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NASA NTRS1989

A dual-armed free flyer

This summary presents the main conclusions and results of the design studies conducted by a group of 13 students at the University of Maryland. The students, all participants in the spring 1989 course ENEE418 in the Electrical Engineering Department, met weekly in a two-hour class to discuss and evaluate design alternatives. The main problem considered was the design and control of a planar testbed simulating a free-flying space robot for applications in satellite servicing. This project grew out of the 1988 class where a dual-armed free flyer (DAFF) was designed and partially built. This year, a group of six students continued the development of the DAFF, achieving computer-controlled motion of the DAFF's arms. All fabrication and testing of the DAFF is being conducted in the Intelligent Servosystems Laboratory at the University of Maryland. While the work related to the design and development of the DAFF is the main subject of the report, it should be noted that other students in the ENEE418 class have investigated additional issues related to manipulation in space. For example, one group studied a new parallel linkage based manipulator for fine motion applications such as in assembly operations in space. They investigated the mechanism's kinematics, its reachable workspace, and precision of applying forces and torques. In yet another project, a student set out to measure and map the friction characteristics of the actuators used in the Modular Dextrous Hand, which has been recently developed in the Intelligent Servosystems Laboratory. The results are expected to help compensate for this friction, which is a highly nonlinear disturbance and presents significant problems in high-precision, low-speed operations. This summary continues with the discussion of the results obtained by the group of students who have been working on developing the DAFF testbed.

Beck, D.↗

NASA NTRS1985

A solid-state framework for terrestrial upper mantles

A framework is proposed for understanding the upper mantle structure of the terrestrial planets which is based on solid state dislocation processes. It is proposed that the base of the lithosphere on any planet with a Mg-Fe silicate rich upper mantle is defined by the threshold temperatures of low energy dislocation glide systems in olivine and the pyroxenes. This threshold temperature is approximately 1100 to 1200 K and is directly tied to the mobility of olivine and pyroxene dislocations. Using this definition, all terrestrial planets of the Inner Solar System are expected to have mantle lithospheres. Second, the anomalous properties of the asthenosphere in the Earth are related to the rheological properties of an olivine rich differentiate approximately 220 km thick. All of these properties can be the result of the abundant low energy glide systems in olivine which are mobile at temperatures above approximately 1200 K. It is proposed that a true planetary asthenosphere must be both olivine rich and at temperatures above approximately 1200 K. This mode for the asthenosphere does not require partial melting and is therefore less sensitive to the constraints of volatile content in the upper mantle. The thickness of planetary lithospheres is estimated as is maximum thickness of the olivine rich differentiate on each planet. The cooling of the olivine rich layer on Mars (and hence disappearance of the asthenosphere) may coincide with the cessation of global tectonic activity approximately 1 Gyr ago.

Estey, L.↗