In this paper' we describe an underwater robot with three movement functions: 1) swimming, 2) crawling on a floor and 3) climbing on a wall. These functions help the robot avoid obstacles and thus improve its ability to access narrow spaces. The swimming stability of the robot is also improved by increasing the distance between the center of gravity and the center of buoyancy. We propose a simple attitude control technique for making the robot adhere to a wall after it is rotated by thrusters. Attitude control is realized by sequence control, and appropriate control parameters are determined by considering the control disturbance and stability of the robot. We confirm experimentally that the robot swims stably and adheres to a vertical wall by virtue of the simple sequence control.
A grid-connected inverter is indispensable for photovoltaic power generation and smart grid systems, and it must be designed for stable operation. The impedance method based on the Nyquist criterion is often utilized to analyze the stability of grid-connected inverter systems. The impedance method is based on the eigen values of the product of the inverter output admittance and the line impedance matrices in the frequency domain. However, the frequency characteristics have so far been derived only for inverters with analog control systems. A new frequency analysis method for inverters with digital control systems is proposed in this paper. First, a stability analysis example for a three-phase LCL-type inverter controlled digitally is shown and the results are compared and validated with those by simulation using a Saber simulator. Finally, they are also compared and validated with experimental results digitally controlled by a DSP-based system.
This paper presents dynamic analysis and control of an isolated dual-active-bridge (DAB) dc-dc converter. Conventional control methods for the DAB converter may cause dc offsets in both inductor current and transformer magnetizing current in transient states. The dc offset in the inductor current introduces an excessive peak current through the switching devices. The dc offset in the magnetizing current cannot be neglected and may induce magnetic flux saturation. Conventional phase-shift control methods simultaneously turn on and off the diagonal switches in each H-bridge converter to produce a square-wave voltage with a 50% duty ratio. In contrast, the proposed method independently controls the diagonal switches to modify the duty ratios in transient states. The theoretical analysis derives the requirements of the switching angles for eliminating dc offsets in both inductor and magnetizing currents with a settling time as short as half a switching period. The results of experiments using a 5-kW, 20-kHz experimental system verify the validity of the proposed control method.
The adoption of train automatic stop control (TASC) device is expected to increase according to high demand of platform door system. The challenges of TASC are (1) reduction of adjustment man-hour at TASC introduction and (2) maintenance of stopping position accuracy. These challenges are caused by the difficulty of obtaining vehicle characteristics accurately in real-time. To solve the difficulty, a function for automatic parameter tuning for TASC devices has been developed. The function learns vehicle characteristics from real-time driving data and controls vehicle deceleration based on the learned vehicle characteristics. The developed function was evaluated on a test vehicle, and the results showed that it acquired vehicle characteristics accurately and stopped the test vehicle within a stopping accuracy of ±35cm. In addition, it was confirmed that calculation time was within 70s. This was less than the target time of 80s, which is comparable to the average driving time between two stations on a metro line.
On the AC motor drive system with the PWM inverter, a surge voltage appearing on the motor terminals (differential-mode motor-surge-voltage) is a serious problem. In order to reduce the surge voltage, a motor-surge-voltage suppression method using a surge suppression cable that the authors have developed has already been proposed. However, another type of motor surge voltage appearing between the motor windings and the motor frame (common-mode motor surge voltage) still remains. This paper presents a novel motor-surge-voltage suppression method using a surge suppression cable. The method enables suppression of both the differential-mode and the common-mode motor surge voltage, and its effectiveness is confirmed by the experiment. Moreover, a numerical analysis of the motor surge voltage is performed to obtain optimum parameters of the surge suppression cable for greater reduction of each.
In this paper, a technique is proposed for alleviating the thermal concentration on specific switching devices by using a zero-sequence voltage in a 3-level inverter that drives a permanent magnet synchronous motor (PMSM) in a zero-speed and high-torque condition. The use of the PMSM in home electronics and industrial products has become widespread, since it can realize miniaturization and a high efficiency drive. however, a large DC current flows when the PMSM is used in a zero-speed and high-torque condition, for example, in the hill-start of electric vehicles, start or stop of elevators, or servo lock of servo pressing machines, etc. In these cases, the current flows in the specific switching devices and heat generation is locally concentrated. This problem is unavoidable in a conventional 2-level inverter. A technique that can change the current path in a 3-level inverter and control the losses generated in the switching devices is proposed. We evaluated the effects of the proposed technique through a circuit simulation in which the electrical characteristics of a commercially available power device were applied. The proposed technique can reduce the highest loss of the switching devices in the 3-level inverter by about 40% as compared to that in a conventional inverter. Moreover, a method is proposed that suppresses the neutral point potential variation between the power supplies. This method can also reduce the maximum loss of the specific switching devices by about 30%.
This paper proposes a new high-efficiency PV converter for grid connection through a high-leg delta transformer, which is composed of a symmetrically-connected boost converter and three half-bridge inverters. One of the three half-bridge inverters is connected to the boost converter, and the others are directly connected to the PV terminals. This circuit configuration enables to reduce the power losses in both boost converter and inverter. This paper also proposes a new cooperative control method between the symmetrically-connected boost converter and inverter. The control method can reduce the average switching frequency to 75% of that in a conventional one, resulting in a great reduction in switching power loss. Experimental results show that the proposed circuit improves its European efficiency from 91.6% to 94.5%.
This paper focuses on the circuit capable of reducing the number of switching elements (six-switch inverter) in comparison to a common multi-level neutral point clamped inverter. Waveform improvement is carried out by increasing the number of levels of the output voltage. This is possible by providing the ratio of the input voltage. The simulated and experimental results demonstrate the effectiveness of the six-switch inverter.
In this paper, a design of an axial type switched reluctance motor (SRM) is presented that considers the magnetic characteristics of soft magnetic composite (SMC) material. These characteristics are changed by the press and cutting processes for producing the motor core. It is shown that the magnetic characteristics of the vertical and the parallel flux direction differ according to the direction of the press, and are degraded by the cutting process. The proposed design method can be accurately simulated by using the magnetic characteristics in both the vertical and the parallel flux direction in an axial SRM motor.