Image forming apparatus

An image forming apparatus has a plurality of image forming stations capable of forming different images which are to be correctly registered and superimposed. Elimination of possible image misregistration in each image station is executed on the basis of the result of detection of register marks which are formed by each image forming station. The correction is executed at a timing which is independent from image forming sequences performed by each imgae forming station in accordance with image signals, e.g., during warming-up of the apparatus after the initial application of the power immediately after the input of image forming operation start instruction through a start key, or after completion of production of a predetermined number of copies counted from the last misregistration correcting operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus for forming 
images on a transfer material,and specifically relates to an image forming 
apparatus for forming superimposed images. 
2. Related Background Art 
The applicant of the present invention has proposed a number of color image 
forming apparatuses for obtaining a full color image by arranging a 
plurality of image holding devices (photo-sensitive drums or the like) in 
parallel. 
Such apparatuses are described, for exmple, in Japanese Unexamined Patent 
Publication (Kokai) No. 58-23074 and Japanese Unexamined Patent 
Publication (Kokai) No. 58-95361 (corresponding to U.S. Pat. No. 
4,591,903); Japanese Unexamined Patent Publication (Kokai) No. 58-95362, 
Japanese Unexmined Patent Publication (Kokai) No. 58-154856, Japanese 
Unexamined Patent Publication (Kokai) No. 58-207021, and Japanese 
Unexmined Patent Publication (Kokai) No. 59-31976 (corresponding to U.S. 
patent application No. 521,832 filed Aug. 10, 1983); and Japanese 
Unexamined Patent Publication (Kokai) No. 59-46659, Japanese Unexamined 
Patent Publication (Kokai) No. 59-50460, Japanese Unexamined Patent 
Publication (Kokai) No. 59-42879, all of which are incorporated herein by 
reference. 
In an image forming apparatus of this type, an overlapping aberration 
(chromatic aberration) among respective colors at the time of 
multitransfer becomes an extremely large problem. 
This problem will be explained in more detail with specific reference to 
FIGS. 12 to 17. 
FIG. 12 schematically shows a 4-drum type full-color image forming 
apparatus. The apparatus has image forming stations 101C, 101M, 101Y and 
101Bk for forming images of cyan, magenta, yellow and black colors, 
respectively. These image forming stations 101C, 101M, 101Y and 101Bk are 
respectively provided with photosensitive drums 102C, 102M, 102Y and 102Bk 
and optical scanning means 103C, 103M, 103Y and 103Bk, as well as 
developing units, cleaners and other parts. In operation, a transfer 
material or sheet S is conveyed in the direction of an arrow by a conveyor 
belt 112 through these image forming stations so that images of cyan, 
magenta, yellow and black colors are successively formed in an overlapping 
manner, whereby a full color image is formed on the transfer material S. 
The apparatus has a mark detector 111 disposed downstream from the image 
forming station 101Bk as viewed in the direction of movement of the 
transfer material, more specifically, at a point which is spaced from the 
center of the photosensitive drum 101Bk by a distance l.sub.4. As will be 
seen from FIG. 12, a constant interval (l.sub.1 =l.sub.2 =l.sub.3) is left 
between the photosensitive drums of the adjacent image forming stations. 
The mark detector 111 is capable of detecting register marks which have 
been formed by the photosensitive drums 102C, 102M, 102Y and 102Bk of the 
successive image forming stations 101C, 101M, 101Y and 101Bk and 
transferred to the conveyor belt 112. These register marks are used as 
misregistration images indicative of any misregistration of images of the 
respective colors. 
In this type of image forming apparatus having a plurality of image forming 
stations 101C, 101M, 101Y and 101Bk, images of the respective colors are 
successively formed on the same surface of the same transfer material S. 
Any deviation of the actual image transfer position from the designated 
position in each image forming station causes problems such as 
misregistration of the images of different colors or overlapping of the 
images of different colors, with the result that the quality of the 
reproduced image is seriously impaired due to degradation in the color or 
an unacceptably large misregistration. 
Misregistration of color images takes place in various forms. For instance, 
misregistration takes place in the direction of conveyance of the transfer 
material indicated by an arrow A in FIG. 13(a). This misregistration is 
referred to as "top margin misregistration". Misregistration also takes 
place in the direction of scan of image indicated by an arrow B in FIG. 
13(b). This type of misregistration will be referred to as "left margin 
misregistration". Misregistration can take place also in oblique 
direction, as shown in FIG. 13(c). This type of misregistration will be 
referred to as "oblique misregistration". FIG. 13(d) illustrates 
misregistration attributable to error in magnification. Thus, the 
misregistration of the type shown in FIG. 13(d) will be referred to as 
"magnification error misregistration". Usually, misregistration occurs in 
the form of combination of two or more of these four types of 
misregistration. 
The top margin misregistration shown in FIG. 8(a) is mainly attributable to 
deviation in the time when image formation begins in the respective image 
forming stations 101C, 101M, 101Y and 101Bk. The left margin 
misregistration shown in FIG. 13(b) is usually caused by deviations in the 
time when writing of an image begins, i.e., deviations in the timing of 
start of each main scan of the image, in the respective image forming 
stations 101C, 101M, 101Y and 101Bk. Referring now to the oblique 
misregistration, this type of misregistration is chiefly attributed either 
to angular offset .theta..sub.1 see FIGS. 14(a) to 14(c)) in the mount of 
scanning optical systems and angular offset .theta..sub.2 (see FIGS. 15(a) 
to 15(c)) of the axes of the respective photosensitive drums 101C, 102M, 
102Y and 102Bk. Finally, the magnification error misregistration shown in 
FIG. 8(d) is attributable to error .DELTA.L in the length of the optical 
path between the optical scanning system and the photosensitive drum 102C, 
102M, 102Y or 102Bk in each image forming station, i.e., the difference in 
the length of the scanning line expressed by 2.times..delta.S, as shown in 
FIGS. 16 and 17. 
Various measures have been taken in order to eliminate these four types of 
misregistration. For instance, electrical adjusting means are used to 
electrically adjust the scan timing by a light beam so as to eliminate top 
margin misregistration and left margin misregistration. On the other hand, 
for the purpose of eliminating oblique misregistration and magnification 
error misregistration, means are used for adjustably mounting the optical 
scanning units (referred to as "scanners" hereinafter) and the 
photosensitive drums 102C, 102M, 102Y and 102Bk so as to allow the 
positions and angles of these units to be adjusted to eliminate these 
misregistrations. Thus, the mounting positions and the mounting angles of 
the scanners and photosensitive drums directly affect the oblique 
misregistration and magnification error misregistration, so that these 
misregistrations can be eliminated by adjusting the mounting positions and 
angles of the scanners and drums, as well as positions and angles of 
reflection mirrors which are disposed in the optical paths. 
The top margin misregistration and the left margin misregistration may take 
place as temporal changes during long use of the image forming apparatus, 
and such temporal changes can be corrected rather easily by electrical 
adjusting means. However, adjustment of mounting positions and/or angles 
of the scanners, drums and reflection mirrors, which are adjusted for the 
purpose of eliminating oblique misregistration and magnification error 
misregistration, is very difficult. This is because a highly delicate and 
minute adjustment required since accurate the adjustment must be accurate 
down to the order of pixel size which is as small as 62 micrometers. 
Misregistration in the respective image forming station is caused also by 
other indefinite factors. For instance, misregistration may be caused by 
unstable running characteristics of the conveyor belt 112, e.g., winding 
and offsetting, reproducibility of mounting of the photosensitive drum 
after dismounting, and so forth. In addition, when the image forming 
apparatus is a laser beam printer, the top margin and left margin tend to 
fluctuate due to characteristics peculiar to this type of printer. It is 
also to be pointed out that the positional relationship between the 
photosensitive member and the optical system in each image forming 
station, which has been initially set correctly after final set-up and 
adjustment before installation, may be lost due to strains of structural 
parts of the apparatus which are liable to occur when the apparatus is 
moved or transported to another location. Such a change in the positional 
relationship, even if it is very minute, undesirably causes 
misregistration of color images. Readjustment for restoring the correct 
positional relationship is very complicated and difficult to conduct. 
In an image forming apparatus which is designed to form images with a much 
higher resolution than ordinary electrophotographic machines, e.g., a 
laser beam printer which is capable of forming dots at a very small pitch 
such as 16 dots/mm, the misregistration of color images is caused even by 
a very small expansion or contraction of the structural parts of the 
apparatus attributable to a change in the ambient air temperature, as well 
as by a temporal change. 
In order to obviate these problems, it has been proposed to correct any 
misregistration of color images formed by the respective photosensitive 
drums 102C, 102M, 102Y and 102Bk, by enabling a highly precise detection 
of color image misregistrations through detection of register marks which 
are formed in the respective image forming stations and transferred 
simultaneously with the color images to the carrier or conveyor such as a 
transfer belt, intermediate transfer member, rolled paper sheet, cut sheet 
or the like, typically a conveyor belt 112 (see FIG. 7). 
The apparatus having such a correcting function, however, still encounters 
the following problem, due to the location of the mark detector 111 at a 
position which is spaced in the downstream direction by the distance 
l.sub.4 from the axis of the most downstream photosensitive drum 102Bk. 
Namely, a considerably long time is required for the mark detector 111 to 
detect all the register marks corresponding to the photosensitive drums 
102C, 102M, 102Y and 102Bk formed on the conveyor belt 112. For instance, 
representing the running speed of the conveyor belt 112 by P (mm/sec), a 
considerably long time (l.sub.1 +l.sub.2 +.sub.3 +l.sub.4)/P seconds is 
required for the register mark formed by the photosensitive drum 102C to 
reach the mark detector 111. If the correction of misregistration has to 
be executed for each of successive copies, the feed of copy paper has to 
be done with a time interval which is not shorter than (l.sub.1 +l.sub.2 
+l.sub.3 +l.sub.4)/P seconds. Thus, starting of each copying operation is 
seriously degraded. 
If the apparatus is designed such that correction of misregistration is 
effected upon each detection of a register mark, particularly when the 
design is such that the misregistration correction and image formation are 
executed simultaneously, a problem is encountered in that the color images 
of diffrent colors formed in the successive image forming stations are 
partially registered and partially misregistered, so that the hue of the 
final color image is changed with the result that the quality of the final 
color image is seriously degraded. 
This problem is serious particularly in the case where the misregistration 
is the one which is caused by deviation of the optical scanning pitch, 
i.e., when the misregistration is the left margin misregistration. In this 
case, if the correction is executed during formation of the image, 
distinctive regular unevenness of image density is caused in a region 
which is to be reproduced in a uniform halftone, resulting in a critical 
defect in the final image. 
If the detected misregistration is the oblique misregistration or the 
magnification error misregistration, correction of such misregistration 
requires geometrical rearrangement or relocation of the constituent parts. 
If such a relocation is executed during the image forming operation, there 
is a risk that the quality of the final color image is seriously impaired 
because the change in the hue and the regular unevenness are multiplied by 
each other as a result of vibration caused by the relocation. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an image 
forming apparatus which is capable of stably forming an image of a high 
quality, thereby overcoming the above-described problems encountered with 
the prior art. 
Another object of the present invention is to provide an image forming 
apparatus which is capable of always starting an image forming sequence in 
a state in which image misregistration has been corrected, thereby to 
ensure that the reproduced image has no misregistration among images of 
different colors. 
To this end, the present invention provides in one aspect an image forming 
apparatus which has correction means capable of correcting, independently 
of an image forming sequence in each image forming station, 
misregistration of image in accordance with image misregistration 
information concerning image misregistration at each image forming 
station. 
In operation, when misregistration is detected, the correction means starts 
to effect the correcting operation at a predetermined time that is 
independent from the image forming sequence of each image forming station. 
Still another object of the present invention is to provide an image 
forming appratus which is capable of correcting any image misregistration 
without causing the rate of operation of the apparatus to decrease. 
To this end, according to another aspect of the present invention, there is 
provided an image forming apparatus having image forming means capable of 
forming an image in accordance with image signals, correction means for 
correcting any misregistration of an image formed by said image forming 
means, and control means for controlling the correction means such that 
the correction of misregistration of the image is executed in a period 
between the instant at which power is applied to the apparatus and the 
instant at which warm-up of the apparatus is completed. 
A further object of the present invention is to provide an image forming 
apparatus which is capable of performing correction of any image 
misregistration each time a predetermined number of copies has been 
produced. 
A still further object of the present invention is to provide an image 
forming apparatus which is capable of correcting any image misregistration 
in response to an input from input means that is capable of giving image 
forming sequence instructions. 
These and other objects, features and advantages of the present invention 
will become clear from the following description of the preferred 
embodiments when the same is read in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view of a 4-drum type full-color image forming 
apparatus embodying an image forming apparatus in accordance with the 
present invention. 
As in the case of the known image forming apparatus explained before in 
connection with FIG. 12, the embodiment is so designed that color images 
of cyan, magenta, yellow and black colors are successively formed on the 
same transfer material as the transfer material passes through the 
respective color image forming stations, such that the color images are 
properly superposed, whereby a full-color image is obtained. 
Referring to this Figure, the color image forming stations have respective 
photosensitive drums 1C, 1M, 1Y and 1Bk having toners of the respective 
colors: namely, cyan, magenta, yellow and black. These photosensitive 
drums 1C, 1M, 1Y and 1Bk are arranged to rotate in the direction of the 
arrows shown therein. The image forming stations have primary chargers 
surrounding the respective photosensitive drums 1C, 1M, 1Y and 1Bk and 
capable of uniformly charging these photosensitive drums, optical scanning 
units 3C, 3M, 3Y as image writing means (latent image forming means), 
developing units (not shown) for developing the latent images, cleaners, 
transfer chargers and so forth. Numerals 4C, 4M, 4Y and 4Bk denote, 
respectively, scanning mirrors which are capable of focusing beams from 
the optical scanning units 3C, 3M, 3Y and 3Bk on the associated 
photosensitive drums 1C, 1M, 1Y and 1Bk. These scanning mirrors 4C, 4M, 4Y 
and 4Bk are movable horizontally and vertically as viewed in the drawings 
by actuators which will be described later. A stack of transfer paper 
sheet 5 is set on one side of the apparatus. The transfer paper sheets are 
fed one by one into the apparatus by means of feed rollers 5a and register 
rollers 2. The transfer paper sheet 5 is then conveyed by an endless 
conveyor belt 7 which moves in an endless manner along a path presented by 
power-driven conveyor rollers 6a to 6c. The upper surface of conveyor belt 
7 runs in the direction of an arrow A at a constant speed P (mm/sec). 
The use of the conveyor belt 7, however, is only illustrative. Namely, the 
conveyor belt 7 may be substituted by an intermediate transfer member, a 
paper sheet unwound from a paper roll, cut sheet and so forth. 
A series of successive register marks 9C, 9M, 9Y, 9Bk and 10C, 10M, 10Y, 
10Bk is formed at opposing peripheral edges of conveyor 7 at image forming 
stations 1C, 1M, 1Y, 1Bk, respectively. 
A cleaner 8 is capable of erasing these register mark images 9C, 9M, 9Y, 
9Bk and 10C, 10M, 10Y and 10Bk which have been transferred to the conveyor 
belt 7. Reference numerals 11 and 12 denote mark detectors each being 
composed of, for example, a CCD (charge coupled device). These mark 
detectors 11 and 12 have a construction similar to that of an image 
reading sensor ordinarily used in facsimile machines, and are disposed 
downstream from the final image forming station as viewd in the direction 
of movement of the transfer paper sheet 5. The mark detectors 11 and 12 
detect, at the downstream side of the final image forming stations, the 
successive register mark images which have been transferred to 
predetermined portions of the conveyor belt 7, and deliver register mark 
image data concerning the detected register mark images to a controller 15 
which will be described later. The mark detectors include illuminating 
lamps 13, 13 for illuminating the register marks and lenses 14, 14 through 
which the register mark images are focused on the detectors 11 and 12. The 
controller 15 also serves as correction means for correcting any image 
misregistration. To this end, the controller 15 has a ROM 15b which store 
reference register mark image data. The controller 15 computes, on the 
basis of the register mark image data derived from the mark detectors 11, 
12 and the reference register mark images stored beforehand, correction 
data necessary for correcting any positional offset, magnification error 
and scanning inclination in each image forming station, i.e., various 
types of image misregistration in each image forming station. The 
controller 15 further operates to deliver the computed correction data to 
a driver of a later-mentioned actuator so as to effect correction of the 
positional offset, magnification error and scanning inclination in each 
image forming station. 
The controller 15 has, in addition to the ROM 15b which stores data 
necessary for computing misregistration correction date, a CPU 15a which 
controls the whole apparatus, a RAM 15c which is used as a work area, an 
oscillator 15d, a counter circuit 15e and so forth. In operation, the 
controller detects image misregistration in each image forming station by 
comparing the register mark image data of the respective colors output 
from the mark detectors 11, 12 with the reference register mark image data 
stored in the ROM 15b, and computes the position offset correction amount 
peculiar to each image forming station. 
The controller 15 controls the timing of operation of a correcting 
mechanism, e.g., later-mentioned actuator, as well as timings at which 
operations are started for adjusting the top margin and the left margin, 
in such a manner that the operation for correcting the misregistration 
corresponding to the computed amount of position offset correction is 
executed at a time independent from the image forming sequence performed 
in any image forming station, e.g., in a period between the turning on of 
the power supply and the completion of warm-up of the apparatus, before 
commencement of image forming sequence, or when a predetermined number of 
image forming sequences have been completed. In particular, correcting 
operations including geometrical relocation of parts must be executed when 
the image forming stations are out of the image forming sequences. 
The register mark images 9C, 9M, 9Y and 9Bk are transferred to the conveyor 
belt 7 such that they are arranged along and substantially in parallel 
with one longitudinal edge of the conveyor belt 7 at a predetermined 
pitch. Similarly, the register mark images 10C, 10M, 10Y and 10Bk are 
transferred to the conveyor belt 7 such that they are arranged along and 
substantially in parallel with the other longitudinal edge of the conveyor 
belt 7 at a predetermined pitch. 
FIG. 2 is a perspective view presenting details of any one of the image 
forming stations 1C, 1M, 1Y and 1Bk shown in FIG. 1, and particularly 
illustrating the positional relationship between the scanning mirror shown 
in FIG. 1 and the scanning unit in the resepective image forming stations. 
In this Figure, similar reference numerals are used to denote the same 
parts or members as those appearing in FIG. 1. The same arrangement is 
used in each image forming station, and this Figure show specifically the 
arrangements for the magenta, yellow, and black stations. 
Referring to this Figure, a laser beam LB emitted from a laser 22 is 
deflected by a polygonal mirror 21 rotating at a constant angular velocity 
and is focused through an f.theta. lens 20 onto a photosensitive member 
such as a photosensitive drum 1C. These parts 20 to 22 are housed in an 
optical system housing denoted by 23. 
More specifically, the laser beam LB emitted from the laser 22 and focused 
through the f.theta. lens 20 emanates through a slit 23a formed in a wall 
of the housing 23. The optical scanning unit further has a reflector 
including a first reflection mirror 24a and a second reflection mirror 24b 
which substantially perpendicularly faces the first reflection mirror 24a. 
The first reflection mirror 24a and the second reflection mirror 24b in 
combination provide a reflector 24 which serves as each of the scanning 
mirrors 4C, 4M, 4Y and 4Bk shown in FIG. 1. These parts are arranged such 
that the laser beam from the associated lasers 22C, 22M, 22Y or 22Bk (not 
shown) is focused on the photosensitive drum 1C, 1M, 1Y or 1Bk through the 
first reflection mirror 24a and the second reflection mirror 24b. The 
laser beam LB is produced in accordance with image signals which are 
produced by an image reader (not shown) capable of reading an original 
image. 
The actuator mentioned before includes, although not exclusively, a linear 
step actuator denoted by 25. The linear step actuator 25 is capable of 
moving the reflector 24 composed of a unitary structure including the 
first and the second reflection mirrors 24a and 24b in the vertical 
direction as indicated by a double-headed arrow a in FIG. 2, in accordance 
with the number of steps output from the controller 15. The actuator also 
includes linear step actuators 26 and 27 which are operable independently 
of each other so as to move the associated ends of the reflector 24 back 
and forth as indicated by double-headed arrows b by an amount 
corresponding to the number of steps output from the controller 15. 
Each of the linear step actuators 25 to 27 is a stepping motor whose shaft 
is moved lineary. For instance, the linear step actuator may have 
trapezoidal screw threads on the rotor and the output shaft as is the case 
of motors which are ordinarily used in driving recording/reproducing heads 
in floppy-disk drives. It is to be noted, however, that the actuator need 
not be of linear motion type. For instance, each of the linear step 
actuators 25 to 27 may be substituted by an ordinary rotary type stepping 
motor. In such a case, a lead screw thread is formed on the output shaft 
of the motor and a threaded member is engaged with the threaded output 
shaft so as to move linearly back and forth in accordance with the 
rotation of the output shaft. It will be understood that such an 
alternative actuator produces the same effect as the linear step actuator. 
When the screw thread formed on the output shaft of the motor is, for 
example, 4P0.5 (nominal diameter 4 mm, pitch 0.5 mm) while the step angle 
of the stepping motor may be 48 steps/360.degree., the amount of feed DS 
per step is calculated as PS=0.5/48=10.42 um/step. Thus, the reflector 24 
can be moved back and forth by amount of 10.42 um in response to each step 
input to the motor. 
Numeral 28 denotes a scanning mirror which is capable of introducing the 
laser beam LB immediately before entering the image area to a beam 
detector 29. The beam detector 29 is adapted for generating a horizontal 
synchronizing signal BD which determines the timing of start of writing in 
the direction of main scan on the photosensitive drum 1. It is possible to 
adjust the left margin of the print by adjusting the timing of issue of 
the horizontal synchronizing signal BD. 
The operation of the actuators 25 to 27 of FIG. 2 will be described with 
reference to FIGS. 3(a) to 3(c) which show various types of image 
misregistration on the image carrier. Referring to these Figures, the 
transfer material 5 is conveyed in the direction of an arrow A, i.e., in 
the direction of running of the conveyor belt 7. 
Assuming here that the actuator 25 is driven in the direction a.sub.1 which 
is toward the direction of emission of the laser beam LB, the reflector 24 
is moved substantially translationally in the direction a so that the 
length of the optical path to the photosensitive drum 1C is shortened. 
Conversely, the length of the optical path is increased when the actuator 
25 is driven in the direction a.sub.2. Since the laser beam LB has a 
predetermined angle of divergence, it is possible to vary the length of 
the scanning by the laser beam LB on the photosensitive drum 1 between 
m.sub.0 (solid line) and m.sub.1 (broken line) as shown in FIG. 3(a). 
Lines m.sub.0 and m.sub.1 are shown spaced a part for the sake of clarity. 
It is assumed here that the actuators 26 and 27 are simultaneously driven 
in the same direction, e.g., in the direction b.sub.1. In such a case, the 
reflector 24 is translationally moved in the direction b which is 
perpendicular to the direction a.sub.1 of the movement effected by the 
actuator 25. In consequence, the scanning line m.sub.0 is translationally 
moved to the position of the scanning line m.sub.2 in FIG. 3(b). 
It is also possible to change the inclination of the scanning line, such as 
from the inclination of scanning line m.sub.0 to inclination of scanning 
line m.sub.3 in FIG. 3(c), by operating only one of the actuators 26 and 
27 or by operating these actuators in opposite directions, e.g., by 
operating the actuator 26 in the direction b.sub.1 while driving the 
actuator 27 in the direction b.sub.2. 
Thus, the described embodiment incorporates a reflector 24 which is 
composed of a pair of perpendicularly facing mirrors 24a and 24b and which 
is disposed in the path of light between the optical scanning unit and the 
photosensitive drum 1. With this arrangement, it is possible to adjust the 
length of the scanning optical path and the position of scanning by the 
laser beam independently of each other, by suitably controlling the 
position of the reflector 24 by selective operation of the actuators 25 
and 26, 27. Namely, when the reflector 24 having pair of orthogonally 
opposing mirrors is moved in the direction a, the length of optical path 
for the laser beam LB can be changed without causing any change in the 
position of the scanning line focused on the photosensitive drum 1. On the 
other hand, when the reflector 24 is moved in the direction b, the 
position on the photosensitive drum 1 where the beam is focused and the 
angle of the scanning line can be varied without causing any change in the 
length of optical path of the laser beam LB. 
In the described embodiment, each of four image forming stations in the 
4-drum type full-color printer is equipped with the reflector 24 and the 
actuator system capable of adjusting the position of the reflector 24, as 
well as mirror 28 and detector 29, so that the oblique error and the 
magnification error misregistration attributable to inclination of the 
scanning line and a difference in the length of optical path, top margin 
misregistration and left margin misregistration are correctable for each 
of the successive transfer paper sheets and independently in each of the 
image forming stations. 
A description will now be given of the process for correcting image 
misregistration, with specific reference to FIG. 4. 
FIG. 4 is a block diagram illustrative of the process for correcting the 
image misregistration executed by the controller 15 explained before in 
connection with FIG. 1. Thus, in FIG. 4, the same reference numerals are 
used to denote the same parts or members as those appearing in FIG. 1. 
Although the following description specifially mentions the cyan image 
forming station, it is to be understood that the same process applies to 
each of the other image forming stations, i.e., the magenta, yellow and 
black image forming stations. 
Referring to FIG. 4, a register roller start (driving) signal RON is 
produced when the register rollers 2, 2 shown in FIG. 1 are started. A 
signal BDC, which is a BD signal for the cyan color, is generated when the 
laser beam LB coming through the beam scanning mirror 28C is detected by 
the beam detector 29C. For instance, when the laser beam LB from the laser 
22 of the cyan station is detected by the beam detector 29C, the beam 
detector 29C delivers the BD signal BDC to the controller 15, and the 
scanning of the photosensitive drum 1C with the laser beam LB in the 
direction of main scan is commenced by making use of the beam detection 
signal BDC as the reference. 
Then, register mark image 9C and 10C are formed in accordance with a 
program stored in the ROM 15b of the controller 15 and the thus formed 
register mark image are transferred to predetermined regions of the 
conveyor belt 7 which is running at a constant speed in accordance with 
the register roller driving signal RON. The transferred register mark 
images 9C and 10C are moved in the direction of the arrow A so as to reach 
and be read by the mark detectors 11 and 12 which are disposed downstream 
of the final image forming station 1Bk. The controller 15 beforehand 
stores the cyan register mark image data which are to be used as 
reference. These data correspond to broken-line reference marks MC1 and 
MC2 shown in FIG. 4. 
The register mark images have, for example, cross-like form. The controller 
15 has a cyan image memory which is designed for storing the register mark 
image data concerning the register mark images 9C and 10C which are read 
by the mark detectors 11 and 12. The storage of the register mark image 
data is executed under the control of the controller 15 in synchronization 
with predetermined reference clock signals from the moment at which the 
register roller driving signal RON is issued. The controller then 
determines the position of the central pixel of the data in the direction 
A of the main scan and the central pixel of the data in the direction B of 
sub-scan. The controller then determines the amounts D1 and D2 of 
difference between the positions of the central pixels and the positions 
of the central pixels of the reference marks MC1 and MC2 in the direction 
of main scan in terms of numbers of pixels, as well as the amounts D3 and 
D4 of the difference between the positions of the central pixels of the 
reference marks MC1 and MC2 in the direction of the sub-scan in terms of 
number of pixels. 
In consequence, the controller 15 recognizes that there is an offset of 
left margin, i.e., left margin misregistration, in amount of D3 in terms 
of number of pixels and an offset of the top margin, i.e., top margin 
misregistration, in amount of D1 in terms of the number of pixels. The 
controller 15 further recognizes the degree of inclination of the scan 
line as the difference (D2-D1) between the amouts D2 and D1 of offset of 
the respective central pixels of the register image data from the central 
pixels of the associated reference marks as well as the magnification 
error as the difference (D4-D3) between the amounts D4 and D3 of offset of 
the central pixels of the respective register mark images from the central 
pixels of the associated reference marks. 
The offset amounts D1, D3 and the difference values (D2-D1) and (D4-D3) are 
stored in the RAM 15C and are used in the adjustment of the left margin 
for the actual image forming operation. More specifically, a left margin 
control output for cyan (DELAYC) is issued in such a manner as to negate 
or cancel the offset amount D3 after the receipt of the BD signal BDC so 
that the timing of writing of image in accordance with the cyan image data 
stored in the image memory is delayed after the detection of the laser 
beam LB by the beam detector 29 so as to set the left margin at a 
predetermined position, thus effecting correction of left margin 
misregistration. 
On the other hand, the correction of the top margin misregistration is 
effected by operating the actuators 26C and 27C in accordance with the 
amount D1 thereby to set the top margin on the position of the central 
pixel of the reference mark MC1. This can be conducted by delivering, to 
the stepping motor actuator drive circuit 30C, a top margin control signal 
for cyan (TC) which corresponds to the number of the steps necessary for 
cancelling the offset amount D1 of the central pixel. As a result, the 
actuators 26C and 27C are operated back and forth by an equal amount so 
that the scanning mirror 4C is moved translationally thereby correcting 
the position of the top margin. 
Correction for the inclination of scanning line is executed in the 
following manner. Namely, the controller 15 functions to operate the 
actuators 26C and 27C in accordance with the difference value (D2-D1) so 
as to set the scanning line in alignment with a predetermined axis line. 
This can be executed by delivering, to the stepping motor actuator drive 
circuit 30C, an inclination control output for cyan (IC) which corresponds 
to the number of steps necessary for negating or cancelling the difference 
value (D2-D1). In consequence, the actuators 26C and 27C operate in the 
horizontal direction in different amounts so as to change the posture of 
the scanning mirror 4C thereby to eliminate any inclination of the 
scanning line. 
The magnification error misregistration can be corrected by operating the 
actuator 25C in accordance with the difference amount (D4-D3) so as to 
make the image magnification value. This can be executed by delivering, to 
the stepping motor actuator drive circuit 30C, a magnification control 
ouput from cyan (RC) which corresponds to the number of steps necessary 
for cancelling the difference value (D4-D3). As a result, the actuator 25C 
operates to move the scanning mirror 4C up and down, thereby adjusting the 
length of the optical path leading from the laser 22 so as to correct the 
misregistration attributable to the magnification error. 
The operation of the arrangement shown in FIG. 4 will be described with 
reference to the timing chart shown in FIG. 5. 
Referring to FIG. 5, counting of reference clock signals CLK produced by 
the oscillator 15d is commenced in synchronization with the register 
roller start (driving) signal RON. A period t.sub.c is for detection of 
the register marks for cyan color and corresponds to the time length 
required for the counter circuit 15e to count the reference clock signals 
CLK after the issuance of registr roller start signal RON. Thus, the fact 
that the mark detectors 11 and 12 detect the register mark images 9C and 
10C after the expiration of the detection period t.sub.c means that there 
is ni image misregistration in regard to the cyan color. Thus, the 
detection period t.sub.c corresponds to the time between the instant at 
which the register roller start signal RON is issued until the instant at 
which the reference marks MC1 and MC2 shown in FIG. 4 are detected. A mark 
detection output MO1 is produced when the mark detector 11 has read the 
register mark image 9C, while mark detection output MO2 is produced when 
the register mark image 10C is read by the mark register 12. 
As will be understood from this Figure, any image misregistration taking 
place, for instance, in the cyan image forming station having the 
photosensitive drum 1C, the mark detectors cannot detect imaginary 
reference marks MC1 and MC2 shown in FIG. 4 when the detection period 
t.sub.c has expired after the issue of the register roller start signal 
RON, and the timings at which the mark detectors 11 and 12 detect the 
register mark images 9C and 10C fluctuate as shown in FIG. 5. The 
controller 15 thertefore computes the central pixel offset amounts D1 and 
D2 from the time lengths t.sub.1 and t.sub.2 shown in FIG. 4, and produces 
a correction control signal for correcting the image misregistration, 
e.g., the topmargin control signal TC in accordance with the thus computed 
central pixel offset amounts D1 and D2. The controller 15 then deliers 
this correction signal to the stepping motor actuator drive circuit 30C so 
that the top margin is set at the correct position. 
Numeral 32 in FIG. 4 denotes a power supply switch for supplying electrical 
power to the apparatus, while 31 denotes an inverter which delivers a 
later-mentioned signal B. The inverter operates such that the leel of the 
signal B is set to H (High) level when the power supply switch 32 is 
turned on. Numeral 34 denotes a start key for giving instruction for 
starting the image forming operation. An inverter 33 is designed to set a 
later-mentioned signal STR to H level when the start key 34 is turned on. 
A laser driver 35 is capable of controlling the laser 22 which emits the 
laser beam. 
Various controls for starting correcting operations performed in this 
embodiment will be described with reference to FIGS. 4 to 13. 
(First Correction Control Process) 
FIG. 4 is a timing chart explanatory of a first correction control process 
performed by the apparatus of this embodiment. The signal B set at H level 
indicates that the power supply switch 32 in FIG. 4 has been turned on. A 
symbol VC(C) represents a cyan image writing signal. The laser driver 35 
is operated in synchronization with the rise of the image writing signal 
VC(C) so that register mark images 9C and 10C are written on the 
photosensitive drum 1C and the thus written register mark images are 
transferred to the conveyor belt 7 when a predetermined time has passed 
after they are written on the photosensitive drum 1C. Similarly, magenta 
register mark images 9M and 10M are written on the photosensitive drum 1M 
in synchronization with the rise of an image writing signal VC(M). The 
images 9M and 10M are transferred to the conveyor belt 7 after elapse of a 
predetermined time. Similarly, yellow register mark images 9Y and 10Y are 
written on the photosensitive drum 1Y in synchronization with the rise of 
a yellow image writing signal VC(Y). The images 9Y and 10Y are transferred 
to the conveyor belt 7 after elapse of a predetermined time. Finally, 
black register mark images 9Bk and 10Bk are written on the photosensitive 
drum 1Bk in synchronization with the rise of a black image writing signal 
VC(Bk). The images 9Bk and 10Bk are transferred to the conveyor belt 7 
after elapse of a predetermined time. Upon detection of the register mark 
image 9C after elapse of a time T.sub.C from the issue of the register 
roller drive signal RON, the mark detector 11 produces a mark detection 
output signal CD1. Similarly, the register marks 9M, 9Y and 9Bk are 
sequentially detected at timings t.sub.m, t.sub.y and t.sub.Bk which are 
not shown. Symbol CD2 represents a mark detection output produced by the 
detector 12. The detector 12 successively detects the register mark images 
10C, 10M, 10Y and 10Bk. 
The controller 15 delivers to the actuators 25C, 26C and 27C (see FIGS. 2 
and 4) aforementioned correction control signals TC, IC and RC in 
synchronization with the rise of a feedback timing signal FB(C) which 
indicates the timing of feedback control for commencing the correction. At 
the same time, the controller adjusts the vertical and horizontal 
synchronization for determining the left margin and top margin. The 
correction process is completed when the feedback timing signal FB(C) 
falls. 
Similarly, the controller 15 delivers correction control signals to the 
actuators associated with the magenta photosensitive drum 1M (see FIGS. 2 
and 4) in synchronization with the rise of a feedback timing signal FB(M) 
which indicates the timing of feedback control for commencing the 
correction. At the same time, the controller adjusts the vertical and 
horizontal synchronization for determining the left margin and top margin. 
The correction process is completed when the feedback timing signal FB(M) 
falls. 
Similarly, the controller 15 delivers correction control signals to the 
actuators associated with the yellow photosensitive drum 1Y of FIG. 2 in 
synchronization with the rise of a feedback timing signal FB(Y) which 
indicates the timing of feedback control for commencing the correction. At 
the same time, the controller adjusts the vertical and horizontal 
synchronization for determining the left margin and top margin. The 
correction process is completed when the feedback timing signal FB(Y) 
falls. 
Finally, the controller 15 delivers correction control signals to the 
actuators associated with the black photosensitive drum 1Bk (see FIGS. 2 
and 4) in synchronization with the rise of a feedback timing signal FB(Bk) 
which indicates the timing of feedback control for commencing the 
correction. At the same time, the controller adjusts the vertical and 
horizontal synchronization for determining the left margin and top margin. 
The correction process is completed when the feedback timing signal FB(Bk) 
falls. 
An operation ready signal RDY indicates that the warm-up of the apparatus 
is finished. An ordinary image forming sequence is commenced at the 
instant t.sub.s at which the print start signal STR is set to H level by 
pressing of the start key 34 after the completion of the warm-up of the 
apparatus. 
FIG. 7 is a flow chart illustrating the operation described in connection 
with FIG. 6. This flow is stored in the ROM 15b and is executed under the 
control of the CPU 15a. 
FIG. 7 is a flow chart illustrating the operation described in connection 
with FIG. 6. This flow is stored in the ROM 15b and is executed under the 
control of the CPU 15a. 
Step S1 determines whether the power supply input signal B has been set to 
H level. The process proceeds to Step 32 if the power supply switch 32 has 
been turned on. Step 32 judges whether the register roller drive signal 
has been set to H level. If the answer is YES, the process proceeds to 
Step S3 in which the counter circuit 15e is started so as to commence 
counting of the clock signals from the oscillator 15d. In Step S4, the 
image writing signal VC(C) for forming the register mark is set to H level 
so that the laser driver 35 is started. Similarly, image writing signals 
VC(M), VC(Y) and VC(Bk) are successively set to H level so that register 
mark images 9C, 10C, 9M, 19M, 9Y, 10Y, 9Bk and 10Bk are independently 
formed on predetermined portions of the photosensitive drums 1C, 1M, 1Y 
and 1Bk of the respective image forming stations in synchronization with 
the image writing signals VC(C), VC(M), VC(Y) and VC(Bk). After elapse of 
a predetermined time, these register mark images are transferred to the 
conveyor belt 7 which is running at a constant speed. 
Step S5 judges whether the value counted by the counter circuit 15e, which 
has started in synchronization with the start of the register rollers 2, 
2, has exceeded a value which corresponds to the timing to at which the 
register mark images 9C and 10C are expected to be detected. If this value 
is exceeded, i.e., when the time t.sub.c has elapsed, the process proceeds 
to Step S6 in which register mark images 9C and 10C are detected by the 
mark detectors 11 and 12. Similarly, in Steps S7 to S12, register mark 
images formed in the respective image forming stations are detected at the 
respective timings. In Step S13, data concerning the register marks 
detected in Steps S5 to S12 are stored in the RAM 15c. In Step S14, 
misregistration correction amounts are computed for the respective image 
forming sections. In Step S15, the thus computed misregistration 
correction amounts, e.g., the signals TC, IC and RC in the case of the 
cyan image forming section, are delivered to the driver circuit 30C. 
In response to these signals representing the correction amounts, the 
misregistration is corrected in each image forming station, in accordance 
with the correction feedback signals FB(C), FB(M), FB(Y) and FB(Bk). More 
specifically, in each image forming section, correction control signals 
are delivered to the actuators in concert with the correction feedback 
signals so as to move the scanning mirror or reflector vertically and 
horizontally thereby to eliminate any top margin misregistration, oblique 
misregistration attributable to inclination of scanning line and 
magnification error misregistration. 
Step S16 judges whether the misregistration correction is finished. If the 
correction has been finished, the process proceeds to Step S17 in which 
the warm-up completion signal RDY is set to H level, thus confirming 
completion of the warm-up of the apparatus. 
Then, ordinary image forming sequence is started when the print start 
signal STR is set to H level as a result of start input through the start 
key 34, in Steps S18 and S19. In Step S19, the timing of delivery of the 
image signal is controlled in accordance with the magnification data 
DELAYC, DELAYM, DELAYY, DELAYBK stored in the ram 15c so as to correct any 
left margin misregistration. Thus, according to the first correction 
control process, the image registration in all the image forming stations 
are corrected until the warm-up of the apparatus is completed. In 
consequence, image misregistration of various types, which have taken 
place before the power is turned for various reasons such as a change in 
the environmental condition can be collectively corrected. This ensures 
that the copy which is obtained for the first time after the start of the 
apparatus has a high quality of reproduced image which contains no image 
misregistration. 
It is also to be noted that the first correction control process is 
advantageous in that the correction of image misregistration is completed 
during warming up the apparatus, so that it is not necessary to set a 
special sequence time for the correction of the image misregistration. 
This contributes to an improvement in the rate of operation of the 
apparatus. 
When one or more of the photosensitive drums 1C, 1M, 1Y and 1Bk have been 
renewed after the preceding image forming operation, there is a risk that 
the new drum or drums may have been set such that their generating lines 
are inclined from the correct direction of generating line. This, however, 
does not cause any trouble because the correcting operation for 
eliminating any undesirable effects of such mounting error is 
automatically executed before the next image forming operation is started. 
Thus, the user is relieved from troublesome maintenance work which 
otherwise would be necessitated to eliminate such undesirable effect. 
(Second Correction Control Process) 
FIG. 8 is a timing chart illustrating the second correction control process 
in accordance with the present invention. In this Figure, the same 
reference numerals are used to denote the same things as those in FIG. 6. 
As will be seen from this Figure, in the second correction control process, 
image misregistration correcting sequence is forcibly executed after the 
print start signal STR is set high as a result of actuating start key 34. 
The correcting operation itself is the same as that in the first 
correction control process. The second correction control process is 
illustrated in FIG. 9. 
Step S21 judges whether the print start signal STR has been set high (H). 
The process proceeds to Step S22 if the start key 34 has been turned on. 
Step S22 judges whether the register roller drive signal has been set to H 
level. If the answer is YES, the process proceeds to Step S23 in which the 
counter circuit 15e is started so as to commence counting of the clock 
signals from the oscillator 15d. In Step S24, the image writing signal 
VC(C) for forming the register mark is set to H level so that the laser 
driver 35 is started. Similarly, image writing signals VC(M), VC(Y) and 
VC(Bk) are successively set to H level so that register mark images 9C, 
10C, 9M, 10M, 10Y, 9Bk and 10Bk are independently formed on predetermined 
portions of the photosensitive drums 1C, 1M, 1Y and 1Bk of the respective 
image forming stations in synchronization with the image writing signals 
VC(C), VC(M), VC(Y) and VC(Bk). After elapse of a predetermined time, 
these register mark images are transferred to the conveyor belt 7 which is 
running at a constant speed. 
Step S25 judges whether the value counted by the 
counter circuit 15e, which has started in synchronization with the start of 
the register rollers 2, 2, has exceeded a value which corresponds to the 
timing t.sub.c at which the register mark images 9C and 10C are expected 
to be detected. If this value is exceeded, i.e.,when the time t.sub.c has 
elapsed, the process proceeds to Step S26 in which register mark images 9C 
and 10C are detected by the mark detectors 11 and 12. Similarly, in Steps 
S27 to S32, register mark images formed in the respective image forming 
stations are detected at the respective timings. In Step S33, data 
concerning the register marks detected in Steps S25 to S32 are stored in 
the RAM 15c. In Step S34, misregistration correction amounts are computed 
for the respective image forming stations. In Step S35, the thus computed 
misregistration correction amounts, e.g., the signals TC, IC and RC in the 
case of the cyan image forming station, are delivered to the driver 
circuit 30. 
In response to these signals representing the correction amounts, the 
misregistration is corrected in each image forming station, in accordance 
with the correction feedback signals FB(C), FB(M), FB(Y) and FB(Bk). More 
specifically, in each image forming station, correction control signals 
are delivered to the actuators in concert with the correction feedback 
signals so as to move the scanning mirror or reflector vertically and 
horizontally thereby to eliminate a by top margin misregistration, oblique 
misregistration attributable to inclination of scanning line and 
magnification error misregistration. 
Step S36 judges whether the correction of misregistration has been finished 
with all image forming stations. If the answer is YES, the process 
proceeds to Step S37 in which ordinary image forming sequence is started. 
Step S37 also executes control of output of the image signal in accordance 
with the misregistration data DELAYC, DELAYM, DELAYY, DELAYBk stored in 
the RAM 15c thereby to correct any left margin misregistration. 
Thus, according to the second correction control process, the image 
misregistration correction sequence is executed before an ordinary image 
sequence is started each time the print start signal STR rises. This 
second correction control process, in addition to the advantages brought 
about by the first correction control process, provides the following 
advantages. There is a risk that a cause of image misregistration may have 
occurred after completion of the preceding image forming sequence due to, 
for example, relocation of the whole image forming apparatus. It will be 
seen that the second correction control process eliminates any image 
misregistration attributable to such a case, so that the image forming 
apparatus can produce color copy images of good quality regardless of any 
change in the environmental or installation condition. 
(Third Correction Control Process) 
FIG. 10 is a timing chart illustrative of a third correction control 
process which can be employed in the described embodiment. In this Figure, 
the same reference numerals are used to denote the same things as those in 
FIG. 6. 
As will be seen from this Figure, the image misregistration correction is 
completed at an instant t.sub.s and the apparatus waits for the input of 
the next print start signal STR. When the print start signal STR rises at 
an instant t.sub.s(1), the apparatus starts the image forming sequence to 
produce a registered number of copies. Then, the apparatus again waits for 
the next print start signal STR. This sequence is repeated each time the 
print start signal STR rises. 
In this third correction control process, operation for correcting image 
misregistration is executed each time a predetermined total number of 
copies has been produced. However, if the registered number of copies for 
a particular original has not been produced yet when the total number of 
copies produced by the apparatus has reached the predetermined number, the 
copying operation is continued until the registered number of copies is 
obtained with the present original image, and the time misregistration 
correction is executed only after the registered number of copies has been 
obtained. 
Referring to FIG. 10, it is assumed that an image forming sequence is 
started in response to a print start signal STR at an instant t.sub.s (n). 
On the other hand, the number of copies produced after the last 
misregistration correcting operation conducted at instant t.sub.s is 
counted. When the counted value, i.e., the total number of copies produced 
since the last misregistration correcting operation has exceeded a 
predetermined number which maybe variable and which is stored in a memory 
during the present image forming operation, image misregistration 
correcting operation is executed at an instant tE at which the image 
forming operation triggered by the start signal STR issued at the instant 
t.sub.s(n) is completed. The total number of copies produced since the 
last misregistration correction is counted by the counter circuit 15e 
shown in FIG. 4 and is compared by the CPU 15a with the value stored in 
the RAM 15c. 
The third correction control process will be explained with reference to a 
flow chart shown in FIG. 11. 
In Step S41, the predetermined number of copies has been produced so that 
the misregistration correction is executed. The operation executed in Step 
S41 is the same as those executed in Steps S2 to S16 in the flow of FIG. 7 
and Steps S22 to S36 of the flow shown in FIG. 9. 
This Step S41 also clears the counter 15e. Step judges whether the signal 
STR has been set high (H), i.e., whether the start key 34 has been 
pressed. Upon judging that the start signal STR has been set high, the 
process proceeds to Step S43 in which the printing operation is executed. 
In Step S44, the content of the counter circuit 15e is incremented by one 
in response to production of each copy. Step S45 judges whether the value 
counted in the counter circuit 15e has reached the predetermined 
number.i.e., the total copy number which necessitates the image 
misregistration correction. When the predetermined number has been 
reached, a flag F is set in the RAM 15c in Step S46. If the judgment in 
Step S45 has proved that the count value has not reached yet the 
predetermined number, the process proceeds to Step S47 without setting the 
flag F. Step S47 judges whether the number of copies which has been 
registered prior to the operation of the start key in Step S42 has been 
reached, i.e., whether the registered number of copies of a particular 
original has been finished. 
If the registered number of copies has not been finished yet, the process 
returns to Step S43 in which the copying operation is continued. 
Conversely, if copies of the registered number have been obtained, the 
process proceeds to Step S48 which judges whether the flag F has been set 
or not. If the flag F has not been set yet, the controller decides that 
the misregistration correction need not be executed at this instant, and 
returns the process to Step S42 so that the image forming apparatus waits 
for the next input of the start signal STR through the start key 34. 
Conversely, if the flag F has been set, the flag F is reset in Step S49 
and operation for correcting image misregistration is executed in Step 
S50. 
As will be understood from the foregoing description, in the third 
correction control process, the total number of copies produce since the 
last misregistration correcting operation is counted and the next 
misregistration correction is executed when the apparatus is in the 
waiting condition after the predetermined number of copies has been 
obtained. It is therefore possible to effect misregistration correction 
even when the image forming apparatus is required to operate continuously 
to produce a large number of copies. In consequence, clear copy image can 
stably be obtained without causing the rate of operation to decrease, even 
when the apparatus is used frequently. As has been described, the image 
forming apparatus of the present invention has misregistration correction 
means which is capable of effecting correction of misregistration detected 
by detecting means for each image forming station, at a timing which is 
independent from the image forming sequence performed in each image 
forming station. The misregistration correction operation, therefore, is 
not executed when the image forming sequence is being executed in any of 
the image forming station, and the image forming sequence can be started 
always in a state in which the causes of image misregistration have been 
eliminated. It is therefore possible to obtain clear color copy images 
with good balance and hue. In addition, since the misregistration 
correction is executed in a predetermined period prior to the start of 
ordinary image forming sequence or in a period in which the image forming 
apparatus is in the waiting condition, necessary image forming operations 
can be performed without being interrupted by the misregistration 
correcting operation, thus ensuring a high rate of use of the image 
producing apparatus. 
Although the invention has been described through its specific terms, it is 
to be understood that the described embodiments are only illustrative and 
various changes and modifications are possible without departing from the 
scope of the invention which is limited solely by the appended claims.