JP2743408B2 - Laser beam scanning speed correction method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning speed of a laser beam scanning optical system which is incorporated in a laser printer, a facsimile, or the like, and scans a recording medium with a laser beam modulated based on image information. It relates to a correction method.

2. Description of the Related Art Conventionally, a laser beam scanning optical system basically includes a semiconductor laser as a light source, a deflector such as a polygon mirror,
It is composed of a θ lens. The deflector is for scanning the laser beam emitted from the semiconductor laser at a uniform angular velocity in one plane, and as it is, a difference in scanning speed occurs from the center to the end on the main scanning line of the recording medium, Dot pitches when exposing at isochronous timing vary. The fθ lens is provided to correct such a difference in scanning speed and to keep the dot pitch constant when exposure is performed at isochronous timing over the entire scanning area.

By the way, the fθ lens is a combination of various concave lenses, convex lenses, and the like, has a complicated design, has a large number of polished surfaces, is difficult to achieve processing accuracy, and is expensive.

On the other hand, when the fθ lens is not used, the image height (y) on the recording medium is given by the following equation.

y = Ltan θ where L: distance from deflection point to recording medium Since the deflector is driven to rotate at a constant angular velocity, the scanning speed (V) on the recording medium is given by the following equation.

V (θ) ∝dy / dθ = L / cos ² θ That is, the scanning speed increases according to the equation as the deflection angle (θ) increases. If attempt is made whether the corrected as scanned by this as if constant speed may be changed in proportion to the period of the synchronizing control clock of the laser beam to cos ² theta.

JP-A-54-12853, JP-A-58-87965, JP-A-61-128869, JP-A-60-36620, and JP-A-61-184050 disclose the techniques disclosed in JP-A-61-184050. , Fθ lens is omitted, and the main scanning speed is to be corrected based on the above equation.

However, if the period of the laser beam synchronization control clock is corrected in accordance with the fluctuation of the scanning speed as in the above-described conventional technology, 420 MHz which is 100 times the current modulation frequency of 4.2 MHz is used. Requires about a reference clock. As a practical matter, using such a high frequency as a reference clock requires a frequency at which circuit components can follow at most.
Considering that it is about 100 MHz, a low-speed printing machine is impossible apart from it. For this reason, it is conceivable to take measures such as setting the distance (L) from the deflection point to the recording medium to be long, or significantly reducing the frequency of the reference clock. However, this results in a poor correction state.

Therefore, an object of the present invention is to provide a method capable of sufficiently correcting the scanning speed without setting the frequency of the reference clock too high and without impairing high-speed printing.

Means for Solving the Problems In order to solve the above problems, a laser beam scanning speed correction method according to the present invention has a laser beam control signal having a plurality of cycles to synchronize the turning on and off of a laser beam. Changing the density of the control signals by changing the ratio of mixing and using the plurality of cycles of the laser beam control signals as the scanning area shifts from the center to the end of the main scanning line. It is characterized by.

More specifically, for example, the density gradient of the cycle of the plurality of laser beam synchronization control signals is continuously distributed from the center to the end of the scanning area, or the scanning area is divided into several blocks, and the block is divided into blocks. The composition ratio of a plurality of periods within is set to a constant gradient from the center to the end of the scanning area. Alternatively, the distribution state of a plurality of periods is repeated in a certain pattern in the divided block as described above, and the period distribution is set to be substantially uniformly dispersed in the block. Within the divided block, the block is further divided into several areas according to the types of the plurality of cycles, and one area is scanned at one type of the cycle according to the composition ratio of the cycles. Alternatively, the values of the periods occupying the area in a certain block are arranged in the descending order or the descending order.

The control signal for synchronizing the on / off of the laser beam is
It is not always necessary to make the change continuously, but may be set to a plurality of periods. By changing the density of the control signal as the scanning area shifts from the center to the end, the control signal density is changed. Fluctuations in the scanning speed on the main scanning line due to angular velocity scanning are effectively corrected. At this time, the frequency of the reference clock need only be about 10 times that of the commonly used 4.2 MHz, and a correction effect equivalent to that of the reference clock that is about 100 times can be obtained.

Embodiment FIG. 1 shows a schematic configuration of a laser beam scanning optical system for implementing a scanning speed correction method according to the present invention, and (1)
Is a semiconductor laser, (2) is a collimator lens, (3) is a cylindrical lens, (4) is a polygon mirror,
(5) is a toroidal lens, (6) and (7) are folding mirrors, and (8) is a photosensitive drum.

The semiconductor laser (1) emits a modulated divergent light beam driven and modulated by a drive circuit (30) shown in FIG. This divergent light beam is converged or corrected to a parallel light beam by passing through the collimator lens (2). Polygon mirror (4)
Is rotated by a motor (not shown) at a constant speed in the direction of arrow (a) with its central axis as a fulcrum, continuously reflects the laser beam from the semiconductor laser (1) on each surface, and forms a uniform angular velocity in one plane. Reflected by The cylindrical lens (3) and the toroidal lens (5) are for correcting pitch unevenness in the sub-scanning direction due to surface tilt of the polygon mirror (4) (vertical error of each surface). The laser beam reflected by the polygon mirror (4) is reflected by the return mirrors (6) and (7), is focused on the exposure drum (8), scans on the main scanning line (9), and scans the main scanning line (9). This is called main scanning. The photosensitive drum (8) is driven to rotate at a constant speed in the direction of the arrow (b), and scanning by this rotation is called sub-scanning, and a latent image is formed on the photosensitive drum (8) by main scanning and sub-scanning. It is formed.

By the way, in the laser beam scanning optical system having the above configuration, when modulation is performed using a laser control signal having a constant period, a latent image on the main scanning line (9) is shown in FIG. State. In FIG. 2, (y ₁ ) and (y ₁ ′) are printing intervals in the main scanning direction corresponding to one cycle (T ₁ ) of the control signal of the laser beam. Since the scanning speed is higher at the end than at the center of the scanning area, y ₁ <y ₁ ′ as is clear from the above equation. (Y ₂ ) and (y ₂ ′) are printing in the main scanning direction corresponding to the time (T ₂ ) during which the semiconductor laser (1) is on in one cycle (T ₁ ) of the laser beam control signal. Length (dot length), and y ₂ <y ₂ ′.

In this embodiment, the scanning area is divided into seven blocks from the center to the end of the main scanning line (9) under the following specifications, and the cycle of the laser beam control signal is changed for each block.

Specification System speed: 35mm / sec Effective scanning width: 210mm (A4 size width dimension) Distance from deflection point to recording medium: 400mm Printing density: 300dots / inch Polygon surface number: 6 The scanning speed is corrected for specifications of 6 or more. If not, the modulation frequency of the laser beam is about 4.2 MHz. By the way, the above equation gives a speed ratio with respect to the deflection angle (θ). When this is converted into a speed ratio with respect to the image height, the following equation is derived.

V (y) ∝1 / cos ² ｛tan ^-1 (y / L)｝ Therefore, the control clock cycle (T) is set as T (y) ∝cos ² ｛tan ^-1 (y / L)｝. Is satisfied, the scanning speed can be equally corrected from the center to the end of the scanning area.

Table 1 below shows the control clock cycle as a ratio for each image height of 5 mm with respect to the above specification.

If the control signal is corrected based on Table 1 at the rate shown in Table 2 below, the printing accuracy in the main scanning direction can be suppressed to within ± 0.5%.

When the scanning speed is corrected according to the conventional example described in the section of the problem to be solved by the above-mentioned invention, the latent image on the main scanning line (9) is in the state shown in FIG. 2B. . That is, the printing interval (y ₁ ′) in the main scanning direction becomes substantially equal to (y ₁ ) within an error range of about 1%. However, the printing length (y ₂ ′) becomes longer at the end, that is, y ₂ <y ₂ ′, unless correction is made. However, in this kind of speed correction, it is necessary to oscillate the reference clock at a high frequency of 420 MHz, which is 100 times the modulation frequency of 4.2 MHz when no correction is performed, which is practically difficult as described above. .

Therefore, in the present embodiment, based on the above specifications, and the same block division as in Table 1, the normal modulation frequency 4.
Oscillates a reference clock 10 times higher than 2 MHz, and this clock
The composition ratio between the portion where one dot is printed at 10 counts and the portion where one dot is printed at 9 counts was set as shown in Table 3 below.

The above composition ratios are distributed so as to be distributed almost uniformly in each block. As a result, the main scanning line (9)
The above latent image is in the state shown in FIG. That is, y ₁ ′ <y ₁ at a portion where one laser beam control signal is generated at 9 counts of the reference clock, and y ₁ <y ₁ ″ at a portion where one laser beam control signal is generated at 10 counts. As shown in Table 3 above, the composition ratio of (y ₁ ′) and (y ₁ ″) is given a density gradient for each block (scanning area), thereby macroscopically increasing the printing interval from the center to the end. If it is viewed uniformly over, for example, the unit of 5 mm, it is possible to correct the scanning speed to the same extent as when oscillating a 100-fold reference clock.

Although the above-described correction state has been described with reference to FIG. 2 in which the correction state is partially enlarged, a supplementary description will be given here.

In this embodiment, the number of dots recorded on one main scanning line in the scanning optical system having the above specifications is (210 / 25.4) ×
300 = 2480. The increase (Δy) of the scanning length per cycle of the laser beam control signal at the center and the end of the scanning area when no correction is performed is given by the following equation from the above equation.

Here, when y = 105 and L = 400 are substituted into the equation, (Δ
y) is 6.89%. That is, the length of 100 at the center extends to about 107 at the end. Ideally, the clock cycle T (y) should be changed in accordance with the equation. However, when the correction is performed by dividing the block into seven blocks as in the present embodiment, the cycle of the control signal for the image height (y) becomes as shown in FIG. The relationship is as shown.

The number of dots included in each block is as shown in Table 4 below corresponding to the value of the image height.

As is clear from Table 4, even in the seventh block having the smallest number of dots, the image height is 8.5 mm (96.5 to 105 mm).
The included 100 dots for the length, according to the configuration ratio of the laser beam control signal cycle in this embodiment, occupies 40% of the (y ₁ ") occupies the other 60% (y ₁ ′) Despite being printed at 10/9 intervals,
Since (y ₁ ′) is equal to both and there are about 12 dots per 1 mm, the printed result is Is visually observed as if each dot was weighted average. Microscopically, for y ₁ = 1, y ₁ ′ = 0.962 y ₁ ″ = 1.0069, but the difference between these lengths is (1.069−0.962) × 25.4 / 300 = 0.09 mm. It is a size that does not cause any problem visually.

Specifically, for each dot, whether the dot should be regarded as one dot at 10 counts of the reference clock or whether it should be regarded as one dot at 9 counts is shown in FIG. It is convenient to store information for 2480 dots in the read-only memory (23) shown in FIG. That is, the first dot is controlled so that ten clocks are input and turned on, and the second dot is controlled so that nine clocks are input and turned on.

Ideally, the two types of periods (y ₁ ′) and (y ₁ ″) are randomly distributed, but for the sake of simplicity, the first half of each block prints one dot with a clock of 10 counts and the second half. May be controlled so as to print one dot at 9 counts.The configuration of the control signal for the image height value in this case is shown in Table 5 corresponding to Table 4 above.

By the way, the states of the latent images when the two types of periods are randomly dispersed in each block as described above and when the two types of periods are separated in the first half and the second half are schematically shown in FIG. 4 and FIG. . FIG. 4 shows a state of random dispersion, and FIG. 5 shows a state of separation. Both shaded portions indicate portions corresponding to 1 dot printing at 9 counts, and other portions correspond to 1 dot printing at 10 counts. Show. FIG. 4 shows that the pattern shown is repeated uniformly for each block, and FIG. 5 shows that the first and second half regions in each block are uniformly 1 dot at 10 counts and 1 dot at 9 counts. Is printed.

By the way, the print length in the main scanning direction (y ₂ ), (y ₂ ′)
The (pulse width) is not corrected only by the above-described correction of the scanning speed, and y ₂ <y ₂ ′ as shown in FIG. 2C. Therefore, the print length (pulse width) is also corrected at the same time, that is, the pulse width of the portion (T ₂ ) corresponding to the ON time of the semiconductor laser (1) in one cycle (T ₁ ) of the laser beam control signal. Is preferably controlled so as to be substantially proportional to cos ² θ with respect to the deflection angle (θ) as the position shifts from the center to the end of the main scanning line (9). According to this correction, 'as shown in, y ₂ in FIG. 2 (C)' ≒ y _2, and the printing width of each dot is approximately equal toward the end from the central portion.

Although not shown in FIG. 2, the illuminance (exposure amount) of the laser beam emitted from the semiconductor laser (1) decreases in proportion to cos ² θ from the center to the end. Therefore, it is preferable to control the emission intensity of the semiconductor laser (1) so as to be substantially proportional to 1 / cos ² θ with respect to the deflection angle (θ). According to this correction, the exposure amount for each dot becomes substantially equal from the center to the end of the main scanning line (9).

FIG. 6 shows a control circuit, and a scanning speed correction circuit (20).
Is the reference clock oscillator (21), address counter (2
2), read-only memory (23), latch circuit (2
4) It consists of a clock oscillator (25) and a programmable counter (26). The composition ratio of a plurality of periods is stored in a read-only memory (23), and the density gradient of this composition ratio is programmable counter (26). Is controlled by
The semiconductor laser (1) is turned on and off by a drive circuit (30).

FIG. 7 shows a control circuit including a pulse width correction circuit (40) for correcting the pulse width (dot length) shown by (C ') in FIG. The pulse width correction circuit (40) consists of a reference clock oscillator (41), an address counter (42), a read-only memory (43), a latch circuit (44), and a modulator (4
The output from the scanning speed correction circuit (20) is input to a modulator (45). Here, co for correcting the pulse width of the portion where the semiconductor laser (1) is turned on.
The coefficient almost proportional to s ² θ is read-only memory (43)
Is stored as data.

FIG. 8 shows an exposure amount correction circuit (50) in addition to the circuit of FIG.
2 shows a control circuit to which is added. The exposure correction circuit (50) controls the input current of the semiconductor laser (1).
Reference clock oscillator (51), address counter (52),
The output from the pulse width correction circuit (40), which comprises a read only memory (53), a latch circuit (54), and a power controller (55) and is connected to the scanning speed correction circuit (20), is a power controller (55) Is input to Here, a coefficient substantially proportional to 1 / cos ² θ for correcting the exposure amount is stored as data in the read-only memory (53).

As is apparent from the above description, according to the present invention, a laser beam control signal having a plurality of cycles is provided for synchronizing the on / off of the laser beam, and the scanning area is located at the center of the main scanning line. With the transition from to the end, the density of the control signal is changed by changing the ratio of mixing and using the laser beam control signals of the plurality of cycles, so that the fθ lens is not used. In addition to being able to effectively correct the scanning speed on the main scanning line due to the deflection angle (θ) from the center to the end, the frequency of the reference clock for controlling the on / off of the laser beam can be adjusted to the conventional value. It can be reduced to about 1/10 or less of the proposed correction method.

[Brief description of the drawings]

1 shows an embodiment of a scanning speed correction method according to the present invention, FIG. 1 is a perspective view showing a schematic configuration of a laser beam scanning optical system, and FIG. 2 shows a state of a latent image on a main scanning line. FIG. 3 is a graph showing the period of the laser control signal with respect to the image height, FIGS. 4 and 5 are explanatory diagrams showing the ratios of laser control signals having different periods, and FIG. 6 is a block diagram showing a control circuit FIG. 7, FIG. 7, and FIG. 8 are block diagrams showing modified examples of the control circuit. (1) ... semiconductor laser, (4) ... polygon mirror,
(8) photoconductor drum, (9) main scanning line,
(20) ... Scan speed correction circuit.

Claims (1)

(57) [Claims] 1. A laser beam scanning optical system for deflecting a laser beam modulated based on image information at a constant angular velocity by a deflector and scanning on a recording medium to synchronize on / off of a laser beam. Having a laser beam control signal having a period of, and changing the ratio of mixing and using the laser beam control signals having the plurality of periods as the scanning area shifts from the center to the end of the main scanning line. And changing the density of the control signal according to the above method.