Fixed Dwell Time Research Articles

A comparative study on the accuracy of an automated multi-parameter high-dose-rate brachytherapy quality assurance system

This study aims to explore the measuring accuracy of an automated high-dose-rate brachytherapy (HDR BT) quality assurance system (hereinafter referred to as OriQA system) that could provide simultaneous measurements of dwell position, dwell time and source strength in a single test. Elekta-Flexitron was selected as the HDR BT afterloader, and measured the farthest transmission distance by source position simulator. Within the effective distance, we set dwell positions in two groups, specifically, plan_1 with an interval of 2.00 cm: 106.00 cm, 108.00 cm, 110.00 cm, 102.00 cm, 104.00 cm, 106.00 cm, 108.00 cm, and plan_2 with an interval of 0.20 cm: 111.40 cm, 111.60 cm, 111.80 cm, 112.00 cm, 112.20 cm, 112.40 cm, 112.60 cm. The measured values collected from OriQA device were compared with the data from traditional HDR BT QA tool (source position check ruler). Three groups of dwell time were set, specifically, plan_1: with fixed dwell time 2.00 s, plan_3 with interval 0.10 s: 3.70 s, 3.80 s, 3.90 s, 4.00 s, 4.10 s, 4.20 s, 4.30 s, and plan_4: fixed dwell time 5.00 s. Source strength was also measured continuously for a period of 10 weeks, and compared with the data collected from the well-chamber. The OriQA system showed reliable performance in the evaluation of measuring accuracy. The adjusted farthest transmission distance was 119.33 cm. And according to the time-position graph, the maximum deviation of dwell position between the measured and planned data was 0.34 mm, which was smaller than that between the ruler and planed data (0.75 mm). The maximum deviation of dwell time between the measured and planned data was 0.05 s. Fitting analysis revealed linear relationship between source strength Sk and the intensity detector response, with R2 = 0.9877, and intensity deviation fluctuated between 1.14% and 2.19%. Compared with the traditional QA tool, the OriQA device provided improved measuring accuracy and efficiency QA procedure for HDR BT.

Stochastic stability of positive Markov jump linear systems with fixed dwell time

This paper studies the stochastic stability of positive Markov jump linear systems with a fixed dwell time. By constructing an auxiliary system that originated from the initial system with state jumps, sufficient and necessary conditions of stochastic stability for positive Markov jump linear systems are obtained with both exactly known and partially known transition rates. The main idea in the latter case is applying a convex combination to convert bilinear programming into linear programming problems. On this basis, multiple piecewise linear co-positive Lyapunov functions are provided to achieve less conservative results. Then state feedback controller is designed to stabilize the positive Markov jump linear systems by solving linear programming problems. Numerical examples are presented to illustrate the viability of our conclusions.

Bumpless Transfer H∞ Anti-Disturbance Control of Switching Markovian LPV Systems Under the Hybrid Switching.

This article focuses on the bumpless transfer H∞ anti-disturbance control problem for switching Markovian LPV systems under a hybrid switching law. A parameter-dependent multiple piecewise disturbance observer-based bumpless transfer control strategy is put forward to reject multiple disturbances and reduce switching bumps. First, a hybrid switching law making full use of determinacy and randomness is proposed to improve the bumpless transfer anti-disturbance level by introducing a fixed dwell time in random switching. Second, a generalized bumpless transfer anti-disturbance specification is given to describe the switching quality at the switching points of switching Markovian LPV systems. Third, a solvability condition is established for the bumpless transfer H∞ anti-disturbance control problem, and a parameter-dependent multiple piecewise disturbance observer-based bumpless transfer controller is designed. Finally, an application example has been supplied to demonstrate the availability of the developed method.

Design of resilient reliable control for uncertain periodic piecewise systems with time-varying delay and disturbances

In this paper, the stabilization problem for a class of periodic piecewise linear systems with uncertainties, time-varying delay, actuator faults and disturbances is discussed. Precisely, the resilient reliable control scheme is designed to stabilize the addressed system with satisfactory disturbance attenuation index. Moreover, the concerned system is a kind of periodic system which consists of many subsystems with constant state dynamics. Based on fixed dwell time approach, the addressed system is divided into many piecewise subsystems and activated periodically. Depending on the division of subintervals, the sufficient conditions are derived by constructing a proper Lyapunov–Krasovskii functional with piecewise time-varying positive definite matrices. Further, the periodic time-varying control gain matrices for individual subsystems are obtained via solving the developed constraints. Specifically, the potential and impact of the developed control scheme is examined by presenting two numerical examples including the periodic piecewise vibration systems.

Dynamic Bayesian Adjustment of Dwell Time for Faster Eye Typing.

Eye typing is a hands-free method of human computer interaction, which is especially useful for people with upper limb disabilities. Users select a desired key by gazing at it in an image of a keyboard for a fixed dwell time. There is a tradeoff in selecting the dwell time; shorter dwell times lead to errors due to unintentional selections, while longer dwell times lead to a slow input speed. We propose to speed up eye typing while maintaining low error by dynamically adjusting the dwell time for each letter based on the past input history. More likely letters are assigned shorter dwell times. Our method is based on a probabilistic generative model of gaze, which enables us to assign dwell times using a principled model that requires only a few free parameters. We evaluate our model on both able-bodied subjects and subjects with a spinal cord injury (SCI). Compared to the standard dwell time method, we find consistent increases in typing speed in both cases. e.g., 41.8% faster typing for able-bodied subjects on a transcription task and 49.5% faster typing for SCI subjects in a chatbot task. We observed more inter-subject variability for SCI subjects.

Source shot noise mitigation in focused ion beam microscopy by time-resolved measurement

Focused ion beam microscopy suffers from source shot noise – random variation in the number of incident ions in any fixed dwell time – along with random variation in the number of detected secondary electrons per incident ion. This multiplicity of sources of randomness increases the variance of the measurements and thus worsens the trade-off between incident ion dose and image accuracy. Repeated measurement with low dwell time, without changing the total ion dose, is a way to introduce time resolution to this form of microscopy. Through theoretical analyses and Monte Carlo simulations, we show that three ways to process time-resolved measurements result in mean-squared error (MSE) improvements compared to the conventional method of having no time resolution. In particular, maximum likelihood estimation provides reduction in MSE or reduction in required dose by a multiplicative factor approximately equal to the secondary electron yield. This improvement factor is similar to complete mitigation of source shot noise. Experiments with a helium ion microscope are consistent with the analyses and suggest accuracy improvement for a fixed source dose by a factor of about 4.

Perovskite ceramic membrane separator with improved biofouling resistance for yeast-based microbial fuel cells

Ceramic-derived components in microbial fuel cells (MFCs) aim to substitute Nafion 117. Ceramic membranes are a cheaper alternative and powders’ tailoring has beneficial flexibility for proper surface morphology, porosity, controlled permeability, and water uptake. In this work, differently from the fine fired ceramics or clay materials, barium-cerium-gadolinium oxides (BCGO) powders, co-doped with lithium (Li) or cobalt (Co), are synthesized at a low temperature and then sintered at 1400 and 1500 °C to form the ceramic surface. Results show that the pore size is always within a few microns, porosity can be varied greatly with firing temperature at fixed dwell time, and the use of Li or Co can give smooth or corrugated porous particles, respectively. The permeability tests show that the BCGO doped with Li cannot control the water flux from yeast extract-peptone-d-glucose medium (YPD) with the yeast, and the absolute amount of biofouling is higher than that of Nafion 117. BCGO doped with 5 mol% Co exhibits good permeability and lowers absolute biofouling due to the unique surface morphology of parent powders. Thus, the ceramic separators based on BCGO doped with Co can be an attractive alternative to expensive Nafion.

Investigations of Process Parameters on the Mechanical Properties of MTV Decoy Flare Pellets for Defence Applications

The Infrared (IR) guided missiles have posed serious threats to aircrafts flying even at considerably high altitude. An effective counter measure to this imminent threat of the heat seeking missiles is IR decoy flares constituting of Magnesium/Teflon/Viton (MTV) based pyrotechnic composition. The provisioning of MTV flares primarily for defence applications globally is controlled by very few foreign firms who have ensured that the manufacturing techniques and performance parameters are closely guarded owing to economic as well as strategic reasons. Hence, there are limited research papers available which illustrates the mechanical pressing of the MTV composition to form pellets and the performance evaluation of these pellets.The present paper is an attempt to study the density of 50 mm diameter pellets by varying the charge mass of pellets and the applied load to compact the MTV composition. The applied load is varied in steps of 1 ton for a fixed pyrotechnic charge mass mixture of 100 gm for a fixed dwell time of 15 seconds. It is observed that the density of pellets increased with increase in application of load up to an applied load of 8 tons. This phenomenon is owing to reduction in porosity of the mixture as well as brittle fracture of magnesium particles facilitating optimisation of density of pellets. Similar phenomenon was observed by varying the charge mass and the optimum density was obtained for a L/D ratio nearing unity. Thus, the range of process parameters which is found to be of interest for development of pellets of this dimension is 100 gm to 140 gm of charge mass and 6 tons to 8 tons of applied load.

Using Variable Dwell Time to Accelerate Gaze-Based Web Browsing with Two-Step Selection

ABSTRACT In order to avoid the “Midas Touch” problem, gaze-based interfaces for selection often introduce a dwell time: a fixed amount of time the user must fixate upon an object before it is selected. Past interfaces have used a uniform dwell time across all objects. Here, we propose a gaze-based browser using a two-step selection policy with variable dwell time. In the first step, a command (e.g., “back” or “select”) is chosen from a menu using a dwell time that is constant across the different commands. In the second step, if the “select” command is chosen, the user selects a hyperlink using a dwell time that varies between different hyperlinks. We assign shorter dwell times to more likely hyperlinks and longer dwell times to less likely hyperlinks. In order to infer the likelihood each hyperlink will be selected, we have developed a probabilistic model of natural gaze behavior while surfing the web. We have evaluated a number of heuristic and probabilistic methods for varying the dwell times using both simulation and experiment. Our results demonstrate that varying dwell time improves the user experience in comparison with fixed dwell time, resulting in fewer errors and increased speed. While all of the methods for varying dwell time resulted in improved performance, the probabilistic models yielded much greater gains than the simple heuristics. The best performing model reduces error rate by 50% compared to 100ms uniform dwell time while maintaining a similar response time. It reduces response time by 60% compared to 300ms uniform dwell time while maintaining a similar error rate.

Comparative study of image contrast in scanning electron microscope and helium ion microscope.

Images of Ga+ -implanted amorphous silicon layers in a 110 n-type silicon substrate have been collected by a range of detectors in a scanning electron microscope and a helium ion microscope. The effects of the implantation dose and imaging parameters (beam energy, dwell time, etc.) on the image contrast were investigated. We demonstrate a similar relationship for both the helium ion microscope Everhart-Thornley and scanning electron microscope Inlens detectors between the contrast of the images and the Ga+ density and imaging parameters. These results also show that dynamic charging effects have a significant impact on the quantification of the helium ion microscope and scanning electron microscope contrast.

Photon-Efficient Computational 3-D and Reflectivity Imaging With Single-Photon Detectors

Capturing depth and reflectivity images at low light levels from active illumination of a scene has wide-ranging applications. Conventionally, even with single-photon detectors, hundreds of photon detections are needed at each pixel to mitigate Poisson noise. We develop a robust method for estimating depth and reflectivity using on the order of 1 detected photon per pixel averaged over the scene. Our computational imager combines physically accurate single-photon counting statistics with exploitation of the spatial correlations present in real-world reflectivity and 3D structure. Experiments conducted in the presence of strong background light demonstrate that our computational imager is able to accurately recover scene depth and reflectivity, while traditional maximum-likelihood based imaging methods lead to estimates that are highly noisy. Our framework increases photon efficiency 100-fold over traditional processing and also improves, somewhat, upon first-photon imaging under a total acquisition time constraint in raster-scanned operation. Thus our new imager will be useful for rapid, low-power, and noise-tolerant active optical imaging, and its fixed dwell time will facilitate parallelization through use of a detector array.

Dwell‐time approach to ℒ 2 ‐induced gain analysis of discrete‐time switched systems subject to actuator saturation

This study investigates ℒ2 -induced gain from the input disturbance to output of discrete-time switched linear systems subject to actuator saturation. With the state feedback gains adopted beforehand, the authors establish the linear matrix inequalities formulation which estimates the restricted ℒ2 -induced gain for this class of switched systems by dwell-time approach. Then the authors investigate the influence of parameter variations on the restricted ℒ2 -induced gain, and present a state feedback design method for a fixed dwell time and an upper bound of restricted ℒ2 -induced gain. In addition, the authors investigate the qualitative relationship between disturbances rejection capability and dwell time. A numerical example is provided to illustrate the effectiveness of our obtained results.

Constrained Optimization of Coincidence Detector Parameters for GPS Acquisition

Acquisition of code division multiple access (CDMA) signals can be carried out using fixed or variable dwell times. The choice of acquisition strategy is a trade-off between performance and computational complexity, and a number of variable parameters (number of dwells, dwell lengths and thresholds) must be carefully designed. In this work, the performance of a variable dwell time coincidence detector is analysed in terms of its mean time to verify the acquisition of Global Positioning System (GPS) L1 C/A signals. A new expression for the variance of the coincidence detector dwell time is derived. A general framework for the optimisation of dwell parameters for the coincidence detector is outlined. Both immediate- and non-immediate reject cases are analysed and compared to the fixed dwell time single dwell strategy, which is, itself, a special case of the coincidence detector. Case-specific numerical optimisation results are illustrated across a range of signal-to-noise ratios and general rules-of-thumb for the best choice of parameters are highlighted.

On the Asymptotic Performance of Threshold-Based Acquisition Systems in Multipath Fading Channels

The asymptotic performance of timing acquisition systems having fixed dwell time in multipath fading channels is investigated. The detrimental effect of the multipath channel fading on the acquisition performance is isolated by considering the asymptotic performance as the average signal-to-noise ratio (SNR) increases. It is found that for any threshold such that the average probability of false alarm is less than a given tolerance, the channel fading results in a lower bound on the asymptotic average probability of miss which is nontrivial for a variety of fading scenarios. A threshold-based direct-sequence spread-spectrum signal acquisition system is considered and it is found that the detrimental effect of channel fading on asymptotic acquisition performance, albeit nontrivial, is not very significant. The asymptotic acquisition performance of two threshold-based acquisition schemes for ultra-wideband (UWB) signals with time-hopping (TH) spreading are also evaluated and compared. For both schemes, the detrimental effect of the channel fading on the asymptotic acquisition performance turns out to be significant.

Multiple Dwell Serial Search: Performance and Application to Direct Sequence Code Acquisition

The technique of multiple dwell serial search is described and analyzed. The advantage of the multiple dwell procedure is that the examination interval need not be fixed, allowing incorrect cells to be quickly discarded, which in turn results in a shorter search time than is possible with a fixed dwell time procedure. This type of search scheme is particularly useful for direct sequence code acquisition in a spread-spectrum communication system. An expression for the generating function is obtained from a flow graph representation of the multiple dwell technique. The generating function is used to develop expressions for the mean and variance of the search time in terms of the following parameters: the dwell times, the detection probability, the false alarm probability, and the false alarm penalty time. Coherent detector characteristics are then used to investigate the performance of the multiple dwell technique for direct sequence code acquisition. It is shown that the multiple dwell procedure can significantly reduce the expected acquisition time from that obtained with a single dwell system. The most significant improvement is obtained by using a two-dwell system. Additional but nominal improvement is gained when more than two dwells are employed.

Performance Analysis for the Expanding Search PN Acquisition Algorithm

An approach is presented for approximating the cumulative probability distribution of the acquisition time of the serial PN search algorithm. The results are applicable to variable as well as fixed dwell time systems. The theory is developed for the case where some a priori information on the PN code epoch is available (reacquisition problem or acquisition of very long codes). The special case of a search over the whole code is also treated. The accuracy of the approximation is demonstrated by comparing with published exact results for the fixed dwell time algorithm.