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The impedance-ratio criterion is widely adopted for analyzing the small-signal stability of systems with cascaded power electronic converters. However, the impedance ratio is formed differently for different cascaded systems. In this paper, a generic impedance-sum criterion is proposed for the stability of general power electronic systems with two converters/sources. The two converters/sources can be in parallel operation, such as parallel-operated voltage-controlled inverter systems in microgrids and parallel-operated current-controlled inverter systems, or in series (cascaded) operation, such as cascaded DC/DC converter systems and grid-connected current-controlled converters. In this paper, it is shown at first that in order for a converter/source to be stable both the impedance and its inverse should be stable, that is, the impedance should not have any right-half-plane (RHP) zeros or RHP poles. Then, it is shown that a system with two individually stable converters/sources is stable if and only if the sum of the individual impedances does not have any RHP zeros or, equivalently, the impedance sum does not encircle the origin clockwise. This generic criterion is then demonstrated via applications covering all four possible cases. Published version

An S-band wideband high power GaN-based power amplifier is presented in this work. The designed amplifier size is 11mmX 20mm, the interconnection between discrete transistor and matching networks is achieved by gold bonding wires. The internally matched power amplifier has presented the pulse output power of over 350 W over 2.7-3.6 GHz frequency band. Experimental results show that the power amplifier with drain voltage of 48 V, can be achieved more than 60% power added efficiency and 350W output power under the pulse-width 300 us and 10% duty-cycle.

This work proposes the large-scale adoption of self-synchronized universal droop controller (SUDC)-based inverters to enable ancillary services for different modes of distribution system operations. The IEEE 123 bus system was modeled on a real-time simulator to study the performance of large-scale adoption of SUDC inverters in a distribution system. The resulting data collected shows that the voltage and the frequency were regulated within ranges, such as less than 5% for voltage and less than 0.5% for frequency, under different load variations and grid operations. Also, the black start was achieved within 0.4 s without any voltage overshoot. Through the simulation and validation on a small microgrid and the IEEE 123 bus distribution system, it can be concluded that the SUDC was successfully adopted to regulate the voltage and the frequency within the given ranges, and black start achieved within 1 s without voltage overshoot for different modes of distribution system operations.

This paper presents an impedance modeling and small-signal stability analysis of the grid-tied Universal Droop Control (UDC) inverter. The -impedance model of the UDC inverter is derived analytically and, which is verified by simulation. The impedance model is used to determine the stability of the grid-tied UDC inverter. Simulation and experimental results are provided to further verify the theoretical analysis. © 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

This paper presents a new control scheme for the back-to-back converter in a wind energy conversion system (WECS) that is equipped with a permanent magnetic synchronous generator (PMSG). The self-synchronized universal droop controller (SUDC) is utilized for both the rotor-side converter (RSC) and the grid-side converter (GSC). The DC-link voltage is regulated by the RSC instead of the GSC. The GSC is responsible for the maximum power point tracking (MPPT) and the grid integration. The control scheme does not involve the commonly-used abc/dq transformation, the phase-locked loop (PLL), or the motor encoder usually used in the field-oriented control to measure the rotor position/angle. The control scheme does not need the generator parameters either, therefore it is simple, and it is robust to system parameters. An experimental platform is built up to test the proposed control scheme and the results have demonstrated the effectiveness and good performance of the control scheme.

This paper proposes a new high-efficiency miniaturized S-band internally matched high power amplifier (HPA), which developed with 24mm AlGaN/GaN high-electron mobility transistors. Load-pull method is used to design the amplifier, the out network matched to compromise between power added efficiency (PAE) and output power. Experimental results show that the class C GaN HPA can be realized more than 62% PAE and 120W output power over the band of 2.6-3.7 GHz, with drain voltage of 32 V, pulse-width of 3ms and duty-cycle of 30%. The HPA size is 6.6mm×15mm.

We present two moment matching-based solutions to the problem of rational approximations of a finite-impulse response (FIR) system: a direct moment matching approximation performed on the FIR system and a sequential moment matching approximation performed on a block split model of a FIR system. Imposing moment matching conditions yields families of parametrized, low-order, rational approximations of the FIR filters, with guaranteed low frequency behavior. We show that the block rational implementation from [1], [2] is a particular case of the sequential moment matching result. The theory is illustrated with an example of a FIR system, where we select the free parameters such that the approximations are stable and more accurate. We compare the proposed approximations with existing rational implementations of the FIR system as well as with Loewner matrices-based approximations.

Robust droop controller belongs to the virtual synchronous machine family, which guarantees the accurate power sharing and an autonomous grid support for the inverters in parallel. In this paper, a precise small signal model of the parallel inverter system controlled by the robust droop controller is constructed. Problems such as the influence of the control and system parameters and system harmonic responses are analyzed based on eigenvalue evaluations. The results reveal the operation characteristics of the system and are further verified through simulations in PSCAD/EMTDC.

In this paper, a dual-buck voltage divider is further studied to provide two arbitrary, instead of balanced, voltage outputs. The two voltage outputs can be the same or different and are robust against parameter drift. The low-frequency ripples in one output are significantly reduced by actively diverting low-frequency ripple currents away from the corresponding output. Note that these are achieved by designing an advanced controller, without changing the topology. The controller consists of a PI controller to split the voltage, a repetitive controller and a resonant controller to deal with the low-frequency ripples at different frequencies. Experimental results are presented to validate the effectiveness of the proposed strategy.

In this paper, an uncertainty and disturbance estimator (UDE)-based robust control strategy is developed for AC/DC converters to achieve accurate DC-link voltage regulation. The models of both DC-link voltage dynamics and power delivering are derived firstly. The UDE strategy is introduced into both voltage-loop design and power-loop design to handle the uncertainties (e.g., the effects of model/parametric uncertainties), and external disturbances (e.g., variations of both amplitude and frequency in the grid, or load change). The proposed control strategy simplifies the parameter tuning and algorithm implementation, and offers a very robust disturbance rejection capability without additional synchronization units. Simulation results are provided to show the effectiveness of the proposed strategy.

This paper presents a control-loop stability analysis and parameter design of the grid-connected virtual synchronous machine (VSM) inverters that enables the VSM operate in a very weak grid condition. In order to study the stability and design the parameters, a line-frequency-averaged small-signal model of the VSM inverter has been derived. A transfer function of the control-loop has been obtained, including the coupling resulting from the inherent nature of the VSM as both active power and reactive power related to the voltage and frequency. An example of parameter design is presented by using the derived model. It is shown that the designed parameters ensure the stability and robustness of the VSM inverters connected into a weak grid with a wide range of the grid impedance variation.

In this paper the robust droop controller based virtual synchronous machines are implemented in a complete system level simulation to validate their active participation in the power system regulation and the benefits they can bring to the power grid. The IEEE 12-bus system with four conventional synchronous generators are adopted as the test system. The system structure and the robust droop controllers are briefly introduced. The simulations are carried out where the entire system operates in the normal condition and after the disturbances. The simulation results are discussed and the advantages of the virtual synchronous machines are illustrated through comparisons.

This paper presents a new anti-islanding protection (AIP) scheme for inverters in integration of distributed generation (DG) sources into the grid. The main advantage of this proposed AIP scheme is that it can be embedded into the control of DG inverters without a need of additional part in the hardware of the inverter and any change in the controller. The AIP scheme does not rely on a communication network. It detects the islanding condition based on continuous monitoring of the active power, the reactive power, and the voltage magnitude mismatch, which is shown to be an alternative to the grid impedance measurement technique. The AIP scheme isolates the inverter from the grid after the grid experiences an islanding condition for all possible scenarios of islanding. A detailed modeling of its implementation in a DG inverter is presented and the performance of the AIP scheme is verified by simulations and experiments.

Grid voltage parameters, including the amplitude, frequency and phase angle, are crucial information for real-time control and protection in power systems. Currently, the phase-locked loop methods are widely used for voltage parameters estimation. However, they could hardly achieve accurate estimation in the presence of unbalanced grid voltage and harmonic disturbance. In this paper, a novel three-phase sinusoid-locked loop (SLL) approach is proposed to address the estimation issue of the unbalanced grid voltage parameters. Firstly, the unbalanced three-phase grid voltage is decomposed as the positive and negative sequences based on the instantaneous symmetrical components theory. Then, two SLLs based on the principles of the virtual synchronous machine (VSM) are developed for the positive and negative sequence grid voltages, respectively. Following the idea of SLL, the output voltage of the VSM is locked with the input grid voltage when the real power and reactive power are both driven to zero. Eventually, the unbalanced three-phase grid voltage is reconstructed with the estimated parameters of the positive and negative sequence voltages. Simulation results demonstrate that the proposed SLL approach achieves excellent estimation performance with respect to the unbalanced grid voltage.

In this paper, a bounded integral controller (BIC) is developed for nonlinear multiple-input and multiple-output (MIMO) systems to handle the sum of squares constraint of control inputs. The BIC guarantees the inputs constraint independently from the plant parameters, and keeps the properties of traditional integral controller (IC) with eliminating tracking errors and achieving disturbances rejection. The closed-loop stability is analyzed with the input-to-state (practical) stability (ISpS). Two simulation cases with practical applications are provided to demonstrate the effectiveness of the BIC design.

This paper proposes an output feedback trajectory tracking control based on the uncertainty and disturbance estimator (UDE) technique. The studied nonlinear system is single-input-single-output (SISO), bounded-input-bounded-output (BIBO), and Lipschitz smooth. The main idea is that the nonlinear system is firstly approximated by a proper first-order linear system plus a lumped uncertainty term, which is estimated and compensated through the UDE. The proposed control technique is modeling-free, and only uses the information of system output and the spectrum of the lumped uncertainty term. It also relaxes the structural constraint in the conventional UDE-based robust control. In order to improve the tracking performance, the internal model principle is adopted in the design of the filter in the UDE. The proposed control design is successfully applied to the position control of a piezoelectric nanopositioning system with hysteresis nonlinearity. The experimental results show that the piezoelectric stage can achieve a fine tracking at 1100Hz, which is 37.93% of the lowest resonant frequency, a significant improvement compared to the commercially available range at 1%–10%.

Grid voltage parameters, including the amplitude, frequency and phase angle of the grid voltage, are essential for real-time control and protection in power systems. Current approaches mainly focus on adopting a phase-locked loop or its derivatives for parameters estimation. However, they could hardly achieve fast response and are sensitive to voltage variations and disturbances. In this paper, the estimation of grid voltage parameters is recast as an observer design problem. An exosystem model of the sinusoidal voltage is established at first. Then, a framework for the observer-based methods is proposed for the robust estimation of single-phase grid voltage parameters. Moreover, a second-order sliding mode observer (SMO), which actually has the lowest order among all possible observers, is designed to estimate the parameters. Based on the Lyapunov theory, the asymptotic stability of the closed-loop SMO is proved. Compared with two other typical methods, simulations are carried out to illustrate the effectiveness and robustness of the proposed approach in the presence of voltage variations and external disturbances. In addition, the proposed estimation approach may also be valuable for online fault detection of voltage sag and low-frequency oscillation.

In this paper, a control framework is proposed to render a power electronic converter passive by using the port-Hamiltonian (PH) theory. As a result, the stability of the system is guaranteed if the rest of the system that the power electronic converter connects to is passive. The controller consists of two control loops, which form a passive port-Hamiltonian (PH) system, and one lossless block, which interconnects the passive block with the plant. In order to facilitate the implementation of the framework, a new word quorte is coined to represent the quantity that is dual to the torque. Simulation results are presented to demonstrate the effectiveness of the control framework in both grid-connected and islanded modes.

Modular multilevel converters (MMC) have been a research focus in high-voltage power conversion, covering modulation techniques, capacitor voltage balancing and circulating current control. In this paper, a control method is proposed to control MMCs to behave like virtual synchronous machines (VSM). It can be operated in the set mode and the droop mode. The dynamics, implementation and operation of the VSM-like MMC are introduced and analyzed. In particular, how the VSM control algorithm works with the circulating current control in MMC is proposed. In this paper, three proportional-resonant (PR) controllers are adopted to inject second-order harmonics to the MMC in order to suppress second-order circulating current. The simulation results are provided to verify the frequency and voltage droop mechanisms, the circulating current suppression of the VSM-like MMC.

In today's energy grid there are very few options to handle the severe energy swings of renewable energy sources. A Synchronverter is designed to allow an asynchronous energy source like wind mills and solar panels to appear as a synchronous energy sources. To this day, the designs have been analog with large size and large assortment of components. This paper proposes a digital design that matches the proposed analog design's output, while using common digital components in order to lower cost and size. The primary focus for the digital design is to use the most optimal mathematical block solutions for improved performance. An FPGA is used as the base component while implementing common FIR integrators. For the mathematical sine and cosine functions, the CORDIC method is used due to its efficient calculation of the sine and cosine functions. Using scaling and 8-bit multiplication blocks, the data integrity can be maintained, while the keeping scale to a minimum. Overall this paper provides a digital synchronverter solution that is optimal for small FPGA use, translating to small dedicated chips, with peak performance.

It is very challenging to control a DC-DC boost converter, due to its non-minimum phase phenomenon, model uncertainties (e.g., the parasitics), and external disturbances (e.g., input voltage change or load change). In this paper, a uncertainty and disturbance estimator (UDE)-based current-mode control (CMC) is proposed to address these problems. A new voltage dynamics is developed to avoid the non-minimum phase phenomenon. Then the UDE algorithm is adopted for both voltage-loop control design and current-loop control design to deal with model uncertainties, external disturbances, and the inductor current estimation error. Furthermore, a new control framework based on this UDE-based CMC is proposed for the grid integration of solar systems with voltage protection. Experimental results are provided to demonstrate the effectiveness and robustness of the proposed strategies.

Design the control system of EWVCM-600 vacuum casting machine according to the principle of vacuum injection. EWVCM-600 vacuum-injection machine control system is mainly consisted of the mechanical structure and circuit structures. The part of mechanical structure completes the pouring cup holders, stirring, pouring each operation by turning the handwheel. Dumping Action of container material uses a chain drive to achieve. Due to the systems for the process is relatively fixed, and the control operation is relatively simple, thus the control part applies the simple and stability relay to control start or stop of the vacuum pump and start or stop of gas equipment. AC variable speed motor drives the agitator, the stirring speed is controlled by operating the governor on the panel. Analyse and test the working principle and process of EWVCM-600 vacuum-injection machine, and design the mechanical structure and circuit diagrams. It achieves the desired objectives.

In this paper, a recently-reported single-phase rectifier with two outputs (RECTO) is further improved to reduce the current stress of the neutral inductor in the rectifier. The reduction is achieved by moving the neutral inductor away from the path of the grid current. As a result, the inductor only carries the differential current of the dual loads. Since the maximum value of the differential current is much smaller than that of the grid current, the current stress of the neutral inductor can be significantly reduced, and the size of the inductor becomes much smaller, which helps improve the power density of the RECTO. In theory, the current stress can be reduced by at least three times and the inductor size by at least nine times. It is worth noting that the current stress of the switches and the other features of the RECTO, e.g., operation principles, independent DC outputs and unity power factor, are not affected. Comparative experimental results are presented to demonstrate the reduction.

In conventional droop control, accurate proportional load sharing could not be achieved among parallel operated inverters due to the mismatch of output impedance and system disturbances. In this paper, an uncertainty and disturbance estimator (UDE)-based robust droop control strategy is proposed for accurate proportional load sharing, particularly reactive power sharing, among parallel operated L-inverters. The reactive power dynamics is derived from power delivery equation passed by a low-pass filter, while the reactive power reference is designed with the feedback of load voltage. The reactive power control is developed based on UDE-based method. The model nonlinearity and uncertainty (e.g., power angle), and system operation disturbances (e.g., change of output impedance, and load change), can be estimated and compensated by this UDE-based robust droop control. Experimental validation is provided to show the effectiveness of the proposed method.

For cascaded system, interactions among individually designed power subsystems may cause the instability of the whole system. It has been shown recently that the series-virtual-impedance (SVI) control strategy can stabilize the cascaded system via shaping the load input impedance in a very small frequency to minimize its impact on the original load converter. In this paper, a further investigation of the SVI control strategy is carried out by two-port network analysis. Firstly, it is pointed out that, though the impact of the SVI control strategy is limited in a very small frequency, it still deteriorates the performance of the original load converter. In order to overcome this drawback, this paper proposes source-side series-virtual-impedance (SSVI) control strategy, which move the series-virtual-impedance of the traditional SVI control strategy from load side to source side by changing the control of the source converter instead of the load converter. With the proposed SSVI control strategy, the cascaded system not only can solve its instability problem, but also can improve the performance of its source converter. Finally, a 100W 48V-32V-24V experimental system is fabricated to validate the SSVI control strategy.1

In this paper, the uncertainty and disturbance estimator (UDE)-based control is adopted for a class of nonlinear systems with mismatched uncertainties and input saturation. By integrating with the backstepping approach, the robustness for the mismatched uncertainties is guaranteed by the recursive structure of backstepping. The “disturbance-like” terms which consist of the unknown dynamics, disturbances and the system intermediate tracking errors are estimated by low-pass filters with the knowledge of spectrum information. An auxiliary system is constructed to deal with the input saturation while only the system order information is known. Stability analysis of the closed-loop system shows that the uniformly ultimate boundedness is guaranteed. A numerical example is provided to illustrate the effectiveness of the proposed method.

A current-limiting controller with nonlinear dynamics is proposed in this paper for single-phase grid-tied inverters. The inverter is connected to the grid through an LCL filter and it is proven that the proposed controller can achieve accurate real and reactive power regulation. By suitably selecting the controller parameters, it is shown by using the nonlinear input-to-state stability theory that the inverter current remains below a given value at all times. This is achieved without external limiters, additional switches or monitoring devices and the controller remains a continuous-time system guaranteeing the boundedness of the system states. Guidelines for selecting the controller parameters are also given to provide a complete controller design procedure. Simulation results of a single-phase grid-tied inverter are presented to verify the desired power regulation of the proposed controller and its current-limiting capability.

In this paper, an uncertainty and disturbance estimator (UDE)-based control is proposed to achieve accurate power flow control for grid-tied inverters (GTI). The power delivering dynamics is introduced at first after considering both frequency dynamics and voltage dynamics. Then the UDE algorithm is adopted to regulate both output voltage amplitude and frequency for accurate real power and reactive power control. With the adoption of the UDE method, the model uncertainty (e.g., power angle) and external disturbance (e.g., variations of grid frequency and grid voltage) can be compensated automatically. Moreover, this UDE-based dynamic power flow control can achieve self-synchronization without an extra synchronization unit (e.g., a phase-locked-loop) when the inverter is connected to the grid. In addition, the proposed controller is easy for implementation and parameter tuning through the designs of desired tracking error dynamics and UDE filters, while having the flexibility and performance of advanced control methodologies. The asymptotic stability of the closed-loop system is analyzed. Experimental results are provided for validation.

This paper proposes a new method for the tracking control problem of strict-feedback systems with mismatched disturbances. The method, called the uncertainty disturbance estimator (UDE), is introduced into the conventional backstepping scheme to handle the mismatched disturbances due to its excellent performance in handling uncertainties and disturbances yet a simple control scheme. The dynamic surface control (DSC) is adopted to overcome the “explosion of complexity” problem in the backstepping scheme. Compared to the existing work in the literature, the backstepping deviation term between the synthetic virtual control and system state, is lumped into the “disturbance-like” term, together with the DSC filtering error and mismatched disturbances. The lumped term is then handled by the UDE method at each step, which simplifies the control design and analysis. In addition, the marriage of UDE with backstepping also relaxes the structural constraint of the UDE design for mismatched uncertainties. The uniformly ultimate boundedness of the closed-loop system is proved. Both numerical simulation and experimental validation are carried out to demonstrate the effectiveness of the proposed approach.

A nonlinear controller with an inherent current-limiting capability is presented in this paper for different types of dc/dc power converters (boost, buck-boost). The proposed controller is based on the idea of applying a dynamic virtual resistance in series with the inductor of the converter, which varies according to a nonlinear dynamical system. It is shown that the proposed approach acts independently from the converter parameters (inductance, capacitance) or the load and has a generic structure that can be used to achieve different regulation scenarios, e.g. voltage, current or power regulation. Based on the nonlinear model of the boost and the buck-boost converter, it is analytically proven that the inductor current remains always bounded below a given maximum value using input-to-state stability theory under a suitable choice of the controller parameters. Hence, the proposed control strategy offers an inherent protection property since the power of the converter is limited below a given value during transients or unrealistic power demands. Simulation results for both types of dc/dc converters are presented to verify the desired controller performance.

Two typical load shaping stabilization methods, stabilization via amplitude compensation (SAC) and stabilization via phase compensation (SPC), are firstly analysed and compared in this paper. It is shown that, though both SAC and SPC can stabilize the cascaded system, the system utilizing the SAC is unconditionally stable but conditionally stable when utilizing the SPC. Therefore, the cascaded system with SAC is more stable than that with SPC. In addition, considering the SAC can be realized by both the parallel-virtual-impedance (PVI) and series-virtual-impedance (SVI) control strategies, the PVI / SVI control strategies are also compared in this paper. The results shows that, the PVI control strategy has inevitable limitation when realizing the SAC during the whole load and input voltage range of the load converter, however the SVI control strategy does not have limitation. Therefore, compared to the PVI control strategy, the SVI control strategy is more suitable for the SAC. Moreover, in order to stably connect the load converter to different source converters without changing its internal structure, an adaptive SVI control strategy has been further proposed in this paper. Finally, the adaptive SVI strategy has been experimentally verified on an actual 100 W cascaded system.

In this paper, a cascaded control scheme for single-phase inverters connected to a load is proposed to improve the power quality and shape the output impedance. The proposed strategy consists of a proportional controller in the inner loop and the Uncertainty and Disturbance Estimator (UDE) in the outer loop. An advanced modulation method is adopted to minimize the control implementation delay and both the current and the voltage control loops are analytically investigated. Simulation and experimental results are presented to validate the theory.

This paper describes the structure and the function of the main components of the elevator, and then detailed describes drag principle and control principle for the elevator. AC motor speed control method is selected, then describe and choose the basic principle of AC two-speed, AC voltage regulator control and AC variable voltage variable frequency speed control. Finally, design the elevator project program based on VVVF control system by PLC and inverter.

Cascaded Buck conversion system is a typical structure in dc distributed power system (DPS). It is known that, the interaction between its individually designed power modules/subsystems may cause the instability problem of the whole system. The traditional solution is to design this system by Middlebrook criterion. However, authors found that, even if the entire system meets Middlebrook criterion, the fast scale instability problem may still occurs. In order to analyse and solve this problem, this paper presents a fast scale model for the cascaded buck conversion system. By this model, the essential reason for fast scale intsability problem is explained and a phase-shifted-carrier (PSC) solution is also proposed. Finally a 48 V-24 V-5 V 200 W cascaded system is built, and the experimental results confirmed the theoretical analysis.1

Synchronverters are grid-friendly inverters that mimic the operation of conventional synchronous generators. In this paper, the non-linear stability of the synchronverter in the sense of boundedness is investigated and proven for the first time. To this end, initially, the complete non-linear model of the synchronverter is obtained and then the field-excitation current loop of the synchronverter is implemented using a bounded integral control (BIC) structure, as proposed by the authors in the literature. While maintaining the original synchronverter operation near the rated value, the BIC additionally guarantees that the field-excitation current will remain bounded in a given range. Using non-linear analysis and input-to-state stability theory, it is proven that the frequency dynamics will remain bounded which leads to a bounded synchronverter voltage and finally results in the stability of the closed-loop system in the sense of boundedness. Extensive simulation results verify that the proposed synchronverter with the BIC approximates the behavior of the original synchronverter near the rated value with a guaranteed bounded performance.

In this work, a non-linear controller with an inherent current-limiting property is proposed to achieve dc output voltage regulation and unity power factor operation for single-phase ac/dc converters. The proposed current-limiting non-linear controller (CLNC) acts independently from the system parameters and is proven to guarantee closed-loop system stability using non-linear control theory. Without requiring the instantaneous measurement of the grid voltage, a PLL or external limiters, the CLNC additionally guarantees that the input current will always remain below a given maximum value, even during transients. This represents a significant superiority with respect to the existing techniques, since a given limit for the input current is achieved without changing the controller dynamics or using external protection circuits. Extended simulation results using both Matlab/Simulink and a real-time digital simulator are presented to verify the proposed controller performance under several changes of the reference voltage.

Robust and precise control of piezoelectric actuators is quite challenging due to the existence of strong hysteresis nonlinearities. In this paper, a control strategy is proposed based on the uncertainty and disturbance estimator (UDE) to improve the performance of the positioning control of piezoelectric actuators. Compared to the existing hysteresis inversion based or other robust control strategies, the UDE-based controller can achieve excellent positioning precision without the knowledge of the bound and shape of hysteresis. Simulation results are presented to illustrate the effectiveness of the UDE-based controller, where the system dynamics of the piezoelectric actuator are characterized by a second order linear system preceded by a symmetric or an asymmetric generalized Prandtl-Ishlinskii (GPI) input hysteresis model. The extraordinary capability of the UDE-based controller is further demonstrated on other smart material-based actuators, e.g., magnetostrictive, shape memory alloys (SMA), where the hysteresis is characterized by a saturated GPI model.