Frequency Characteristic Curve Research Articles

Research on the Control and Modulation Scheme for a Novel Five-Switch Current Source Inverter

Different from the voltage source inverter (VSI), the current source inverter (CSI) can boost the voltage and eliminate the additional passive filter and dead time. However, the DC-side inductor current is not a real current source and is generated by a DC voltage supply and an inductor. Under different switching states, the DC-side inductor will be charged or discharged, which leads to the DC-side inductor current being discontinuous or increasing. To solve the control problem of the DC-side inductor current of the CSI, a novel single-phase CSI topology with five switching tubes for grid-connected applications is proposed. Firstly, the reference calculation method and the hysteresis loop control scheme for the DC-side inductor current are proposed, and the adjustable and constant DC-side inductor current are obtained. Since the PWM signals cannot be directly implemented to the switching tubes, the modulation strategy for the single-phase CSI is proposed in this paper. Then, an active damping method based on the feedback capacitor voltage is presented to suppress the resonance peak caused by the LC filter on the grid side. Finally, the math model of the AC-side structure is established, and the optimal proportional-resonant controller parameters’ design method is explored by the amplitude–frequency characteristic curves. The simulation and experiment are implemented for the proposed CSI topology. The results show that a high-quality power with a good control performance can be obtained with the proposed CSI topology.

Research on Nonlinear Vibration of Dual Mass Flywheel Considering Piecewise Linear Stiffness and Damping

Nonlinear torsional vibration differential equation of the nested arc-shaped short spring dual mass flywheel (DMF) is established, considering the piecewise linear stiffness and damping of the spring. The first-order approximate analytical solution under sinusoidal excitation and the amplitude–frequency characteristic function are obtained by means of the average method which verified by the Runge–Kutta (R–K) method. The effects of the parameters of input excitation, inertia, and piecewise linear stiffness and damping of DMF on the resonant amplitude, resonant frequency band, and equivalent linear natural frequency of the system are analyzed. The results show that the amplitude–frequency characteristic curve bending and jumping with the changes of excitation frequency and the peak of resonant amplitude can be obviously reduced by increasing the inertia of the primary flywheel and decreasing the inertia of the secondary flywheel. The complex nonlinear dynamic phenomena such as Period 1, quasi-periodic, and chaos are obtained by analyzing the forced vibration response under the different excitation frequencies.

Analysis of factors influencing electric energy measurement in power systems

This article focuses on summarizing and analyzing the causes of errors in electric energy meters under harmonic conditions, clarifying the relationship between errors and frequency characteristic curves, and pointing out that the positive and negative errors mainly depend on the direction of each harmonic power relative to the fundamental power, the properties of the load, the differences in physical properties of electric energy meter manufacturing and material formation, etc.

Modeling and Simulation of the Stepping Motor Operation Curve

To improve the open-loop control performance of stepper motors, research was conducted on their operating curves. The acceleration-velocity characteristic equations of three common operating curves were established and compared with the torque frequency characteristic curves of stepper motors. Based on this, an improved segmented correction fitting S-shaped (SCFS) curve design method based on a high torque utilization sine curve was proposed to solve the problem of rotor blockage and overshoot caused by uneven acceleration during the operation of stepper motors. In the simulation of the stepper motor load motion system, the SCFS curve was compared with common curves. The simulation results showed that the position tracking error was reduced by approximately 82% compared to the trapezoidal curve and approximately 70% compared to the exponential curve. Research has found that this method is suitable for the design of high-precision motor operating curves and has a high application value in the field of precision motion control.

Analysis on Thermo-Electrical Principal Parametric Resonance of an Axially Moving Piezoelectric Thin Plate

In this paper, principal parametric resonance of an axially moving piezoelectric rectangular thin plate under thermal and electric field is investigated. Based on Kirchhoff–Love plate theory and Von Karman theory, the transverse vibration differential equation of a piezoelectric rectangular thin plate under thermal and electric field is derived by using Hamilton’s principle. The dimensionless vibration equation of piezoelectric rectangular thin plate with parametric excitations is discretized by Galerkin’s method. Then, the multiple scales method is applied to derive amplitude-frequency response equation and the stability conditions of the steady-state solution are obtained by Lyapunov stability theory. Numerical method is used to find the influences of specific parameters on the vibration performance and stability of the system. Based on the global bifurcation diagram and corresponding response diagram, the influences of bifurcation control parameters on the nonlinear dynamic characteristics of the system are discussed. Numerical results illustrate that the system amplitude frequency characteristic curve presents soft spring characteristics. There are periodic and chaotic motions with the increase of velocity and the central temperature difference, and the decrease of plate thickness and velocity will result in the decrease of chaotic threshold. The results also show that increase the velocity perturbation amplitude can prolong the chaotic motion.

Electrohydraulic Servo Shaker Control Based on the Single Layer Flexible Structure Load

As a kind of equipment commonly applied in laboratory vibration test simulation, the shaker has been widely used in many important fields of scientific research on experimental engineering vibration. It is employed for the numerical simulation of seismic activities of large buildings to quickly and quantitatively detect and calculate building components related to different building forces, analyze and study various changes of on-site structural forms and the influence mechanism of common earthquake damages or deformations with the actual existence of large-scale seismic activities, as well as to test the antiseismic strengthening structure of buildings to reduce the impact of destructive earthquakes on buildings losses. In general, the load of the shaker is regarded as a rigid structure. But in terms of the load of an electrohydraulic servo shaker with a flexible structure, the system control tends to be nonlinear accompanied by the output signals distorted. In order to achieve the goal of improving the waveform accuracy of shaking tables, it is necessary to combine the corresponding control algorithm strategy. Hence, in compliance with the working principles and functions of shaking table systems, the reasons for the coupling between flexible structure loads and shaking tables are carefully analyzed in this paper. Meanwhile, a control strategy based on the Luenberger observer control method is proposed to effectively realize the coupling suppression within the system and make the output signals of the shaking table reach the required accuracy. Finally, by comparing the frequency characteristic curves of the system before and after the introduction of the Luenberger observer, as well as the output and input analog signals, it can be seen that the Luenberger observer can effectively suppress the impact of oscillations within the system and reproduce the accuracy of the input signals.

Dynamics analysis for a class of fractional Duffing systems with nonlinear time delay terms

The dynamic characteristics of the fractional Duffing system with nonlinear time delay term are studied according to the average method and the harmonic balance method. First, the steady‐state solutions of the fractional Duffing system with a nonlinear time delay term are obtained by applying the average method and the harmonic balance method. The analytical expressions of the amplitude–frequency characteristic curves under the average method and the harmonic balance method are also established. Second, the stability criterion of the system is obtained according to the indirect method of studying the motion stability problem, and the concept of the stability condition parameter is proposed in this process. Third, the transient solution of the fractional Duffing system with a nonlinear time delay term is obtained utilizing the average method, and the approximate analytical solution of the fractional Duffing system with nonlinear time delay term is obtained. Finally, the amplitude–frequency characteristic curves of the different fractional differential terms, coefficients of different fractional differential terms, magnitude of different time delay quantities, and feedback strength of different delay are analyzed by numerical simulation. Furthermore, the amplitude–frequency characteristic curves obtained by the average method and the harmonic balance method under different order of fractional differential term are compared and analyzed by numerical simulation. The time sequence and phase diagrams of the system under different order of fractional differential term and different time delay quantities are analyzed by numerical simulation. In addition, the correctness of the analytical analysis is verified by comparing the analytical results with the numerical simulation results under the average method and the harmonic balance method, respectively, by numerical simulation.

Study on Hopf bifurcation types and dominant parameters of grid‐connected voltage source converter considering non‐linear saturation elements

AbstractSubsynchronous oscillation events of grid‐connected voltage source converter (VSC) pose a potential risk to the security and stability of power systems. It is particularly challenging to reveal the subsynchronous oscillation mechanism precisely because the grid‐connected VSC consists of non‐linear components such as saturation. The describing function and the generalized Nyquist criterion are used in this paper to investigate the bifurcation types and dominant parameters of grid‐connected VSC. First, the condition of a supercritical Hopf bifurcation is investigated, which generates subsynchronous oscillations. Then, the dominant parameters of supercritical Hopf bifurcation are deduced, including the grid strength, the active power reference, or the proportional coefficient of phase‐locked loop. Finally, it is examined how the saturation's limit value impacts the limit cycle's amplitude and stability. The findings demonstrate that a system's stability depends on whether its closed‐loop frequency characteristic curve intersects the describing function's negative reciprocal curve. When the two curves intersect, the supercritical Hopf bifurcation appears, and changing the limit value can affect the limit cycle's amplitude. However, if the two curves do not intersect, altering the limit value cannot cause the supercritical Hopf bifurcation, and the subsynchronous oscillation will not happen. Variations in grid strength, active power reference, or proportional coefficient of phase‐locked loop will significantly affect the Nyquist curve, determining if supercritical Hopf bifurcation appears.

Ripple Voltage Suppression of Dual Active Bridge Converter in Electric Locomotive Using H∞ Loop Shaping Control Method

The next-generation electric locomotive drive system has the problem of double grid frequency ripple voltage in the DC side of a single-phase PWM rectifier. In this paper, the intermediate stage dual-active-bridge DC-DC converter is studied, and an H∞ loop shaping ripple voltage suppression control strategy is proposed to restrain the harmonic voltage and optimize the dynamic and robust performance of the closed-loop system. Firstly, the mathematical model of the DAB closed-loop control system with fluctuating input voltage is established. Then, based on the H∞ loop shaping control theory, the dynamic performance, the ability of ripple voltage suppression, and the robust performance of the system are analyzed in the frequency domain. According to the results, a reasonable compensator is selected to reconstruct the magnitude frequency characteristic curve of the nominal system. After reconstruction, the solution of the robust stability margin of the corrected object is transformed into the H∞ robust control problem under the coprime factor uncertainty. The suboptimal H∞ robust controller is solved to obtain the best robustness boundary of the constructed system. Finally, the H∞ loop shaping ripple voltage suppression controller is obtained by combining the H∞ controller with the compensator. The experimental environment is set up in MATLAB-Simulink software. Compared with the traditional voltage loop control method and the input voltage feedforward control method, the dynamic performance of the proposed control strategy is slightly improved, and while within the allowable robust boundaries of the system, it has better ripple voltage suppression performance. When the system magnitude frequency curve is outside the tolerant robust stability margin, the ripple voltage suppression performance fails, but the harmonic content of output voltage is still far less than the other two control methods. The experiment results also prove the correctness of the theoretical analysis and that the proposed control method is effective and proper.

Adaptive inertia control of hybrid energy storage system based on Butterworth filter

The large-scale access of new energy to the power grid leads to a significant decrease in the inertia level of the power system, which seriously threatens the system frequency stability. By considering the power complementary characteristics of the supercapacitor and battery, an inertia adaptive control is proposed based on the amplitude–frequency characteristic curves of the Butterworth filter to integrate the inertia level of the system. For this strategy, the virtual inertia voltage control is used to adjust the power output of the supercapacitor to smooth the high-frequency disturbance and imitate the inertia response of the synchronous generators (SGs), which can reduce the number of SGs movements near the dead zone, and at the same time, the virtual synchronous generator (VSG) control is adopted to regulate the power output of the hybrid energy storage system (HESS) to participate in primary frequency regulation. The notable feature of this method is to achieve a smooth switching between the virtual inertia voltage control of the supercapacitor and the VSG control, which can ensure the DC side voltage level and AC side grid frequency level, by constructing an adaptive control model of inertia weight coefficients according to the Butterworth filter. Finally, a regional frequency regulation model with the HESS is built under the MATLAB/SIMULINK environment to verify the effectiveness of the proposed approach.

Research on Harmonic Transfer Characteristics of Capacitive Voltage Transformer and Design of Testing Device

Capacitive voltage transformers (CVTs), as the main voltage detection devices in the power grid, can accurately measure power frequency voltage, but CVT’s measurement of harmonic voltage will pass errors, resulting in CVT cannot accurately monitor the harmonic voltage in the power grid. Based on the current status of CVT, this paper studies the structure of CVT and the harmonic voltage transfer characteristics of CVT, and analyzes the frequency characteristic curve of CVT by using MATLAB simulation. Based on the study of the CVT frequency characteristic curve, this paper proposes a method of series voltage divider capacitor C3 at the ground terminal of CVT low voltage capacitance to indirectly measure the harmonic voltage on the primary side of CVT and verifies the accuracy of this method. Finally, this paper presents a preliminary design scheme for the series capacitor voltage division method.

Determination of intracellular electrical parameters in bioelectrical impedance analysis

Creating a frequency characteristic curve for bioelectrical impedance analysis involves analyzing electrical resistance (impedance) values at different frequencies. This curve is used to evaluate the electrical properties of biological tissues or fluids. In the article, the impedance of the intracellular environment was evaluated and frequency dependence was established based on the electrical circuit model of the biological system. Xc = f(f), Z = f(f), φ= f(f) tgδ= f(f), dependencies were created for different frequency values of the intracellular environment in the Matlab program through a series-connected RC circuit. Regression and general statistical analyzes were performed. The most appropriate frequency range was determined to evaluate the impedance of the intracellular environment. The correlation relationship of electrical and dielectric parameters of the intracellular environment was also taken into account. Purpose: The main goal of the research is the selection of the optimal frequency range for bioimpedance analysis. In particular, the selection of the most optimal frequency was carried out in the assessment of intracellular electrical activity. The frequency range is determined depending on the electrical and dielectric parameters of the cell. Materials and Methods: Using the Matlab program, the equivalent circuit model of the cell was assembled. Frequency dependence of electrical and dielectric parameters characterizing intracellular processes was performed by model-based measurements. Conclusion: According to the obtained value, regression and general statistical analyzes were performed for the dependencies Xc=f(f), Z=f(f), φ=f(f), tgδ=f(f). Based on analysis of variance (Analysis of Variance-ANOVA) of the output variables, statistical parameters were determined for each dependency.

System identification and mechanical resonance frequency suppression for servo control used in single gimbal control moment gyroscope.

Effective identification of the control model is one of the key aspects in improving the performance of the single gimbal control moment gyroscope (SGCMG) servo system. The accuracy and stability of the servo system can be improved by studying system identification and mechanical resonance frequency. In this study, firstly, the SGCMG gimbal servo system was simplified to a two-mass block model. The theoretical mathematical model of the system's transfer function and mechanical resonance frequency was derived. Secondly, this paper studied the effective suppression method for mechanical resonance. Thirdly, the mathematical model of the orthogonal correlation analysis method was deduced for system identification. Then, an experimental platform was investigated to obtain the frequency characteristic curve and the transfer function. Finally, the frequency characteristic curve obtained using the transfer function model was plotted and compared with the frequency characteristic curve obtained experimentally. Our results indicate that the orthogonal correlation analysis has a high identification accuracy.

Vibration Characteristics of Rolling Mill System under Constraints of the Nonlinear Spring Force and Friction Force from Hydraulic Cylinder

Considering the two kinds of nonlinear constraints of rolling mill hydraulic cylinder, spring force and friction force, the vibration model of rolling mill system is established. The amplitude frequency response equations are obtained by using the average method. Comparing the time history curves of vertical vibration displacement of rolling mill system under the nonlinear spring force and friction force, the amplitude frequency characteristic curves are simulated. The external excitation amplitude is viewed as the bifurcation parameter, and the system bifurcation response changing with the external excitation amplitude is analyzed. The influence of the external excitation amplitude on the system stability is studied. The results indicate that the increase of the nonlinear spring force makes the rolling mill system’s unstable area to become wider, and the influence on the rolling mill system of nonlinear friction force behaves as the damping characteristics; the vibration of rolling mill system is alternating between the periodic, period-doubling, and the chaotic motion. The research results provide a theoretical support for restraining the vibration of the rolling mill system in the actual production process.

Investigation on the unsteadiness of centrifugal compressor exposed to pulsating backpressure

Centrifugal compressor is exposed to pulsating backpressure due to the movement of intake valves in internal combustion engine. The performance of compressor deviates from the steady performance map, which affects the matching between turbocharger and engine. The behavior of compressor system at pulsation conditions are investigated via an in-house developed 1D unsteady code validated by experimental results. The influence of pulse frequency, magnitude and compressor characteristic curve on the compressor transient responses, including filling-emptying effect and wave dynamics, are analyzed. Results show that the strength of wave dynamics grows stronger with the increasing of pulse frequency, while the strength of filling-emptying effect increases first then decreases. The rise of pulse magnitude results in an almost linearly increasing of filling-emptying effect, while it can hardly affect the wave dynamics. Furthermore, the influence of pulsation magnitude and frequency represents the influence of local pressure gradient, and a correlation as quadratic curve can be evaluated between the pressure gradient and compressor unsteadiness. On the other hand, the influence of operating point, including the average mass flow rate and the slope of characteristic curve, is confirmed to be evidently smaller, comparing to the influence of pulsation frequency and magnitude. This study is helpful to estimate the behavior of compressor and the discrepancy of performance when operating at unsteady environment or matched with engine.

To transform maifanite powders from insulators to conductors by Sm gaseous penetration technology

For the preparation of low-cost and eco-friendly conductive powders, a novel rare earth gaseous penetration technology was applied to modify maifanite powders. After 5-h penetration at 600 °C, the resistivity of Sm-modified maifanite powders has been greatly reduced from 1.95 × 1010 Ω m to 3.67 Ω m. And the resistivity characteristics of grain interior and grain boundary were discussed by analyzing the Nyquist plots and the impedance frequency characteristic curves. After penetration, the grain resistance and grain boundary resistance of maifanite were reduced to 50 Ω and 900 Ω, respectively. Furtherly, the composition and structure of Sm-modified maifanite powders were analyzed; then, the conductive mechanism of the materials was discussed in detail. SEM patterns showed that the fine particles in Sm-modified maifanite powders increased after penetration, which indicates that rare earth ions have a crushing effect on maifanite powders and improve the conductive properties of maifanite powders. XRD and XPS patterns showed that Ca2+ and Na+ ions were successfully replaced by Sm3+ ions, which is the main reason for improving the electrical conductivity of maifanite powders. Moreover, thermogravimetric analysis displays that Sm-modified maifanite powders have excellent thermal stability, which indicates their widely application prospect in electronic industry.

Acoustical study on spectral characteristics of teachers’ English vocalization

With the increase of English teaching courses, the specific vocal characteristics of teachers in English teaching are studied and used as the basic data for the design of teachers’ spatial acoustics. In order to study this problem, three different English voice materials were read in a anechoic room under three sound intensity levels: large, medium and small, respectively, so as to analyse the equivalent continuous sound pressure level (SPL) and frequency characteristic curves of male and female teachers at 0.3m. In the low frequency band, the SPL increases as the frequency increases. In the mid-frequency band, the SPL reaches the first peak, then decreases briefly as the frequency increases, then increases again to the second peak, and then decreases again briefly as the frequency increases. Then increase again to the third peak. In the high frequency band, the SPL decreases as the frequency increases.

Early warning analysis of online vibration fault characteristics of motor base screw loosening based on similarity measurement theory

When small- and medium-sized motors are running, abnormal vibration phenomenon caused by various reasons often occur, and continuous development may cause serious motor failure. Therefore, online monitoring and warning for motor vibration is very important. Among them, the looseness of the base fixing screw is the most common and typical cause of abnormal vibration of the motor. Based on the similarity measurement theory, a new method for predicting the fault characteristics of the motor base screw loosening online vibration is proposed in this paper. Firstly, the causes of motor vibration signals are analyzed, and the vibration characteristics of different causes are classified. The vibration fault characteristics of the pedestal fixing screws are mainly studied. The vibration signal generated by the looseness of the base fixing screw is collected by the designed real-time online vibration monitoring system. Then, a fast Fourier transform of the vibration signal is carried out to obtain the corresponding amplitude and frequency characteristic curve. By using the two schemes of ‘Cosine similarity and modulus ratio combination’ and ‘Correlation coefficient combined with vector norm,’ the vibration fault characteristics are analyzed and compared, the vibration fault characteristic signal is extracted, and the vibration reason is determined and alerted by combining fault characteristic classification. Finally, this method is used to compare and test the working condition of the motor frame fixing screw loosening and the normal operation of the motor. The experimental results show that the algorithm which combines correlation coefficient with 1-norm has the advantages of simple algorithm and high accuracy. The method also provides new solutions and ideas for online warning of abnormal vibration caused by other reasons.

Noise source identification of three pulleys and one belt drive system based on sound array technology

This paper analyzed the noise distribution of three pulleys and one belt system theoretically and experimentally. Aiming at the influence of the tensioner on the transmission noise of the synchronous belt, on the premise of theoretical analysis of the influence of the tensioner on the transmission noise of the synchronous belt, the noise test of the synchronous belt transmission system with and without the tensioner was carried out under the same experimental conditions. Based on the principle of acoustic array measurement, a three-pulley and one belt noise test device was designed. The noise pressure distribution nephogram and amplitude–frequency characteristic curve were obtained by noise tests at different speeds. Through the comparison of the results of two groups of tests, the influence rule of the tensioner on the transmission noise of the synchronous belt was obtained. The results show that the tensioner can effectively avoid the resonance of the synchronous belt, and the noise amplitude of the three-pulley and one belt drive system is 3 dB higher after the tensioner is installed. It provides a basis for vibration and noise reduction of the engine timing transmission system.

Research on dynamic behavior of torsional absorber in powertrain system considering nonlinear factors

The influence of nonlinear factors on the dynamic behavior of the powertrain torsional damper is studied by using the average method. First, the structure and working principle of dual mass flywheel are analyzed, and the nonlinear dynamic model of dual mass flywheel is established. Then the frequency response curve is obtained by the average method, and the accuracy of the solution is verified by the RK4 method. Finally, the effect of the parameters of dual mass flywheel on the amplitude–frequency characteristics is studied. The results show that the average method is suitable for the nonlinear vibration analysis of the dual mass flywheel, and the amplitude–frequency characteristic curve of the system has the characteristics of turning to the high frequency at the segment. Reducing the excitation amplitude, increasing the viscous damping, or increasing the Coulomb friction damping can effectively reduce the resonance peak value of the dual mass flywheel, and increasing the angular clearance is beneficial to further reduce the resonance frequency of the dual mass flywheel.