V. Karkaletsis, C. Spyropoulos, and G. Petasis. volume 21 of Intelligent Systems, Control and Automation: Science and Engineering, chapter 12, page 131--142. Springer Berlin / Heidelberg, (1999)Presented at the 3rd European Robotics Intelligent Systems & Control Conference (EURISCON’98), June 22--25 1998, Athens, Greece..