Hafiz Ahmed

The University of Sheffield | Sheffield · Advanced Manufacturing Research Centre with Boeing (AMRC)

Using measurements of valve activity (i.e., the distance between the two valves) in populations of bivalves under natural environmental conditions (16 oysters in the Bay of Arcachon, France, in 2007, 2013, and 2014), an algorithm for an automatic detection of the spawning period of oysters is proposed in this brief. Spawning observations are import...

In this article, motivated by a recent work of R. Brockett [1], a robust global synchronization problem of multistable Brockett oscillators has been studied within an Input-to-State Stability (ISS) framework. Two synchronization protocols are designed with respect to compact invariant sets of the unperturbed Brockett oscillator. The conditions obta...

Many phase locked-loop (PLL) or frequency locked-loop (FLL) use quadrature signals (directly or indirectly) as the input variables. Generalized integrator (GI) is a popular quadrature signal generator (QSG) available in the literature. GI is also widely used in various industrial applications. In addition to being a QSG, GI also works as kind of ba...

Atomic force microscopy (AFM) plays an important role in nanoscale imaging application. AFM works by oscillating a micro-cantilever on the surface of the sample being scanned. In this process, estimating the amplitude of the cantilever deflection signal plays an important role in characterizing the topography of the surface. Existing approaches on...

This paper presents an enhanced frequency adaptive demodulation technique for grid-synchronization of grid-connected converters (GCC) in variable frequency condition. Demodulation works by generating demodulated voltages which contain undesired double frequency components. As a result, high-order low-pass filters (LPF) with high cutoff frequency ar...

Global interest in advanced reactors has been reignited by recent investments in small modular reactors and micro-reactor design. The use of digital devices is essential for meeting the size and modularity requirements of small modular reactor controls. By fully digitizing the small modular reactor control systems, critical information can be obtai...

In order to feed the ever-increasing global population, current food production must increase significantly in the near future. Increasing food production by expanding farmland or by farming intensification would lead to increasing land-use demand, which comes at the cost of biodiversity, increasing pressure on vulnerable ecosystems, and requiring...

Urban power systems rely on intricate wire networks, known as the power grid, which form the essential electric infrastructure in cities. While these networks transmit electricity from power plants to consumers, they are vulnerable to faults caused by manufacturing errors and improper installation, posing risks to system integrity. Thus, accurate i...

The encoder and resolver are commonly used rotor angle position sensors for controlling permanent magnet synchronous motors (PMSMs). However, they require significant axial space. This issue can be avoided with an embedded magnetic encoder (EME), which offers precise rotor angle estimation using a few linear Hall sensors in the motor cavity while o...

In photovoltaic (PV) systems, partial shading is a major issue that may cause power losses, hot spots, and PV modules damage. Thus, PV array dynamic reconfiguration approaches based on irradiance equalization (IEq) between rows have been proposed to alleviate the shading effect thereby improving PV power production. However, the existing IEq-based...

DC offset (DCO) in the input signal causes estimation inaccuracy in the phase-locked loop (PLL) output. This letter provides a solution to DCO rejection while simultaneously offering frequency ramp tracking capability to a single-phase PLL, which is an integral part of the renewable energy-interfaced grid-connected converter control system. This ha...

In voltage‐controlled voltage source inverters (VSIs)‐based microgrids (MGs), the inner control is of prime interest task for guaranteeing safe and stable operation. In this paper, an in‐depth investigation of the modelling, control design, and analysis of the voltage and current inner control loops intended for single‐phase voltage‐controlled VSIs...

This paper proposes a delayed signal-based demodulation algorithm that enables a fast and accurate extraction of the positive/negative sequences and DC-offset in the grid voltage under a frequency drift. Moreover, a simplified mathematical formalism is proposed to reduce the computational burden for real-time implementation. The algorithm is incorp...

Extensive testing is crucial to ensure exhaust emission compliance for high-horsepower (H-hp) commercial engines. Back pressure sensors, integral to the exhaust system, are prone to producing noisy measurements due to the turbulent nature of the exhaust gas flow and other testing inaccuracies, mandating the use of a noise reduction filter. The time...

Conventional SOGI-PLL, designed for DC offset (DCO) free measurements, experiences significant accuracy degradation in the presence of DCO. To tackle this issue, literature offers various solutions, including low- and high-order methods. This letter focuses onlow-order methods due to their computational and implementation simplicity. It has been mo...

This letter explores accurate frequency and phase tracking for single-phase power grid applications. While the conventional type-2 phase-locked loop (PLL) offers fast dynamic response, it exhibits steady-state tracking error in the presence of frequency ramp caused by rapid changes in power generation and/or consumption. The higher-order type-3 PLL...

Various faults can cause voltage sag in the power grid at different voltage levels across the network. Balanced or unbalanced voltage sags lead to grid instability by tripping off a large number of wind or solar power plants from the electric power network. This is particularly problematic to maintain the stability of renewable energy-rich converte...

We address the problem of attack signal estimation in industrial control systems that are subjected to actuator false data injection attack (FDIA) and where the sensor measurements are corrupted by non-negligible high-frequency measurement noise. The actuator FDIA signal is categorized as disturbance to be estimated and subsequently compensated, ba...

This paper implements Broyden's Method Assisted Differential Evolution (BMADE) algorithm for eliminating third and fifth harmonics in the output voltage of 7‐level cascaded H‐Bridge multilevel inverter (CHB‐MLI) using selective harmonic elimination technique. Calculations of switching angles are done for various modulation index values (M). Initial...

Quasi type-1 phase-locked loop (QT1-PLL) has become very popular in recent times for grid-connected converters owing to its simple structure and fast dynamic response. By means of the use of a half-cycle moving average filter (MAF), this PLL can completely eliminate all the nominal frequency odd-order harmonics. However, the performance deteriorate...

Tidal stream turbines (TST) operate in a harsh marine condition. TSTs interact with the tide to produce electricity, where the tide rotates the turbine and the turbine in return rotates the electrical generator for electricity production as shown in Figure 2.1. In order to maximise the energy production from the tide and also to make the system rob...

This paper proposes an extended state observer (ESO)‐based second‐order sliding mode control (SMC) for dynamic voltage restorers (DVRs). Unlike the conventional first‐order SMC and some second‐order SMC (SOSMC) methods that suffer from chattering, the proposed control method can alleviate chattering and achieve finite‐time convergence. Chattering s...

Integrating renewable energy sources into an unbalanced distribution network requires fast and accurate extraction of fundamental frequency positive‐ and negative‐sequence components from the unbalanced three‐phase grid voltage signals. For this purpose, various methods are already available in the literature. Out of them, delayed signal cancellati...

Because of their advantages, speed sensorless induction motor drives have received a lot of attention in the academic and industrial communities over the last decade. Despite the recent developments in this area, the performance of sensorless variable-speed IM drives in ultra-low and zero electrical rotor speeds and regeneration mode remains a big...

Distribution-grid connected electric vehicle charging stations draw nonlinear current, which causes power quality issues including harmonic distortion, DC-link fluctuation etc. Recent literature found that a unified power quality conditioner with superconducting magnetic energy storage (UPQC-SMES) can alleviate charging induced power quality issues...

In the available literature, there are several methods and techniques are employed to extract the unknown parameters of solar cell. The performance of the method used is validated on the basis of Root Mean Square Error (RMSE) value. This work aims to promote a decent basis for proper investigation and the implementation of an optimization algorithm...

Diesel engine combustion releases many harmful components, thus there are continuous efforts into improving the efficiency of these engines and reducing the harmful gasses and particulates to meet the emission authorities targets. To develop and sell new engine-related products, these engines are required to run and to be audited in diesel engine t...

Computation of active and reactive powers is a crucial step in droop‐controlled single‐phase voltage source inverters (VSIs) in standalone microgrid since the performance and stability of the power‐sharing strategy are strongly influenced by its speed and accuracy, especially in the case of non‐linear loads. Here, an improved performance of power‐s...

In smart greenhouse farming, artificially adjustable light qualities (colours) play an important role to promote plant growth. While it is known that these light qualities have significant impacts on plant development, the suitable combinations enabling the plant to grow at its best are yet to be systematically identified. This study fills this gap...

In recent years, countries across the world have started developing small modular reactors (SMRs), nuclear reactors that generally produce around 300 megawatts of electricity (MWe). Many believe this type of reactor could be key in helping countries achieve their net-zero goals, as they are theoretically less expensive and safer than their larger c...

Tidal stream turbine (TST) has the potential to become a key enabling technology for sustainable electricity supply in remote islands and coastal communities. This type of turbines are typically employed under water and subject to various external disturbances. These external disturbances make the development of control system for TST very challeng...

This paper presents a continuous terminal sliding mode controller (CTSMC) for single-phase dynamic voltage restorers (DVRs). The presented controller can substantially eliminate the chattering compared to the first-order sliding mode counterpart while achieving faster convergence speed compared to the super twisting counterpart. Moreover, the conti...

This article studies the problem of fundamental frequency positive-sequence component separation in unbalanced three-phase systems for grid-integration applications. We propose a-simple-to-implement approach involving a modified delayed signal cancellation method and moving average filtering. A delay-based linear-regression framework is considered...

In order to reach the ambitious net-zero emission target by 2050, various technological solutions need to be developed to ensure efficient utilisation of energy. Commercial and residential buildings are a big source of greenhouse gas emissions, where efficient utilisation of energy can play a major role towards decarbonisation of the buildings sect...

The advancement in blockchain applications in the smart sustainable city infrastructure is evaluated in this paper through a comprehensive mapping review. The evaluation is carried out by posing four research questions that address current developments in blockchain technology in the context of smart cities and point out areas where additional stud...

Hand-eye calibration is an important step in controlling a vision-guided robot in applications like part assembly, bin picking and inspection operations etc. Many methods for estimating hand-eye transformations have been proposed in literature with varying degrees of complexity and accuracy. However, the success of a vision-guided application is hi...

This paper develops an advanced scheme, modelling, and analysis of power flow control intended for grid-connected droop-controlled VSIs within a single-phase microgrid (MG). The proposed control scheme includes a power calculation method based on an enhanced second-order generalized integrator frequency-locked loop (ESOGI-FLL). Contrary to the exis...

A proportional-resonant (PR)-based current control strategy for grid-connected packed-E cell (PEC) inverter is presented. Unlike the existing control strategies, which are based on proportional-integral controller, developed for packed-U cell and PEC inverters, the proposed PR-based control strategy achieves zero steady-state error in the grid curr...

Deep learning algorithms provide plausible benefits for efficient prediction and analysis of nuclear reactor safety phenomena. However, research works that discuss the critical challenges with deep learning models from the reactor safety perspective are limited. This article presents the state-of-the-art in deep learning application in nuclear reac...

The adaptability of maximum power point tracking (MPPT) of a solar PV system is important for integration to a microgrid. Depending on what fixed step-size the MPPT controller implements, there is an impact on settling time to reach the maximum power point (MPP) and the steady state operation for conventional tracking techniques. This paper present...

With a strong expansion of transportation electrification, electric vehicle charging systems are becoming very important part of the electrified powertrain. This paper proposes a sliding mode based adaptive linear neuron (ADALINE)-proportional resonant (PR) control solution to enhance performance of Vienna rectifier (VR), an AC-DC converter, as a c...

A grid-synchronization-based fundamental frequency positive-sequence (FFPS) and negative-sequence (FFNS) amplitudes estimation technique is proposed for unbalanced and distorted grid. In this technique, the sequence amplitudes are extracted by extracting the phase-angle of the FFPS and FFNS components. The extracted phase-angles have DC and double...

Topics of interest include, but are not limited to:-Novel topologies of reduced switch multilevel converters-Improved design and analysis of reduced switch multilevel converters-Efficiency enhancement of reduced switch multilevel converters-Power quality improvement of reduced switch multilevel converters-Advanced modulation techniques based reduce...

The increased adoption of renewable energy sources is reducing the overall inertial response capability of the modern electric power grid, which translates into larger frequency variation in both transient and pseudo-steady-state operation. Energy storage system (ESS) has the potential to play a significant role in regulating the frequency of more...

Out of various moving average filter (MAF)-based phase-locked-loop (PLL), quasi type-1 PLL (QT1-PLL) is widely adopted due to its fast dynamic performance, implementation simplicity, and harmonics rejection abilities. However, the performance of QT1-PLL deteriorates in the presence of an off-nominal frequency unbalanced grid voltage component. More...

Delayed signal cancellation (DSC) is a popular method to separate the fundamental frequency positive- and negative-sequence components from unbalanced three-phase signals. Conventional DSC can separate the sequence components using a quarter-cycle delay. Fast DSC tools can achieve the same with less than a quarter-cycle delay. However, neither conv...

This article proposes a robust and enhanced three-phase fundamental frequency estimation algorithm for the electric aircraft grid. Unlike conventional frequency-locked loops, the digital signal processing technique-based frequency estimator proposed relies on the storage of five consecutive samples of the fundamental grid voltage signal. Furthermor...

This paper proposes a exponential moving average extended Kalman filter (EMAF-EKF) for the purpose of battery state-of-charge (SOC) estimation. The proposed approach relies on a second-order RC circuit model of the Lithium-ion battery. In the proposed approach, first, exponential moving average filters are applied to the various input signals that...

Parametric estimation of a biased harmonic signal is a significant technical challenge for many engineering applications. Such a problem is particularly important for electric utility grid-connected power electronic converters. This article utilizes a linear regression model of the signal to solve this interesting practical problem. A continuous-ti...

Complex coefficient filters (CCF) are widely utilized for the phase-locked loop (PLL) in three-phase power systems, while its usage is largely sparse in the single-phase domain. Single-phase CCF-PLLs so far reported in the literature do not yield satisfactory performances at off-nominal frequency conditions, which is a crucial PLL attribute. Whethe...

Electric vehicle (EV) charging stations draw nonlinear currents, which makes the distribution network unbalanced, distorted, and results in power quality (PQ) issues. These PQ issues are mitigated in this work through high-performance control of the shunt active power filter (SAPF). In the proposed method, a linear Kalman filter (LKF) has been appl...

Smart greenhouse farming has emerged as one of the solutions to global food security, where farming productivity can be managed and improved in an automated manner. While it is known that plant development is highly dependent on the quantity and quality of light exposure, the specific impact of the different light properties is yet to be fully unde...

This paper considers the reference signal generation problem for multi-functional operation of single-phase dynamic voltage restorer. For this purpose, a single-phase quasi type-1 phase-locked loop (QT1-PLL) is proposed. The pre-loop filter part of this PLL is composed of frequency-fixed delayed signal cancellation method and a two-stage all-pass f...

This paper presents a fast phase-locking and amplitude detection scheme for single-phase applications. The proposed scheme employs a pseudo-rectification process in order to generate even harmonics from the grid signal. The DC-offset component present in the grid signal increases the sensitivity of the pseudo-rectification process, and the design o...

In recent times, permanent magnet assisted synchronous reluctance motors (PMaSRM) have been considered as suitable traction motors for electric vehicle applications. In this type of machine, where the share of reluctance torque is more significant than the excitation torque, it is more appropriate to use a control strategy that can fully utilize th...

Electric vehicles (EVs) have received massive consideration in the automotive industries due to their improved performance, efficiency and capability to minimize global warming and carbon emission impacts. The utilization of EVs has several potential benefits, such as increased use of renewable energy, less dependency on fossil-fuel-based power gen...

Hand-eye calibration enables proper perception of the environment in which a vision guided robot operates. Additionally, it enables the mapping of the scene in the robots frame. Proper hand-eye calibration is crucial when sub-millimetre perceptual accuracy is needed. For example, in robot assisted surgery, a poorly calibrated robot would cause dama...

In this paper, a simple control methodology is proposed to stabilize the position of the sprung mass of the quarter car system. The simplicity of the structure of the proposal facilitates the application in real systems. Such methodology allows five different continuous sliding-mode controllers to be selected by means of two similar designs to miti...

Fast and accurate frequency estimation of an unbalanced three-phase power system in the presence of measurement offset is a challenging task. A simple to implement and tune method is proposed in this work through a delay-based linear-regression framework. Using two challenging scenarios, we verify the performance of our procedure in the presence of...

Grid synchronization techniques play a significant role in integrating renewable energy sources to the electric power grid. In this context, estimating the phase and frequency of the grid voltage signal is an interesting problem. Out of various techniques available in the literature, Linear Kalman Filter (LKF) is one of the most popular one. In thi...

Many systems in the natural and physical world often work in unison with similar other systems. This process of simultaneous operation is known as synchronization. In the past few decades, owing to this phenomenon’s importance, extensive research efforts have been made. However, many of the existing results consider the systems are identical and/or...

Phase-locked loops (PLL) are widely used in the synchronization of grid interfaced power converters. One solution is based on orthogonal signal generation (OSG), which requires the grid frequency information for their appropriate operation.This paper developed a new solution to achieve the PLL function for single-phase grid interconnection but erad...

Li-ion batteries are essential component in the current generation of electric vehicles. However, further pushing electric vehicles are concerned with battery life. Since the temperature dictates battery lifetime, it is crucial to manage the heat and keep the temperature at an acceptable range within the battery pack. The benefit of a cooling syste...

Development of advanced grid-synchronization technique for unbalanced and distorted grid is considered in this paper. In this context, self-tuning filter (STF) is considered to extract the fundamental component from the measured unbalanced and distorted voltages. Standard STF considers balanced grid voltages which is not always possible to ensure i...

This paper proposes a quasi open-loop grid synchronization technique-based control scheme for active power filter. The proposed filtering technique does not require any direct frequency feedback which has the advantage of unconditional stability. Based on the reference a current, a proportional integral and hysteresis band current control scheme is...

Various faults and disturbances often make the grid unbalanced and distorted. To make the grid-connected converters resilient to unbalanced and distorted grid, real-time estimation of grid voltage sequences are essential. Adaptive-observer based grid-synchronization techniques are very suitable for this purpose. In this paper, a comparative analysi...

This paper presents a super twisting sliding mode control (ST-SMC) for single-phase dynamic voltage restorers (DVRs). Unlike the conventional first-order sliding mode controller, the proposed ST-SMC technique eliminates the need for differentiating the compensation voltage in the sliding surface function while keeping the merits of first-order SMC....

Fast and accurate grid synchronization is very important to integrate renewable energy systems to traditional electric power grid. In this context, frequency-locked loop (FLL) became very popular in recent time as a grid synchronization technique. Numerous FLL techniques are available in the literature. Out of them, adaptive observer-based FLL tech...

The goal of this paper is to accurately estimate three-phase grid voltage parameters under distorted and unbalanced voltage conditions. For this purpose, a novel phase locked loop (PLL) using parallel second-order generalized integrator (Parallel-SOGIs) is developed. The proposed technique can accomplish grid synchronization under distorted and unb...

Control of grid-connected converters (GCC) is a challenging task in adverse grid conditions. Due to increasing presence of inverter-interfaced renewable energy sources and nonlinear loads, adverse grid conditions are often can not be avoided. Self-tuning filter (STF) is an widely used technique for the control of GCC in adverse conditions. However,...

Estimation of instantaneous phase and frequency of harmonics-polluted single-phase grid voltage signal have been studied in this paper. The proposed approach uses a frequency adaptive Luenberger-sliding mode observer. Using Lyapunov stability theory, a frequency adaptation law has been proposed. The proposed frequency adaptive observer technique is...

This paper studies the grid-synchronization problem of three-phase system. Second-order adaptive filters are a popular tool for grid-synchronization. In this context, reduced-order generalized integrator has attracted some attention in recent time. However, existing implementations can not control directly the closed-loop poles (real and imaginary)...

This paper proposes the estimation of parameters and symmetrical components of unbalanced grid using adaptive observer framework. Recent adaptive observers proposed in the literature doesn't employ any gain normalization in their frequency estimation loop. This can be problematic in the presence of large voltage dip. This paper proposes a solution...

This paper aims to address the DC offset rejection problem in grid synchronization algorithm. A simple approach to estimate the unknown grid frequency in the presence of DC offset is proposed for this purpose. Some of the existing techniques available in the literature use either low-pass filter or an additional integrator to eliminate the DC offse...

This paper deals with a comparative study of two phasor estimators based on the least square (LS) and the linear Kalman filter (KF) methods, while assuming that the fundamental frequency is unknown. To solve this issue, the maximum likelihood technique is used with an iterative Newton-Raphson-based algorithm that allows minimizing the likelihood fu...

This paper studies the phase and frequency estimation problem of single-phase grid voltage signal in the presence of DC offset and harmonics. For this purpose, a novel parameterized linear model of the grid voltage signal is considered where the unknown frequency of the grid is considered as the parameter. Based on the developed model, a linear obs...

Grid-connected converters are an important class of power electronic systems. Many power and energy applications require grid-connected converters. To control grid-connected converters, precise information of the grid voltage frequency and phase are required. Gradient estimators can be very useful in this regard. They are suitable to estimate the f...

This Letter proposes demodulation type PLL for phase and frequency estimation of single‐phase system that can reject DC offset. Using results from the adaptive estimation literature, this Letter proposes a linear parametric model‐based initial phase angle estimation approach. Then by using differentiation and integration operation on the estimated...

Grid synchronization plays an important role in the grid integration of renewable energy sources. To achieve grid synchronization, accurate information of the grid voltage signal parameters are needed. Motivated by this important practical application, this paper proposes a state observer-based approach for the parameter estimation of unbalanced th...

This manuscript describes the design and numerical implementation of a novel composite differential neural network aimed to estimate nonlinear uncertain systems. A differential neural network (DNN) with a composite feedback matrix approximates the structure of non-linear uncertain systems. The feedback matrix is assumed to belong to a convex set as...

Circular limit cycle oscillator- frequency locked loop (CLO-FLL) is a recently proposed advanced nonlinear technique for the parameter estimation of grid voltage signal. It has fast convergence and can handle DC bias/offset. Nevertheless, the performance degrades in the presence of harmonics. In this letter, we propose the application of pre-loop f...

Harmonic robust grid synchronization technique has been studied in this paper. This implies the estimation of phase and frequency of single-phase grid voltage signal. Motivated by recent developments in linear harmonic oscillator based technique, this paper first propose an adaptive observer based solution that has the potential to speed-up the dyn...