MEI Technologies Inc.

Greenbelt, MD, United States

MEI Technologies Inc.

Greenbelt, MD, United States
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England S.,MEI Technologies Inc. | Rajulu S.,NASA
Advances in Intelligent Systems and Computing | Year: 2017

NASA’s endeavors in human spaceflight rely on extensive volumes of human-systems integration requirements to ensure mission success. These requirements protect space hardware accommodation for the full range of potential crewmembers, but cannot cover every possible action and contingency in detail. This study was undertaken in response to questions from various strength requirement users who were unclear how to apply idealized strength requirements that did not map well to the complex loading scenarios that crewmembers would encounter. Three of the most commonly occurring questions from stakeholders were selected to be investigated by human testing and human modeling. Preliminary findings indicate that deviation from nominal postures can affect compliance with strength requirements positively or negatively, depending on the nature of the deviation. Human modeling offers some avenues for quickly addressing requirement verification questions, but is limited by the fidelity of the model and environment. © Springer International Publishing Switzerland 2017.


Batson C.D.,MEI Technologies Inc. | Brady R.A.,Wyle | Peters B.T.,Wyle | Ploutz-Snyder R.J.,Universities Space Research Association | And 3 more authors.
Experimental Brain Research | Year: 2011

Recent evidence shows that the ability to adapt to a novel discordant sensorimotor environment can be increased through prior training. We aimed to determine whether gait adaptability could be increased and then retained using a training system comprised of a treadmill placed on a motion base facing a virtual visual scene that provided a variety of sensory challenges that served as training modalities. Ten healthy adults participated in three training sessions during which they walked on a treadmill at 1.1 m/s while receiving discordant support-surface and visual manipulations. Upon completion, participants were presented with a novel sensorimotor challenge designed to test for transfer of adaptive skills. During this test, stride frequency, reaction time, and heart rate data were collected as measures of postural stability, cognitive load, and anxiety, respectively. Compared to 10 untrained controls, trained participants showed enhanced overall performance on the Novel Test, which was administered 20 min after their final training session. Subjects in both groups had greater stride frequency, reaction time, and heart rate when exposed to the new sensory environment; however, these increases were less pronounced in the trained subjects than in the controls. The Novel Test was re-administered to both groups 1 week, 1 month, 3 months, and 6 months later. Trained subjects maintained their level of performance for 6 months. Untrained subjects continued to improve in these measures at each subsequent test session, suggesting that a lasting sensorimotor adaptability training effect can be achieved with very short, repeated exposures to discordant sensory conditions. © 2011 Springer-Verlag (outside the USA).


Brady R.A.,Wyle | Peters B.T.,Wyle | Batson C.D.,MEI Technologies Inc. | Ploutz-Snyder R.,Universities Space Research Association | And 2 more authors.
Experimental Brain Research | Year: 2012

As part of a larger gait adaptability training study, we designed a program that presented combinations of visual flow and support-surface manipulations to investigate the response of healthy adults to walking on a treadmill in novel discordant sensorimotor conditions. A visual dependence score was determined for each subject, and this score was used to explore how visual dependency was linked to locomotor performance (1) during three training sessions and (2) in a new discordant environment presented at the conclusion of training. Performance measures included reaction time (RT), stride frequency (SF), and heart rate (HR), which respectively served as indicators of cognitive load, postural stability, and anxiety. We hypothesized that training would affect performance measures differently for highly visually dependent individuals than for their less visually dependent counterparts. A seemingly unrelated estimation analysis of RT, SF, and HR revealed a significant omnibus interaction of visual dependency by session (p < 0.001), suggesting that the magnitude of differences in these measures across training day 1 (TD1), training day 3 (TD3), and exposure to a novel test is dependent on subjects' levels of visual dependency. The RT result, in particular, suggested that highly visually dependent subjects successfully trained to one set of sensory discordant conditions but were unable to apply their adapted skills when introduced to a new sensory discordant environment. This finding augments rationale for developing customized gait training programs that are tailored to an individual. It highlights one factor-personal level of visual dependency-to consider when designing training conditions for a subject or patient. Finally, the link between visual dependency and locomotor performance may offer predictive insight regarding which subjects in a normal population will require more training when preparing for specific novel locomotor conditions. © 2012 Springer-Verlag.


Wang B.L.,MEI Technologies Inc.
9th Annual International Energy Conversion Engineering Conference, IECEC 2011 | Year: 2011

This paper describes a novel fault detection technique for hybrid DC/DC converter oscillation diagnosis. The technique is based on principles of feedback control loop oscillation and RF signal modulations, and is realized by using signal spectral analysis. Realcircuit simulation and analytical study reveal critical factors of the oscillation and indicate significant correlations between the spectral analysis method and the gain/phase margin method. A stability diagnosis index (SDI) is developed as a quantitative measure to accurately assign a degree of stability to the DC/DC converter. This technique is capable of detecting oscillation at an early stage without interfering with DC/DC converter's normal operation and without limitations of probing to the converter.


Weachock R.J.,NASA | Liu D.,MEI Technologies Inc.
Proceedings - CARTS International 2013: The 33rd Symposium for Passive Electronic Components | Year: 2013

Leakage current measurements of BaTiO3-based X7R multilayer ceramic capacitors (MLCCs) with base-metal electrodes (BMEs) have revealed three distinct failure modes: avalanche breakdown (ABD), thermal runaway (TRA), and slow degradation. Failure analysis (FA) was performed for a number of BME capacitors that failed with the aforementioned three failure modes. The samples that failed with ABD had damage sites that were easily found and that were characterized by the existence of incompletely burned binder particles that were surrounded by transverse cracks that extended through several layers of electrodes from the damaged site, clearly a sequence caused by thermal damage. The samples that failed with TRA also had a particle-like processing flaw with high carbon content, but the flaw was smaller than that of the ABD failure samples. The failure site was also surrounded with extensive transverse cracks that extended through many dielectric layers. Degraded dielectric and conglomerates of nickel spheres were also revealed, indicating a severe thermal event that generated excessive heat and that resulted in the melting of the local dielectric and electrodes. There is no fundamental physical difference between ABD and TRA failures for BME capacitors. The failure mode is a combination of ABD and TRA and is referred to as "catastrophic." The failure analysis on the samples that failed with a slow degradation indicates a failure process that can be described as follows: The electromigration of oxygen vacancies not only gives rise to a gradual increase of leakage current against stress time, but also changes the initial stoichiometry of BaTiO3 grains and causes the local hollowing and melting of dielectric grains and the formation of cracks. The molten dielectric dissolves the internal nickel electrodes and transports the nickel along the transverse cracks, which causes the resistive short. Some of the cracks with dielectric degradation will eventually result in a catastrophic failure. The failure process involves a localized high temperature that can melt both dielectric and nickel. There was no evidence of nickel migration in the BME capacitors, even under highly accelerated life stress conditions. The failure analysis indicates that failure mechanism in BME capacitors with BaTiO3 dielectrics can be more accurately described as a two-stage dielectric wearout that begins with a slow dielectric degradation, characterized by a gradual increase in leakage current with stress time, and followed by a thermally dominated catastrophic breakdown (either ABD or TRA).


Liu D.,MEI Technologies Inc.
Proceedings - CARTS International 2013: The 33rd Symposium for Passive Electronic Components | Year: 2013

An improved highly accelerated life stress testing (HALST) and modeling method was developed and applied to evaluate the reliability of BME capacitors. General reliabilities of multilayer ceramic capacitors (MLCCs) with precious-metal electrodes (PMEs) and base-metal electrodes (BMEs) are discussed. A combination of leakage current and mean-time-to-failure (MTTF) measurements under accelerated life stress conditions have been used to distinguish and separate the MTTF data into two failure groups: slow degradation and catastrophic. The slow degradation failures, characterized by a near-linear leakage increase against stress time, fit well to an exponential model over an applied field. A characteristic exponential growth time, TJZ, is defined to describe the reliability life of this failure mode. The two separated MTTF data groups have been fitted to the 2-parameter Weibull model. When data points in the catastrophic subset are used for reliability modeling, the data points of the slow degradation subset are treated as suspensions, and vice versa. MTTF of most BME capacitors reveals an exponential dependence on an applied electric field due to the mixed failure modes. The initial MTTF data for slow degradation failures appears to follow the exponential law, and that for catastrophic failures follows the conventional power law. The reliability model developed with respect to mixed failure modes and acceleration factors agrees well with the HALST results - not only with the MTTF data, but also with the failure modes (catastrophic or slow degradation). BX life has been used to replace MTTF for predicting the reliability life of BME capacitors at 125°C and 2× rated voltage (Vr), the condition that all MLCCs are subject to pass at at least 1, 000 hours life test for consideration for high-reliability space applications. This B0.8 approach can be used to select BME capacitors that exhibit the potential for passing the regular life test when evaluated using the quick turnaround HALST method developed in this work.


Krainak M.A.,NASA | Sun X.,NASA | Yang G.,NASA | Lu W.,MEI Technologies Inc.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

Silicon avalanche photodiode (APD) detectors have been used in most space lidar receivers to date with a sensitivity that is typically hundreds of photons per pulse at 1064 nm, and is limited by the quantum efficiency, APD gain noise, dark current, and preamplifier noise. We have purchased and tested InGaAs avalanche photodiode based receivers from several US vendors as possible alternatives. We present our measurement results and a comparison of their performance to our baseline silicon APD. Using a multichannel scalar instrument, we observed undesired dark counts in some devices, even though the APDs were biased below the breakdown voltage. These effects are typically associated with over-biased Geiger-mode photoncounting, but we demonstrate that the probability distribution indicates their necessity at the high gains typically associated with operation slightly below the breakdown voltage. We measured the following parameters for our 0.8 mm diameter baseline silicon APD receiver: excess noise factor 2.5, bandwidth 210 MHz, minimum detectable pulse (10 ns) in incident photons 110 photons, noise equivalent power 30 fW/rt-Hz. We present our test procedures and results for the InGaAs based APD receivers. © 2010 Copyright SPIE - The International Society for Optical Engineering.


Donhang Liu D.,MEI Technologies Inc.
CARTS USA 2011 | Year: 2011

Power-on failure has been the prevalent failure mechanism for solid tantalum capacitors in decoupling applications. A surge step stress test (SSST) has been previously applied to identify the critical stress level of a capacitor batch to give some predictability to the poweron failure mechanism [1]. But SSST can also be viewed as an electrically destructive test under a time-varying stress (voltage). It consists of rapidly charging the capacitor with incremental voltage increases, through a low resistance in series, until the capacitor under test is electrically shorted. When the reliability of capacitors is evaluated, a highly accelerated life test (HALT) is usually adopted since it is a time-efficient method of determining the failure mechanism; however, a destructive test under a time-varying stress such as SSST is even more time efficient. It usually takes days or weeks to complete a HALT test, but it only takes minutes for a time-varying stress test to produce failures. The advantage of incorporating a specific time-varying stress profile into a statistical model is significant in providing an alternative life test method for quickly revealing the failure mechanism in capacitors In this paper, a time-varying stress that mimics a typical SSST has been incorporated into the Weibull model to characterize the failure mechanism in different types of capacitors. The SSST circuit and transient conditions for correctly surge testing capacitors are discussed. Finally, the SSST was applied for testing Ta capacitors, polymer aluminum capacitors (PA capacitors), and multi-layer ceramic (MLC) capacitors with both precious metal electrodes (PME) and base metal electrodes (BME). The test results are found to be directly associated with the dielectric layer breakdown in Ta and PA capacitors and are independent of the capacitor values, the way the capacitors were built, and the capacitors' manufacturers. The test results also show that MLC capacitors exhibit surge breakdown voltages much higher than the rated voltage and that the breakdown field is inversely proportional to the dielectric layer thickness. The SSST data can also be used to comparatively evaluate the voltage robustness of capacitors for decoupling applications.


Allen G.R.,Jet Propulsion Laboratory | Adell P.C.,Jet Propulsion Laboratory | Chen D.,MEI Technologies Inc. | Musil P.,M.S. Kennedy Corporation
IEEE Transactions on Nuclear Science | Year: 2012

Recommendations are provided for Single Event Transient (SET) testing of low dropout (LDO) PNP series linear voltage regulators. A combination of SPICE circuit simulations and pulsed laser irradiations are used to demonstrate that the equivalent series resistance (ESR), loading conditions and regulator stability are the key elements that govern LDO regulator's SET response. Pulsed laser testing of several flight-like candidates shows similar SET trends and dependences. Due to the additional circuits introduced in an LDO design, we reveal the existence of a thermal shutdown mode that can be triggered by a single event that is also load dependent. © 1963-2012 IEEE.


Rogers J.S.,MEI Technologies Inc.
Proceedings of the ACM SIGAda Annual International Conference; SIGAda | Year: 2011

The programming languages currently most popular among software engineers for writing safety critical applications are C and, more recently, C++. The Ada language has been designed with software safety in mind. Although Ada is not perfect concerning safety critical programming, it is far better than C or C++. There have been definitions of subsets of C for safety critical applications, such as MISRA C. Similarly, there are several attempts at defining a safe subset of C++, including MISRA C++ and the Joint Strike Fighter (JSF) Avionics C++ coding standards. The most commonly used safety critical subset of Ada is SPARK. SPARK provides a statically provable fully deterministic subset of Ada. The C and C++ safety critical subsets attempt to achieve a level of safety similar to the full Ada language. That attempt generally fails. This paper concentrates on a comparing the C++ language, including portions of the JSF C++ standard and those features inherited from C, with the full Ada language. © 2011 ACM.

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