JPS6029929B2 - Drive method of galvanometer type optical deflector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a device that scans the surface of a photoreceptor with directional light such as a laser beam and displays characters or images on the surface. This invention relates to a driving method for a galvanometer-type optical deflector for deflecting horizontally or vertically.

Galvanometer type optical deflector (hereinafter referred to as galvano mirror)
As one of the optical deflection elements, it occupies a very important position along with rotating polygon mirrors, acousto-optic elements, etc., but in practical terms it is based on the inertial motion based on the existence of a mechanical resonance frequency unique to the element. It is extremely difficult to simultaneously satisfy high speed, linearity of the scanning section, good scanning efficiency, and stability of mirror deflection. This trend is remarkable.

For this reason, although galvano mirrors are more economical than other deflection means, they have rarely been put to practical use as deflection elements, especially horizontal deflection elements that require high speed. Below, we will explain in detail the high speed and linearity of the galvano mirror, as well as the scanning efficiency and stability of deflection.

When a galvano mirror is driven with a current having a waveform other than a sine wave, a significant vibration component will always appear due to the natural frequency of the galvano mirror itself, unless some special operation is applied to the drive system. FIG. 1 shows the deflection angle 8 of the mirror when a sawtooth wave drive current 1 is applied to the galvanometer mirror, and the mirror oscillates freely at a period of approximately 1/ra. If the next drive current is applied before this vibration component is sufficiently attenuated, the vibration component will overlap in the scanning section, impairing the linearity of the scanning section, and further inducing instability in the deflection characteristics of the mirror. The extra vibration of the mirror due to the superposition of components from the natural frequency is called "ringing," but in order to suppress the deterioration of the linearity of the scanning section due to this ringing to within the permissible limit, it is necessary to use a frequency far lower than fo ( The galvanic/mirror is forced to drive at a maximum speed of about 5% of its capacity. However, even with deflection using a galvano mirror, scanning at a high frequency is required in most cases, and a driving method of simply applying a driving current has not been practical. In particular, the high scanning frequency,
Horizontal deflection, which requires good linearity of the scan path and improved scanning efficiency (i.e., larger effective scan angle, shorter retrace time), is a much more pressing issue than vertical deflection, which requires a relatively low frequency. Met. Moreover, in reality, in order to increase the resolution of the scanning line, the situation becomes even more severe due to the addition of the adverse condition of a non-negligible drop in the natural frequency coefficient due to the use of a large mirror. Several methods have been explored to alleviate such ringing of the galvano mirror and the deterioration in linearity of the scanning portion due to the ringing.

There are two main ways to do this: one is to add some special manipulation to the drive signal waveform (hereinafter referred to as signal processing), and the other is to apply feedback to the drive circuit or insert an appropriate damping resistor to override the system. There are two methods: damping and suppressing the natural frequency spectrum (hereinafter referred to as damping). The main galvanometer mirror drive systems using signal processing include the General Scanning Company's system (hereinafter abbreviated as the G.S. system) and the Japan Broadcasting Corporation's system (hereinafter abbreviated as the NHK system) shown in Japanese Patent Application Laid-Open No. 50-9444. There are two options:

As shown in Fig. 2, the G/S method uses a monomer that has a constant slope from 0 to t2, when the retrace time is 0 and the deflection direction is reversed to move on to the next scan. A drive current 1 with one step or a single horizontal step is provided. In this case, the consumption effect of ringing energy at time t=to~ra' is quite insufficient, so
It is very difficult to suppress ringing under under-braking, and it is essential to place the system under deep over-braking in order to keep the vibration component caused by W within an allowable range. The degree of damping is 2 to 3 times longer than under-damping.
The ratio of effective scanning time to the driving cycle decreases as the driving frequency increases (speeding up). As a result, if the same resolution is to be maintained, the optical modulation efficiency will decrease due to an increase in the optical intensity modulation frequency, and the light source output will have to be increased. Moreover, the linearity at the start of scanning is deteriorated, and the effective scanning angle with good linearity in the scanning portion is reduced, which limits the effective scanning time in horizontal scanning. In any case, G.
In the S method, due to poor signal processing settings, there is no choice but to rely heavily on dynamic damping, and even if it can be adjusted to the best condition, a non-negligible vibration component of the natural frequency fo throughout the scanning section The contamination of the line and the deterioration of the linearity at the start of scanning are semi-inevitable, and this is not an effective means for achieving both high speed and linearity.
Next, in the NHK system, as shown in Figure 3, the retrace time to
A staircase wave drive current 1 having two steps is applied to ~t4, and the subsequent step t and ~t4 levels are set in a negative direction relative to the reference level of the sawtooth wave.

Compared to the above G/S method, this method shortens return time,
The linearity is improved. However, if the damping is increased, the retrace time will inevitably increase, although not as much as the G/S method, and the height of the bias of the second pulse will change in a negative direction relative to the reference level. Good linearity cannot be obtained unless it is set quite deep. Next, the scanning efficiency, good shake characteristics, and shake stability of the galvanic mirror will be explained.

As already explained in FIG. 1, the characteristics of the galvano mirror involve free damping vibration with a period of 1/ra. When a galvano mirror having such characteristics is used as a scanning means, the time from to to t2 is considered as the substantial retrace time,
From the point of view of scanning efficiency and stability, it is best to proceed to the next scan at time t2 to t5. However,
In order to smoothly transition to linear scanning during the same period, it is necessary that the original free vibration characteristics of the galvanometer mirror and the velocity vector of the desired linear scanning characteristic match at the scanning transition point to create a continuous straight line. This is an essential condition for realizing the conversion to scanning. The point at which this condition is satisfied at time ra ~ t5 is the two points t; t3, and in Figure 1 (from now on,
(referred to as the first point and the second point, respectively), the first point is close to the direction reversal time t = ra', and the second point
The point exists just before t=t5, which is the first peak after the inversion. As is clear from Fig. 1, which of these two points should be selected is that the scanning period and scanning angle should be as large as possible, so the scanning start from t=etc., that is, the scanning transition angle, is set at the first point. It is preferable. Regarding the above two methods, the G/S method explores the first point scan, and the NHK method explores the second point scan.
However, since a large amount of damping is applied to the former, the NHK method is actually shorter and the retrace time is longer. However, since the NHK system uses second point scanning, it is true that it is forced to sacrifice scanning efficiency compared to first point scanning. In addition, as mentioned above, since the bias of the second pulse is set to be negative with respect to the reference level, there is a very strong tendency for the swing speed to increase from time ra to t4, and the bias of the second pulse is set to be negative. If it is set deeply in the direction, the time occupied by the total deflection angle; t2
The ratio of the transition angle at ~t4 increases, and generally the efficiency of use of the deflection angle cannot be said to be good. In particular, when trying to draw characters made up of dots using a laser beam, etc. on the photoreceptor surface, the dots can be displayed on the photoreceptor surface at equal spatial intervals rather than at equal intervals; The tolerance regarding linearity has become significantly wider, and it is sufficient if it can be considered that there are almost no differences in the density distribution depending on the location when visually observed. Therefore, not only the regular scanning period but also the scanning transition period can be included in the scanning period by making the scanning speed slightly slower than the original scanning speed.
This has the great advantage of achieving a deflection angle utilization efficiency of close to 50% and being able to obtain the remaining scanning time after subtracting the retrace time from one cycle as effective scanning time. In the NHK method, the scanning transition angle is originally large (second point), and furthermore, compared to the normal scanning speed, the deflection angular velocity at the scanning transition part tends to be very large, and the deviation from the original scanning starting point It is essentially impossible to apply this method because a clear difference in concentration distribution occurs. The present invention
This was done in view of the above matters, and in order to obtain good linearity in the scanning period and high stability in runout, very effective signal processing was introduced in the retrace period, and as much as possible, t= By absorbing the potential energy that is the cause of free vibrations stored in the We aim to reduce line time close to the theoretical limit, significantly improve linearity at the beginning of the scan, and also take into account the use of the scan transition section mentioned earlier, so that the transition speed is the same as the regular scan speed. The purpose of the present invention is to provide a drive system that achieves speeds as low as 100% or slower than that of the previous 2000s. That is, regarding the driving method of the galvanometer type optical deflector, a staircase pulse is interposed in the retrace period, and this staircase pulse has two steps at a level higher than the reference level of the drive waveform, and a second step. is characterized in that the level is lower than that of the previous step. The method of the present invention will be explained below.

First, let us consider the direction and magnitude of the inertial force of the galvanometer mirror at time t=to to t5 using FIG. Due to the action from normal damped free vibration, the direction of the inertial force is reversed near t=chi, that is, in the middle between t=ra and t2, and near t=t, the acceleration of the mirror becomes minimum, and the deflection speed is is maximum towards the bottom. In this way, the mechanical mode of the galvano mirror changes completely with the boundary at t=L between t=to and t2, and the absolute amount of attenuation of potential energy is between t=to and t.
Since 2 is the largest, the female period is to~t,,
It was decided to divide the pulse into two sections, t, - ra', and insert a staircase pulse having a single step into each of these two sections. Therefore, by setting the first step level near the center of the total amplitude of the drive waveform or above it, a strong braking force is applied to the downward inertial force of the mirror, and t=
Taking advantage of the fact that the downward speed toward the scanning start point is at its maximum, the drive current level is lowered all at once, promoting a reduction in the retrace period. On top of that, further time; t=t,
In a typical case, a second staircase pulse having a level approximately half of the first staircase pulse step level is introduced in the second section consisting of ~, thereby preventing a reduction in the braking effect;
Eliminates residual inertial energy. This is shown in Figure 4, where the braking center is determined by the biases A, , A2, and the first step wave consumes most of the inertial energy, shortening the retrace period while the second A staircase wave of 200 ms gives another control effect, and at the same time, the swing speed is rapidly reduced to prepare for the next scan after t= et al. In the end, by introducing such signal processing, the galvanometer mirror can be as close as possible to the resting state in which it has completely released its potential energy during the direction change at t:, etc., and the next scan after t=t2 This makes it possible to follow the drive waveform fairly faithfully in this area. Further, unlike the conventional method, since the dependence on damping is small, it is possible to bring the retrace time almost to the theoretical limit. However, the point to be noted in the above explanation is that the physical parameters of the two-stage insertion step pulse have redundancy, and by mutually adjusting the parameters between each step pulse, a favorable driving state can be maintained. It is possible to do so. As mentioned above, in the method of the present invention, the first point scan is intended, so in order to perform the first point scan most easily and reliably without compromising scanning efficiency, the scanning slope of the drive current waveform is adjusted. By changing this, it is possible to adjust the desired steady-state angular velocity of the mirror during scanning, match the scanning start timing to the first point, and obtain extremely stable deflection characteristics. In FIG. 4, t=t2 and so on are scan transition periods, and t=t3 corresponds to the first point. According to the method of the present invention, the ringing suppression effect is very large, and since it is a first point scan, the scan transition angle is small.
Depending on the adjustment, the ratio of the transition angle to the total deflection angle can be almost ignored. Furthermore, the fact that the speed distribution in this section is slower than the scanning speed can also be a great advantage. Next, the procedure for implementing the method of the present invention will be explained.

According to FIG. 4, the drive signal variable parameters are five: height of the step level; A, A2; time width; t2; and scan gradient for the two insertion pulses. In the present invention, the damping mode is adjusted by feeding back the difference component between the drive current and the terminal voltage of the coil to the drive amplifier, but in addition, damping can also be a variable parameter. Because the ringing suppression force is large, the dependence on damping is low, and the experimental fact that damping improves the linearity during the scan transition period (t2~ et al.) and immediately after the start of scanning, as well as during the blanking period. From the point of view of shortening, the galvano mirror is driven in a damping mode near critical damping or completely under damping. From the analogy of damped free vibration, the runout characteristics can be determined by the time;
Since it can be regarded as approximately 1/ra), it is set as ding, 10 2 ± 1/gen 10 Q. Typical settings for variable parameters include step level height; A, A2 is A, total amplitude: A.
Similarly, the time width is set to be around 50% of A, and the time widths are set to be approximately equal to 72. As described above, after setting the drive waveform in advance before starting the drive, first step A,...
7, and then proceed to the second step, A2, 72. After completing the above operations, adjust the scanning start timing. This is equivalent to slightly changing the size of the original sawtooth wave before signal processing is applied. Of these, the first step plays a major role in suppressing ringing, and the second step is effective in finely adjusting linearity, and these two operations serve as ringing suppression measures. The third operation, adjusting the running start timing, concludes this series of operations, and if this setting is neglected, new ringing will occur and linearity will be impaired. Through the above-described series of operations, it is possible to obtain runout characteristics that fully satisfy all of ringing suppression, good linearity, shortening/retrace time, high scanning efficiency, and excellent stability. It should be noted that the setting values for each parameter are only representative models, and each parameter has considerable flexibility. However, unless the damping is changed significantly, the mirror shake characteristics will not change much, so 7,...? 2. Once the scanning gradient is set, there is almost no need to modify it again, and once the step level A of the first pulse is adjusted, there is no need to change it much. After all, slight variations in the linearity of the galvanometer mirror itself due to changes over time in the drive signal system can be completely corrected by finely adjusting the step level of the second pulse. In this way, once the drive parameters with the optimum run-out characteristics are set, the only variable parameter that can be chosen after that is A2, and it becomes a reality to maintain the optimum scanning characteristics for a long period of time. This method can be said to be extremely superior in terms of reliability and simplicity. In implementing the method of the present invention, the general
Model G-612 manufactured by Scanning Co., Ltd. was used. The natural frequency when no mirror is installed is approximately 2.1 KH2, but when the method of the present invention was tried with a scanning frequency of IKHZ, the retrace period was 320 seconds, and the area with good linearity was 600 seconds. Second or more was obtained. The invention method detailed above will be temporarily referred to as the first invention method. This first invention method sets the height of the step level of the two-step staircase pulse in the positive direction with respect to the reference level of the mirror tooth wave, and provides two different levels of control to the galvano mirror. As a result, it has become possible to obtain better runout characteristics and especially higher scanning efficiency than the conventional method using signal processing.Here, we will further develop the first invented method to improve the scanning transition period. :A new method will be described which is intended to further improve the linearity of the scanning period with the aim of improving the linearity between t2 and t3. This second invention method is as shown in Figure 5.
By further inserting a staircase wave having a third step from t2 to t2 (scan transition period), the linearity of not only the scan period but also the scan transition period is improved.
With this second invention method, the slight slowness in the scan transition period in the first invention method has been improved by adjusting the new third step pulse level, and the fourth step pulse level has been improved.
It is also possible to eliminate the very small amount of residual potential energy that exists at the time of direction change in the figure; t2.
As a result, it is possible to further improve overall linearity. In addition to the variable parameters A, , A2, 7, ., 72 in the first invention method, the variable parameters in the method of the present invention include a third pulse number 73. In the method of the present invention, the scan gradient used to adjust the scan start timing is replaced by ↑3, and in the end, the number of parameters increases by only one compared to the first method of the invention, simplifying the adjustment. is well preserved. Although this second invention method is intended for first point scanning, it is also possible to switch to second point scanning. Further, depending on the adjustment of each parameter, the time t=t2 in FIG. 4 or 5 may not necessarily coincide with the actual direction change point of the galvanic mirror. This is largely due to the influence of damping. In addition, in order to obtain stronger braking force, a method of inserting a staircase wave with more steps into the retrace time and scan transition period can be considered, but the adjustment will suddenly become complicated and difficult, and the retrace time It is impractical because it may become large. As explained above, according to the drive method of the present invention, the galvano mirror as a deflection element can be scanned at high speed (approximately 20% to 50% of fo) that can withstand horizontal scanning, and has a very short retrace time (
1/false Q), good linearity due to nearly complete suppression of ringing, and high scanning efficiency due to a large deflection angle, and can be driven with characteristics that satisfy these simultaneously.

In particular, it is superior to conventional methods in terms of high scanning efficiency.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the basic deflection characteristics of the galvano mirror under insufficient braking, Figures 2 and 3 are diagrams showing the drive current waveform of the conventional method and the vibration characteristics of the galvano mirror, and Figure 4 5 is a diagram showing the drive current waveform of the drive method according to the present invention and the deflection characteristics of the galvanometer mirror, and FIG. 5 is a diagram showing the drive current waveform of another drive method according to the present invention. 1... Drive current of galvanometer mirror, 8...
- Deflection angle of galvano mirror. Figure 5 Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. A step pulse is interposed in the retrace time of a drive waveform that drives a galvanometer type optical deflector, and this step pulse has two steps at a level higher than the reference level of the drive waveform. death,
The pulse of the first step starts from the retrace start time t_0 to the first reversal of the inertial force direction of the free damping vibration of the galvanometer mirror t.
The pulse of the second step has a time width of _1, and the pulse of the second step has a time width following the pulse of the first step until the first direction reversal time t_2 of the free damped oscillation, and the pulse of the first step A driving method for a galvanometer-type optical deflector, which is characterized by a lower level than the current level. 2. A galvanometer-type optical deflection method according to claim 1, characterized in that one step pulse is interposed after the second step pulse during the scanning transition period of the drive waveform. The drive method of the device.