[1] Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan
[2] Technical Univ. of Szczecin, Electrical Engineering Dept., ul. Sikorskiego 37, 70-310 Szczecin, Poland
[3] Electrotechnical Institute, ul. Pozaryskiego 28, 04-703 Warszawa, Poland
[4] Satobenec Inc., 1-7-1 Maizurumachi, Oita 870-8678, Japan