Automatic diaphragm control system means for preventing energization of

Abstract

A diaphragm-adjusting motor varies the setting of an adjustable diaphragm and includes a drive winding and a damping winding. A negative-feedback control circuit generates a scene-light-dependent error signal and energizes the drive winding of the adjusting motor in dependence upon the error signal. During adjusting motion of the adjusting motor, a damping voltage is induced in the damping winding. The damping voltage is utilized to modify the error signal in a sense opposing energization of the adjusting motor, so as to damp the adjusting motion of the adjusting motor and thereby prevent adjustment overshoots during normal operation. When the error signal reaches a preselected value, further energization of the drive winding of the adjusting motor is prevented. During normal operation, the lowering of the error signal effected by the damping voltage as the adjusting motor performs an automatic adjustment prevents the error signal from reaching the preselected value. However, when the adjusting motor reaches an extreme of its adjustment range and comes to a standstill, the damping voltage becomes zero. The error signal is no longer lowered by the damping voltage. If the error signal is still non-zero, the error signal will exceed the preselected value. This is detected, and further energization of the adjusting motor is prevented.