Mechanical & Fluid Systems

Measuring Volume of Incompressible Liquid in a Rigid Tank

The measurement is unaffected by the shape of the liquid or tank. A technique for measuring the volume of an incompressible liquid in a rigid tank involves measurement of the total volume of gas in those parts of the tank not occupied by the liquid. The volume of liquid is then computed by subtracting V from the total volume of the tank and the associated plumbing.

Posted in: Mechanics, Briefs, TSP

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Vision-Based Maneuvering and Manipulation by a Mobile Robot

Mobility is used to augment limited dexterity. A small mobile robot equipped with a stereoscopic machine vision system and two manipulator arms that have limited degrees of freedom has been given the ability to perform moderately dexterous manipulation autonomously, under control by an onboard computer. The approach taken in this development has been one of formulating vision-based control software to utilize the mobility of the vehicle to compensate for the limitation on the dexterity of the manipulator arms. Although the goal was selected visually, it is tracked onboard using information about its shape; in particular, the target is assumed to be a local elevation maximum (i.e., the highest point within a small patch of area).

Posted in: Mechanics, Briefs, TSP

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Algorithms for Collision Avoidant Formation Flying

A report discusses algorithms for realtime planning of translation paths of multiple spacecraft flying in formation. The algorithm takes account of requirements to avoid collisions while operating within resource constraints (e.g., not calling for an acceleration greater than maximum possible) and striving for optimality (e.g., completing a change of formation in minimum time or at minimum energy cost). The optimality/collision- avoidance problem is formulated as a parameter-optimization problem, in which the translation path of each spacecraft is parameterized by polynomial functions of time. It is shown that this parameterization is the key to the solution of the parameter-optimization problem in that it enables decoupling of the collision-avoidance and accelerationlimit constraints, thereby making it possible to solve the problem in two stages. In the first stage, one constructs feasible paths that satisfy only the collision-avoidance constraints subject to certain optimality criteria. It is shown that the acceleration- limit constraints can be imposed a posteriori to compute the required maneuver duration such that at least one acceleration component is saturated. This also enables construction of paths that require minimum time in the class of solutions being considered.

Posted in: Mechanics, Briefs, TSP

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Integrated Colloid Thrusters for Microspacecraft

A report proposes the development of a microfabricated, integrated colloid thruster as a prototype of devices for propulsion and control of the attitudes of microspacecraft. (In a colloid thruster, a beam of positively charged, microscopic droplets is extracted electrohydrodynamically from a column of liquid and accelerated electrostatically to produce thrust.) Unlike other electrical thrusters, colloid thrusters are amenable to extreme miniaturization. The direction of thrust would be controlled electronically through selective activation of accelerator electrodes, eliminating the need for mechanical gimbals.

Posted in: Mechanics, Briefs, TSP

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Improvements in a Piezoelectrically Actuated Microvalve

A report discusses the continuing development of a normally closed, piezoelectrically actuated valve fabricated mostly by micromachining of silicon. The design and operation of the microvalve as described in the instant report are basically the same as those of the version described in “Improved Piezoelectrically Actuated Microvalve” (NPO-30158), NASA Tech Briefs, Vol. 26, No. 1 (January 2002), page 29. Major elements of design described in both the instant report and the cited prior article include (1) a pressure-aided sealing configuration that contributes to the desired normally-closed mode of operation and (2) knife-edge sealing rings that reduce susceptibility to trapping of particles and the consequent leakage. The report also presents additional information concerning details of design and fabrication, including, notably, additional justification for knife-edge (in contradistinction to blunt-cross-section) sealing rings: The knife-edge sealing rings provide greater sealing pressure at a given sealing force, thereby reducing the leak rate and even making it possible to achieve an adequate seal with a hard seat. A potential additional advantage of the knife-edge/hard-seat design is that contact pressures may be high enough to crush contaminant particles, thereby reducing the leakage attributable to contaminants.

Posted in: Mechanics, Briefs, TSP

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Low-Energy Transfer From Near-Earth to Near-Moon Orbit

A report presents a theoretical approach to designing a low-energy transfer of a spacecraft from an orbit around the Earth to ballistic capture into an orbit around the Moon. The approach is based partly on the one presented in “Low-Energy Interplanetary Transfers Using Lagrangian Points” (NPO-20377), NASA Tech Briefs, Vol. 23, No. 11 (November 1999), page 22. The approach involves consideration of the stable and unstable manifolds of the periodic orbits around the Lagrangian points L1 and L2 of the Sun/Earth and Earth/Moon systems. (The Lagrangian points are five points, located in the orbital plane of two massive bodies, where a much less massive body can remain in equilibrium relative to the massive bodies.)

Posted in: Mechanics, Briefs, TSP

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Software for Designing Actively Controlled Structures

One program offers capabilities heretofore available only in separate programs. SMARTCOM is a computer program for the analysis and design of actively controlled “smart” structures. Typically, an actively controlled “smart” structure incorporates piezoelectric sensors and actuators that are used, in conjunction with an electrical control system, to damp vibrations. As is the case for other structures, the analysis and design of actively controlled “smart” structures is often best accomplished with the help of finite-element computer programs. Unfortunately, prior finite-element codes do not offer coupled analyses of the mechanical, electrical, and thermal properties of “smart”-structure materials. Also, they are not directly linked with control software, making it necessary to use separate finite-element and control programs to analyze controlled structures. Furthermore, the programs used heretofore to design “smart” structures do not offer capabilities for optimization or for probabilistic or fuzzy analysis.

Posted in: Mechanics, Briefs, TSP

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