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This study aimed to compare the effect of needle puncture and chondroitinase ABC (ChABC) injection on inducing intervertebral disc (IVD) degeneration (IVDD) in rabbits.Sixteen New Zealand white rabbits were used in this study. Briefly, the rabbits were divided into four groups. In the annulus fibrosis (AF) needle puncture group, a 16-G needle was used to puncture the L5-6 and L6-7 IVDs, while in the sham group, these IVDs were not punctured. In the ChABC group, 30 μL 0.5 Unit/mL ChABC was injected into L5-6 and L6-7 IVDs using a 26-G needle, while in the vehicle group, these IVDs were injected with 30 μL phosphate-buffered saline (PBS). X-ray and MRI scans were performed at the 4th, 12th and 16th weeks postoperatively. Histological, immunohistochemical and biochemical analyses were performed at the 16th week postoperatively.Both needle puncture and ChABC successfully established IVDD in rabbits at 4th, 12th and 16th weeks, confirmed by X-ray and MRI scan. The progression of IVDD went in a time-dependent manner. The IVDD in the ChABC group was less severe than in the needle puncture group throughout the study. Aggrecan and type II collagen significantly decreased, while tumor necrosis factor-α and superoxide dismutase 2 increased in the needle puncture and ChABC groups, compared with the sham and PBS groups.Both AF needle puncture and ChABC injection can successfully induce IVDD in rabbits. Compared with ChABC injection, AF needle puncture can induce more severe IVDD. more...

Stator and rotor cores of interior permanent magnet (IPM) motors are usually saturated, and the saturation distribution and saturation depth vary with the stator current. Also, their rotor structures are complex; hence the saturation cannot be accurately reflected by a saturation factor. These considerations lead to existing equivalent air-gap length models of induction motors and inset permanent magnet (PM) motors no longer suitable for IPM motors. Therefore, based on the consideration of stator and rotor saturation, this paper proposes the equivalent air-gap length model of IPM motors expressed by magnetic potentials on the stator inner surface and the rotor outer surface. The proposed equivalent air-gap length model can accurately reflect the influences of complex structure and severe saturation of IPM motors. Compared with existing models, the proposed one is no longer uniquely determined by motor structure parameters; it is also a function of stator current and rotor position. Furthermore, the obtained equivalent air-gap length model is combined with the winding function and the modified winding function to solve the inductances under different stator currents and different rotor eccentricity states. Finally, the proposed method is verified by both finite element analysis (FEA) and experiments. more...

This article analyzes the realis converbs and irrealis converbs in Hwari (Dpa’ ris) Tibetan, an Amdo Tibetan dialect spoken in Tianzhu Tibetan Autonomous County, Wuwei City, Gansu Province, China. One peculiarity of Hwari Tibetan morphosyntax is the existence of a large set of converbal suffixes. A survey of previous descriptions reveals a lack of consensus in the analysis of these converbs. They have been included in the framework of Written Tibetan too often, which has led to confusion regarding their typological status. A primary objective of our paper is to rectify this approach by describing the language on its own terms by comparing the morphosyntactic behavior of eight selected suffixes with regard to relevant criteria, such as realis and irrealis. This empirical research feeds back into the theoretical discussion, as it shows that the Hwari Tibetan converbs can be described using conventional typological concepts if construed appropriately. Our second objective is to explain the semantic and pragmatic properties of these converbs, particularly -na and -ta. The conclusion is that the two converb types are located along a continuum from realis/irrealis to positive/negative, thus constituting an instantiation of syntax-semantics isomorphism. The clause chaining in Hwari Tibetan is representative. By describing the converbs in Hwari Tibetan from the perspective of realis and irrealis, and by focusing on the syntactic properties and pragmatic features of -Ce in realis converbs and -Ca in irrealis converbs, it is revealed that the irrealis converbs -na and -ta function in a unique way in Hwari Tibetan. more...

Open-winding permanent magnet synchronous machines (OW-PMSMs) with a common dc bus have gained increasing research focus in recent years due to its advantages including multilevel modulation and flexible control capability. However, a low impedance zero-sequence electrical path exists in this type of configuration. Significant uncontrolled zero-sequence current will result in unexpected power losses, reduced efficiency, and undesired high-frequency torque ripples. The linear proportional resonant controller has been previously proved to be an effective solution to suppress the zero-sequence current. However, the zero-sequence current regulation performances will be degraded, even become unstable, when the switching to operating frequency ratio is reduced, i.e., when the switching frequency is decreased or the machine operating frequency is increased. This article addresses this issue using a discrete-time controller, which is designed based on virtual three-phase expansion. The exact closed-form hold-equivalent discrete model, including the zero-order hold characteristic and the control delay, of OW-PMSMs is derived. The zero-sequence components are expanded to virtual three phases and, then, converted to direct signals by means of reference frame transformation. A synchronous-frame state-space framework for OW-PMSMs is designed directly in the discrete-time domain, which can obtain improved control performances with decreased switching frequency and increased operating frequency. Relevant experimental validations and performance comparisons based on a 2.3-kW OW-PMSM setup is presented to confirm the effectiveness of the proposed controller. more...

This article introduces a method to achieve the variable action period (VAP) of voltage vector in the finite control set model predictive control (FCS-MPC). In the conventional FCS-MPC, the control behavior is limited since the action period of a voltage vector is normally fixed to the sampling period. This article aims to enhance the control performances by means of regulating the action period of selected voltage vectors, together with the selection of an optimal control action. A VAP-based predictive flux control strategy is thus proposed. The optimal voltage vector and its optimal action period are both obtained by means of a two-step cost function minimization. In this manner, the future states of the control objectives are taken into account in the construction of cost function. The action period for the selected voltage vectors is no longer fixed to the sampling period, which provides an additional control freedom to obtain improved reference tracking behaviors. Experimental validations and comparison studies demonstrate that enhanced steady-state performances as well as excellent dynamic behaviors can both be obtained with the proposed strategy adopted. more...

Open-end winding permanent magnet synchronous machines (OW-PMSMs) supplied by dual inverters have gained increasing research focus due to its advantages in multilevel modulation, flexible control capability, and enhanced fault-tolerant performance. Zero-sequence current is one of the most concerned issues when the common dc bus configuration is adopted. Feedback control of zero-sequence current has been previously proved to be an effective solution to suppress zero-sequence current. However, due to the existence of current measurement errors, dc disturbance and periodic disturbances will inevitably occur in zero-sequence current, which may result in obvious deteriorations of the suppression performance of zero-sequence current. Zero-sequence current circulates in the OW-PMSM, which will lead to unexpected power losses and reduced efficiency. In addition, current measurement errors also cause increased periodic disturbances in the torque and the rotor speed, resulting in torque ripples and speed fluctuations. To address these problems, the exact closed-form hold-equivalent model of OW-PMSMs is derived, and the specific periodic disturbances introduced by current measurement errors are analyzed. Subsequently, zero-sequence current controller and speed controller are designed with enhanced mitigation capability of periodic disturbances resulting from current measurement errors. Its effectiveness under different operation conditions is confirmed experimentally on a 2.3-kW OW-PMSM setup. more...

In the control of permanent magnet synchronous machines (PMSMs), predictive controllers have received considerable attention due to the enhanced dynamic responses. However, periodic disturbances caused by dead-time effects and sampling errors deteriorate the performances of PMSMs. In order to suppress periodic disturbances, one typical control structure is to adopt several corresponding resonant controllers connected in parallel with the main controller. Even though excellent steady-state performances can be obtained, coupling effects between reference tracking and periodic disturbance attenuation can be observed, leading to significant transient oscillations under the situation of a reference step change. In order to realize decoupling between reference tracking and disturbance attenuation, this paper proposes a dual-loop predictive control structure, in which the reference tracking is only determined by the primary predictive controller. Periodic disturbance attenuation is achieved through an embedded disturbance rejection loop. By applying the proposed controller, periodic disturbances are strongly attenuated while smooth and fast transient performances are simultaneously guaranteed. Furthermore, a wide range of low frequency harmonics can be successfully suppressed, including but not limited to the main periodic harmonics. The steady-state performance, dynamic responses, and robustness against parameter variations of the proposed controller are verified through experiments based on a 2.3-kW PMSM. more...

The semicontrolled open-winding permanent magnet synchronous generators (OW-PMSGs) with a common dc bus system have received more attention in recent years due to its advantages of reduced cost and simpler power supply. Good control performances could be achieved under the zero power factor angle (PFA). However, current clamping problem is normally presented under non-zero PFA condition, resulting in unexpected torque pulsations and voltage pulse disturbances. Meanwhile, misjudgment of voltage vectors and subsequent deteriorations in steady-state performances can be observed at current zero-crossing points. In order to improve the control performances of semicontrolled OW-PMSGs, a virtual zero-crossing detection method is proposed in this paper to effectively suppress the oscillation of voltage vectors at current zero-crossing points. In addition, a segment injection strategy of third-order harmonic current is proposed. With the proposed strategy adopted, the current clamping problem can be successfully solved, and more importantly, the torque ripple and third-order harmonic current could be simultaneously suppressed. Relevant experimental validations and performance comparisons based on a 2.3-kW semicontrolled OW-PMSG system are presented to verify the proposed control strategies. more...

Model predictive control (MPC) strategies have attracted wide attention due to its inherently high dynamic characteristics and flexible control capability. When the MPC is adopted to regulate the currents of permanent magnet synchronous machines, the cost function is typically constructed based on tracking errors of current components. Reduced switching frequency can be obtained by means of minimizing a revised cost function which includes a penalty item related to the predicted switching frequency. This will inevitably affect current tracking performances and, meanwhile, increase the computational burden due to the required long-horizon minimization. In order to solve this problem, this paper proposes a predictive current controller which inherently takes the converter zero-order-hold characteristics and system delay into account. Besides, with the aim of simultaneously realizing system behavior prediction and disturbance estimation, a structure of parallel extended-state observers is proposed. Theoretical analyses and experimental validations are both conducted to confirm its effectiveness. It is demonstrated that, under the situation of reduced switching frequency, improved control performances can be expected when the proposed controller is adopted, including highly dynamic responses inherently presented by typical predictive controllers and enhanced steady-state performances with the help of parallel observation structure. more...

The mass fraction burned (MFB) of the gas mixture in combustion process as the function of the combustion duration time, reflects the percentage of fuel consumed at the every moment in the combustion process, and is an important factor for analysing of the fuel combustion performance, and it has high engineering value for evaluating engine design parameters and operating performance and for improving the combustion process in-cylinder combustion process. The ion current signal directly originated from the fuel combustion process in the cylinder, and contained much information about the combustion process; the MFB of fuel in combustion process can be obtained based on the correlation between the MFB and ion current signal. In this paper, the ion current method is constructed to calculate the MFB of fuel in combustion process under different initial conditions (initial pressure from 1 bar to 3 bar, equivalence ratio from 0.8 to 1.2, hydrogen mass fraction from 0% to 10%, and CO2 dilution ratio from 0% to 10%), the method based on the waveform parameters of ion current signal for obtaining the efficiency factor and the form factor of Wiebe function, and further to slove the mass fraction burned of combustion process. The mass fraction burned is obtained through ion current method reduced assumptions error and improved the calculation accuracy compared with the well-known Rassweiler–Withrow method, and the ion current method has an advantage that not requires an high-cost sensor and some rather complex measuring instruments. The ion current method is a simple but powerful calculation model that provide a new technical path for analysing and online obtaining the MFB of fuel in combustion process. more...

The characteristics of the ionization current, pressure, and flame schlieren photos during the combustion process of natural gas-hydrogen/CO2 blends were studied using high-speed schlieren data acquisition technology and flame ionization analysis methods. According to the ion current waveform, pressure waveform, and flame propagation photos, the relationship of the characteristic parameters between the ion current and pressure were analysed using available experimental data. The ionization current waveform can reflect not only the entire combustion process (including ignition phase, the front flame phase and the post flame phase) but also the effect of the initial pressure, equivalence ratios, hydrogen fraction and CO2 dilution ratios on the combustion process. This results of this study show that the amplitude and characteristic timing of the ion current can be used to predict the change of the chamber pressure. The ion current and pressure will increase with the increase of the initial pressure, equivalence ratio and hydrogen fractions, but the parameters will decrease with the increase of the CO2 dilution ratio. The characteristic timing of the ion current and pressure peaks is advanced with the increase of the equivalence ratio and hydrogen fractions; however, the peaks are postponed by the increase in the initial pressure and CO2 dilution ratio. The corresponding relationship of the characteristic timing between the ion current and pressure is basically unchanged. The results of this study provide a new approach for exactly controlling the combustion process. more...

In this paper, an observer-based predictive vector-resonant current control strategy for permanent magnet synchronous machines is proposed, which can effectively improve system performance in case of disturbances caused by converter dead-time effect and model mismatch. Taking the external periodic, nonperiodic disturbances, and system delay into consideration, the system model in discrete-time domain is built. This model is then embedded with vector-resonant scheme with the aim of suppressing periodic disturbances resulting from converter dead-time effect and rotor flux harmonics. An extended-state observer is constructed in order to predict system behavior and to estimate the nonperiodic disturbances caused by parameter variations. The overall control law is then derived on basis of cost function minimization. With the proposed method adopted, the undesired anomalous peak around the resonant frequency in the frequency responses can be avoided. Moreover, enhanced transient performances and wider stability margin can also be obtained. The effectiveness and advantages of the proposed strategy are verified by frequency-domain analysis and comparative experimental studies. more...

In this study, the flame propagation characteristics of premixed natural gas–hydrogen–air mixtures were studied in constant volume combustion bomb by using the high-speed schlieren photography system. The flame radius, laminar flame propagation speed and the flame stretch rate were obtained under different initial pressure, temperature, equivalence ratios and hydrogen fractions. Meanwhile, the flame stability and their influencing factors were obtained by analyzing the Markstein length and the flame propagation schlieren photos under various combustion conditions. The results show that the stretched laminar propagation speed increases with the increase of the initial temperature and hydrogen fraction of the mixture, and will decreases with the increase of the initial pressure. Meanwhile, according to the Markstein length and the flame propagation pictures, the flame stability decreases with the increase of the temperature and hydrogen fraction, and the slight flaws occurred at the early stage; at larger flame radius, the flame stability is more sensitive to the variation of the initial temperature and hydrogen fraction than to that of initial pressure and equivalence ratio. more...

In this paper, a predictive direct power control (DPC) method without sector information and voltage vector selection is proposed for three-phase grid-connected converters. Different from conventional predictive direct power controllers, the proposed strategy always adopts fixed voltage vectors, instead of selecting voltage vectors according to the angular information of the grid-voltage vector or the virtual-flux vector. The converter switching time is obtained by minimizing the squared errors of the instantaneous active and reactive power. Switch duty cycles are then calculated and equivalently reconstructed. The proposed strategy not only presents rapid dynamic response due to the use of the predictive controller, but also possesses excellent steady-state performance as a result of duty cycle reconstruction. The main advantage of the proposed control scheme, compared with the classical DPC strategies, is that it is unnecessary to use sector information of the grid-voltage vector, and selection of active voltage vectors is also not required here. Therefore, incorrect selection of voltage vectors and the resulting performance deterioration are surely avoided, without the need of any additional compensation measures. An in-depth comparison and experimental assessment are given to validate the effectiveness of the proposed strategy. more...

In the wind energy conversion system (WECS) based on the permanent magnet synchronous generator, the topology that a wind generator is connected to a voltage source inverter through a diode rectifier and a boost chopper is widely adopted for its low cost and high reliability. However, with the increasing power level of the WECS, the power switch device of a boost chopper in the topology has to endure high instantaneous voltage and current stress, thus reducing the system reliability. Although the traditional multiple boost converter reduces the transmission power level of single circuit units in terms of structure, it is still limited by narrow speed regulation range. Thus, a novel switchable-cascaded-mode boost chopper is proposed in this paper. The operation principle of the new converter and the generator speed control strategy are analyzed. At last, the effectiveness of the new converter is verified with an experimental prototype. The experimental results demonstrate that speed range is extended by using the new topology as generator-side converter in the permanent magnet wind generation system. more...

A predictive current controller with an extended-state observer (ESO) is proposed for grid integration of wind energy systems. In each sampling period, the proposed strategy calculates the converter switching time that minimizes a cost function defined as a sum of squared current errors, leading to constant switching frequency. To achieve excellent dynamic performance, the impact of sampling delay is analyzed, and detailed compensation methods are proposed. In addition, an ESO is constructed to suppress parameter variations and modeling errors, which affect the performance of the controller. The parameter tuning and the stability of the observer are analyzed. The proposed strategy not only presents rapid dynamic response due to the use of the predictive current controller but also possesses robust control performance as a result of the observation algorithm. Simulation and experimental results are given to validate the effectiveness of the proposed solution. more...

Due to the excellent dynamic performance, the Finite Control Set Model Predictive Control has been widely used in various types of converters. However, when Finite Control Set Model Predictive Control is adopted, the switching frequency of converters varies significantly with system operating conditions. Consequently, constant-frequency predictive control strategy has been proposed. Two active voltage vectors and a zero voltage vector are selected within each sampling period. The action time sequence is then calculated. Due to the unsymmetrical distribution of current variation rates around zero, the calculated value of the voltage-vector action time will turn up negative. According to common sense, the voltage-vector action time is greater than or equal to zero. The action time is normally forced to zero whenever a negative value is predicted, resulting in the control failure and performance deterioration. To solve this problem, this paper proposes modified strategy. The modified strategy examines the action time calculated out. When negative action time comes out, the modified strategy reselects the active voltage vector accordingly, instead of forcing the action time to be zero. Optimized action time sequence is further determined by minimizing the cost function. The effectiveness of the modified strategy is clearly verified by experimental tests, and analytical remarks are all founded in practical results. more...

This paper presents a robust model predictive current controller with a disturbance observer (DO-MPC) for three-phase voltage source PWM rectifier. The new algorithm is operated with constant switching frequency (CF-MPC). In order to minimize instantaneous d- and q-axes current errors in every sampling period, CF-MPC is implemented by selecting appropriate voltage vector sequence and calculating duty cycles. The fundamental of this algorithm is discussed and the instantaneous variation rates of d- and q-axes currents are deduced when each converter voltage vector is applied in six different sectors. A Luenberger observer is constructed for parameter mismatch and model uncertainty which affect the performance of the MPC. The gains of the disturbance observer are determined by root-locus analysis. Moreover, the stability of the disturbance observer is analyzed when there are errors in the inductor filter parameter. The proposed method has an inherent rapid dynamic response as a result of the MPC controller, as well as robust control performance with respect to the disturbance due to use of the combined observation algorithm. Simulation and experimental results on a 1.1 kW VSR are conducted to validate the effectiveness of the proposed solution. more...

The Halbach magnet array has become more and more attractive in many electromagnetic engineering domains such as electrical motors for its potential features such as self-shielding and providing sinusoidal field distribution. This paper develops analytical models formulated in polar coordinates for predicting the field distribution of a segmented Halbach magnet cylinder with or without back iron. Based on the analytical models, the relationships between the air-gap flux density and four design parameters including pole-pair number, segment number per pole, permanent-magnet radial dimension, and air-gap length are analyzed. Further, the Taguchi method is employed to identify the settings of design parameters and determine the parameters which have a significant effect on the field distribution. The analytical models are verified by taking advantage of the finite-element method (FEM), which shows that all the results can be of considerable use in the design of permanent-magnet machines. more...

A novel adaptive current controller for DFIG-based wind turbines is introduced in this paper. The attractiveness of the proposed strategy results from its ability to actively estimate and actively compensate for the plant dynamics and external disturbances in real time. Thus, the control strategy can successfully drive the rotor current to track the reference value, ensuring that the performance degradation caused by grid disturbances, cross-coupling terms and parameter uncertainties can be successfully suppressed. Besides, the two-parameter tuning feature makes the control strategy practical and easy to implement in commercial wind turbines. To quantify the controller performances, the transfer function description of the controller is derived. General disturbance rejection, robustness against parameter uncertainties, bandwidth and stability are also addressed. Simulation results, together with the time-domain responses, proved the stability and the strong robustness of the control system against parameter uncertainties and grid disturbances. Significant tracking and disturbance rejection performances are achieved. more...

With increasing wind power penetration, transient responses of doubly-fed-induction-generator (DFIG) based wind turbines gain attentive focus. Accurate prediction of transient performance of DFIG under grid faults is required with increasing wind power penetration. Taking into account the main flux saturation and deep-bar effect, this paper concentrates on transient responses and stability of the DFIG system under symmetrical grid faults. Their roles played in the enhancement of system transient stability are clarified. The analyses proposed contribute greatly to proper selection, design and coordination of protection devices and control strategies as well as stability studies. more...

A novel control strategy is proposed in this paper for the rotor side converter (RSC) of doubly-fed induction generator (DFIG)-based wind power generation systems. It is supposed to enhance the low-voltage ride-through (LVRT) capability of DFIGs during great-level grid voltage dips. The strategy consists of a proportional-resonant (PR) controller and auxiliary PR controllers. The auxiliary controllers compensate the output voltage of the RSC in case of grid faults, thus limiting the rotor inrush current of DFIG and meeting the requirements of LVRT. Sequential-component decompositions of current are not required in the control system to improve the response of system. Since the resonant compensator is a double-side integrator, the auxiliary controllers can be simplified through coordinate transformation. The feasibility of the control strategy is validated by simulation on a 1.5 MW wind-turbine driven DFIG system. The impact of the RSC converter voltage rating on the LVRT capability of DFIG is investigated. Meanwhile, the influence of angular frequency detection and control parameters are also discussed. Compared with traditional vector control schemes based on PI current controllers, the presented control strategy effectively suppress rotor current and reduce oscillations of DFIG power and torque under grid faults. more...