1. Department of Mechanics and Vibration, Politehnica University of Timișoara, 300222 Timișoara, Romania
2. Department of Electromechanics and Vibration, Center for Advanced and Fundamental Technical Research, Romania Academy, 300223 Timișoara, Romania
3. Department of Mathematics, Politehnica University of Timișoara, 300006 Timișoara, Romania
4. Department of Applied Electronics, Politehnica University of Timișoara, 300223 Timișoara, Romania