Color shift correcting structure of image forming apparatus

BACKGROUND OF THE INVENTION

1 Technical Field of the Invention

The present invention relates generally to an image-forming apparatus which is designed to convert an image carried by an image carrier into a visual image and transfer it onto a print sheet and which may be employed in copying machines, facsimile machines, printers, or information processing systems using electrophotographic printing techniques, and more particularly to an improved color shift correcting structure of such an image-forming apparatus.

2 Background Art

Typical color image-forming apparatuses are designed to form a multi-colored image by overlaying yellow, magenta, cyan, and block images in registration on a print sheet. A positional shift between the different color images on the print sheet will, thus, result in a shift between colors. In order to eliminate such a color shift, Japanese Patent No. 2505206 teaches printing marks continuously on a transfer belt for correcting a shift between color images. This structure is designed to correct locations of images formed by exposure of photosensitive drums on which the marks are to be printed and not the speed of the transfer belt. The distances from a sensor detecting the marks to each photosensitive drum and an image-transferred location of the transfer belt are long. A change in speed of the transfer belt in a long cycle (e.g., 1 m or more) may thus be corrected, but it is difficult to correct one in a short cycle (e.g., several tens mm).

The marks are first formed on the photosensitive drums and then transferred to the transfer belt. It is, thus, difficult to determine whether a positional shift of the marks arises from a change in speed of the transfer belt or a change in speed of the photosensitive drum, thereby resulting in a difficulty in correcting the speed of the transfer belt using the marks.

Japanese Patent First Publication Nos. 8-211693, 6-144631, and 9-62047 teach a belt speed correction structure in which two sensor are arrayed in lengthwise direction of a transfer belt to measure the time required for one of marks printed on the transfer belt to pass the two sensors for calculating the speed of the transfer belt.

The structure as taught in Japanese Patent First Publication No. 8-211693 prints a plurality of magnetic marks along the periphery of the transfer belt. Therefore, if the belt is partly deformed due to, for example, residual stress of the belt created during manufacture, it will result in a change in pitch of the marks, which makes it difficult to measure the speed of the belt accurately.

The structures as taught in Japanese Patent First Publication Nos. 6-144631 and 9-62047 print a plurality of marks with toner. The surface of a transfer belt is usually cleaned by a cleaner blade. A problem is, thus, encountered in that the marks may erased by friction between themselves and a cleaner blade. An additional problem is also encountered in that a consumption of toner is increased.

Typical color image-forming apparatus also have a sheet conveyer belt for transporting a print sheet to which images are to be transferred from a transfer belt. The sheet conveyer belt is usually moved by a drive roller. The drive roller is increased in temperature by radiation of heat from a neighboring fixer, leading to thermal expansion, which will result in a change in diameter of the drive roller. This change will cause the speed of the sheet conveyer belt to be changed, resulting in a shift between color images.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.

It is another object of the present invention to provide an image-forming apparatus designed to measure the speed of image carriers such as photosensitive drums, a transfer belt, and a sheet conveyer belt accurately and eliminate positional shifts between colored images for avoiding a shift between colors.

According to one aspect of the invention, there is provided an image-forming apparatus which comprises: (a) an image carrier carrying images printed on one of a surface of the image carrier and a surface of a printing medium held on the image carrier, the image carrier having at least the surface made of a dielectric material; (b) an electrostatic mark writing device writing electrostatic marks at predetermined intervals on the image carrier during movement of the image carrier; (c) a detecting device detecting the electrostatic marks written on the image carrier during the movement of the image carrier to provide signals indicative thereof; and (d) a correction device correcting a speed of the movement of the image carrier based on the signals outputted from the detecting device.

In the preferred mode of the invention, the electrostatic mark writing device includes a plurality of electrostatic mark writing units which are arrayed in a direction of the movement of the image carrier and which form the electrostatic marks on the image carrier, respectively.

Each of the electrostatic mark writing units writes a series of the electrostatic marks at regular intervals. The intervals in the electrostatic mark writing units are different from each other. The correcting device performs a correcting operation based on the degree to which the electrostatic marks written by the electrostatic mark writing units are overlaid.

The detecting device may alternatively include a plurality of detecting units which are arrayed in a direction of the movement of the image carrier and which detect the electrostatic images, respectively.

According to another aspect of the invention, there is provided an image-forming apparatus which comprises: (a) an image carrier carrying images printed on one of a surface of the image carrier and a surface of a printing medium held on the image carrier, the image carrier having at least the surface made of a dielectric material; (b) an electrostatic mark writing device writing electrostatic marks at predetermined intervals on the image carrier during movement of the image carrier; (c) a detecting device detecting the electrostatic marks written on the image carrier during the movement of the image carrier to provide signals indicative thereof; and (d) a correction device correcting a location where each of the images is printed based on the signals outputted from the detecting device.

In the preferred mode of the invention, the electrostatic mark writing device includes a plurality of electrostatic mark writing units which are arrayed in a direction of the movement of the image carrier and which print the electrostatic marks on the image carrier, respectively.

Each of the electrostatic mark writing units writes a series of the electrostatic marks at regular intervals. The intervals in the electrostatic mark writing units are different from each other. The correcting device performs a correcting operation based on the degree to which the electrostatic marks written by the electrostatic mark writing units are overlaid.

The detecting device may alternatively include a plurality of detecting units which are arrayed in a direction of the movement of the image carrier and which detect the electrostatic images, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.

In the drawings:

FIG. 1

is a cross sectional view which shows an image-forming device according to the first embodiment of the invention;

FIG. 2

is a perspective view which shows a detecting device and a mark writing device installed above a conveyer belt;

FIG. 3

is an enlarged view of the detecting device and the mark writing device in

FIG. 2

;

FIG. 4

is a circuit structure of an image-forming device of the first embodiment;

FIG. 5

is an illustration which shows waveform of a signal A inputted to a mark writing device and waveform of a signal B outputted from a detecting device;

FIG. 6

is a flowchart of a program performed to correct the speed of a conveyer belt;

FIG. 7

is a perspective view which shows a detecting device and a mark writing device installed above a conveyer belt according to the second embodiment of the invention;

FIG. 8

is an enlarged view of the detecting device and the mark writing device in

FIG. 7

;

FIG. 9

is a side view which shows a detecting device and a mark writing device installed above a conveyer belt according to the third embodiment of the invention;

FIG.

10

(

a

) shows pulse trains applied to electrodes of a mark writing device;

FIG.

10

(

b

) shows a current signal outputted from an electrode of a detecting device;

FIG. 11

is a flowchart of a program performed by an image-forming device of the third embodiment;

FIG. 12

is a block diagram which shows an image-forming device according to the fourth embodiment of the invention;

FIG. 13

is a block diagram which shows a circuit structure designed to determine a correction value for correcting a locational shift of each print assembly;

FIG.

14

(

a

) shows a bit map memory space;

FIG.

14

(

b

) shows a locational shift data;

FIG.

14

(

c

) shows a correction data; and

FIG.

14

(

d

) shows a result of printing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like numbers refer to like parts in several views, particularly to

FIG. 1

, there is shown an image-forming device

60

according to the invention which will be discussed below as a full color printer.

The image-forming device

60

has four print assemblies

68

Y,

68

M,

68

C, and

68

K arranged in line and an endless conveyer belt

1

traveling through the print assemblies

68

Y to

68

K. The conveyer belt

1

is made of a synthetic resin material such as fluorine-contained polymers (e.g., Polyvinylidene Fluoriden) which has high durability and ability to hold a print sheet P statically. The conveyer belt

1

is wound around four rollers

10

a,

10

b,

10

c,

and

10

d.

The roller

10

a

is a drive roller which also works as an AC discharge roller to remove a charge from the conveyer belt

1

. The roller

10

b

is a driven roller and also works as an electrifying roller to electrifies the conveyer belt

1

. The rollers

10

c

and

10

d

are guide rollers. The roller

10

d

also works as a tension roller which provides a given degree of tension to the conveyer belt

1

.

A hopper

70

is disposed beneath the conveyer belt

1

which stores therein a stack of print sheets P. A pickup roller

72

picks up one of the print sheets P from the hopper

70

and carries it to the conveyer belt

1

through a pair of paper feed rollers

74

. The print sheet P is transported by the conveyer belt

1

to the print assemblies

68

Y,

68

M,

68

C, and

68

K in which full color images are printed. After printing, the print sheet P is fed to a fixing device

76

and ejected through a guide roller (not shown) installed on an end cover

66

toward a stacker provided on an upper surface of a top cover

64

.

The conveyer belt

1

is charged by the driven roller

10

b

to hold thereon the print sheet P when it reaches the conveyer belt

1

through the driven roller

10

b.

The print sheet P is, thus, moved to a printing station in a given positional relation to the conveyer belt

1

. The driven roller

10

a,

as described above, works as the discharge roller which removes charges from the conveyer belt

1

, thereby causing the print sheet P to be separated from the conveyer belt

1

when it reaches the driven roller

10

a.

The print sheet P is next carried to the fixing device

76

.

The print assemblies

68

Y,

68

M,

68

C, and

68

K are identical in structure. The primary distinction between them is the color of toner particles contained therein. Specifically, the print assembly

68

Y stores therein developer containing yellow toner particles to form a yellow toner image on the print sheet P transported by the conveyer belt

1

. The print assembly

68

M stores therein developer containing magenta toner particles to form a magenta toner image on the print sheet P transported by the conveyer belt

1

. The print assembly

68

C stores therein developer containing cyan toner particles to form a cyan toner image on the print sheet P transported by the conveyer belt

1

.

When the print sheet P enters, as shown in

FIG. 1

, the print station at the driven roller

10

b

and passes through the print assemblies

68

Y,

68

M,

68

C, and

68

K, yellow, magenta, cyan, and black toner images are overlaid to form a full color image on the print sheet P. The print sheet P is next transported from the driven roller

10

a

to the heat roller type fixing device

76

in which the full color image is thermally fixed on the print sheet P.

Each of the print assemblies

68

Y,

68

M,

68

C, and

68

K is designed as an image-transferring unit which includes a photosensitive drum

78

, a charging roller (not shown), a developing device (not shown), and a toner cleaner (not shown). Each image-forming unit is installed detachably on a frame

62

.

The conventional tandem color printers wherein a print sheet is transported by a conveyer belt to a print station in which a plurality of image transfer units are placed in line have problems in that a belt drive shaft of a driven roller for a fixing device is heated by radiation of heat from the fixing device located near the belt drive shaft, leading to a local rise in temperature of the conveyer belt, and the conveyer belt is cooled as moving away from the belt drive shaft, which results in a considerable change in temperature of the conveyer belt, thereby causing the speed of the conveyer belt to be changed. The change in speed of the conveyer belt will cause the timing where each of Y, M, C, and K toner images is transferred to the print sheet to be shifted from the timing where the print sheet passes a corresponding one of the image transfer units, thereby resulting in a shift between colors.

Specifically, the main factor of the change in speed of the conveyer belt is thermal expansion of the conveyer belt caused by transfer of the heat from the belt drive shaft.

FIGS. 2 and 3

show a mark writing device

2

and a detecting device

3

. The mark writing device

2

writes marks in the form of electrostatic images on the conveyer belt

1

transporting the print sheet P to which toner images are to be transferred from the print assemblies

68

Y,

68

M,

68

C, and

68

K. The detecting device

3

detects the marks written on the conveyer belt

1

and outputs a signal indicative thereof to a correction circuit

4

, as will be discussed later in FIG.

4

.

The conveyer belt

1

is, as described above, made of a dielectric material such as Polyvinylidene Fluoriden. The application of charges to the conveyer belt

1

through the mark writing device

2

, thus, causes an electrostatic mark to be formed on a charged portion of the conveyer belt

1

. The conveyer belt

1

is moved by the driven roller

10

a.

The driven roller

10

a

is supported by a shaft

12

and turned by a belt motor

14

. The speed of the conveyer belt

1

is regulated by controlling the rotation of the belt motor

14

.

The mark writing device

2

consists of, as shown in

FIG. 2

, a blade-shaped electrode

20

and a charging circuit

22

. The electrode

20

is disposed above the conveyer belt upstream of the detecting device

3

. The charging circuit

22

applies higher voltages to the electrode

20

cyclically in the form of pulses. Specifically, the charging circuit

22

is responsive to pulse signals A, as shown in

FIG. 5

, outputted at regular intervals from a sensor controller

48

of a print engine controller

40

, as will be described later in

FIG. 4

, to apply the higher voltages to the electrode

20

, forming the electrostatic marks on the conveyer belt

1

.

The detecting device

3

consists of a pair of blade-shaped electrodes

30

a

and

30

b

and a pair of high impedance-operational amplifiers

32

a

and

32

b.

The electrodes

30

a

and

30

b

are disposed above the conveyer belt

1

downstream of the mark writing device

2

and connected to the operational amplifiers

32

a

and

32

b,

respectively. Under the principle of a surface potential meter, passage of the electrostatic mark beneath the electrodes

30

a

and

30

b

will cause the current to be produced in the electrodes

30

a

and

30

b

which is, in turn, amplified by the operational amplifiers

32

a

and

32

b

and outputted as a signal B to the sensor controller

48

of the print engine controller

40

.

The correction circuit

4

includes, as shown in

FIG. 4

, a main control circuit

42

which is the part of the print engine controller

40

and a motor controller

44

. The correction circuit

4

determines the speed of the conveyer belt

1

using the signals B inputted to the sensor controller

48

from the detecting device

3

and controls the speed of the belt motor

14

to bring the speed of the conveyer belt

1

into agreement with a target one.

FIG. 4

shows a circuit structure of the image-forming device

60

.

The image-forming device

60

includes a controller

50

and a print engine controller

40

. The controller

50

has an operator panel

52

and communicates with a host computer to separate a color image to be printed into primary color images (i.e., toner images) and to control the whole operation of the image-forming device

60

. The print engine controller

40

controls mechanisms of the image-forming device

60

and includes the main control circuit

42

. The main control circuit

42

forms the part of the correction circuit

4

which measures and controls the speed of the conveyer belt

1

. The print engine controller

40

also includes the motor controller

44

, the photosensitive drum controller

46

, and the sensor controller

48

. The motor controller

44

operates the belt motor

14

and regulates the speed thereof. The photosensitive drum controller

46

controls the speed of the photosensitive drum

78

of each of the print assemblies

68

Y,

68

M,

68

C, and

68

K. The sensor controller

48

outputs the signals A at regular intervals to the charging circuit

22

of the mark writing device

2

and receives the signals B outputted from the operational amplifiers

32

a

and

32

b

of the detecting device

3

.

FIG. 5

shows waveforms of the signal A inputted to the charging circuit

22

of the mark writing device

2

from the sensor controller

48

and the signal B outputted from the operational amplifiers

32

a

and

32

b

. Upon passage of the electrostatic mark formed on the conveyer belt

1

, the electrode

30

a

of the detecting device

3

first produces a current signal Bia, after which the electrode

30

b

produces a current signal Bib at a time interval Ti away from the current signal Bia. In

FIG. 5

, each time interval Ti is defined between rising edges of the current signals Bia and Bib. The correction circuit

4

measures the time interval Ti in response to inputs of the current signals Bia and Bib and determines the speed S

1

of the conveyer belt

1

based on a relation of S

1

=L

1

/Ti where L

1

is a preselected distance between the electrodes

30

a

and

30

b

. Subsequently, the correction circuit

4

compares the speed S

1

and a nominal speed S to determine a difference therebetween and controls the frequency of a pulse signal inputted to the motor controller

44

so as to eliminate the speed difference. Specifically, when the time interval Ti is longer, the correction circuit

4

increases the speed of the conveyer belt

1

, while when the time interval Ti is shorter, the correction circuit

4

decreases the speed of the conveyer belt

1

.

FIG. 6

is a flowchart of a program or sequence of logical steps performed by the image-forming device

60

.

Upon input of a print request signal after the image-forming device

60

is turned on, and a given printer initializing operation is completed, the routine proceeds to step

101

wherein the belt motor

14

is turned on. The routine proceeds to step

102

wherein a nominal phase excitation switching time τ

0

is set in a belt timer built in the motor controller

44

. The routine proceeds to step

103

wherein the rotation of the belt motor

14

is started.

The routine proceeds to step

104

wherein the sensor controller

48

starts to apply the pulse signals A at regular intervals to the electrode

20

of the mark writing device

2

. The routine proceeds to step

105

wherein it is determined whether a rising edge of the current signal Bia outputted from the electrode

30

a

of the detecting device

3

is detected or not. If a NO answer is obtained, then the routine repeats step

105

. Alternatively, if a YES answer is obtained, then the routine proceeds to step

106

wherein a rising edge of the current signal Bib outputted from the electrode

30

b

of the detecting device

3

is detected or not. If a NO answer is obtained, then the routine repeats step

106

. Alternatively, if a YES answer is obtained, then the routine proceeds to step

107

wherein the time interval Ti between the rising edges of the current signals Bia and Bib is determined. The routine proceeds to step

108

wherein a nominal phase excitation switching time τ

1

is determined using an equation of τ

1

=τ

0

×(T

0

/T

I

) where T

I

is a minimal value of a time interval between the current signals Bia and Bib.

The routine proceeds to step

109

wherein the nominal phase excitation switching time τ

1

is set in the belt timer to control the frequency of pulse signals inputted to the motor controller

44

, bringing the speed of the conveyer belt

1

into agreement with the nominal speed (i.e., a target speed). The routine proceeds to step

110

wherein it is determined whether the conveyer belt

1

is stopped or not. If a NO answer is obtained, then the routine returns back to step

105

. Alternatively, if a YES answer is obtained, then the routine proceeds to step

111

wherein the application of the pulse signals A to the electrode

20

of the mark writing device

2

is stopped. The routine proceeds to step

112

wherein the belt motor

14

is turn, off.

The image-forming device

60

also measures the speed of the photosensitive drum

78

of each of the print assemblies

68

Y,

68

M,

68

C, and

68

K in the same manner as described above and brings it into agreement with a target one.

FIG. 7

shows the second embodiment of the invention.

The mark writing device

2

consists of a pair of blade-shaped electrodes

20

a

and

20

b

and charging circuits

22

a

and

22

b,

as shown in FIG.

8

. The electrodes

20

a

and

20

b

are disposed above the conveyer belt upstream of the detecting device

3

. The charging circuits

22

a

and

22

b

apply higher voltages to the electrodes

20

a

and

20

b

cyclically in the form of pulses. Specifically, each of the charging circuits

22

a

and

22

b

is responsive to the pulse signals A, as shown in

FIG. 5

, outputted at regular intervals from the sensor controller

48

of the print engine controller

40

to apply the higher voltages to one of the electrodes

20

a

and

20

b,

forming the electrostatic marks on the conveyer belt

1

.

The detecting device

3

consists of a single blade-shaped electrode

30

and a high impedance-operational amplifier

32

, as shown in FIG.

8

. The electrode

30

is disposed above the conveyer belt

1

downstream of the mark writing device

2

and connected to the operational amplifier

32

. The passage of each electrostatic mark beneath the electrode

30

will cause the current to be produced which is, in turn, amplified by the operational amplifier

32

and outputted as the signal B to the sensor controller

48

of the print engine controller

40

. The sensor controller

48

detects rising edges of two consecutive signals B, that is, the current signals Bia and Bib produced by two consecutive marks formed on the conveyer belt

1

by the electrodes

20

a

and

20

b

to determine, like the first embodiment, the time interval Ti between the current signals Bia and Bib.

The correction circuit

4

measures the time interval Ti in response to inputs of the current signals Bia and Bib and determines the speed S

2

of the conveyer belt

1

based on a relation of S

2

=L

2

/Ti where L

1

is a preselected distance between the electrodes

20

a

and

20

b

of the mark writing device

2

. Subsequently, the correction circuit

4

compares the speed S

2

and the nominal speed S to determine a difference therebetween and controls the frequency of a pulse signal inputted to the motor controller

44

so as to eliminate the speed difference.

Other arrangements and operations are identical with those of the first embodiment, and explanation thereof in detail will be omitted here.

FIG. 9

shows the mark writing device

2

according to the third embodiment of the invention which is different from the one shown in

FIGS. 7 and 8

in that electrostatic marks are formed on the conveyer belt

1

at a given time interval therebetween Other arrangements are identical, and explanation thereof in detail will be omitted here.

The charging circuits

22

a

and

22

b

are responsive to the pulse signals A

1

and A

2

outputted from the sensor controller

48

of the print engine controller

40

to apply the higher voltages to the electrodes

20

a

and

20

b,

respectively. Specifically, a train of the pulse signals A

1

, as shown in FIG.

10

(

a

), are inputted to the electrode

20

a

to form first electrostatic marks on the conveyer belt

1

, while a train of the pulse signals A

2

are inputted to the electrode

20

b

at a preselected time interval away from the train of the pulse signals A

1

to form second electrostatic marks on the conveyer belt

1

. The time interval between the pulse signals A

1

and the pulse signals A

2

is so determined that a first one of the second electrostatic marks is formed in registration with a first one of the first electrostatic marks.

The waveforms of the signals B produced by the electrode

30

of the detecting device

3

are shown in FIG.

10

(

b

). In FIG.

10

(

b

), a first one of the second electrostatic marks is in registration with a first one of the first electrostatic marks. In this case, the signals B produced by the first and second electrostatic marks overlap with each other to have a waveform as shown on the leftmost side of the FIG.

10

(

b

). The central waveform in FIG.

10

(

b

) is produced when a shift in registration between the first and second electrostatic marks becomes the greatest. The rightmost waveform is produced when the first and second electrostatic marks are formed in registration. Specifically, this embodiment measures a phase shift between the signals B through the Fourier transform to determine a difference between an actual speed of the conveyer belt

1

and a nominal speed thereof. This difference may alternatively be determined based on a shift between the centers of the signals B.

FIG. 11

is a flowchart of a program performed by the image-forming device

60

of the third embodiment.

Upon input of a print request signal after the image-forming device

60

is turned on, and a given printer initializing operation is completed, the routine proceeds to step

201

wherein the belt motor

14

is turned on. The routine proceeds to step

202

wherein a nominal phase excitation switching time τ

0

is set in the belt timer built in the motor controller

44

. The routine proceeds to step

203

wherein the rotation of the belt motor

14

is started.

The routine proceeds to step

204

wherein the sensor controller

48

starts to apply a train of the pulse signals A

1

to the electrode

20

a

of the mark writing device

2

. Simultaneously, the routine proceeds to step

205

and waits a preselected time of L

2

/V

0

where L

2

is the distance between the electrodes

20

a

and

20

b,

and V

0

is the speed of the conveyer belt

1

. The routine proceeds to step

206

wherein a train of the pulse signals A

2

to the electrode

20

b

of the mark writing device

2

.

After steps

204

and

206

, the routine proceeds to step

207

wherein the detecting device

3

picks up the current signal B through the electrode

30

. The routine proceeds to step

208

wherein a speed difference is determined using the Fourier transform, and the nominal phase excitation switching time τ

i

is determined in a manner as described below.

Assuming a sensor output waveform D(t) is sampled from time t

1

to time t

2

in a cycle Ts, following conversions are performed.

t=t

1

→

x=

0

t=t

2

→

x

=(

t

2

−

t

1

)/

Ts−n

D

(

t

)→

d

′(

x

)

If a component to be added to a sine wave is defined as S, and a component to be added to a cosine wave is defined as C, we obtain

S = \sum_{X = 1}^{n} D^{'} (x) \sin (2 π x / n)

C = \sum_{X = 1}^{n} D^{'} (x) \cos (2 π x / n)

Thus, from the cosine wave, the phase difference may be determined as φ=ArcTan(S/C)[radian].

Next, using the thus determined phase difference φ, the speed deviation Δv/V

0

is determined. The cycle of the pulse signals A

1

is so set that the center-to-center distance L between the mark No.

1

and the mark No.

6

, as shown in FIG.

10

(

a

), produced by the pulse signals A

1

is equal to the center-to-center distance between the mark No.

1

and the mark No.

7

produced by the pulse signals A

2

. If a phase difference between the pulse signals A

1

and A

2

is zero (0), the mark No.

4

produced by one of the pulse signals A

2

appears at an intermediate location between the marks No.

3

and No.

4

produced by the pulse signals A

1

. Specifically, the mark No.

4

produced by the pulse signal A

2

is shifted from the marks No.

3

and No.

4

produced by the pulse signals A

1

, so that the signal B shows the waveform closest to a −cosine wave. Alternatively, if the mark No.

4

produced by the pulse signal A

2

is in registration with the mark No. produced by the pulse signal A

1

, the signal B shows the waveform close to a cosine wave.

Comparing between the former and the latter case, it is found that the movement of the mark produced by the pulse signal A

2

by a distance (=L/6/2) of half a distance (=L/6) between the marks No.

3

and No.

4

produced by the pulse signals A

1

causes the signal B to have ±cosine waveform components, resulting in a phase shift of 180°=π[rad]. Thus, a relation between the phase shift and a shift in registration between the marks is expressed by δ=L/6/2/π(φ−π). Since when the shift in registration between the marks is zero (0), the signal B has, as described above, −cosine waveform, the term (φ−π) is provided for conversion of the phase difference derived based on the cosine waveform.

The shift in registration between the marks occurs during movement of the conveyer belt

1

by the distance L

2

. The speed deviation may, thus, be expressed by Δv/V

0

=L/6/2/π(φ−π) /L

2

. After the speed deviation is calculated using this equation, the nominal phase excitation switching time τ

i

is determined using the relation of τ

i

=τ

0

×(1+Δv/V

0

)

After step

208

, the routine proceeds to step

209

wherein the nominal phase excitation switching time τ

1

is set in the belt timer to control the frequency of pulse signals inputted to the motor controller

44

, bringing the speed of the conveyer belt

1

into agreement with the nominal speed (i.e., a target speed). The routine proceeds to step

210

wherein it is determined whether the conveyer belt

1

has been stopped or not. If a NO answer is obtained, then the routine returns back to steps

205

and

204

. Alternatively, if a YES answer is obtained, then the routine proceeds to step

211

wherein the application of the pulse signals A

1

and A

2

to the electrodes

20

a

and

20

b

of the mark writing device

2

is stopped. The routine proceeds to step

212

wherein the belt motor

14

is turned off.

The image-forming device

60

also measures the speed of the photosensitive drum

78

of each of the print assemblies

68

Y,

68

M,

68

C, and

68

K in the same manner as described above and brings it into agreement with a target one.

FIG. 12

shows an image-forming device according to the fourth embodiment of the invention which is different from the first embodiment in that the position of each of the print assemblies

68

Y to

68

K) is changed to correct a location where each toner image is transferred onto the print sheet P carried by the conveyer belt

1

. The same reference numbers as employed in the first embodiments refer to the same parts, and explanation thereof in detail will be omitted here.

The print assemblies

68

Y,

68

M,

68

C, and

68

K are supported by the positioning members

80

Y,

81

Y,

80

M,

80

M

1

,

80

C,

81

C,

80

K, and

81

K. The positioning members

80

Y,

81

Y,

80

M,

81

M,

80

C,

81

C,

80

K, and

81

K are installed on a device casing (not shown) so as to be movable only in a lateral direction, as viewed in the drawing. Portions of the positioning members

80

Y to

81

K opposite portions thereof supporting the print assemblies

68

Y to

68

K are provided with gears connected mechanically to print assembly drive motors

84

Y,

85

Y,

84

M,

85

M,

84

C,

85

C,

84

K, and

85

K through intermediate gears

82

Y,

83

Y,

82

M,

83

M,

82

C,

83

C,

82

K, and

83

K.

In operation, when the print assembly drive motor

84

Y is rotated in a counterclockwise direction, as viewed in the drawing, and the print assembly drive motor

85

Y is rotated in a clockwise direction through the same angular steps as those of the print assembly drive motor

84

Y, the positioning members

80

Y and

81

Y are moved in parallel in the left direction to shift the print assembly

68

Y away from the print assemblies

68

M,

68

C, and

68

K. In other words, intervals between the print assembly

68

Y and the other three print assemblies

68

M,

68

C, and

68

K are increased, respectively. Conversely, when the print assembly drive motor

84

Y is rotated in the clockwise direction, and the print assembly drive motor

85

Y is rotated in the counterclockwise direction, the positioning members

80

Y and

81

Y are moved in parallel in the right direction to shorten the intervals between the print assembly

68

Y and the other three print assemblies

68

M,

68

C, and

68

K.

The image-forming device of this embodiments has a circuit structure, as shown in

FIG. 13

, which is designed to determine a correction value for correcting a locational shift of each of the print assemblies

68

Y to

68

K. The image-forming device includes a correction mark forming circuit

24

, a correction value determining circuit

400

, a correction value storage

402

, and an image shift correcting circuit

404

. The functions of these circuits are realized by a sensor processing MPU (not shown) installed in the print engine controller

40

and an MPU (not shown) installed in controller

50

.

For determining a correction value used in correcting an image shift, the correction mark forming circuit

24

forms two series of color-mixed marks on the conveyer belt

1

using a yellow LED array

28

Y, a magenta LED array

28

M, cyan LED array

28

C, and a black LED array

28

K which are activated by an LED driver

26

. The series of color-mixed marks are transferred, respectively, to lateral ends of the conveyer belt which correspond to a start and an end portion of a main scanning line extending perpendicular to a traveling direction of the conveyer belt. Two sensors

34

a

and

34

b

are disposed near the conveyer belt to detect the series of color-mixed marks, respectively, and output signals indicative thereof to the correction value determining circuit

400

. The image shift correction in this embodiment is achieved by determining correction values for correcting image shifts of yellow (Y), magenta (M), and cyan (C) images from a black (B) image which presents the strongest contrast to the other Y, M, and C images.

Specifically, the correction mark forming circuit

24

stores therein print information for the series of color-mixed marks having patterns, as will be described later, and activates the four LED arrays

28

Y,

28

M,

28

C, and

28

K in parallel to transfer or form on the conveyer belt a K/C mixed mark, a K/M mixed mark, and a K/Y mixed mark each of which is made up of two different color electrostatic images overlaid with a slight shift. The print information may be formed by bit-mapped patterns or alternatively be prepared as vector information. In case of the vector information, the LED driver

26

expands it to produce bit-mapped data for writing the color-mixed marks.

The correction value determining circuit

400

reads outputs of the sensors

34

a

and

34

b

and determines the phase of a brightness pattern of each series of the K/C mixed mark, the K/M mixed mark, and the K/Y mixed mark to calculate correction values for correcting image shifts of the C image, the M image, and the Y image from the K images in the K/C mixed mark, the K/M mixed mark, and the K/Y mixed mark, respectively. Specifically, the correction value determining circuit

400

determines a first correction value ΔX corresponding to a shift of each of the C image, the M image, and the Y image from the K image in a direction of the main scanning line, a second correction value ΔY corresponding to a shift of each of the C image, the M image, and the Y image from the K image in a direction of a sub-scanning line, and a third correction value ΔZ (skew correction value) corresponding to a shift of each of the C image, the M image, and the Y image from the K image in an oblique direction.

In order to increase the accuracy of calculating the first, second, third correction values ΔX, ΔY, and ΔZ based on the brightness patterns of the K/C mixed mark, the K/M mixed mark, and the K/Y mixed mark detected by the sensors

34

a

and

34

b,

the correction value determining circuit

400

performs a Fourier transform and derives Fourier coefficients a and b to determine the phase φ for determining the correction values ΔX, ΔY, and ΔZ.

The correction values derived in the correction value determining circuit

400

are listed in a cyan table, a magenta table, and a yellow table stored in the correction value storage

402

.

The image shift correcting circuit

404

performs an address conversion for image shift correction using the correction values stored in the correction value storage

402

when image data in image memories

406

Y,

406

M,

406

C, and

406

K are expanded.

FIGS.

14

(

a

) to

14

(

d

) show the principle of the image shift correction based on the first, second, and third correction values ΔX, ΔY, and ΔZ derived in the correction value determining circuit

400

.

A detected locational shift of a correction target print line

100

is converted into a locational shift in a bit map memory space

110

, as shown in FIG.

14

(

a

), defined by a pixel matrix made up of a horizontal array of pixels extending in the main scanning direction x and a vertical array of pixels extending in the sub-scanning direction y. In the shown case, an optimum print line

102

is already known, and the correction target print line

100

is defined in relation to the optimum print line

102

. Specifically, using the first, second, and third correction values ΔX, ΔY, and ΔZ, the correction target print line

100

is defined in the bit map memory space

110

and converted into image data to produce, as shown in FIG.

14

(

b

), locational shift data

104

a,

104

b,

and

104

c.

Next, correction data

106

a,

106

b,

and

106

c,

as shown in FIG.

14

(

c

), are determined. The correction data

106

a

to

106

c

are equivalent to the locational shift data

104

a,

104

b,

and

104

c,

that is, the correction target print line

100

which is reversed in a minus direction to be symmetrical with respect to the optimum print line

102

and shifted to the minus direction of the main scanning (i.e., the left direction as viewed in the drawings) by the first correction value ΔX.

Using the thus determined correction data

106

a

to

106

c,

the image shift correcting circuit

404

activates the LED array

20

Y,

28

M,

28

C, and

28

K to form an image shift-corrected print line

108

, as shown in FIG.

14

(

d

), which matches up with the optimum print line

102

.

In this embodiment, a pitch P

1

of the K images of the K/C mixed mark, the K/M mixed mark, and the K/Y mixed mark arrayed on the conveyer belt is different from a pitch P

2

of other arrayed images, e.g., the C images arrayed in a lengthwise direction of the conveyer belt so that the brightness pattern of an array of the marks may show a sine wave when the shift in registration between the marks is zero, but the present invention is not limited to the same. The marks may be arrayed in any form as long as a train of the pulse signals A

2

, as shown in FIG.

10

(

a

), is shifted from a train of the pulse signals A

1

, and a brightness pattern is derived which has a single maximum or minimum value as a singular point for detecting a phase conversion in a range of the leading and trailing marks defined by ±π, as determined in step

208

of FIG.

11

.

For example, pitches of the K images and C images may be determined at random or as a function of a brightness pattern which has a minimum or maximum value in a range of ±π.

The above embodiment improves the accuracy of calculating the correction values by determining the phase φ using the discrete Fourier transform based on the fact that a brightness pattern of a two-color mixed image represents a sine curve, but may determine each image shift based on the level of a brightness pattern of scattered light based on the fact that the image shift can be determined by identifying the mark corresponding to a minimum value Lmin in the brightness pattern defined by levels of scattered light.

The image-forming device of this embodiment determines an actual speed of the conveyer belt

1

in the same manner as in the first embodiment and changes the locations of the print assemblies

68

Y,

68

M,

68

C, and

68

K through the positioning members

80

Y,

81

Y,

80

M,

81

M,

80

C,

81

C,

80

K, and

81

K to control the time the print sheet P passes each of the print assemblies

68

Y to

68

K, thereby correcting the locations of the images to be transferred to the print sheet P on the conveyer belt to eliminate a shift between colors.

The K images of the K/C mixed mark, the K/M mixed mark, and the K/Y mixed mark may also be formed on each of the photosensitive drums

78

to control the speed thereof for minimizing the image shifts.

While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.

For example, the electrodes

20

,

20

a,

20

b,

22

,

22

a,

22

b,

30

,

30

a,

and

30

b

are disposed above an upper surface of the conveyer belt

1

in the above embodiments, but may alternatively be installed beneath a back surface of the conveyer belt

1

, thereby keeping fee from foreign substances such as toner or paper powders to the electrodes. Two detecting devices

3

may be installed one on each side of the conveyer belt

1

. In this case, the accuracy of detecting the marks printed on the conveyer belt

1

is improved by using an average value of the current signals B outputted from the two detecting devices

3

.

The present invention is not limited to the speed correction of the conveyer belt

1

and the photosensitive drums

78

only. For example, it is possible to correct the speed of an intermediate transfer belt to which color images formed by the photosensitive drums are transferred in the same manner as described above.

Number	Date	Country
0 816 930	Jan 1998	EP
0 866 603	Sep 1998	EP
5-197244	Aug 1993	JP
6-144631	May 1994	JP
8-211693	Aug 1996	JP
9-62047	Mar 1997	JP

Color shift correcting structure of image forming apparatus

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

US Referenced Citations (1)

Foreign Referenced Citations (6)

Non-Patent Literature Citations (1)