劉 康志

Networked control systems (NCSs) have received much attention in the field of robot teleoperation, telesurgery, and other applications. In an NCS, it is important to compress data transmitted over a limited communication network while preserving the data required for control. We address a quantized feedback system in which the output of a plant is quantized with a variable quantization interval. For such a system, we present a novel optimal quantized feedback control. The proposed approach derives not only an optimal input, but also an optimal quantization interval in the quantizer simultaneously using the model predictive control. The approach also satisfies constraints on the state, input, and quantization interval. The effectiveness of the proposed approach is validated through cart-positioning experiments.

This letter addresses reduction of the energy capacity of energy storage systems (ESSs) using H∞ control theory. The ESSs have been widely used for smoothing fluctuations of wind generator outputs. We propose a direct and efficient method to design controllers that achieve the minimization considering the trade-off between the energy capacity of the ESSs and frequency variation in power systems.

The burden distribution process is an important and efficient measure to maintain the stable operation of the blast furnace. An accurate burden distribution model will reveal the impact on the internal furnace state and help to optimize the blast furnace production index. This article reviews the recent development of the modeling and control techniques in the burden distribution process. The current modeling methods of the blast furnace burden distribution can mainly be divided into the following types: the mechanism based method, the physical scale model-based experiments and the data-driven method. However, most of the existing modeling methods are not applicable to general blast furnaces because it depends on the specific furnace structure and parameters. Furthermore, with the advancement in measurement technology, sensors now provide rich amount of online measurement of the blast furnace iron-making process. This makes the data analysis more challenging. It is imperative to establish new modeling methods for the burden distribution process. Therefore, this paper points out the new trends in modeling and control of the blast furnace burden distribution process. First, a dynamic clustering method based on dynamic time warping and adaptive resonance theory is introduced. Second, the inverse dynamic model-based burden distribution control is developed. Furthermore, a multi-model-based switch for modeling the fluctuating blast furnace process is formulated. Finally, the reinforcement learning method for the dynamic optimization of the production index is recommended.

GyroWheel is an innovative device that combines the actuating capabilities of a control moment gyro with the rate sensing capabilities of a tuned rotor gyro by using a spinning flex-gimbal system. However, in the process of the ground test, the existence of aerodynamic disturbance is inevitable, which hinders the improvement of the specification performance and control accuracy. A vacuum tank test is a possible candidate but is sometimes unrealistic due to the substantial increase in costs and complexity involved. In this paper, the aerodynamic drag problem with respect to the 3-DOF flex-gimbal GyroWheel system is investigated by simulation analysis and experimental verification. Concretely, the angular momentum envelope property of the spinning rotor system is studied and its integral dynamical model is deduced based on the physical configuration of the GyroWheel system with an appropriately defined coordinate system. In the sequel, the fluid numerical model is established and the model geometries are checked with FLUENT software. According to the diversity and time-varying properties of the rotor motions in three-dimensions, the airflow field around the GyroWheel rotor is analyzed by simulation with respect to its varying angular velocity and tilt angle. The IPC-based experimental platform is introduced, and the properties of aerodynamic drag in the ground test condition are obtained through comparing the simulation with experimental results.

In this paper, a compound control which combines a spatial internal model with a linear extended state observer (LESO) is proposed for continuously rotating machines. It is implemented digitally based on a uniform time sampling. An angular position-based internal model is synthesized in the spatial domain to suppress the spatially periodic disturbances existing in rotary machines, which is actuated with a fixed sampling period by introducing the angular position to the internal model in real-time. Further, an LESO is designed to estimate non-periodic disturbances and uncertain dynamics acting on the system so as to reduce the steady state error of the position output with respect to ramp angular position reference input. Comparisons and experimental results are presented to illustrate the feasibility and effectiveness of the compound control method. (C) 2016 Elsevier Ltd. All rights reserved.

Recently, wind power has been attracting much attention because of its positive effect on the global warming. However, wind power generation causes frequency variations in electric power system. Therefore, battery systems for smoothing fluctuations of wind generator output are needed. Though battery control systems with simple low-pass filter (LPF) have been reported so far widely, filters other than LPF have not been studied sufficiently. This paper proposes a new method for designing energy capacity and control system of battery system with a new type of filter, which is based on the frequency characteristics of power system.

This paper deals with the vector control, including both the direct vector control (DVC) and the indirect vector control (IdVC), of induction motors. It is well known that the estimation of rotor flux plays a fundamental role in the DVC and the estimation of rotor resistance is vital in the slip compensation of the IdVC. In these estimations, the precision is significantly affected by the motor resistances. Therefore, online estimation of motor resistances is indispensable in practice. For a fast estimation of motor resistances, it is necessary to slow down the convergence rate of the current estimate. On the other hand, for a fast estimation of the rotor flux, it is necessary to speed up its convergence rate. It is very difficult to realize such a trade-off in convergence rates in a full order observer. In this paper, we propose to decouple the current observer from the flux observer so as to realize independent convergence rates. Then, the resistance estimation algorithm is applied to both DVC and IdVC. In particular, in the application to IdVC the flux observer needs not be used, which leads to a simpler structure. Meanwhile, independent convergence rates of current observer and flux observer yield an improved performance. A superior performance in the torque and flux responses in both cases is verified by numerous simulations.

This paper addresses the stabilization problem of linear systems subject to input saturation. The major purpose is to introduce nonlinearity into control laws so as to expand the design freedom for performance enhancement. To this end, a new approach is developed which involves a partial differential matrix inequality (PDMI). A class of stabilizing feedback laws is explicitly obtained by solving this PDMI analytically and the feedback laws are parameterized by a nonlinear function. Further, it is revealed that any linear observer can be used to realize the output feedback stabilization. Numerical examples, including the seek control of a hard disk drive, show that the introduced nonlinearity does contribute to the improvement of system performance. The application to integral control is also discussed. (C) 2013 Elsevier Ltd. All rights reserved.

The direct torque control (DTC) of ac motors leads to a faster torque response with a small number of switching operations in inverters when compared with a conventional approach such as vector control. DTC exhibits a hybrid nature in the sense that the system is composed of continuous variables of torque and flux and involves discrete switching in the inverter. The output of the inverter is limited to the finite discrete values at each instance of sampling. Model predictive control (MPC) is applied to the system so that an optimal switching sequence is derived subject to the given constraints. The proposed MPC-based approach reduces the torque ripple with a small number of switching operations. The effectiveness of the proposed MPDTC approach is verified through simulations and experiments by comparing the proposed approach with conventional DTC.

The next-generation fine stage of the wafer scanner needs a suitable actuator to meet the requirements of high speed, high acceleration, and high precision. The voice coil actuator is no longer the best choice because of its large size and the heat dissipation is difficult to solve. The reluctance actuator can provide a big force based on a unique property of small volume and low current, making it a very suitable candidate. But the strong nonlinearity such as the hysteresis between the current and force limits the reluctance actuator applications in nanometer positioning. This paper proposes a nonlinear current control configuration with hysteresis compensation using the adaptive multilayer neural network. Simulation results show that the hysteresis compensator is effective in overcoming the hysteresis and is promising in precision control applications.

This paper describes a new model predictive control method for buck DC-DC converters. The objective is to achieve a good trade-off between fast response and low overshoot in the transient response of the output voltage. In the well-known PWM-PI method, such a trade-off is difficult to attain. The proposed model predictive control is based on an analytic algorithm that can be easily implemented online. Further, a PI outer loop is introduced to improve the robustness against load uncertainty. Moreover, the advantage of the proposed model predictive control is verified experimentally. The experimental results show that the proposed method outperforms the PWM-PI method.

In this paper, a model-based control method for RTP (Rapid Thermal Processing) system is proposed. RTP system is a device used to heat semiconductor wafers uniformly. It can heat wafers rapidly and reduce the processing time. However, it is very difficult for a RTP system to achieve the extremely stringent requirement on temperature uniformity (±1[K] or less) without suitably controlling and coordinating the radiative heat flux from the halogen lamps. In this research, the location of lamps is optimized first. Then, a feedback control system is designed which provides optimal temperature control during the thermal processing. The design is carried out based on a linearized model and H₂ optimal control. The effect of sensor location is also investigated. Numerous simulations show that satisfactory performances are obtained in both the tracking and the uniformity of wafer temperature.

DC-DC converters by nature present hybrid behavior, which is described by a set of discrete modes associated with continuous dynamics. The control objective is accomplished by switching among the discrete modes. This paper presents a new model predictive control approach to optimize the performance of DC-DC converters. The proposed control algorithm is tailored to DC-DC converters, making full use of the fact of finite number of modes to transform the performance optimization problem into a combinatory optimization task. This approach has made the on-line computation very simple and effective for implementation.

In this paper, a new control approach is proposed Cor the transient stabilization of a single-machine infinite-bus Power system. The proposed method is based oil linear parameter varying (LPV) modeling of the nonlinear power system and the design of a gain-scheduled Output feedback controller. It is well-known that when large disturbances or it fault occurs, the nonlinearity inherent in power systems call no longer be ignored. The proposed method call handle the nonlinear model directly. First, we show that the nonlinear model call be transformed equivalently into all LPV system with the rotor angle as the scheduling parameter. Then, a gain-scheduled output feedback controller is designed based on robust pole placement and L-2-gain minimization. Simulation results verily that the proposed method is better than well-tuned conventional power system stabilizer (PSS) control. (C) 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The Takagi-Sugeno (T-S) model is one of the most powerful methods in describing nonlinear systems. Because the T-S model can be easily converted into an LPV system, Lyapunov stability analysis and controller design reduce to solving an LMI problem. However, since the constructed T-S model is generally approximation, how to construct a T-S model with less approximation error is important. In this paper, we propose a modeling method for T-S model which yields no error for some class of nonlinear systems, and discuss some expansion as well as estimation of error bound for general nonlinear systems. (c) 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(1): 52-59, 2009; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20503

In this paper, the stabilization problem of a class of on-off control systems is studied. First a gobal stability condition for the state feedback system is derived, which is characterized by a partial differential matrix inequality. Then, a class of state feedback laws which includes nonlinear laws is designed by solving related partial differential equations analytically. Further, the output feedback control design based on a state observer is discussed.

In this paper, a gain-scheduling controller for automotive vehicles is proposed. This controller is to protect the vehicle from spin and realize the improved cornering performance. Since the vehicle unstabilization is mainly caused by the cornering force saturation of rear tire. This saturation is considered explicity in design by using LPV modelling method. And a gain-scheduling controller is designed based on linear matrix inequality (LMI) approach. Simulation shows that the vehicle stability performance is improved significantly as compared with robust control based on linearized model.

This paper studies the feedback control of a class of second order linear systems which include the double integrator and are actuated by on-off input. This class of systems includes many important applications such as the orientation control of satellites which are actuated by thrustors. A class of feedback control laws are proposed which guarantee the global stability. The basic approach in stability analysis is to use continuous and piecewise smooth Lyapunov functions and Filippov theory.

In this brief, a simple control approach is proposed for the multichain systems based on a switching algorithm of two sets of steering laws. Global exponential convergence to any prescribed norm bound (epsilon-convergence) is guaranteed, and the convergence rate is explicitly given. This approach has a distinctive feature that one of the inputs (driving input) is free. This feature plays a key role in achieving realistic parking tasks. For the design of steering laws, a method based on backstepping is proposed and analyzed in detail. Further, this control algorithm is applied to the automatic parking, systems. The corresponding system package is developed which is composed of control laws for the steering angle and the driving velocity, algorithms for various kinds of parking tasks under steering angle and parking space limitations. A prototype system is built to validate this package, and the system works well in experiments subject to various parking situations.

This paper aims at developing a practical automatic parking technology for 4-wheeled cars, which is able to park cars subject to parking space constraint and steering angle saturation. The feedback law is designed based on the switching control method proposed by the authors for chained systems. The focus of this paper is to prove a condition on the initial state under which the stability is guaranteed even when the steering angle is saturated. After that, parking algorithms are proposed based on this condition. The proposed method is validated by numerous parking experiments.

In this paper, the posture stabilization and longitudinal motion control of a unicycle robot is presented. This unicycle robot has two gyroscopes acting as an actuator for the lateral stabilization and a wheel as an actuator for the longitudinal stabilization. The system is modeled based on Lagrangian Dynamics and system identification method. A gain-scheduled robust control method is applied to enhance its disturbance attenuation ability so as to achieve better performance in posture stabilization and longitudinal motion control. Satisfactory experimental results are obtained. Copyright © 2006 by The Japan Society of Mechanical Engineers.

In this note, a simple derivation of the Riccati equation based solutions to the standard H∞ control problem, namely the well-known Glover-Doyle solution and DGKF solution is given based on LMI solution. It is hoped that this will be helpful in deepening the understanding of Riccati equation solutions. © 2005 Elsevier B.V. All rights reserved.

Takagi-Sugeno(T-S) model is one of the most powerful methods in describing nonlinear systems. Because T-S model can be easily converted into an LPV system, Lyapunov stability analysis and controller design reduce to solving LMI problem. However, since the constructed T-S Model is generally approximation, how to construct an T-S model with less approximation error is important. In this paper, we propose a modelling method for T-S model which yields no error for some class of nonlinear systems, and discuss some expansion as well as estimation of error bound for general nonlinear systems.

The control of position and orientation angle of 4-wheeled cars is the central part in the development of automatic parking systems which is going to play an important role in ITS. There have been many approaches to this problem from a viewpoint of application of nonholonomic control theory. But there are still some practical problems such as steering angle limitation that need to be addressed. In this paper, a method is proposed to deal with the steering control with a steering angle limitation. The basic idea is to contruct a circle family and let the car approach the circle family that leads to the destination asymptotically. Both theoretical and experimental results are described.

It is an important problem in multiarea power systems to attenuate the frequency deviation due to load change. Since power systems are spatially dispersive, decentralized control is more practical than lumped control. In this paper, we propose two robust decentralized control schemes for this problem. The difficulty with this problem is that the linearized model of a multiarea power system with frequency deviations as outputs is a MIMO system that has a transmission zero at s = 0. They key idea is to change the output of one subsystem so that the resulting new plant does not have any zero at s = 0. The new outputs are chosen carefully so that the regulation of the new outputs can guarantee the regulation of all frequency deviations and tieline power. Further, parameter uncertainty is taken into account in the controller design to achieve robustness. Simulation shows that the proposed methods are effective. (C) 2002 Wiley Periodicals, Inc.

In this paper we investigate parameter dependent Lyapunov functions for the linear systems with a uncertain constant real parameter in terms of geometry. First, we formulate the surface on which all parameter dependent Lyapunov matrices for a given uncertain system exist. Next, after defining a Riemannian metric on the surface, we derive a method to obtain a parameter dependent Lyapunov matrix as a geodesic on the surface.

It is an important problem in multi area power systems to attenuate the frequency deviation due to la change. Since power systems are spatially dispersive, decentralized control is more practical than lump control. In this paper, we propose two robust decentralized control schemes for this problem. The difficul with this problem is that the linearized model of a multi area power system with frequency deviations outputs is an MIMO system that has a transmission zero at s=0. The key idea is to change the output one subsystem so that the resulting new plant does not have any zero at s=0. The new outputs are chos, carefully so that the regulation of the new outputs can guarantee the regulation of all frequency deviatio and tie-line power. Further, parameter uncertainty is taken into account in the controller design to achie robustness. Simulation shows the proposed methods are effective.

In this paper, we consider an H∞ problem using unstable weighting functions. Such weights are used to achieve asymptotically disturbance attenuation and/or asymptotically reference tracking, and they cannot be handled by the standard H∞ control theory. For this problem, we present a necessary and sufficient condition to exist a solution using Riccati inequalities. This condition is derived under no unnecessary assumption to enlarge the applicability to various control problems. Moreover, introducing the assumption that the image of the zero vectors of G12 and G21 are unique, we obtain a LMI condition in order to check the solvability numerically. A numerical example is presented to demonstrate how to calculate a controller.

In this paper, we derive an LMI solution to general script H sign2 suboptimal control problems without any unnecessary assumptions. Necessary and sufficient conditions for the script H sign2 norm of the closed-loop transfer matrix to be bounded by a given number is given by 4 coupled LMIs. A suboptimal controller is also obtained. The obtained solution applies to both nonsingular and singular cases. © 1999 Elsevier Science B.V. All rights reserved.

In recent years, various electronic control devices have been used in order to improve the performance of cars. One purpose is to make the driving comfortable. In automatic transmission cars, torque fluctuation occurs due to gear shifting, this worsens the drivability. In this research, we try to reduce the fluctuation of output torque of transmission via feedback control. H_∞ control theory is used in the design of control system. The effectiveness of the designed control system is verified by experiment.

An automatic transmission without a one-way clutch for a small sized, light weight automatic transmission is presented. The factor of torque fluctuation occurrence during shifting of the transmission increases so that the shifting is executed by controlling two wet clutches electronically in place of the one-way clutch and the wet clutch. Therefore, it is necessary to develop a new smooth gear shift control technology for clutch-to-clutch shifting on an automatic transmission without a one-way clutch. The control technology has desirable clutch-to-clutch shift control, learning control and robust control which apply to accurate signals obtained by an observation method. Smooth shifts during clutch-to-clutch shifting can be realized by recognizing clutch change-over time using a calculated acceleration and an input/output speed ratio of the transmission. Moreover, smooth shifting characteristics in the oil temperature range from 30 °C to 120 °C are obtained by using one robust controller. Copyright © 1999 Society of Automotive Engineers, Inc.

In this paper, the robust tracking performance for a class of uncertain plants with triangular structure is considered. A decentralized feedback configuration is adopted in order to preserve this triangular structure in the closed loop system and enable the asymptotic tracking with as less servomechanism as possible. The necessary and sufficient condition for the robust asymptotic tracking subject to this decentralized configuration is derived. Synthesis based on the obtained condition will yield servomechanism of order lower than that obtained by the Internal-Model Principle.

In this paper, we consider an H_∞ problem with unstable weighting functions. Such weightings are used to achieve asymptotically disturbance attenuation and/or asymptotically tracking to reference input. Such problems cannot be handled by the standardH_∞ control theory. To this problem, we present a necessary and sufficient condition for the existence of a solution by using Riccati inequalities. This condition needs no assumptions on the jω invariant zeros of G₁₂ and G₂₁.

In this study, the extended H_&infin;, control, proposed in Ref. 6), is applied to the frequency control of a multi area power generation plant. More specifically, we consider a two area electric power generation system, with two generators, two loads and a tie-line connecting them. The purpose is to reject the frequency deviations due to step load disturbance.<BR>This is an interesting control problem because of the existence of a zero at s=0 in the multivariable plant that imposes a constraint on the steady state output y (&infin;). The paper discusses how to construct a suitable generalized plant for such a problem in order to design an H&infin; optimized control system that is able to reject step load disturbances asymptotically.<BR>Using the H_&infin; approach, the controller is derived to achieve the disturbance rejection and to be robust to parameter uncertainty of the plant. The performances of the extended H&infin; controller and the LQG controller are compared. It is shown that the H_&infin; controller is superior to the LQG controller in both nominal case and perturbed case.

Mine train control system is consisted of a winder, a train which transports many people to minebottom, and a long wire rope which connects a winder and a train. The mine train control system reported in this paper is the system with the longest rope of about 6700m in the world. This system often generate a rolling of train when the train stops at the station, when using conventional control. In convensional systems, two mine train control systems of the length of 1/2 of all mining galleries were used in order to ristrict the rolling. In these systems, change time was necessary. However, this change time become unnecessary by using a system with the length of 6700m. When using this system, it is the main subject how to control the bigger rolling. We solved the problem on the rolling control of this system by using H_&infin; control, and got the result that the travel time of the train made 2/3 compared with that of the convensional systems using two mine train control systems, and the train speed of 420m/min which is the maximum speed in the world was achieved.

When an MIMO plant has zeros at s=0, it will be shown that this imposes a constraint on the steady state output of the plant. In the control design using unstable weight, the structure of unstable weight can not be selected freely in this case. The required structure is exposed in this note. We then propose a particular generalized plant structure for an optimal servo design such as the one using H_∞ control.

In this paper, we propose an approach that treats the synthesis of servo systems as well as the estimation of unstable plants as ℋ2 control problems with unstable weights. This approach is based on the notion of comprehensive stability and the parameterization of comprehensively stabilizing controllers. The complete result for the nonsingular case is obtained. This provides an ℋ2 optimal synthesis method for general linear servo problems and estimation problems of unstable plants. © 1997 Elsevier Science B.V.

This paper derives comprehensive stabilty condition for plants having unstable weighting functions and investigates the structure of controllers with an application to the extended H₂ control problem.

Two-degree-of-freedom (TDOF) system has a characteristic that the feedback property such as disturbance attenuation and the reference response property can be designed separately. But in general the order of TDOF controllers tend to be higher compared with that of one-degree-of-freedom (ODOF) controllers. For this reason, a new design method of the TDOF system using the dynamical model of the feedback controller was proposed by Y. Chida. In this method, the reference response achieved by the feedback controller can be improved by the TDOF controller whose order is same as that of the feedback controller. The TDOF controller is designed as such that minimizes a square integral performance index. But the choice of the weighting parameter of the performance index is difficult and some try-and-error is needed to get a good performance.<br>In this paper, we propose an alternative design method for the TDOF controller proposed by Y. Chida. We introduce a model transfer function and the controller is designed so that the output of the two-degree-of-freedom system follows the output of the model transfer function. Designers need only to choose the model transfer function which has desired time response.<br>Further, we apply this method to the 2-mass control system to improve the reference response. Simulation results are presented to demonstrate the control effects.

The present paper considers a variant of the standard ℋ∞control problem which allows one to use weighting functions having jω poles. Using the solution, one can design ℋ∞controllers having prescribed jω poles such as internal models. To solve the problem, the authors propose a new requirement of closedloop stability, called essential stability, and alternative standing assumptions imposed on the generalized plant. To write the results in the form of the so-called 2-ARE solution, the authors introduce the notion of quasi-stabilizing solutions to the algebraic Riccati equations arising in ℋ∞control. The solution involves the same Riccati equations and parametrization of the controllers given by Glover and Doyle except the stability requirement on the solutions to the Riccati equations.

This paper gives the parameterization of all controllers achieving the comprehensive stability which is the stability required both in regulation problems and in servo problems of linear systems. The parameters in the parameterization of comprehensive stabilizing controllers are analyzed. Further, it is shown that subject to some conditions, the obtained comprehensive stabilizing controllers possess an Internal-Model-like structure. It is also proven that in such cases the condition of comprehensive stabilizability implicitly indicates that the plant does not have transmission zeros capable of canceling the poles of the Internal-Model, as was well understood.

A 2-mass system connected by a non-ideal shaft has resonance modes, and it is difficult to design a controller suppressing the vibration in the face of parameter variation. This paper proposes a method of μ-synthesis based on the descriptor form representation to deal with parameter uncertainties independently. First, a class of parameter uncertainty given in the descriptor form is considered, and the paper shows how to achieve robust stability and robust performance for this class of perturbed systems. Then this method is applied to a 2-mass system with parameter uncertainties of the torsional constant and the load inertia. By using this method these two physical uncertainties are treated independently, and a controller is designed to achieve not only robust stability but also robust disturbance suppression. In addition, a two-degree-of-freedom control scheme is used to achieve a good transient response. Simulation results are presented to demonstrate the control effects.

In recent years, linear control theories, especially the linear optimal control theories, have come to be developed in a general feedback framework which includes the models of disturbances and plant uncertainties that are are usually known as weighting functions. In such framework, the stability common to both regulator and servomechanism problems requires that the weighted closed loop transfer matrix be stable in addition to the internal stability of the actual feedback system. Since some weighting functions may be unstable, this stability requirement is not always solvable even when the plant is internally stabilizable. Such stability requirement is termed as comprehensive stability and analyzed in this paper. The necessary and sufficient condition is derived in a extremely general case and stated in terms of the invariant-zeros of the given generalized plant which contains the dynamics of the plant and weighting functions. Also, a class of controllers achieving the comprehensive stability is parameterized. This result is believed to form a foundation for a unified control theory of regulator and servomechanism.

A 2-mass system connected by a non ideal shaft has resonance modes and it is difficult to design a controller suppressing the vibration in the face of parameter variation. In this paper, we propose a method of μ-Synthesis based on the descriptor form representation to deal with parameter uncertainties independently. We first consider a class of parameter uncertainty given in the descriptor form and show how to achieve robust stability and robust performance for this class of perturbed systems. Then we apply this method to the 2-mass spring system with parameter uncertainties of the torsional constant and the load inertia. By using this method these two physical uncertainties are treated independently and a controller is designed to achieve not only robust stability but also robust disturbance suppression. In addition, we also use two-degree freedom control scheme to achieve a good transient response. Simulation results are presented to demonstrate the control effects. © 1994, The Institute of Electrical Engineers of Japan. All rights reserved.