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At least 397 records · Page 22

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NASA NTRS1989

Design Of Feedforward Controllers For Multivariable Plants

Controllers based on simple low-order transfer functions. Mathematical criteria derived for design of feedforward controllers for class of multiple-input/multiple-output linear plants. Represented by simple low-order transfer functions, obtained without reconstruction of states of commands and disturbances. Enables plant to track command while remaining unresponsive to disturbance in steady state. Feedback controller added independently to stabilize plant or to make control system less susceptible to variations in parameters of plant.

Seraji, Homayoun↗

NASA NTRS1974

Lunar electrical conductivity

It is pointed out that the lunar magnetometer experiment has made important contributions to studies of the lunar interior. Numerical inversions of the lunar electromagnetic response have been carried out, taking into account a void region behind the moon. The amplitude of the transfer function of an eight-layer model is considered along with a model of the temperature distribution inside the moon and the amplitude of the transfer function of a semiconductor lunar model.

Leavy, D.↗

NASA NTRS1979

Choice of method for discretization of continuous systems

The paper discusses and compares several common methods of discretizing the transfer function of a continuous control system so that a digital computer can be used. The discretization effect on the frequency response attenuation of the simple transfer function G(s) = 1/(s + 1) is illustrated for the Tusting, Boxer-Thaler, Madwed, linear segment approximation, and stair-step with half period advanced methods. The input frequency must be significantly lower than one-half the sampling error to have negligible discretization error. Phase differences due to discretization are also plotted, and it is seen that the zero-order hold contributes much more phase shift than that due to discretization. In situations where zero-order hold must follow a computer, the method of discretization of a continuous system is not a major factor if phase shift is important.

Mcvey, E. S.↗

NASA NTRS1993

Gravity and suspension effects on the dynamics of controlled structures

The effects of gravity on the actuators, sensors and structural plant of a controlled flexible structure are investigated. These influences include gravity stiffening, gravity induced deformations, gravity influence on inertial sensors and actuators, and the interaction with the suspension system. For each of the four influences, a simplified model is derived, from which the appropriate non-dimensional parameter can be identified, and the magnitude of the influence estimated. The construction of a detailed numerical model, which includes the four gravity influences, is outlined. This modeling procedure is then applied to a simplified model of the Middeck Active Control Eperiment (MACE). The influence on the poles and input-output transfer function is examined for several variations in the plant. The modeling procedure is also applied to the MACE experimental hardware. Experimental and model-based transfer functions are compared to demonstrate the significant improvement by incorporating gravity effects.

Rey, Daniel A.↗

NASA NTRS1974

The response of a propeller anemometer to turbulent flow with the mean wind vector perpendicular to the axis of rotation

Determination of the properties of the system transfer function of the vertical-velocity propeller anemometer. A treatment of this problem is presented which is based on an analysis of estimates of the system transfer function derived from estimates of spectra calculated from time histories of indicated vertical velocity and on estimates of the associated input Eulerian vertical-velocity frequency spectra based on the hypothesis that instrument degradation of the Fourier components occurs in the inertial subrange.

Fichtl, G. H.↗

NASA NTRS1977

Calibration of a universal indicated turbulence system

Theoretical and experimental work on a Universal Indicated Turbulence Meter is described. A mathematical transfer function from turbulence input to output indication was developed. A random ergodic process and a Gaussian turbulence distribution were assumed. A calibration technique based on this transfer function was developed. The computer contains a variable gain amplifier to make the system output independent of average velocity. The range over which this independence holds was determined. An optimum dynamic response was obtained for the tubulation between the system pitot tube and pressure transducer by making dynamic response measurements for orifices of various lengths and diameters at the source end.

Chapin, W. G.↗

NASA NTRS1977

Unsteady aerodynamic modeling and active aeroelastic control

Unsteady aerodynamic modeling techniques are developed and applied to the study of active control of elastic vehicles. The problem of active control of a supercritical flutter mode poses a definite design goal stability, and is treated in detail. The transfer functions relating the arbitrary airfoil motions to the airloads are derived from the Laplace transforms of the linearized airload expressions for incompressible two dimensional flow. The transfer function relating the motions to the circulatory part of these loads is recognized as the Theodorsen function extended to complex values of reduced frequency, and is termed the generalized Theodorsen function. Inversion of the Laplace transforms yields exact transient airloads and airfoil motions. Exact root loci of aeroelastic modes are calculated, providing quantitative information regarding subcritical and supercritical flutter conditions.

Edwards, J. W.↗

NASA NTRS1980

Interaction of oscillations of channel flow and flow separation at duct discontinuities

A two dimensional experiment was used to study the flow fluctuations in the separated region resulting from a wall slot step (convergent step). This geometry corresponds to the geometry present in a segmented solid rocket motor in the region of the ends of adjoining segments. A modulated tunnel flow of controlled oscillation frequency was used, and the effect on flow fluctuations in the separated region was examined by hot wire velocity measurements. The transfer function between free stream oscillations and separated flow fluctuations at the driven frequency was determined over a range of frequencies and main stream flow velocities. The transfer function exhibited a frequency dependence with maximum in the 10 - 25 Hz range, with the maximum occurring at higher frequency when the mean flow velocity was higher.

Price, E. W.↗

NASA NTRS1986

Power cepstrum technique with application to model helicopter acoustic data

The application of the power cepstrum to measured helicopter-rotor acoustic data is investigated. A previously applied correction to the reconstructed spectrum is shown to be incorrect. For an exact echoed signal, the amplitude of the cepstrum echo spike at the delay time is linearly related to the echo relative amplitude in the time domain. If the measured spectrum is not entirely from the source signal, the cepstrum will not yield the desired echo characteristics and a cepstral aliasing may occur because of the effective sample rate in the frequency domain. The spectral analysis bandwidth must be less than one-half the echo ripple frequency or cepstral aliasing can occur. The power cepstrum editing technique is a useful tool for removing some of the contamination because of acoustic reflections from measured rotor acoustic spectra. The cepstrum editing yields an improved estimate of the free field spectrum, but the correction process is limited by the lack of accurate knowledge of the echo transfer function. An alternate procedure, which does not require cepstral editing, is proposed which allows the complete correction of a contaminated spectrum through use of both the transfer function and delay time of the echo process.

Martin, R. M.↗

NASA NTRS1991

An exact algebraic solution of the infimum in H-infinity optimization with output feedback

This paper presents a simple and noniterative procedure for the computation of the exact value of the infimum in the standard H-infinity-optimal control with output feedback. The problem formulation is general and does not place any restrictions on the direct feedthrough terms between the control input and the controlled output variables, and between the disturbance input and the measurement output variables. The method is applicable to systems that satisfy (1) the transfer function from the control input to the controlled output is right-invertible and has no invariant zeros on the j(w) axis and, (2) the transfer function from the disturbance to the measurement output is left-invertible and has no invariant zeros on the j(w) axis. A set of necessary and sufficient conditions for the solvability of H-infinity-almost disturbance decoupling problem via measurement feedback with internal stability is also given.

Chen, Ben M.↗

NASA NTRSDate not supplied

Design of a Thruster Vector Servo-Elastic Filter

This presents a method to improve the thruster vector servo elasticity (TVSE) stability. TVSE is the coupling between the TVC system engine dynamics and the vehicle flexible modes. It can lead to instability of the flexible modes due to the energy injection from the thrust. For large boosters with heavy TVC engine, TVSE is more likely to cause instability. This instability is part of the plant dynamics and is not detected by the flight control. In this paper, the TVSE is treated as a disturbance rejection problem with the inertial force of the flexible motion viewed as an external disturbance to the TVC actuator control system. By effectively rejecting this disturbance, the engine can be decoupled from the flexible dynamics thus improving TVSE stability. The actuator disturbance transfer function is modified by inserting a TVSE filter into the control system. The filter aims to reduce the magnitude and introduce phase lead of the disturbance transfer function over the TVSE sensitive frequency range while maintaining adequate stability margins of the actuator control system. A prefilter is also designed for good command tracking. The filter and prefilter resulting from this method are usually of low order and can be included to the existing actuator controller if the controller is programmable. This method was demonstrated for a hydraulic Simplex actuator model. Its applications to SLS B1 and B2 TVC actuator are also described. B2 actuator controller is modified based on this method. The tradeoff between TVSE stability and the control system stability can be balanced by adjusting the amount of phase lead introduced by the TVSE filter. This method provides a useful way to solve the TVSE instability issue without costly and time-consuming a TVC system redesign. This method is based on the basic control principles of loop shaping and disturbance rejection and has been proven to be effective under proper circum-stances.

Thrust vector control↗

NASA NTRS1971

Fundamental limitations on V/STOL terminal guidance due to aircraft characteristics

A review is given of limitations on approach flight paths of V/STOL aircraft, including limits on descent angle due to maximum drag/lift ratio. A method of calculating maximum drag/lift ratio of tilt-wing and deflected slipstream aircraft is presented. Derivatives and transfer functions for the CL-84 tilt-wing and X-22A tilt-duct aircraft are presented. For the unaugmented CL-84 in steep descents the transfer function relating descent angle to thrust contains a right-half plane zero. Using optimal control theory, it is shown that this zero causes a serious degradation in the accuracy with which steep flight paths can be followed in the presence of gusts.

Wolkovitch, J.↗

NASA NTRS1981

Application of the MNA design method to a nonlinear turbofan engine

Using nonlinear digital simulation as a representative model of the dynamic operation of the QCSEE turbofan engine, a feedback control system is designed by variable frequency design techniques. Transfer functions are generated for each of five power level settings covering the range of operation from approach power to full throttle (62.5% to 100% full power). These transfer functions are then used by an interactive control system design synthesis program to provide a closed loop feedback control using the multivariable Nyquist array and extensions to multivariable Bode diagrams and Nichols charts.

Leininger, G. G.↗

NASA NTRS1988

Remote structural damage detection via substructuring

An approach to structural damage detection which utilizes the concept of a substructure transfer function matrix is proposed. The major advantage of this approach is that structural changes localized within a small volume can be detected by studying the changes in the transfer function for a relatively small substructure containing the damaged portion.

Walton, William B.↗

NASA NTRS1994

Robustness properties of LQG optimized compensators for collocated rate sensors

In this paper we study the robustness with respect to stability of the closed-loop system with collocated rate sensor using LQG (mean square rate) optimized compensators. Our main result is that the transmission zeros of the compensator are precisely the structure modes when the actuator/sensor locations are 'pinned' and/or 'clamped': i.e., motion in the direction sensed is not allowed. We have stability even under parameter mismatch, except in the unlikely situation where such a mode frequency of the assumed system coincides with an undamped mode frequency of the real system and the corresponding mode shape is an eigenvector of the compensator transfer function matrix at that frequency. For a truncated modal model - such as that of the NASA LaRC Phase Zero Evolutionary model - the transmission zeros of the corresponding compensator transfer function can be interpreted as the structure modes when motion in the directions sensed is prohibited.

Balakrishnan, A. V.↗

NASA NTRS1996

The Use of Structural-Acoustic Techniques to Assess Potential Structural Damage From Sonic Booms

The potential impact of supersonic operations includes structural damage from the sonic boom overpressure. This paper describes a study of how structural-acoustic modeling and testing techniques may be used to assess the potential for such damage in the absence of actual flyovers. Procedures are described whereby transfer functions relating structural response to sonic boom signature may be obtained with a stationary acoustic source and appropriate data processing. Further, by invoking structural-acoustic reciprocity, these transfer functions may also be acquired by measuring the radiated sound from the structure under a mechanical drive. The approach is based on the fundamental assumption of linearity, both with regard to the (acoustic) propagation of the boom in the vicinity of the structure and to the structure's response. Practical issues revolve around acoustic far field and source directivity requirements. The technique was implemented on a specially fabricated test structure at Edwards AFB, CA with the support of Wyle Laboratories, Inc. Blank shots from a cannon served as our acoustic source and taps from an instrumented hammer generated the mechanical drive. Simulated response functions were constructed. Results of comparisons with corresponding measurements recorded during dedicated supersonic flyovers with F-15 aircraft are presented for a number of sensor placements.

Garrelick, Joel↗

NASA NTRS1995

Dynamics and control of instrumented harmonic drives

Since torque in harmonic drives is transmitted by a pure couple, harmonic drives do not generate radial forces and therefore can be instrumented with torque sensors without interference from radial forces. The installation of torque sensors on the stationary component of harmonic drives (the Flexipline cup in this research work) produce backdrivability needed for robotic and telerobotic compliant maneuvers. Backdrivability of a harmonic drive, when used as torque increaser, means that the output shaft can be rotated via finite amount of torque. A high ratio harmonic drive is non-backdrivable because its output shaft cannot be turned by applying a torque on it. This article first develops the dynamic behavior of a harmonic drive, in particular the non-backdrivability, in terms of a sensitivity transfer function. The instrumentation of the harmonic drive with torque sensor is then described. This leads to a description of the control architecture which allows modulation of the sensitivity transfer function within the limits established by the closed-loop stability. A set of experiments on an active hand controller, powered by a DC motor coupled to an instrumented harmonic drive, is given to exhibit this method's limitations.

NASA Discipline Space Human Factors↗

NASA NTRS1987

Frequency-response identification of XV-15 tilt-rotor aircraft dynamics

The timely design and development of the next generation of tilt-rotor aircraft (JVX) depend heavily on the in-depth understanding of existing XV-15 dynamics and the availability of fully validated simulation models. Previous studies have considered aircraft and simulation trim characteristics, but analyses of basic flight vehicle dynamics were limited to qualitative pilot evaluation. The present study has the following objectives: documentation and evaluation of XV-15 bare-airframe dynamics; comparison of aircraft and simulation responses; and development of a validated transfer-function description of the XV-15 needed for future studies. A nonparametric frequency-response approach is used which does not depend on assumed model order or structure. Transfer-function representations are subsequently derived which fit the frequency responses in the bandwidth of greatest concern for piloted handling-qualities and control-system applications.

Tischler, Mark B.↗