DE4447627C2 - Method for generating pixel signals - Google Patents

Abstract

The invention relates to a method for generating a pixel clock for a scanner, which has a laser radiation source and a rotating gyroscope, which deflects a light beam directed along its gyro axis onto a part-cylindrical drum. In this case, a gyro motor (224) is rotated at a constant speed and its angular velocity encoded to produce a first frequency at a given resolution. The first frequency is divided by a given constant C to produce a second frequency at a second resolution. The constant is chosen so that the second resolution is substantially higher than the first resolution, and the second frequency is used as a pixel clock for outputting the image data to the laser radiation source (26) to expose a medium sheet having a photosensitive surface.

Description

The invention relates to a method for generating pixel signals The preamble of claim 1. Such a method is known from DE 43 05 183 A1 known.

High efficiency in exposing record media through scanning is achieved through the use of drum plotters that have a rotatable Using a mirror, the light beam from a laser source down to directs the photosensitive recording medium attached to the drum support surface is held. The record carriers consist of a photosensitive Layer, z. As an emulsion, on a base material of polyester or alumi minium.

For a good print result can be achieved with such Drum plotter the rotational movement of the mirror and the pixel clock frequency the pixel signals are synchronized with each other. This brings a high  Circuit complexity and makes high demands on the synchronization of the Mirror.

The object of the invention is to provide a method with which simple Way a good print result can be achieved and that only low requirements to the synchronization of the mirror.

This object is achieved by a method according to claim 1. advantageous Further developments of the invention are the subject of the dependent claims.

In a method according to the invention, the pixel clock frequency is directly off derived from the rotation of the mirror. This can be independent of the Mirror speed synchronizes the pixel clock frequency with the mirror rotation ren, which allows a good print result.

In the following the invention will become more apparent with reference to the drawings described in which:

Fig. 1 is a perspective view of a device with plotter and transport system with housing,

FIG. 2 is a perspective view of the device without housing shown in FIG. 1; FIG.

Fig. 3 is a side view of a drum and a rotatable mirror;

Fig. 4 is a partial front view of Drehspiegelpositioniersystems,

FIG. 5 is a side view of a turn mirror carriage; FIG.

Fig. 6 is a plan view of the rotary mirror carriage is ground at remote guide,

Fig. 7 is a schematic of the general system control of the device;

Fig. 8 is a diagram for generating the pixel clock frequency, and

Fig. 9 is a diagram for the control of the linear displacement of the drehba ren mirror.

In Fig. 1, a photoplotter and transport system is shown. The system comprises a device 2 and a computer 4 which is connected to a is in the device 2 associated system controller 6 for processing of the computing ner 4 charged image information and for controllably moving the compo of the system components in accordance with numerical control commands ver is responsible. The computer 4 and the system controller 6 are connected by a STEADER data line, the z. B. may be an RS-232 C-line. The device 2 shown in Fig. 1 is suitable for light-tight applications. For this purpose, it is housed against the ingress of light in a housing 5 with light-tight doors 3 . Further, in one side of the housing, an opening 1 for discharging recording media is mounted.

The device 2 consists, as shown in Fig. 2, of a transport system 10 and a photoplotter 12 , which are each held in a box-like frame structure 14 and 16 and are connected as modules. They can also be operated separately.

The photoplotter 12 includes a drum 18 having a part-cylindrical support surface 20 for holding a record carrier in a given orientation relative to a point along the central axis Z. It should be noted here that the term "partially cylindrical" means that the holding surface 20 along the central axis Z has a uniform radius of curvature, which gives it a curved shape in cross section. Holes 23 in the support surface 20 serve to draw the Aufzeich tion carrier 22 with negative pressure on the drum 18 . The drum 18 includes for this purpose a conduit system under the holding surface 20 in conjunction with the holes 23 and a suction pump (not shown).

The plotter 12 includes a scanning device 24 having a laser radiation source 26 to the drum 18 and a Lichtumlenkspiegel 28 which is also secured to the frame 16 in order to deflect the beam emitted from the laser radiation source 26 light beam so that in the longitudinal direction of the central axis Z it of Trom mel 18 is located. The scanning device 24 further includes a rotating mechanism 30 having an off-axis parabolic mirror, which directs along the Z-axis ge focused light beam on the support surface 20, sweeps along a given Bo gens in raster format on the recording medium 22 and focused on the surface. The rotating mechanism 30 is movably controlled on the center axis Z by a web system, not shown, attached to the drum 18 .

In Figs. 3 and 6, it can be seen that the rotary mechanism 30 is suspended on a holding system 184 which is fixed to a side edge of the drum 18. Das Halteelement 184 ist in der Fig. 3 und 6 dargestellt. The retainer assembly 184 includes a Überhängblock 186 on which a very smooth rectangular web 188 extending parallel to the central axis Z, a rotary Spie gelwagen 182 which moves along the track 188, and positioning 190 are mounted, which for controlled positioning of the carriage 182 are provided with very well-defined steps along the central axis Z drivingly between the carriage 182 and the track 188 . The turntable mechanism 30 borne by the carriage 182 consists of a motor 24 , a parabolic mirror 226 having a geometric center located on the center axis Z, and an encoder 228 for sensing the rotational steps of the motor and converting them into pulse signals for the system controller 6 .

The rotary mirror carriage 182 includes, as best shown in FIGS. 5 and 6, a base member 192 and an integrally molded mounting flange 194 projecting downwardly from the base member 192 . The mounting flange 194 has an opening 196 for fastening the motor 224 to the carriage 182 . The base member 192 has, as shown in Fig. 5, in vertical cross section a U-shaped upper portion 198 for receiving the web portion 188 and a front wall 208 and a bottom surface 213. The base member 192 includes a plurality of magnetic bearings 202 . Each magnetic bearing 202 is mounted in a bore 204 with a screw 206 . The magnetic bearings 202 form sliding bearing surfaces 209 and 210 along opposite sides of the bottom surface 213 of the U-förmi are arranged gene segment 188, to pull the base member 192 upward to the metallic web 188th Similarly, the front, upstanding wall 208 of the U-shaped portion is provided with magnetic bearings 203 which are fixed in the same manner as the magnetic bearings 202 . The combined effect of the front wall and bottom surface bearings keeps the carriage 182 in contact with the track 188 in the U direction and laterally in contact with the track 188 in the I direction. The rotary mirror carriage 182 is displaced against the forces generated by the magnetic bearings 202 and 203 under the controlled linear positioning force of the positioning devices 190 . These include, as best shown in Fig. 4, a stepper motor 211 , which is BEFE to the web 188 Stigt, a Präzisionsleitspindel 212 which extends parallel to the central axis Z he and a 1: 10 reduction gear through the stepping motor 211st is driven, and a drive nut 214 , which serves to convert the rotational movement of the stepping motor 211 in a linear positioning movement between the carriage 182 and the lead screw 212 . The lead screw has such a length that the carriage 182 can be parked outside of the range of motion of the secondary carriage 34 when a record carrier 22 is placed on the drum 18 . In this way, a return of the rotating mirror carriage 182 is avoided to a starting position before the transport of the recording medium 22 ver and therefore increases the throughput in the system. The drive nut 214 is fixed to the base member 192 and the mounting flange 194 concentrically with the through hole 223 formed therein.

The web 188 is fastened with connecting screws 218 to be the Überhängblock 186, so that the opposite surfaces of the web 188 and of the block are pressed together 186th In order during the scanning of the effects of relative thermal expansion between the web 188 and the Überhäng block to reduce 186, inwardly directed cutouts 220 are formed in the Überhängblock 186 and extend parallel to the Z axis, a portion of the length of the Überhängblockfläche of the Keep track 188 separate.

The drum 18 is held on the frame 12 at four supporting brackets 227 disposed at the four corners of the rectangular base surface of the drum 18 . The brackets 227 are each connected to a source of pressurized air and substantially act as one way actuators lifting the drum 18 from its lowered, hard, holding state on the frame 12 . In its lowered state, when the retainers 227 are not urged, the weight of the drum 18 rests on hard stops that make up a portion of each retainer 227 , thereby ensuring the repeatability of the transport of the record carriers to and from the support surface 20 . However, in operation, compressed air is introduced into each holder 227 to hold the drum 18 on an air bag. This arrangement favors the damping of vibrations that could be transferred from other driven components to the photoplotter 12 . For this purpose, the laser radiation source 26 and the Lichtumlenkspiegel 28 are both attached to the drum 18 .

Fig. 7 shows a schematic of the system controller 6 with a main processor 250 , the z. B. may be a MOTOROLA 6800 processor, a sub-processor 252 for controlling the movements of the transport system 10 and an interface 254 for controlling more specific operations of Fotoplotters 12th

The processor 250 controls the speed of the mirror motor 224 by a suitable amplifier 256 to ensure a constant mirror speed. The processor 250 also controls the stepper motor 211 of the linear drive according to the scheme shown in Fig. 8 and generates the pixel clock for the laser 26 to scan the image. The sub-processor 252 controls the drive axis motors of the transport system 10 and the linear drive motors with multiple controllers 260 , which in turn are controlled by a motion control circuit 258 which is connected by a suitable data bus 262 to the sub-processor 252 . The interface 254 is responsible inter alia for querying the edge detector 114 to determine a reference point for the beginning of a scanning process.

In the photoplotter 12 , the diameter of the scanning beam is varied according to the desired resolution. For example, the photoplotter 12 may be suitable for optional scanning at a resolution of 3810 dots per inch, 2540 dots per inch, 1270 dots per inch, or 1905 dots per inch. In this case, the diameter of the scanning beam is changed according to the selected resolution as follows:

resolution diameter 3810 8 microns 2540 10 microns 1905 15 microns 1270 20 microns

In addition, the sub-processor 252 is used to select the scanning beam diameter according to the required resolution specified as a parameter of the scanning. In this case, a scanning beam with a fixed diameter can be generated with the laser, which is then either reduced or enlarged with a selected optics to a given diameter. Alternatively, the laser itself may generate a variable diameter scanning beam in accordance with commands directly issued by the sub-processor 252 . Such a LASAR is available from Special Optics Inc. of Little Falls, New Jersey, under model no. 56-30-2-8X and sold under the trade designation "Variable Zoom Beam Expander".

In Fig. 8 and 9, a scheme for generating a pixel clock frequency is shown, with the pixel information from the central computer 4 to the LASAR transmis gene, and for generating a drive frequency for driving the stepping motor 211 , the rate of the end of a scan line and the beginning of the next following line corresponds. As shown in Fig. 8, the pixel clock frequency is generated from the output of the mirror encoder 228 , which is used as the reference clock. The mirror encoder further has an angular position detector which is used by the controller 6 as an indicator of when a scan line for a given scan begins. However, as a pixel clock for driving the LASAR, a much higher frequency is required because the encoder is divided into only 1000 steps per revolution. Therefore, the output signal (first signal) of the encoder is supplied to a phase locked loop 311 to increase the frequency (first frequency) by a certain constant C. The frequency (second frequency) of the signal thus generated (second signal) corresponds to the maximum printing resolution of the system and is then divided by a data selector 314 to produce the pixel clock frequency for the desired resolution.

Since the system uses the output of the mirror encoder as its reference clock, means are provided to operate the mirror motor 224 accurately at a desired speed. This includes a master clock 300 with egg ner clock frequency of 20 MHz, which is divided by a suitable circuit 302 to generate a 20 kHz clock frequency, which is then passed through a phase detector 301 . The phase differential is filtered and then input to the amplifier 256 which drives the motor 224 and controls its speed. In this way, the mirror speed is kept constant. As indicated above, the system is suitable for scanning with various resolution. The sampling then takes place at the system with a pixel clock frequency corresponding to the respectively selected resolution, as indicated below:

Resolution (dpi) Pixel clock frequency (MHz) 3810 54.8 2540 36.6 1905 27,40 1270 18.30

The speed of the mirror motor is selected for the highest resolution listed above. That is, the mirror motor 224 rotates at a speed of 200 revolutions per second, thereby generating a frequency of 200 KHz based on 200 U / s × 1000 pulses / U and inputting the phase locked loop 311 to provide an appropriate frequency for the to produce the highest resolution. In the embodiment, the system has a rotating mirror which can generate a faster reference clock for certain applications. That is, the data rate of 54.8 MHz for the resolution of 3810 dpi is an upper limit in clock generation and simultaneously adjusts the sampling rate to 200 scanning lines per second. However, the pixel clock frequencies at 1270dpi, 1905dpi and 2540dpi are significantly less so that a scan rate of 300 scan lines per second or an equally high mirror speed can be used. This is desirable because at a higher sampling rate, the imaging or imaging time is reduced.

The phase locked loop 311 has a phase detector 310 , a low pass filter 313, and a voltage controlled oscillator 312 whose output is fed to the data selector 314 . The phase detector 310 generates depending on whether a phase lag or a phase advance is detected, a positive or a negative output signal. The low-pass filter 313 filters out noise or stray signals from the voltage-controlled oscillator 312 which produces the output frequency used as the pixel clock. A digital frequency divider 315 divides the output frequency of the oscillator 311 by the constant C and returns it to the phase detector 310 . The phase detector 310 couples the phase of the input signal from the encoder 228 with that of the divided frequency signal from the frequency divider 315 .

The output signal of the voltage-controlled oscillator 311 is input to the data selector 314 or multiplexer, the output of which finally controls, as a pixel clock, the transfer of the pixel information by a shift register 316 for the laser 26 . In the described system, the image dot clock frequency is variable depending on the resolution selected for a scanning operation. Therefore, the data selector 314 may suitably change its input frequency for the particular resolution selected except the 3810 resolution. This is accomplished by dividing the input frequency by 2, for a resolution of 1905 dpi, by dividing the input frequency by 3 for a resolution of 1270 dpi, or by multiplying the input frequency by two-thirds (2/3) for a resolution of 2540 dpi.

FIG. 9 shows a scheme for the control of the stepping motor 211 . This system has a differential data accumulator 378 with an adder 380 , the output of which is supplied to a clocked register 382 . The adder 380 consists of a 42-bit accumulator having substantially two inputs and an output 390 which drives the stepper motor 211 . At one input, the addie 380 receives the output of the register 382 , which receives at 383 the pixel clock frequency or photoplotter data rate for the highest print resolution regardless of the selected resolution as a clock signal. The second input to the Addie rer 380 is removed from a data bus 384 and by three separate re gister 386 supplied to enter the adder 380 a constant N. This constant can be changed as a system parameter for changing the frequency that drives the stepping motor 211 . In the embodiment, the system has a 16-bit data bus through which the constant N is input to the three registers 386 , each having a size of 16 bits. When a 42-bit accumulator is used as the adder 380 , its output 390 is defined by the following equation:

The constant number N is input to an input of the adder 380 . The second input of the adder 380 is the previous result. At each clock 383 , the register 382 inputs the previous result of the adder 380 as an input value, which then adds the constant N. The adder 380 adds the constant N until its content exceeds ⁴² . Then, the adder 380 outputs an output signal 390 having the frequency F (out). Thus, the frequency F (out) is equal to the frequency in F (in) multiplied by the constant N by 2 ⁴² . Thus, if N is 2 ⁴⁰ , the output frequency F (out) at 390 would be equal to a quarter of the frequency F (in). The output signal 390 is thus an overflow signal of the adder 380 and is used to operate the motor 211 ver.

In the above, a photoplotter and a transport system have been preferred Embodiment described. However, there are several modifications and Supplements possible. For example, the mirror speed and thus the Fre frequency of the reference clock can be increased to a higher throughput on to effect the drawing media. Finally, in the preferred embodiment example used stepper motors. But the driven parts can also be powered by servo-controlled motors.

Claims (8)

A method of generating pixel signals having a pixel clock frequency for a scanning device having a laser radiation source and a rotatable mirror,
in which a sheet-shaped recording medium is scanned pointwise in accordance with the pixel signal,
characterized in that
from the rotation of the mirror, a first signal is generated at a first frequency, which is a multiple of the mirror speed,
that a second signal is generated with a second frequency which is greater by a factor C than the first frequency,
that the second signal is held in a fixed phase relation to the first signal by a control loop,
and that the pixel clock frequency by dividing the second frequency he testifies. 2. The method according to claim 1, characterized in that the first signal is generated by an encoder on the motor of the rotatable mirror. 3. The method according to claim 1 or 2, characterized in that the rotating bare mirror deflects a light beam directed along its axis of rotation on a part-cylindrical support surface ( 20 ) for the sheet-shaped Aufzeich voltage carrier ( 22 ). 4. The method according to any one of the preceding claims, characterized in that pixel signals in accordance with the pixel clock frequency to the La serstrahlungsquelle ( 26 ) are transmitted. 5. The method according to any one of the preceding claims, characterized in that the control loop has a phase detector ( 310 ), a Fre quenzteiler ( 315 ) and a voltage controlled oscillator ( 312 ), that the voltage controlled oscillator generates the second signal that the Fre quenzteiler ( 315 ) generates a third signal having a third frequency which is smaller by a factor of C than the second frequency, and in that the phase detector ( 310 ) compares the phase of the third signal with the phase of the first signal Si and the voltage controlled oscillator ( 312 ) corresponding to the comparison result transmits a drive signal. 6. The method according to any one of the preceding claims, characterized characterized in that the scanning device, the sheet-shaped recording medium optionally with at least a first or a second resolution abta stet. 7. The method according to claim 6, characterized in that the ge associated resolution associated pixel clock frequency from the second Fre quency by sharing with a data selector ( 314 ) is generated. 8. The method according to any one of the preceding claims, characterized in that a differential data accumulator ( 378 ) counts the signal pulses of the second signal and when exceeding a predetermined count a linear drive stepping motor ( 211 ) causes the rotatable Spie gel along the axis of the part-cylindrical support surface ( 20 ).