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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

JWST Lifting System

A document describes designing, building, testing, and certifying a customized crane (Lifting Device LD) with a strong back (cradle) to facilitate the installation of long wall panels and short door panels for the GHe phase of the James Webb Space Telescope (JWST). The LD controls are variable-frequency drive controls designed to be adjustable for very slow and very-short-distance movements throughout the installation. The LD has a lift beam with an electric actuator attached at the end. The actuator attaches to a rectangular strong back (cradle) for lifting the long wall panels and short door panels from a lower angle into the vertical position inside the chamber, and then rotating around the chamber for installation onto the existing ceiling and floor. The LD rotates 360 (in very small increments) in both clockwise and counterclockwise directions. Eight lifting pads are on the top ring with 2-in. (.5-cm) eye holes spaced evenly around the ring to allow for the device to be suspended by three crane hoists from the top of the chamber. The LD is operated by remote controls that allow for a single, slow mode for booming the load in and out, with slow and very slow modes for rotating the load.

Tolleson, William↗
NASA NTRS1947

Frequency-response method for determination of dynamic stability characteristics of airplanes with automatic controls

A frequency-response method for determining the critical control-gearing and hunting oscillations of airplanes with automatic pilots is presented. The method is graphical and has several advantages over the standard numerical procedure based on Routh's discriminant. The chief advantage of the method is that direct use can be made of the measured response characteristics of the automatic pilot. This feature is especially useful in determining the existence, amplitude, and frequency of the hunting oscillations that may be present when the automatic pilot has nonlinear dynamic characteristics. Several examples are worked out to illustrate the application of the frequency-response method in determining the effect of automatic-pilot lag or lead on critical control gearing and in determining the amplitude and frequency hunting. It is shown that the method may be applied to the case of a control geared to airplane motions about two axes.

Greenberg, Harry↗
NASA NTRS1972

Characteristics of hingeless rotors with hub moment feedback controls including experimental rotor frequency response, Volume 1

Wind tunnel tests to determine the dynamic characteristics of hingeless rotors with hub moment feedback controls and to acquire experimental hingeless rotor transfer functions are discussed. Rotor transfer functions were calculated from data acquired during open loop frequency response tests. The transfer functions are linear and present the rotor longitudinal and lateral frequency responses to collective pitch, longitudinal cyclic pitch, and lateral cyclic pitch. The theoretical analysis was based on the rigid blade flapping model coupled with appropriate control system and cyclic pitch actuator equations of motion.

Kuczynski, W. A.↗
NASA NTRS2008

Re-active Passive (RAP) Devices for Control of Noise Transmission through a Panel

Re-Active Passive (RAP) devices have been developed to control low frequency (<1000 Hz) noise transmission through a panel. These devices use a combination of active, re-active, and passive technologies packaged into a single unit to control a broad frequency range utilizing the strength of each technology over its best suited frequency range. The RAP device uses passive constrained layer damping to cover the relatively high frequency range (>200 Hz), reactive distributed vibration absorber) to cover the medium frequency range (75 to 250 Hz), and active control for controlling low frequencies (<200 Hz). The device was applied to control noise transmission through a panel mounted in a transmission loss test facility. Experimental results are presented for the bare panel, and combinations of passive treatment, reactive treatment, and active control. Results indicate that three RAP devices were able to increase the overall broadband (15-1000 Hz) transmission loss by 9.4 dB. These three devices added a total of 285 grams to the panel mass of 6.0 kg, or approximately 5%, not including control electronics.

Carneal, James P.↗
NASA NTRS1991

Frequency weighted system identification and linear quadratic controller design

Application of filters for frequency weighting of Markov parameters (pulse response functions) is described in relation to system/observer identification. The time domain identification approach recovers a model which has a pulse response weighted according to frequency. The identified model is composed of the original system and filters. The augmented system is in a form which can be used directly for frequency weighted linear quadratic controller design. Data from either single or multiple experiments can be used to recover the Markov parameters. Measured acceleration signals from a truss structure are used for system identification and the model obtained is used for frequency weighted controller design. The procedure makes the identification and controler design complementary problems.

Horta, Lucas G.↗
NASA NTRS1989

On-line frequency domain information for control of a flexible-link robot with varying payload

Experimental results are given for the endpoint position control of a single-link, very flexible robot arm carrying an unknown, varying payload. The control objective is to maintain endpoint position accuracy in the presence of flexure effects after rapid movement due to a rigid body slew-angle commanded position. Fast, simple, and efficient frequency-domain schemes are used for online controller gain adjustment within an effective scheduling framework. Only endpoint acceleration and motor shaft angle measurements are utilized in relatively simple control laws where the appropriate gains have been scheduled as correlated to modal frequency information corresponding to a varying, unknown payload.

Yurkovich, Stephen↗
NASA NTRS1975

Variable frequency inverter for ac induction motors with torque, speed and braking control

A variable frequency inverter was designed for driving an ac induction motor which varies the frequency and voltage to the motor windings in response to varying torque requirements for the motor so that the applied voltage amplitude and frequency are of optimal value for any motor load and speed requirement. The slip frequency of the motor is caused to vary proportionally to the torque and feedback is provided so that the most efficient operating voltage is applied to the motor. Winding current surge is limited and a controlled negative slip causes motor braking and return of load energy to a dc power source.

Nola, F. J.↗
NASA NTRS1973

A spaceborne receiver for measuring electromagnetic field intensity

Description of a very accurately controlled receiver for monitoring the electromagnetic radiations in both existing and projected space communication bands. Based on analysis of the existing and projected space communication bands, 108 to 174 MHz, 240 to 478 MHz, and 1535 to 1665 MHz were covered. The receiver achieves accurate control via a digitally tuned synthesizer and a wide range of digital control including frequency band coverage and gain control selection. Digital memory was provided to store 16 separate digital command instructions which can be programmed via a command data link. The receiver provides for transmission to the ground of both a predetection signal and signals in digital format, which in turn, were provided by sampling and analog-to-digital conversions.

Reich, B. W.↗
NASA NTRS1975

The effects of localized damping on structural response

The effect of localized structural damping on the excitability of higher order normal modes of the large space telescope was investigated. A preprocessor computer program was developed to incorporate Voigt structural joint damping models in a NASTRAN finite-element dynamic model. A postprocessor computer program was developed to select critical modes for low-frequency attitude control problems and for higher frequency fine-stabilization problems. The mode selection is accomplished by ranking the flexible modes based on coefficients for rate gyro, position gyro, and optical sensors, and on image-plane motions due to sinusoidal or random power spectral density force and torque inputs.

Merchant, D. H.↗
NASA NTRS1990

Infrared detector responsivity measured simultaneously at multiple frequencies

The responsivity (the ratio of the output electrical signal to the incident radiation power) of an IR detector is measured at several frequencies simultaneously. A computer-controlled variable frequency chopper is used. Chopper modulation of a blackbody is square-wave-like, hence measurements can be taken not only at the fundamental frequency but also at the harmonics. The technique gives excellent agreement with the classical technique of measuring the fundamental response alone at a set of modulation frequencies. With a small enough aperture and a high enough chopper speed, agreement can be within five percent out to 31 times the fundamental frequency. The technique allows the rapid and automated acquisition of the frequency response of IR detectors, which may then be screened for responsivity, noise, and noise equivalent power.

Brasunas, John C.↗
NASA NTRS1955

An Experimental Investigation of the Hinge-moment Characteristics of a Constant-chord Control Surface Oscillating at High Frequency

The results of an experimental investigation of the hinge-moment characteristics of a constant-chord control surface oscillating at high frequency is presented. The control surface was mounted on an aspect-ratio-2 triangular wing. The aerodynamic restoring-moment coefficient and damping-moment coefficient were determined at a frequency of 260 cycles per second for a Mach number range of 0.6 to 0.8 and 1.3 to 1.9 at angles of attack of 5 degrees and 10 degrees. The test results showed linear theory to be a reliable guide to the prediction of the trend of the restoring-moment coefficient with Mach number for the supersonic speed range of the investigation but overestimated the magnitude of the coefficient. The experimental values of the damping-moment coefficient were, for the most part, more positive than those indicated by the theory and, for some conditions, could lead to instability of the control surface. Comparison of the results of this investigation with those of previous investigations at 0 and 50 cycles per second showed that frequency had little effect on the restoring-moment coefficient. The damping-moment coefficient was similarly insensitive to frequency at an oscillation amplitude of plus-or-minus 1.0 degrees but at an amplitude ofplus-or-minus 2.5 degrees the results showed a destabilizing shift with increasing frequency.

Reese, David E JR↗
NASA NTRS1993

A wide-band fiber optic frequency distribution system employing thermally controlled phase compensation

An active wide-band fiber optic frequency distribution system employing a thermally controlled phase compensator to stabilize phase variations induced by environmental temperature changes is described. The distribution system utilizes bidirectional dual wavelength transmission to provide optical feedback of induced phase variations of 100 MHz signals propagating along the distribution cable. The phase compensation considered differs from earlier narrow-band phase compensation designs in that it uses a thermally controlled fiber delay coil rather than a VCO or phase modulation to compensate for induced phase variations. Two advantages of the wide-band system over earlier designs are (1) that it provides phase compensation for all transmitted frequencies, and (2) the compensation is applied after the optical interface rather than electronically ahead of it as in earlier schemes. Experimental results on the first prototype shows that the thermal stabilizer reduces phase variations and Allan deviation by a factor of forty over an equivalent uncompensated fiber optic distribution system.

Johnson, Dean↗
NASA NTRS1999

Frequency-Weighting Filter Selection, for H2 Control of Microgravity Isolation Systems: A Consideration of the "Implicit Frequency Weighting" Problem

Many space-science experiments need an active isolation system to provide them with the requisite microgravity environment. The isolation systems planned for use with the International Space Station (ISS) have been appropriately modeled using relative position, relative velocity, and acceleration states. In theory, frequency-weighting design filters can be applied to these state-space models, in order to develop optimal H2 or mixed-norm controllers with desired stability and performance characteristics. In practice, however, since there is a kinematic relationship among the various states, any frequency weighting applied to one state will implicitly weight other states. These implicit frequency-weighting effects must be considered, for intelligent frequency-weighting filter assignment. This paper suggests a rational approach to the assignment of frequency-weighting design filters, in the presence of the kinematic coupling among states that exists in the microgravity vibration isolation problem.

Hampton, Roy David↗
NASA NTRS2000

Frequency-Weighting Filter Selection for H2 Control of Microgravity Isolation Systems: A Consideration of the "Implicit Frequency Weighting" Problem

Many space-science experiments need an active isolation system to provide them with the requisite microgravity environment. The isolation system planned for use with the International Space Station (ISS) have been appropriately modeled using relative position, relative velocity, and acceleration states. In theory, frequency-weighting design filters can be applied to them state-space models, In order to develop optimal H2 or mixed-norm controllers with desired stability and performance characteristics. In practice. however, since there Is a kinematic relationship among the various states. any frequency weighting applied to one state will implicitly weight other states. These implicit frequency-weighting effects must be considered, for intelligent frequency-weighting filter assignment. This paper suggests a rational approach to the assignment of frequency-weighting design filters, in the presence of the kinematic coupling among states that exists in the microgravity vibration isolation problem.

Hampton, R. David↗
NASA NTRS1991

Resolved rate and torque control schemes for large scale space based kinematically redundant manipulators

Resolved rate control of kinematically redundant ground based manipulators is a challenging problem. The structural, actuator, and control loop frequency characteristics of industrial grade robots generally allow operation with resolved rate control; a rate command is achievable with good accuracy. However, space based manipulators are different, typically have less structural stiffness, more motor and joint friction, and lower control loop cycle frequencies. These undesirable characteristics present a considerable Point of Resolution (POR) control problem for space based, kinematically redundant manipulators for the following reason: a kinematically redundant manipulator requires an arbitrary constraint to solve for the joint rate commands. A space manipulator will not respond to joint rate commands because of these characteristics. A space based manipulator simulation, including free end rigid body dynamics, motor dynamics, motor striction/friction, gearbox backlash, joint striction/friction, and Space Station Remote Manipulator System type configuration parameters, is used to evaluate the performance of a documented resolved rate control law. Alternate schemes which include torque control are also evaluated.

Robert W Bailey↗