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Your search keyword '"Calvert, A H"' showing total 7 results
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7 results on '"Calvert, A H"'

6. Propulsion System Development for the Iodine Satellite (iSAT) Demonstration Mission

Author

Polzin, Kurt A, Peeples, Stephen R, Seixal, Joao F, Mauro, Stephanie L, Lewis, Brandon L, Jerman, Gregory A, Calvert, Derek H, Dankanich, John, Kamhawi, Hani, Hickman, Tyler A, Szabo, James, Pote, Bruce, and Lee, Lauren

Subjects
Ground Support Systems And Facilities (Space), Spacecraft Propulsion And Power, Propellants And Fuels, Cybernetics, Artificial Intelligence And Robotics
Abstract

The development and testing of a 200-W iodine-fed Hall thruster propulsion system that will be flown on a 12-U CubeSat is described. The switch in propellant from more traditional xenon gas to solid iodine yields the advantage of high density, low pressure propellant storage but introduces new requirements that must be addressed in the design and operation of the propulsion system. The thruster materials have been modified from a previously-flown xenon Hall thruster to make it compatible with iodine vapor. The cathode incorporated into this design additionally requires little or no heating to initiate the discharge, reducing the power needed to start the thruster. The feed system produces iodine vapor in the propellant reservoir through sublimation and then controls the flow to the anode and cathode of the thruster using a pair of proportional flow control valves. The propellant feeding process is controlled by the power processing unit, with feedback control on the anode flow rate provided through a measure of the thruster discharge current. Thermal modeling indicates that it may be difficult to sufficiently heat the iodine if it loses contact with the propellant reservoir walls, serving to motivate future testing of that scenario to verify the modeling result and develop potential mitigation strategies. Preliminary, short-duration materials testing has thus-far indicated that several materials may be acceptable for prolonged contact with iodine vapor, motivating longer-duration testing. A propellant loading procedure is presented that aims to minimize the contaminants in the feed system and propellant reservoir. Finally, an 80-hour duration test being performed to gain experience operating the thruster over long durations and multiple restarts is discussed.

Published
2015

7. Effects of the availability of accurate proprioceptive information on older adults’ postural sway and muscle co-contraction

Author

Craig, Chesney, Calvert, Glenn H M, Doumas, Michail, Craig, Chesney, Calvert, Glenn H M, and Doumas, Michail

Abstract

During conditions of increased postural instability, older adults exhibit greater lower-limb muscle co-contraction. This response has been interpreted as a compensatory postural strategy, which may be used to i006Ecrease proprioceptive information from muscle spindles or to stiffen the lower limb as a general response to minimise postural sway. The current study aimed to test these two hypotheses by investigating use of muscle co-contraction during sensory transitions that manipulated proprioceptive input. Surface EMG was recorded from the bilateral tibialis anterior and gastrocnemius medialis muscles, in young (aged 18-30) and older adults (aged 68-80) during blind-folded postural assessment. This commenced on a fixed platform (baseline: 2mins), followed by 3-minutes on a sway-referenced platform (adaptation) and a final 3-minutes on a fixed platform again (reintegration). Sensory reweighting was slower in older adults, as shown by a significantly larger and longer postural sway aftereffect once a stable platform was restored. Muscle co-contraction showed similar aftereffects, whereby older adults showed a larger increase in co-contraction once the stable platform had been restored, compared to young adults. This co-contraction aftereffect did not return to baseline until after 1 minute. Our evidence for high muscle co-contraction during the reintroduction of veridical proprioceptive input suggests that increased co-contraction in older adults is not dependent on contemporaneous proprioceptive input. Rather, it is more likely that co-contraction is a general postural strategy used to minimize postural sway, which is increased during this sensory transition. Future research should examine whether muscle co-contraction is typically a reactive or anticipatory response.

Published
2017

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