Ahmed Mohamed - Academia.edu (original) (raw)

Papers by Ahmed Mohamed

Abstract In this paper, converter topologies, and their control, that can be used as an interface... more Abstract In this paper, converter topologies, and their control, that can be used as an interface between fuel cells and the DC bus in a DC zonal electric distribution system (DC ZEDS) have been investigated. Performance of the conventional DC-DC boost converter ...

This paper presents two modeling techniques for the prediction and monitoring of the characterist... more This paper presents two modeling techniques for the prediction and monitoring of the characteristics of transformer oil. The first employs artificial neural network (ANN) and the second employs non-linear modeling (nlm). The proposed techniques are implemented for predicting the transformer oil residual operating time (t rot) which is defined as the service period after which the breakdown voltage (BDV) violates the limits given in the standard specifications. The selection of the most influential characteristics on residual operating time (t rot) in the proposed techniques is obtained by statistical analysis. The non-linear model depends on linear combination of non-linear functions for each characteristic. The ANN technique for modeling these characteristics preserves the non-linear relationship between these characteristics and (t rot). The results are compared with previously published modeling techniques namely multiple linear regression and polynomial regression models. Different evaluation indices have been used to justify the superiority of the proposed modeling techniques for predicting (t rot).

In this paper, some of the aspects related to the connectivity of DC microgrids to the main grid ... more In this paper, some of the aspects related to the connectivity of DC microgrids to the main grid are investigated. A prototype system has been designed and implemented to address these aspects. The described system is dependent mainly on sustainable energy sources. Hence, a special care has been given to dealing with this kind of sources while designing different components

ABSTRACT Proportional integral derivative (PID) controllers are usually used to control DC-DC boo... more ABSTRACT Proportional integral derivative (PID) controllers are usually used to control DC-DC boost converters in PV systems. However, they have to be tuned based on certain defined operating range using averaged mathematical models. Loading conditions have great effect on PI controllers; PI controllers are subjected to failure under dramatic load changes. This limits the PI controller's operating range. Moreover, transient and steady state response both get affected by changing the operating range. This paper presents a novel smart-PID controller for optimal control of DC-DC boost converter used as voltage controller in PV systems. This proposed controller maximizes the stable operating range by using genetic algorithms (GA) to tune the PID parameters ultimately at various loading conditions. Then, a fuzzy logic approach is used to add a factor of intelligence to the controller such that it can move among different values of proportional gain (Kp), derivative gain (Kd) and integral gain (Ki) based on the system conditions. This controller allows optimal control of boost converter at any loading condition with no need to retune parameters or possibility of failure. Moreover, the paper presents a novel technique to move between the PI and PID configurations of the controller such that the minimum overshoot and ripple are obtained, which makes the controller very applicable for PV systems supplying sensitive loads. The controlled boost converter is used as an interface between photovoltaic (PV) panels and the loads connected to them. It converts any input voltage within its operating range into a constant output voltage that is suitable for load feeding. The proposed smart controller adapts the duty cycle of the boost converter based on input voltage and loading conditions such that it outputs a constant output voltage. A prototype system has been developed to verify the applicability of the proposed controller. Moreover, simulation and experimental results both confirm its v alidity as an effective and reliable controller for boost converters in PV systems and the possibility to use it in different applications.

ABSTRACT Text summarization is an important problem, which has numerous applications. This proble... more ABSTRACT Text summarization is an important problem, which has numerous applications. This problem has been extensively studied and many approaches have been proposed in the literature for its solution. In this paper, we investigate a new approach that employs meta-search. In particular, summaries from several summarizers are evaluated and a new summary is formed from these summaries. Clearly, this meta-summarizer will produce a better summary than any of the others since it exploits the special features of all the sources. We have employed data from Document Understanding Conference 2002 (DUC-2002) and 5 different summarizers in our experiments. We have also created our own summarizer

Numerous outbreaks of cholera have occurred in Kenya since 1971. To more fully understand the epi... more Numerous outbreaks of cholera have occurred in Kenya since 1971. To more fully understand the epidemiology of cholera in Kenya, we analyzed the genetic relationships among 170 Vibrio cholerae O1 isolates at 5 loci containing variable tandem repeats. The isolates were collected during January 2009-May 2010 from various geographic areas throughout the country. The isolates grouped genetically into 5 clonal complexes, each comprising a series of genotypes that differed by an allelic change at a single locus. No obvious correlation between the geographic locations of the isolates and their genotypes was observed. Nevertheless, geographic differentiation of the clonal complexes occurred. Our analyses showed that multiple genetic lineages of V. cholerae were simultaneously infecting persons in Kenya. This fi nding is consistent with the simultaneous emergence of multiple distinct genetic lineages of V. cholerae from endemic environmental reservoirs rather than recent introduction and spread by travelers.

Physica A Statistical …, 2001

An interesting and challenging research subject in the field of nonlinear dynamics is the study o... more An interesting and challenging research subject in the field of nonlinear dynamics is the study of chaotic behavior in systems of more than two degrees of freedom. In this work we study fixed points, strange attractors, chaotic behavior and the problem of chaos control for complex ...

J. Am. Chem. …, 2005

The discovery of the extraordinary activity exhibited by small gold clusters on certain oxide sup... more The discovery of the extraordinary activity exhibited by small gold clusters on certain oxide supports 1,2 has stimulated considerable research 3-11 to address the origin of this unusual behavior. We report, relevant to this question, a study of the oxidation of CO to CO 2 over ...

Journal of Applied Sciences, 2011

A prospective study. Our purpose was to evaluate prospectively the complications, clinical outcom... more A prospective study. Our purpose was to evaluate prospectively the complications, clinical outcomes, and self-reported quality of life in a relatively homogenous group of adults aged ≥ 40 years undergoing primary surgical treatment for scoliosis. Relatively few reports have examined surgical outcomes in adult patients with scoliosis, especially adults aged ≥ 40 years, whose outcomes may differ because of more rigid curves and more frequent and severe comorbidities. Although most studies have shown patient benefits despite high complication rates after such surgery, most were retrospective and conducted before the introduction of third-generation instrumentation techniques. We prospectively studied a consecutive series of 35 patients of age ≥ 40 years (average age, 56.3 years) undergoing primary surgery for scoliosis by one surgeon. Most of our patients (86%) had at least one comorbidity. We collected complete radiographic measurements and Oswestry Disability Index, Short Form 36, and Scoliosis Research Society 22 questionnaires before surgery and at each follow-up, and recorded the number and type of complications. Outcomes were assessed in the context of complications, degree of correction, and procedure characteristics to detect significant (P < 0.05) correlations. The overall complications rate was 49%; 26% of the patients had a major complication and 31% had a minor one. There were no deaths. Coronal curve correction was 30.8° (61%) on average. There were statistically significant postoperative improvements in Oswestry Disability Index, Short Form 36, and Scoliosis Research Society 22 scores. Patients whose fusions ended at L4 or L5 showed greater improvements in some of the Short Form 36 component scores than patients whose fusions involved the sacrum (P = 0.041). There were no significant differences in outcomes related to presence of complications or operative staging. Adults ≥ 40 years with symptomatic scoliosis benefit from surgical treatment, despite the high complication rate.

Descriptive, retrospective cohort analysis. To evaluate the presentation, etiology, and treatment... more Descriptive, retrospective cohort analysis. To evaluate the presentation, etiology, and treatment of surgical site infections (SSI) after spinal surgery. SSI after spine surgery is frequently seen. Small case control studies have been published reporting the results of treatment options of SSI. We performed this study to identify the most common clinical and laboratory presentation of a SSI, the most frequently seen infective organism, and evaluate the effectiveness of current treatment. All patients who underwent spinal surgery at our institution for diagnosis other than infection between June 1996 and December 2005 (N=3174) were reviewed. All cases of SSI were identified. Patient and operative characteristics were reviewed. Infection type (deep or superficial), treatment course, laboratory and culture results were abstracted. A total of 132 cases of SSI (84 deep and 48 superficial) were identified. About 72.7% of the SSI were detected as outpatients an average 28.7 days (deep, 29.9; superficial, 25.2) after the index procedure. Wound drainage was the most common complaint (68.2%). C-reactive protein level was elevated in 98.0%, erythrocyte sedimentation rate was elevated in 94.4%, but only 48.6% had elevated white blood cell count. Staphylococcus aureus was isolated in 72.6% of deep and 85.7% of superficial positive cultures. Seventy-six percent of deep SSI could be treated with a single debridement to clear the SSI. Instrumentation was retained or primarily exchanged if loose in all cases. Around 72.9% of superficial SSI were treated without formal debridement in the operating room. Antibiotic treatment was longer in deep SSI (40.8 vs. 19.6 days). Deep SSI following spinal surgery was effectively treated with single stage debridement and intravenous antibiotics. Superficial SSI could be treated effectively with local wound care and oral antibiotic therapy.

The Journal of …, 2008

To examine risk factors associated with chorioamnionitis. We conducted a retrospective cohort stu... more To examine risk factors associated with chorioamnionitis. We conducted a retrospective cohort study using data on women who delivered prematurely (< 37 weeks of gestation) over a 12-year period. Eleven potential risk factors were identified. Subjects were stratified according to their blood type into 2 groups: group 1, subjects with anti-B antibodies (blood types A and O), and group 2, subjects without anti-B antibodies (blood types B and AB). Univariate, bivariate, and logistic regression analyses were done to examine risk factors for chorioamnionitis while controlling for confounders. The study included 2879 subjects, 96 of whom (3.3%) were diagnosed with chorioamnionitis. Chorioamnionitis increased significantly with alcohol use (adjusted odds ratio [AOR] = 4.7), prolonged rupture of membranes (ROM) (AOR = 4.16), anemia (AOR = 2.17), and group 1 status (AOR = 1.88). Advanced maternal age was protective of chorioamnionitis (AOR = 0.96). Alcohol consumption during pregnancy, prolonged ROM, anemia, and blood types A and O are associated with increased risk for chorioamnionitis; advanced maternal age, with decreased risk. Further studies are needed to determine the efficacy of early prenatal care in the control of anemia and to examine its affect on the incidence of chorioamnionitis.

Social research has consistently reported that Black Minority Ethnic (BME) communities, which inc... more Social research has consistently reported that Black Minority Ethnic (BME) communities, which include Somalians, are not accessing psychological therapy (Department of Health, 2007; Sainsbury Centre for Mental Health, 2004). In response, the Department of Health has introduced a new initiative called, "Improving Access to Psychological Th erapy" (IAPT); part of its

The international journal of …, 2011

… (ICIP), 2009 16th …, 2009

Journal of Alloys and …, 2004

Nanostructured materials such as MCM-41 modified with rare earth ions have been studied due to th... more Nanostructured materials such as MCM-41 modified with rare earth ions have been studied due to their potential for a variety of applications. Rare earth trivalent ions in some solid compounds emit light at characteristic wavelengths due to infra 4f or inter 4f–5d transitions. The ...

Abstract In this paper, converter topologies, and their control, that can be used as an interface... more Abstract In this paper, converter topologies, and their control, that can be used as an interface between fuel cells and the DC bus in a DC zonal electric distribution system (DC ZEDS) have been investigated. Performance of the conventional DC-DC boost converter ...

This paper presents two modeling techniques for the prediction and monitoring of the characterist... more This paper presents two modeling techniques for the prediction and monitoring of the characteristics of transformer oil. The first employs artificial neural network (ANN) and the second employs non-linear modeling (nlm). The proposed techniques are implemented for predicting the transformer oil residual operating time (t rot) which is defined as the service period after which the breakdown voltage (BDV) violates the limits given in the standard specifications. The selection of the most influential characteristics on residual operating time (t rot) in the proposed techniques is obtained by statistical analysis. The non-linear model depends on linear combination of non-linear functions for each characteristic. The ANN technique for modeling these characteristics preserves the non-linear relationship between these characteristics and (t rot). The results are compared with previously published modeling techniques namely multiple linear regression and polynomial regression models. Different evaluation indices have been used to justify the superiority of the proposed modeling techniques for predicting (t rot).

In this paper, some of the aspects related to the connectivity of DC microgrids to the main grid ... more In this paper, some of the aspects related to the connectivity of DC microgrids to the main grid are investigated. A prototype system has been designed and implemented to address these aspects. The described system is dependent mainly on sustainable energy sources. Hence, a special care has been given to dealing with this kind of sources while designing different components

ABSTRACT Proportional integral derivative (PID) controllers are usually used to control DC-DC boo... more ABSTRACT Proportional integral derivative (PID) controllers are usually used to control DC-DC boost converters in PV systems. However, they have to be tuned based on certain defined operating range using averaged mathematical models. Loading conditions have great effect on PI controllers; PI controllers are subjected to failure under dramatic load changes. This limits the PI controller's operating range. Moreover, transient and steady state response both get affected by changing the operating range. This paper presents a novel smart-PID controller for optimal control of DC-DC boost converter used as voltage controller in PV systems. This proposed controller maximizes the stable operating range by using genetic algorithms (GA) to tune the PID parameters ultimately at various loading conditions. Then, a fuzzy logic approach is used to add a factor of intelligence to the controller such that it can move among different values of proportional gain (Kp), derivative gain (Kd) and integral gain (Ki) based on the system conditions. This controller allows optimal control of boost converter at any loading condition with no need to retune parameters or possibility of failure. Moreover, the paper presents a novel technique to move between the PI and PID configurations of the controller such that the minimum overshoot and ripple are obtained, which makes the controller very applicable for PV systems supplying sensitive loads. The controlled boost converter is used as an interface between photovoltaic (PV) panels and the loads connected to them. It converts any input voltage within its operating range into a constant output voltage that is suitable for load feeding. The proposed smart controller adapts the duty cycle of the boost converter based on input voltage and loading conditions such that it outputs a constant output voltage. A prototype system has been developed to verify the applicability of the proposed controller. Moreover, simulation and experimental results both confirm its v alidity as an effective and reliable controller for boost converters in PV systems and the possibility to use it in different applications.

ABSTRACT Text summarization is an important problem, which has numerous applications. This proble... more ABSTRACT Text summarization is an important problem, which has numerous applications. This problem has been extensively studied and many approaches have been proposed in the literature for its solution. In this paper, we investigate a new approach that employs meta-search. In particular, summaries from several summarizers are evaluated and a new summary is formed from these summaries. Clearly, this meta-summarizer will produce a better summary than any of the others since it exploits the special features of all the sources. We have employed data from Document Understanding Conference 2002 (DUC-2002) and 5 different summarizers in our experiments. We have also created our own summarizer

Numerous outbreaks of cholera have occurred in Kenya since 1971. To more fully understand the epi... more Numerous outbreaks of cholera have occurred in Kenya since 1971. To more fully understand the epidemiology of cholera in Kenya, we analyzed the genetic relationships among 170 Vibrio cholerae O1 isolates at 5 loci containing variable tandem repeats. The isolates were collected during January 2009-May 2010 from various geographic areas throughout the country. The isolates grouped genetically into 5 clonal complexes, each comprising a series of genotypes that differed by an allelic change at a single locus. No obvious correlation between the geographic locations of the isolates and their genotypes was observed. Nevertheless, geographic differentiation of the clonal complexes occurred. Our analyses showed that multiple genetic lineages of V. cholerae were simultaneously infecting persons in Kenya. This fi nding is consistent with the simultaneous emergence of multiple distinct genetic lineages of V. cholerae from endemic environmental reservoirs rather than recent introduction and spread by travelers.

Physica A Statistical …, 2001

An interesting and challenging research subject in the field of nonlinear dynamics is the study o... more An interesting and challenging research subject in the field of nonlinear dynamics is the study of chaotic behavior in systems of more than two degrees of freedom. In this work we study fixed points, strange attractors, chaotic behavior and the problem of chaos control for complex ...

J. Am. Chem. …, 2005

The discovery of the extraordinary activity exhibited by small gold clusters on certain oxide sup... more The discovery of the extraordinary activity exhibited by small gold clusters on certain oxide supports 1,2 has stimulated considerable research 3-11 to address the origin of this unusual behavior. We report, relevant to this question, a study of the oxidation of CO to CO 2 over ...

Journal of Applied Sciences, 2011

A prospective study. Our purpose was to evaluate prospectively the complications, clinical outcom... more A prospective study. Our purpose was to evaluate prospectively the complications, clinical outcomes, and self-reported quality of life in a relatively homogenous group of adults aged ≥ 40 years undergoing primary surgical treatment for scoliosis. Relatively few reports have examined surgical outcomes in adult patients with scoliosis, especially adults aged ≥ 40 years, whose outcomes may differ because of more rigid curves and more frequent and severe comorbidities. Although most studies have shown patient benefits despite high complication rates after such surgery, most were retrospective and conducted before the introduction of third-generation instrumentation techniques. We prospectively studied a consecutive series of 35 patients of age ≥ 40 years (average age, 56.3 years) undergoing primary surgery for scoliosis by one surgeon. Most of our patients (86%) had at least one comorbidity. We collected complete radiographic measurements and Oswestry Disability Index, Short Form 36, and Scoliosis Research Society 22 questionnaires before surgery and at each follow-up, and recorded the number and type of complications. Outcomes were assessed in the context of complications, degree of correction, and procedure characteristics to detect significant (P < 0.05) correlations. The overall complications rate was 49%; 26% of the patients had a major complication and 31% had a minor one. There were no deaths. Coronal curve correction was 30.8° (61%) on average. There were statistically significant postoperative improvements in Oswestry Disability Index, Short Form 36, and Scoliosis Research Society 22 scores. Patients whose fusions ended at L4 or L5 showed greater improvements in some of the Short Form 36 component scores than patients whose fusions involved the sacrum (P = 0.041). There were no significant differences in outcomes related to presence of complications or operative staging. Adults ≥ 40 years with symptomatic scoliosis benefit from surgical treatment, despite the high complication rate.

Descriptive, retrospective cohort analysis. To evaluate the presentation, etiology, and treatment... more Descriptive, retrospective cohort analysis. To evaluate the presentation, etiology, and treatment of surgical site infections (SSI) after spinal surgery. SSI after spine surgery is frequently seen. Small case control studies have been published reporting the results of treatment options of SSI. We performed this study to identify the most common clinical and laboratory presentation of a SSI, the most frequently seen infective organism, and evaluate the effectiveness of current treatment. All patients who underwent spinal surgery at our institution for diagnosis other than infection between June 1996 and December 2005 (N=3174) were reviewed. All cases of SSI were identified. Patient and operative characteristics were reviewed. Infection type (deep or superficial), treatment course, laboratory and culture results were abstracted. A total of 132 cases of SSI (84 deep and 48 superficial) were identified. About 72.7% of the SSI were detected as outpatients an average 28.7 days (deep, 29.9; superficial, 25.2) after the index procedure. Wound drainage was the most common complaint (68.2%). C-reactive protein level was elevated in 98.0%, erythrocyte sedimentation rate was elevated in 94.4%, but only 48.6% had elevated white blood cell count. Staphylococcus aureus was isolated in 72.6% of deep and 85.7% of superficial positive cultures. Seventy-six percent of deep SSI could be treated with a single debridement to clear the SSI. Instrumentation was retained or primarily exchanged if loose in all cases. Around 72.9% of superficial SSI were treated without formal debridement in the operating room. Antibiotic treatment was longer in deep SSI (40.8 vs. 19.6 days). Deep SSI following spinal surgery was effectively treated with single stage debridement and intravenous antibiotics. Superficial SSI could be treated effectively with local wound care and oral antibiotic therapy.

The Journal of …, 2008

To examine risk factors associated with chorioamnionitis. We conducted a retrospective cohort stu... more To examine risk factors associated with chorioamnionitis. We conducted a retrospective cohort study using data on women who delivered prematurely (< 37 weeks of gestation) over a 12-year period. Eleven potential risk factors were identified. Subjects were stratified according to their blood type into 2 groups: group 1, subjects with anti-B antibodies (blood types A and O), and group 2, subjects without anti-B antibodies (blood types B and AB). Univariate, bivariate, and logistic regression analyses were done to examine risk factors for chorioamnionitis while controlling for confounders. The study included 2879 subjects, 96 of whom (3.3%) were diagnosed with chorioamnionitis. Chorioamnionitis increased significantly with alcohol use (adjusted odds ratio [AOR] = 4.7), prolonged rupture of membranes (ROM) (AOR = 4.16), anemia (AOR = 2.17), and group 1 status (AOR = 1.88). Advanced maternal age was protective of chorioamnionitis (AOR = 0.96). Alcohol consumption during pregnancy, prolonged ROM, anemia, and blood types A and O are associated with increased risk for chorioamnionitis; advanced maternal age, with decreased risk. Further studies are needed to determine the efficacy of early prenatal care in the control of anemia and to examine its affect on the incidence of chorioamnionitis.

Social research has consistently reported that Black Minority Ethnic (BME) communities, which inc... more Social research has consistently reported that Black Minority Ethnic (BME) communities, which include Somalians, are not accessing psychological therapy (Department of Health, 2007; Sainsbury Centre for Mental Health, 2004). In response, the Department of Health has introduced a new initiative called, "Improving Access to Psychological Th erapy" (IAPT); part of its

The international journal of …, 2011

… (ICIP), 2009 16th …, 2009

Journal of Alloys and …, 2004

Nanostructured materials such as MCM-41 modified with rare earth ions have been studied due to th... more Nanostructured materials such as MCM-41 modified with rare earth ions have been studied due to their potential for a variety of applications. Rare earth trivalent ions in some solid compounds emit light at characteristic wavelengths due to infra 4f or inter 4f–5d transitions. The ...