1. Department of Automation of Productions Systems, Faculty of Mining Electromechanic, University of Mining and Geology “St. Ivan Rilski”, Studentski Grad, Prof. Boyan Kamenov 1, 1000 Sofia, Bulgaria
2. Faculty of Electrical Engineering, Electronics and Automation, University of Ruse “Angel Kanchev”, 8 Studentska Str., 7004 Ruse, Bulgaria