M. Moshrefi-Torbati

Physical and geometrical interpretation of fractional operators

In this paper an interpretation of fractional operators in the time domain is given. The interpretation is based on the four concepts of fractal geometry, linear filters, construction of a Cantor set and physical realisation of fractional operators. It is concluded here that fractional operators may be grouped as filters with partial memory that fall between two extreme types of filters with complete memory and those with no memory. Fractional operators are capable of modelling systems with partial loss or partial dissipation. The fractional order of a fractional integral is an indication of the remaining or preserved energy of a signal passing through such system. Similarly, the fractional order of a differentiator reflects the rate at which a portion of the energy has been lost.

Signal processing techniques applied to human sleep EEG signals - a review

Abstract A bewildering variety of methods for analysing sleep EEG signals can be found in the literature. This article provides an overview of these methods and offers guidelines for choosing appropriate signal processing techniques. The review considers the three key stages required for the analysis of sleep EEGs namely, pre-processing, feature extraction, and feature classification. The pre-processing section describes the most frequently used signal processing techniques that deal with preparation of the sleep EEG signal prior to further analysis. The feature extraction and classification sections are also dedicated to highlight the most commonly used signal analysis methods used for characterising and classifying the sleep EEGs. Performance criteria of the addressed techniques are given where appropriate. The online supplementary materials accompanying this article comprise an extended taxonomy table for each section, which contains the relevant signal processing techniques, their brief descriptions (including their pros and cons where possible) and their specific applications in the field of sleep EEG analysis. In order to further increase the readability of the article, signal processing techniques are also categorised in tabular format based on their application in intensively researched sleep areas such as sleep staging, transient pattern detection and sleep disordered breathing diagnosis.

Effect of tempering conditions on the mechanical properties of ductile cast iron with dual matrix structure (DMS)

In this work, the effects of tempering time and temperature on the mechanical properties of a new class of ductile cast iron is investigated. The results show that for a tempering temperature range of 450–500°C, there is a sudden rise in the impact strength and ductility. By increasing the tempering temperature, the ultimate tensile strength drops initially, and within the range of 400–500°C, remains almost constant and is then followed by a decrease. With increasing the tempering time, the ultimate tensile strength and yield stress decrease for tempering periods up to 120 min, the impact strength increases for periods up to 90 min and ductility increases for periods up to 120 min.

Passive vibration control of a satellite boom structure by geometric optimization using genetic algorithm

In this paper, the superior mid-frequency vibration isolation of a geometrically optimized lightweight structure is demonstrated. The initial structure under test here was a 4.5 m long satellite boom consisting of 10 identical bays with equilateral triangular cross-sections. An unusual geometric variant of this, with inherent isolation characteristics, has been designed by the use of genetic algorithm (GA) methods. In order to obtain the best design, the joints in the boom were allowed to move around by 20% of the length of each bay (i.e., ±9 cm in all three translational directions). This work is based on results from a Fortran code (which was derived from receptance analysis) that are fully validated against detailed finite element (FE) models of the structure. The experimental forced response of the regular boom structure has been measured and compared with predicted curves. Finally, having obtained the geometrically optimized boom structure, its experimental response is compared with the theoretical results predicted by the receptance method. It is shown that the average of 30 dB isolation in the vibration energy transfer between the ends of the network of beams, over a 100 Hz bandwidth predicted in the design process, is achieved experimentally in an essentially undamped structure.

Output power and efficiency of electromagnetic energy harvesting systems with constrained range of motion

In some energy harvesting systems, the maximum displacement of the seismic mass is limited due to the physical constraints of the device. This is especially the case where energy is harvested from a vibration source with large oscillation amplitude (e.g., marine environment). For the design of inertial systems, the maximum permissible displacement of the mass is a limiting condition. In this paper the maximum output power and the corresponding efficiency of linear and rotational electromagnetic energy harvesting systems with a constrained range of motion are investigated. A unified form of output power and efficiency is presented to compare the performance of constrained linear and rotational systems. It is found that rotational energy harvesting systems have a greater capability in transferring energy to the load resistance than linear directly coupled systems, due to the presence of an extra design variable, namely the ball screw lead. Also, in this paper it is shown that for a defined environmental condition and a given proof mass with constrained throw, the amount of power delivered to the electrical load by a rotational system can be higher than the amount delivered by a linear system. The criterion that guarantees this favourable design has been obtained.

Constrained Design Optimization of Vibration Energy Harvesting Devices

Existing design criteria for vibration energy harvesting systems provide guidance on the appropriate selection of the seismic mass and load resistance. To harvest maximum power in resonant devices, the mass needs to be as large as possible and the load resistance needs to be equal to the sum of the internal resistance of the generator and the mechanical damping equivalent resistance. However, it is shown in this paper that these rules produce suboptimum results for applications where there is a constraint on the relative displacement of the seismic mass, which is often the case. When the displacement is constrained, increasing the mass beyond a certain limit reduces the amount of harvested power. The optimum load resistance in this case is shown to be equal to the generator’s internal resistance. These criteria are extended to those devices that harvest energy from a low-frequency vibration by utilizing an interface that transforms the input motion to higher frequencies. For such cases, the optimum load resistance and the corresponding transmission ratio are derived.

Characterization of new PEEK/HA composites with 3D HA network fabricated by extrusion freeforming

Addition of bioactive materials such as calcium phosphates or Bioglass, and incorporation of porosity into polyetheretherketone (PEEK) has been identified as an effective approach to improve bone-implant interfaces and osseointegration of PEEK-based devices. In this paper, a novel production technique based on the extrusion freeforming method is proposed that yields a bioactive PEEK/hydroxyapatite (PEEK/HA) composite with a unique configuration in which the bioactive phase (i.e., HA) distribution is computer-controlled within a PEEK matrix. The 100% interconnectivity of the HA network in the biocomposite confers an advantage over alternative forms of other microstructural configurations. Moreover, the technique can be employed to produce porous PEEK structures with controlled pore size and distribution, facilitating greater cellular infiltration and biological integration of PEEK composites within patient tissue. The results of unconfined, uniaxial compressive tests on these new PEEK/HA biocomposites with 40% HA under both static and cyclic mode were promising, showing the composites possess yield and compressive strength within the range of human cortical bone suitable for load bearing applications. In addition, preliminary evidence supporting initial biological safety of the new technique developed is demonstrated in this paper. Sufficient cell attachment, sustained viability in contact with the sample over a seven-day period, evidence of cell bridging and matrix deposition all confirmed excellent biocompatibility.

Simulation of a solar powered air compressor

In this paper we discuss the simulation of a solar powered air compressor comprising a cylinder and a compressor coupled to a vector controlled induction motor which is fed from a photovoltaic array with maximum power point tracking (MPPT). The induction motor speed is controlled to maintain a constant pressure in the cylinder. In the model, the effect of temperature difference between the inside and outside of the cylinder on compressor efficiency is taken into account. The model is used to investigate the operation of the system and provide an insight into the effect of temperature, load and output flow rate on system efficiency. The model will help with design of the control system.

Maximum Power Point Tracking of a Small-Scale Compressed Air Energy Storage System

This paper is concerned with the maximum power tracking of a pneumatically driven electric generator in a standalone small-scale compressed air energy storage system. In this system, an air motor is used to drive a permanent-magnet direct-current generator, whose output is controlled by a buck converter supplying power to a resistive load. The output power of the converter is controlled such that the air motor operates at a speed corresponding to the maximum power. A maximum power point (MPP) tracking controller employs a hybrid perturb and observe method. The rate of change of the converters output power with respect to its duty cycle and the change of the power and the duty cycle are used to correct the search direction under transient input power fluctuations. Small speed step changes are used in the vicinity of the MPP to improve the accuracy of the search algorithm. However, relatively coarse speed step changes are used when the operating point is far from the MPP to improve the dynamic response of the controller and to increase its speed of convergence. The analysis and design of the controller are based on a small injected-absorbed current signal model of the power converter. The controller is experimentally implemented using a real-time digital signal processor system. Test results are presented to validate the proposed design and to demonstrate its capabilities.

Energy harvesting from a rotational transducer under random excitation

This paper evaluates the performance of a proposed device for harvesting energy from the vertical motion of boats and yachts under broadband and band-limited random vibrations. The device comprises a sprung mass coupled to an electrical generator through a ball screw. The mathematical equations describing the dynamics of the system are derived. Then by utilizing the theory of random vibration, the frequency response function of the system is obtained. This is used to derive an expression for the mean power produced by the harvester when it is subjected to broadband and band-limited stationary Gaussian white noise. The power expressions are derived in dimensional form to provide an insightful understanding of the effect of the physical parameters of the system on output power. An expression for the optimum load resistance to harvest maximum power under random excitation is also derived and validated by conducting Monte-Carlo simulation. The discussion presented in the paper provides guidelines for designers to maximize the expected harvested power from a system under broadband and band-limited random excitations. Also, based on the method developed in this paper, the output power of a rotational harvester subjected to the vertical excitation of a sailing boat is obtained.