Engineering topics

Gawronski, W.

Publications and source records attributed to Gawronski, W..

At least 37 records · Page 2

NASA NTRS1993

Design of the reduced LQG compensator for the DSS-13 antenna

A linear-quadratic-Gaussian (LQG) compensator design procedure is proposed for the DSS-13 antenna. The procedure is based on two properties. It is shown that tracking and flexible motion of the antenna are almost independent (the separation property). As a consequence, compensators for the flexible and tracking parts can be designed separately. It is shown also that the balanced LQG compensator's effort is evenly divided between the controller and the estimator. This allows a minimization of the compensator order, which is important for implementation purposes. An efficient compensator reduction procedure that gives a stable low-order compensator of satisfactory performance is introduced. This approach is illustrated with a detailed compensator design for the DSS-13 antenna. The implementation of this compensator design requires an update of the antenna model.

Gawronski, W.↗

NASA NTRS1993

Design of the reduced LQG compensator for the Deep Space Network antennas

A linear quadratic Gaussian (LQG) compensator design procedure is proposed for the NASA Deep Space Network (DSN) antennas. The procedure is based on two properties of antennas. It is shown that tracking and flexible motion of the DSN antennas are almost independent (the separation property), and that for the balanced LQG compensator its performance is evenly divided between the controller and the estimator (balancing property). As a consequence compensators for the flexible and tracking parts can be designed separately, and the compensator order is reduced significantly, which is important for implementation purposes. The approach is illustrated with the detailed compensator design for the DSS-13 antenna.

Gawronski, W.↗

NASA NTRS1993

Modal grammian approach to actuator and sensor placement for flexible structures

The problem of selecting optimal locations of actuators and sensors for the control of flexible structures is addressed. Novel geometrical and physical interpretations are given for both controllability and observability grammians which enhances the intuitive aspects of the actuator and sensor placement problem. The explicit form of controllability and observability grammians for flexible structures is used to define modal grammian coefficients, which provides the basis for an actuator and sensor placement technique. The relationship of the proposed metric to three different existing criteria are given. The method is demonstrated on a model of an experimental large flexible structure.

Lim, K. B.↗

NASA NTRS1993

Beam-Waveguide Antenna Servo Design Issues for Tracking Low-Earth-Orbiting Satellites

The upcoming NASA missions will require tracking of low-orbit satellites. As a consequence, NASA antennas will be required to track satellites at higher rates than for the current deep space missions. This paper investigates servo design issues for the 34-m beam-waveguide (BWG) antennas that track low-orbit satellites. This includes the upgrade of the servo with the feedforward loop, monopulse controller design, and tracking error reduction through proper choice of elevation pinion location, through application of a notch filter, and through the elevation drive amplifier gain adjustment. Finally, improvement of the signal-to-noise ratio through averaging of the oversampled monopulse signal is presented.

Mellstrom, J. A.↗

NASA NTRS1992

Parameter and configuration study of the DSS-13 antenna drives

The effects of different elevation and azimuth drive configurations on DSS-13 antenna performance are presented as well as a study of gearbox stiffness and motor inertia. Small motor inertia and rigid gearboxes would improve the pointing accuracy up to a certain limit. The limit is imposed by critical values of gearbox stiffness and motor inertia introduced in the article. The critical values depend on the lowest structural frequency of the rate-loop model. The tracking performance can be improved by raising gearbox stiffness to the critical stiffness and reducing motor inertia to the critical inertia. An azimuth drive configuration with four driven wheels was also investigated. For the four-wheel drive configuration in azimuth, the cross-coupling effects are reduced and wind disturbance rejection properties improved. Pointing is improved substantially in the cross-elevation but is relatively unaffected in the elevation direction. More significant improvements can be achieved through either structural redesign (stiffening the structure) or new control algorithms or control concepts, which would eliminate the effect of flexible deformations on the antenna pointing accuracy. Although the study is performed for the DSS-13 antenna, the results can be extended for other DSN antennas.

Gawronski, W.↗

NASA NTRS1992

Feed-forward control upgrade of the deep space network antennas

In order to improve the accuracy of high-rate tracking of NASA's DSN antennas, the position-loop controller has been upgraded with a feed-forward loop. Conditions for perfect and approximate tracking with the feed-forward loop are presented. The feed-forward loop improves tracking performance and preserves wind disturbance rejection properties of the previous closed-loop system.

Gawronski, W.↗

NASA NTRS1992

Sequential design of a linear quadratic controller for the Deep Space Network antennas

A new linear quadratic controller design procedure is proposed for the NASA/JPL Deep Space Network antennas. The antenna model is divided into a tracking subsystem and a flexible subsystem. Controllers for the flexible and tracking parts are designed separately by adjusting the performance index weights. Ad hoc weights are chosen for the tracking part of the controller and the weights of the flexible part are adjusted. Next, the gains of the tracking part are determined, followed by the flexible controller final tune-up. In addition, the controller for the flexible part is designed separately for each mode; thus the design procedure consists of weight adjustment for small-size subsystems. Since the controller gains are obtained by adjusting the performance index weights, determination of the weight effect on system performance is a crucial task. A method of determining this effect that allows an on-line improvement of the tracking performance is presented in this article. The procedure is illustrated with the control system design for the Deep Space Station (DSS)-13 antenna.

Gawronski, W.↗

NASA NTRS1992

Pointing-error simulations of the DSS-13 antenna due to wind disturbances

Accurate spacecraft tracking by the NASA Deep Space Network (DSN) antennas must be assured during changing weather conditions. Wind disturbances are the main source of tracking errors. The development of a wind-force model and simulations of wind-induced pointing errors of DSN antennas are presented. The antenna model includes the antenna structure, the elevation and azimuth servos, and the tracking controller. Simulation results show that pointing errors due to wind gusts are of the same order as errors due to static wind pressure and that these errors (similar to those of static wind pressure) satisfy the velocity quadratic law. The presented methodology is used for wind-disturbance estimation and for the design of an antenna controller with wind-disturbance rejection properties.

Gawronski, W.↗

NASA NTRS1992

Wind-induced dynamics of the deep space network antennas

Accurate spacecraft tracking by the NASA Deep Space Network (DSN) antennas must be assured during changing weather conditions. Wind disturbances are the main source of tracking errors. This paper presents the development of a wind force model, and simulations of wind-induced pointing errors of DSN antennas. The antenna model includes the antenna structure, the elevation and azimuth servos, and the tracking controller. Simulation results show that pointing errors due to wind gusts are of the same order as due to wind static pressure, and that (similarly to the static wind pressure) these errors satisfy the velocity quadratic law. The methodology presented is used for wind disturbance estimation, and for design of an antenna controller with wind disturbance rejection properties.

Gawronski, W.↗

NASA NTRS1992

Controllability and observability of flexible structures with proof-mass actuators

Relationships between Hankel singular values of a structure with a proof-mass actuator to those of a structure with an ideal actuator are analyzed. They indicate that the proof-mass actuator has little influence on Hankel singular values of a structure if the natural frequency of the actuator is much lower than the structural frequencies. This fact significantly simplifies controllability/observability analysis and model reduction of flexible structures. Results from numerical simulations verify the conclusions.

Gawronski, W.↗

NASA NTRS1992

Predictive controller and estimator for NASA Deep Space Network antennas

A new design procedure is presented for a predictive controller that significantly improves antenna tracking performance. The predictive controller uses future values of the stored output command to generate the control signal. For antennas tracking stars or spacecraft, these values are known in advance, hence the predictive control scheme is easily implemented in this case. The predictive controller is designed for tracking control of the the NASA/JPL 70-m antenna. On-axis tracking is considered, where the output is taken on the encoder, or tachometer. Simulation results show a significant improvement in performance over the LQ controller.

Gawronski, W.↗

NASA NTRS1991

Modeling and simulations of the DSS 13 antenna control system

A model of the antenna control system for the azimuth and elevation axes of the Deep Space Station (DSS) 13 antenna is developed. This model is used for simulation of elevation and azimuth dynamics, cross-coupled dynamics, and radio-frequency pointing error due to both input commands and wind disturbances. This model also serves as a tool for the antenna controller design. A modal state-space model of the antenna structure was obtained from its finite-element model with a free-rotating tipping structure and alidade. Model reduction techniques applied separately for the antenna structure, elevation, and azimuth drives, and rate-loop model reduce the system order to one-third of that of the original, while preserving its significant dynamic properties. Extensive simulation results illustrate properties of the model.

Gawronski, W.↗

NASA NTRS1991

Elevation control system model for the DSS 13 antenna

In order to meet the requirements for precision pointing of 34-m antennas, adequate control design and simulation software have to be developed along with a detailed description of the supporting analytical tools. This article describes a control system model for the elevation drive of the DSS 13 antenna. The model allows one to simulate elevation dynamics, cross-coupled dynamics in azimuth and elevation, and RF pointing error. A modal state-space model of the antenna structure was obtained from its finite-element model with a free rotating tipping structure. Model reduction techniques were applied separately for the antenna model and rate-loop model, thereby reducing the system order to one-third of the original one while preserving its dynamic properties. Extensive simulation results illustrate properties of the model.

Gawronski, W.↗

NASA NTRS1991

Predictive control and estimation algorithms for the NASA/JPL 70-meter antennas

A modified output prediction procedure and a new controller design is presented based on the predictive control law. Also, a new predictive estimator is developed to complement the controller and to enhance system performance. The predictive controller is designed and applied to the tracking control of the Deep Space Network 70 m antennas. Simulation results show significant improvement in tracking performance over the linear quadratic controller and estimator presently in use.

Gawronski, W.↗

NASA NTRS1991

Predictive control and estimation algorithms for the NASA/JPL Deep Space Network antennas

A modified output prediction procedure, and a new controller design based on the predictive control law are presented. Also, the predictive estimator is developed to complement the controller, and to enhance the system performance. The predictive controller was designed and applied to the tracking control of the National Aeronautics and Space Administration (NASA)/Jet Propulsion Laboratory (JPL) 70-m antenna. Simulation results show significant improvement in tracking performance over the linear quadratic controller and estimator presently in use.

Gawronski, W.↗

NASA NTRS1991

Predictive control and estimation - State space approach

A modified output prediction procedure and a new controller design based on the predictive control law are presented. A new predictive estimator enhances system performance. The predictive controller was designed and applied to the tracking control of the NASA/JPL 70-m antenna. Simulation results show significant improvement in tracking performance over the linear quadratic controller and estimator presently in use.

Gawronski, W.↗

NASA NTRS1990

On dynamics and control of multi-link flexible space manipulators

In this paper dynamics, inverse dynamics, and control problems for multi-link flexible space manipulators are presented. In deriving the flexible manipulator dynamics the following are assumed: flexible deformations are relatively small; angular rates of the links are much smaller than their fundamental frequencies; nonlinear terms (centrifugal and Coriolis forces) in the flexible manipulator model are the same as those in the rigid body model. These assumptions are reasonable for large space manipulators, such as the space crane. Flexible displacements are measured with respect to the rigid body configuration, for which a linear time-varying system is obtained. The inverse dynamics problem consists of determination of joint torques, given tip trajectory, such that joint angles in flexible configuration are equal to the angles in the rigid body configuration. The manipulator control system consists of the feedforward compensation and feedback control loops. Simulation results of a two-link space crane with large payload show that the performance of this linearized dynamics and control approach is reasonable and robust subject to parameter variations during slew operations.

Gawronski, W.↗

NASA NTRS1990

A Hankel singular value approach for identification of low level dynamics in flexible structures

The intensity of dynamics in a flexible structure is characterized by its Hankel singular values. Low level dynamics may be caused by generic properties of a structure or by its actuator and sensor configurations. In the latter case, it is shown that the dynamics can significantly be amplified by small perturbations of actuator and/or sensor configurations. In the presented examples, the variations of sensor configuration of 0.05 or less caused an increase of Hankel singular values of order 1000. This approach is useful for model reduction and model order determination in the presence of noise.

Gawronski, W.↗