Scanned image calibration device and method thereof for adjusting a scan frequency

A device for calibrating scanned image includes a scanning unit arranged in a paper transmitting path for capturing texts and images on a document and transferring into electrical signals, a set of transmitting device which includes at least a first transmitting roller arranged upstream to the scanning unit, and a second transmitting roller arranged downstream to the scanning unit. There is no other transmitting roller arranged between the first and the second transmitting rollers, the distance between the first and the second transmitting rollers is shorter than the length of the document, and the second transmitting roller is faster than the first transmitting roller. A paper sensor is arranged in the paper transmitting path for detecting the position and the velocity of the document, and a control unit is connected with the paper sensor and the scanning unit to adjust the scan frequency in response to the position and the velocity of the document.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for calibrating scanned image and a method thereof, especially to the device and method for calibrating image by digital control.

2. The Related Art

Referring toFIG. 1, a common automatic document feeder uses a plurality of rollers22′,23′ to transmit documents from an input tray to a scanning device30′ for capturing the images and texts printed on a document. In order to keep the document flat, the roller23′ arranged downstream rolls faster than the roller22′ arranged upstream. By using the velocity difference of the upstream and downstream rollers22′,23′ to pull and flatten the document.

Referring toFIG. 2, when the document10passes through the scanning device30′, the scanning device30′ will capture a stripe of image from the surface of the document. In order to describe more easily, said stripe of image will be defined as scan lines60below. Because the scanning device30′ captures the image in a constant frequency, the distance between two successive scan lines60is also the same. After the whole document is transmitted through the scan device30′, all of the scan lines60will be put together and form a completed image.

Referring toFIG. 3, because the completed scan image is assembled with each of scan lines60, and the distance between each scan lines60will change with the transmitting speed of the document10, so the scale of the scanned image along the transmitting direction will change greatly according to the acceleration or deceleration of the document. And the most common reason of velocity change is caused by the velocity difference of the upstream roller22′ and the downstream roller23′. When the following end of the document10is leaving the upstream roller22′, the document10will accelerate instantly with the pulling of the downstream roller23′ and enlarge the distance between each scan line60.

To avoid the scale of the scanned image changing, there are a variety of technical solutions for preventing the document from accelerating when the following edge thereof leaves the upstream roller22′. Such as the solution disclosed in the U.S. Pat. No. 3,988,817 uses different types of material to construct rollers, so the document can leave the roller much more slowly, and thus avoid the prompt acceleration. And solution disclosed in the U.S. Pat. No. 6,530,569 uses an actuator which is connected with the roller to move the roller away from the document slowly when the document is leaving the roller, and thus avoid the prompt acceleration.

However, the roller constructed by two or more kinds of material is apt to fail to work when the softer part wears out, on the other hand, mounting an additional actuator will increase the product cost. Therefore, it is necessary to improve the conventional automatic document feeder.

SUMMARY OF THE INVENTION

An objective of this invention is to provide an image calibrating device for a document scanner which is easy to be maintain and to be produced.

To achieve the objective, the image calibrating device includes a scanning unit arranged in a paper transmitting path for capturing texts and images on the document and transferring into electrical signals, a set of transmitting device including at least a first transmitting roller and a second transmitting roller that rotates faster than the first transmitting roller, the first transmitting roller being arranged upstream to the scanning unit, the second transmitting roller being arranged downstream to the scanning unit, the distance between the first and the second transmitting rollers being shorter than the length of document, and there is no other transmitting roller being arranged between the first and the second transmitting rollers, a paper sensor arranged in the paper transmitting path for detecting the position and the velocity of the document, and a control unit connected with the paper sensor and the scanning unit to adjust the scan frequency in response to the position and the velocity of the document.

And the method for calibrating scanned image is described below:

transmitting the document through the scanning unit page by page with the first and the second transmitting rollers, wherein the first transmitting roller being arranged upstream to the scanning unit, the second transmitting roller which rotates faster than the first roller being arranged downstream to the scanning unit, there is no other transmitting roller being arranged between the first and the second transmitting roller, the distance between the first and the second transmitting roller being shorter than the length of the document, detecting the position and the velocity of the document with the document sensor, starting the scanning unit to capture the text and image on the document when the document sensor detecting the document being passing through the scanning unit, and adjusting the scan frequency of the scanning unit when the document sensor detecting the transmitting state or velocity of the document changed.

In summary, the device in this invention calibrates scanned image by adjusting the scan frequency according to the transmitting state and the velocity changes of the document. Therefore, there is no need for the image calibrating device in this invention to use any additional actuator and components, so it will not failure due to mechanical wear. Thus the image calibrating device in this invention is easy to be produced and maintain.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 4, it is known that the document will accelerate when the following end of a document leaves the transmitting roller, and thus make the scanned image shorter than the original image printed on the document. In order to solve said problem, the scanned image calibrating device in this invention includes: a scanning unit30arranged in the transmitting path21for scanning and transferring image and texts on the original into electrical signals. A transmitting device20that includes at least a first transmitting roller22and a second transmitting roller23, wherein the first transmitting roller22is arranged upstream to the scanning unit30, the second transmitting roller23is arranged downstream to the scanning unit30, there is no other transmitting roller settled between the first and second transmitting rollers22,23, the second transmitting roller23rotates faster than the first transmitting roller22, and the distance there between is shorter than the length of the document. A document sensor21is arranged in the transmitting path21for detecting the position and the velocity of the document, and a control unit50is connected with the scanning unit30and the paper sensor40for adjusting the scan frequency of the scanning unit30in response to the velocity or position change of the document (not shown) to calibrate defects caused by velocity change of the document.

The paper sensor40in this embodiment includes a paper pass sensor41and a scan sensor42, the paper pass sensor41is arranged next to the first transmitting roller22and the scan sensor42is arranged next to the scanning unit30. The paper pass sensor41and the scan sensor42are arranged to detect the moment of the document when the document passes through the first transmitting roller22and when the document leaves the scanning unit30respectively. When the document is passing through the first transmitting roller22or leaving the scanning unit30, the paper sensor40will send a signal to the control unit50to adjust the scanning mode.

Wherein, the scan sensor42is a micro switch sensor which will be active when the document passes through the scan sensor42, and the paper pass sensor41is an ultrasonic sensor which can measure thickness of the document. Therefore, the paper pass sensor41in this invention can detect both statements of document passing through the paper pass sensor41and multi-feed.

Referring toFIG. 5, the process of calibrating scanned image are shown as follow:

When the automatic feeding scanner starts to scan, the document will be transmitted through the transmitting path21page by page with the transmitting device20, meanwhile the paper sensor40arranged along the transmitting path21also starts to detect the position and the velocity of the document. When the document is passing through the scanning unit30, the scan sensor42will send a scan signal to the control unit50, and after the control unit50receives the scan signal, the control unit50will control the scanning unit30to scan in response. Furthermore, when the transmitting state of the document (such as transmitting velocity) changes, the paper pass sensor41will send an adjusting signal to the control unit50, and in response, the control unit50will adjust the scan frequency of the scanning unit30. After the document finishes the scanning and leaves the scanning unit50, all of the scan lines will be put together to form a complete image. By adjusting the scan frequency according to the transmitting state, the image calibrating device in this invention can solve the image defects effectively.

In this embodiment, in order to prevent the document from prompt acceleration while the following end thereof leaves the first transmitting roller22and causes the scale of the scanned image to change, it is necessary to measure the moment when the document starts to accelerate, and the process is shown as follow:

Referring toFIG. 6, when the scanning unit30starts to scan, the scan sensor42is to monitor the leading end of the document. When the document is passing through the scanning unit30, the scan sensor42will be triggered to send a scan signal to the control unit50. After receiving the scan signal, the scanning unit30will start scanning in response.

Referring toFIG. 7, because the document will accelerate instantly when the following end of the document leaves the first transmitting roller22, it is possible to determine when the document will accelerate according to the leaving movement detection. In other words, the moment that the scan sensor42still can detect the document and the paper pass sensor41cannot detect the document is the moment that the document is going to accelerate. By then, the detection of the paper passing through the sensor41will send an adjusting signal to the control unit50, and the adjusting signal requests the control unit50to increase the scan frequency of the scanning unit in response.

Referring toFIGS. 6 to 9, in order to avoid the exposure value being changed on account of the scan frequency being accelerated, the process for adjusting the scan frequency in this embodiment is shown as follow:

Before the following end of the document10leaves the first transmitting roller22, the scanning unit30scans the document10in original frequency. At this duration, the scanning unit30turns on and off intermittently, and for describing easily, the period that scanning unit30turns on will be defined as the first scan period (A), and the period that scanning unit30turns off will be defined as stop period (B) in the following description. Meanwhile the light source (not shown) also turns on and off intermittently to obtain the correct exposure, and the period that the light source turns on is defined as light on period (C) and the period that the light source turns off is defined as the first light off period (D). In this embodiment, the first scan period (A) is settled to eight units of time, the stop period (B) is settled to one unit of time, the light on period (C) is settled to five units of time and the first light off period (D) is settled to four units of time.

And when the following end of the document10leaves the first transmitting roller22and accelerates the document10, the scan frequency will be increased to coordinate the velocity of the document10. In this embodiment, the period that the scanning unit30remains turning on and the period that the light source remains turning off are changed by the same length of time, the adjusted period will be defined as the second scan period (A′) and the second light off period (D′) in the following description. In order to increase the scan frequency of the scanning unit30, the second scan period is reduced to six units of time, the stop period still is one unit of time, the light on period also is unchanged to avoid changing the exposure value, and the second light off period is reduced to two units of time.

As a result, the changed value of scan period (A′-A) is equal to the changed value of light off period (D-D′), so the scan frequency and the flash frequency of the light source increase synchronously. And both the first and second scan periods (A, A′) are longer than the light on period (C) which maintains constant. It makes the exposure value remain constant regardless of the change of the scan frequency.

In summary, the device in this invention calibrates scanned image by adjusting the scan frequency according to the transmitting state and the velocity changes of the document. Therefore, the image calibrating device in this invention doesn't have to use any additional actuator and components to prevent failure due to mechanical wear. Thus the image calibrating device in this invention is easy to be produced and maintain.