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At least 1,603 records · Page 89

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NASA NTRS1989

Impedance hand controllers for increasing efficiency in teleoperations

An impedance hand controller with direct force feedback is examined as an alternative to bilateral force reflection in teleoperations involving force contact. Experimentation revealed an operator preference for direct force feedback which provided a better feel of contact with the environment. The advantages of variable arm impedance were also made clear in tracking tests where subjects preferred the larger hand controller inertias made possible by the acceleration feedback loop in the master arm. The ability to decouple the hand controller impedance from the slave arm dynamics is expected to be even more significant when the inertial properties of various payloads in the slave arm are considered.

Carignan, Craig↗

NASA NTRS1987

Effect of control sampling rates on model-based manipulator control schemes

The effect of changing the control sampling period on the performance of the computed-torque and independent joint control schemes is discussed. While the former utilizes the complete dynamics model of the manipulator, the latter assumes a decoupled and linear model of the manipulator dynamics. Researchers discuss the design of controller gains for both the computed-torque and the independent joint control schemes and establish a framework for comparing their trajectory tracking performance. Experiments show that within each scheme the trajectory tracking accuracy varies slightly with the change of the sampling rate. However, at low sampling rates the computed-torque scheme outperforms the independent joint control scheme. Based on experimental results, researchers also conclusively establish the importance of high sampling rates as they result in an increased stiffness of the system.

Khosla, P. K.↗

NASA NTRS1987

Dynamics and control of coordinated multiple manipulators

A technique is presented for controlling multiple manipulators which are holding a single object and therefore form a closed kinematic chain. The object, which may or may not be in contact with a rigid environment, is assumed to be held rigidly by robot end-effectors. The derivation is based on setting up constraint equations which reduce the 6 x n degrees of freedom of a manipulators each having six joints. Additional constraint equations are considered when one or more of the degrees of freedom of the object is reduced due to external constraints. Utilizing the operational space dynamics equations, a decoupling controller is designed to control both the position and the interaction forces of the object with the environment. Finally, simulation results for the control of a pair of two-link manipulators are presented.

Hayati, S. A.↗

NASA NTRS1987

Simple robust control laws for robot manipulators. Part 1: Non-adaptive case

A new class of exponentially stabilizing control laws for joint level control of robot arms is introduced. It has been recently recognized that the nonlinear dynamics associated with robotic manipulators have certain inherent passivity properties. More specifically, the derivation of the robotic dynamic equations from the Hamilton's principle gives rise to natural Lyapunov functions for control design based on total energy considerations. Through a slight modification of the energy Lyapunov function and the use of a convenient lemma to handle third order terms in the Lyapunov function derivatives, closed loop exponential stability for both the set point and tracking control problem is demonstrated. The exponential convergence property also leads to robustness with respect to frictions, bounded modeling errors and instrument noise. In one new design, the nonlinear terms are decoupled from real-time measurements which completely removes the requirement for on-line computation of nonlinear terms in the controller implementation. In general, the new class of control laws offers alternatives to the more conventional computed torque method, providing tradeoffs between robustness, computation and convergence properties. Furthermore, these control laws have the unique feature that they can be adapted in a very simple fashion to achieve asymptotically stable adaptive control.

Wen, J. T.↗

NASA NTRS1989

A torus bifurcation theorem with symmetry

Hopf bifurcation in the presence of symmetry, in situations where the normal form equations decouple into phase/amplitude equations is described. A theorem showing that in general such degeneracies are expected to lead to secondary torus bifurcations is proved. By applying this theorem to the case of degenerate Hopf bifurcation with triangular symmetry it is proved that in codimension two there exist regions of parameter space where two branches of asymptotically stable two-tori coexist but where no stable periodic solutions are present. Although a theory was not derived for degenerate Hopf bifurcations in the presence of symmetry, examples are presented that would have to be accounted for by any such general theory.

Vangils, S. A.↗

NASA NTRS1988

Modelling and control of two coordinated robot arms

Two coordinated robot arms are modeled by considering the two arms as working on the same object simultaneously and as a closed kinematic chain. In both formulations, a novel dynamic control method is used which is based on feedback linearization and simultaneous output decoupling.

Tarn, T. J.↗

NASA NTRS1988

A control scheme for trajectory tracking of robot manipulators

The authors put forward a simple scheme for multivariable control of robot manipulators. The scheme is composed of an inner-loop stabilizing controller and an outerloop tracking controller. The inner loop utilizes a multivariable PD (proportional differential) controller to stabilize the robot by placing the poles at some desired locations. The outer loop uses a multivariable PID (proportional integral differential) controller to achieve input-output decoupling and trajectory tracking. The gains of the PD and PID controllers are related directly to the linearized robot model by simple closed-form expressions. The controller gains are updated online to cope with variations in the robot model for gross motion and payload change. Alternatively, the use of high-gain controllers for gross motion and payload change are discussed.

Tarokh, M.↗

NASA NTRS1988

Vibrational-vibrational coupling in air at low humidities

Calculations of sound absorption in air are traditionally based on the assumption that molecular relaxations in N2 and O2 are independent. In binary mixtures of these two gases, however, they are not independent; rather, molecular relaxation is known to be controlled by a very strong vibrational-vibrational (V-V) coupling, which influences both the relaxation frequencies and the relaxation strengths. This article shows that small concentrations of the air constituents CO2 and H2O, which themselves possess a strong V-V coupling to N2 and O2, serve to decouple the N2 and O2 relaxations. To characterize the N2-O2 coupling a coupling strength is derived which depends upon the constituent concentrations and the related reaction rate constants. It is found that the molecular relaxations associated with N2 and O2 in air experience a gradual transition from strong to weak coupling as the humidity increases beyond approximately 0.001 mole percent.

Zuckerwar, Allan J.↗

NASA NTRS1988

Singular perturbation and time scale approaches in discrete control systems

After considering a singularly perturbed discrete control system, a singular perturbation approach is used to obtain outer and correction subsystems. A time scale approach is then applied via block diagonalization transformations to decouple the system into slow and fast subsystems. To a zeroth-order approximation, the singular perturbation and time-scale approaches are found to yield equivalent results.

Naidu, D. S.↗

NASA NTRS1988

Position and force control of coordinated multiple arms

A technique is presented for controlling multiple manipulators which are holding a single object and therefore form a closed kinematic chain. The object, which may or may not be in contact with a rigid environment, is assumed to be held rigidly by n robot end-effectors. The derivation is based on setting up constraint equations which reduce the 6 x n degrees of freedom of n manipulators each having six joints. Additional constraint equations are considered when one or more degrees of freedom of the object are reduced due to external constraints. Utilizing the operational space dynamic equations, a decoupling controller is designed to control both the position and the interaction forces of the object with the environment. Simulation results for the control of a pair of two-link manipulators are presented.

Hayati, Samad A.↗

NASA NTRS1989

Interaction of a vortex and a premixed flame

The interaction of a vortex structure and a premixed flame is studied. The presence of pressure gradients in the vortex and density gradients in the flame result in a complicated interaction. This interaction has been examined when the flame and vortex are fully coupled and in two special cases where they are decoupled: a frozen flame case and a frozen vortex case. In the frozen flame case the main effect of the flame on the vortex is through the barocline torque term. This has been modeled for high Damkoehler numbers. In the frozen vortex case the main effect, at moderate Damkoehler numbers, is to convect the flame around the vortex. At low Damkoehler numbers, depending on the length scales, pockets of unburned gas can form or the flame structure can be significantly changed. The two frozen cases provide a basis for understanding the full interaction.

Ferziger, Joel H.↗

NASA NTRS1989

Viscous-inviscid interaction and local grid refinement via truncation error injection

A methodology is presented which makes it possible to decouple a complex problem having multiple disparate length scales into problems of single length scale so that they can be solved more efficiently on a computer. The method is applied to a viscous transonic flow over an airfoil. It is found that accurate prediction of the flow over an airfoil can be obtained by solving the Euler equations on a relatively coarse global grid with viscous effects computed separately on a boundary-layer type grid and injected into the global grid solution as a combination of vorticity and trucation error.

Goble, Brian D.↗

NASA NTRS1989

Global sensitivity analysis in control-augmented structural synthesis

In this paper, an integrated approach to structural/control design is proposed in which variables in both the passive (structural) and active (control) disciplines of an optimization process are changed simultaneously. The global sensitivity equation (GSE) method of Sobieszczanski-Sobieski (1988) is used to obtain the behavior sensitivity derivatives necessary for the linear approximations used in the parallel multidisciplinary synthesis problem. The GSE allows for the decoupling of large systems into smaller subsystems and thus makes it possible to determine the local sensitivities of each subsystem's outputs to its inputs and parameters. The advantages in using the GSE method are demonstrated using a finite-element representation of a truss structure equipped with active lateral displacement controllers, which is undergoing forced vibration.

Bloebaum, Christina L.↗

NASA NTRS1988

A novel approach to the modelling and control of flexible robot arms

A general dynamic model of a two-link Euler-Bernoulli beam flexible robot arm is presented in the form of partial-differential-integral equations. Observations are made on important properties of the dynamic model. The resulting infinite-dimensional system is then input-output decoupled and partially linearized by a diffeomorphic state transformation and nonlinear state feedback. The local stability issue is addressed for a one-link flexible robot arm.

Ding, Xuru↗

NASA NTRS1988

Slew-induced deformation shaping

Computed torques for pointing and tracking require compensation for slew-induced structural, forebody/aftbody, or optical train alignment deformations. Thus even if only line-of-sight variables are to be commanded, full state feedback is needed. The solution proposed is to decouple by feedforward of the line-of-sight slew dynamics into the deformation control loop. It is shown how arbitrarily few actuators are needed for such deformation shaping, at the cost of higher differentiability of the reference line-of-sight dynamics. The low-rate, single-axis case is developed in detail, and its extension to high rates and multiple axes by global feedback linearization is outlined.

Dwyer, T. A. W., III↗

NASA NTRS1989

Globally optimal maneuver of distributed systems

This paper examines the globally optimal maneuver of distributed systems undergoing large overall rigid-body motion and small relative elastic motion. Using floating coordinates the rigid-body motion and the elastic motion are decoupled thereby allowing the globally optimal maneuver problem to be separated into components associated with the rigid-body and elastic motions. The maneuvers are performed using distributed maneuvering forces based on modal measurements. The modal measurements are extracted from the physical measurements using modal filters. Rest-to-rest maneuvers of a uniform beam illustrate the decentralized nature of the globally optimal solutions.

Silverberg, Larry↗

NASA NTRS1989

Improvements and applications of a streamwise upwind algorithm

An improved streamwise upwind algorithm has been used to study conical flow fields. In the present method, additional terms have been introduced in the cross-flow direction to prevent solution decoupling in supersonic flows, and the local Mach number is taken into account in order to evaluate the rotated differencing. It is found that the formula captures oblique shock waves in the same manner as Roe's (1986) formula, has good convergence properties, and accurately computes shear flows.

Obayashi, Shigeru↗

NASA NTRS1989

Cometary plasma boundaries

The solar wind starts to interact with comets at distances from the nucleus of several million kilometers. The nature of the interaction changes as a function of cometocentric distance. Several dynamically important boundaries have been observed in the cometary plasma environment by instruments on several spacecraft: (1) the bow shock marks the transition from supersonic to subsonic solar wind flow, (2) the cometopause was observed at a distance of about 100,000 km from Comet Halley where the flow begins to stagnate and where charge exchange with neutrals becomes important, (3) the diamagnetic cavity boundary (i.e., contact surface, ionopause) separates magnetized and unmagnetized cometary plasma, (4) the magnetotail boundary defines the tail lobes, (5) the plasma sheet boundary defines the extent of the plasma sheet, and (6) the density enhancement layer was observed at a distance of 10,000 km from Comet Halley and might be located where the neutrals and plasma thermally decouple.

Cravens, T. E.↗