JPH0580261A - Laser beam scanning position detection device - Google Patents

Abstract

PURPOSE:To provide the laser beam scanning position detection device of simple constitution which can accurately detect the timing where a laser beam passes a specific position. CONSTITUTION:A laser beam is detected by a photodetector 15, whose current output is converted by a current-voltage converting circuit 17 into a voltage signal (a). A differentiating circuit 19 differentiates the signal (a) and outputs a signal (b) and a zerocross circuit 20 makes the signal (b) into pulses by a zero-cross method. An edge detecting circuit 21 is triggered with an edge of the pulsated signal (c) and outputs a pulse signal (d). A waveform shaping circuit 18, on the other hand, outputs a signal (e) which goes up to a high level when the signal (a) is higher than the reference voltage and goes down to a low level when the signal (a) is lower than the reference voltage. A latch circuit 23 latches the signal (e) as data by using the pulse signal (d) as a clock. Consequently, a pulse signal (f) is obtained which rises at the timing of the peak of the signal (a) to accurately indicates the timing where the laser beam passes through the photodetector 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam scanning position which is preferably used for determining the print start timing of a laser beam printer by detecting the timing when the laser beam passes a predetermined position. The present invention relates to a detection device.

[0002]

2. Description of the Related Art Conventionally, in a certain type of laser beam printer, a change in process condition is corrected by positively changing a laser power during operation. However, in such a laser beam printer, In order to determine the writing position of characters and images in the main scanning direction, the laser beam is detected by a photodetector such as a photodiode,
The rising edge of the output signal is used as a reference. However, such a laser beam printer has a problem that the writing start position is displaced when the laser power is changed.

In order to alleviate such a problem and improve the accuracy of the writing start position, the output signal of the photodetector provided on the beam scanning line is amplified and shaped into a sync pulse, and the writing start position is determined based on the sync pulse. The method of making a decision is generally adopted. An example of such a laser beam printer is shown in FIG.

In FIG. 5, a laser beam emitted from a laser oscillator 1 is main-scanned by a rotary polygon mirror, that is, a polygon mirror 2, and enters a recording surface 4 through an f-θ lens 3.
The image is recorded. Further, the photodetector 5 is arranged on the main scanning line on the scanning start side, and its output signal is output as the sync pulse signal S via the amplification / waveform shaping circuit 6.

FIG. 6 shows the details of the amplification / waveform shaping circuit 6. When a laser beam is incident on the photodetector 5, a photocurrent proportional to the amount of incident light is output from the photodetector 5.
The current is converted into a voltage by the current / voltage conversion circuit composed of the operational amplifier 7 and the resistor R1 and input to the non-inverting input terminal of the comparator 8. Comparator 8
The voltage determined by the resistors R2 and R3 is applied to the inverting input terminal of the signal, and the signal from the current / voltage conversion circuit is compared with this voltage by the comparator 8 and shaped into a pulse signal to obtain the synchronization pulse signal S. Is output.

By the way, in general, the tilt angle on each surface of the polygon mirror is not constant, the presence of dust in the air, and the laser power itself fluctuates. The output signal level of the photodetector for detecting the beam is not constant, but changes as shown in FIG. 7, for example. That is, in this case, the signal level output from the photodetector 5 and input to the comparator 8 fluctuates as shown in FIG. 7. As a result, the timing of the sync pulse signal S fluctuates and the write start position is changed. It becomes difficult to make an accurate decision.

FIG. 8 shows this state in detail.
Waveforms S1 and S2 are input signal waveforms of the comparator 8 when the incident light on the photodetector 5 is strong and weak, respectively. The straight line L indicates the voltage applied to the inverting input terminal of the comparator 8. As is clear from the figure, the pulse width of the sync pulse signal S output from the comparator 8 is
The waveforms S1 and S2 are different from each other. As a result, when such a synchronization pulse signal S is used, the image recording start time fluctuates by Δt, resulting in deterioration of image quality.

In order to solve such a problem, for example, the pulse width of the output pulse from the photodetector is counted, and the write start position is determined according to the counted value (Japanese Patent Laid-Open No. 61-61).
25363) or a method of detecting the light quantity of the laser beam and feeding it back to the waveform shaping circuit (Japanese Patent Laid-Open No. 60-23).
3614) and a method of narrowing the pulse width of a spread input waveform to reduce the influence of variations in the input signal (Japanese Patent Laid-Open No. 58-13069).

[0009]

However, in order to improve the accuracy of writing position determination by the above method,
The frequency of the clock signal for counting has to be increased so as to be within the writing error tolerance range (Japanese Patent Laid-Open No. 61-25363), and a detection period for detecting the light amount is required and the circuit is required. The configuration is complicated (Japanese Patent Laid-Open No. 60-233614), and the adjustment of each part is complicated and delicate. When the input voltage level changes significantly, the delay time is constant and the fluctuation cannot be completely suppressed. (Japanese Patent Laid-Open No. 58-13069) has problems.

The present invention has been made in view of the above-mentioned conventional problems, and provides a laser beam scanning position detecting device having a relatively simple structure and capable of accurately detecting the timing at which a laser beam passes a predetermined position. The task is to do.

[0011]

In order to achieve the above-mentioned object, a laser beam scanning position detecting device of the present invention is a photodetector which is arranged at a predetermined position through which a laser beam passes and which detects the optical output of the laser beam. A peak detecting means for detecting the peak time of the detection signal of the photodetector, and the detected peak.
Pulse generation means for generating a pulse based on the clock time.

[0012]

In the laser beam scanning position detecting apparatus of the present invention, when the photodetector arranged at a predetermined position through which the laser beam passes detects the optical output of the laser beam, the peak detecting means causes the photodetector to detect the laser beam. The peak time of the detection signal is detected. The pulse generating means generates a pulse based on the peak time thus detected. Here, for example, the mismatch of the tilt angles on the respective surfaces of the polygon mirror as described above,
Even if the level of the light output detected by the photodetector or the amplitude of the detected waveform changes due to the presence of dust in the air or the fluctuation of the laser power itself, as shown in FIG. The peak time t0 of the detection signal of does not change. Therefore, the pulse generated by the pulse generation means based on the peak time of the detection signal of the photodetector always accurately represents the timing when the laser beam passes through the photodetector.

The peak detecting means can be composed of, for example, a differentiating circuit for differentiating the detection signal of the photodetector. The pulse generation means includes, for example, a zero-cross circuit that shapes the waveform of the output signal by the zero-cross method, an edge detection circuit that outputs a pulse whose logic level changes at the timing of the edge of the output signal of the zero-cross circuit, and the photodetector. Can be composed of a waveform shaping circuit that compares the detection signal of 1) with a predetermined reference level to shape the waveform, and a latch circuit that latches and outputs the output signal of the waveform shaping circuit with the pulse of the edge detection circuit.

As a result of the above, the laser beam scanning position detecting device of the present invention has a relatively simple structure and can accurately detect the timing at which the laser beam passes a predetermined position.

From the following examples of the present invention, such effects of the present invention will be further clarified, and further other effects of the present invention will be clarified.

[0016]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 shows a laser beam scanning position detecting device according to an embodiment of the present invention.

In FIG. 1, the laser beam scanning position detecting device comprises a photodetector 15, a current / voltage conversion circuit 17, and a peak-to-peak converter.
Differentiating circuit 19 that constitutes an example of the pulse detector, a waveform shaping circuit 18 that constitutes an example of the pulse generator, and a zero-cross circuit.
20, an edge detection circuit 21, and a latch circuit 23.

When a laser beam is incident on the photodetector 15,
The photodetector 15 outputs a photoelectric current proportional to the amount of incident light, which is converted into a voltage by the current / voltage conversion circuit 17 including the operational amplifier 7 and the resistor R1 shown in FIG. 6, for example. The output signal a of the current / voltage conversion circuit 17 has a waveform as shown in FIG. 2, and the signal a is differentiated by the differentiating circuit 19 to detect the peak time t0 and becomes the signal b. Here, according to the configuration of the present embodiment,
The output signal a of the voltage conversion circuit 17 is output as shown in FIG.
It has been confirmed that the peak time t0 of the signal waveform does not change even if the power is changed.

The signal b is input to the zero-cross circuit 20, where it is waveform-shaped into a pulse signal. That is, the positive part of the signal b becomes low level and the negative part becomes high level, so that the signal c which always rises to high level at the timing t0 of the peak of the signal a is obtained. The narrow pulse portion of the signal c is caused by the small amplitude portion of the signal b, and is unstable because the amplitude of the signal b is small. The edge detection circuit 21 detects the rising edge of this signal c and outputs a pulse signal d having a predetermined pulse width at that timing.

On the other hand, the waveform shaping circuit 18 compares the output signal a of the photodetector 15 with the reference voltage COMPV, and outputs the pulse signal e which becomes high level when the signal a exceeds the reference voltage. Here, the reference voltage COMPV is a voltage comparison level determined by the resistors R2 and R3 of the waveform shaping circuit 18. Then, the latch circuit 23 receives the pulse signal e as data and the output signal d of the edge detection circuit 21 as a trigger signal, and latches the pulse signal e at the rising timing of the signal d. As a result, the signal f, which becomes a high level at the timing t0 and then becomes a low level by being triggered by the pulse of the unstable region of the signal d, is output from the latch circuit 23 as a timing detection signal.

The rising timing of the pulse signal f is determined at the first zero cross point of the output signal b of the differentiating circuit 19, and this zero cross point coincides with the peak timing t0 of the output signal a of the current / voltage converting circuit 17. Therefore, the rising timing of the pulse signal f coincides with the peak of the signal a regardless of the amplitude of the signal a.

FIG. 3 shows signals of respective portions when the laser beam incident on the photodetector 15 is weak and the amplitude of the output signal a of the current / voltage conversion circuit 17 is small. As is apparent from FIG. 3, even when the amplitude of the output signal a is small, the differentiation circuit
19 outputs the signal b whose first zero-cross point coincides with the timing of the peak of the signal a, and the zero-cross circuit 20 outputs the signal c which becomes high level at the first zero-cross point. Then, the edge detection circuit 21 outputs the pulse signal d which becomes high level in synchronization with the rising of the signal c, and the latch circuit 23
Receives the pulse signal e from the waveform shaping circuit 18 as data and the output signal d of the edge detection circuit 21 as a trigger signal, and latches the pulse signal e at the rising timing of the signal d. As a result, the pulse signal f which becomes high level at the timing t0 is obtained.

FIG. 4 is a circuit diagram showing in detail each part following the current / voltage conversion circuit 17. As shown in FIG. 4, the differentiation circuit
19 can be configured by a kind of high-pass filter using an operational amplifier, and the zero-cross circuit 20 can be configured by a comparator that compares the signal b with a voltage of 0V. And
The edge detection circuit 21 can be configured by a one-shot multivibrator, the latch circuit 23 can be configured by a D-type flip-flop, and the waveform shaping circuit 18 can be configured by a comparator that compares the signal a with a reference voltage.

In this embodiment, the zero cross circuit 20 is
It is assumed that the signal c that becomes low level in the positive part of the signal b and becomes high level in the negative part is output, but as a zero cross circuit, on the contrary, it becomes low level in the negative part of the signal b and becomes positive in the positive part. It is also possible to use a device that outputs a high level signal. In that case, the edge detection circuit 21 may detect the trailing edge of the signal c.

It is also possible to use a gate circuit such as an AND circuit instead of the latch circuit 23. In that case, the first pulse of the signal d is taken out and output as the signal f.

Further, it is possible to change the voltage comparison level by the waveform shaping circuit 18 according to the level of the signal a, which can further improve the reliability.

When this laser beam scanning position detecting device is used in a laser beam printer, the pulse signal f is used as a synchronizing signal for starting printing, and as a result, the writing start position is accurately determined and a high quality image is obtained. It becomes possible to obtain. Not only when the scanning optical system of the laser beam is composed of the polygon mirror and the f-θ lens as shown in FIG. 5, but also the hologram scanning optical system which realizes these functions by one means. The present invention is effective in the case where

As described above, according to the present embodiment, the timing at which the logic level of the timing detection signal (signal f) changes coincides with the timing of the edge of the signal d which is the output pulse of the edge detection circuit 11, and is applied. Since the edge timing of the signal d coincides with the zero-cross point of the signal c which is the output signal of the zero-cross circuit 20, that is, the peak time of the detection signal of the photodetector 15, the timing when the logic level of the timing detection signal changes is It coincides with the timing of the peak of the detection signal of the device 15. Then, the peak time t0 of the detection signal of the photodetector 15 does not change even if the intensity of the laser beam changes or the amplitude of the detection signal of the photodetector 15 changes, as shown in FIG. The timing detection signal always accurately represents the timing when the laser beam passes through the photodetector 15.

[0029]

As described in detail above, according to the laser beam scanning position detecting device of the present invention, the photodetector is arranged at a predetermined position through which the laser beam passes, and detects the optical output of the laser beam. The peak detection means detects the peak time of the detection signal of the photodetector, and the pulse generation means generates a pulse based on the detected peak time, so that the pulse generated by the pulse generation means. Will always accurately represent the timing at which the laser beam passes through the photodetector.

As a result, according to the laser beam scanning position detecting device of the present invention, the timing when the laser beam passes the predetermined position can be accurately detected with a relatively simple structure. Therefore, when the laser beam scanning position detecting device of the present invention is used in, for example, a laser beam printer, an accurate image recording start point can be obtained, and high quality printing can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of a laser beam scanning position detecting device according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing signals of respective parts of the laser beam scanning position detecting device of FIG.

FIG. 3 is a waveform diagram showing signals of respective portions in the laser beam scanning position detecting device of FIG. 1 when the light amount of the laser beam is small.

FIG. 4 is a circuit diagram showing in detail a main part of the laser beam scanning position detecting device of FIG.

FIG. 5 is a configuration diagram showing a conventional laser beam printer.

FIG. 6 is a circuit diagram of an amplification unit that constitutes the laser beam printer of FIG.
It is a circuit diagram which shows a waveform shaping circuit.

FIG. 7 is a waveform diagram showing an output signal of a laser beam photodetector.

FIG. 8 is a waveform diagram for explaining the operation of the amplification / waveform shaping circuit in FIG.

[Explanation of symbols]

 15 Photodetector 17 Current / voltage conversion circuit 18 Waveform shaping circuit 19 Differentiation circuit 20 Zero cross circuit 21 Edge detection circuit 23 Latch circuit

Claims (2)

[Claims] 1. A photodetector which is arranged at a predetermined position through which a laser beam passes and which detects an optical output of the laser beam, and a peak detecting means which detects a peak time of a detection signal of the photodetector. A laser beam scanning position detecting device, comprising: pulse generating means for generating a pulse based on the detected peak time. 2. The peak detecting means includes a differentiating circuit for differentiating the detection signal of the photodetector, and the pulse generating means includes a zero-cross circuit for shaping the output signal of the differentiating circuit by a zero-cross method. An edge detection circuit that outputs a pulse whose logic level changes at the edge timing of the output signal of the zero-cross circuit, and a waveform shaping circuit that shapes the waveform of the detection signal of the photodetector by comparing it with a predetermined reference level, A laser beam scanning position detecting device, comprising: a latch circuit that latches an output signal of the waveform shaping circuit with a pulse of the edge detecting circuit and outputs the latched signal.