1. Department of Machine and Precision Engineering, Technical University of Gabrovo, 4 H. Dimitar Str., 5300 Gabrovo, Bulgaria
2. Department of Precision Engineering and Measurement Instruments, Technical University of Sofia, 1756 Sofia, Bulgaria
3. Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
4. Department of Mathematics, Informatics and Natural Sciences, Technical University of Gabrovo, 4 H. Dimitar Str., 5300 Gabrovo, Bulgaria
5. Department of Information Computer Technologies and Mathematics, Ukrainian Engineering Pedagogics Academy, 16 Universitetskaya St., 61003 Kharkov, Ukraine