Ahmad Rahmati - Academia.edu (original) (raw)

Papers by Ahmad Rahmati

Differential protection relays may false tripping for external faults due to current transformer ... more Differential protection relays may false tripping for external faults due to current transformer saturation (CT). This paper presents an adaptive technique for transformer differential protection against saturation of current transformers, based on differential currents. A Wavelet Index (WI) by wavelet Transform has been devised using extracted feature from differential currents due to saturation currents. In this technique, with beginning of saturation of current transformers, the restrain currents in area zone of the differential relay will increase. The proposed algorithm is evaluated by using various simulated internal fault on a power transformer, saturation current cases and different magnetizing inrush currents that has been modeled using Electromagnetic Transients Program software. Results of evaluation study show that, proposed wavelet based differential protection scheme can stable in CT saturation events. The obtained results demonstrate precise operation of the proposed algorithm in different conditions.

Hysteresis motors behave very much like to classical synchronous motors, except that the motor ma... more Hysteresis motors behave very much like to classical synchronous motors, except that the motor magnetic pole definition varies slightly with torque. This paper presents a study on the hunting of a radial-flux type, high speed hysteresis motor caused by the hysteresis material of the rotor. Firstly, a nonlinear dynamic model of the PWM-controlled hysteresis motor drive is developed in d-q axis. Then the effects of magnetic properties of hysteresis rotor materials are investigated. Experimental results verify that B-H curve of hysteresis material have significant effect on the rotor hunting in steady state rather than other parameters.

Plasma Science & Technology, 2010

A sintered Ti13Cu87 target was sputtered by reactive direct current (DC) magnetron sputtering wit... more A sintered Ti13Cu87 target was sputtered by reactive direct current (DC) magnetron sputtering with a gas mixture of argon/nitrogen for different sputtering powers. Titanium-copper-nitrogen thin films were deposited on Si (111), glass slide and potassium bromide (KBr) substrates. Phase analysis and structural properties of titanium-copper-nitrogen thin films were studied by X-ray diffraction (XRD). The chemical bonding was characterized by Fourier transform infrared (FTIR) spectroscopy. The results from XRD show that the observed phases are nano-crystallite cubic anti rhenium oxide (anti ReO3) structures of titanium doped Cu3N (Ti:Cu3N) and nano-crystallite face centered cubic (fcc) structures of copper. Scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDX) were used to determine the film morphology and atomic titanium/copper ratio, respectively. The films possess continuous and agglomerated structure with an atomic titanium/copper ratio (~ 0.07) below that of the original target (~ 0.15). The transmittance spectra of the composite films were measured in the range of 360 nm to 1100 nm. Film thickness, refractive index and extinction coefficient were extracted from the measured transmittance using a reverse engineering method. In the visible range, the higher absorption coefficient of the films prepared at lower sputtering power indicates more nitrification in comparison to those prepared at higher sputtering power. This is consistent with the formation of larger Ti:Cu3N crystallites at lower sputtering power. The deposition rate vs. sputtering power shows an abrupt transition from metallic mode to poisoned mode. A complicated behavior of the films' resistivity upon sputtering power is shown.

Vacuum

Ti:Cu 3 N thin films were deposited on Si(111), quartz, and glass slide substrates by DC magnetro... more Ti:Cu 3 N thin films were deposited on Si(111), quartz, and glass slide substrates by DC magnetron sputtering in molecular nitrogen ambient. The structural properties of Ti:Cu 3 N thin films were studied by X-ray diffraction (XRD) analysis. XRD measurements show diffraction band with peaks close to the and diffraction lines of cubic anti-ReO 3 structure of Cu 3 N. The Ti:Cu 3 N nano-crystalline size is in the range 22-27 nm. Lattice constant expansion reflects Ti incorporation causing the excess nitrogen to occur. Surface morphology shows that the N richness suppresses the grain growth. The optical absorption spectra indicate a remarkable shift to higher energies of the absorption edge due to higher N concentration and quantum size effect. Photoluminescence (PL) measurement shows interstitial N excess and Ti impurity produce shallow and deep levels, respectively. Thermal stability of the Ti:Cu 3 N films annealed at 300 and 400°C is improved in comparison with that of Ti free Cu 3 N films.

IEEE Transactions on Mobile Computing, 2011

Context information brings new opportunities for efficient and effective system resource manageme... more Context information brings new opportunities for efficient and effective system resource management of mobile devices. In this work we focus on the use of context information to achieve energy-efficient, ubiquitous wireless connectivity. Our field-collected data show that the energy cost of network interfaces poses a great challenge to ubiquitous connectivity, despite decent availability of cellular networks. We propose to leverage the complementary strengths of Wi-Fi and cellular interfaces by automatically selecting the most efficient one based on context information. We formulate the selection of wireless interfaces as a statistical decision problem. The challenge is to accurately estimate Wi-Fi network conditions without powering up the network interface. We explore the use of different context information, including time, history, cellular network conditions, and device motion, to statistically estimate Wi-Fi network conditions with negligible overhead. We evaluate several contextbased algorithms for the estimation and prediction of current and future network conditions. Simulations using field-collected traces show that our network estimation algorithms can improve the average battery lifetime of a commercial mobile phone for an ECG reporting application by 40%, very close to the estimated theoretical upper bound of 42%. Furthermore, our most effective algorithm can predict Wi-Fi availability for one and ten hours into the future with 95% and 90% accuracy, respectively.

Mobile Computing and Communications Review, 2011

In this poster we describe initial findings regarding Internet usage characteristics, in particul... more In this poster we describe initial findings regarding Internet usage characteristics, in particular TCP flows from a field study with 27 iPhone 3GS users. We present details regarding their usage characteristics, and provide a solution for migrating flows between different networks and/or network interfaces without requiring infrastructure support or changes to current applications and protocols, with minimal impact to the

Pervasive and Mobile Computing, 2009

Mobile phone users have to deal with limited battery lifetime through a reciprocal process we cal... more Mobile phone users have to deal with limited battery lifetime through a reciprocal process we call human-battery interaction. We conducted three user studies in order to understand human-battery interaction and discover the problems in existing designs that prevent users from effectively dealing with the limited battery lifetime. The studies include a largescale international survey, two long-term field trials including quantitative battery logging and qualitative inquiries, and structured interviews with twenty additional mobile phone users. We evaluated various aspects of human-battery interaction, including charging behavior, battery indicators, user interfaces for power-saving settings, user knowledge, and user reaction. We find that mobile phone users can be categorized into two types regarding human-battery interaction and often have inadequate knowledge regarding phone power characteristics. We provide qualitative and quantitative evidence that problems in stateof-the-art user interfaces have led to under-utilized power-saving settings, under-utilized battery energy, and dissatisfied users. Our findings provide insights into improving mobile phone design for users to effectively deal with the limited battery lifetime. Our work is the first to systematically address human-battery interaction on mobile phones and is complementary to the extensive research on energy-efficient design for a longer battery lifetime.

IEEE Transactions on Mobile Computing, 2010

Mobile phones are truly personal devices loaded with personal data such as photos, contacts, and ... more Mobile phones are truly personal devices loaded with personal data such as photos, contacts, and call history. Yet it is often necessary or desirable to share our phones with others. This is especially true as mobile phones are integrating features conventionally provided by other dedicated devices, from MP3 players to games consoles. Yet existing phones assume a single user and provide little protection for private data and applications when a phone is shared. That is, when we lend our phones to others, we give away complete access. In this work, we present xShare, a protection solution to address this problem. xShare allows phone owners to rapidly specify what they want to share and place the phone into a restricted mode where only the data and applications intended for sharing can be accessed. We first present two formative user studies and derive the design requirements of xShare. We then offer the design of xShare based on file-level access control. We describe the implementation of xShare on Windows Mobile and report a comprehensive evaluation, including performance measurements, usability, and a one-month field trial.

Consumer electronics and mobile devices intended for pervasive applications are often subject to ... more Consumer electronics and mobile devices intended for pervasive applications are often subject to shaking that makes their screen difficult to read. To address this usability challenge, we present oShake, a system for screen content stabilization.

Behaviour & Information Technology, 2012

The present report is an empirical analysis of smartphone personalisation. We collected data from... more The present report is an empirical analysis of smartphone personalisation. We collected data from two groups of users to measure how they adapt the content, interface and physical appearance of their devices. This user-driven personalisation is measured with a simple heuristic approach to quantify the behaviour. Using these scores, we explore how users differ from each other in how they

IEEE Transactions on Circuits and Systems for Video Technology, 2011

In this paper, we present a comprehensive study on sensor-assisted video encoding (SaVE) schemes ... more In this paper, we present a comprehensive study on sensor-assisted video encoding (SaVE) schemes for video capturing on mobile devices in real-world environments. Our purpose is to reduce the computational complexity of video encoding by leveraging sensors that are increasingly available on mobile devices, e.g., accelerometers and digital compasses. Motion estimation is a key component of video encoding. In this paper, SaVE calculates the rotational movement of a camera (on mobile devices) and then infers the global motion in the camera imager. SaVE subsequently employs the estimated global motion as predictors to simplify motion estimation algorithms for state-of-the-art H.264/AVC video coding. We have constructed a prototype of SaVE and evaluated its performance with a pair of accelerometers, a digital compass, and their combination. Our experimental results show that SaVE can significantly reduce the computations of motion estimation while achieving equal or better video quality. Our results also show that SaVE has a strong noise-resistant capability. Therefore, it can be practically employed in real-world environments.

We present results from a longitudinal study of 34 iPhone 3GS users, called LiveLab. LiveLab coll... more We present results from a longitudinal study of 34 iPhone 3GS users, called LiveLab. LiveLab collected unprecedented usage data through an in-device, programmable logger and several structured interviews with the participants throughout the study. We have four objectives in writing this paper: (i) share the findings with the research community; (ii) provide insights guiding the design of smartphone systems and applications; (iii) demonstrate the power of prudently designed longitudinal field studies and the power of advanced research methods; and (iv) raise important questions that the research community can help answer in a collaborative, multidisciplinary manner.

Mobile phone users have to deal with limited battery lifetime through a reciprocal process we cal... more Mobile phone users have to deal with limited battery lifetime through a reciprocal process we call human-battery interaction (HBI). We conducted three user studies in order to understand HBI and discover the problems in existing mobile phone designs. The studies include a large-scale international survey, a onemonth field data collection including quantitative battery logging and qualitative inquiries from ten mobile phone users, and structured interviews with twenty additional mobile phone users. We evaluated various aspects of HBI, including charging behavior, battery indicators, user interfaces for power-saving settings, user knowledge, and user reaction. We find that mobile phone users can be categorized into two types regarding HBI and often have inadequate knowledge regarding phone power characteristics. We provide qualitative and quantitative evidence that problems in state-of-the-art user interfaces has led to underutilized power-saving settings, under-utilized battery energy, and dissatisfied users. Our findings provide insights into improving mobile phone design for users to effectively deal with the limited battery lifetime. Our work is the first to systematically address HBI on mobile phones and is complementary to the extensive research on energy-efficient design for a longer battery lifetime.

Radio Frequency Identification (RFID) technology has the potential to dramatically improve numero... more Radio Frequency Identification (RFID) technology has the potential to dramatically improve numerous industrial practices. However, it still faces many challenges, including security and reliability, which may limit its use in many application scenarios. While security has received considerable attention, reliability has escaped much of the research scrutiny. In this work, we investigate the reliability challenges in RFID-based tracking applications, where objects (e.g., pallets, packages, and people) tagged with low-cost passive RFID tags pass by the RFID reader's read zone. Our experiments show that the reliability of tag identification is affected by several factors, including the inter-tag distance, the distance between the tag and antenna, the orientation of the tag with respect to the antenna, and the location of the tag on the object. We demonstrate that RFID system reliability can be significantly improved with the ap

We report a user study of over four months on the non-voice usage of mobile phones by teens from ... more We report a user study of over four months on the non-voice usage of mobile phones by teens from an underserved urban community in the USA where a community-wide, open-access Wi-Fi network exists. We instrumented the phones to record quantitative information regarding their usage and location in a privacy-respecting manner. We conducted focus group meetings and interviewed participants regularly for qualitative data. We present our findings on what applications our participants used and how their usage changed over time. The findings highlight the challenges to evaluating the usability of mobile systems and the value of long-term methodologies. Based on our findings, we analyze the unique values of mobile phones, as a platform technology. Our study shows that the usage is highly mobile, location-dependent, and serves multiple social purposes for the participants. Furthermore, we present concrete findings on how to perform and analyze similar user studies on mobile phones, including four contributing factors to usage evolution, and provide guidelines for their design and evaluation.

Motion estimation is a key component of modern video encoding and is very compute-intensive. We p... more Motion estimation is a key component of modern video encoding and is very compute-intensive. We present a novel Sensor-assisted Video Encoding (SaVE) method to reduce the computational complexity of motion estimation in H.264/AVC encoders, leveraging accelerometers and digital compasses that are increasingly available on mobile devices. Using these sensors, SaVE calculates the rotational movement of a camera and then infers the global motion in the camera image sensor; it subsequently employs the estimated global motion to simplify the state-of-the-art motion estimation algorithms, UMHS and EPZS used in H.264/AVC encoders. We have constructed a prototype of SaVE and report extensive evaluation of it. Our experimental results show that SaVE can reduce the computations of UMHS and EPZS algorithms by up to 27% and 18%, respectively, while achieving the same or better video quality.

Plasma Science & Technology, 2010

Vacuum

IEEE Transactions on Mobile Computing, 2011

Mobile Computing and Communications Review, 2011

Pervasive and Mobile Computing, 2009

IEEE Transactions on Mobile Computing, 2010

Behaviour & Information Technology, 2012

IEEE Transactions on Circuits and Systems for Video Technology, 2011

Mobile phone users have to deal with limited battery lifetime through a reciprocal process we cal... more Mobile phone users have to deal with limited battery lifetime through a reciprocal process we call human-battery interaction (HBI). We conducted three user studies in order to understand HBI and discover the problems in existing mobile phone designs. The studies include a large-scale international survey, a onemonth field data collection including quantitative battery logging and qualitative inquiries from ten mobile phone users, and structured interviews with twenty additional mobile phone users. We evaluated various aspects of HBI, including charging behavior, battery indicators, user interfaces for power-saving settings, user knowledge, and user reaction. We find that mobile phone users can be categorized into two types regarding HBI and often have inadequate knowledge regarding phone power characteristics. We provide qualitative and quantitative evidence that problems in state-of-the-art user interfaces has led to underutilized power-saving settings, under-utilized battery energy, and dissatisfied users. Our findings provide insights into improving mobile phone design for users to effectively deal with the limited battery lifetime. Our work is the first to systematically address HBI on mobile phones and is complementary to the extensive research on energy-efficient design for a longer battery lifetime.