JPH01200319A - Laser beam scanning device - Google Patents

Abstract

PURPOSE:To correct the scanning irregularity of an image resulting from the use of a rotary polygon mirror by controlling the frequency of a clock for modulating a laser beam in picture element units according to the check result of the state of a surface to be scanned which is one scan before. CONSTITUTION:One surface of the rotary polygon mirror 22 is scanned with a laser beam of a 1st system and then scanned with a laser beam of a 2nd system. Then, the detection output of a 2nd scanning start photodetector 35 becomes a 1st input to a function oscillator 58 and another voltage-controlled oscillator 59. The voltage outputted by a 2nd scanning end photodetector 36 becomes a 2nd input to the voltage- controlled oscillator 59. Further, the voltage outputted by the function generator 58 is added to the output voltage of a DC converter 47 by an adder 55 to generate a 3rd input to the voltage-controlled oscillator 59. Then, the oscillation output 67 of the voltage-controlled oscillator 59 is supplied to a frequency divider 68, a clock 69 which is obtained by frequency division is corrected in response to variation in scanning speed, and an information signal of each picture element is read out in synchronism with the clock 69. Consequently, proper correction corresponding to variation in the rotating speed of the rotary polygon mirror and the states of respective surfaces is enabled.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a laser beam scanning device used in a recording device that records images by manipulating a laser beam, and particularly relates to a laser beam scanning device used in a recording device that records images by manipulating a laser beam. The present invention relates to a laser beam scanning device that prevents such problems.

``Prior Art'' Recording devices that record images by scanning a laser beam on a photoreceptor and forming an electrostatic latent image are widely used as output devices for some types of copying machines and convenience stores. It looks like this. A laser beam scanning device is used as a device for manipulating a laser beam onto a scanned medium such as a photoreceptor. A rotating polygon mirror is usually used as a mechanical part for scanning a laser beam in a laser beam scanning device.

``Problem to be Solved by the Invention'' In a laser beam scanning device, if the plane accuracy of the rotating polygon mirror varies or uneven rotation occurs in the motor that rotates the rotating polygon mirror, images will be distorted. Therefore,
Several proposals have been made to prevent such inconveniences from occurring.

For example, in Japanese Patent Laid-Open No. 56-69665, variations in the scanning speed of a laser beam are detected on one surface of a rotating polygon mirror. Then, on the surface scanned on the photoreceptor,
The pixel clock pulse is controlled accordingly. According to this circuit, the surface of the rotating polygon mirror that detects fluctuations in scanning speed and the surface that feeds back the detected amount are continuous in chronological order, so uneven rotation of the motor that rotates the rotating polygon mirror can be avoided. Of these, those that occur at relatively large cycles can be well compensated for. However, since, for example, the flat area of each surface of a rotating polygon mirror differs from surface to surface, there is no measure to be taken against such factors that cause sudden speed fluctuations in each scanning period.

Another problem with laser beam scanning devices is that the image may expand or contract depending on the main scanning position of the scanned medium. FIG. 3 is for explaining this. Let the angular velocity of the rotating polygon mirror 11 be ω, and the length of the laser beam between the rotating polygon mirror 11 and the scanning start point on the surface 12 of the medium to be scanned be F. The scanning speed ■ of the laser beam on the scanned medium 12 is given by the following (when the deflection angle is θ)
1) It can be expressed by the following equation.

Vt (θ) = (1) FCQS θ...
(1) Therefore, the scanning speed V of the laser beam on the scanned medium 12 depends on COS θ,
If this continues, the image will be relatively shrunk in the central portion of the scanned medium 12, and the pixels will be expanded in the peripheral portion. Optical systems for correcting such distortions have been proposed, but these systems have the drawbacks of being complex and expensive.

SUMMARY OF THE INVENTION An object of the present invention is to provide a laser beam scanning device that can correct image scanning unevenness caused by the use of a rotating polygon mirror using a simple optical system.

"Means for Solving the Problems" The present invention includes (i) a laser beam modulator that modulates a laser beam according to an image information signal, (11) a rotating polygon mirror that scans the laser beam, and (iii) a rotating polygon mirror that scans the laser beam. A device equipped with a scanning motor that rotates this rotating polygon mirror (i
v) scanning speed monitoring means for detecting variations in scanning speed;
(v) clock generation means for generating a clock for modulating the laser beam pixel by pixel; and (vi) clock frequency control means for controlling the frequency of the clock according to the scanning speed monitored by the scanning speed monitoring means. A laser beam scanning device is provided.

Here, the scanning speed monitoring means includes a laser beam incidence means that makes the laser beam incident on a surface one surface before the surface that reflects the laser beam on the medium to be scanned, and measures the time taken for one scan of this laser beam. It may be of the kind that comprises a scanning time measuring means and a time comparing means for comparing the time measured by the scanning time measuring means with the time before one scan.

That is, in the present invention, the state of the surface to be scanned in the rotating polygon mirror before one scan is checked, and the frequency of the clock for modulating the laser beam pixel by pixel is controlled according to the result.

"Example" The present invention will be described in detail with reference to Examples below.

FIG. 1 shows the main parts of a laser beam scanning device in one embodiment of the present invention. This device includes a rotating polygon mirror 22 rotated by a scanning motor 21.

Two systems of laser beams are prepared to enter the rotating polygon mirror 22. The first semiconductor laser driving device 23 drives the first semiconductor laser 24 to output a laser beam belonging to the first system. This laser beam is focused by the first collimating lens 25 and is incident on a predetermined surface of the rotating polygon mirror 22. The incident laser beam moves in the direction of arrow 26A due to the change in the inclination of the surface as the rotating polygon mirror 22 rotates.
The reflection direction is sequentially changed from the direction of arrow 26B to the direction of arrow 26B. In the direction of arrow 26A, which can be considered as the scan start position, there is a first scan start photodetector 27, and in the direction of arrow 26B, which can be considered as the scan end position, there is a first scan end photodetector 28. each is placed. In addition, the rotating polygon mirror 22
A lens 29 for correcting the optical system is arranged between the photodetectors 27 and 28.

On the other hand, the second semiconductor laser driving device 31 drives the second semiconductor laser 32 to output a laser beam belonging to the second system. This laser beam is focused by the second collimating lens 33 and is incident on the surface of the rotating polygon mirror 22 next to the surface described above. The incident laser beam sequentially changes its reflection direction from the direction of arrow 34A to the direction of arrow 34B due to variations in the inclination of the surface as the rotating polygon mirror 22 rotates. A second scan start photodetector 35 is located in the direction of arrow 34A, which can be considered as the scan start position, and a second scan end photodetector 36 is located in the direction of arrow 34B, which can be considered as the scan end position. A lens 37 for correcting the optical system is arranged between the rotating polygon mirror 22 and these photodetectors 35 and 36.

Now, with the optical system configured as above, one of the rotating polygon mirrors 22
Focusing on one surface, this surface is first scanned by the first system of laser beams, and then scanned by the second system of laser beams. The second system of laser beams scans a scanned medium 38 consisting of a photoreceptor drum, etc., and the image scanning situation is corrected based on the scanning results of the first system of laser beams. Become.

In order to make such a correction possible, the detection output of the first scanning start photodetector 27 is manually inputted to a first system optical scanning pulse generator 41, from which a start pulse 42 is output. It looks like this. The start pulse 42 is
This becomes an input to the counter 43. Further, the detection output of the first scanning end photodetector 28 is manually inputted to a second first system optical scanning pulse generator 44, from which a stop pulse 45 is output. Stop pulse 45 becomes one input of phase comparator 46.

Phase comparator 46, charge pump/DA converter 47
, a voltage controlled oscillator (VCO) 48, and the above-mentioned counter 43 constitute a loop circuit, which detects the scanning speed of the reflected light 26 of the rotating polygon mirror 22. That is, the laser beam reaches the first scanning starting photodetector 27.
When the start pulse 42 is input and the start pulse 42 is output, the counter 43 starts counting. This counter 43 is preset to count up when it counts a predetermined number of pulses. When the counter 43 counts up, a count end signal 49 is output, which serves as the other input for the phase comparator 46. The phase comparator 46 compares the arrival timing of the count end signal 49 and the stop pulse 45 and outputs the result as a phase difference signal 51.

For example, the stop pulse 45 is better than the stop pulse 49.
Let us consider a case where the signal is input to the phase comparator 46 earlier than the above. In such a case, the rotational speed of the rotating polygon mirror 22 is relatively slow.

At this time, the charge pump/DA converter 47 increases its output voltage (1). As a result, the voltage controlled oscillator 48 lowers the frequency of the pulse signal 53 that it outputs. As a result, the time it takes for the counter 43 to count the predetermined number of pulses becomes longer,
If the situation is the same in the next scan, the count end signal 49
and the stop pulse 45 have the same phase.

On the other hand, the count end signal 49 is the start pulse 45.
If the input voltage is input to the phase comparator 46 later than that, the charge pump/DA converter 47 lowers its output voltage (2). This causes the voltage controlled oscillator 48 to increase the frequency of the pulse signal 53 that it outputs. As a result, the time it takes for the counter 43 to count the predetermined number of pulses described above becomes shorter, and if the situation is the same in the next scan, the count end signal 49 and the stop pulse 4
5 are in phase.

Unlike these, if there is no variation in the rotation speed of the rotating polygon mirror 22, the count end signal 49 is input to the phase comparator 46 at the same time as the stop pulse 45.

In this way, when these are in phase, the charge pump/DA converter 47 does not change its output voltage (1), and the frequency of the pulse signal 53 output from the voltage controlled oscillator 48 is kept constant.

From the above results, charge pump/DA converter 47
It can be seen that the output voltage ■1 is maintained at a voltage dependent on the rotation speed of the rotating polygon mirror 22.

This output voltage ■1 becomes one addition value of the adder 55.

By the way, the detection output of the second scanning start photodetector 35 is as follows.
The first second system optical scanning pulse generator 56 is manually powered, and a start pulse 57 is output from there. Start pulse 57 becomes the first input of function oscillator 58 and another voltage controlled oscillator 59. Further, the detection output of the second scanning end photodetector 36 is input to a second second system optical scanning pulse generator 61, from which a stop pulse 62 is output. The stop pulse 62 becomes the second input of the voltage controlled oscillator 59.

The function oscillator 58 is a circuit for generating a function for correcting the non-uniformity of the scanning speed of the laser beam in the main scanning direction as explained in FIG. 3, and plays the role of an fθ lens. The function oscillator 58 synchronizes with the clock signal 65 output from the reference clock generator 64 when the start pulse 57 is input manually, and generates a voltage 2 as a correction value at each time point as a function. This voltage (2) increases as the medium 38 to be scanned is scanned, reaches a peak at the center of the main scan, and then exhibits a change in which it decreases. In other words, the charge pump
When the output voltage ■1 of the DA converter 47 is constant,
The function generator 58 generates a voltage 2 so as to satisfy the relationship expressed by the following equation (2).

(2) Here, the clock frequency f refers to the frequency of the clock output from the voltage controlled oscillator 59.

The voltage (2) outputted from the function generator 58 is added to the output voltage (2) of the charge pump/DA converter 47 in an adder 55. The addition result VCIIT becomes the third power of the voltage controlled oscillator 59.

The voltage controlled oscillator 59 starts oscillating when the start pulse 57 is supplied. The oscillation frequency is the addition result ■. It changes over time depending on the time. In other words, the addition result VCIn
When the voltage is low, the oscillation frequency becomes high,
When the voltage is high, the oscillation frequency becomes low. Then, the oscillation is stopped when the stop pulse 62 is supplied. The oscillation output 67 of such a voltage controlled oscillator 59 is
The signal is supplied to a frequency divider 68 and frequency-divided. The resulting click 69 has a high frequency when the laser beam starts scanning the scanned medium 22, decreases toward the center, reaches the bottom at the center, and then gradually increases in frequency. This completes 1942 minutes of scanning.

Clock 69 is supplied to memory buffer 71 .

Incidentally, the clock signal 65 output from the reference clock generator 64 is also supplied to the information signal generator 72. The information signal generator 72 receives the scanned medium 38
This is the part that generates the signal that is the basis of the information recorded on the top. The information signal 73 for each pixel output from the information signal generator 72 in synchronization with the clock signal 65 is written into the memory buffer 71. The memory buffer 71 reads out these information signals for each pixel in synchronization with the clock 69 and supplies them as an image signal 74 to the second semiconductor laser driving device 31 . The second semiconductor laser driving device 31 modulates the laser beam according to this image signal 74 and outputs the modulated laser beam.

Therefore, when fishing on a boat, the image signal 74 is output in sections at a relatively high speed at the periphery of the scanned medium 38, and is output at a relatively low speed at the center. As a result, control is performed such that the scanned length of each pixel on the scanned medium 38 becomes equal. In addition, the rotating polygon mirror 22
Based on the state of the surface checked by the first system of laser beams, scanning for recording is performed using the second system of laser beams. Therefore, corrections for fluctuations in scanning speed due to variations in planar accuracy of individual surfaces of the rotating polygon mirror 22 are also performed in the same way.

FIG. 2 is for explaining the timing control of the laser beam scanning device. Here, the figure a-1 to a
-3 relates to the first system of laser beams. That is, a-1 in the same figure shows the timing at which the start pulse 42 is output, and a-2 in the same figure shows the timing at which the stop pulse 45 is output. A-3 in the figure shows the timing at which the count end signal 49 is output from the counter 43.

In FIG. 2, b-1 to b-3 relate to the second system of laser beams. The output voltage ■1 of the charge pump/DA converter 47 shown in b-1 of the same figure changes when the generation timing of the stop pulse 45° and the count end signal 49 shown in a-2 and a-3 of the same figure deviates. It can be seen that it is rising or falling depending on the situation. As a result, the output voltage ■1 as a correction value in each scanning period determined by the pair of start pulse 57 and stop pulse 62 shown in b-2 and b-3 in the same figure.
will be determined.

Next, in FIG. 2, c-1 represents the change in the voltage 2 output from the function generator 58. When the output voltage ■1 shown in b-1 in the same figure is added to this voltage ■2, the addition result ■ shown in c-2 in the same figure is obtained. It becomes a. 3 shows the generation time periods of the clock 69 output from the frequency divider 68, which are determined by the start pulse 57 and the stop pulse 62 as described above. The clock 69 is not based on a constant oscillation frequency, but is the result of addition ■. , varies based on
As a result, each pixel is arranged at equal intervals in the main scanning direction on the scanned medium 38.

In the laser beam scanning device of the embodiment described above, a semiconductor laser is used, but the invention is not limited to this. Further, although two systems of semiconductor lasers are used, two systems of laser beams may be generated from one laser generator using an optical branching means such as a half mirror.

"Effects of the Invention" As described above, according to the present invention, since the state of the surface to be scanned by the rotating polygon mirror before one scan is checked while actually scanning, fluctuations in the rotational speed of the rotating polygon mirror and Appropriate correction can be made according to the condition of each surface, the yield of scanning motors and rotating polygon mirrors can be improved, and the cost of the apparatus can be reduced.

[Brief explanation of the drawing]

Figures 1 and 2 are for explaining one embodiment of the present invention, of which Figure 1 is a schematic configuration diagram showing the main parts of a laser beam scanning device, and Figure 2 shows various timings of the device. The timing chart shown in FIG. 3 is an explanatory diagram for explaining the cause of image distortion in the laser beam scanning device. 21... Scanning motor, 22... Rotating polygon mirror, 24... First semiconductor laser, 27...
・First scan start photodetector, 28...First scan end photodetector, 31...Semiconductor laser drive device, 32...Second photodetector Semiconductor laser, 38.
... Scanned medium, 43 ... Counter, 46 ... Phase comparator, 48.59 ... Voltage controlled oscillator, 68 ...
...Frequency divider. Applicant Fuji Xerox Co., Ltd. Agent
Patent Attorney Umeo Yamauchi Figure 2 (a-s)
~49 Figure 3

Claims (1)

[Claims] 1. A laser beam modulator that modulates a laser beam according to an image information signal, a rotating polygon mirror that scans the laser beam, a scanning motor that rotates this rotating polygon mirror, and this scanning motor scanning speed monitoring means for monitoring fluctuations in the rotational speed of the rotating polygon mirror and fluctuations in scanning speed due to variations in planar accuracy of the rotating polygonal mirror; clock generating means for generating a clock for modulating the laser beam pixel by pixel; A laser beam scanning device comprising clock frequency control means for controlling the frequency of the clock according to the rotational speed monitored by the monitoring means. 2. The scanning speed monitoring means includes a laser beam incidence means that makes the laser beam incident on a surface one surface in front of the surface on which the laser beam is reflected on the medium to be scanned, and measures the time taken for one scan of this laser beam. 2. The laser beam scanning device according to claim 1, further comprising a scanning time measuring means for measuring the scanning time, and a time comparing means for comparing the time measured by the scanning time measuring means with the time before one scanning.