PURPOSE: To shorten the printing density switching time without adding any extra device by monitoring rotation abnormality by using a timer for rotation abnormality detection while the rotational frequency of a rotary polygon mirror is switched.
CONSTITUTION: When the rotary polygon mirror which rotates at a rotational frequency P1 is switched to a rotational frequency P2, a driving signal is turned OFF at time t1 first and then the rotational frequency of the rotary polygon mirror decreases gradually from P1. Here, a timer value for the rotational frequency P2 of following rotation is set in the timer for rotation abnormality detection and an error is monitored. When the rotational frequency of the rotary polygon mirror decreases below P2 at time t2, the timer turns ON an error signal. Here, it is recognized that the rotational frequency of the polygon mirror is below P2 and when the driving signal is turned ON next time, the polygon mirror is driven still at P2. Then the system is put in wait for a time T2 until the polygon mirror becomes stable at the rotational frequency P2. Consequently, the rotational frequency of the rotary polygon mirror can be switched only in a time (T1+T2).
ISHIKAWA MASAMI
TSUCHIYA KIYOMI
SANO YOSHIHIKO
