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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Dynamic Reconstruction and Multivariable Control for Force-Actuated, Thin Facesheet Adaptive Optics

The Multiple Mirror Telescope (MMT) under development at the University of Arizona takes a new approach in adaptive optics placing a large (0.65 m) force-actuated, thin facesheet deformable mirror at the secondary of an astronomical telescope, thus reducing the effects of emissivity which are important in IR astronomy. However, The large size of the mirror and low stiffness actuators used drive the natural frequencies of the mirror down into the bandwidth of the atmospheric distortion. Conventional adaptive optics takes a quasi-static approach to controlling the, deformable mirror. However, flexibility within the control bandwidth calls for a new approach to adaptive optics. Dynamic influence functions are used to characterize the influence of each actuator on the surface of the deformable mirror. A linearized model of atmospheric distortion is combined with dynamic influence functions to produce a dynamic reconstructor. This dynamic reconstructor is recognized as an optimal control problem. Solving the optimal control problem for a system with hundreds of actuators and sensors is formidable. Exploiting the circularly symmetric geometry of the mirror, and a suitable model of atmospheric distortion, the control problem is divided into a number of smaller decoupled control problems using circulant matrix theory. A hierarchic control scheme which seeks to emulate the quasi-static control approach that is generally used in adaptive optics is compared to the proposed dynamic reconstruction technique. Although dynamic reconstruction requires somewhat more computational power to implement, it achieves better performance with less power usage, and is less sensitive than the hierarchic technique.

Grocott, Simon C. O.↗
NASA NTRS2001

Root elongation against a constant force: experiment with a computerized feedback-controlled device

Axial force was applied to the root tip of corn (Zea mays L. cv. Merit) seedlings using a computerized, feedback-controlled mechanical device. The system's feedback capability allowed continuous control of a constant tip load, and the attached displacement transducer provided the time course of root elongation. Loads up to 7.5 g decreased the root elongation rate by 0.13 mm h-1 g-1, but loads 7.5 to 17.5 g decreased the growth rate by only 0.04 mm h-1 g-1. Loads higher than 18 g stopped root elongation completely. Measurement of the cross-sectional areas of the root tips indicated that the 18 g load had applied about 0.98 MPa of axial pressure to the root, thereby exceeding the root's ability to respond with increased turgor pressure. Recorded time-lapse images of loaded roots showed that radial thickening (swelling) occurred behind the root cap, whose cross-sectional area increased with tip load.

Non-NASA Center↗
NASA NTRS1986

Passive control of aerodynamically forced vibrations of supersonic turbomachine rotors by splitter blades

An aeroelastic model is developed to examine the use of splitter blades as a passive detuning mechanism for flow induced forced response of unstalled supersonic turbomachine rotors. The splitters introduce aerodynamic and structural detuning to the rotor design. The relationship between aerodynamic and structural detuning and the location and chord lengths of splitters is analyzed. The model is applied to the flow induced response of four 12-blade rotors with Verdon's (1973) Cascade B flow geometry. The data reveal that for gusts characterized by forward and backward traveling waves the splitters generally decrease the maximum amplitudes of response; however, for some gust load interblade phase angles, such as -180 deg and 120 deg the splitters did not reduce the amplitudes of response.

Fleeter, S.↗
NASA NTRS1992

Exoskeleton master controller with force-reflecting telepresence

A thorough understanding of the requirements for successful master-slave robotic systems is becoming increasingly desirable. Such systems can aid in the accomplishment of tasks that are hazardous or inaccessible to humans. Although a history of use has proven master-slave systems to be viable, system requirements and the impact of specifications on the human factors side of system performance are not well known. In support of the next phase of teleoperation research being conducted at the Armstrong Research Laboratory, a force-reflecting, seven degree of freedom exoskeleton for master-slave teleoperation has been concepted, and is presently being developed. The exoskeleton has a unique kinematic structure that complements the structure of the human arm. It provides a natural means for teleoperating a dexterous, possibly redundant manipulator. It allows ease of use without operator fatigue and faithfully follows human arm and wrist motions. Reflected forces and moments are remotely transmitted to the operator hand grip using a cable transmission scheme. This paper presents the exoskeleton concept and development results to date. Conceptual design, hardware, algorithms, computer architecture, and software are covered.

Burke, James B.↗
NASA NTRS1999

Thermal Noise Reduction of Mechanical Oscillators by Actively Controlled External Dissipative Forces

We show that the thermal fluctuations of very soft mechanical oscillators, such as the cantilever in an atomic force microscope (AFM), can be reduced without changing the stiffness of the spring or having to lower the environment temperature. We derive a theoretical relationship between the thermal fluctuations of an oscillator and an actively external-dissipative force. This relationship is verified by experiments with an AFM cantilever where the external active force is coupled through a magnetic field. With simple instrumentation, we have reduced the thermal noise amplitude of the cantilever by a factor of 3.4, achieving an apparent temperature of 25 K with the environment at 295K. This active noise reduction approach can significantly improve the accuracy of static position or static force measurements in a number of practical applications.

Liang, Shoudan↗
NASA NTRS1991

An inverse dynamic tracking control for bracing a flexible manipulator

The goal of this research is to produce a braced manipulator that is flexible. In order to brace the manipulator against a surface, a tip position tracking control becomes essential to avoid large impacts on contact, and to control the force after contact. If we consider the characteristics of the flexible manipulator, the control task may be characterized as a tracking control of a nonlinear, noncolocated, nonminimum-phase system with uncertainties. First, the author introduces the staged positioning concept and provides a conceptual background for the bracing strategy. Second, a single link flexible manipulator is modeled by the assumed-modes method, and the validity of the model is verified with experimental results. Third, a time domain inverse dynamic method is proposed to cancel out the nonminimum-phase positive zeros which cannot be cancelled by feedback controls. Fourth, a tracking controller is designed to control the end point of a flexible manipulator without any overshoot or residual vibrations. Its perfect tracking or asymptotic tracking performance is analyzed and discussed for zero and non-zero initial condition cases. Finally, impact phenomena are investigated, and contact force control of a flexible bracing manipulator is presented to set up a control scenerio for such a manipulator. The proposed control scheme is implemented on a single link flexible manipulator, and analyzed and evaluated with simulations and experiments.

Source record↗
NASA NTRS1992

Active control of sound transmission/radiation from elastic plates by vibration inputs. II - Experiments

Actively controlled harmonic force inputs were applied experimentally to reduce the sound transmitted through an elastic circular plate. The control implementation used a time domain least mean square adaptive algorithm with two error sensors. The control forces were applied directly to the plate by point force vibration inputs, while the error information and performance were measured in the radiated acoustic field by microphones. Test cases were also performed in which the error sensors were accelerometers mounted on the plate. When accelerometers were used as error sensors, the controller performance was degraded; leading to the conclusion that minimizing plate motion does not necessarily lead to an associated decrease in radiated sound levels. In contrast, the results show excellent attenuation of the transmitted sound field when microphone error sensors were used. This result was consistent over a range of frequencies. In addition, the experimental results are compared to previously derived analytical results and the effect of using a point or global minimization scheme is discussed.

Metcalf, V. L.↗
NASA NTRS1991

Robot vibration control using inertial damping forces

This paper concerns the suppression of the vibration of a large flexible robot by inertial forces of a small robot which is located at the tip of the large robot. A controller for generating damping forces to a large robot is designed based on the two time scale model. The controller does not need to calculate the quasi-steady variables and is efficient in computation. Simulation results show the effectiveness of the inertial forces and the controller designed.

Lee, Soo Han↗
NASA NTRS1989

Robot vibration control using inertial damping forces

The suppression is examined of the vibration of a large flexible robot by inertial forces of a small robot which is located at the tip of the large robot. A controller for generating damping forces to a large robot is designed based on the two time scale mode. The controller does not need to calculate the quasi-steady state variables and is efficient in computation. Simulation results show the effectiveness of the inertial forces and the controller designed.

Lee, Soo Han↗
NASA NTRS1992

Force analysis of magnetic bearings with power-saving controls

Most magnetic bearing control schemes use a bias current with a superimposed control current to linearize the relationship between the control current and the force it delivers. For most operating conditions, the existence of the bias current requires more power than alternative methods that do not use conventional bias. Two such methods are examined which diminish or eliminate bias current. In the typical bias control scheme it is found that for a harmonic control force command into a voltage limited transconductance amplifier, the desired force output is obtained only up to certain combinations of force amplitude and frequency. Above these values, the force amplitude is reduced and a phase lag occurs. The power saving alternative control schemes typically exhibit such deficiencies at even lower command frequencies and amplitudes. To assess the severity of these effects, a time history analysis of the force output is performed for the bias method and the alternative methods. Results of the analysis show that the alternative approaches may be viable. The various control methods examined were mathematically modeled using nondimensionalized variables to facilitate comparison of the various methods.

Johnson, Dexter↗
DOE OSTI2020

Iterative Learning Control for Video-rate Atomic Force Microscopy

We present a control scheme for video-rate atomic force microscopy with rosette pattern. The controller structure involves a feedback internal-model-based controller and a feedforward iterative learning controller. The iterative learning controller is designed to improve tracking performance of the feedback-controlled scanner by rejecting the repetitive disturbances arising from the system nonlinearities. We investigate the performance of two inversion techniques for constructing the learning filter. We conduct tracking experiments using a two-degree-of-freedom microelectromechanical system (MEMS) nanopositioner at frame rates ranging from 5 to 20 frames per second. Furthermore, the results reveal that the algorithm converges rapidly and the iterative learning controller significantly reduces both the transient and steady-state tracking errors. We acquire and report a series of high-resolution time-lapsed video-rate AFM images with the rosette pattern.

42 ENGINEERING↗
NASA NTRS1989

Force-reflective teleoperated system with shared and compliant control capabilities

The force-reflecting teleoperator breadboard is described. It is the first system among available Research and Development systems with the following combined capabilities: (1) The master input device is not a replica of the slave arm. It is a general purpose device which can be applied to the control of different robot arms through proper mathematical transformations. (2) Force reflection generated in the master hand controller is referenced to forces and moments measured by a six DOF force-moment sensor at the base of the robot hand. (3) The system permits a smooth spectrum of operations between full manual, shared manual and automatic, and full automatic (called traded) control. (4) The system can be operated with variable compliance or stiffness in force-reflecting control. Some of the key points of the system are the data handling and computing architecture, the communication method, and the handling of mathematical transformations. The architecture is a fully synchronized pipeline. The communication method achieves optimal use of a parallel communication channel between the local and remote computing nodes. A time delay box is also implemented in this communication channel permitting experiments with up to 8 sec time delay. The mathematical transformations are computed faster than 1 msec so that control at each node can be operated at 1 kHz servo rate without interpolation. This results in an overall force-reflecting loop rate of 200 Hz.

Szakaly, Z.↗
NASA NTRS1992

The optimization of force inputs for active structural acoustic control using a neural network

This paper investigates the use of a neural network to determine which force actuators, of a multi-actuator array, are best activated in order to achieve structural-acoustic control. The concept is demonstrated using a cylinder/cavity model on which the control forces, produced by piezoelectric actuators, are applied with the objective of reducing the interior noise. A two-layer neural network is employed and the back propagation solution is compared with the results calculated by a conventional, least-squares optimization analysis. The ability of the neural network to accurately and efficiently control actuator activation for interior noise reduction is demonstrated.

Cabell, R. H.↗