Mechanical & Fluid Systems

Lightweight, Low-Backlash Robot Wrist With Epicyclic Drive

Several design features contribute synergistically to versatility and efficiency. A unique lightweight wrist with three degrees of rotational freedom has been developed as a prototype of wrists for future anthropomorphic robots that would perform a variety of tasks on Earth and in outer space. The three degrees of freedom (two rolling, one bending) intersect at the center of the wrist. Included in the wrist is a power transmission with an epicyclic ring-gear configuration that enables efficient packaging and provides a wide internal passage through the center of rotation for routing of wires and drive cables. The power transmission combines lowbacklash planetary gearing and a tripleinput differential with a triple-loadpath, cable-driven output stage that generates minimal radial bearing loads and no thrust (that is, axial) bearing loads.

Posted in: Mechanics, Briefs

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Program Computes Pointing Corrections for a Radiotelescope

TLC is a computer program that determines corrections for radiotelescopepointing errors associated with tilts and elastic deformations. These errors occur because for rotation in azimuth, a radiotelescope is mounted on wheels that move on a circular track that deviates from perfect flatness. TLC processes radiotelescope field data through smoothing, filtering, segment-fitting, trend-removal, Fourier-transform, and high-passfiltering algorithms to generate a lookup table that contains the pointing corrections. The field data in question are readouts from four inclinometers, the relative positions of the inclinometers, and readouts from an azimuth-angle encoder. Written in the Matlab software system, TLC is a user-friendly program that provides a graphical user interface that enables even an unfamiliar user to proceed, step by step, to the final result.

Posted in: Mechanics, Briefs, TSP

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Improved Lifetime for Stirling Cryogenic Coolers

Core-technology advances extend maintenance-free operating life of coolers to more than 50,000 hours. A Stirling cooler is a mechanical system that approximates the ideal gas cycle. Often associated with airborne or portable cooling applications that require compact design and low input power, Stirling coolers are suitable for such applications because of their high efficiency. Compared to Gifford-McMahon and Joule-Thomson cycles, Stirling offers more than twice the cooling performance in the capacity range from 1-100 W. The coolers have a variety of military and commercial applications in infrared receivers and thermal imaging equipment.

Posted in: Briefs

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Improved Piezoelectrically Actuated Microvalve

The improvements are intended to ensure less leakage and true normally-closed operation. Figure 1. The Previous Version of the Valve, like the present version, was opened by applying a voltage that caused the piezoelectric actuator to contract slightly.Efforts are underway to implement an improved design of the device described in “Normally Closed, Piezoelectrically Actuated Microvalve” (NPO-20782), NASA Tech Briefs, Vol. 25, No. 1 (January 2001), page 39. To recapitulate: This valve is being developed as a prototype of valves in microfluidic systems and other microelectromechanical systems (MEMS). The version of the valve reported in the cited previous article (see Figure 1) included a base (which contained a seat, an inlet, and an outlet), a diaphragm, and an actuator. With the exception of the actuator, the parts were micromachined from silicon. The actuator consisted of a stack of piezoelectric disks in a rigid housing. To make the diaphragm apply a large sealing force on the inlet and outlet, the piezoelectric stack was compressed into a slightly contracted condition during assembly of the valve. Application of a voltage across the stack caused the stack to contract into an even more compressed condition, lifting the diaphragm away from the seat, thereby creating a narrow channel between the inlet and outlet.

Posted in: Mechanics, Briefs, TSP

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Instrumented Bolt Measures Bending Moments Within Itself

The direction as well as the magnitude of bending can be determined. The Ultrabend bolt is a specially designed bolt instrumented with strain gauges (see figure) that are connected into twin Wheatstone-bridge circuits. The geometric arrangement of the strain gauges is such that by suitable electrical switching of the Wheatstone-bridge circuits, these circuits can be made to either (1) suppress responses to bending and torsional stresses while putting out signals indicative of axial preload or (2) suppress responses to axial and torsional stresses while putting out signals indicative of the magnitude and direction of the bending moment in the bolt. Switching between these two measurement modes is accomplished by use of field-effect transistors controlled by a logic circuit.

Posted in: Mechanics, Briefs

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Launching Payloads Into Orbit at Relatively Low Cost

A report proposes the development of a system for launching payloads into orbit at about one-fifth the cost per unit payload weight of current systems. The system would be based on the formerly secret PILOT microsatellite- launching system developed in response to the Soviet launch of Sputnik-1. The PILOT system was a solid-fuel, aerodynamically spun and spin-stabilized, five-stage rocket with onboard controls including little more than an optoelectronic horizon sensor and a timer for triggering the second and fifth stages, respectively. The proposal calls for four improvements over the PILOT system to enable control of orbital parameters:

Posted in: Briefs, TSP

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Improved Quick-Release Pin Mechanism

An improved quick-release pin mechanism supplants a prior such mechanism in which the pin bears a shear load to hold two objects together. The prior mechanism, of a ball-locking design, can fail when vibrations cause balls to fall out. The load-bearing pin is an outer tube with a handle at one end (hereafter denoted the near end). Within the outer tube is a spring- loaded inner tube that includes a handle at its near end and a pivoting tab at its far end. The pin is inserted through holes in the objects to be retained and the inner tube is pushed against an offset pivot inside the outer tube to make the tab rotate outward so that it protrudes past the outer diameter of the outer tube, and the spring load maintains this configuration so that the pin cannot be withdrawn through the holes. Pushing the handles together against the spring load moves the locking tab out far enough that the tab becomes free to rotate inward. Then releasing the inner-tube handle causes the tab to be pulled into a resting position inside the outer tube. The pin can then be pulled out through the holes.

Posted in: Briefs

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