Electrostatic copying method including compensation for photoconductor fatigue and dark recovery

Claim 1

In a method of producing toner-developed electrostatic images involving the repetitive performance of a copying cycle comprising the steps of electrostatically charging a photoconductive layer by means of a corona discharge at a determined voltage level, information-wise photo-exposing said charged photoconductive layer to electromagnetic radiation to which it is sensitive, applying electrostatically charged toner particles to said exposed photoconductor to develop the resulting electrostatic charge pattern as a toner image, information-wise transferring the applied toner image to a receptor, and restoring the photoconductive layer to a rest potential preparatory to the next cycle; and after completion of one series of copying cycles, but before the completion of the next series, maintaining said photoconductor in the dark for a randomly varying time period, the improvement wherein: (i) during the performance of a given series of copying cycles following immediately one after another, the number of performed copying cycles in said series is registered by electronic means as they are performed and an output signal corresponding to said number is generated; (ii) after the performance of one such series, the period of time that said photoconductor is maintained in the dark before the start of the next series is measured and an output signal corresponding to said time period is generated; and (iii)the respective output signals (i) and (ii) are applied as input signals to electronic control means which (a) on the basis of an experimentally developed relation establishing the actual charge surface potential of said photoconductor as a function of the number of copying cycles performed in a series, provides a measure of the voltage change on said photoconductor surface resulting from the actual number of cycles in said given series, (b) on the basis of an experimentally developed relation establishing the actual change in surface potential of said photoconductor as a function of the duration of the time period to which said photoconductor is maintained in the dark between said series of cycles, provides a measure of the voltage change in the surface potential of said photoconductor after the elapse of the actual period of time said photoconductor is maintained in the dark, (c) said two measures are combined to generate a control signal of a magnitude indicative of the overall change in surface potential of said photoconductor from the beginning of one series of cycles to the beginning of the next series, and (d) the voltage level of said corona discharge is regulated to a determined level substantially according to the magnitude of said control signal, whereby said corona discharge voltage level is controlled so as at least substantially to compensate for variations in the surface potential of the photoconductive layer resulting from both the number of copy cycles performed in a given series and the duration of the dark maintenance period between that series and the next.