz rate. The resulting wire frame image imparts both position, velocity, distance and altitude information to the pilots. Using a laser light source guarantees high luminance levels and provides large depths of field. This large depth of field allows for unique packaging arrangements and cost saving attributes. The LTP has total dome coverage via a computer-controlled, servo-driven, gimb-alled two-axis assembly that projects the wire frame aircraft image onto the dome surface. To unburden the host computer, all dome-to-dome communication, real world-to-dome coordinate transformations and all geometry corrections are done by a special purpose high-speed computer called a Dome Master. Each dome has one Dome Master that can drive up to six LTP's. This paper will deal with the technical aspects of the design and development of the LTP and Dome Master as a low cost air combat training system.