Sharif Digital Repository

In this work, the 3D proton dose profile is calculated in a homogenous water phantom using a Monte Carlo application developed with the Geant4 toolkit. The effect of the ridge filter material (for SOBP widths of 6, 9 and 12 cm) on the homogeneity of the dose distribution, secondary neutron production and beam efficiency are investigated in a single ring wobbling irradiation system. The energy spectrum of secondary neutrons per primary proton at various locations around the phantom surface is calculated. The simulation revealed that most of the produced neutrons are released at slight angles which enable them to reach the patient and consequently to be hazardous. Also, the homogeneity of the... 

The online condition assessment of circuit breakers (CBs) has been increasingly requested by power utilities in recent years. Trip and close coil current (CC) signature is an effective and noninvasive parameter in CB online condition monitoring. This paper gives an insight into the impacts of the various failures on the coil current waveform, as well as on the CB operation time. The failures and their causes are categorized based on the outcome of these investigations. Finally, a new algorithm using trip/close CC is proposed to detect the mode and cause of CB incipient failures. In this study, the CC patterns are acquired by measurements carried out on (healthy and faulty) CBs of 72.5-kV and... 

Redundancy Allocation Problem (RAP) is one way to increase system reliability. In most of the models developed so far for the RAP, system components are considered to have a binary state consisting of "working perfect" or "completely failed". However, to suit the real-world applications, this assumption has been relaxed in this paper, such that components can have three states. Moreover, a Bi-Objective RAP (BORAP) is modeled for a system with serial subsystems, in which non-repairable tri-state components of each subsystem are configured in parallel and the subsystem works under k-out-of-n policy. Furthermore, to enhance system reliability, technical and organizational activities that can... 

The redundancy allocation problem (RAP) is a useful method to enhance system reliability. In most works involving RAP, failure rates of the system components are assumed to follow either exponential or k-Erlang distributions. In real world problems however, many systems have components with increasing failure rates. This indicates that as time passes by, the failure rates of the system components increase in comparison to their initial failure rates. In this paper, the redundancy allocation problem of a series-parallel system with components having an increasing failure rate based on Weibull distribution is investigated. An optimization method via simulation is proposed for modeling and a... 

The effects of weld nugget size and expulsion on the performance of low carbon steel resistance spot weld have been investigated in the present paper. Failure mode, peak load and failure energy obtained in tensile-shear test have been used to describe the performance of spot weld. The influence of voids and porosity as well as electrode indentation associated with expulsion on peak load and failure energy is discussed. The results showed that although expulsion does not reduce the load carrying capacity of spot welds, it decreases their energy absorption capability which was attributed to the change of failure location due to excessive electrode indentation associated with expulsion. © 2008... 

In this study, the failure analysis of a shaft of a unit, which had been failed during the service, was studied. This shaft was welded to the blades, and their chemical compositions were determined by spectroscopy method. In order to determine the root cause of failure and contribution factors, different investigation methods including visual examination, optical microscopy and scanning electron microscope analysis, and hardness test were carried out. It was concluded that the shaft was failed due to fatigue. The failure was caused by improper welding of blades to the shaft. At the end, in order to prevent or decelerate such failure, some recommended remedies were suggested  

Fast and accurate fault detection is the primary step and one of the most important tasks in fault tolerant converters. In this paper, a fast and simple method is proposed to detect and diagnosis the faulty cell in a cascaded H-bridge multilevel inverter under a short circuit fault. In this method, the reference voltage is calculated using switching control pulses and DC-Link voltages. The comparison result of the output voltage and the reference voltage is used in conjunction with active cell pulses to detect the faulty cell. To achieve this goal, the cell which is active when the Fault signal turns to “0” is detected as the faulty cell. Furthermore, consideration of generating the active... 

Importance measures are integral parts of risk assessment for risk-informed decision making. Because the parameters of a risk model, such as the component failure rates, are functions of time and a perturbation (change) in their values can occur during the mission time, time dependence must be considered in the evaluation of the importance measures. In this paper, it is shown that the change in system performance at time t, and consequently the importance of the parameters at time t, depends on the parameters perturbation time and their value functions during the system mission time. We consider a nonhomogeneous continuous time Markov model of a series-parallel system to propose the... 

In this article, we present a cost-effective solution for switching short-circuit fault diagnosis in a three-phase quasi-Z-source inverter (qZSI). The fault is announced to the processor utilizing a peripheral circuit that covers the fault detection in all switches of the inverter. After the fault detection, a nonmaskable interrupt is activated, cutting the CPU routine, and a fault-diagnosis algorithm is initiated. The exact location of the failed switch is identified through the proposed algorithm. The entire process is accomplished in advance of the critical overcurrent condition, which typically arises after a short-circuit fault. Thanks to this overtake, the proposed technique prevents... 

Paraplegic users of mechanical walking orthoses, e.g. advanced reciprocating gait orthosis (ARGO), often face high energy expenditure and extreme upper body loading during locomotion. We studied the effect of kinematical pattern on the mechanical performance of paraplegic locomotion, in search for an improved gait pattern that leads to lower muscular efforts. A three-dimensional, four segment, six-degrees-of-freedom skeletal model of the advanced reciprocating gait orthosis-assisted paraplegic locomotion was developed based on the data acquired from an experimental study on a single subject. The effect of muscles was represented by ideal joint torque generators. A response surface analysis... 

Public transport is amongst critical infrastructures in modern cities, especially megacities, home to millions of people. The reliability of these systems is highly crucial for both citizens and service providers. If service providers overlook system reliability, a considerable amount of expenses will be wasted. Several factors such as vehicle failure, accident, lack of budget weather factors, and traffic congestion cause unreliability, among which vehicle failure plays a prominent role. The brake system is the most vulnerable and vital component of a public transportation bus. Brake reliability depends on driver's expertise, component quality, passenger loading, line situation, etc.... 

Power transformer inner insulation system is a very critical component. Its degradation may pose apparatus to fail while in service. On the other hand, experimental experiences prove that partial discharges are a major source of insulation failure in power transformers. If the deterioration of the insulation system caused by PD activity can be detected at an early stage, preventive maintenance measures may be taken. Because of the complex structure of the transformer, accurate PD location is difficult and is one of the challenges power utilities are faced with. This problem comes to be vital in open access systems. In this paper a theory for locating partial discharge and its propagation... 

Augmenting the security of cryptographic algorithms by protecting them against side-channel active attacks (and natural faults) is essential in cryptographic engineering. BLAKE algorithm is an efficient hash function which has been developed based on Bernstein's ChaCha stream cipher. Because of the fact that Google has chosen ChaCha along with Bernstein's Poly1305 message authentication code as a replacement for RC4 in TLS for Internet security, BLAKE's implementation is of paramount importance. In this paper, we present high-performance fault detection schemes for BLAKE. Specifically, for the round function, two fault diagnosis approaches are developed and analyzed in terms of error... 

The failure analysis of a ruptured superheater tube after 20. years service in the oil-fueled boiler, as the typical problems in power plants, was investigated. A thin-lipped rupture at failed region was observed in superheater tube. By measuring the tube's wall thicknesses far from failed region, non-uniformity was seen. The suggested main root cause of failure was fireside corrosion of the tube during the service. Because of low grade of used fuel, sodium, sulfur, and vanadium elements were observed at the outer surface, which caused continuously scale formation and reduction of wall thickness, by metal consumption. In addition, it seems that it has been worsened by occurrence of long-term... 

In the present work, failure analysis of an automatic coupler in railway carriages which failed in service and caused derailment, was carried out. An evaluation of the failed coupler was undertaken to assess its integrity, including visual examination, photo documentation, fractography, chemical analysis, hardness measurement, tensile and charpy impact testing and microstructural characterization. Results show that some casting defects (e.g. mis-run, cold lap, hot crack and inclusion) resulted in poor mechanical properties. Mis-runs were also responsible for fatigue crack nucleation from two different locations. Beach marks can be observed on the fracture surface. According to the results of... 

Journal-bearings play a significant role in industrial applications and the necessity of condition monitoring with nondestructive tests is increasing. This paper deals a proper fault detection technique based on power spectral density (PSD) of vibration signals in combination with K-Nearest Neighbor and Support Vector Machine (SVM). The frequency domain vibration signals of an internal combustion engine with three journal-bearing conditions were gained, corresponding to, (i) normal, (ii) corrosion and (iii) excessive wear. The features of the PSD values of vibration signals were extracted using statistical and vibration parameters. The extracted features were used as inputs to the KNN and... 

Predicting the residual-life (RL) of rolling element bearings (REBs) is an essential procedure of rotating machinery condition monitoring, in industries. Since the bearing fault propagation has a complicated stochastic nature, the multi-phase phenomenon is probable to happen in the degradation process. By occurrence of the multi-phase phenomenon, the degradation process experiences different phases, with distinct failure rate in each phase. Therefore, the mentioned phenomenon leads to large error in the prediction results of single-phase degradation models. The main concern of the present paper is to consider the effect of multi-phase phenomenon on the prediction confidence level of REBs. In... 

The purpose of this study was to investigate the main causes of the co-fracture of sixteen connecting bolts of a filter press cylinder-piston system. Stress state of the bolts during the service conditions has been analyzed and the failure reasons were determined from the fractography analysis and gathered information. According to the obtained results, it was concluded that the bolts had failed by the fatigue mechanism. It seems that insufficient torque was used during assembly  

A fully electrically tunable microwave photonic filter is realized by the implementation of delay lines based on frequency-Time conversion. The frequency response and free spectral range (FSR) of the filter can be engineered by a simple electrical tuning of the delay lines. The method has the capability of being integrated on a silicon photonic platform. In the experiment, a 2-Tap tunable microwave photonic filter with a 3-dB bandwidth of 2.55 GHz, a FSR of 4.016 GHz, a FSR maximum tuning range from -354 MHz to 354 MHz and a full FSR translation range is achieved  

In this paper, we introduce a new approach to fault detection and isolation for robot manipulators. Our technique is based on using a new simplified Euler-Lagrange (EL) equation that reduces complexity of the proposed fault detection method. The proposed approach isolates the faults and is capable of handling the uncertainty in manipulator gravity vector. It is shown that the effect of uncalibrated torque sensor measurement is asymptotically rejected in the detection process. A simulation example is presented to illustrate the results. © 2009 IEEE