[0001] This invention relates to xerographic machines.
[0002] In document copier machines of the xerographic type, charged latent images are produced
on a photoreceptive material and then developed through the application of a developer
mix. Where the photoreceptive material is separate from the copy paper itself, a transfer
of the developed image to the copy paper takes place with subsequent fusing of the
developed image to the paper. A common type of developer mix currently in use in such
machines is comprised of a carrier material, such as a magnetic bead, coated with
a coloured powdery substance called toner. It is the toner which is attracted to the
charged, latent image to develop that image and it is the toner which is then transferred
from the latent image to the copy paper (where the copy paper is separate from the
photoreceptive material). Finally, it is the toner which is then fused to the copy
paper to produce the finished copy.
[0003] It is apparent from the procedure outlined above that toner is a supply item which
must be periodically replenished in the developer mix since the toner is carried out
of the machine on the copy paper as a reproduced image. It is also apparent that the
concentration of toner particles in the developer mix is significant to good development
of the latent image since too light a toner concentration will result in too light
a developed image and too heavy a toner concentration will result in too dark a developed
image.
[0004] Other variables which seriously affect copy quality include the image voltage of
the photoconductor and the bias voltage on the developer. Many other variables factor
into these basic quantities, for example, the quality of the original, the cleanliness
of the optical system, and the condition of the photoconductor.
[0005] The prior art includes U.S. Patents 2,956,487 and 3,348,522. U.S. Patent 2,956,487
provides a toner concentration control system where the reflectivity of the document
image to be reproduced is used as a measure of toner density. This system appears
subject to difficulty since reflectivity readings will change dependent upon the quality
of the original. U.S. Patent 3,348,522 discloses a toner concentration control scheme
in which a special test image is developed outside the image area used for reproducing
document copies. In this latter patent separate reflectivity-sensing devices are used
to simultaneously sense light reflected from a single light source, one sensing device
to establish a voltage indicative of clear photoconductor outside the image area and
the other to establish a voltage indicative of the test area which, as noted above,
is also outside the image area. U.S. Patent 3,348,523 is essentially similar to U.S.
Patent 3,348,522.
[0006] U.S. Patent 3,926,338 discloses a circuit for use in a toner concentration control
scheme. In this patent thermally insensitive photodetectors must be used since the
large amount of heat generated during machine operation affects the accuracy of toner
concentration control readings. Similarly, this patent says that a stable amplifying
circuit, stable referring to temperature stability, must be used in order to avoid
destruction of the validity of the sensed signal.
[0007] According to the present invention, there is provided a xerographic copier characterised
by means for producing on a photoconductive imaging surface a substantially uniformly
toned test area bordered by a substantially untoned area, sensing means for sensing
the optical density of said areas to produce first and second sensor signals representing
respectively the density of said untoned and toned areas, and control means responsive
to said first and second sensor signals to provide output signals whenever the first
and second sensor signals differ by more than a predetermined amount.
[0008] By way of example, an embodiment of the invention will now be described with reference
to the drawings, in which:-
FIGURE 1 is a schematic layout of a xerographic machine in which the present invention
may be embodied;
FIGURE 2 shows the optical system and a photoconductive drum in the machine of FIGURE
1;
FIGURE 3 is an idealized perspective view of components in the paper path of the machine;
FIGURE 4 shows the reflectivity-sensing elements of the toner concentration control
device;
FIGURE 5 shows the layout of the photoconductor with the location of the bare reference
area and the developed test area within the document reproduction image area;
FIGURE 6 shows the circuit for processing the reference and test information; and
FIGURE 7 shows the layout of the photoconductor for minimizing the need for special
cycles in long, multi-copy runs.
[0009] FIGURE 1 shows a typical electrophotographic machine of the transfer type. Copy paper
is fed from either paper bin 10 or paper bin 11 along guides 12 in the paper path
to a transfer station 13A located just above transfer corona 13. At that station an
image is placed upon the copy paper. The copy paper continues through the fusing rolls
15 and 16 where the image is firmly attached to the copy paper. The paper continues
along path 17 into a movable deflector 18 and from there into one of the collator
bins 19.
[0010] In order to produce an image on the photoconductive surface 26, an image of a document
to be copied is transferred to the photoconductive surface 26 through an optics module
25 producing that image on the photoconductive surface 26 at exposure station 27.
As the drum 20 continues to rotate in the direction A, developer 23 develops the image
which is then transferred to the copy paper. As the photoconductor continues to rotate
it comes under the influence of preclean corona 22 and erase lamp 24 which discharge
all of the remaining charged areas on the photoconductor. The photoconductor continues
to pass around and through the developing station 23 (which is also a cleaning station
in this embodiment) until it reaches the charge corona 21 where the photoconductor
26 is again charged prior to receiving another image at exposure station 27.
[0011] FIGURE 2 is a perspective view of the optics system showing the document glass 50
upon which a document to be copied is placed. An illumination lamp 40 is housed in
a reflector 41. Sample light rays 42 and 43 emanate from lamp 40 and are directed
from dichroic mirror 44 to the document glass 50 whereat a line of light 45 is produced.
Sample light rays 42 and 43 are reflected from the document placed on the document
glass to reflective surface 46; from there to reflective surface 47 to reflective
surface 48 and thence through lens 9 to another reflective surface 49. From mirror
49 the light rays are finally reflected through opening 51 in wall 52 to reach photoconductor
26 whereat a line of light 45' is produced. In that manner a replica of the information
contained in the line of light 45 on the glass platen 50 is produced on the photoconductor
26 at 45'. The entire length of a document placed on document glass 50 is scanned
by motion of lamp 40 and the mirrors 44, 46, 47 and 48. By traversing the line of
light 45 across the document at the same speed at which the line of light 45' is moved
across photoconductor 26 by rotation of drum 20, a 1:1 copy of the document can be
produced on the photoconductor 26.
[0012] FIGURE 3 shows the various elements in the paper path in perspective. Here a copy
sheet 31 is shown with its trailing edge 31A in the paper path at guides 12. The copy
paper is receiving an image at transfer station 13A and is in the process of having
that image fused to itself by fuser rolls 15 and 16. The leading edge 31B of the copy
paper is about to leave the document copier and proceed into the collator 19 which
is represented in simplified form.
[0013] After an image is transferred to the copy paper, the photoconductor 26 continues
to rotate until it comes under the influence of preclean corona 22 which applies a
charge to the photoconductive surface to neutralize the remaining charge thereon.
Photoconductor 26 continues to rotate until the photoconductor comes under the influence
of an erase light 24' in housing 24. The erase light produces illumination across
the entirety of the photoconductor 26 in order to complete the discharge of any remaining
areas on the photoconductive surface which have not been neutralized by the preclean
corona 22. After passing under erase lamp 24', the photoconductor continues through
the cleaning station of developer/cleaner 23, wherein any remaining toner powder not
transferred to copy paper is cleaned from the photoconductor prior to the beginning
of the next copy cycle.
[0014] In the next copy cycle the charge corona 21 lays down a uniform charge across photoconductor
26 which charge is variably removed when the image of the document is placed on the
photoconductor at the exposure station 27 shown in FIGURE 1. Preclean corona 32 and
erase lamp 24' are off during this cycle.
[0015] When the toner concentration control cycle is run, and if the result indicates a
need to add toner to the developer, a signal is sent to replenisher 35 which holds
a supply of toner and operates to dump a measured amount into the developer. In that
manner, the toner density of the developer mix is replenished. Any suitable replenisher
mechanism may be used including the replenisher described in IBM Technical Disclosure
Bulletin, Vol. 17, No. 12, pp. 3516, 3517.
[0016] FIGURE 3 shows a housing 32 containing the toner concentration control sensing system
shown in FIGURES 4 and 6. When it is desired to sense for the concentration of toner
in the developer mix the photoconductor is charged as usual at the charge corona 21,
but no image is placed on the charged photoconductor at exposure station 27. Instead,
on this cycle, the erase lamp 24' remains on, discharging all of the charge which
has been laid down by charge corona 21 in order to provide bare photoconductor for
a reference test area. However, the erase lamp 24' is momentarily interrupted to produce
a charged stripe toned sample for a test area. If the lamp 24' is comprised of an
array of light-emitting diodes, the array can be segmented such that only a few of
the LEDs are momentarily turned off and therefore only a small "patch" of charge remains
on the photoconductor at the conclusion of this part of the cycle. If a fluorescent
tube is used as the erase lamp 24', momentarily reducing its energization to a low
level will produce a "stripe" of charge remaining on the photoconductor at the conclusion
of this part of the cycle.
[0017] Whether a stripe of charge or a patch of charge is produced, the charged test area
continues to rotate in the direction A until it reaches the developer 23 where toner
is placed onto the charged area to produce a toned sample test area. No copy paper
need be present at transfer station 13A in the test cycle, thus allowing the developed
test area to continue its rotation in direction A until it approaches the toner concentration
control housing 32. At this point, referring now to FIGURE 4, a light-emitting diode
(LED) or other suitable light source 33 is energized to produce light rays which reflect
off the toned sample test area 35 and are reflected to a photosensor 34. It should
be noted that the toned image could be transferred to copy paper, if desired. The
reflectance of the developed and transferred stripe (or patch) would then be sensed
by locating sensors on the paper path. It should also be noted that the principles
of this system work well with photosensitive paper, i.e., electrophotographic machines
in which the image is exposed directly onto the copy paper rather than through a transfer
station.
[0018] FIGURE 5 shows the layout of the photoconductor 26 with an image area 28 outlined
therein. A developed patch 30 has been produced within the image area 28. FIGURE 2
shows apparatus for producing patch 30. As described above, erase lamp 24' is momentarily
interrupted to produce a stripe of charge. While the above description designated
45' as a line of light producing an image on photoconductor 26, suppose now that during
the test cycle the line or stripe 45' is used to designate a stripe of charge produced
by momentarily interrupting lamp 24'. Suppose also that document lamp 40 is turned
on during the test cycle so that light from lamp 40 will erase the stripe of charge
45' unless it is interrupted. Such an interruption is made possible by the provision
of shutter 36 which is shown in FIGURE 2 as dropping across slot 51 in wall 52. Shutter
36 is actuated by solenoid 38. As a result, light from lamp 40 is blocked away from
photoconductor 26 by shutter 36, thus producing a stripe of charge 37. Of course,
erase lamp 24' will erase all of stripe 37 except for patch 30. In that manner, a
patch instead of a stripe can be produced. Note that slot 51 should be positioned
close to the photoconductive surface 26.
[0019] As FIGURE 5 demonstrates, placing the test area 30 within image area 28 necessitates
skipping the production of a copy during a long, multi-copy run since a density test
should be taken periodically, for example, after 20 copies. On short runs the test
is taken on the run-out cycle at the conclusion of document reproduction. FIGURE 7
shows the layout of photoconductor 26 illustrating a technique for avoiding the need
for skipping copies even when operating a long, multi-copy run. If the machine has
the capability of producing two different size copies, for example, 216 x 280 m.m.
and 216 x 355 m.m. the extra 3-inch part of the image area 28 can be used for the
density test without skipping a copy. FIGURE 7 shows the timing considerations needed
for the erase lamp, the document lamp, and the shutter 36 of FIGURE 2.
[0020] If a segmented LED array is used for the erase lamp, or if a stripe of charge is
produced instead of a patch 30, the production of the test area is obtained by turning
off the document lamp at the conclusion of viewing the 11-inch document and momentarily
interrupting the erase lamp as shown on FIGURE 7. Of course, no shutter is used in
that case.
[0021] In order to produce a reference voltage, when the proper time in the sequential operation
of the machine has arrived, the logic control of the machine provides a signal to
trigger the viewing of a reference sample. This is accomplished by energizing LED
33 in the following manner. The logic signal results in triggering a transistor switch
(not shown) which connects the reference sample input line 60 to ground. As a consequence,
the voltage on the negative input of OP AMP 61 is dropped from approximately 8 volts
to about ground potential. This causes the negative input of OP AMP 61 to switch from
a value higher than the positive input to one that is lower resulting in an inversion
of OP AMP output from low to high on line 62. That output is then fed back to the
positive input to lock the OP AMP 61 in a high output condition avoiding oscillations.
The output voltage on line 62 is applied to transistor Q2 to turn that transistor
on, thus closing a circuit from the 24-volt source through the light-emitting diode
33 and transistor Q2 to ground. The result is to provide light from the LED 33 to
the photocell 34 at the precise time in the machine cycle to reflect light rays from
the bare photoconductor to photocell 34.
[0022] In order to produce a sensed toned sample voltage, when the proper time in the machine
cycle is reached to direct light upon the toned sample a logic signal is provided
to turn on a transistor switch, not shown, to connect the toned sample input line
to ground. This results in lowering the negative input on OP AMP 63 from approximately
8 volts to ground potential and causes the output on line 64 to go high. The signal
on line 64 turns on the transistor Ql, causing the light-emitting diode to conduct
through the transistor Ql to ground. Note that the resistance levels connected with
the transistor Ql are significantly lower than the resistances associated with transistor
Q2. As a result, the current level through transistor Ql is significantly higher than
the current level through Q2, thus creating a more intense light from LED 33 when
the toned sample is viewed. The reason for this is that the bare photoconductor will
reflect a higher light level than the toned photoconductor. It was recognized that
the reflected light intensities exciting the photocell must be kept at a nearly equal
level whether viewing a bare sample or a toned sample. The reason for this is to avoid
the non-linearities which occur in photocell excitations from reception of different
light levels to avoid the non-linearities in circuit response and to guarantee high
signal levels whether viewing the bright reference sample or the dark toned sample
in order to improve noise immunity. In a system which is designed to be relatively
free from variations in component sensitivities, this is an important feature.
[0023] Referring now to the circuit of photocell 34, note that OP AMP 65 is connected as
a transconductance amplifier. With photocell 34 off only a small dark current flow
exists between the output of OP AMP 65 and the negative input. However, when the photocell
is excited, the current flow is substantially increased causing a significant voltage
drop across resistors R16 and R17 creating a voltage level at line 66 of perhaps 1
or 2 volts. Zener diode 67 limits the voltage level which can occur at line 66 to
8.5 volts, i.e., a swing of 8.5 volts from the photocell unexcited value. Assuming
a photocell excited voltage level of 2 volts at line 66, the change from 0 volts to
2 volts is coupled through capacitor 68 to an integrating circuit comprised of OP
AMP 69, capacitor 70, field effect transistor (FET) Q5 and the associated resistances.
Under ordinary conditions 16 volts is placed on the input of OP AMP 69 resulting in
an output of 16 volts at line 71. When a light source excites the photocell, resulting
in a voltage of, for example, 2 volts on line 66, a two-volt swing appears across
the capacitor 68 and is placed on the capacitor 70, resulting in a ramping down of
the voltage on line 71 from 16 volts to 14 volts. If a bare (reference) sample is
being taken the output of OP AMP 61 biases diode 72 to turn on FET Q6 during the bare
sample period. Thus the 14 volts on line 71 passes through FET Q6 and is placed on
capacitor 73. That voltage is stored until such time as the toned sample is taken
by photocell 34.
[0024] When the toned sample is taken, there should again be a 2-volt potential produced
on line 66 if the density of the toned sample is approximately correct. This is true
because of the balancing of current flow in photocell 34 regardless of whether a reference
sample or a toned sample is being taken (due to the different current levels through
LED 33 as explained above). Thus a 2-volt swing again appears across capacitor 68
resulting in a 2-volt potential drop across capacitor 70, causing the voltage of line
71 to ramp down from 16 to 14 volts. During the toned sample input period FET Q7 is
turned on and FET Q6 remains off. Thus the 14 volts present on capacitor 73, that
is, the reference voltage, is placed on the positive inputs of OP AMPS 74 and 75,
while the toned sample input present on line 71 is connected directly to the negative
input of OP AMP 74, and is connected through a voltage divider network to the negative
input of OP AMP 75. If, for example, resistance levels R21 and R22 were equal, the
potential at the negative input of OP AMP 75 would be the difference of 14 volts on
line 71 and the 16 volts input, that is, 15 volts.
[0025] At OP AMP 74, the 14-volt reference signal is placed on the positive input while
the 14-volt toned sample signal is placed on the negative input. Since there is no
differential, the output of OP AMP 74
indicates that the toner concentration condition is correct and the toner low signal
remains off. Similarly, at OP AMP 75, the bare sample input is 14 volts, the toned
sample input is 15 volts, and therefore the toner extra low signal remains off.
[0026] Suppose, however, that the toner density of the toned patch was too light. The result
would be an excessive reflection of light from that patch, causing a high excitation
of photocell 34 and resulting in a potential at line 66 of, for example, 4 volts.
In this example a 4-volt swing would appear across capacitor 68, thus causing a ramping
of the voltage at line 71 from 16 volts to 12 volts. Now the 12 volts appears directly
on the negative input of OP AMP 74 and is compared to the 14 volts on the positive
input, creating a high output, thus turning on the "toner low" signal. OP AMP 16 is
designed to register when a 30 millivolt difference appears, and thus the low output
signal will now be energized. At OP AMP 75, the toned sample signal of 12 volts on
line 71 is divided against 16 volts and if the resistances R21 and R22 were equal,
would cause 14 volts to appear at the negative input of OP AMP 75. Since both inputs
are 14 volts, the toner extra low signal remains off.
[0027] Suppose now that the toned sample was so light that the photocell excited to such
a degree that a 6-volt swing was experienced on line 66, thus sending the voltage
on line 66 from 0 volts to 6 volts. That 6-volt swing causes a ramping of the voltage
on line 71 from 16 volts to 10 volts. When the 10 volts is divided with the 16 volts
(again assuming equal R21 and R22 values) a voltage of 13 volts is placed on the negative
input of OP AMP 75. When this 13-volt signal is compared to the 14-volt reference,
the toner extra low output signal is turned on.
[0028] During regular operation of the machine, i.e., when there is no interruption for
a test cycle, it is desirable to provide a checking signal in order to determine that
the test network is in operating order. That is provided by the portion of the circuit
including transistor Q8. Note that when transistor Q8 is turned on the negative input
to OP AMP 75 is grounded and thus turns high the output of OP AMP 75. As a consequence,
the toner extra low signal is turned on. At the same time the voltage levels at OP
AMP 74 keep the toner low output signal off. This creates an unusual condition of
having the toner extra low signal on while the toner low signal is off. This condition
is forced by the operation of transistor Q8, and thus any change in this condition
during the operation of the machine will signify to the machine logic that something
is wrong in the test circuit. Note that transistor Q8 is turned on by a high output
from OP AMP 76. A high output from OP AMP 76 is present whenever the output of OP
AMP 77 is high (neglecting the RC time delay). OP AMP 77 is high when the negative
input is lower than the input on the positive side. Note that since line 66 is at
0 volts during regular operation, the voltage at the negative input of OP AMP 77 is
lower than the positive side under normal conditions. Note, however, that when a bare
or toned sample is taken, voltage on line 66 rises, thus turning off the high output
from OP AMP 77, turning off the high output from OP AMP 76 and thus opening the circuit
of transistor Q8.
[0029] Another quality test available through this circuit is that if the photoconductor
has become so coated with toner that when the bare sample is taken it actually is
a darkened sample, there will be only a small amount of light from LED 33 appearing
at the photocell 34. It will be a much lower photocell excitation than expected, consequently,
the voltage on line 66 does not change significantly, and thus even though a bare
sample is being taken, transistor Q8 is not turned off since line 66 does not change
significantly higher from its regular value. Therefore the output of OP AMP 77 remains
high and transistor Q8 remains on. In this situation, the logic senses the fact that
the toner extra low output signal from OP AMP 75 has remained on even though it should
have gone off when entering the test sequence. This informs the logic that a darkened
photoconductor condition is present and that remedial steps are needed. Consequently,
the circuit of transistor Q8 performs a darkened photoconductor check as well as indicating
the presence of problems in the test circuit itself.
[0030] Upon testing for toner density, if the toner low signal is activated, the toner replenisher
35 (FIGURE 3) operates to dump a quantity of toner into developer 23. If both the
toner low and the toner extra low signals are activated, a variety of possibilities
for further action are present, depending on machine design. For example, the first
subsequent action would probably be to check a "cartridge empty" signal from the toner
replenisher 35. If it is empty, a call for the key operator of the machine is in order.
However, if the replenisher has an adequate toner supply, the next action might be
to shut the machine down. Alternatively, there might be repeated toner density checks
after a few more copies until the toner extra low signal is no longer active. At some
point, if the extra low signal remains activated, the machine would be shut down.
[0031] As stated above, a test cycle can be run on the shut-down cycle when only small numbers
of reproductions are called for during a reproduction run. Special test cycles with
reproductions skipped may be used only during long, multi-copy runs. Providing the
specific control circuitry for interrupting machine operation to provide special test
cycles at the proper time is dependent upon the requirements of a particular machine.
Such circuit design is well within the skill of the art and does not comprise a part
of the instant invention. Similarly, control apparatus for receiving the toner low
and toner extra low signals to actuate the replenisher are well within the skill of
the art and not a part of the invention herein.
1. A xerographic copier characterised by means (22, 23, 24, 25, 36) for producing
on a photoconductive imaging surface (26) a substantially uniformly toned test area
(30) bordered by a substantially untoned area, sensing means (32) for sensing the
optical density of said areas to produce first and second sensor signals representing
respectively the density of said untoned and toned areas, and control means (Figure
6) responsive to said first and second sensor signals to provide output signals whenever
the first and second sensor signals differ by more than a predetermined amount.
2. A xerographic copier as claimed in claim 1, in which said photoconductive imaging
surface is a reusable surface from which toned images are transferred to copy sheets,
characterised in that said test area is within the portion (28) of the imaging surface
used for copy reproductions.
3. A xerographic copier as claimed in claim 2, in which said test area is formed during
a test cycle in which no copy is reproduced.
4. A xerographic copier as claimed in claim 3, in which said test cycle is run upon
the completion of a copy production run of less than a predetermined number of copies.
5. A xerographic copier as claimed in claim 4, in which said test cycle is run by
interrupting the production of copies in a copy production run of more than said predetermined
number of copies.
6. A xerographic copier as claimed in any of the previous claims, including a developer
station (23) operable to direct a developer mix, comprising carrier particles and
toner on to the imaging surface, and a toner replenisher device (35) operable in response
to said output signals to replenish said mix with toner.
7. A xerographic copier as claimed in any of the previous claims, in which said control
means is responsive to a said first sensor signal indicating an optical density of
said untoned area greater than a predetermined amount to provide an output signal
indicative of contamination of the imaging surface.
8. A xerographic copier as claimed in any of the previous claims in which said areas
are produced by operation of a pre-clean corona (22) and selective operation of an
erase lamp (24).
9. A xerographic copier as claimed in claim 8, including a shutter (36) associated
with the erase lamp and effective therewith to define said test area.