Discrete-time Sliding Mode Observer Research Articles

Capacitor Voltage Estimation of MMC Using a Discrete-Time Sliding Mode Observer Based on Discrete Model Approach

The Modular Multilevel Converter (MMC) is widely used for high and medium voltage applications due to its several advantages over other multilevel topologies. The number of voltage levels that can be obtained at the AC output can be increased at the expense of greater number of submodule (SM) capacitors. As the capacitor voltages need to be monitored for voltage balancing, the MMC system requires a large number of voltage sensors. This increases capital investment for the MMC system and failure points due to increased hardware interfaces. So far, a few works have been reported that uses observation techniques to either completely eliminate or reduce the number of sensors. This paper presents a Discrete-Time Sliding Mode Observer (DTSMO) for estimating the capacitor voltages and thus possibly eliminating all the voltage sensors of the SM capacitors. A switching function based discrete mathematical model is used for the design framework of the proposed DTSMO. The errors in observation are taken as important performance indicators under both transient and steady state conditions. It has also been observed that the proposed DTSMO delivers robust performance in presence of parametric uncertainty of capacitance values. The control algorithm is simulated in MATLAB/Simulink to validate the performance of the proposed observer. It is further validated through experimental results in a hardware prototype.

Discrete-Time SMO Sensorless Control of Current Source Converter-Fed PMSM Drives With Low Switching Frequency

In this article, a sensorless control method for medium- and high-speed operation is proposed for a current source converter (CSC)-fed permanent magnet synchronous machine (PMSM) with low switching frequency. The low switching frequency as well as inverter-side capacitor filter can cause great challenges in controller and rotor speed/position observer design, especially under higher speed operation due to the limited updates per fundamental cycle and approaching to the LC resonant point. To improve the system dynamic performance and attenuate the LC resonant, a multi-loop controller with capacitor voltage control was added into conventional field-oriented control. Moreover, an exact discrete-time sliding mode observer based sensorless strategy with adaptive filter is proposed to improve the rotor speed and position estimation accuracy as well as robustness to system uncertainty. The effectiveness of the proposed method is verified on a transformerless CSC-fed PMSM drives with both simulation and experiment.

Sensorless Control of CSC-fed PMSM Drives with Low Switching frequency for Electrical Submersible Pump Application

In this article, current source converter (CSC)-fed permanent magnet synchronous motor (PMSM) drives are investigated for electrical submersible pump (ESP) applications. The CSC features motor-friendly waveform with low dv/dt , which is very suitable for long cable application. High-speed ESP by adopting a PMSM has higher efficiency and shorter length and diameter compared with the counterpart adopting induction motor. Sensorless control instead of mechanical sensor can further reduce the system size/cost and improve reliability due to the harsh downhole environment. However, low switching frequency and an inverter-side output filter cause great challenge in controller and observer design. These kinds of influence become more serious under a higher speed region due to the limited updates per fundamental cycle. To overcome these challenges, the dynamic capacitor voltage control is employed in a field-oriented control scheme to enhance the system dynamic response. Moreover, a discrete-time sliding mode observer based rotor position estimation method is proposed to reduce estimation error and improve parameter uncertainty robustness. The effectiveness of the proposed method is verified on a CSC-fed PMSM with simulation and experimental results.

Novel Predictive Stator Flux Control Techniques for PMSM Drives

In this paper, a predictive stator flux control (PSFC) algorithm for permanent magnet synchronous motor (PMSM) drives is proposed, which can eliminate the influence of flux linkage parameter perturbation and rotor position error. First, the sensitivity of conventional predictive current control (PCC) to the flux linkage parameter and rotor position is analyzed. Then, the novel composite discrete time sliding mode observer (SMO) based on stator flux state is designed, which can estimate the flux linkage parameter perturbation, rotor position error, and load torque simultaneously. Finally, a novel PSFC method is developed, which can enhance the robustness of PCC against flux linkage parameter perturbation and rotor position error by using composite discrete time SMO. Simulation and experimental results indicate that the proposed PSFC can achieve low stator current harmonics, low torque ripple, and excellent steady-state performance under the flux linkage parameter perturbation and rotor position error.

Discrete-time sliding mode observer for the state estimation of a manoeuvring target

This article addresses the problem of estimating the position, velocity, and acceleration of a manoeuvring target from noisy position measurements. A discrete-time sliding mode observer is designed to handle unmeasured disturbance input and measurement noise. A first-order linear dynamics is considered for target acceleration. The acceleration input command and the pole of the first-order acceleration dynamics are considered to be unknown parameters with known upper bounds. A finite non-zero boundary layer is employed to reduce the chattering phenomenon typically associated with sliding mode observers. Analysis of estimation error dynamics is presented for the case where the discrete-time sliding mode observer is operating outside the boundary layer and also within the boundary layer. An algorithm is developed for obtaining the observer gain vector that guarantees the stability of the error dynamics. Numerical simulations and experimental results are presented to validate the stability and performance of the proposed observer.

Full-Speed Range Encoderless Control for Salient-Pole PMSM with a Novel Full-Order SMO

For salient-pole permanent magnet synchronous motor (PMSM), the amplitude of extended back electromotive force (EEMF) is determined by rotor speed, stator current and its derivative value. Theoretically, even at extremely low speed, the back EEMF can be detected if the current in q-axis is changing. However, it is difficult to detect the EEMF precisely due to the current at low speed. In this paper, novel full-order multi-input and multi-output discrete-time sliding mode observer (SMO) is built to detect the rotor position. With the proposed rotor position estimation technique, the motor can start up from standstill and reverse between positive and negative directions without a position sensor. The proposed method was evaluated by experiment.

Fault Tolerant Predictive Control Based on Discrete-Time Sliding Mode Observer for Quadrotor UAV

An active fault-tolerant control scheme for a quadrotor unmanned aerial vehicle (UAV) with actuators faults is presented in this paper. The proposed scheme is based on model predictive control (MPC) and the discrete-time sliding mode observer. Considering the impact of disturbances on fault diagnosis, a discrete-time sliding mode observer with simple structure and strong robustness against the disturbances is designed to isolate the actuator faults and estimate the control effectiveness factors accurately. Using the fault diagnosis information, a model predictive active fault tolerant controller with embedded integrator is proposed to compensate parameter uncertainty and bounded disturbances in the realistic control system of the quadrotor. The advantages of the proposed control scheme are the ability of dealing with the control constraints, improving the fault-tolerant control precision and getting better real-time and anti-interference performance. The algorithm comparison experimental results on the quadrotor semi-physical simulation platform validate the feasibility and effectiveness of the proposed control scheme.

State of Charge Estimation of Lithium-Ion Batteries Using a Discrete-Time Nonlinear Observer

In this paper, a novel approach using the discrete-time nonlinear observer (DNLO) is proposed for state-of-charge (SOC) estimation of lithium-ion batteries (LIBs). To design the DNLO for SOC estimation, the state equations based on a second-order resistor–capacitor equivalent circuit model are derived to simulate the dynamic behaviors of an LIB. Considering the hysteresis characteristic of the battery, the model parameters depend on the SOC and the direction of battery current simultaneously, and then, the exponential-function fitting method is adopted to identify the offline results of the parameters. The ninth-order polynomial function is adopted to represent the highly nonlinear relationship between the open-circuit voltage and the SOC. The Lyapunov stability theory is used to prove the convergence of the proposed DNLO. The performance of the proposed method is also verified by the experiments based on the hybrid pulse power characteristic test, which indicates that compared with the extended Kalman filter (EKF) and the discrete-time sliding mode observer (SMO) algorithms, the proposed observer has better performance in reducing the computation cost, improving the estimation accuracy and enhancing the convergence capability.

Combined Discrete-time Sliding Mode and Disturbance Observer for Current Control of Induction Motors

This paper proposes a composite control strategy which combines a discrete-time sliding mode controller with a disturbance observer aiming to decouple current control of vector oriented induction motor drives. The stator current control is carried out through an indirect field orientation in a dq reference frame rotating at synchronous speed. The cross-coupling variables of the induction motor stator currents at synchronous reference frame are modeled as disturbances, and these values are estimated using a discrete-time disturbance observer. The digital implementation delay was included in the plant model formulation, resulting in a control law suitable to direct implementation in microcontrollers and digital signal processors. Then, the nominal and decoupled part of the induction machine model is used for the design of the sliding mode controller, and the additional variables are modeled as disturbances. The cross-coupling variables are observed and used in the control law. The convergence analysis is presented in discrete-time domain. Simulation and experimental results are presented to validate the theoretical analysis, and they show the good performance of the proposed method.

Dead-Time Effect Analysis and Compensation for a Sliding-Mode Position Observer-Based Sensorless IPMSM Control System

This paper presents an extended electromotive-force-based discrete-time sliding-mode observer (DSMO) for rotor position/speed sensorless control of interior permanent-magnet synchronous machines (IPMSMs). Without using voltage sensors to measure IPMSM terminal voltages, the reference voltages generated by the vector control system are used as inputs for the DSMO. However, due to the inverter dead-time effect, the mismatch between the reference voltages and actual terminal voltages will degrade the performance of DSMO. In this paper, the periodically oscillating rotor position estimation error caused by the dead-time is first analyzed. Then, a dead-time compensation scheme is proposed to mitigate this position estimation error. The proposed DSMO with the dead-time compensation scheme is validated on an IPMSM control system used in heavy-duty, off-road, hybrid, and electric vehicles.

Second-Order Discrete-Time Sliding Mode Observer for State of Charge Determination Based on a Dynamic Resistance Li-Ion Battery Model

A second-order discrete-time sliding mode observer (DSMO)-based method is proposed to estimate the state of charge (SOC) of a Li-ion battery. Unlike the first-order sliding mode approach, the proposed method eliminates the chattering phenomenon in SOC estimation. Further, a battery model with a dynamic resistance is also proposed to improve the accuracy of the battery model. Similar to actual battery behavior, the resistance parameters in this model are changed by both the magnitude of the discharge current and the SOC level. Validation of the dynamic resistance model is performed through pulse current discharge tests at two different SOC levels. Our experimental results show that the proposed estimation method not only enhances the estimation accuracy but also eliminates the chattering phenomenon. The SOC estimation performance of the second-order DSMO is compared with that of the first-order DSMO.

Discrete-time sliding mode speed observer for sensorless control of induction motor drives

This study investigates the rotor speed estimation problem for induction motor drives. The authors propose the design of a scheme based on a discrete-time sliding mode observer which provides the rotor speed estimative. A new algorithm for discrete-time rotor speed estimation is developed and analysed. The conditions for the existence of a discrete-time sliding switching hyperplane are analysed. Moreover, conventional algorithms aiming at the chattering reduction and high-frequency switching on discrete-time implementation are discussed for use with the proposed technique. The stability analysis and parameter convergence of the proposed method are investigated for discrete-time solution. The algorithms developed are tested by experimental results based on fixed-point digital signal processor (DSP) platform (TMS320F2812). Therefore the results demonstrate the good performance of the proposed scheme.

Unknown Input Observers for Uncertain Systems: A Unifying Approach

In this paper, a unifying view of existing disturbance decoupling observers is proposed. The different approaches of linear algebraic and geometric design methods and variable structure/sliding modes methods are put together into a single framework. A systematic view is posed in some already existing disperse results, concerning the class of admissible disturbances, the role of the disturbance input matrix, the mechanism of disturbance decoupling and the achievable error dynamics in particular cases of the general model. Some results that fill gaps in knowledge in current research fields are highlighted: a discrete-time sliding mode observer with disturbance decoupling properties, a general form of continuous-time sliding mode observers, the link between sliding modes and linear unknown input observers, and a bound for the degrees of freedom in the error dynamic assignment for some particular cases of the general observer. A heuristic procedure is also proposed dealing with the invariant system zeros.