Sheet detecting apparatus with correction means for sheet surface condition and sheet thickness

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet detecting apparatus, a sheet conveying apparatus and an image forming apparatus to improve detecting accuracy of a sheet.

2. Related Background Art

[Background Art 1]

In the past, as a sheet sensor (sheet position detecting means)

1

used for conveyance control of a sheet S in an image forming apparatus and the like, for example, as shown in

FIG. 27

, a sensor constituted by a photo-interrupter

3

and a mechanical flag

2

contacted with the sheet S conveyed in a sheet conveying direction indicated by the arrow is well known.

[Background Art 2]

Further, in such a sheet sensor of contact type, although disadvantages such as chattering and time lag upon contacting of the sensor with the sheet may occur, when the conveying speed of the sheet is relatively low, since such disadvantages do not arise any problem, such a sheet sensor has been used in many apparatuses.

However, when the above-mentioned disadvantages arise serious problem or when the sheet conveying speed is high, detection of the sheet S is effected by using a sensor

4

of reflection type comprised of a light emitting element

5

and a light receiving element

6

as shown in FIG.

28

.

[Background Art 3]

In an image forming apparatus such as a copying machine handling a sheet, in many cases, exclusive sheets (transfer materials) recommended by the maker of the image forming apparatus are used. However, in the present days in which various user needs exists, it has been requested that various kinds of sheets be used as transfer materials for the copying machines, other than the sheets recommended by the maker.

In the past, regarding such various sheets, the sheet has been conveyed under sheet conveyance control (for example, conveying timing upon registration and/or conveying speed) similar to the exclusive sheet. Alternatively, for example, when a thick sheet is used, in some cases, user or service man has effected the setting of the thick sheet with respect to a predetermined sheet tray through an operating portion of the copying machine to change the conveying timing.

However, in the above-mentioned background art 1, if any play is generated between the mechanical flag

2

and a shaft as a fulcrum

7

for the flag for a long term use or if an operating point is changed in dependence upon a conveying condition (for example, position along a thickness direction in a conveying path) of the sheet to be detected, error of sheet detecting accuracy of about±2 to 3 mm may occur.

In the sensor

4

of reflection type in the background art 2, depending upon the kind of the sheet, detecting error of about 1.3 mm occurred in the range of a detecting point (fully described in connection with first embodiment shown in FIG.

5

).

Further, in the sensor of reflection type, detecting error due to a sheet passing position (distance between the sensor of reflection type and the sheet) may cause serious problems.

FIGS. 29A and 29B

are explanatory views showing a relationship between a height direction distance with respect to the sheet (this height direction distance can be considered as deviation in the sheet thickness direction when the sheet is passed through the conveying path or as change in distance with respect to the sensor caused by the thickness of the sheet itself) and output of a sheet position detecting sensor, where

FIG. 29A

shows output OP of the sheet position detecting sensor with respect to the height direction distance h, and

FIG. 29B

shows outputs OP of the sheet position detecting sensor with respect to a passing time t when the sheet is passed through the sheet position detecting sensor with heights h

1

and h

2

.

As shown in

FIGS. 29A and 29B

, for example, when it is assumed that a timing that the sheet reaches the sheet position detecting sensor corresponds to a timing that the output OP of the sheet position detecting sensor becomes

01

, the arrival detection timing becomes t

1

when the sheet height is h

1

and becomes t

2

when the sheet height is h

2

.

In many cases, although such detecting errors normally do not arise any serious problem, when the sheet conveying speed is requested to be further increased or when the conveying accuracy is requested to be further improved, such errors cannot be neglected.

In the above-mentioned background art 3, regarding various kinds of sheets, when the sheet is conveyed at the uniform conveying timing, since a load and a friction force acting on conveying means are differentiated depending upon the kind of sheet, poor sheet conveyance may occur.

Particularly, in a process for forming a loop in a leading end portion of the sheet at a registration portion to effect correction of skew-feed of the sheet and registration of the leading end of the sheet with an image, due to dispersion in sheet stopped positions depending upon the difference in sheet kinds, loop amounts in the sheets are differentiated, with the result that function of the registration means may be worsened and sheet jam may occur.

Further, when the user or service man effects the setting of the thick sheet with respect to the predetermined sheet tray through the operating portion of the copying machine to change the conveying timing, not only operability and workability may be worsened but also it may be difficult to cope with various sheets.

SUMMARY OF THE INVENTION

The present invention aims to eliminate the above-mentioned conventional drawbacks and an object of the present invention is to make higher accurate sheet detection possible thereby to improve reliability of stable sheet conveyance under high speed sheet conveyance and small sheet-to-sheet distance control by reducing a detecting error due to difference in sheet kind and by reducing a detecting error due to difference in sheet conveying position (distance with respect to a sensor of reflection type), when the sensor of reflection type is used as means for detecting the fact that a sheet being conveyed reaches a predetermined position.

Another object of the present invention is to improve reliability of stable sheet conveyance by making proper sheet conveyance control possible for various sheets without worsening operability and workability of users and by detecting the fact that a sheet is deviated from an allowable value permitting stable sheet conveyance and by effecting sheet conveyance control on the basis of a detecting result.

To achieve the above object, a sheet detecting apparatus according to the present invention comprises sheet position detecting means for detecting a fact that a sheet being conveyed in a conveying path reaches a predetermined position, sheet surface detecting means for detecting a surface condition of the sheet, and correction means for correcting a detection result of the sheet position detecting means in accordance with the surface condition of the sheet detected by the sheet surface detecting means.

Further, the present invention may provide a sheet detecting apparatus comprising sheet position detecting means for detecting a fact that a sheet being conveyed in a conveying path reaches a predetermined position, sheet height detecting means disposed in the vicinity of the sheet position detecting means and adapted to detect a distance between the sheet position detecting means and the sheet in a direction perpendicular to a sheet conveying plane, and correction means for correcting a detection result of the sheet position detecting means in accordance with the distance detected by the sheet height detecting means.

Further, the present invention may provide a sheet detecting apparatus comprising sheet position detecting means for detecting a fact that a sheet being conveyed in a conveying path reaches a predetermined position, sheet surface detecting means for detecting a surface condition of the sheet, sheet thickness detecting means for detecting a thickness of the sheet, and correction means for correcting a detection result of the sheet position detecting means in accordance with a detection result of the surface condition of the sheet detected by the sheet surface detecting means and the thickness of the sheet.

The sheet position detecting means includes a light emitting element for illuminating light onto the sheet being conveyed, and a light receiving element for receiving light reflected from the sheet, and, when an output value of the light receiving element for converting the received light into an electric signal exceeds a predetermined threshold level, a sheet detection signal is emitted, and the correction means preferably correct a sheet detecting timing of the sheet position detecting means.

It is preferable that timer means counted in association with the output of the sheet position detecting means is provided, and the correction means correct a set value of the timer means.

Further, it is preferable that the sheet position detecting means emit a sheet detection signal when the output value of the light receiving element for converting the received light into the electric signal exceeds the predetermined threshold level, and the correction means alters the threshold level.

Further, it is preferable that the sheet surface detecting means includes a light emitting element for illuminating light onto the sheet, polarization separation means for separating light reflected from the sheet into two polarization components, and light receiving elements for receiving the separated two polarization components respectively, and the surface condition of the sheet is detected on the basis of a calculation value obtained from output values of the two polarization components.

The calculation value may be a value obtained by dividing a difference between the two polarization components by sum of the two polarization components.

Judging means for judging as sheet judgement error if the calculation value is deviated from a predetermined range may be provided.

The sheet surface detecting means may detect reflectance with respect to the sheet.

It is preferable that the sheet surface detecting means are disposed at an upstream side of the sheet position detecting means in the conveying path.

In a sheet conveying apparatus according to the present invention, the above-mentioned sheet detecting apparatus is provided in the conveying path through which the sheet is conveyed by conveying means, and conveyance control means for controlling the conveying means on the basis of the corrected detection result of the sheet position detecting means is provided.

The conveyance control means may alter conveyance distance between the sheets given by the conveying means.

The conveyance control means may alter a sheet conveying speed of the conveying means.

The conveyance control means may effect control to alter a sheet stopping timing when the sheet is stopped by the conveying means.

It is preferable that the sheet stopping timing is a sheet stopping operation of the conveying means when a loop is formed in the sheet by abutting the leading end of the sheet against a registration member (disposed in the conveying path) by the conveying means or a conveyance starting operation for conveying the sheet from the registration member.

Further, in a sheet conveying apparatus according to the present invention, the above-mentioned sheet detecting apparatus is provided in the conveying path through which the sheet is conveyed by conveying means, and a sheet stopping timing associated with a sheet stopping operation of the conveying means or a conveyance starting operation (for conveying the sheet from the registration member) when a loop is formed in the sheet by abutting the leading end of the sheet against a registration member (disposed in the conveying path) by the conveying means is altered on the basis of the corrected detection result of the sheet position detecting means and kind and thickness of the sheet detected by the sheet surface detecting means and the sheet thickness detecting means and loop forming amount control information corresponding to pre-set kind and thickness of sheet.

The sheet stopping timing may be altered so that the loop forming amount becomes constant.

It is preferable that the sheet stopping timing associated with the sheet stopping operation of the conveying means or a conveyance starting operation (for conveying the sheet from the registration member) is altered so that the loop forming amount control information corresponding to pre-set kind and thickness of sheet becomes loop forming amount corresponding to sheets having different kind or thickness.

An image forming apparatus according to the present invention comprises the above-mentioned sheet conveying apparatus, and image forming means for forming an image on the sheet conveyed by the sheet conveying apparatus.

With this arrangement, sheet detecting accuracy can be enhanced, and the reliability of stable sheet conveyance under high speed sheet conveyance and small sheet-to-sheet distance control can be achieved.

Further, the proper sheet conveyance control for various sheets can be performed, thereby achieving the stable sheet conveyance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a block diagram of a control system according to a first embodiment of the present invention;

FIG. 2

is a sectional view of an image forming apparatus;

FIG. 3

is a view showing an arrangement of an operating portion;

FIGS. 4A

,

4

B and

4

C are views showing message displays;

FIGS. 5A and 5B

are views showing a property of a sheet position detecting sensor;

FIG. 6

is a view showing a sheet surface detecting sensor;

FIGS. 7A and 7B

are views showing an output calculation result of the sheet surface detecting sensor;

FIG. 8

is a view for explaining correction values corresponding to kinds of sheets;

FIGS. 9A and 9B

are a flowchart and a table for correction control respectively;

FIG. 10

is a flowchart for correction control of a registration loop amount;

FIGS. 11A and 11B

are views showing a property of a sheet position detecting sensor;

FIG. 12

is a view for explaining threshold levels;

FIGS. 13A and 13B

are a flowchart and a table for correction control for changing the threshold level respctively;

FIG. 14

is a sectional view of an image forming apparatus according to a third embodiment of the present invention;

FIG. 15

is a block diagram of a control system of the image forming apparatus according to the third embodiment;

FIG. 16

is a constructural view showing various detecting sensors;

FIG. 17

is a constructural view of a sheet position detecting sensor;

FIG. 18

is a constructural view of a sheet surface detecting sensor;

FIG. 19

is a constructural view of a sheet thickness detecting sensor;

FIG. 20

is a constructural view of a sheet height detecting sensor;

FIGS. 21A and 21B

are views showing an output calculation result of the sheet surface detecting sensor;

FIGS. 22A

,

22

B,

22

C and

22

D are views output of the sheet height detecting sensor and correction thereof;

FIG. 23

is a flowchart for sheet stopping control in registration;

FIG. 24

is a flowchart for sheet stopping control according to a fourth embodiment of the present invention;

FIG. 25

is a view showing correction values of a sheet stopping timing according to a fifth embodiment of the present invention;

FIG. 26

is a flowchart for setting the sheet stopping timing according to the fifth embodiment;

FIG. 27

is a view for explaining a conventional sheet position detecting sensor;

FIG. 28

is a view for explaining a sheet position detecting sensor; and

FIGS. 29A and 29B

are views for explaining outputs of the sheet position detecting sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

Now, a first embodiment of the present invention will be explained with reference to the accompanying drawings.

FIG. 1

is a block diagram showing an example of a control system of an image forming apparatus to which the present invention can be applied. In

FIG. 1

, a control circuit (controller)

1004

acting as correction means includes a CPU

1004

a

, a ROM

1004

b

and a RAM

1004

c

and serves to control a copying sequence on the basis of program stored in the ROM

1004

b.

A thermistor

302

serves to detect a surface temperature of a fixing roller

144

(FIG.

2

), and a value A/D-converted by an A/D converter

301

is inputted to the control circuit

1004

. The control circuit

1004

effects control on the basis of a detection value of the thermistor

302

so that the surface temperature of the fixing roller

144

becomes a predetermined value.

A sheet surface detecting sensor

312

comprises an S-polarized light receiving element

312

-

1

and a P-polarized light receiving element

312

-

2

, and a value A/D-converted by the A/D converter

301

is inputted to the control circuit

1004

. The control circuit

1004

performs calculation on the basis of detection values of the S-polarized light receiving element

312

-

1

and the P-polarized light receiving element

312

-

2

, thereby detecting a sheet surface, i.e., a sheet kind. Detailed explanation will be made later.

A sheet position detecting sensor

313

acts as sheet position detecting means, and a value A/D-converted by the A/D converter

301

is inputted to the control circuit

1004

. The control circuit

1004

performs calculation on the basis of a detection value of the sheet position detecting sensor

313

and a correction value in accordance with the sheet kind detected by the sheet surface detecting sensor

312

, thereby detecting an edge of the sheet. The correction calculation will be fully described later.

A high-voltage controlling portion

303

serves to control charging systems such as a primary charger

106

and a transfer charger

110

and a high-voltage unit

304

for applying predetermined potential to a developing device

109

and the like.

A motor controlling portion

305

serves to control driving of various stepping motors and a motor

306

such as a main drive motor.

A DC load controlling portion

307

serves to control driving of a solenoid for a pick-up roller

116

, a clutch for registration rollers

120

and a fan.

Sensors

308

such as a sheet jam detecting sensor and the like are connected to the control circuit

1004

. An AC driver

1000

serves to control AC load

309

such as an original illumination lamp

103

and AC power supply for a fixing heater

310

. Further, it serves to detect abnormality of the original illumination lamp

103

and the fixing heater

310

and to turn off a main switch

1001

having a shut-off function.

A power supply

1100

serves to supply DC power source to the control circuit

1004

, and commercial power source is inputted from an input power supply plug

311

to the power supply

1100

through the main switch

1001

and a door switch

1007

.

A paper deck

124

is a sheet feeding device for increasing the number of stacked sheets, a feeder

200

is an auto original feeder for automatically setting a plurality of originals, and a sorter

250

is a sorting device for sorting discharged sheets.

An operating portion

600

includes a touch panel

703

, an LCD

701

and a back light

702

and is connected to the control circuit

1004

.

FIG. 2

is a sectional view showing an example of an image forming apparatus according to the present invention. In

FIG. 2

, the circulating automatic original feeding device or feeder (referred to an “RDF” hereinafter)

200

for automatically feeding the originals and the sorting device or sorter

250

are detachably combined with a main body

100

of the image forming apparatus.

In

FIG. 2

, the image forming apparatus comprises an original glass stand (original stocking plate)

101

, and an optical system (image reading means)

102

including an exposure lamp

103

for illuminating the original, a scanning mirror, a lens

143

and a motor

104

. The original is illuminated by the exposure lamp

103

while effecting the scanning by the motor

104

, and light reflected from the original is illuminated onto a photosensitive drum

105

through the scanning mirror and the lens

143

.

Around the photosensitive drum

105

, there are disposed a primary charger

106

, a blank exposure unit

107

, a potential sensor

108

, a developing device

109

, a transfer charger

110

, a separating charger

111

, and a cleaning device

112

. All of these elements including the photosensitive drum

105

constitute image recording means.

The photosensitive drum

105

is rotated in a direction indicated by the arrow in

FIG. 2

by means of a main motor

113

and is corona-charged by the primary charger

106

. When the light reflected from the original in the optical system

102

is illuminated on the photosensitive drum, an electrostatic latent image is formed on the drum. The electrostatic latent image is developed by the developing device

109

to be visualized as a toner image.

On the other hand, after a leading end of the sheet fed from an upper cassette

114

or a lower cassette

115

by a sheet feed roller

118

or

119

via a pick-up roller

116

or

117

to the main body

100

is aligned with a leading end of the toner image by the registration rollers

120

, the sheet is fed to the photosensitive drum

105

. Then, the toner image is transferred onto the sheet by the transfer charger

110

.

After the transferring, the sheet is separated from the photosensitive drum

105

by the separating charger

111

, and the separated sheet is directed, by a conveying belt

121

, to a fixing device

122

, where the toner image is fixed to the sheet by heat and pressure. Thereafter, the sheet is discharged out of the main body

100

by a pair of discharge rollers

123

. On the other hand, the surface of the photosensitive drum

105

is cleaned by the cleaning device

112

.

Within the main body

100

, there is provided a paper deck

124

capable of containing four thousands (4000) sheets, for example. A lifter

125

of the paper deck

124

is lifted in accordance with the amount of the sheets so that an uppermost sheet always abuts against the sheet feeding roller

126

. Further, a multi-manual feed tray

150

through which the operator can perform manual sheet feed is disposed above the paper deck

124

.

Further, in

FIG. 2

, a sheet discharge flapper

127

serves to switch between a path for both-sided recording or multi-recording and a path for sheet discharge (sorter

250

). The sheet discharged from the pair of discharge rollers

123

is switched toward the path for both-sided recording or multi-recording by the sheet discharge flapper

127

. A lower conveying path

128

serves to turn over the sheet discharged from the pair of discharge rollers

123

through a surface reversing path

129

and to direct the sheet to a re-feed tray

130

.

A multi flapper

131

serves to switch between the path for both-sided recording and the path for multi-recording, and, by inclining the flapper toward the left, the sheet is directly directed to the lower conveying path

128

without passing through the surface reversing path

129

.

A sheet feeding roller

132

serves to feed the sheet toward the photosensitive drum

105

through a path

133

. A pair of discharge rollers

134

are disposed in the vicinity of the sheet discharge flapper

127

and serves to discharge the sheet switched toward the discharge side by the sheet discharge flapper

127

out of the image forming apparatus.

In the both-sided recording (both-sided copying) and the multi-recording (multi-copying), the sheet discharge flapper

127

is lifted, so that the copied sheet is contained in the re-feed tray

130

through the lower conveying path

128

and the surface reversing path

129

in a state that the surfaces of the sheet are reversed or turned over.

In this case, in the both-sided recording, the multi flapper

131

is inclined to the right, and, in the multi-recording, the multi flapper

131

is inclined to the left. In the next both-sided recording and the multi-recording, the sheets contained in the re-feed tray

130

are directed one by one from a lowermost sheet to the pair of registration rollers

120

of the main body by the sheet feeding roller

132

through the path

133

.

When the sheet is discharged from the main body while being turned over, the sheet discharge flapper

127

is lifted and the multi flapper

131

is inclined to the right, so that the copied sheet is conveyed to the surface reversing path

129

, and, after a trailing end of the sheet passes through a first feed roller

140

, the sheet is conveyed toward a second feed roller

141

by a reversing roller

142

and is discharged out of the image forming apparatus while being turned over by the pair of discharge rollers

134

. Incidentally, the sheet conveying paths include a plurality of sensors (not shown) which perform sheet jam detection such as detection (delay jam detection) for detecting the fact that the sheet does not reach till a predetermined timing and detection (trap jam detection) for detecting the fact that the sheet does not leave the sensor till a predetermined timing. If the sheet jam is detected, after a predetermined operation, the apparatus is stopped and predetermined sheet jam display is effected.

The sheet surface detecting sensor

312

includes the S-polarized light receiving element and the P-polarized light receiving element, as will be described later. On the basis of detection value of the S-polarized light receiving element and the P-polarized light receiving element, calculation is effected, thereby detecting the sheet kind through sheet surface detection.

The detection value of the sheet position detecting sensor

313

is corrected on the basis of the sheet kind detected by the sheet surface detecting sensor

312

, thereby detecting the leading end of the sheet.

FIG. 3

shows an arrangement of an operating portion

600

provided on the main body

100

.

In

FIG. 3

, the operating portion includes a reset key

601

used when a copy mode is selected to a standard mode, a stop key

602

used when the copying is interrupted or stopped, a start key

603

used when the copying is started, a ten-key

604

used for setting the number of copies and inputting numerical value, and a clear key

605

used when the copy number is selected to one (1) sheet and when the numerical value inputted from the ten-key is cleared.

A personal identification number key

606

is used when a personal identification number mode is set. A pre-heating key

607

is used when a pre-heating mode is set and cleared. In the setting of the pre-heating mode, the temperature of the fixing device can be lowered to reduce the power consumption. An interruption key

608

is used when interruption copy is effected. A user mode key

609

is used when a user mode (such as setting of specification and timer setting) is set and cleared. A guide key

610

is used when explanation of modes is displayed.

The message display

700

includes the LCD

701

comprised of a transmission type LCD (liquid crystal display) unit of 320×240 dots, and the back light

702

comprised of a cold cathode tube, and a touch panel

703

having key matrix of 15×20 is arranged on the surface of the display, so that display of various modes, display of various conditions and key display corresponding to the key input from the touch panel are effected.

FIGS. 4A

,

4

B and

4

C show displays and key arrangements of the message display

700

, where

FIG. 4A

shows a standard picture plane,

FIG. 4B

shows a jam clearance picture plane, and

FIG. 4C

shows a picture plane instructing “close door after jam clearance”.

In

FIG. 4A

, a displaying portion

710

serves to display magnification, sheet feeding cassette selection and copy number. A reduction/enlargement key

711

is used when reduction or enlargement is desired. An isometric key

712

is used when the magnification is returned to isometric (100%). A sheet selection key

713

is used when an auto sheet selection, an upper cassette

114

, a lower cassette

115

, a paper deck

124

or a multi-manual feed

150

is selected.

A zoom key

714

is used when a zoom mode is set. A slightly reduction key

715

is used when the image of the original is copied with slight reduction (for example, 93%). A copy density key

716

is used when copy density is adjusted manually. An AE key

717

is used when an AE (auto density adjustment) is set and cleared.

A sorter key

718

is used when a sort mode, a staple sort mode or a group mode is selected. A two-sided copying key

719

is used when a two-sided copying mode is set and cleared. An application mode key

720

is used when application modes which are not indicated in the standard picture plane are selected.

With the above-mentioned arrangement, in the image forming apparatus according to the first embodiment shown by the block diagram of

FIG. 1

, high accurate sheet leading end detection and correction control will be explained hereinbelow.

The CPU

1004

a

serves to effect correction control, the ROM

1004

b

stores contents of control, and the RAM

1004

c

serves to read and write the data. The reference numeral

312

-

1

denotes an S-polarized light receiving element:

312

-

2

denotes a P-polarized light receiving element;

313

denotes a sheet position detecting sensor; and

301

denotes an A/D converter.

With the arrangement as mentioned above, the correction control in which the kind of the sheet is judged by the sheet surface detecting sensor

312

and output of the sheet position detecting sensor

313

is corrected on the basis of the detected kind of sheet will be described hereinbelow.

First of all, the sheet position detecting sensor

313

connected to the A/D converter will be explained. The sheet position detecting sensor

313

explained here detects a leading end of a moving object to be detected. However, if reflectance of the surface of the moving object differs, a detecting point will also differ in principle, for example, due to difference in kind of sheet.

FIGS. 5A and 5B

show property (difference in detecting point due to difference in reflectance) of the sheet position detecting sensor

313

. In the graph, the abscissa indicates a distance between a leading end of a moving object to be detected and an end of the sheet position detecting sensor

313

, i.e., a distance from which the sheet position detecting sensor

313

starts to detect the object. The ordinate indicates output value of the sheet surface detecting sensor

312

, i.e., relative photoelectric current IL (%) which is obtained by converting light (illuminated from the light emitting element of the sheet position detecting element

313

) reflected from the object to be detected and received by the light receiving element into an electric signal as an electric current value.

Further, in the graph, the relative photoelectric current IL (%) indicated by the ordinate is relative photoelectric current when detection level of an object having reflectance of 100% is 100%.

As can be seen from the graph, in a threshold level of 20%, when a glossy paper having reflectance of 90% advances from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

by 2.5 mm, the sheet detecting point is reached.

Similarly, in the threshold level of 20%, when a plain paper having reflectance of 80% advances from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

by 2.75 mm, the sheet detecting point is reached.

In the threshold level of 20%, when a recycled paper having reflectance of 70% advances from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

by 3 mm, the sheet detecting point is reached.

In the threshold level of 20%, when an OHP sheet advances from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

by 3.8 mm, the sheet detecting point is reached.

Next, the sheet surface detecting sensor

312

shown in

FIG. 6

will be explained.

First of all, the detecting principle will be described with reference to the accompanying drawings. Light is incident on the sheet (object to be detected) at a predetermined incident angle. In this case, the reflectances of the P-polarized component and the S-polarized component differ from each other in dependence upon the difference in surface condition of the object to be detected. By comparing a ratio between light amounts of the P-polarized component and the S-polarized component of the reflected light by utilizing such difference in reflectance, the kind of the sheet (object to be detected) is judged.

The sheet surface detecting sensor comprises an S-polarized light receiving element

312

-

1

, a P-polarized light receiving element

312

-

2

, a polarizing beam splitter

312

-

3

for dividing the polarized light components of light, and a light emitting element (LED)

312

-

4

.

With the arrangement shown in

FIG. 6

, sum/difference of the output signal PD

2

of the S-polarized light receiving element

312

-

1

and the output signal PD

1

of the P-polarized light receiving element

312

-

2

is calculated as an output calculation value Px of the sheet surface detecting sensor from the following equation:

Px

=(

PD

2

−

PD

1

)/(

PD

2

+

PD

1

)

Calculation results from the above equation are shown in

FIGS. 7A and 7B

.

In

FIG. 7A

, the abscissa indicates a distance from the sensor. The ordinate indicates a value calculated on the basis of the above equation. As can be seen from

FIG. 7A

, when the distance between the sheet (object to be detected) and the sheet surface detecting sensor

312

is 5 mm (central value), the glossy paper (sheet) having reflectance of 90% has a value of 0.06, the plain paper (sheet) having reflectance of 80% has a value of 0.14, the recycled paper (sheet) having reflectance of 70% has a value of 0.22, and the OHP sheet has a value of 0.63.

FIG. 8

shows a relationship between the kinds of sheet detected by the sheet surface detecting sensor

312

and the correction amounts for correcting the difference in detecting point (distance) of the sheet position detecting sensor

313

.

Calculation or judgement of the correction value based on the kind of sheet shown in

FIG. 8

will be explained with reference to

FIG. 9A

(flowchart) and

FIG. 9B

(table).

First of all, it is judged whether or not there is a correction calculation area (step

9

-

1

). If the correction calculation area, a correction value Lx is calculated on the basis of the following equation (step

9

-

2

). However, Px is defined by the above-mentioned equation (Px=(PD

2

−PD

1

)/(PD

2

+PD

1

)):

Correction value Lx=(Px+0.62)/0.28 (mm)

In this correction calculation area, since the detecting accuracy of the sheet position detecting sensor

313

is high, small sheet-to-sheet distance control is effected to control so that a sheet-to-sheet distance becomes 20 mm (step

9

-

10

).

Then, it is judged whether or not the sheet is an OHP sheet (step

9

-

3

). Regarding the judgement for the OHP sheet, if the output calculation value Px of the sheet surface detecting sensor

312

is 0.6≦Px≦0.7, it is judged as the OHP sheet, and the correction is effected with the correction value of 3.8 mm (fixed value).

If it is judged as the OHP sheet, since the detecting accuracy of the sheet position detecting sensor

313

is low, great sheet-to-sheet distance control is effected to control so that a sheet-to-sheet distance becomes 30 mm (step

9

-

10

).

Further, if the output calculation value Px of the sheet surface detecting sensor

312

is 0.3<Px<0.6, it is judged as an upper limit fixed value (step

9

-

5

), and the correction value when the output calculation value Px of the sheet surface detecting sensor

312

is 0.3 (i.e., 3 mm) is used as the fixed value (step

9

-

6

).

If it is judged as the correction fixed value area, since the detecting accuracy of the sheet position detecting sensor

313

is low, great sheet-to-sheet distance control is effected to control so that a sheet-to-sheet distance becomes 30 mm (step

9

-

10

).

If the output calculation value Px of the sheet surface detecting sensor

312

is 0<Px<0.08, it is judged as a correction lower limit fixed value area (step

9

-

7

), and the correction value when the output calculation value Px of the sheet surface detecting sensor

312

is 0.08 (i.e., 2.5 mm) is used as the fixed value (step

9

-

8

). If it is judged as this correction fixed value area, since the detecting accuracy of the sheet position detecting sensor

313

is low, great sheet-to-sheet distance control is effected to control so that a sheet-to-sheet distance becomes 30 mm (step

9

-

10

).

Further, if the output calcuation value Px of the sheet surface detecting sensor

312

is 0.7<Px or Px≦0, since the judgement of the kind of sheet is impossible, it is judged as correction error (step

9

-

9

). Further, since the sheet exceeds a design allowable value, a message “sheet judgement error” is displayed on the display

700

of the operating portion

600

(step

9

-

12

).

Next, a flowchart for effecting correction of a registration loop amount when sheet leading end registration is controlled based on the kind of sheet for each sheet supplying will be explained with reference to FIG.

10

. The sheet leading end registration explained hereinbelow means control in which a leading end of the sheet is aligned with an image leading end of the latent image on the surface of the photosensitive drum

105

.

Further, the registration loop amount means a loop amount required in the control in which the sheet is once abutted against the stopped registration rollers

120

and is then intactly conveyed to form a loop in the sheet to correct skew-feed of the sheet and the image leading end of the latent image on the photosensitive drum

105

is aligned with the leading end of the sheet by rotating (turning ON) the registration rollers

120

at a predetermined timing.

In the actual operation, a copy job is started by depressing a copy button of the operating portion to detect a surface of a sheet (step

10

-

1

). The above-mentioned sheet kind correction value is calculated (step

10

-

2

) and the pre-determined loop amount L

1

is corrected on the basis of the kind of the sheet.

For example, when the registration loop amount is 10 mm, the correction amount of the glossy paper (sheet) having reflectance of 90% is 2.5 mm. That is to say, a position that advances from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

by 2.5 mm becomes the detecting point.

Thus, when the correction loop amount L (=L

1

−Lx) is calculated by subtracting the correction value of 2.5 mm read by the sheet position detecting sensor

313

from the pre-determined loop amount of 10 mm, a loop amount after correction becomes 7.5 mm.

Similarly, the correction amount Lx of the plain paper is 2.75 mm. Thus, when the correction loop amount L (=L

1

−Lx) is calculated, a loop amount after correction becomes 7.25 mm. After the loop amount correction calculation, the sheet supplying is started (step

10

-

4

), and the above-mentioned sheet leading end registration is controlled (step

10

-

5

).

[Second Embodiment]

Now, a second embodiment of the present invention will be explained with reference to the accompanying drawings. In the first embodiment, while an example that the sheet detecting point (distance) of the sheet position detecting sensor

313

is corrected on the basis of the kind of sheet was explained, in the second embodiment, the correction is effected by changing a sheet detecting threshold level of the sheet position detecting sensor

313

on the basis of the kind of the sheet.

FIGS. 11A and 11B

show property (difference in detecting point due to difference in reflectance) of the sheet position detecting sensor

313

. In the graph, the abscissa indicates a distance between a leading end of a sheet (a moving object to be detected) and an end of the sensor, i.e., a distance from which the sensor starts to detect the object. The ordinate indicates output value of the sensor, i.e., relative photoelectric current IL (%) which is obtained by converting light (illuminated from the light emitting element of the sheet position detecting element

313

) reflected from the sheet (object to be detected) and received by the light receiving element into an electric signal as a current value. Further, in the graph, the relative photoelectric current IL (%) indicated by the ordinate is relative photoelectric current when detection level of an object having reflectance of 100% is 100%.

As can be seen from the graph, in a threshold level of 20%, when the OHP sheet (object to be detected) advances from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

by 3.8 mm, the sheet detecting point is reached.

Regarding the glossy paper having reflectance of 90%, a threshold level is selected to 90% to have the same detecting point as that of the OHP sheet. Similarly, the threshold level of the plain paper having reflectance of 80% is selected to 70% and the threshold level of the recycled paper having reflectance of 70% is selected to 50%.

In accordance with

FIG. 7A

explained in connection with the first embodiment, the kind of the sheet is judged from the output of the sheet surface detecting sensor

312

and the detection value of the sheet position detecting sensor

313

is corrected on the basis of the detected kind of the sheet. Now, the correction control will be described.

FIG. 12

is a graph showing a relationship between the threshold level and the output calculation value of the sheet surface detecting sensor when the kind of the sheet is judged by the sheet surface detecting sensor

312

and the threshold level is changed in accordance with the detected kind of the sheet so that the detecting point of the sheet position detecting sensor

313

becomes the same regardless of the kind of the sheet.

On the basis of this relationship, the correction control effected by changing the threshold level based on the kind of the sheet will be explained with reference to

FIG. 13A

(flowchart) and

FIG. 13B

(table).

First of all, it is judged whether or not there is a correction calculation area (step

14

-

1

). If the correction calculation area, a correction value S

1

is calculated on the basis of the following equation (step

14

-

2

). However, Px is defined by the above-mentioned equation (Px=(PD

2

−PD

1

)/(PD

2

+PD

1

)):

Threshold level correction value

S

1

=(−Px+0.57)/0.007

In this correction calculation area, since the detecting accuracy of the sheet position detecting sensor

313

is high, high speed sheet conveyance control is effected to control so that a sheet conveying speed becomes 416 mm/sec (step

14

-

10

).

Then, it is judged whether or not the sheet is an OHP sheet (step

14

-

3

). Regarding the judgement for the OHP sheet, if the output calculation value Px of the sheet surface detecting sensor

312

is 0.6≦Px≦0.7, it is judged as the OHP sheet, and the threshold correction value of 20 is selected as a fixed value.

If it is judged as the OHP sheet, since the detecting accuracy of the sheet position detecting sensor

313

is low, low speed sheet conveyance control is effected so that a sheet conveying speed becomes 277 mm/sec (step

14

-

11

).

Further, if the output calculation value Px of the sheet surface detecting sensor

312

is 0.3<Px<0.6, it is judged as an upper limit fixed value area (step

14

-

5

), and the threshold level correction value when the output calculation value Px of the sheet surface detecting sensor

312

is 0.3 (i.e., 50) is used as the fixed value (step

14

-

6

).

If it is judged as this correction fixed value area, since the detecting accuracy of the sheet position detecting sensor

313

is low, low speed sheet conveyance control is effected so that a sheet conveying speed becomes 277 mm/sec (step

14

-

11

).

If the output calculation value Px of the sheet surface detecting sensor

312

is 0<Px<0.08, it is judged as a lower limit fixed value area (step

14

-

7

), and the threshold corection value when the output calculation value Px of the sheet surface detecting sensor

312

is 0.08 (i.e., 90) is used as the fixed value (step

14

-

8

).

If it is judged as this correction fixed value area, since the detecting accuracy of the sheet position detecting sensor

313

is low, low speed sheet conveyance control is effected so that a sheet conveying speed becomes 277 mm/sec (step

14

-

11

).

Further, if the output calculation value Px of the sheet surface detecting sensor

312

is 0.7<Px or Px≦0, since the judgement of the kind of sheet is impossible, it is judged as correction error (step

14

-

9

). Further, since the sheet exceeds a design allowable value, a message “sheet judgement error” is displayed on the display

700

of the operating portion

600

(step

14

-

12

).

As mentioned above, in the second embodiment, since the correction control is effected so that the detecting point of the reflection type sensor

313

becomes constant (3.8 mm) regardless of the kind of the sheet, for example, when the predetermined loop amount L

1

in the registration is 10 mm, the correction value Lx may be 3.8 mm uniformly.

That is to say, since the sheet detecting point is a location advanced by 3.8 mm from the end (front side of the sensor in the sheet feeding direction) of the sheet position detecting sensor

313

, when the predetermined loop amount L

1

in the registration is 10 mm, the loop amount after correction becomes 6.2 mm (=10 mm−3.8 mm) by subtracting Lx=3.8 mm from L

1

=10 mm).

Further, in the above-mentioned first and second embodiments, while the correction control in registration was explained, it should be understood that the present invention can be applied to a sheet sensor in a feeder portion for the sheets or the originals within the image forming apparatus or a sheet sensor for paper handling in a sorter effecting discharge sort after the image transferring to achieve the stable sheet conveyance by effecting correction based on the kind of sheet.

[Third Embodiment]

FIG. 14

is a sectional view of an image forming apparatus according to a third embodiment of the present invention. In

FIG. 14

, the same elements as those in the first embodiment are designated by the same reference numerals and detailed explanation thereof will be omitted. Here, characteristics of the third embodiment will mainly be described.

In this image forming apparatus, there are provided the following sheet detecting sensors in a sheet conveying path. Such sensors are a deck sheet surface detecting sensor

147

, an upper stage sheet surface detecting sensor

148

, a lower stage sheet surface detecting sensor

149

, a manual feed sheet surface detecting sensor

151

(when discrimination is not necessitated, these sensors will merely be referred to as “sheet surface detecting sensor (

147

,

148

,

149

,

151

)” hereinafter), a sheet thickness detecting sensor

152

, a sheet height detecting sensor

153

and a sheet position detecting sensor

154

.

Further, in a conveying path ahead of the registration rollers

120

, there is provided a thickness detecting roller

145

. A pair of conveying rollers

146

serves to convey the sheet from a paper deck

124

.

FIG. 15

is a block diagram showing an example of a control system of the image forming apparatus having such sensors. In

FIG. 15

, a control circuit (controller)

1004

includes a CPU

1004

a

, a ROM

1004

b

and a RAM

1004

c

and serves to control a copying sequence on the basis of program stored in the ROM

1004

b.

A thermistor

302

serves to detect a surface temperature of a fixing roller

144

, and a value A/D-converted by an A/D converter

301

is inputted to the control circuit

1004

. The control circuit

1004

effects control on the basis of a detection value of the thermistor

302

so that the surface temperature of the fixing roller

144

becomes a predetermined value.

Further, the deck sheet surface detecting sensor

147

, the upper stage sheet surface detecting sensor

148

, the lower stage sheet surface detecting sensor

149

, the manual feed sheet surface detecting sensor

151

, the sheet thickness detecting sensor

152

, the sheet height detecting sensor

153

and the sheet position detecting sensor

154

are connected to the A/D converter

301

, and input values from these sensors are A/D-converted and are inputted to the control circuit

1004

.

A high-voltage controlling portion

303

serves to control charging systems such as a primary charger

106

and a transfer charger

110

and a high-voltage unit

304

for applying predetermined potential to a developing device

109

and the like.

A motor controlling portion

305

serves to control driving of various stepping motors and a motor

306

such as a main drive motor.

A DC load controlling portion

307

serves to control driving of a solenoid for a pick-up roller

116

, a clutch for registration rollers

120

and a fan. Sensors

308

such as a sheet jam detecting sensor are connected to the control circuit

1004

.

An AC driver

1000

serves to control AC load

309

such as an exposure lamp

103

and AC power supply for a fixing heater

310

. Further, it serves to detect abnormality of the exposure lamp

103

and the fixing heater

310

and to turn off a main switch

1001

having a shut-off function.

A power supply

1100

serves to supply DC power source to the control circuit

1004

, and commercial power source is inputted from an input power supply plug

311

to the power supply

1100

through the main switch

1001

and a door switch

1007

.

A paper deck

124

is a sheet feeding device for increasing the number of stacked sheets, a feeder

200

is an auto original feeder for automatically setting a plurality of originals, and a sorter

250

is a sorting device for sorting discharged sheets.

As described in connection with

FIG. 3

, an operating portion

600

includes a touch panel

703

, an LCD

701

and a back light

702

and is connected to the control circuit

1004

.

FIG. 16

is a constructural view showing various sheet detecting sensors provided in the sheet conveying path according to the present invention. A sheet S contained in the paper deck

124

is conveyed through the conveying path by a sheet feeding roller

126

and a pair of conveying rollers

146

. After the kind of the sheet (plain paper, glossy paper, recycled paper or OHP sheet) is ascertained by the deck sheet surface detecting sensor

147

as will be described later in connection with

FIGS. 21A and 21B

, the sheet is conveyed to the thickness detecting roller

145

, where a thickness of the sheet S is detected by the sheet thickness detecting sensor

152

as will be described later in connection with FIG.

19

.

A leading end of the sheet S left the thickness detecting roller

145

is detected by the sheet position detecting sensor

154

as will be described later in connection with

FIG. 17

, in front of the registration rollers

120

. In this case, as will be described later in connection with

FIGS. 22A

,

22

B,

22

C and

22

D, the detection value of the sheet position detecting sensor

154

is corrected by the sheet height detecting sensor

153

disposed in the vicinity of the sheet position detecting sensor

154

.

Similar to the paper deck

124

, kinds of sheets contained in a multi manual feed tray

150

, an upper stage cassette

114

and a lower stage cassette

115

are detected by the manual feed sheet surface detecting sensor

151

, the upper stage sheet surface detecting sensor

148

and the lower stage sheet surface detecting sensor

149

, respectively.

FIGS. 17

,

18

,

19

and

20

are explanatory views for various sheet detecting sensors according to the present invention.

FIG. 17

shows the sheet position detecting sensor

154

,

FIG. 18

shows the sheet surface detecting sensor

147

,

148

,

149

and

151

,

FIG. 19

shows the sheet thickness detecting sensor

152

and

FIG. 20

shows the sheet height detecting sensor

153

.

In

FIG. 17

, in the sheet position detecting sensor

154

, a light beam emitted from an LED

2201

driven by the control circuit

1004

is reflected by the sheet S on the sheet position detecting sensor

154

and is inputted to a PD (photo-diode)

2202

.

The inputted light beam is conveyed into an electric signal by the PD

2202

and is A/D-converted by the A/D converter

301

and is inputted to the CPU

1004

a

of the control circuit

1004

.

If there is no sheet S on the sheet position detecting sensor

154

, since the light beam is not inputted to the PD

2202

, the arrival of the sheet S is detected by the fact that the light beam is inputted to the PD

2202

.

Next, the sheet surface detecting sensors

147

,

148

,

149

and

151

will be explained. In

FIG. 18

, a light beam emitted from an LED

2301

driven by the control circuit

1004

enters on the sheet S (object to be detected) at a predetermined incident angle.

The incident light beam is reflected by the sheet S and is divided into a P-polarized light component and an S-polarized light component by a PBS (polarizing beam splitter)

2304

. The P-polarized light component is inputted to a PD

2

(

2302

) and the S-polarized light component is inputted to a PD

1

(

2303

). The inputted P-polarized light component and S-polarized light component are converted into electric signals by the PD

2

(

2302

) and PD

1

(

2303

), respectively, and are A/D-converted by the A/D converter

301

and are inputted to the CPU

1004

a

of the control circuit

1004

.

Due to difference in surface condition of the sheet, reflectances of the P-polarized light component and the S-polarized light component differ from each other. By comparing a ratio between light amounts of the P-polarized light component and the S-polarized light component of the reflected light by utilizing the above fact, a kind of the sheet is judged.

The CPU

1004

a

calculates sum/difference of an A/D-converted value pd

2

of the S-polarized light component and an A/D-converted value pd

1

of the P-polarized light component as an output calculation value Px of the sheet surface detecting sensor from the following equation:

Px

(

pd

2

−

pd

1

)/(

pd

2

+

pd

1

)

Calculation results from the above equation are shown in FIG.

21

A.

In

FIG. 21A

, the abscissa indicates a distance from the sensor and the ordinate indicates a value calculated on the basis of the above equation. As can be seen from

FIG. 21A

, when the distance between the sheet (object to be detected) and the sheet surface detecting sensors

147

,

148

,

149

,

151

is 5 mm (central value), the glossy paper (sheet) having reflectance of 90% has a value of 0.06, the plain paper (sheet) having reflectance of 80% has a value of 0.14, the recycled paper (sheet) having reflectance of 70% has a value of 0.22, and the OHP sheet has a value of 0.63.

In the illustrated embodiment, regarding the sheet thickness detecting sensor

152

, by using a PSD (semiconductor position detecting element), a thickness of the sheet is detected by detecting a distance between the PSD and the thickness detecting roller

145

.

In

FIG. 19

, the light beam emitted from an LED

2002

driven by an LED driving circuit

2003

in response to a signal from the control circuit

1004

is passed through a lens

2001

and is reflected by the thickness detecting roller

145

and is inputted to a PSD

2005

through a lens

2004

.

The light beam inputted to the PSD

2005

is inputted to a PSD driving and signal processing circuit

2006

, where a gravity center position of the spot light is calculated and sample-held, and then the light is A/D-converted by the A/D converter

301

and is inputted to the CPU

1004

a

of the control circuit

1004

.

Here, when the sheet is a plain paper, the light beam from the LED

2002

is inputted to the PSD

2005

through a path shown by the broken line; however, when the sheet is a thick paper shown by the broken line, the light beam is inputted to the PSD

2005

through a path shown by the solid line. Thus, by difference in gravity center position of the spot light inputted to the PSD

2005

, the thickness of the sheet is detected.

In the illustrated embodiment, regarding the sheet height detecting sensor

153

, by using a PSD (semiconductor position detecting element), a height of the conveying sheet is detected by detecting a distance between the PSD and the sheet.

In

FIG. 20

, the light beam emitted from an LED

2102

driven by an LED driving circuit

2103

in response to a signal from the control circuit

1004

is passed through a lens

2101

and is reflected by the sheet and is inputted to a PSD

2105

through a lens

2104

.

The light beam inputted to the PSD

2105

is inputted to a PSD driving and signal processing circuit

2106

, where a gravity center position of the spot light is calculated and sample-held, and then the light is A/D-converted by the A/D converter

301

and is inputted to the CPU

1004

a

of the control circuit

1004

.

Here, normally, the light beam from the LED

2102

is inputted to the PSD

2105

through a path shown by the solid line; however, when the sheet is floating or lifted as shown by the broken line, the light beam is inputted to the PSD

2105

through a path shown by the broken line. Thus, by difference in gravity center position of the spot light inputted to the PSD

2105

, the height of the sheet is detected.

FIGS. 22A

,

22

B,

22

C,

22

D are explanatory views showing a relationship between the distance from the sheet in the sheet height direction and the output of the sheet position detecting sensor

154

. Particularly,

FIG. 22A

shows a relationship between a distance h in the height direction and the output OP of the sheet position detecting sensor

154

,

FIG. 22B

shows passing times t and the output OP of the sheet position detecting sensor

154

when the sheets are passing through the sheet position detecting sensor

154

at heights h

1

, h

2

.

As shown in

FIG. 22C

, when it is assumed that the timing that the sheets reaches the sheet position detecting sensor

154

corresponds to

01

(output OP of the sheet position detecting sensor

154

), at the height h

1

, the arrival detecting timing becomes t

1

, and, at the height h

2

, the arrival detecting timing becomes t

2

. In this way, the detecting timing is varied with the sheet conveying height.

Thus, the height of the sheet is detected by the sheet height detecting sensor

153

disposed in the vicinity of the sheet position detecting sensor

154

, and the timing that the sheet reaches onto the sheet position detecting sensor

154

is corrected.

In the illustrated embodiment, the times t regarding the heights h of the sheet as shown in

FIG. 22C

are previously stored in the ROM

1004

b

of the control circuit

1004

, when the output OP of the sheet position detecting sensor

154

is detected as the predetermined value

01

, the CPU

1004

a

reads out the data from the ROM

1004

b

, thereby correcting the timing.

More specifically, for example, when the timing for stopping the sheet at the registration roller

120

portion is selected to T seconds after the sheet was detected by the sheet position detecting sensor

154

, if the output h of the sheet height detecting sensor

153

is h

1

, after (T−t

1

) seconds are elapsed, the sheet is stopped; whereas, if the output of the sheet height detecting sensor is h

2

, after (T−t

2

) seconds are elapsed, the sheet is stopped.

Incidentally, in

FIG. 22C

, although only two data are shown for explanation, a plurality of data corresponding to the number of detection units required to the heights h and times t can be stored.

Further, as shown in

FIG. 22D

, the outputs OP of the sheet position detecting sensor

154

with respect to the heights h of the sheet based on the time t

1

may be previously stored in the ROM

1004

b

of the control circuit

1004

, and the CPU

1004

a

may judge the arrival of the sheet in accordance with the height h of the sheet, when the output OP of the sheet position detecting sensor

154

becomes a predetermined value (for example, output

01

at the height h

1

, output

02

at the height h

2

).

Incidentally, similar to

FIG. 22C

, also in

FIG. 22D

, although only two data are shown for explanation, a plurality of data corresponding to the number of detection units required to the heights h and times t can be stored.

Further, in place of the fact that the data shown in

FIG. 22C

or

FIG. 22D

is stored in the ROM

1004

b

, the graph shown in

FIG. 22A

or

FIG. 22B

may be directly calculated by the CPU

1004

a

by using a calculation formula.

Further, in place of the fact that the data shown in

FIG. 22C

or

FIG. 22D

is stored in the ROM

1004

b

, the adjusted values may be stored at a factory or a market by using a non-volatile memory which can be re-written.

FIG. 23

shows an example of a flowchart for control procedure of the control circuit

1004

of the image forming apparatus according to the present invention regarding control in which the sheet is stopped at the registration roller

120

portion.

First of all, when the sequence is started, in a step S

101

, if the output of the sheet position detecting sensor

154

is judged as the predetermined value

01

, it is judged as the arrival of the sheet, and the sheet height detecting sensor

153

is inputted (step S

102

), and the correction value explained in connection with

FIGS. 22A

to

22

D is set in a timer (step S

103

), and the timer is started.

When the timer is counted up (step S

104

), the conveyance of the sheet is stopped (step S

105

), and the sequence is ended.

(Fourth embodiment)

FIG. 24

shows a flowchart for control procedure of a control circuit

1004

of an image forming apparatus according to a fourth embodiment of the present invention and shows an example that the threshold level of the output of the sheet position detecting sensor

154

is changed in the control for stopping the sheet at the registration roller

120

portion.

First of all, when the sequence is started, in a step S

111

, the output of the sheet height detecting sensor

153

is inputted, and the threshold level from which the sheet position detecting sensor

154

explained in connection with

FIG. 22

detects the arrival of the sheet is determined (step S

112

).

Then it is judged whether the output of the sheet position detecting sensor

154

reaches the threshold level determined in the step S

112

or not (step S

113

). If reached, a timer for stopping the sheet is set (step S

114

), and the timer is started.

When the timer is counted up (step S

115

), the conveyance of the sheet is stopped (step S

115

, and the sequence is ended.

(Fifth embodiment)

In a fifth embodiment of the present invention, in an image forming apparatus having the same construction as that in the third embodiment, another control method for controlling a sheet stopping timing will be explained.

FIG. 25

shows correction values for the sheet stopping timing determined on the bases of the outputs of the sheet surface detecting sensors

147

,

148

,

149

,

151

and the output of the sheet thickness detecting sensor

152

. In this embodiment, the correction values are previously stored in the ROM

1004

b

of the control circuit

1004

.

In this embodiment, the judgement of surfaceness (recycled paper, plain paper, glossy paper or OHP sheet) of the sheet based on the outputs of the sheet surface detecting sensor

147

,

148

,

149

,

151

is effected by the CPU

1004

a

of the control circuit

1004

on the bases of the pre-determined threshold levels.

Further, the judgement of the thickness of the sheet based on the output of the sheet thickness detecting sensor

152

is effected by the CPU

1004

a

of the control circuit

1004

on the bases of the pre-determined threshold levels (for thin paper, plain paper and thick paper).

From the judgement results regarding the surfaceness and thickness, the CPU

1004

a

reads out, for example, data t

12

(in case of recycled paper having a normal thickness) or t

33

(in case of thick glossy paper) from the ROM

1004

b

and effects control for stopping the sheet being conveyed at timing (t−t

12

) or (t−t

33

), where t is a reference value of stopping time.

Incidentally, in the above embodiment, while the control is effected by using the data in which the surfaceness is divided into four and the thickness is divided into three was explained, if necessary, the surfaceness and the thickness may be divided more finely.

Further, the correction value for sheet stopping timing may be common data in all cases where the sheet is stopped in the conveying path or correction data may be given for respective stopping positions.

Particularly, regarding the control in the sheet leading end loop formation in the registration roller

120

portion, data which always give the constant loop amount even when the surfaceness and the thickness of the sheet are changed may be used or data which give optimum loop amount in accordance with the surfaceness and the thickness of the sheet may be used, and, thus, various data can be provided in accordance with the feature of the apparatus.

Further, in place of the fact that the data shown in

FIG. 25

is stored in the ROM

1004

b

, the adjusted values may be stored at a factory or a market by using a non-volatile memory which can be re-written.

FIG. 26

shows an example of a flowchart for control procedure of the control circuit

1004

of the image forming apparatus according to the present invention regarding control in which the sheet is stopped at the registration roller

120

portion.

First of all, when the sequence is started, in a step S

121

, it is judged whether the sheet reaches the sheet position detecting sensor

154

. If reached, as described in connection with

FIG. 25

, a correction value txy is determined in accordance with either output of the sheet surface detecting sensor

147

,

148

,

149

or

151

depending upon the selected and used sheet feeding portion and the output of the sheet thickness detecting sensor

152

(step S

122

), and (t−txy) is set in the timer with respect to the reference value t (step S

123

).

Then, it is judged whether the timer is counted up (step S

124

). If counted up, in a step S

125

, the sheet is stopped, and the sequence is ended.

As mentioned above, according to the present invention, detection errors due to difference in kind and thickness of the sheet to be detected and difference in conveying height can be reduced, thereby detecting the sheet with high accuracy. The high speed conveyance and small sheet-to-sheet distance control requirements in the sheet conveyance can be satisfied, and the stable sheet conveyance can be achieved.

By judging the fact that the sheet being conveyed is deviated from the allowable range, poor conveyance can be prevented.

The conveyance suitable to the kind and thickness of the sheet can be controlled by the conveyance control means, operator's setting operation and workability for setting the kind of sheet can be improved.

For example, when the loop is formed in the sheet after the sheet is temporarily stopped at the registration portion, the sheet stopping timings of various sheets can be controlled more accurately and more properly, thereby permitting the stable sheet conveyance.

Number	Name	Date	Kind
4025187	Taylor et al.	May 1977	A
4503960	Koeleman et al.	Mar 1985	A
5153655	Suzuki et al.	Oct 1992	A
5157444	Mori et al.	Oct 1992	A
5187521	Shimizu et al.	Feb 1993	A
5612776	Machino et al.	Mar 1997	A
5678127	Suga	Oct 1997	A
5745661	Koh et al.	Apr 1998	A
5913099	Kamei et al.	Jun 1999	A
6002906	Hino et al.	Dec 1999	A
6070023	Kataoka	May 2000	A

Sheet detecting apparatus with correction means for sheet surface condition and sheet thickness

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Abstract

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Priority Claims (1)

US Referenced Citations (11)