Copy quality monitoring

An apparatus that monitors the quality of copies reproduced by a printing machine. A signal of an image to be reproduced by the printing machine is transmitted to an electronic subsystem. The electronic subsystem converts the signal to a reference bitmap and transmits the reference bitmap to a raster output scanner in the printing machine. The printing machine reproduces the image on a copy sheet. The image reproduced on the copy sheet is scanned by a raster input scanner and a scanned bitmap generated. A comparison is made between the scanned bitmap and the reference bitmap to determine if the image reproduced on the copy sheet is at an acceptable quality level.

This invention relates generally to a an apparatus for monitoring copy 
quality, and more particularly concerns an electrophotographic printing 
machine in which a comparison is made between a signal sensing the image 
reproduced on a copy sheet and an input signal to the printing machine of 
the desired copy to be reproduced to determine if the image reproduced on 
the copy sheet is at an acceptable quality level. 
Generally, the process of electrophotographic printing includes charging a 
photoconductive member to a substantially uniform potential so as to 
sensitize the surface thereof. The charged portion of the photoconductive 
surface is exposed to a light image corresponding to the copy desired to 
be reproduced. This records an electrostatic latent image on the 
photoconductive surface. After the electrostatic latent image is recorded 
on the photoconductive surface, the latent image is developed by bringing 
a developer mixture into contact therewith. A common type of developer 
comprises carrier granules having toner particles adhering 
triboelectrically thereto. This two-component mixture is brought into 
contact with the photoconductive surface. The toner particles are 
attracted from the carrier granules to the latent image. This forms a 
toner powder image on the photoconductive surface which is subsequently 
transferred to a copy sheet. The toner powder image is heated to fuse it 
to the copy sheet. 
Remotely controlled printing machines, in particular high volume printing 
machines, produce large numbers of copies or prints without immediate 
operator inspection. If there is a defect in the prints, e.g. a deletion, 
the operator may find the defect only after an entire batch of prints have 
been printed. This may require that hundreds of pages need to be 
reprinted. Thus, it is desirable to detect the occurrence of a defect, 
such as a deletion, and to terminate the printing run. Inasmuch as 
electrophotographic printing machines are being used as magnetic ink 
character recognition printers (MICR) for printing checks, it is necessary 
to determine that extra checks have not been printed. Security can be 
maintained by providing an automatic reconciliation between the number of 
checks printed and the number requested to be printed. An audit trail can 
be provided by determining the total number of checks reproduced and the 
number of defective checks. Various approaches have been devised to 
measure copy defects, the following disclosures appear to be relevant: 
U.S. Pat. No. 4,674,863, Patentee: Tomosada et al., Issued: June 23, 1987. 
U.S. Pat. No. 4,745,434, Patentee: Shimomura et al., Issued: May 17, 1988. 
U.S. Pat. No. 4,802,231, Patentee: Davis, Issued: Jan. 31, 1989. 
The relevant portions of the foregoing patents may be briefly summarized as 
follows: 
U.S. Pat. No. 4,674,863 discloses a light receiving sensor, which detects 
the amount of light reflected from an original document being reproduced 
by the printing machine, and an electrical potential sensor, which detects 
the state of the photoconductive drum. These signals are used to control 
the voltage applied to the exposure lamp. 
U.S. Pat. No. 4,745,434 describes a photosensor located behind the lens of 
a copier to detect the density of an original being reproduced. The output 
from the photosensor is transmitted to a microcomputer which controls 
developer bias so that the toner image is formed in response to the 
detected density of the original. Such a control is carried out for each 
segment of the length of the original. 
U.S. Pat. No. 4,802,231 discloses a pattern recognition error reduction 
system. Errors are reduced by by creating independent error templates 
which corresponds to patterns which tend to be erroneously matched and 
linked. These templates are linked to specified reference templates which 
are stored for comparison. 
In accordance with one aspect of the present invention, there is provided 
an apparatus for monitoring the quality of copies reproduced by a printing 
machine. The apparatus includes means for transmitting a signal of an 
image to be reproduced to the printing machine. In response to this 
signal, the printing machine reproduces the image on a copy sheet. Means 
are provided for sensing the image reproduced on the copy sheet and 
generating a signal indicative thereof. Means compare the signal from the 
sensing means with the signal from the transmitting means to determine if 
the image reproduced on the copy sheet is at an acceptable quality level. 
Pursuant to another aspect of the present invention, there is provided an 
electrophotographic printing machine of the type in which an image 
developed on a photoconductive member is transferred to a sheet and fused 
thereto. The improvement includes means for transmitting a signal of the 
image to the printing machine. Means are provided for sensing the 
developed image fused on the copy sheet and generating a signal indicative 
thereof. Means compare the signal from the sensing means with the signal 
from the transmitting means to determine if the image reproduced on the 
copy sheet is at an acceptable quality level.

While the present invention will be described in connection with a 
preferred embodiment thereof, it will be understood that it is not 
intended to limit the invention to that embodiment. On the contrary, it is 
intended to cover all alternatives, modifications, and equivalents as may 
be included within the spirit and scope of the invention as defined by the 
appended claims. 
Referring now to FIG. 1, the electrophotographic printing machine employs a 
belt 10 having a photoconductive surface 12 deposited on a conductive 
substrate 14. Preferably, photoconductive surface 12 is made from a 
selenium alloy. Conductive substrate 14 is made preferably from an 
aluminum alloy which is electrically grounded. Belt 10 moves in the 
direction of arrow 16 to advance successive portions of photoconductive 
surface 12 sequentially through the various processing stations disposed 
about the path of movement thereof. Belt 10 is entrained about stripping 
roller 18, tensioning roller 20 and drive roller 22. Drive roller 22 is 
mounted rotatably in engagement with belt 10. Motor 24 rotates roller 22 
to advance belt 10 in the direction of arrow 16. Roller 22 is coupled to 
motor 24 by suitable means, such as a drive belt. Belt 10 is maintained in 
tension by a pair of springs (not shown) resiliently urging tensioning 
roller 20 against belt 10 with the desired spring force. Stripping roller 
18 and tensioning roller 20 are mounted to rotate freely. 
Initially, a portion of belt 10 passes through charging station A. At 
charging station A, a corona generating device, indicated generally by the 
reference numeral 26 charges photoconductive surface 12 to a relatively 
high, substantially uniform potential. After photoconductive surface 12 of 
belt 10 is charged, the charged portion thereof is advanced through 
exposure station B. 
At exposure station B, an electronic subsystem (ESS), indicated generally 
by the reference numeral 28, receives the image data flow and processes 
this data to convert it to a bitmap of the image which is transmitted to a 
raster output scanner (ROS), indicated generally by the reference numeral 
30. Preferably, ESS 28 is a self-contained, dedicated minicomputer. The 
image data flow transmitted to ESS 28 may originate from a computer. This 
enables the electrophotographic printing machine to serve as a remotely 
located printer for one or more computers. For example, the printer may be 
coupled to a plurality of personal computers or workstations, such as the 
Model No. 6085 manufactured by the Xerox Corporation, on a local area 
network. Alternatively, the printer may serve as a dedicated printer for a 
high speed main frame computer. The signal from ESS 28 corresponding to 
the bitmap of the image desired to be reproduced by the printing machine 
is transmitted to ROS 30. ROS 30 includes a laser with rotating polygon 
mirror blocks. Preferably, a nine facet polygon is used. The ROS 
illuminates the charged portion of photoconductive belt 20 at a rate of 
about 300 pixels per inch. The ROS will expose the photoconductive belt to 
record an electrostatic latent image thereon corresponding to the bitmap 
of the image received from ESS 28. In another embodiment, ESS 28 is 
connected to a raster input scanner (RIS). The RIS has an original 
document positioned thereat. The RIS has document illumination lamps, 
optics, a scanning drive, and photosensing elements, such as a CCD array, 
i.e. a charge coupled device. The RIS captures the entire image from the 
original document and converts it to a series of raster scan lines which 
are transmitted as electrical signals to ESS 28. ESS 28 processes the 
signals received from the RIS and converts them to a bitmap of the image 
which is transmitted to ROS 30. ROS 30 exposes the charged portion of the 
photoconductive belt to record an electrostatic latent image thereon 
corresponding to the bitmap of the image received from ESS 28. 
After the electrostatic latent image has been recorded on photoconductive 
surface 12, belt 10 advances the latent image to development station C. At 
development station C, a magnetic brush development system, indicated by 
the reference numeral 38, advances developer material into contact with 
the latent image. Preferably, magnetic brush development system 38 
includes two magnetic brush developer rollers 40 and 42. Rollers 40 and 42 
advance developer material into contact with the latent image. These 
developer rollers form a brush of carrier granules and toner particles 
extending outwardly therefrom. The latent image attracts toner particles 
from the carrier granules forming a toner powder image thereon. As 
successive electrostatic latent images are developed, toner particles are 
depleted from the developer material. A toner particle dispenser, 
indicated generally by the reference numeral 44, dispenses toner particles 
into developer housing 46 of developer unit 38. 
With continued reference to FIG. 1, after the electrostatic latent image is 
developed, belt 10 advances the toner powder image to transfer station D. 
A copy sheet 48 is advanced to transfer station D by sheet feeding 
apparatus 50. Preferably, sheet feeding apparatus 50 includes a feed roll 
52 contacting the uppermost sheet of stack 54. Feed roll 52 rotates to 
advance the uppermost sheet from stack 54 into chute 56. Chute 56 directs 
the advancing sheet of support material into contact with photoconductive 
surface 12 of belt 10 in a timed sequence so that the toner powder image 
formed thereon contacts the advancing sheet at transfer station D. 
Transfer station D includes a corona generating device 58 which sprays 
ions onto the back side of sheet 48. This attracts the toner powder image 
from photoconductive surface 12 to sheet 48. After transfer, sheet 48 
continues to move in the direction of arrow 60 onto a conveyor (not shown) 
which advances sheet 48 to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 62, which permanently affixes the transferred powder 
image to sheet 48. Fuser assembly 60 includes a heated fuser roller 64 and 
a back-up roller 66. Sheet 48 passes between fuser roller 64 and back-up 
roller 66 with the toner powder image contacting fuser roller 64. In this 
manner, the toner powder image is permanently affixed to sheet 48. 
After fusing, sheet 48 advances through chute 68. A RIS, indicated 
generally by the reference numeral 32, senses the image fused to the copy 
sheet and transmits a signal corresponding to the fused Image. RIS 32 has 
document illumination lamps, optics, and photosensing elements, such as a 
CCD array, i.e. a charge coupled device. The RIS has a resolution of about 
47 pixels per inch. The raster scan line signal from RIS 32 is converted 
to a bitmap of the image fused to the copy sheet. The bitmap of the image 
fused to the copy sheet is compared to the bitmap of the image transmitted 
to ESS 30 to determine if the image fused to the copy sheet is of 
acceptable quality. In the event the image fused to the copy sheet is 
beneath the acceptable quality level, a fault is indicated and displayed 
to the operator at the workstation of the computer coupled to the ESS 
and/or on the printing machine console. With continued reference to FIG. 
1, chute 68 advances sheet 48 to catch tray 72 for subsequent removal from 
the printing machine by the operator. 
After the copy sheet is separated from photoconductive surface 12 of belt 
10, the residual toner particles adhering to photoconductive surface 12 
are removed therefrom at cleaning station F. Cleaning station F includes a 
rotatably mounted fibrous brush 74 in contact with photoconductive surface 
12. The particles are cleaned from photoconductive surface 12 by the 
rotation of brush 74 in contact therewith. Subsequent to cleaning, a 
discharge lamp (not shown) floods photoconductive surface 12 with light to 
dissipate any residual electrostatic charge remaining thereon prior to the 
charging thereof for the next successive imaging cycle. 
It is believed that the foregoing description is sufficient for purposes of 
the present application to illustrate the general operation of an 
electrophotographic printing machine incorporating the features of the 
present invention therein. 
Referring now to FIG. 2, there is shown a block diagram of the quality 
control system used in the FIG. 1 printing machine. As depicted thereat, 
ESS 28 converts the incoming signal into a bitmap of the image desired to 
be reproduced by the printing machine. The image bitmap is transmitted 
from ESS 28 to ROS 30. In addition, the bitmap of the desired image its 
also transmitted to an image recognition and error detection minicomputer, 
indicated generally by the reference numeral 70. RIS 32 transmits raster 
scan lines of the image fused to the copy sheet to image recognition and 
error detection minicomputer 70. Minicomputer 70 compares the bitmap of 
the desired image (reference bitmap) to the bitmap of the fused image 
(scanned bitmap) and determines if the quality level of the fused image is 
acceptable. Minicomputer 70 identifies all the solid and background areas 
which are larger than a preset threshold. For example, if the threshold is 
5 millimeters, there areas would be background areas around the text and 
spaces between the paragraphs. Regular text areas and halftones or thin 
lines are excluded. High solid area density portions are identified 
similarly. If there are no solid areas present, the text areas which 
appear statistically uniform may also be identified for approximate 
evaluation of maximum density. Average area coverage and the anticipated 
average ratio of light reflected by the text area to the light reflected 
by the background is used for this calculation. As soon as the fused image 
is scanned by RIS 32, the pattern recognition algorithms register the 
scanned bitmap against the reference bitmap. The procedure is an error 
minimization iteration. An error is recorded whenever the scanned bitmap 
differs from the reference bitmap. In checking solid area and line 
developability, it is assumed that the scanned bitmap has only text and 
background. The difference between the reference bitmap and the scanned 
bitmap may be large resulting in a defect being recorded, or the 
difference may be small resulting in no defect. Alternatively, the 
difference may be in the intermediate range which requires that the 
measurements be repeated or that a test pattern having known solid area 
and line developability be used to verify the results. If the error 
appears inside a uniform area rather than at the border, the coordinates 
are recorded with a higher weight. If the number of spots in this group is 
large, the registration iterations are repeated with such spots excluded. 
Improved match confirms the registration validity and reduces the number 
of of required confirmations. Errors appearing in the boundary region 
between the solid and background areas may be a misregistration of the 
borderline rather than a defect indication. If the error is not a deletion 
along the line between the solid area and background, it is recorded as a 
lower weight error. If the error spots are almost continuous along the 
border line between the solid areas and background areas, and the error 
line is substantially perpendicular to the process direction on the lead 
or trail edge, it is recorded as a higher weight defect. Shape test are 
applied when the error areas are large. At this point minicomputer 70 may 
have stored a record of several types of defects with coordinates and 
weighting levels computed according to algorithms accounting for the 
number of spots or pixels, defect type and passage of subsequent 
verification tests. The defect record may also be compared against shapes, 
repetition intervals and other characteristics of known defects. A 
resident artificial intelligence self diagnostic unit may be used to 
further process the defect record. These levels are compared against 
preset thresholds. The thresholds may be adaptive and depend upon the type 
of print, paper, number of remaining prints in the run, relative humidity, 
etc.. In the event that minicomputer 70 determines that the quality of the 
scan bitmap is not acceptable, a fault condition exits. Under these 
circumstances, minicomputer 70 transmits a fault signal to the computer or 
workstation coupled to the ESS and/or to the console of the printing 
machine. This results in a display on the computer and/or the printing 
machine console indicating that the copy is beneath the acceptable quality 
level. Minicomputer 70 counts the total number of copy sheets and the 
number of copy sheets having defective images. All of these counts are 
displayed. This provides an audit trail enabling tracking of defective and 
acceptable copies. 
In recapitulation, it is evident that the apparatus of the present 
invention compares a bitmap of the desired image with a bitmap of the 
fused image to determine if the fused image is at an acceptable quality 
level. A RIS scans the fused image and transmits raster scan lines to a 
minicomputer which converts the raster scan lines to a scanned bitmap of 
the fused image. The ESS transmits a reference bitmap of the desired image 
to the minicomputer. The minicomputer compares the scanned bit map with 
the reference bitmap to determine defects in the fused image. In the event 
the quality of the fused image is beneath an acceptance threshold, a fault 
signal is transmitted from the minicomputer to the computer or workstation 
and/or printer console for display to the operator. In addition, the 
minicomputer tracks the total number of copy sheets printed by the 
printing machine, and the number of defective copy sheets. This 
information may also be displayed. 
It is, therefore, apparent that there has been provided in accordance with 
the present invention, an apparatus for monitoring copy quality in an 
electrophotographic printing machine that fully satisfies the aims and 
advantages hereinbefore set forth. While this invention has been described 
in conjunction with a specific embodiment thereof, it is evident that many 
alternatives, modifications, and variations will be apparent to those 
skilled in the art. Accordingly, it is intended to embrace all such 
alternatives, modifications and variations that fall within the spirit and 
broad scope of the appended claims.