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NASA NTRS1989

Motion and force control for multiple cooperative manipulators

The motion and force control of multiple robot arms manipulating a commonly held object is addressed. A general control paradigm that decouples the motion and force control problems is introduced. For motion control, there are three natural choices: (1) joint torques, (2) arm-tip force vectors, and (3) the acceleration of a generalized coordinate. Choice (1) allows a class of relatively model-independent control laws by exploiting the Hamiltonian structure of the open-loop system; (2) and (3) require the full model information but produce simpler problems. To resolve the nonuniqueness of the joint torques, two methods are introduced. If the arm and object models are available, the allocation of the desired end-effector control force to the joint actuators can be optimized; otherwise the internal force can be controlled about some set point. It is shown that effective force regulation can be achieved even if little model information is available.

Wen, John T.↗

NASA NTRS1989

Bandwidth-limited robust nonlinear sliding control of pointing and tracking maneuvers

It is shown how multiaxial spacecraft tracking and pointing maneuvers with known control bandwidth and given tracking error bounds can be implemented by variable-structure control in the presence of uncertain vehicle and target dynamics. It is shown how to select a nonlinear sliding surface relating attitude and rate variables, as well as a Lyapunov function in the surface variables that absorbs multiplicative model undertainties, thereby simplifying the computation of control corrections. It is then shown how a boundary layer envelope can be designed, within which the components of the surface error dynamics can be modeled as the outputs of designer-selected decoupled low-pass filters. Closed-loop stability conditions accounting for the coupling between the attitude error dynamics and the surface error dynamics are then obtained.

Dwyer, Thomas A. W., III↗

NASA NTRS1990

Balanced-Bridge Feedback Control Of Motor

Sensitivity to variations in electrical and mechanical characteristics reduced. Proposed control system for motor-driven rotary actuator includes three nested feedback loops which, when properly designed, decoupled from each other. Intended to increase accuracy of control by mitigating such degrading effects as vibrations and variations in electrical and mechanical characteristics of structure rotated. Lends itself to optimization of performance via independent optimization of each of three loops. Includes outer, actuator, and driver feedback loops, configured so that actuator is subsystem, and driver is subsystem of actuator.

Lurie, Boris J.↗

NASA NTRS1990

Multivariable PID Controller For Robotic Manipulator

Gains updated during operation to cope with changes in characteristics and loads. Conceptual multivariable controller for robotic manipulator includes proportional/derivative (PD) controller in inner feedback loop, and proportional/integral/derivative (PID) controller in outer feedback loop. PD controller places poles of transfer function (in Laplace-transform space) of control system for linearized mathematical model of dynamics of robot. PID controller tracks trajectory and decouples input and output.

Seraji, Homayoun↗

NASA NTRS1989

An iterative method for systems of nonlinear hyperbolic equations

An iterative algorithm for the efficient solution of systems of nonlinear hyperbolic equations is presented. Parallelism is evident at several levels. In the formation of the iteration, the equations are decoupled, thereby providing large grain parallelism. Parallelism may also be exploited within the solves for each equation. Convergence of the interation is established via a bounding function argument. Experimental results in two-dimensions are presented.

Scroggs, Jeffrey S.↗

NASA NTRS1989

General approach and scope

This paper describes a joint activity involving NASA and Army researchers at the NASA Langley Research Center to develop optimization procedures aimed at improving the rotor blade design process by integrating appropriate disciplines and accounting for all of the important interactions among the disciplines. The disciplines involved include rotor aerodynamics, rotor dynamics, rotor structures, airframe dynamics, and acoustics. The work is focused on combining these five key disciplines in an optimization procedure capable of designing a rotor system to satisfy multidisciplinary design requirements. Fundamental to the plan is a three-phased approach. In phase 1, the disciplines of blade dynamics, blade aerodynamics, and blade structure will be closely coupled, while acoustics and airframe dynamics will be decoupled and be accounted for as effective constraints on the design for the first three disciplines. In phase 2, acoustics is to be integrated with the first three disciplines. Finally, in phase 3, airframe dynamics will be fully integrated with the other four disciplines. This paper deals with details of the phase 1 approach and includes details of the optimization formulation, design variables, constraints, and objective function, as well as details of discipline interactions, analysis methods, and methods for validating the procedure.

Adelman, Howard M.↗

NASA NTRS1990

Development of shape memory metal as the actuator of a fail safe mechanism

A small, compact, lightweight device was developed using shape memory alloy (SMA) in wire form to actuate a pin-puller that decouples the flanges of two shafts. When the SMA is heated it contracts producing a useful force and stroke. As it cools, it can be reset (elongated in this case) by applying a relatively small force. Resistive heating is accomplished by running a current through the SMA wire for a controlled length of time. The electronics to drive the device are not elaborate or complicated, consisting of a timed current source. The total available contraction is 3 percent of the length of the wire. This device, the engineering properties of the SMA, and the tests performed to verify the design concept are described.

Ford, V. G.↗

NASA NTRS1990

Development of the CLAES instrument aperture door system

The design, assembly, and test processes followed in developing a space-qualified aperture door system are described. A blackbody calibration source is mounted inside the door, requiring the assembly to open and close a minimum of 150 cycles for instrument recalibration. Within the door system are four separate mechanisms, three of which are redundant; a pyro launch latch, a hinge bearing assembly, and a pair of pivot mechanisms. Decoupling devices within the pivot mechanisms allow an active drive unit to automatically overdrive a failed drive unit. The door is also stowable for possible Shuttle retrieval and re-entry. Illustrations and photographs of the flight hardware help acquaint the reader with the design. The aim is to pass on lessons learned in all phases of developing this spaceflight mechanism.

Stubbs, D. M.↗

NASA NTRS1990

A new six-degree-of-freedom force-reflecting hand controller for space telerobotics

A new 6 degree of freedom universal Force Reflecting Hand Controller (FRHC) was designed for use as the man-machine interface in teleoperated and telerobotic flight systems. The features of this new design include highly intuitive operation, excellent kinesthetic feedback, high fidelity force/torque feedback, a kinematically simple structure, mechanically decoupled motion in all 6 DOF, good back-drivability, and zero backlash. In addition, the new design has a much larger work envelope, smaller stowage volume, greater stiffness and responsiveness, and better overlap of the human operator's range of motion than do previous designs. The utility and basic operation of a new, flight prototype FRHC called the Model X is briefly discussed. The design heritage, general design goals, and design implementation of this advanced new generation of FRHCs are presented, followed by a discussion of basic features and the results of initial testing.

Mcaffee, Douglas↗

NASA NTRS1990

H-infinity based integrated flight-propulsion control design for a STOVL aircraft in transition flight

Results are presented from an application of H-infinity control design methodology to a centralized integrated flight/propulsion control (IFPC) system design for a supersonic Short Take-Off and Vertical Landing (STOVL) fighter aircraft in transition flight. The overall design methodology consists of a centralized IFPC controller design with controller partitioning. Only the feedback controller design portion of the methodology is addressed. Design and evaluation vehicle models are summarized, and insight is provided into formulating the H-infinity control problem such that it reflects the IFPC design objectives. The H-infinity controller is shown to provide decoupled command tracking for the design model. The controller order could be significantly reduced by modal residualization of the fast controller modes without any deterioration in performance. A discussion is presented of the areas in which the controller performance needs to be improved, and ways in which these improvements can be achieved within the framework of an H-infinity based linear control design.

Garg, Sanjay↗

NASA NTRS1990

Numerical investigation of the thermal stratification in cryogenic tanks subjected to wall heat flux

The flow pattern and thermal stratification of a cryogenic cylindrical tank are numerically studied. The tank sidewall is subjected to either a uniform heat-flux or two discrete levels of uniform heat-flux at the upper and lower halves of the tank wall. The tank bottom is kept at a constant temperature controlled by the heat exchanger of a thermodynamic vent system. The tank pressure is also assumed constant resulting in a constant saturation temperature at the interface which is higher than the tank bottom temperature. The effects of vapor motion and vapor superheat on the mass and heat transfer processes at the interface are assumed negligible such that the calculations of liquid region can be decoupled from the vapor region. Dimensionless steady-state conservation equations are solved by a finite-difference method. The effects of modified Rayleigh number, Prandtl number, tank aspect ratio, wall heat-flux parameter, and wall heat-flux distribution on the liquid velocity and temperature fields are investigated. Also, their effects on the rate of heat transfer through the interface and the tank bottom are examined.

Lin, Chin-Shun↗

NASA NTRS1990

Structural analysis of stratocumulus convection

The 1 and 20 Hz data are examined from the Electra flights made on July 5, 1987. The flight legs consisted of seven horizontal turbulent legs at the inversion, midcloud, and below clouds, plus 4 soundings made within the same period. The Rosemont temperature sensor and the top and bottom dewpoint sensors were used to measure temperature and humidity at 1 Hz. Inversion structure and entrainment; local dynamics and large scale forcing; convective elements; and decoupling of cloud and subcloud are discussed in relationship to the results of the Electra flight.

Siems, S. T.↗

NASA NTRS1990

Microwave background distortions from domain walls

Domain walls arising in a cosmic phase transition after decoupling were recently proposed as seeds for the formation of large scale structure. The distortion induced in the microwave background radiation is calculated in dependence of the wall thickness, surface density, scalar field potential, cosmic redshift and the velocity of the wall. It was found that the maximal redshift distortion for both spherical and planar walls is of the order pi G sigma H(sup -1)(sub 0), where sigma is the surface energy density and H(sup -1)(sub 0) the Hubble parameter. It was also found that, for a wall thickness smaller than the horizon, walls can be treated as infinitely thin, i.e., the redshift distortion is independent of the wall thickness and the specific form of the scalar potential. For planar walls moving with a Lorentz-factor gamma the redshift distortion is enhanced by gamma cubed.

Goetz, Guenter↗

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, C.↗

NASA NTRS1989

Thermal state of an ice shell on Europa

The thickness of the ice shell presently hypothesized for Europa, which is decoupled from a silicate core by a liquid water layer, is calculated as a function of colatitude and longitude under suitable assumptions for two plausible ice rheology behaviors: that of Maxwell type, and that of generalized flow-law rheology. Due to the dissipation rate's pronounced temperature dependence, virtually all tidal dissipation is found to concentrate in the lowest few kilometers of the shell. While for some parameter choices an insulating regolith that raises the near-surface temperature by more than a few tens of degrees C may stabilize the shell against polar wander, a modest regolith may enhance the probability of such wander's occurrence through the reduction of the shell's retarding friction.

Ojakangas, Gregory W.↗

NASA NTRS1989

Polar wander of an ice shell on Europa

The present consideration of a hypothesized ice shell around Europa, which is decoupled from the silicate core by a liquid water layer and possesses a spatially varying thermal equilibrium thickness profile, proceeds through the development of equations for variations in the inertia tensor of a body when second-harmonic-degree topography is added to the crustal base. Attention is given to a realistic model in which the shell and ocean are assumed to undergo reorientations as a single entity independently of the core, but subject to viscous dissipation within the shell. Shell friction is in this case noted to preclude polar wander, unless a low conductivity regolith increases the near-surface temperature by a few tens of degrees C; the ice beneath the regolith would then behave viscously on the time-scale of polar wander.

Ojakangas, Gregory W.↗

NASA NTRS1989

Numerical investigation of a plasma beam entering transverse magnetic fields

Two-dimensional electrostatic and electromagnetic particle-in-cell codes are used to investigate plasma beam propagation across transverse magnetic fields. Both large- and small-ion-gyroradius beams are studied in the case of small beam momentum or energy (low drift kinetic beta). Large-ion-gyroradius beams with a large dielectric constant epsilon much greater than (M/m) to the 1/2 (where M/m is the ion-to-electron mass ratio) are found to propagate across the magnetic field via E x B drifts at nearly the initial injection velocity. Beam degradation and undulations are observed, in agreement with previous experimental and analytical results. When epsilon is of order (M/m) to the 1/2 the plasma beam propagates across field lines at only half its initial velocity and loses its coherent structure. When epsilon is much less than (M/m) to the 1/2 the beam particles decouple at the magnetic field boundary, scattering the electrons and deflecting the ions slightly.

Koga, J.↗

NASA NTRS1989

Interstellar matter in early-type galaxies - Optical observations

Results of optical observations of 26 bright elliptical galaxies selected on the basis of IRAS data are discussed. Optical broadband imaging (using B and R filters) and narrow-band imaging (using H-alpha interference filters) have been performed to study dust patches and ionized gas. Long-split spectroscopy has also been made to determine gas kinematics and relative line ratios. The spectroscopic data confirm the presence and distribution of interstellar matter (dust lanes and ionized gas) seen in the direct imaging. Decoupled kinematics of interstellar gas and stars favors an external origin of the interstellar matter. However, for one isolated galaxy, an internal origin is not excluded. The rotation curves determined by optical emission lines are symmetric around the center in most galaxies observed. Galaxy masses and mass-to-light ratios are estimated using the rotation curves of the ionized gas.

Kim, Dong-Woo↗