This paper presents a current control strategy for a single-phase PWM AC/DC converter. The input current is sinusoidal and an effective power factor is unity. Since the control variables of the single phase converter are sinusoidal, the conventional control strategy requires high control gains to achieve zero steady state error. However, the gains are limited because of sampling delay.
To solve the problem, the authors propose a new current control scheme of a single-phase PWM converter which has two special features: One is that a fast response of current is achieved by introducing a converter model in the controller. Another one is that the steady-state AC current control error is canceled even if the parameter differences between the converter model and the actual converter exist. Moreover, the steady-state current error can be eliminated under low gains by a feedback loop of the fundamental component of current error.
The experimental data show the proposed control to be effective.

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A direct-drive (DD) arm system is one of the most attractive and interesting research subjects in the field of robot engineering. Up till now, many DD robot arm systems with vertical axis have been adopted for industrial applications, but very few systems with horizontal axis because of the difficulty to reduce the speed fluctuation caused by the arm weight. The PID control without any additional control process has very poor response characteristics because of the continuous load fluctuation and the time delay in the system, The accuracy is consequently, not satisfactory. Therefore, we proposed a repetitive control with learning process (position-memory type) to drive the horizontal axis robot arm with constant speed, which is based on the periodicity of the arm action.
We apply this control method to the position control in which the arm reciprocates between arbitrary two points. We propose a novel control strategy in which time-memory type learning process is introduced in combination with the position-memory type learning process in order to improve the positioning accuracy. We also discuss about self tuning of the compensation rate in the system.
The control method is very simple and powerful. The availability of this repetitive control method is compared with experiments.

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Super high speed electrical machines for electrical vehicles, compressors and co-generation systems have been studied. The speed range of the machines is 10, 000_??_30, 000 rpm. This paper describes a basic structure, analysis and characteristics of a newly developed super high speed machine, which rotates at 20, 000 rpm. The new machine is an axial-field type with a high energy permanent magnet (Nd-Fe-B). The stator has mold coils of air gap winding which can be divided in half. The rotor has two discs sandwiched by the mold coils. Many columnar magnets are embeded in the duralumin discs (non magnetic material) of the rotor. As the rotor is yokeless, the magnetic circuit of field is composed of back yokes of the stator and the permanent magnets of the rotor. This construction allows us to make the considerably light rotor which can withstand large centrifugal force at high speed. Then a little amature reaction, little cogging torque and small noise are realized. The analysis and test results show that the machine has the good stability at high speed and the capacity of about 7kW at 20, 000 rpm is achieved.

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With the advance of superconducting wires for a. c. power use, researches on superconducting power transformers are increasing. These transformers can be divided into two types; iron-core type^{(1) (2)} and air-core type^(3)_??_(9). The latter type has advantages such as absence of iron losses and magnetic saturation, and it has more possibility of reduction of size and weight. But the air-core transformer has a large magnetizing current due to the absence of iron core. Thus, researches have been carried out on the possibility of using the air-core transformer as a transformer that can also serve as a shunt reactor in a power transmission system. However, the operating characteristics of the air-core transformer, such as voltage regulation and reactance voltage, are not sure at present.
In this paper, the equivalent circuit without losses is first proposed. Since this equivalent circuit is expressed by means of the magnetic coupling factor and self-inductances of windings, the effect of these parameters on the transformer characteristics can be investigated with ease. Then, based on this equivalent circuit, the per-unit expressions for the air-core transformer characteristics are derived and the characteristics are analyzed in detail. The validity of the theoretical results are confirmed by experimental results obtained by the use of an experimental superconducting transformer.

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A new type of magnetic suspension system is proposed which uses a permanent magnet. In this system, the attractive force is adjusted by controlling the reluctance of the magnetic circuit. Since no electromagnet is necessary, this system is effective for saving energy and avoiding heat generation. The composition of this paper is as follows. First, we explain the principle of this suspension system and introduce various types of magnetic circuits for reluctance control. Second, we examine the possibility of one of them from the viewpoint of linear control theory. Third, we develop an experimental device based on the proposed suspension method. The reluctance of this device is controlled by the air gap which is between the magnet and the suspended object, and which is adjust by driving a permanent magnet by piezoelectric actuator. Experimental results support well the theoretical predictions.

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To design DC electromagnets in an optimal manner, it is necessary to obtain the dynamic behavior accurately. A new method to calculate the dynamic behavior using the finite element method with edge elements has been developed. In order to verify its accuracy, the static and the dynamic behaviors of a clapper type electromagnet are calculated. The 3-D electromagnetic field analysis is carried out using the vector finite element with tetrahedral edge variables. The force is obtained by using the Maxwell stress tensor. The displacement of the armature of the electromagnet is obtained by solving the motion equation of the moving parts. The calculated results of the edge elements and that of the nodal elements are compared with the measurements. Moreover, it is clarified that the forces and the dynamic behavior can be accurately calculated using the edge elements.

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This paper deals with a high speed direct drive hydraulic servo valve (linear servo valve, LSV) using linear DC motor (LDM). The nozzle-flapper type hydraulic servo valves and proportional valves driven linear electromagnetic solenoid are widely used in factory automation. High speed and high precision hydraulic valves have been required, because these conventional valves involve problems in efficiency, contamination and low-speed.
In this paper, the following points are discussed:
(1) The static thrust distribution of the LDM in the range of rated stroke ±0.5mm of the LSV have good flat characteristics. The static thrust of the LDM is not in proportion to exciting current, because of magnetic force generated by permanent magnet and yoke. It is clarified that this phenomenon is not matter in practical application for the LSV.
(2) The flow force characteristics acting on the LDM are evaluated on two spools. The flow force of an improved spool is decreased by circulating the flow in a compensation bucket, and the flow force of a conventional spool is not decreased. The maximum flow force of the improved spool is equal 27% to that of the conventional spool.
(3) The LSV exhibits a dynamic response of a 90° phase lag at 500 Hz. It is clarified that the dynamic response of the LSV is above 67% higher than that of the conventional hydraulic valves. The LSV is able to reduce the control power including electric and hydraulic power to above 55% in comparison with the nozzle-flapper type hydraulic servo valves.

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