The invention relates to a mechanism wherein a relative rotary angle of a driving motor is detected by use of an encoder, and, if an error occurs in a servo control action when the driving motor is servo-controlled in accordance with a deviation value of the relative rotary angle from a desired rotary angle of the driving motor, then the servo control action is brought to an emergency stop. A potentiometer for detecting an absolute rotary angle of the driving motor is connected to a rotary shaft of the driving motor and said emergency stop is effected when a difference between said absolute rotary angle and said relative rotary angle exceeds a preset tolerance value, so that an error in the servo control action can be reliably prevented.