Orman, K., A. Basci, and A. Derdiyok. “Speed and Direction Angle Control of Four Wheel Drive Skid-Steered Mobile Robot by Using Fractional Order PI Controller”. Elektronika Ir Elektrotechnika, vol. 22, no. 5, Oct. 2016, pp. 14 -19, doi:10.5755/j01.eie.22.5.16337.