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  • Other Sources  (7,580)
  • ASTROPHYSICS  (6,235)
  • MECHANICAL ENGINEERING  (1,345)
  • General Chemistry
  • Inorganic Chemistry
  • 1990-1994  (7,580)
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  • 1
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    An ensemble pulsar time (1993)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Millisecond pulsars are galactic objects that exhibit a very stable spinning period. Several tens of these celestial clocks have now been discovered, which opens the possibility that an average time scale may be deduced through a long-term stability algorithm. Such an ensemble average makes it possible to reduce the level of the instabilities originating from the pulsars or from other sources of noise, which are unknown but independent. The basis for such an algorithm is presented and applied to real pulsar data. It is shown that pulsar time could shortly become more stable than the present atomic time, for averaging times of a few years. Pulsar time can also be used as a flywheel to maintain the accuracy of atomic time in case of temporary failure of the primary standards, or to transfer the improved accuracy of future standards back to the present.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center, The 24th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 73-86
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  • 2
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    Control and applications of cooperating disparate robotic manipulators relevant to nuclear waste management (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Remote handling in nuclear waste management requires a robotic system with precise motion as well as a large workspace. The concept of a small arm mounted on the end of a large arm may satisfy such needs. However, the performance of such a serial configuration lacks payload capacity which is a crucial factor for handling a massive object. Also, this configuration induces more flexibility on the structure. To overcome these problems, the topology of bracing the tip of the small arm (not the large arm) and having an end effector in the middle of the chain is proposed in this paper. Also, control of these cooperating disparate manipulators is accomplished in computer simulations. Thus, this robotic system can have the accuracy of the small arm, and at the same time, it can have the payload capacity and large workspace of the large arm.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 8 p
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  • 3
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    Robot vibration control using inertial damping forces (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: 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.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 8 p
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  • 4
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    A time domain inverse dynamic method for the end point tracking control of a flexible manipulator (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: The inverse dynamic equation of a flexible manipulator was solved in the time domain. By dividing the inverse system equation into the causal part and the anticausal part, we calculated the torque and the trajectories of all state variables for a given end point trajectory. The interpretation of this method in the frequency domain was explained in detail using the two-sided Laplace transform and the convolution integral. The open loop control of the inverse dynamic method shows an excellent result in simulation. For real applications, a practical control strategy is proposed by adding a feedback tracking control loop to the inverse dynamic feedforward control, and its good experimental performance is presented.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 28 p
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  • 5
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    Control for two cooperating disparate manipulators (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: This research seeks to provide an advanced force control strategy for a flexible manipulator. Two main issues with force control of flexible arms are discussed: non-collocation and robustness.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 2 p
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  • 6
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    Contact control for advanced applications of light weight arms (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Many applications of robotic and teleoperated manipulator arms require operation in contact and non-contact regimes. This paper deals with both regimes and the transition between them with special attention given to problems of flexibility in the links and drives. This is referred to as contact control. Inverse dynamics is used to plan the tip motion of the flexible link so that the free motion can stop very near the contact surface without collision due to overshoot. Contact must occur at a very low speed since the high frequency impact forces are too sudden to be affected by any feedback generated torques applied to a joint at the other end of the link. The effect of approach velocity and surface properties are discussed. Force tracking is implemented by commands to the deflection states of the link and the contact force. This enables a natural transition between tip position and tip force control that is not possible when the arm is treated as rigid. The effect of feedback gain, force trajectory, and desired final force are of particular interest and are studied. Experimental results are presented on a one link arm and the system performance in the overall contact task is analyzed. Extension to multi-link cases with potential applications are discussed.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 8 p
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  • 7
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    Control of a flexible bracing manipulator (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: Many applications of robotic manipulator arms require operation in contact and noncontact regimes. Control of impact between the arm's tip and the environment has been largely ignored in prior research. The impact phenomena was investigated through simulation and experiment for the realization of the bracing strategy, and the key factors of the behavior were understood well. The approaching velocity is dominant parameter for the magnitude of the impact force. The impact is also affected by the compliance of the environmental surface.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 2 p
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  • 8
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    Control of a small working robot on a large flexible manipulator for suppressing vibrations (1991)
    In:  CASI
    Details
    Publication Date: 2004-12-03
    Description: The short term objective of this research is the completion of experimental configuration of the Small Articulated Robot (SAM) and the derivations of the actuator dynamics of the Robotic Arm, Large and Flexible (RALF). In order to control vibrations SAM should have larger bandwidth than that of the vibrations. The bandwidth of SAM consist of 3 parts; structural rigidity, processing speed of controller, and motor speed. The structural rigidity was increased to a reasonably high value by attaching aluminum angles at weak points and replacing thin side plates by thicker ones. The high processing speed of the controller was achieved by using parallel processors (three 68000 process, three interface board, and one main processor (IBM-XT)). Maximum joint speed and acceleration of SAM is known as about 4 rad/s and 15 rad/sq s. Hence SAM can move only .04 rad at 3 Hz which is the natural frequency of RALF. This will be checked by experiment.
    Keywords: MECHANICAL ENGINEERING
    Type: Manipulation Strategies for Massive Space Payloads; 2 p
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  • 9
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    Stirling engine - Approach for long-term durability assessment (1992)
    In:  Other Sources
    Details
    Publication Date: 2011-12-09
    Description: The approach employed by NASA Lewis for the long-term durability assessment of the Stirling engine hot-section components is summarized. The approach consists of: preliminary structural assessment; development of a viscoplastic constitutive model to accurately determine material behavior under high-temperature thermomechanical loads; an experimental program to characterize material constants for the viscoplastic constitutive model; finite-element thermal analysis and structural analysis using a viscoplastic constitutive model to obtain stress/strain/temperature at the critical location of the hot-section components for life assessment; and development of a life prediction model applicable for long-term durability assessment at high temperatures. The approach should aid in the provision of long-term structural durability and reliability of Stirling engines.
    Keywords: MECHANICAL ENGINEERING
    Type: In: IECEC '92; Proceedings of the 27th Intersociety Energy Conversion Engineering Conference, San Diego, CA, Aug. 3-7, 1992. Vol. 5 (A93-25851 09-44); p. 5.209-5.214.
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  • 10
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    Modal simulation of gear box vibration with experimental correlation (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-12-09
    Keywords: MECHANICAL ENGINEERING
    Type: Journal of Propulsion and Power (ISSN 0748-4658); 9; 2; p. 301-306.
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