The position of the brightest point in the stick-slip movement: (a) coordinate Y of the brightest point, which is in a stable position, (b) coordinate X of the brightest point, which changes periodically in the phase III of stable stick-slip movement, which can be divided into three stages in one cycle: stable sticking stage (I), where the position is relatively stable, fretting stage (II), and cracking stage (III), as shown in panel (c).