JPH11188918A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve high resolution by a simple and low cost structure in an image forming apparatus utilizing an electrophotographic method. SOLUTION: A light source having a small light emitting section of an LED, a driving circuit and a lens are provided on an identical substrate. Head sections 1a, 1b each having the substrate in a case are provided at a front face and a rear face of a photosensitive body 4, respectively. The head sections 1a, 1b on the front and rear faces are disposed such that image forming positions of the head sections 1a, 1b are shifted with each other by a half of a dot pitch of the light emitting section in a main scanning direction of the photosensitive body 4. The head sections 1a, 1b are scanned by a stepping motor or the like, then a latent image is formed on the photosensitive body 4 by virtue of the light emission of the light source at the scanned position. The latent image is developed by a developing device 5 to be transferred to a recording paper, then the image is fixed by a fixing device 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic method, such as a copying machine, a facsimile, a bar code printing machine, a printer and a plotter.

[0002]

2. Description of the Related Art FIG. 6 is a diagram showing the configuration of an electrophotographic system having a digital optical system which is currently in practical use. In the figure, 101 is a driving device of the LED writing light source 102,
203 is a charger, 204 is a discharge lamp, 205 is a cleaner, 206 is a stacker serving as a paper discharge tray, 207 is a fixing device, 208 is a peeler, 209 is a transfer device, 210 is a paper feed cassette, 211 is a developing device, 212 Denotes a photosensitive drum.

The conventional electrophotographic system uses an LED array as the light source 102 and controls the electric charge on the photosensitive drum 112 by causing only the necessary components to emit light from the LEDs of the minute light emitting section. Then, LE is set so that electric charges are left in the image area.
D irradiates light, and attaches a toner having an opposite charge to the remaining charge on the photoconductor. Alternatively, light is irradiated so as to erase the image portion, and a toner having the same charge as the charge on the photoconductor is attached. Then, an image can be recorded by transferring the attached toner to paper or the like.

FIGS. 7 to 9 show L used in the above system.
FIG. 7 is a diagram showing an ED writing light source, and FIG.
FIG. 8 shows the structure of the LED head, and FIG. 9 shows a cross section of the LED array module.

In FIG. 7, reference numeral 121 denotes an LED array chip,
Reference numeral 22 denotes a driver (IC) for driving the LED array chip 121, reference numeral 123 denotes a substrate on which those components and an SLA (focusing rod lens array) 124 are mounted, and reference numeral 125 denotes an SL.
A fixing metal fitting, 126 is an FPC (flexible wiring board) with GEP lining, 127 is an FPC press-fitting metal, 128 is a connector, and 129 is a base board supporting the above components.

In FIG. 8, reference numeral 131 denotes the above-mentioned SLA, 1
32 is an SLA holder, 133 is an LED module part, 1
Reference numeral 34 denotes a circuit board, and 135 denotes a mechanical unit.
1 is a common electrode wiring pattern, 142 is an individual electrode wiring pattern, 143 is W / B (wire bonding), 144
Is a conductive paste, 145 is a driver for the LED 146, 1
47 is a ceramic substrate.

As shown in FIG. 1, an LED array chip 121 and a driver 122 are die-bonded to a substrate 123. The electrodes between the electrodes on the LED array and the substrate wiring pattern and the electrodes on the driver 122 are wire-bonded with gold wires or the like.

The driver 122 includes a logic circuit section such as a shift register and a latch, and a drive circuit using transistors.

Then, the substrate 123, the SLA 124, and the SLA fixing bracket 125 are attached, fixed to a heat sink, and connected to a circuit portion by a wire harness or the like.

[0010] This system uses an LED to charge the photoreceptor.
The latent image is formed by controlling the array, and the toner is adhered to the charge remaining on the photosensitive member or to a portion where the charge does not remain, thereby performing development. Then, the toner is transferred to paper or the like and transferred by applying heat or pressure to complete image recording.

For this reason, the resolution is limited by the pitch size of the LED dots as the light source.

The resolution practically used in the writing light source of the LED at present is mainly 300-600 DPI.
In order to increase the resolution, it is necessary to reduce the pitch of the LED dots. This includes miniaturization of LED dots, increase in total number of LED dots, miniaturization of wiring on LED chips and wiring on substrates, miniaturization of bonding pads, increase in total number of bonding pads and increase in the number of connections,
Due to an increase in the number of driver ICs, an increase in the amount of heat generation, etc., an increase in cost, a decrease in yield, and a decrease in reliability occur.

For example, in the case of 300 DPI, the pitch is 4
If the yield is 90% at 8.7 μm, the pitch becomes 42.4 μm and the yield decreases to 81% at 600 DPI where the number of light emission is doubled. In addition to this, a reduction in yield due to miniaturization of the width between light emitters and the width between bonding pads of about 10 μm and an increase in output variation are added, so that the total yield further decreases.

Further, this LED has a Ga on a GaAs substrate.
A dot is formed by having an epitaxial layer of AsP and diffusing impurities into the GaAsP. In the current method of forming wiring by a dry process using a metal such as Al in accordance with the dots, it is difficult to increase the resolution beyond 600 DPI because it approaches the limit of processing technology.

FIG. 10 shows an example of the pattern of the LED chip. The wire bond electrode portions 152 of each light emitting portion 151 are arranged in a staggered manner. FIG.
Is a diagram showing a structural example of the LED, 153 is an aluminum electrode, 154 is an insulating tank, and 155 is an Au-Ge-Ni electrode.

FIG. 12 shows the L constituting the driver 122.
FIG. 3 is a diagram illustrating a basic configuration of an ED drive circuit. This circuit is
As described above, the driver section 16 includes the register 161 to which the image signal is input, the latch 162, the AND gate 164 connected to the memory (ROM), and the driver section 165.
5 drives the LED array 166.

Further, as the number of LED dots increases, variation between dots increases. For this reason, variation correction is performed. As a typical method of this variation correction,
There are (1) rank selection of LED chips, (2) correction by electrical control, and the like. Further, the method (2) includes, as shown in FIG. 13, correction by the LED chip unit control of (a) and the LED dot unit control of (b).

FIG. 13 is a diagram showing an energization time control circuit for correcting the above-mentioned variation, wherein 171 is a shift register, 172 and 172a and 172b are latch circuits, Q1 and Q2 are AND gates, and Tr1 and Tr2 are transistors. Is shown.

The exposure correction conditions by the above circuit are as follows. First, when the exposure energy of the LED is E, the light emission output of the LED is P ₀ , and the LED energization time is τ, it is expressed by the following equation.

E = P ₀ τ (1) When the correction energizing time of each LED is τ (k), the following equation is established.

Τ (k) = T ₀ + (m−k) t (2) T ₀ : Common energizing time m: Number of correction steps t: Basic unit of additional time for correction k: Natural number (1 to m) From the above equations (1) and (2), they are expressed as follows.

E = {T ₀ + (m−k) t} P ₀ (3) Here, assuming that the exposure energies of an arbitrary LED emission output value B _k are E (k) and E (k−1), The following equation is obtained from the equation (3).

E (k) = {T ₀ + (m−k) t} B _k (4) E (k−1) = {T ₀ + (m−k + 1) t} B _k (5) The variation ΔE of the exposure energy is represented by (4),
The following is obtained from the equation (5).

ΔE = {E (k) −E (k−1)} / {E (k) + E (k−1)} = 2 / {2T ₀ + (2m−2k + 1) t} (6) This variation ΔE is maximized when m = k. Therefore, ΔE at the maximum is expressed by the following equation.

ΔE = t / (2T ₀ + t) (7) Since T ₀ is an integral multiple of t, the energizing time T ₀ is as follows.

T ₀ = nt (8) n: Integer Normally, to make the variation ΔE within ± 15%, the following equation is obtained from the above equations (7) and (8).

ΔE = 1 / (2n + 1) ≦ 0.15 (9) From the above, n ≧ 2.8, and the minimum energization time is as follows.

T ₀ = 3t (10) The basic unit t for correction is expressed by the following equation.

T = N / df (11) N: total number of dots d: number of data bits expressed in data processing format f: transfer clock frequency The maximum correction time τ (k) max is the one line lighting time T Must be smaller. If T = 1 msec, m is equal to or less than 11 according to the equations (2) and (10). As described above, the corrected exposure energy maximum value and minimum value E
max and Emin are the maximum value Pmax of the LED emission output.
And the minimum value Pmin is as follows.

Emax = 3tPmax (12) Emin = (2 + m) tPmin (13) Here, as the print density increases, the number of dots increases. Accordingly, the basic unit t also extends. However, in order to make the lighting time of one line the same, it is necessary to shorten the energizing time.
For this purpose, it is necessary to make ΔE larger than 15%, or to increase the dot multiplication factor by a and the number of steps m ′ as follows.

M ′ ≦ 1 / at−2 (14) This is because ΔE is to narrow the variation correction range of the LED.
This indicates that it is necessary to suppress the variation of the LED itself to 1 / a in order not to change.

The above description has been made on the case where the exposure amount is corrected by time. However, the correction may be made by using the fact that the light emission output of the LED is proportional to the flowing current. This can be achieved by considering τ (k) as a basic unit of a correction energizing current, TO as a basic unit of a common energizing current, and t as a basic unit of an additional current for correction. Also in this case, since the maximum number m of steps of the correction cannot be changed similarly to the time correction, it is necessary to suppress the variation of the LED chip itself to the present level or less and to satisfy the linearity of the output-current value of the LED itself.

The above correction information is recorded in the ROM and is referred to as needed.

The driving of the LED writing light source is performed by the LE
This is a combination of a D emission output correction circuit and an IC driver, and the basic configuration is shown in FIG. 13 as an example of an LED dot control circuit.

Further, when driving the LED writing light source, it is inevitable to adopt a bit serial transfer format due to its configuration. FIG. 14 shows an interface between the head unit and the printer. The function of each signal is as follows.

DATA: Video input signal, LE
The light emission state of D is indicated.

CLOCK: A sampling clock for DATA and DTP.

DTP: Synchronous signal of DATA signal.

Immediately after DTP is input, sampling of a video input signal is started in synchronization with CLOCK. On and Off of the LED are controlled by the logical level of DATA.

These data are input to the serial IN-parallel OUT shift register 171 and held by the latch circuits 172, 172a, 172b. Here, if the electrodes on the LED array chip are arranged in a zigzag pattern as shown in FIG. 10, the data must be distributed to the odd side and the even side with respect to the LED. For this reason, it is necessary to convert the data input by bit serial into 2-bit parallel data and input it to the IC driver. Therefore, in FIG. 13A, since it is necessary to align the shift data, it is general to use a bidirectional shift register as shown in FIG. 13B and select the L / R terminal for indicating the shift direction.

Then, the correction information is input from the ROM to each IC driver, and after the information is latched by the latch circuit (1) of the IC driver, the print data is input to the shift register. The print data is held in the latch circuit (2), and at the same time, the logical product of the print data and the information of the latch circuit (1) is obtained.
The required exposure energy is emitted by being energized for each LED dot.

The light emitted from this LED dot is shown in FIG.
The SLA 124 shown in (1) forms a dot-shaped light image on the photoreceptor through a 1: 1 imaging lens. In this SLA, light travels meandering sinusoidally with respect to the optical axis. Therefore, light beams having different optical path lengths due to geometrical optics simultaneously travel regardless of the lens length, and form an image as a lens.

If the lens deviates even slightly from the center of the object image distance TC, an MTF (Modulation Transfer Fun
ction) is significantly reduced. This is because, since the selfoc lens is a compound eye lens, if the 1: 1 imaging relationship is disrupted, the overlap of adjacent images is disrupted and the image is degraded. Therefore, it is necessary to have a degree of parallelism that keeps the center of the Selfoc lens and the TC within the standard at all points. Also, when the Selfoc lens is represented by the F value of an optically equivalent spherical lens,
The F value on the center is as follows.

F = (m / 2πK _N ) ^0.5 (1 / θ) (15) m: degree of overlap θ: opening angle N: number of columns K _N is given by the number of columns as follows.

K ₁ = 1 K ₂ = 2- (1/8 m ² ) K ₃ = 3- (3/2 m ² ) K4 = 4- (15/4 m ² ) Also, the image plane illuminance obtained by the Selfoc lens E
Is given by the following equation using the above F value.

E = (τπL / 16) (1 / F) ² (16) τ: Equivalent ratio of lens L: LED luminance As described above, the aperture angle and the number of rows are larger, but the higher the degree of overlap, the darker.

[0047]

However, since the conventional image forming apparatus of the electrophotographic system is constructed as described above, it has the following problems.

(1) Since a plurality of dots arranged in accordance with the resolution are simultaneously emitted, a large amount of heat is generated from the LED and the driver IC, and the substrate, the heat radiating plate and the like are disturbed, resulting in displacement.

(2) As the density increases, the interval between LED chips becomes narrower, and the rate of occurrence of defects due to scribe or dicing deviation increases.

(3) A die bond shift occurs due to a reduction in the LED chip interval, and the defective rate increases due to the chip position shift.

(4) When the density of LED dots is increased and the chips are arranged in a staggered electrode arrangement, serial data sent as image data needs to be 2-bit parallel.

(5) Since the controller for controlling the correction data and the serial data needs to correspond to the number of dots, it is complicated.

(6) Since it is necessary to inspect all the LED dots in the product inspection, the higher the density, the longer the inspection time and the higher the cost.

(7) As the number of dots increases, the processing time per dot is shortened, and the range in which output can be corrected is narrowed.

(8) As the density increases, the range in which the output can be corrected is narrowed, and the variation range in which the LED can be used is narrowed.
This leads to a decrease in ED yield.

(9) If the driving method is static,
Since a current of several tens of A flows when all dots are lit, restrictions are imposed on power supply capacity, responsiveness, and wiring width. Further, the number of layers of the substrate increases due to the increase in the number of dots.

The present invention has been made in view of the above-mentioned problems, and has a simple configuration, makes it easy to correct the variation of the light source, and realizes a low-cost, high-resolution image. It is intended to provide a forming apparatus.

[0058]

An image forming apparatus according to the present invention is configured as follows.

(1) In an image forming apparatus using electrophotography, a light source having a minute light emitting portion on the same substrate, a lens,
And a head having the drive circuit unit is disposed on the front and back of the photoconductor, and the front and rear heads are shifted in the main scanning direction of the photoconductor by の of the light emitting unit pitch to form an image.
An image was formed on the photoreceptor.

(3) In the structure of the above (1), the lens portion is a unit-magnification imaging lens such as a selfoc lens.

(3) In the configuration of the above (1), the lens unit forms an image on the photoreceptor with a monocular lens such as a condenser lens at the same magnification as the dot of the minute light emitting unit.

[0062]

FIG. 1 is a sectional view showing the overall structure of an image forming apparatus according to an embodiment of the present invention. In the figure, 1a, 1
Reference numeral b denotes a head unit provided on the front side and the back side of the photoconductor 4, having a light source (LED) having a small light emitting unit, a lens, and a drive circuit unit on the same substrate, and these are held by a case unit. It has a configuration.

In FIG. 1, 5 is a developing device, 6 is a charging device,
7 is a transfer neutralizer, 8 is a fixing device, 9 is a paper feed cassette, 10
Is a control unit for controlling each unit.

FIG. 2 is a plan view showing an LED chip used in the heads 1a and 1b. FIG. 3 is a diagram showing details of the light emitting unit, and 21 indicates a bonding pad. FIG. 4 is a diagram showing a positional relationship of wire bonding, and reference numeral 22 denotes a driver IC.

In this embodiment, as shown in FIG. 2, a plurality of LED chips emitting light of a single wavelength are provided, this light source is mounted on the same substrate, and an LED having a structure as shown in FIGS.
The ED print head is exposed from both sides of the photoreceptor 4, and emits a specified output according to data at that position.

Here, the arrangement of the head A (head section 1a) and the head B (head section 1b) is set to one of the LED dot pitch.
/ 2 shifted. That is, if the deviation between the heads A and B is 42.3 μm and the scanning pitch is 84.6 μm, the resolution of 600 dpi can be obtained by the heads A and B.

The distribution of the energy output from the LED element is substantially specified on the photosensitive member. The head A and the head B are arranged so as to correspond to A and B in FIG.
The resolution can be increased by overlapping the beams. In this case, the beam diameter needs to be larger than the distance between the heads A and B.

When the light intensity is set so that the potential levels of the LED beam dots A and B are lower than the execution potential level of the execution dot G for the latent image, and the overlapping portion Dg of the beam dots is higher than the execution potential level. In the central portion where the beams do not overlap, no effective dot is formed, and only the superposed Dg portion is latently imaged. Therefore, L
Assuming that the pitch of the ED is 300 dpi, an effective dot G of 600 dpi is formed by using this for the front and back heads.

This LED uses an LED having an emission wavelength that is most sensitive to the wavelength sensitivity of the photosensitive member.
The drive current is suppressed, the temperature rise is suppressed, and the deterioration speed is reduced. For example, when the wavelength sensitivity of the photoconductor is 780 nm, the emission wavelength of the LED is 780 nm GaAs.
GaAsP may be used for the base epitaxial layer.

Next, a plurality of light emitting points of this LED will be described.

In the LED, a p-type semiconductor layer is formed by growing an n-type semiconductor layer GaAsP on an n-GaAs base and then selectively diffusing Zn. This LED has GaAs
In addition to P, it is also possible to form each light emitting dot by epitaxially growing AlGaAs and patterning it by photolithography.

The growth may be liquid phase growth or MBE in addition to the vapor phase growth method. Further, the GaAs base is changed to Si
By using the base, the thermal resistance can be reduced, but in this case, a buffer layer is required between Si and the growth layer.

Here, in the head A, if the moving speed of the photosensitive member 4 is I (mm / s), the printing density is N (dpi), and the printing width is L (mm), the light source moves the printing width L. The line speed V (mm / s) is as follows.

V = NI / 25.4 (21) Therefore, the time T (s) required for one line is expressed by the following equation.

T = 25.4 / NI (22) The number n of dots included in the print width is as follows.

N = NL / 25.4 (23) The light emission time t (s) per light is expressed by (22),
The following is obtained from the equation (23).

T = T / n = (25.4) ² / N ² IL (24) Also, assuming that the emission output of the LED 11 is P (mW / cm ² ) and the transmission efficiency of the optical system is η _T , The above light irradiation energy E (μJ / cm ² ) is given by the following equation.

E = Pη _T t = Pη _T (25.4) ² / N ² IL (25) If the sensitivity of the photoconductor 4 is η _P (cm ² / μJ),
The exposure amount is I / η _P. Therefore, since the light irradiation energy needs to be equal to or more than this exposure amount, the following equation is obtained.

E ≧ I / η _P (26) From the equations (25) and (26), the following equation is obtained.

Pη _T (25.4) ² / N ² IL ≧ I / η _P P ≧ N ² IL / (25.4) ² η _T η _P (27) From the above, the LED emission output is above P I just want it.

Here, the head having the above-mentioned LED has a condenser lens, an equal-magnification coupling lens, or a monocular lens which satisfies the resolution of the light source, and forms the dots of the LED on the photosensitive member.

However, when a monocular lens is used, LE
Since the D light emission point and the imaging point are reversed, it is necessary to invert the data by the range.

[0083]

As described above, according to the present invention, one or a plurality of light source chips are arranged in the head portion, and the head is shifted by 1/2 of the dot pitch between the front surface and the back surface of the photosensitive member. By arranging, it is easy to double the resolution without increasing the manufacturing technology and the position setting accuracy.

Further, as the head is moved, the output correction of the LED to the photosensitive member is complemented by performing the divided exposure, whereby the variation due to the position can be eliminated.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing the overall configuration of an embodiment.

FIG. 2 is a plan view showing a configuration of an LED chip.

FIG. 3 is a plan view showing details of a light emitting unit.

FIG. 4 is an explanatory view showing a wire bonding positional relationship.

FIG. 5 is an explanatory diagram showing a state of high resolution by superposition.

FIG. 6 is a sectional view showing the configuration of an electrophotographic system having a digital optical system.

FIG. 7 is a perspective view showing a configuration of an LED writing light source.

FIG. 8 is an exploded perspective view showing the structure of an LED head.

FIG. 9 is a sectional view of an LED array module.

FIG. 10 is a plan view showing an example of a chip pattern.

FIG. 11 is a sectional view showing an example of the structure of an LED.

FIG. 12 is a block diagram showing a basic configuration of an LED drive circuit.

FIG. 13 is a diagram showing an LED energization time control circuit.

FIG. 14 is an explanatory diagram showing an interface with a printer.

[Explanation of symbols]

 1a Head 1b Head 4 Photoconductor

[Procedure amendment]

[Submission date] February 5, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0015

[Correction method] Change

[Correction contents]

FIG. 10 shows an example of the pattern of the LED chip. The wire bond electrode portions 152 of each light emitting portion 151 are arranged in a staggered manner. FIG.
Is a view showing a structural example of an LED, 153 is an aluminum electrode, 154 is an insulating layer , and 155 is an Au-Ge-Ni electrode.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0046

[Correction method] Change

[Correction contents]

[0046] E = (τπL / 16) ( 1 / F) 2 (16) τ: equivalent ratio L of the lens: aperture angle than the above LED brightness, but brighter the greater the number of columns, darker heavy Do Ri large degree Become.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0078

[Correction method] Change

[Correction contents]

E = Pη _T t = Pη _T (25.4) ² / N ² IL (25) When the sensitivity of the photoconductor 4 is η _P (cm ² / μJ),
Exposure amount is 1 / η _P. Therefore, since the light irradiation energy needs to be equal to or more than this exposure amount, the following equation is obtained.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0079

[Correction method] Change

[Correction contents]

E ≧ 1 / η _P (26) From equations (25) and (26), the following equation is obtained.

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0080

[Correction method] Change

[Correction contents]

Pη _T (25.4) ² / N ² IL ≧ 1 / η _P P ≧ N ² IL / (25.4) ² η _T η _P (27) From the above, the LED emission output is higher than P I just want it.

Claims (3)

[Claims] In an image forming apparatus using electrophotography, a head having a light source, a lens, and a drive circuit thereof having a micro light emitting portion on the same substrate is arranged on the front and back surfaces of a photoreceptor. The front and back heads are 1 /
An image forming apparatus characterized in that an image is formed on a photoconductor by forming an image by shifting the photoconductor in the main scanning direction by two. 2. The image forming apparatus according to claim 1, wherein the lens unit is a unit-magnification imaging lens such as a selfoc lens. 3. The image forming apparatus according to claim 1, wherein the lens unit forms an image on the photoreceptor with a single eye lens such as a condenser lens at the same magnification as the dots of the minute light emitting unit.