Search

Gait adjusting is an effective way to solve leg failure problems and is also the foundation of intelligent self-adjustment. This paper takes a hexagonal type of robot as a prototype and proposes a ‘2-2-1’ gait grouping and gait sequence for the one leg failure case based on the ‘4 + 2’ gait. The longitudinal stability margin is used to evaluate the FLF-A and FLF-B gaits with left front leg failure and FLM-A and FLM-B gait with left middle leg failure. The foothold positions are adjusted by changing the median angle of the stance phase of the body-coxa joint in each leg in order to make sure the centre of gravity is in a stable support polygon. The experiments of the robot walking show that the FLF-A gait and the FLM-B gait are more stable than the FLF-B and FLM-A after adjustment. The pitch and roll angles are maintained within − 3° to 3°, which verify the effectiveness of the stance phase leg median position adjustment strategy. This study could be used to do forward pre-adjustment in the case of one leg failure. [ABSTRACT FROM AUTHOR]

The notion of stability margin (SM) plays an important role in control engineering. For multiple‐input multiple‐output (MIMO) systems, the classic SM is no longer applicable. The one‐loop‐at‐a‐time analysis method may lead to unreliable SMs. Although some robust SM analysis methods are popular in MIMO systems, they are model‐based or not easy‐to‐use in engineering sometimes. In this paper, ℒ2$$ {\mathcal{L}}_2 $$ gain margin and ℒ2$$ {\mathcal{L}}_2 $$ time‐delay margin are defined for linear MIMO systems, and a corresponding SM analysis method is proposed by utilizing a loop transformation and the small‐gain theorem. Most importantly, a data‐driven method for measuring the defined SMs is also presented. As a frequency‐domain method, this method can be used to experimentally obtain the SMs of MIMO systems on model‐free occasions. The proposed SM analysis method and measurement method are simple and practical. Two simulation studies and an experimental test are performed to illustrate the effectiveness and practicability of the proposed method. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of constructing a mathematical model of population density dynamics and the dynamics of forest areas damaged by spongy moth (Lymantria dispar L.) outbreaks in the United States, Europe, Russia, and Japan. The key variable of the model is either the pest population density or the area of forests damaged by spongy moths during a season. This variable can be considered proportional to the total current pest abundance in the study area. For the purposes of modeling, data from a number of different authors was used (see bibliography), as well as data from surveys conducted at the egg or caterpillar stage. The complexity of modeling the dynamics of L. dispar abundance is largely due to the fact that, when studying the dynamics of spongy moth population density, the values of external factors such as parasites, predators, and the amount of available food are often unknown. A simple model was proposed using only two types of data: population density and monthly weather characteristics. Our analysis demonstrated that, even in the absence of knowledge regarding the characteristics of ecosystem components interacting with the spongy moth population (parasites, predators, and the state of forage trees), it is possible to introduce models that characterize the regulatory processes in the population in terms of (i) the presence of negative and positive feedbacks in the system and (ii) the influence of external weather factors. The system under investigation was described as an autoregressive system, whereby the current state of the population is dependent on its state in previous years. The order of autoregression in the system was estimated using the order of the maximum significant partial autocorrelation function. It was found that the regulation of spongy moth population density was characterized by the presence of two feedback loops: positive feedback between the current population density and the population density in the previous season and negative feedback between the current population density and the population density two years ago. To evaluate the model, its stability margin was calculated and found to be directly proportional to the positive feedback coefficient and inversely proportional to the negative feedback coefficient. The model was demonstrated to explain up to 90% of the observed variance of real data. Although the model coefficients for different local populations (North America, Europe, and Asia) differ, the general form of the equation describing both direct data on population densities and indirect data on pest dynamics characterized by areas of stand damage is consistent. Consequently, the form of the ADL model is general, irrespective of the location of the local population. [ABSTRACT FROM AUTHOR]

The experience of a long-term study of the population dynamics of phyllophagous insects in Krasnoturansky pine forest (the southern part of Krasnoyarsk Krai) is discussed in the paper. Over the course of 40 years (1978–2016), regular counts of the abundance of five species of phyllophages were carried out in five landscape structures. Based on the data obtained, models of population dynamics of these species were proposed. The models were based on the idea of the existence of positive and negative feedbacks in populations. This approach made it possible to ignore the impact of parasites on insects. For calculations, an autoregressive (AR) model was proposed, according to which the current density linearly depends on the densities of previous years. The model coefficients were calculated using data from long-term surveys. It has been shown that most populations are characterized by the influence of the density of the past season on the current density in the form of a positive feedback and the influence on the current density of the density of the previous season in the form of a negative feedback. The characteristics of the stability of population dynamics of individual species in various habitats are considered. Stability margin indicators, widely used in control theory, were used as stability characteristics. It has been shown that the margin of resistance for sawflies (Tenthredinidae Latreille) is slightly greater than for other groups of species, which is consistent with the smaller dispersion of the population sizes of these species during long-term censuses. The use of AR models and characteristics of the stock of stability made it possible to classify landscape structures on the territory of the Krasnoturansky forest according to the risks of outbreaks of mass reproduction in these landscapes.

The object of the research is an emergency control system for ensuring the stability of electric power systems (EPS) in case of emergency unbalances. The relevance of the problem of ensuring EPS stability is due to the need to improve the efficiency of emergency control to reduce the risk of system accidents with significant damage. To solve this problem, we propose an algorithm for selecting the volume of control actions based on the principles of adaptive control for predicting the post-emergency mode with an acceptable stability margin. The algorithm of forming the volume of control actions is based on the dependence of the value of control actions on the value of stability reserves estimation by the value of the Jacobi determinant. To build this dependence, the algorithm of searching for the limiting mode by the trajectory of change in the equilibrium position of the steady state of the system from the initial to the limiting one is used. In contrast to the existing algorithms, the proposed algorithm establishes a functional dependence of the control value on the current parameters of the regime or the stability margin, which increases the efficiency of calculations for selecting control actions. Realization of the proposed algorithm is carried out on the basis of the functional scheme according to the data of the vector measurement system. The advantage and novelty of the proposed algorithm is the possibility of eliminating the disadvantages of existing systems of mode automation, the main of which are: – the necessity to perform numerous variant calculations for selecting the volume of control actions; – possible excessive volume of control actions in case of a mismatch of the actual mode with the calculated one [ABSTRACT FROM AUTHOR]

Flight control law parameters should be designed to provide a sufficient stability margin for closed-loop aircraft while ensuring command tracking accuracy. The singular perturbation margin (SPM) and generalized gain margin (GGM), which are generalizations of the classical phase margin (PM) and gain margin (GM), respectively, from a linear time-invariant system to a nonlinear time-varying system, can be used to quantitatively characterize the maximum singular perturbation and regular perturbation allowed to maintain system stability. In this paper, the sliding mode flight control structure and the design parameters of the sliding mode control law are first introduced for an oblique wing aircraft (OWA), the SPM-gauge and GGM-gauge are added to this closed-loop aircraft model, and the analytical expressions of the SPM and GGM are derived with respect to the control law parameters. Second, the stability margin design requirements of closed-loop aircraft in flight control system design specifications are converted into limitations on the SPM and GGM to determine the value range of the flight control law parameters. Then, with the goal of reducing the sum of the approaching time and sliding time, the parameter value combination is selected within the control law parameter range that meets the stability margin requirements, thus forming a flight control law design method for OWA during the wing skewing process. Finally, the designed control law parameters are applied to a sample OWA, and the stability margin of closed-loop aircraft during the wing skewing process is verified. [ABSTRACT FROM AUTHOR]

The operation of the grid-connected inverter (GCI) in weak grid conditions presents a risk of instability due to the presence of high grid impedance and the negative impedance effect of the phase-locked loop (PLL). In response to this issue, this paper introduces an impedance remodeling control strategy for the GCI utilizing the inertia-damping phase-locked loop (ID-PLL). Firstly, the proposed ID-PLL exhibits certain inertia and damping properties in comparison to the traditional PLL (T-PLL), resulting in an enhanced phase margin of the GCI system's equivalent output impedance. Consequently, the adaptation range of the grid impedance increases from 12mH (SCR = 3.00) to 24mH (SCR = 1.37). Next, from the perspective of impedance analysis, this study remodels the PLL impedance and inverter control loop impedance, further improving the amplitude and phase margin of the system's output impedance. As a result, the adaptation range of the grid impedance extends to 33mH (SCR = 1.00), and the transition of grid-connected currents becomes smooth and free of overshoot. Finally, the effectiveness of the proposed control strategy is validated through theoretical analysis and experimental verification.

This work presents a data-driven optimization technique for the optimization of the flame response within a low-order acoustic network model. The methodology exploits the Nyquist criterion to compute a measure of thermoacoustic robustness at targeted frequencies, which serves as an objective value for the optimized system. The method is demonstrated using a simple Rijke tube model coupled with a G -equation solver to model flame dynamics. The approach is shown to efficiently increase the stability margin of the system by modifying the flame transfer function. The methodology is applied to two examples, based on which possible scenarios are discussed and the potential and limitations associated with the practical implementation of the method are analyzed. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of a synthetic jet arranged at the suction surface on the aerodynamic performance of an axial flow compressor, a high-speed subsonic axial flow compressor rotor was numerically simulated in this paper. The results showed that after being excited by the synthetic jet, the flow margin of the compressor decreased by 1.3%, but the peak efficiency increased by 0.47%. After analyzing the internal flow field, it was found that although the alternating blowing and suction effect of the synthetic jet can reduce the separation loss of the suction surface, the radial velocity of the flow is higher when stimulated and converges towards the blade tip, resulting in more severe blockage at the blade tip than in the prototype compressor, leading to a slight decrease in stability margin. To balance the stability margin of the compressor, a coupled flow control numerical simulation combining passive control of the casing treatment and active control of the suction surface synthetic jet was subsequently carried out. The results showed that the coupled flow control could exert their respective advantages, resulting in an increase of 17.84% in the compressor flow margin and a decrease of only 0.2% in peak efficiency. [ABSTRACT FROM AUTHOR]

In response to the constant power negative impedance characteristics on the load side of a ship DC microgrid, leading to voltage oscillation issues in the DC bus, this paper proposes a control optimization method based on impedance reshaping using bus voltage feedback. First, a simplified small-signal diagram of a lithium battery energy storage system converter is analyzed. Combining active damping control technology, an impedance regulator is introduced, and its parameters are optimized to effectively reduce the output impedance magnitude on the power source side. Subsequently, a ship DC microgrid simulation model is constructed using MATLAB R2022a/Simulink for validation, and comparative analysis is conducted on the anti-interference ability of the DC bus voltage before and after impedance reshaping. Finally, a model is built on a semi-physical simulation platform to experimentally verify the proposed method. The research results indicate that the proposed control optimization method can effectively increase the system's stability margin, suppress DC bus oscillations, and enhance the anti-interference ability of the ship DC microgrid's bus voltage when facing significant power load variations. [ABSTRACT FROM AUTHOR]

Purpose: The geometric parameters of the compressor blade have a noteworthy influence on compressor stability, which should be meticulously designed. However, machining inaccuracies cause the blade geometric parameters to deviate from the ideal design, and the geometric deviation exhibits high randomness. Therefore, the purpose of this study is to quantify the uncertainty and analyze the sensitivity of the impact of blade geometric deviation on compressor stability. Design/methodology/approach: In this work, the influence of blade geometric deviation is analyzed based on a subsonic compressor rotor stage, and three-dimensional numerical simulations are used to compute samples with different geometric features. A method of combining Halton sequence and non-intrusive polynomial chaos is adopted to carry out uncertainty quantitative analysis. Sobol' index and Spearman correlation coefficient are used to analysis the sensitivity and correlation between compressor stability and blade geometric deviation, respectively. Findings: The results show that the compressor stability is most sensitive to the tip clearance deviation, whereas deviations in the leading edge radius, trailing edge radius and chord length have minimal impact on the compressor stability. And, the effects of various blade geometric deviations on the compressor stability are basically independent and linearly superimposed. Originality/value: This work provided a new approach for uncertainty quantification in compressor stability analysis. The conclusions obtained in this work provide some reference value for the manufacturing and maintenance of rotor blades. [ABSTRACT FROM AUTHOR]

In contrast with traditional control input design techniques based on mathematical models of the system, in data-driven control approaches, which have recently gained substantial attention, the controller is derived directly from the data that are collected from experiments or observations of the target system. In particular, several data-driven optimal control and model predictive control (MPC) techniques have been proposed. In this paper, it is shown that the recently proposed data-driven LQR (Linear Quadratic Regulator) has a stability margin that is the set of the uncertainties in the control input channels maintaining the closed-loop stability. As an application of the proposed stability margin of the data-driven LQR, the consensus problem is considered. Since the control design for the consensus of multi-agent systems can be reformulated into the robust stabilization of a linear system with uncertainty in the input channel, it is demonstrated that the derived stability margin can be used to design a controller for the consensus of multi-agent systems. [ABSTRACT FROM AUTHOR]

In the past few years and in many applications, linear feedback control has proved to be adequate for the control of various nonlinear systems. Therefore, before attempting the controller design phase, it is natural to ask, "When is a linear controller adequate for a nonlinear system?" This article provides a new nonlinearity analysis method based on the gap metric and the stability margin that is able to quantify the nonlinearity degree of a system once it has an internal model control controller. The internal model control strategy is used since it provides a direct measure of the discrepancy between outputs from the nonlinear system and its internal model when both are subject to the same controller. It is shown that a feedback loop may exhibit much lower nonlinearity than an open-loop nonlinear system by a careful design of a single linear internal model control controller via the gap metric and stability margin, used as model-based analysis tools for linear time-invariant systems. Both simulations and an experimental validation on a semibatch reactor prove the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In a recent work, a definition of stability margin (gain and phase margins) for a class of nonlinear systems (Lur'e systems consisting of a linear time‐invariant (LTI) plant and a sector‐constrained nonlinearity) is proposed based on the famous circle criterion. This definition is indeed interesting because the concept of gain and phase margins has been largely limited to linear systems with a single input and a single output (SISO), but it was further established for Lur'e systems with a particular type of sector constraints (k1=0$$ {k}_1=0 $$, e.g., saturation). In this paper, the previously established concept is extended to cover Lur'e systems with a general type of sector constraints (k1≥0$$ {k}_1\ge 0 $$). It is, however, pointed out that in case of k1=0$$ {k}_1=0 $$, the definitions of phase margin based on the circle criterion can be misleading and need to be modified or replaced perhaps by a time‐delay margin for Lur'e systems including an integrator, for which the phase margin can be trivially zero. [ABSTRACT FROM AUTHOR]

This paper provides a new method to nonlinear control theory, which is developed from the eigenvalue assignment method. The main purpose of this method is to locate the pointwise eigenvalues of the linear-like structure built by freezing the nonlinear systems at a given time instant in a desired disk region. Since the control requirements for the transient response characteristics are the major constraints on the selection of the disk centre and radius, two different update algorithms are also developed to reshape the disk region by changing the disk centre and radius at each time step. The effectiveness of the proposed methods is tested in both simulations and experiments. A validated three-DOF laboratory helicopter is used for experiments. [ABSTRACT FROM AUTHOR]

In recent years, the ultra-low frequency oscillation of power system caused by hydropower units has occurred frequently. It is very important to study the ultra-low frequency oscillation characteristics of hydropower stations for the stable operation of power system. It is worth further research on how to determine the system stability margin in engineering practice. Based on the established model of single machine system with load, the analysis of the characteristic equation of closed-loop transfer function of single machine system shows that the ultra-low frequency oscillation will occur when the ratio of proportional parameter to integral parameter is too small. Based on the single machine with load system model and the equivalent single machine with load model of multi machine system, the ultra-low frequency oscillation mode of the system is analyzed by eigenvalue analysis method. According to the requirements of the damping ratio index, the security domain of the system ultra-low frequency oscillation is constructed, and the risk and stability margin of the system ultra-low frequency oscillation are evaluated by the characteristic values of the system ultra-low frequency oscillation mode.

Flight control law parameters should be designed to provide a sufficient stability margin for closed-loop aircraft while ensuring command tracking accuracy. The singular perturbation margin (SPM) and generalized gain margin (GGM), which are generalizations of the classical phase margin (PM) and gain margin (GM), respectively, from a linear time-invariant system to a nonlinear time-varying system, can be used to quantitatively characterize the maximum singular perturbation and regular perturbation allowed to maintain system stability. In this paper, the sliding mode flight control structure and the design parameters of the sliding mode control law are first introduced for an oblique wing aircraft (OWA), the SPM-gauge and GGM-gauge are added to this closed-loop aircraft model, and the analytical expressions of the SPM and GGM are derived with respect to the control law parameters. Second, the stability margin design requirements of closed-loop aircraft in flight control system design specifications are converted into limitations on the SPM and GGM to determine the value range of the flight control law parameters. Then, with the goal of reducing the sum of the approaching time and sliding time, the parameter value combination is selected within the control law parameter range that meets the stability margin requirements, thus forming a flight control law design method for OWA during the wing skewing process. Finally, the designed control law parameters are applied to a sample OWA, and the stability margin of closed-loop aircraft during the wing skewing process is verified.

Due to its inherent geographical potential for wind/solar power and other renewable energy sources (RES) generation, Tamil Nadu is one of the pioneering Indian states in the early development and utilisation of RES. Tamil Nadu accounts for roughly 25% of India's total wind energy capacity. Increased penetration of RES in a power system indicates that RESs have replaced traditional power plants that have historically managed and stabilised the power system, resulting in novel power flow situations. Grid stability must be enhanced in light of the integration of large-scale RES into the grid. The purpose of this research is to analyse and evaluate the impact of large-scale integration of RES by examining mitigating methods from the most recent academic articles and technical reports in this field. This will aid in identifying the key risks affecting the network's stability margin as a result of the widespread integration of RES generation, allowing for a reasonable increase in the network's stability margin. The study's findings will also help to guide the selection of appropriate mitigation strategies. As the stability margin improves, the future fraction of RES additions to the Tamil Nadu extra high voltage (EHV) grid will be increased significantly. [ABSTRACT FROM AUTHOR]

Background. Machining on lathe machines using automation means is a complex process that depends on the properties of the dynamic system (DS) of the machine, therefore, determining cutting modes based on the minimum value of the attenuation coefficient of the autocorrelation function of vibration acoustic oscillations is an urgent task of machine building. Materials and methods. The vibrations that occur during the processing of critical parts determine the dynamic quality of the machines, the quality of the surface layer and the resistance of the cutting tool, therefore, it is necessary to identify control methods. Increased efficiency of the process is provided by forced cutting modes, which can lead to deterioration of processing quality and premature wear of the cutting tool. The theoretical definition of rational cutting modes causes certain difficulties, so the experimental search for a solution to the problem is most relevant. In order to select rational cutting modes on turning and grinding groups machines, it is proposed to use the stability reserve of the DS of the machine as the most informative characteristic, which should be determined from the transfer function of the DS using the autocorrelation function (ACF) of vibrations. A condition for the identification of the DS machine is the preliminary identification of the ACF, which can be implemented using vibration records during cutting. Pre-filtering of oscillations is carried out to exclude low-frequency range containing frequencies caused by oscillations of machine system elements and to leave frequencies associated with cutting process. Cut modes are assigned to the highest stability margin, which ensures high surface quality. Results and conclusions. There is an unambiguous analytical relationship between the oscillation index and the damping coefficient of the ACF, which makes it possible to calculate the α coefficient, by the value of which it is possible to estimate the stability margin of the ET in various cutting modes and select the most appropriate one. The study of vibrations of machines for processing high-precision parts makes it possible to appropriately control the technological mode, using the level of vibrations as one of the indicators of its quality.

A coupled inductor filter (CIF) is a good candidate for reducing current ripple, and improving the power density of switching power. However, because the resonance frequency of its series branch is easily affected by non-ideal factors, such as the inductance production process, ambient temperature, device aging, among others, it is usually unable to optimize its effect in practice. To solve this problem, by considering the ripple-free input current boost converter as an example, this paper proposes a novel method that adds a variable inductor (VI) into the AC resonant branch of a CIF. The value of the VI is adjusted online to optimize the ripple cancellation effect by comparing the peak-to-peak current ripples in the main power inductor and the AC branch inductor. In the aforementioned scheme, the voltage-controlled current source (VCCS) in the closed loop control circuit is critical for achieving a rapid and stable regulation of the VI. Therefore, a 1/∛s compensator is adopted to provide a sufficient stability margin under various operating conditions. Finally, the input characteristics of a 500 W boost converter are measured and presented in this study. The obtained results demonstrate that the total inductor current ripple peak-to-peak values at each operating point are within 85 mA, which confirms the correctness of the theoretic analyses. [ABSTRACT FROM AUTHOR]

This paper proposes a scheme for auto-tuning a lag-lead compensator to satisfy frequency-domain specifications of a feedback control system. Most previous studies proposed algorithms for tuning either lag or lead compensators. A study presented a self-tuning laglead compensator whose proportional gain, however, needs to be set manually. In contrast to previous studies, a fully automated tuning process for a lag-lead compensator is devised in this paper. In the proposed scheme, the plant's frequency responses at specific frequencies are first estimated, and an iterative algorithm adapts all controller parameters. To facilitate the estimation of high loop gain at a low frequency, a notch filter is introduced to the control loop while estimating this high low-frequency loop gain. Without this mechanism, a real-time data acquisition system of a wide dynamic range would be required. In addition to fulfilling the requirements for the low-frequency loop gain and phase margin, another feature of the proposed scheme is the satisfaction of the required gaincrossover frequency. However, the proposed method supposes that the frequency-domain specifications can be met for a plant using a lag-lead compensator, which is the plant's requirement for the proposed method. Experimental results for controlling a voice-coil motor are reported in this paper, showing that the proposed auto-tuner can practically provide a lag-lead compensated system satisfying frequency-domain performance requirements. [ABSTRACT FROM AUTHOR]

Mathematical models of a large class of electromechanical devices are presented in the form of two-mass systems with elastic bonds. Synthesis of digital control systems for such objects is complicated by the appearance of delays associated with the control system discretization and the presence of resonant frequencies. Introducing an additional dynamic link (filter) into the controller for damping resonant frequencies is a well-known approach, but this reduces the stability margins of the developed control system. In addition, mechanical model parameters can be determined inaccurately or change during device operation. This article presents a method for calculating the parameters of a second-order corrective link, which provides the specified stability margins with mechanical-load variable parameters. A numerical example of using the technique for a servo-drive control system with a synchronous motor with permanent magnets is given. This technique can be applied to the synthesis of controllers of digital control systems for electromechanical devices that ensure stability and control-system robustness in the presence of resonant frequencies. [ABSTRACT FROM AUTHOR]

Introducing Myrmex, a hexapod robot designed for autonomous locomotion on unstructured terrains. It was developed a whole-body kinematics model that enables both open-chain and closed-chain control of the robot. This model enhances the flexibility and mobility of the robot. At the moment, Myrmex relies solely on proprioception (encoders and IMU) for adapting its body posture. Despite these sensory limitations, we have successfully applied the model in static and dynamic situations. Static experiments have showcased increased flexibility, allowing the robot to access previously challenging locations. In dynamic scenarios, the hexapod has demonstrated its capability to traverse irregular terrains, effectively exploiting its body mobility. Myrmex adeptly adjusts its posture according to the terrain topology, maintaining the center of gravity projection along the body’s centerline (i.e., same as it in the level walk), thus preserving stability margins during gait. The robot was able to correct its posture on terrain with gradients in any direction relative to the body structure, with only 3 DoF per leg. Importantly, our experiments have underscored the robot’s ability to maintain its posture even in the face of unexpected disturbances in its legs, emphasizing the robustness and reliability of the model.

In weak grids, control performance of power converters is negatively affected by the significantly varied grid impedance, which can easily lead to degradation of anti-harmonic performance, resonances or even system instability. A rapid control performance monitor is therefore proposed in this work to acquire power converters’ actual performance indexes, i.e., the crossover frequency and phase margin. With the measured real-time information, important functions like health management, control parameters design and adaptive control of power systems can be conveniently realized. The monitor is implemented by perturbating the control loop of the power converter under test with a frequency-autotuned small signal, showing satisfying dynamic response and anti-noise performance. Thanks to the 100% digital implementation, the proposed performance monitor requires no extra hardware and no further cost. The detailed design of the monitor, based on small signal models derivation employing an envelope modeling method, is presented in this article. Extensive experiments were carried out to verify the efficacy of the proposed monitor.

The digital controller of the grid-connected converter achieves closed-loop control by sampling key voltage and current signals. High-frequency switching of power electronic switches introduces high-frequency ripples into the signals, which may cause sampling aliasing. Severe distortion may happen in the output current of the converter because of the sampling aliasing. This paper explains how the aliasing problem arises by analyzing the spectrum of sampling-controlled signals. The explanation shows the severity of the aliasing problem and the necessity of adopting anti-aliasing methods. Next, this paper analyzes the impact of commonly used anti-aliasing low-pass filters on current control stability. From the conclusions, adopting a low-pass anti-aliasing filter reduces the stability margin of the current loop, which needs improvement. So this paper proposes a novel hybrid anti-aliasing filter based on the combination of the notch and low pass analog filters. An experiment on a 6kVA prototype verifies the proposed anti-aliasing method. This paper shows distortion and step-response oscillation of the output current when proposed and traditional anti-aliasing methods adopted. The comparison shows the advantages of the proposed anti-aliasing method.

The regenerative braking in the tram allows the energy to be returned to the power grid through a power inverter. Since the inverter location between the tram and the power grid is not fixed, resulting in a wide variety of impedance networks at grid coupling points, posing a severe threat to the stable operation of the grid-tied inverter (GTI). By independently changing the loop characteristics of the GTI, the adaptive fuzzy PI controller (AFPIC) can adjust according to different impedance network parameters. It is challenging to fulfill the stability margin requirements of GTI under high network impedance since the PI controller has phase lag characteristics. A correction method of series virtual impedance is proposed, which connects the inductive link in a series configuration with the inverter output impedance, correcting the inverter equivalent output impedance from resistance-capacitance to resistance-inductance and improving the system stability margin. Feedforward control is adopted to improve the system's gain in the low-frequency band. Finally, the specific series impedance parameters are obtained by determining the maximum network impedance and setting the minimum phase margin of 45°. The realization of virtual impedance is simulated by conversion to an equivalent control block diagram, and the effectiveness and feasibility of the proposed method are verified by simulation and a 1 kW experimental prototype.

In response to the constant power negative impedance characteristics on the load side of a ship DC microgrid, leading to voltage oscillation issues in the DC bus, this paper proposes a control optimization method based on impedance reshaping using bus voltage feedback. First, a simplified small-signal diagram of a lithium battery energy storage system converter is analyzed. Combining active damping control technology, an impedance regulator is introduced, and its parameters are optimized to effectively reduce the output impedance magnitude on the power source side. Subsequently, a ship DC microgrid simulation model is constructed using MATLAB R2022a/Simulink for validation, and comparative analysis is conducted on the anti-interference ability of the DC bus voltage before and after impedance reshaping. Finally, a model is built on a semi-physical simulation platform to experimentally verify the proposed method. The research results indicate that the proposed control optimization method can effectively increase the system’s stability margin, suppress DC bus oscillations, and enhance the anti-interference ability of the ship DC microgrid’s bus voltage when facing significant power load variations.

In order to investigate the effect of a synthetic jet arranged at the suction surface on the aerodynamic performance of an axial flow compressor, a high-speed subsonic axial flow compressor rotor was numerically simulated in this paper. The results showed that after being excited by the synthetic jet, the flow margin of the compressor decreased by 1.3%, but the peak efficiency increased by 0.47%. After analyzing the internal flow field, it was found that although the alternating blowing and suction effect of the synthetic jet can reduce the separation loss of the suction surface, the radial velocity of the flow is higher when stimulated and converges towards the blade tip, resulting in more severe blockage at the blade tip than in the prototype compressor, leading to a slight decrease in stability margin. To balance the stability margin of the compressor, a coupled flow control numerical simulation combining passive control of the casing treatment and active control of the suction surface synthetic jet was subsequently carried out. The results showed that the coupled flow control could exert their respective advantages, resulting in an increase of 17.84% in the compressor flow margin and a decrease of only 0.2% in peak efficiency.

In contrast with traditional control input design techniques based on mathematical models of the system, in data-driven control approaches, which have recently gained substantial attention, the controller is derived directly from the data that are collected from experiments or observations of the target system. In particular, several data-driven optimal control and model predictive control (MPC) techniques have been proposed. In this paper, it is shown that the recently proposed data-driven LQR (Linear Quadratic Regulator) has a stability margin that is the set of the uncertainties in the control input channels maintaining the closed-loop stability. As an application of the proposed stability margin of the data-driven LQR, the consensus problem is considered. Since the control design for the consensus of multi-agent systems can be reformulated into the robust stabilization of a linear system with uncertainty in the input channel, it is demonstrated that the derived stability margin can be used to design a controller for the consensus of multi-agent systems.

In recent years, the aging of society has led to challenges such as ensuring adequate facility capacity and promoting health maintenance for the elderly. Consequently, there has been a growing demand for rehabilitation and fitness equipment suitable for use in limited spaces such as homes. In a previous study, a hexapod mobile robot, equipped with six tetrahedral-shaped pneumatic soft actuators (TSAs), was developed as a core training device to address these issues. However, the robot's gait was investigated experimentally via a trial-and-error process. Hence, it is necessary to examine the stability of the gait. In this study, a straightforward model is presented for gait analysis of the mobile robot. Furthermore, the stability of the gait is demonstrated based on the analysis, and a more efficient gait with a sufficient stability margin is introduced. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Zhejiang University: Science A is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper provides a new method to nonlinear control theory, which is developed from the eigenvalue assignment method. The main purpose of this method is to locate the pointwise eigenvalues of the linear-like structure built by freezing the nonlinear systems at a given time instant in a desired disk region. Since the control requirements for the transient response characteristics are the major constraints on the selection of the disk centre and radius, two different update algorithms are also developed to reshape the disk region by changing the disk centre and radius at each time step. The effectiveness of the proposed methods is tested in both simulations and experiments. A validated three-DOF laboratory helicopter is used for experiments.

Singular Perturbation Margin (SPM) and Generalized Gain Margin (GGM) are Phase Margin (PM) and Gain Margin (GM) like stability metrics. In this paper, the problem of SPM and GGM assessment for Linear Periodically Time-Varying (LPTV) systems is formulated, which provides a necessary step for the development of the theoretically based and practically efficient SPM and GGM analysis methodology for multivariable Nonlinear Time-Varying (NLTV) systems. Here, Chang transformation makes it possible to reduce the SPM analysis for Hill equations, which is essentially a stability problem of higher order LPTV systems due to the SPM gauge introduced dynamics, to second order LPTV systems. Based upon Floquet Theory, when different types of LPTV SPM gauges are designed, the SPM and GGM assessment methods for the second order and general order LPTV systems are established and stated by the corresponding lemmas, theorems and corollaries. The effectiveness of such methods are illustrated by an examples of a flexible inverted pendulum.

Analytical design of the PID-type controllers for linear plants based on the magnitude optimum criterion usually results in very good control quality and can be applied directly for high-order linear models with dead time, without need of any model reduction. This paper brings an analysis of properties of this tuning method in the case of the PI controller, which shows that it guarantees closed-loop stability and a large stability margin for stable linear plants without zeros, although there are limitations in the case of oscillating plants. In spite of the fact that the magnitude optimum criterion prescribes the closed-loop response only for low frequencies and the stability margin requirements are not explicitly included in the design objective, it reveals that proper open-loop behavior in the middle and high frequency ranges, decisive for the closed-loop stability and robustness, is ensured automatically for the considered class of linear systems if all damping ratios corresponding to poles of the plant transfer function without the dead-time term are sufficiently high.

The present study addresses the population dynamics of five species of phyllophagous forest insects in five habitats located in the Krasnoturansk pine forest (south Middle Siberia). Based on the data of insect surveys obtained during 1979–2016, autoregressive (AR) models of population dynamics have been proposed, with the current population density being dependent on population densities of the preceding years. Methods of calculation of the autoregression order and coefficients of AR equations have been presented. The study shows that, for different insect species in different habitats, the lags between the current population density and the densities of the previous years are not the same. AR equations characterize positive and negative feedbacks regulating population dynamics. By using AR equations, up to 90% of population density variance can be taken into account. Stability margin, which is calculated from coefficients of AR models, has been proposed as a parameter to assess the stability of population dynamics. A small stability margin indicates a high risk of outbreak of an insect species. [ABSTRACT FROM AUTHOR]

Copyright of Control Theory & Applications / Kongzhi Lilun Yu Yinyong is the property of Editorial Department of Control Theory & Applications and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• The ID-PLL structure can provide certain inertia and damping to suppress the issue of significant fluctuations in the output frequency of PLL caused by abrupt changes in the operating state of inverter.Moreover, stability analysis indicates that the ID-PLL structure can also contribute to enhancing the phase margin of the inverter output impedance, enabling the system to gradually adapt to weak grids and even extremely weak grid conditions. • By designing the front-end control of the PLL with PSSIR and the inverter with CLIR, it is possible to further broaden the grid-adaptive range of the inverter without sacrificing the PLL bandwidth. This enhancement improves the steady-state and dynamic performance of the inverter system, achieving smooth and overshoot-free transitions of the grid-connected current, and significantly reducing the frequency fluctuation of the PLL output. • Compared with grid-connected inverter using the T-PLL, the inverter equipped with the ID-PLL and impedance remodeling control strategy can expand the adaptability range to grid impedance nearly three times (SCR reduced from 3 to 1) under the same main circuit parameters. The operation of the grid-connected inverter (GCI) in weak grid conditions presents a risk of instability due to the presence of high grid impedance and the negative impedance effect of the phase-locked loop (PLL). In response to this issue, this paper introduces an impedance remodeling control strategy for the GCI utilizing the inertia-damping phase-locked loop (ID-PLL). Firstly, the proposed ID-PLL exhibits certain inertia and damping properties in comparison to the traditional PLL (T-PLL), resulting in an enhanced phase margin of the GCI system's equivalent output impedance. Consequently, the adaptation range of the grid impedance increases from 12mH (SCR = 3.00) to 24mH (SCR = 1.37). Next, from the perspective of impedance analysis, this study remodels the PLL impedance and inverter control loop impedance, further improving the amplitude and phase margin of the system's output impedance. As a result, the adaptation range of the grid impedance extends to 33mH (SCR = 1.00), and the transition of grid-connected currents becomes smooth and free of overshoot. Finally, the effectiveness of the proposed control strategy is validated through theoretical analysis and experimental verification. [ABSTRACT FROM AUTHOR]

The research is aimed at determining the critical buckling load of the spatial model shell - soil system in the case of inhomogeneous physical and mechanical soil properties along the longitudinal axis of the cylindrical shell in a nonlinear formulations of the task. Methods. The task is solved by a numerical method using a finite element complex ANSYS. Two calculated cases of the spatial model shell - soil system are compiled. The soil is divided into two equal parts with different physical and mechanical properties. The problem was solved in geometrically, physically and constructively nonlinear statement. Nonlinearity is due to the need to find the contact zone through an iterative process and determine the time-varying position of the shell. The soil is modeled by volumetric elements, each consisting of twenty nodes. The shell is modeled by flat elements, each consisting of four nodes. Contact elements of one-side action are used. Critical buckling load are determined relative to the actual load of its own weight. Results. Critical loads are obtained from two calculated cases of the spatial model shell - soil system. There is a comparative analysis of the results. An assessment of the stability margin of the shell relative to the actual load is given.

Reduction of trim loss is essential to raise lift-to-drag ratio and enhance flight control performance for flying wing configuration,so the influence on aerodynamic optimization of transonic flying wing configuration is studied based on discrete adjoint method,and three conditions of 10%,0%,-10%stability margin are considered in the optimization process,respectively.The free-form deformation(FFD)method is used to achieve the parametrization of surface unstructured grid,the control point of FFD is selected to serve as design variable,and the flowfield and adjoint equation is solved to obtain the sensitivity information.The sequential quadratic programming(SQP)algorithm is utilized to obtain the gradient of control points,and the final configuration is obtained by multiple iteration.The results show that the discrete adjoint optimization method can evidently improve the maximum lift-to-drag ratio under the constraint of geometric thickness and moment trim,and the increment is exceeded by 8%at these three conditions.The increment of cruise lift-to-drag ratio appears to reduce with the increase of stability margin while lift coefficient is fixed,the lift-to-drag ratio can increase 5.08%when the stability margin is 10%.

Background. Application of stability margin of dynamic system, integral estimates of autocorrelation functions and spectra of vibro-acoustic oscillations in machine tools is relevant for evaluation of their dynamic quality and purpose of cutting mode, which is theoretically justified by solution of stochastic equation of oscillations in “cutter-detail” system. Materials and methods. An experimental-analytical method is used to substantiate the use of functionals from autocorrelation functions and spectra of vibration-acoustic oscillations in machines to select a rational cutting mode and determine the initial phase of critical tool wear. Results. Results of experimental studies on turning and grinding machines on evaluation of their dynamic quality at various modes of machining and wear of cutter are presented. Conclusions. The results of studies of the stochastic characteristics of oscillations of a dynamic system showed the feasibility of assessing the dynamic quality of automated machine tools. This allows to compare by parameters, to choose a technological mode for processing high-precision parts with a given quality, and to identify the initial phase of catastrophic cutter wear for timely replacement and better use of the resource.

In this paper, the single-stage compressor with circumferential non-uniform tip clearance is experimentally investigated under 180° total pressure distortion for the compressor characteristics and the dynamic stall process. In the special structure of the circumferential non-uniform tip clearance, different circumferential distortion areas are adopted to actively induce the stall. The maximum or minimum flow coefficient near the stall point occurs when the location where the rotor departs the distortion area is at the average tip clearance rather than the maximum or minimum tip clearance. Based on the time-frequency analysis regarding the dynamic stall process at different correspondences between the inlet distortion and the tip clearance, it is found that the rotating frequency of the stall cell that is independent of the location of the distortion area is slightly less than 50% rotor rotating frequency and the large-scale stall inception whose frequency is 4–8 times the rotor rotating frequency occurs. Besides the circumferential phase difference from 90° to 180° between the location where the disturbance occurs and the location where the rotor departs, the distortion area exists. According to the dynamic stall process, the stall interpretation model of circumferential total pressure distortion under the circumferential non-uniform tip clearance is established. [ABSTRACT FROM AUTHOR]

Controlling of voltage and frequency in the microgrid is a challenging issue and many controllers are suggested for this purpose. Proportional-Integral-Derivative (PID) controller is simple and efficient when its coefficients are optimally adjusted. Recently, evolutionary algorithms are used to optimally adjust the PID coefficients. In this paper, four evolutionary algorithms, particle swarm optimization, genetic algorithm, imperialist competitive algorithm and selfish herd optimization, are applied to set the PID controller coefficients and tested in the frequency and voltage control of the microgrid. Settling time, overshoot and stability margin are three main factors in the control response of PID which are considered for generating the fitness function to be optimized in terms of the PID coefficients. The proposed microgrid includes inverter-based distributed generators and different loads. Load change and different faults may cause the degradation of frequency and voltage where using of the PID controller leads to better control of these parameters. Simulation results show the efficiency of all four algorithms where particle swarm optimization performance is better than other three algorithms. Experimental results also verify the simulation results discussed in the paper. [ABSTRACT FROM AUTHOR]