Dead-time Characteristics Research Articles

Determination of Compensation Capacitor Considering the Dead-Time Characteristics for ZVS in Wireless Power Transfer System

In the Wireless Power Transfer (WPT) system, the input impedance characteristics change depending on the load and the air-gap between the coils. Changes in input impedance affect zero voltage switching (ZVS) and dead-time related to stability, efficiency, and electromagnetic interference. Existing studies lack the dead-time analysis according to operating conditions. In addition, this resulted in unnecessary power dissipation as it cannot determine an appropriate compensation capacitance. Thus, it is important to analyze dead-time and determine the compensation capacitance accordingly for ZVS. This paper presents a compensation capacitor selection method for ZVS in a WPT system with various operating conditions based on dead-time analysis. The compensation capacitor that should be adjusted is identified, and its value is calculated considering dead-time, output power, and efficiency. Therefore, the proposed method can stably implement the WPT system and improve the power and overall system efficiency. A 500W WPT system is designed to validate the theoretical analysis. The designed WPT system is measured under various coupling coefficients and load conditions. It is confirmed that the proposed method implements ZVS under various conditions of the WPT system. Experiments show that the proposed method improves overall system efficiency by up to 3.75% and reduces dead-time by 77%.

Process control in multi-stage flash desalination industry

Process control is an essential part of the desalination industry that requires to be operated under optimum conditions to increase the lifetime of the plant and reduce the unit product cost. Improved process control is a cost-effective approach to energy conservation and increased process profitability. The Multi-Stage Flash (MSF) plant involves many complicated operations related to steam, chemicals and seawater. These include variable capacity, slow dynamics, deadtime characteristics due to certain load changes, significant effects of small deviations from design conditions on plant operation, effects of power plant output conditions on the desalination plant, instability due to disturbances in steam supply and water temperature variations. Keeping in view the above criticalities, the selection of an effective control system becomes inevitable. This paper aims at identifying various types of control loop available in an MSF plant, selection of control elements, type of control strategy needed for it and integrating the whole system for supervisory control.

Slat collimator design issues for dual-head coincidence imaging systems

This paper investigates optimum slat collimator design parameters for dual-head coincidence imaging (DHCI) systems. The noise equivalent count (NEC) rate was examined with respect to the activity concentration under various system conditions. All results are derived from Monte Carlo simulations with a digital anthropomorphic (Zubal) phantom. The DHCI system was modeled after the Millennium VG gamma camera (GEMS, Waukesha, WI). The dead-time characteristics of the camera were experimentally determined. Our results suggests that substantial NEC gains can be achieved by varying the slat-to-slat separation, such that the peak of the NEC curve is located at clinically relevant levels (i.e., 0.07 /spl sim/ 0.10 /spl mu/Ci/cc). The NEC was also found to increase with the use of longer slats with appropriately selected slat-to-slat separation. Furthermore, the NEC performance also depends on the count-rate performance (i.e., dead-time losses) of the system. Therefore, as improvements are made to the count-rate capabilities of DHCI systems, the slat geometry should be modified. Further study is required to determine the effect that slat collimator design has on image quality and lesion detection for clinically realistic imaging situations.

The effect of beam time structure on counting detectors in SRS experiments

Counting detector systems are increasingly used in X-ray experiments because of their attractive properties as regards linearity, large dynamic range and simple noise properties. In synchrotron radiation source (SRS) applications of X-ray detectors, counting rates are generally high enough to require dead-time correction. The time structure of an SRS beam interacts with the dead-time characteristics of the detector in a way that the simple stochastic dead-time models cannot always handle. This report generates analytical and Monte Carlo mathematical models which describe the rate performance of any given detector system when used with the typical beam structures encountered in an SRS.

Investigation of deadtime characteristics for simultaneous emission-transmission data acquisition in PET

Modern PET systems generally utilize rotating rod sources and sinogram windowing for transmission scan and are also usually capable of simultaneous emission-transmission data acquisition. The count rate capability of a PET system using BGO is primarily limited by the deadtime of the block detectors. This deadtime is directly related to the singles event rate of the system. In a simultaneous emission-transmission acquisition, the moving rod sources generate a spatially varying singles distribution which rotates synchronously with the rods. The emission activities tend to create a nearly uniform singles distribution which is static or varies slowly depending on the half life of the radioisotope. In this study, the system deadtime is characterized for different sources of activity in a simultaneous acquisition. Typically, there are 60 to 150 MBq Ge-68 in each of the three rods causing a 26% peak-to-peak variation in the instantaneous detector deadtime. The time-averaged deadtime in a transmission sinogram has a variation of 30% peak-to-peak. Rod activities create a 4 to 10% peak-to-peak variation in the time-averaged deadtime in the emission sinogram of a simultaneous scan. The current deadtime correction method results in a 10% error in a simultaneously acquired emission sinogram with 60 MBq in each rod and 74 MBq in a plane source. A spatially variant deadtime correction technique was developed for simultaneous scans. The error was reduced to 4% for the same study.

Investigation of count rate and deadtime characteristics of a high resolution PET system.

Count rate and deadtime characteristics were investigated for a whole body positron emission tomography system by measuring prompt, delayed, and multiple (three or more detectors) coincidence rates and single detector rates as a function of input count rate by imaging a variety of short-lived positron emitting sources as a function of time. Data were collected with cylinder, ring, and point sources for a range of energy thresholds and fields of view. The largest source of deadtime loss involved processes that led to multiple coincidences, which are primarily true or accidental events in coincidence with an unrelated event. In measuring the count rate as a function of time for each type of event, components, with decay constants of 1, 2, or 3 times that of the isotope being measured, could be resolved corresponding to 1 (true events), 2 (accidental and some multiple events), or 3 (multiple events) independent nuclear disintegrations, respectively. Analysis of true, accidental, and multiple coincidence and singles count rate data allowed identification and evaluation of the magnitude of the sources of deadtime losses and provided a basis for a deadtime correction from data available from the PET system.

Application of Channel Multiplier Plates as Image Information Preprocessors

The dead time of a channel plate has been regarded so far as an inevitable defect in many applications; however, it is pointed out in this paper that it can be considered as the memory time during which the input image is stored. Line drawing, moving object detection, and inversion of images were performed by an experimental preprocessor using the dead time characteristics. The device should be recognized as a two-dimensional operator with no scanning and without an electronic computer. The features of the device are very high-speed operation and high sensitivity for dark images.