Electrophotographic apparatus

An electrophotographic apparatus comprising:

• a member having photoconductive substance thereon,
• charge means for electrostatically charging said photoconductive substance,
• image signal generating means for outputting an image signal indicative of an image to be reproduced on said photoconductive substance,
• reference mark signal generating means for outputting an image signal indicative of a reference mark of a high density, a reference mark of a low density and a reference mark of a middle density between the high density and low density to be reproduced on said photoconductive substance,
• exposure means for producing an electrostatic latent image on said photoconductive substance by a light beam on the basis of the outputs of said image signal generating means and said reference mark signal generating means as reference mark toner images,
• target density setting means for setting a low target density, a middle target density and a high target density and outputting data indicative of said target densities,
• developer means for developing said latent image by toner,
• density sensor means for detecting densities of the respective reference mark toner images, and for outputting data indicative of said densities of said reference mark toner images,
• comparator means for comparing said data of the densities of said reference mark toner images with said data of said target densities and outputting data indicative of a low density error, a middle density error and a high density error,
• low density and middle density control means for calculating a variation of a charge voltage for applying to said charge means, a variation of a developer bias voltage for biasing said developer means and a variation of a light intensity control voltage for controlling a light intensity of said exposure means on the basis of said low density error and said middle density error and constants predetermined on the basis of qualitative relations among the density of a reproduced image, said charge voltage, said developer bias voltage and said light intensity control voltage, and for outputting at least one of the variation of the charge voltage having an opposite sign to the sign of said low density error and the variation of said developer bias voltage having the same sign as the sign of said low density error on the basis of said qualitative relations, and for outputting the variation of said light intensity control voltage having the same sign as the middle density error,
• high density control means for calculating a variation of said charge voltage, a variation of said developer bias voltage and a variation of said light intensity control voltage on the basis of said high density error and constants predetermined on the basis of qualitative relations among the density of said reproduced image, said charge voltage, said developer bias voltage and said light intensity control voltage, and outputting one set of the variations selected from a first set of the variation of said charge voltage and the variation of said developer bias voltage having the same signs as the sign of said high density error by maintaining the difference between said charge voltage and said developer bias voltage at a constant, a second set of the variation of said light intensity control voltage having the same sign as the sign of said high density error and a third set of the variation of said light intensity control voltage, the variation of said charge voltage and the variation of said developer bias voltage having the same signs as the sign of said high density error,
• input voltage generating means for generating said charge voltage, said developer bias voltage and said intensity control voltage, and
• input voltage operation means for changing said charge voltage, said developer bias voltage and said light intensity control voltage output by said input voltage generating means on the basis of at least one of variations output from said low density and middle density control means and said high density control means.