Laser diode beam intensity regulation method and apparatus

A printing machine, comprising:

• a photoreceptive surface;
• a charging station for charging said photoreceptive surface to a predetermined potential;
• a first exposure station for exposing said photoreceptive surface to produce a first electrostatic latent images on said photoreceptive surface, said first exposure station being comprised of:
• a time base generator for generating a sequence of pixel periods;
• a laser diode for emitting a laser beam, comprised of a first portion and of a second portion, in response to a laser drive current such that the intensities of said first portion and of said second portion are functions of the magnitude of the laser drive current;
• a drive circuit receiving both drive signals and a current control signal, said drive circuit for applying laser drive current to said laser diode in pixel periods selected by the drive signals and at a magnitude which is a function of the current control signal;
• a raster scanner for sweeping the first portion of the laser beam from a start of scan position, across the photoreceptive surface, and back to the start of scan position in a scan line interval time of N pixel periods;
• a controller for applying a first set of drive signals to the drive circuit in response to video information such that a first latent image is produced on the photoreceptive surface, said controller further for applying a second set of M drive signals in M pixel periods to the drive circuit, wherein the M pixel periods occurs over a number of pixel periods which is greater than N;
• a photosensor illuminated by the second portion of the laser beam, said photosensor for generating a charge current from the M pixel periods, wherein the generated charge current is dependent upon the beam intensity of the second portion of the laser beam;
• a capacitor for receiving the generated charge current and for producing a capacitor voltage from the received generated charge current;
• a comparator for comparing the capacitor voltage with a predetermined value and for producing a comparator signal based upon the comparison, wherein the comparison occurs after the second set of M pixel periods;
• and
• a current control circuit for receiving the comparison signal and for adjusting the current control signal based on the comparison signal such that the intensity of the second part of the beam moves toward a predetermined intensity;
• a first developing station for depositing developing material of a first color on said first electrostatic latent image so as to produce a first toner image on said photoreceptive surface;
• a transfuser member for transferring the first toner image onto a substrate;
• and
• a cleaning station for removing residual debris from the photoreceptive member.

A method of controlling the intensity of a laser beam on a photoreceptive surface, the method comprising the steps of:

• identifying a sequence of pixel periods;
• applying a laser drive current to a laser diode in a first set of selected pixel periods so as to emit a laser beam;
• sweeping a first part of the emitted laser beam from a start of scan position, across a photoreceptive surface, and back to the start of scan position in a scan line interval time which is equal to N pixel periods;
• illuminating a photosensor with a second part of the emitted laser beam and generating a charge current from a second set of M pixel periods, wherein the charge current is dependent upon the beam intensity of the second part of the laser beam;
• charging a capacitor with the generated charge current so as to create a capacitor voltage;
• comparing the capacitor voltage with a predetermined value after the second set of M selected pixel periods, wherein the second set of M selected pixel periods requires more than a scan line interval time to occur;
• adjusting the laser drive current such that the intensity of the second part of the beam moves toward a predetermined intensity.