[1] Northumbria Univ, Fac Engn & Environm, Newcastle Upon Tyne NE1 8ST, Tyne & Wear, England
[2] Univ Duisburg Essen, Inst Automat Control & Complex Syst, D-47057 Duisburg, Germany
[3] Univ Aquila, Dept Informat Engn Comp Sci & Math, I-67100 Laquila, Italy