JPH0683339B2 - Image reader - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

According to the present invention, a document is illuminated by an exposure light source of an optical system, and an optical image of the document is formed in the image pickup unit.
For example, the present invention relates to an image reading device such as a facsimile or a copying machine.

BACKGROUND OF THE INVENTION

In an image reading apparatus, generally, an optical image of a document surface obtained by exposure scanning is imaged on a solid-state image sensor such as an image sensor installed around an optical axis via an imaging lens system. Further, for example, in an image reading apparatus of a multi-color image forming apparatus, an optical image of a document surface obtained by exposure scanning is passed through an image forming lens system and then dispersed by a light splitting means such as a prism provided behind it. The image is formed on an image sensor that receives light in each channel. In the image reading apparatus in which the platen is fixed and the optical system is moved, the document placed on the platen glass is irradiated in a strip shape by the moving light source. An original optical image obtained by being illuminated by the light source is reflected and guided by the first mirror that moves together with the light source and the second and third mirrors provided in the movable mirror unit. Then, the reflected / derived optical image of the original document passes through an image pickup lens fixedly formed, and then is formed on an image sensor which is also fixed behind the image pickup lens. A rod-shaped halogen lamp or a fluorescent tube is used as a light source for illuminating the original surface. Halogen lamps often use fluorescent lamps because they generate a large amount of heat when continuously lit. In an image reading apparatus that scans and exposes a document on a platen glass with an exposure light source of an optical system, a drive motor for driving a scanning moving unit of the optical system is provided inside the apparatus. As the drive motor, a DC motor, an induction motor, etc. are generally used, but in recent years, a stepping motor has been widely used due to the tendency of downsizing of the device and the demand for accurate positioning. When the stepping motor is rotated, the rotation control of the motor is performed stepwise by the pulse, so that the vibration having the reference frequency related to the step angle is generated. The frequency f _{1 of} this vibration corresponds to the rotation speed of the stepping motor N
When rpm and the step angle are θs, it can be expressed as f ₁ = 6N / θs. This f ₁ is the basic drive frequency of the stepping motor. FIG. 6 shows that the rotation speed N = 83.3 rpm and the step angle θ
It is a spectrum analysis diagram of the vibration wave at the time of driving a stepping motor of s = 0.9 °. In the figure, the horizontal axis represents frequency and the vertical axis represents amplitude. The vibration wave when driving this stepping motor has a basic drive frequency f ₁ = 6 × 83.3 / 0.9 ° = 55
5Hz and harmonic components of this basic drive frequency 1/2 ・ f ₁ = 278H
It is composed of z, 1/4 · f ₁ = 138Hz, etc. The natural frequency of the mirror of the optical system may coincide with the above-mentioned fundamental drive frequency of the drive motor and its harmonic components to cause a resonance phenomenon of the mirror. At the time of such a resonance phenomenon, a so-called blur state occurs in which the original light source led out and imaged on the solid-state image pickup device CCD is disturbed by the mirror. The blur amount of the original light image formed on the CCD is 1 pixel pitch = 6
If it exceeds 1/4 of 2.5 μm, the effect on the reproduced image appears and the reproduced image deteriorates. When the mirror of the optical system causes a resonance phenomenon in agreement with the fundamental drive frequency of the drive motor and its harmonic components, the original optical image derived by the mirror is 1/4.
Blurring amounts that exceed the pixel pitch may disturb the reproduced image.

[Problems to be Solved by the Invention]

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and the natural frequency of the mirror of the optical system and the fundamental drive frequency of the drive motor and its harmonic components have different values. To prevent the mirror resonance phenomenon,
It is an object of the present invention to provide an image reading apparatus capable of preventing deterioration of a reproduced image by eliminating blurring of a projected image due to mirror vibration.

[Means for solving problems]

The above object is to provide an image reading apparatus in which an original on a platen glass is scanned and exposed by an exposure light source of an optical system and an optical image of the original is led out into an image pickup unit by a plurality of mirrors of the optical system. , A stepping motor is used as a drive motor for driving the moving part of the optical system, and the natural frequency f ₀ of the mirror and a value obtained by multiplying the basic drive frequency f ₁ of the stepping motor by an integer n, or the natural frequency of the mirror This is achieved by an image reading apparatus characterized in that the frequency f ₀ and the value obtained by dividing the basic driving frequency f ₁ of the stepping motor by an integer n are different. And preferably, the natural frequency f ₀ of the mirror is f ₀ <0.
This is achieved by an image reading device characterized by setting the range represented by 65 · n · f ₁ and f ₀ > 1.35 · n · f ₁ . More preferably, the natural frequency f of the mirror
_0, is achieved by the image reading apparatus being characterized in that set in the range expressed by _{f 0 <0.65 · 1 / n} · f 1 and _{f 0> 1.35 · 1 / n} · f 1. However, n represents the integers 1, 2, 3, ...

【Example】

Prior to the description of the embodiments of the present invention, first, the overall functions of the image reading apparatus will be described. FIG. 3 is a block diagram of the image reading apparatus. In the figure,
Reference numeral 1 is a platen glass, and document 2 is placed on the platen glass 1. The original 2 is illuminated by the fluorescent lamps 5 and 6 provided in the exposure light source unit 40 that moves on the slide rail 3. The movable mirror unit 80 is provided with mirrors 9a and 9b and moves on the slide rail 3, and in combination with the first mirror 7 provided in the exposure light source unit 40, an optical image of the original 2 on the original glass 1 is formed. Imaging unit 20
Derive to. The exposure light source unit 40 and the movable mirror unit 80 are driven in the same direction at a speed of V and 1/2 V by a driving device including a stepping motor 10, pulleys 11, 12, 13, 14 and a wire 15, respectively. . A standard white plate 17 is provided on the back side of the document abutting member 16 at the front end of the document table glass 1 so that a standard white signal can be obtained before the start of scanning the document. The image pickup unit 20 includes a lens 21 as an image pickup lens system, a prism 22 as a photolytic means, a first reading substrate 24, a red channel (hereinafter referred to as R-ch) CCD 27 which is a line image sensor, a second reading substrate 26, and a cyan channel. It is composed of CCD25 (hereinafter referred to as C-ch). The original light image transmitted by the first mirror 7, the mirror 9a and the mirror 9b is focused by the lens 21 and separated into a C-ch image and an R-ch image by a dichroic mirror provided in the prism 22. The C-ch image is formed on the light-receiving surface of the C-ch CCD 25 provided on the first reading substrate 24, and the R-ch image is received by the R-ch CCD 27 provided on the second reading substrate 26. It is imaged on the surface. Commercially available warm white fluorescent lamps are used as the fluorescent lamps 5 and 6 for irradiating the original document in order to prevent emphasis and attenuation of a specific color based on the light source when reading a color original document. In addition, it is turned on by a high frequency power supply of 40 KHz to prevent flicker, and is heated by a heater using a posistor to maintain a constant temperature on the tube wall or to promote warm-up. FIG. 1 is a perspective view of an image reading apparatus according to an embodiment of the present invention, and FIG. 2 is a side sectional view of an image reading apparatus according to an embodiment of the present invention. In these figures, 3 is a slide rail, 7, 9a and 9b are mirrors, 10 is a stepping motor, 2
Reference numeral 0 is an image pickup unit, 25 and 27 are CCDs, and 90 is a side wall of a skeletal casing of the apparatus. The mirrors 7, 9a, 9b reflect and guide an original light image obtained by scanning exposure of the exposure light source into the imaging unit 20. The mirrors 7, 9a, 9b are all supported at both ends in the longitudinal direction. These mirrors 7,9a used in the optical system, the natural frequency f ₀ of 9b ^{_{is, 2πf 0 = (α) 2}} · (EI / ρA) 0.5 I = ωt 3/12 A = ωt where ρ is the mirror The density, E is Young's modulus, t is thickness, α is π / L when both ends are supported, and L is the distance between supporting portions, which can be approximately represented. FIG. 4 is a characteristic diagram showing the natural frequency of the mirror. In the figure, the horizontal axis represents the distance between the support portions in the mirror longitudinal direction, and the vertical axis represents the mirror primary resonance frequency. In the figure, the mirror thickness t is 3mm, 3.5mm, 4mm, 4.5m.
It is a characteristic curve of the result of measuring the primary resonance frequency when changing to m, 5 mm, 5.5 mm, and 6 mm. The stepping motor 10 is driven by a drive circuit (not shown).
It is driven to rotate in both forward and reverse directions with a step angle of 0.9 ° and a rotation speed of 83 rpm. The vibration wave when driving the stepping motor is the basic drive frequency f ₁ = 6 × 83.3 / 0.9 ° = 555Hz and the harmonic components of this basic drive frequency 1/2 ・ f ₁ = 278Hz, 1/4 ・ f ₁ = 138Hz, 2f
It is composed of ₁ = 1110Hz. The solid-state image sensor C-ch CCD25 and R-ch CCD27 has a pixel width of about 7
It is a line image sensor including a picture element of μm. The side wall 90 is an internal partition wall that forms the skeleton housing of the image reading apparatus, and the stepping motor 10 is attached to the outside of the side wall of the skeleton housing. Ie stepping motor 10
Of the stepping motor 10 and the pulley (not shown) at the end of the drive shaft are located inside the side wall 90. Is installed in. The exposure light source unit 40 and the movable mirror unit 80 are a high-density polyethylene laid on both sides of the skeleton housing side wall 90 by means of slide dowels 31 made of polyacetal or PPT resin attached to the bottom surfaces of the respective wings. It slides on a slide rail 3 made of steel. The vibration generated when the stepping motor 10 is driven is transmitted to the exposure light source unit 40 and the movable mirror unit 80 via the skeleton housing side wall 90, the slide rail 3, and the slide dowel 31. It is transmitted to each mirror 7, 9a, 9b via 33. FIG. 5 is a characteristic diagram showing the relationship between the natural frequency and the displacement of the mirror when the stepping motor 10 is driven. In the figure, the horizontal axis represents the natural frequency, and the vertical axis represents the displacement amount of the mirror central portion. Mirror thickness t, distance L between both ends support
To obtain a plurality of natural frequencies by providing several combinations. The plurality of mirrors having the respective frequencies resonate with the fundamental drive frequency of the stepping motor 10 and its harmonic components. For example, as a result of an experiment using a mirror having a distance L = 240 mm between both end supporting portions of the mirror, the condition is the worst when the distance between the mirror and the manuscript is about 350 mm. In that case, near the center of the mirror. When the displacement amount of exceeds 5 μm
The blurring amount of the original light image projected on the CCD becomes larger than the 1/4 pixel pitch, and the reproduced image is disturbed. In Fig. 5, when the mirror center displacement amount of 5 µm is used as a boundary value and the natural frequency of the mirror is set in this region or less, the blur amount of the original light image projected on the CCD is generally about 1 It turns out that it does not exceed the / 4 pixel pitch. The natural frequencies corresponding to the mirror displacement amount of 5 μm are 65% and 135% of the fundamental driving frequency f ₁ of the stepping motor 10 or its harmonic components 1 / n · f ₁ and n · f ₁ , respectively. Therefore, the natural frequency f ₀ of the mirror is f ₀ <0.65 · n · f ₁ and f ₀ > 1.35 · n · f ₁ 1 / n in the n-th harmonic region, and f ₀ <0.65 · It should be set within the range of 1 / n · f ₁ and f ₀ > 1.35 · 1 / n · f ₁ .

【The invention's effect】

According to the present invention, a stepping motor is used as a drive motor for driving the moving part of the optical system, and the natural frequency of the mirror and a value obtained by multiplying the fundamental drive frequency of the stepping motor by an integer n, or the natural frequency of the mirror, Since the value obtained by dividing the basic driving frequency of the motor by the integer n is different, the resonance phenomenon of the mirrors is prevented, the blurring of the original light image derived by these mirrors is reduced, and the clear image without disturbance is obtained. It has become possible to provide an image reading device that can obtain various reproduced images.

[Brief description of drawings]

FIG. 1 is a perspective view of an image reading apparatus according to an embodiment of the present invention,
FIG. 2 is a side sectional view of an image reading device according to an embodiment of the present invention, FIG. 3 is a configuration diagram of the image reading device, FIG. 4 is a characteristic diagram showing a natural frequency of the mirror, and FIG. FIG. 6 is a characteristic diagram showing the relationship between the natural frequency and the displacement amount, and FIG. 6 is a spectrum analysis diagram of the vibration frequency of the stepping motor. 1 ... original glass, 2 ... original 3 ... slide rail, 7 ... first mirror 9a, 9b ... mirror 10 ... stepping motor 20 ... imaging unit, 40 ... exposure light source unit 80 ... movable Mirror unit 90 …… Side wall of skeletal body f ₀ …… Mirror natural frequency, f ₁ …… Basic drive frequency n …… Integer

Claims (1)

[Claims] 1. An image reading apparatus in which an original on a platen glass is scanned and exposed by an exposure light source of an optical system and an optical image of the original is led out into an image pickup unit by a plurality of mirrors of the optical system. In the above, a stepping motor is used as a drive motor for driving the moving part of the optical system, and the natural frequency f ₀ of the mirror and a value obtained by multiplying the fundamental drive frequency f ₁ of the stepping motor by an integer n, or An image reading apparatus characterized in that a natural frequency f ₀ is different from a value obtained by dividing the basic driving frequency f ₁ of the stepping motor by an integer n.