Stator Flux Observer Research Articles

Improved Deadbeat-Direct Torque and Flux Control for PMSM With Less Computation and Enhanced Robustness

Deadbeat-direct torque and flux control (DB-DTFC) significantly improves the torque response performance compared with the conventional direct torque control. However, the existing DB-DTFC algorithm suffers from a relatively heavy computational burden due to its intricate derivation. Thus, two simplified DB-DTFC algorithms for permanent magnet synchronous machines (PMSMs) are proposed in this article to relieve the burden. The first one is based on the stator flux differential, and the second one is based on the complex power of PMSM. Furthermore, the performance of DB-DTFC highly relies on the accuracy of machine parameters. Thus, modified stator flux observer and electromagnetic torque observer are designed to enhance parameter robustness based on disturbance observer theory. The theoretical derivation of the proposed methods is investigated in-depth. Finally, both simulation and experimental results verify that the proposed deadbeat methods can maintain the deadbeat performances with a reduced computational burden. Meanwhile, the modified observer can help the deadbeat algorithms work well when machine parameters change.

Sensorless DTC-SVM applied to an induction motor controlled by a three-level inverter using SMSFO

PurposeThe direct torque control (DTC) of induction motor (IM) drive is featured by high ripples in the electromagnetic torque and stator flux profiles because they are controlled by two hysteresis regulators. Furthermore, the machine flux is not directly measurable. Hence, it is better to reconstitute it from the instantaneous electrical equations of the machine. Once the stator flux is estimated, we can guarantee a reliable sensorless DTC control. Thus, the purpose of this research work is to ensure fast response and full reference tracking of the IM under sensorless DTC strategy with desired dynamic behavior and low ripple levels.Design/methodology/approachIn this work, an improved DTC strategy, which is DTC_SVM_3L, is suggested. The first step of the designed approach is to substitute the conventional inverter feeding the motor with a three-level inverter because it guarantees reduced switching losses, improved quality of voltage waveform and low-current total harmonic distortion rate. The second aim of this paper is to make the IM operate at a constant switching frequency by using the nearest three vectors-based space vector modulation (SVM) technique rather than hysteresis controllers. The third objective of this study is to conceive a sliding-mode stator flux observer, which can improve the control performances by using a sensorless algorithm to get an accurate estimation, and consequently, increase the reliability of the system and decrease the cost of using sensors. The stability of the proposed observer is demonstrated based on the Lyapunov theory. To overcome the load change disturbance in the proposed DTC control strategy, this paper exhibits a comparative assessment of four speed regulation methods: classical proportional and integral (PI) regulator, fuzzy logic PI controller, particle swarm optimization PI controller and backstepping regulator. The entire control algorithm was tested under different disturbances such as stator resistance and load torque variations.FindingsIt was ascertained that the IM, controlled with three-level inverter, exhibits good performances under the proposed DTC-SVM strategy based on a sliding-mode observer. The robustness of the suggested approach against parameter variations is also proved.Originality/valueThe theoretical development of the proposed control strategy is thoroughly described. Then, simulations using Matlab/Simulink software are launched to investigate the merits of the sensorless DTC-SVM command of three-level inverter-fed IM drive with different speed regulators.

Stator Current-Sensorless-Modulated Model Predictive Direct Power Control of a DFIM With Magnetizing Characteristic Identification

This article presents a direct power control method based on modulated model predictive control for a doubly fed induction machine. The modulated predictive control algorithm constructs an optimal voltage vector from two inverter states that give a minimum absolute error in the active and reactive power. This article focuses on the effects of magnetic saturation and its impact on the accuracy of computed reactive power when the stator current sensors are not installed. To reduce the impact of magnetic saturation on reactive power computation, the machine's magnetizing characteristic is identified through a self-commissioning scheme introduced in this article. The identified magnetizing curve is utilized to construct a full-state stator flux observer, which is used to accurately estimate stator currents that appear in the reactive power equation. Experimental results are presented to demonstrate the accuracy of the reactive power computation in the absence of stator current sensors while conserving the rapid transient response offered by a modulated predictive control strategy for active and reactive power regulation.

Digital Implementation of Deadbeat-Direct Torque and Flux Control for Permanent Magnet Synchronous Machines in the M–T Reference Frame

In the traditional deadbeat-direct torque and flux control in the M-T reference frame (DB-DTFC-MT) scheme, the stator flux is calculated with the current model, and one-step delay in the digital system is neglected, which results in poor robustness to parameter variations and a serious oscillation in both the stator flux and torque, especially in the low-speed range. In this article, the digital implementation of DB-DTFC-MT is studied. First, the DB-DTFC-MT scheme considering the one-step delay in the digital system is deduced. Second, digital stator flux observer and current observer are developed to predict the stator flux and current in the next sampling instant. By using the predicted stator flux and torque, the oscillation caused by the one-step delay is eliminated and real deadbeat control is realized. Moreover, the robustness of the system to parameter variations is qualitatively evaluated. Although the system shows some sensitivity to the permanent magnet flux, it has strong robustness to the stator resistance and inductances, especially the d-axis inductance. Hence, a larger estimated d-axis inductance can be used in the system for reducing the pulsation in the d-axis current when tracking sinusoidal torque. All the proposed control designs are validated on a real-time control platform based on dSPACE DS1103.

FPGA‐based real‐time implementation of a direct torque control with second‐order sliding mode control and input–output feedback linearisation for an induction motor drive

A robust direct torque control (DTC) strategy for an induction motor is proposed in this study. In fact, the proposed control strategy is defined by a combination of DTC, space vector modulation (SVM), input–output feedback linearisation (IOFL), a second-order super-twisting speed controller (STSC), and sliding-mode-load torque and stator-flux observers with stator resistance estimation. First, non-linear IOFL is suggested to achieve decoupled flux and torque control, and the SVM technique is utilised to control the inverter switching frequency which decreases the torque ripples and noise. Second, to improve the speed regulation, an STSC is added to an SVM-DTC-IOFL scheme. Furthermore, the sliding mode observers of the stator flux and of the load torque are proposed in order to improve the control performances by reducing uncertainties and to prevent the effects of the stator resistance variations. Indeed, this study presents the importance of implementing the suggested SVM-DTC-IOFL using a field-programmable gate array (FPGA) circuit. The main interest of the FPGA implementation is the decrease in the control loop delay, due to the parallel processing offered by the FPGA. The performances of the proposed control algorithm are investigated by digital simulation using a Xilinx system generator tool and experimental implementation utilising FPGA-Virtex-5-ML507.

Stator flux and load torque observers for PMSM

In this paper, a stator flux observer and constant load torque estimation methodare developed for non-salient permanent magnet synchronous motors. The methods described here are based on implementation of LTI filters and linear regression. The motor resistance and inductance are assumed to be known.

A Stator Flux Observer With Phase Self-Tuning for Direct Torque Control of Permanent Magnet Synchronous Motor

Owing to simple implementation and excellent performance, the conventional stator flux observer based on active flux concept is considered as a competitive scheme for direct torque control system of a permanent magnet synchronous motor. However, it is constructed by a large number of motor parameters, which may vary with the operating points. This leads to weak robustness. To improve its performance, a stator flux observer with phase self-tuning is proposed as an improved version in this article. First, relationship between the stator flux observed values obtained by current and voltage models and observed error of rotor position angle is analyzed. Then, according to the aforementioned analysis, a phase self-tuning link constructed by a proportional-integral regulator is proposed to ensure that the observed coordinate system converges to the actual one even under motor parameters vary conditions. Consequently, the robustness and accuracy of the stator flux observer can be improved. Finally, the stability of the proposed phase self-tuning link is analyzed. Experimental comparison between the conventional stator flux observer based on active flux concept and the proposed improved stator flux observer has been done, and the results verify the feasibility and effectiveness of the proposed stator flux observer.

A Super-Twisting Sliding-Mode Stator Flux Observer for Sensorless Direct Torque and Flux Control of IPMSM

The scheme based on direct torque and flux control (DTFC) as well as active flux is a good choice for the interior permanent magnet synchronous motor (IPMSM) sensorless control. The precision of the stator flux observation is essential for this scheme. However, the performance of traditional observers like pure integrator and the low-pass filter (LPF) is severely deteriorated by disturbances, especially dc offset. Recently, a sliding-mode stator flux observer (SMFO) was proposed to reduce the dc offset in the estimated stator flux. However, it cannot eliminate the dc offset totally and will cause the chattering problem. To solve these problems, a novel super-twisting sliding-mode stator flux observer (STSMFO) is proposed in this paper. Compared with SMFO, STSMFO can reduce the chattering and fully eliminate the dc offset without any amplitude and phase compensation. Then, the precision of the stator flux and rotor position can be greatly improved over a wide speed region. The detailed mathematical analysis has been given for comparing it with another three traditional observers. The numerical simulations and experimental testing with an IPMSM drive platform have been implemented to verify the capability of the proposed sensorless scheme.

Optimization of DTC algorithm based on distributed parameter modelling of PMSM

Accurate mathematical model is the basis and precondition of electric motor control. The existing motor model under concentrated parameters is difficult to describe the harmonic and nonlinear characteristics of the motor, while the motor model under distributed parameters can accurately describe the torque and flux characteristics of the motor precisely at all working points, which provides the foundation for model-based motor control. The stator flux and torque observer have an essential influence on the performance of direct torque control (DTC) for PMSM. The concentrated parameter model of PMSM considering only the fundamental wave vector of the magnetic field for DTC will have obvious deficiencies in the performance such as torque ripple. This paper therefore uses the reconstructed magnetic co-energy (MCE) model to establish a torque and flux observer based on distributed parameters, which is introduced into the DTC algorithm of PMSM as an observer. The optimized DTC algorithm makes full use of the flux and torque harmonic description capabilities of the distributed parameter model, which improves control performance. The superiority of the distributed parameter observer is verified by simulation and bench test.

Comprehensive Diagnosis and Tolerance Strategies for Electrical Faults and Sensor Faults in Dual Three-Phase PMSM Drives

In this paper, fault diagnosis and fault-tolerant control strategies have been studied comprehensively for dual three-phase permanent-magnet synchronous motor (PMSM) drives to improve the reliability. Based on direct torque control (DTC) with space vector modulation, a series of diagnostic and tolerant control methods have been proposed for five types of faults, namely, speed-sensor fault, dc-link voltage-sensor fault, current-sensor fault, open-phase fault, and open-switch fault. First, diagnosis and tolerant schemes are proposed for speed-sensor fault by estimating the rotor angle speed with the rotating speed of stator flux. Second, diagnosis and tolerant schemes are proposed for dc-link voltage-sensor fault by combining the current model based stator flux observer with the voltage model based stator flux observer. Third, a three-step method is designed to diagnose three types of faults related to current signals, namely, current-sensor fault, open-phase fault, and open-switch fault simultaneously. A vector space decomposition based current estimation method is proposed to achieve fault-tolerant control for the current-sensor fault, and the voltage compensation based fault-tolerant control is presented for both open-phase and open-switch faults. The experiments have been taken on a laboratory prototype to verify the effectiveness of the proposed fault diagnosis and fault-tolerant schemes.

Low Switching Frequency Model Predictive Control of Three-Level Inverter-Fed IM Drives With Speed-Sensorless and Field-Weakening Operations

This paper proposes a single-vector-based model predictive control (MPC) for an induction motor drive supplied by a three-level neutral-point-clamped inverter operating at a low switching frequency. Unlike the torque and flux control in the conventional MPC, the proposed MPC tries to minimize the error between the applied voltage vector and a reference voltage vector obtained based on the principle of deadbeat control, thereby reducing the number of weighting factors in the cost function. To reduce the computational burden and restrict the high jumps in both phase and line voltages, two switching tables are proposed and compared for the preselection of candidate voltage vectors. The neutral point potential fluctuation and switching frequency are also included in the cost function to achieve the balance between the upper and lower dc voltages and a relatively low switching frequency. Furthermore, a speed adaptive stator flux observer with a novel gain matrix is proposed to achieve speed-sensorless operation, which has a higher speed and flux estimation accuracy than conventional fixed gains. Finally, the proposed MPC is extended to the field-weakening operation by adjusting the torque and stator flux references online, which significantly widens the speed range and improves the practical value of the MPC. The effectiveness of the proposed method is confirmed by the experimental results obtained at an average switching frequency of less than 600 Hz.

Position sensorless direct torque control for six‐phase permanent magnet synchronous motor under two‐phase open circuit

A position sensor is usually required in the existing fault-tolerant direct torque control (FT-DTC) system for the multiphase permanent magnet synchronous motor (PMSM). Thus, the volume, weight, and cost of the system is increased, and the reliability is decreased. The conventional high-performance position sensorless strategies for the multiphase motor under fault-free condition cannot be directly applied to FT-DTC system. To improve this situation, a high-speed position sensorless FT-DTC strategy for a six-phase symmetrical winding PMSM with two opened phases is proposed in this study. Firstly, modified variables and a new rotating coordinate system are defined to achieve the symmetrical and decoupled mathematical model. Secondly, a stator flux observer based on the modified variables and a new coordinate system is proposed. According to the current model of modified stator flux, the estimated modified stator current can be achieved. Then, the estimated error of the modified stator current is sent to the integrator of the modified stator flux observer to correct the voltage model. Finally, the proposed stator flux observer is applied in FT-DTC for six-phase PMSM with two opened phases to realize the position sensorless operation at high speed. The experimental results verify the feasibility and effectiveness of the proposed strategy.

Robustness Improvement of FCS-MPTC for Induction Machine Drives Using Disturbance Feedforward Compensation Technique

Finite control set-model predictive torque control (FCS-MPTC) has a fast dynamic response because this algorithm directly selects the optimal voltage vector by its cost function for induction machine drives fed by voltage source inverter (VSI). However, belonging to open-loop control paradigm, the FCS-MPTC has torque tracking error due to inevitable load disturbance and mismatched model parameters in reality. In traditional FCS-MPTC, the outer loop, i.e., speed loop, adopts a classic proportional integral (PI) controller, abbreviated as PI-MPTC. The lumped disturbance is only suppressed by a PI controller. However, pole placement of the PI controller is usually designed by cut-and-trial, which is difficult to simultaneously achieve optimal dynamic performance and optimal suppression of lumped disturbance. In this paper, the FCS-MPTC with mismatched parameters is first analyzed. Second, the deficiencies of the traditional PI controller are introduced. Third, disturbance feedforward compensation-based-model predictive torque control (DFCB-MPTC) of induction machine is proposed to compensate lumped disturbance and improve the performance of the system. Furthermore, a simplified stator flux observer is proposed, whose gain matrix is independent of rotor speed. Experimental results verify the feasibility of the proposed DFCB-MPTC. Compared with traditional PI-MPTC, the proposed DFCB-MPTC has better dynamic performance, steady performance, and stronger robustness.

Sensorless Stator Flux Oriented Control for Startup Gas Turbine

This paper proposes a hybrid position estimation method for load commutated inverter (LCI) fed wound field synchronous motor (WFSM) at zero speed up to turbine ignition speed. In this method injection based position estimation method is used at low speed. Then a stator flux observer is applied to firing pulse generation and position estimation.With use of proposed algorithm the problems of other position estimation are reduced and thyristor's commutations at zero up to final speed are well done. In this paper a new structure has been introduced for stator flux oriented control of LCI fed WFSM which improves power factor and dynamic response of speed controller. Moreover, manufacturing cost is reduced. Finally proposed algorithm and structure are simulated in MATLAB Simulink.

Novel Direct Torque Control Based on Space Vector Modulation with Adaptive Stator Flux Observer for Induction Motors using ANFIS

Novel Direct Torque Control Based on Space Vector Modulation with Adaptive Stator Flux Observer for Induction Motors using ANFIS

Low-Sampling-Frequency Stator Flux Linkage Observer for Interior Permanent-Magnet Synchronous Machines

Deadbeat-direct torque and flux control (DB-DTFC) is a direct torque control (DTC) method that provides the fastest possible torque response with smooth and continuous operation even at the voltage limits. DB-DTFC achieves the smoothest possible torque and flux trajectory tracking performance by using the machine inverse model to select volt-second solutions that will produce the desired torque and flux by the end of each pulsewidth modulation (PWM) interval. Since the DB-DTFC inverse model requires accurate steady-state and dynamic flux estimates, discrete-time stator current and flux linkage observers are needed to properly implement DB-DTFC. The existing high-sampling-frequency stator current and flux linkage observers are formed based on approximate discrete-time models for IPMSMs by assuming high-sampling-frequency (10-kHz) operation. This paper presents a more exact discrete-time model for IPMSMs, such that, even at low sampling frequencies, proper stator current and flux linkage observers can be formed to achieve accurate current and flux linkage estimation. The proposed low-sampling-frequency stator current and flux observers are developed using discrete-time modeling with an inverter-sourced volt-second input model. The proposed observers are shown to be very feasible for real-time implementation, and their experimental evaluation documents the improvement achieved compared to high-sampling-frequency observers, even under machine parameter variations.

Torque Ripple Suppression Method for BLDCM Drive Based on Four-Switch Three-Phase Inverter

A novel inverter fault-tolerant control scheme is proposed to drive brushless DC motor. A fault-tolerant inverter and its three fault-tolerant schemes (i.e., phase A fault-tolerant, phase B fault-tolerant, and phase C fault-tolerant) are analyzed. Eight voltage vectors are summarized and a voltage vector selection table is used in the control scheme to improve the midpoint current of the split capacitors. A stator flux observer is proposed. The observer can improve flux estimation, which does not require any speed adaptation mechanism and is immune to speed estimation error. Global stability of the flux observer is guaranteed by the Lyapunov stability analysis. A novel stator resistance estimator is incorporated into the sensorless drive to compensate for the effects of stator resistance variation. DC offset effects are mitigated by introducing an integral component in the observer gains. Finally, a control system based on the control scheme is established. Simulation and experiment results show that the method is correct and feasible.

Stator Flux Observer for Induction Motor Based on Prescribed Convergence Law Algorithm

This paper proposes a stator flux estimation method for induction motor based on Prescribed Convergence Law algorithm. A stator flux observer is designed and applied for direct torque control (DTC) of induction motor. The observer tracks stator current and its differential with Prescribed Convergence Law algorithm of second order sliding mode in order to estimate rotor flux, and then estimate stator flux using the relationship between stator flux and rotor flux. This paper takes the differential of estimated flux error as disturbance and divides the MIMO (Multiple Input Multiple Output) observer model into two separate SISO (single input single output) systems, which simplifies the stability analysis. The observer is applied to DTC of induction motor and achieves a good control effect. Simulation experiment results validate the proposed method.

Reduced Parameter Sensitivity Stator Flux Linkage Observer in Deadbeat-Direct Torque and Flux Control for IPMSMs

Direct torque control (DTC) has become a widely acceptable alternative to field-oriented control. Deadbeat-direct torque and flux control (DB-DTFC) is a significant improvement over the classical DTC methods and provides opportunities for fast simultaneous control of torque and dynamic loss minimization. DB-DTFC uses estimated torque and stator flux magnitude from a stator flux observer as feedback. The constant parameter state observer utilizes the system model to estimate immeasurable physical states or dynamics of the real system. However, the constant parameter stator flux observer is parameter sensitive at low frequencies. This paper presents a flux observer that utilizes disturbance input decoupling (DID) so that even under varying or inaccurately identified machine parameters, stator flux linkage and torque can be accurately estimated. The structure of the DID flux observer is theoretically derived and discussed. The flux estimation error due to the parameter variations is decoupled through the disturbance information from a stator current observer. Comparative evaluation of the DID flux observer versus the constant parameter flux observer has been performed via both simulation and experiments.

Modeling and Sensorless Direct Torque and Flux Control of a Dual-Airgap Axial Flux Permanent-Magnet Machine With Field-Weakening Operation

This paper presents the modeling and motion-sensorless direct torque and flux control of a novel dual-airgap axial-flux permanent-magnet machine optimized for use in flywheel energy storage system (FESS) applications. Independent closed-loop torque and stator flux regulation are performed in the stator flux ( x-y) reference frame via two PI controllers. This facilitates fast torque dynamics, which is critical as far as energy charging/discharging in the FESS is concerned. As FESS applications demand high-speed operation, a new field-weakening algorithm is proposed in this paper. Flux weakening is achieved autonomously once the y-axis voltage exceeds the available inverter voltage. An inherently speed sensorless stator flux observer immune to stator resistance variations and dc-offset effects is also proposed for accurate flux and speed estimation. The proposed observer eliminates the rotary encoder, which in turn reduces the overall weight and cost of the system while improving its reliability. The effectiveness of the proposed control scheme has been verified by simulations and experiments on a machine prototype.