Apparatus for controlling toner concentration in electrophotographic copying machines

Claim 1

In an improved apparatus for controlling the toner concentration in an electrophotographic copying machine having an original carrier plate for supporting an original to be copied at a predetermined location thereon, an electrophotosensitive medium adapted for movement during operation of the copying machine, an optical system positioned between the original carrier plate and the electrophotosensitive medium, means for relatively moving the original carrier plate and the optical system with respect to each other along a path to scan the original and to optically transmit an image of the same onto the medium during movement along said path so as to form an electrostatic image of the original on the medium, a developing device for changing the electrostatic image to a visible toner image and including means for controlling the toner concentration of a developed toner image, and means for measuring the extent of operative movement of the electrophotosensitive medium; said apparatus including a patch of a predetermined uniform density carried on the original carrier plate at a location remote from the predetermined location for supporting the original to be copied, said patch location being such that during said relative movement of the carrier plate and optical system the image of the patch is scanned and transmitted by the optical system onto the electrophotosensitive medium to form thereon an electrostatic image of the patch, and a detector for measuring the density of the developed toner image of the patch and for generating an output signal variable in accordance with image density for enabling control of the toner concentration of the toner image of the original; the improvement comprising; switch means on the copying machine actuatable at a predetermined position of the original carrier plate along said path of relative movement with respect to the optical system adapted when actuated to produce a first control signal, and control means responsive to the movement measuring means and the patch image density detector and said switch means for producing a second control signal at a time corresponding to a predetermined extent of movement of the electrophotosensitive medium following receipt of said first control signal, said second control signal operating the toner concentration control means in accordance with the output of the detector as the moving medium carries the patch toner image to a measurement-enabling position with respect to the detector.