1. Institut fu¨r Allegmeine Mechanik, RWTH Aachen, Templergraben 64, D-52056 Aachen, Germany; igor_andrianov@hotmail.com
2. Technical University of Ło´dz´, Department of Automatics and Biomechanics, 1/15 Stefanowskiego St., 90-924 Ło´dz´, Poland; awrejcew@ck-sg.p.lodz.pl
3. St. Petersburg State University, Box 159, 195256, St. Petersburg, Russia; brem@mail.ru