JPS56108963A - Surface electrometer - Google Patents

Abstract

PURPOSE:To perform DC reproduction of an output AC signal stably by changing the phase of a driving signal of a chopper shielding between an object to be measured and a measuring electrode intermittently thereby obtaining a synchronizing signal, in a device of detecting the surface potential of the object to be measured. CONSTITUTION:When the electric field between the photosensitive drum 47 of a copying machine and an electrometer 67 is intermittently shielded by a tuning fork vibrator 82, an AC signal is induced in an electrode 85. Said AC signal is inputted as a signal S2 to a clamping circuit 113 via a preamplifier 101, a filter 102, an amplifier 103, and a photocoupler 104. On the other hand, the feedback signal S3 from the vibrator 82 is inputted to a synchronizing circuit 108 constituted of a phase shifting circuit 108a and a comparator 108b, and further it is differentiated by a differentiating circuit 111, whereby a synchronizing signal S6 is obtained. The clamping circuit 113 clamps the signal S2 by the synchronizing signal S6 to make a signal S7. The signal S7 is integrated by an integrating circuit 114 and is converted into a DC signal.