Abstract:
A method for digitally controlling power from dual laser diodes in a system that forms a plurality of scan lines in a transverse direction across the width of a photosensitive member by reflecting modulated beams from a plurality of facets of a rotating polygon by detecting the beginning of a scan line and providing a start of scan (SOS) signal representing the detection. Power is digitally controlled from the laser diodes by controlling the power from each of the laser diodes for video ON exposure control and controlling the power from each of the laser diodes for video OFF bias control in order for the bias control to enable constant exposure power during SOS detection.

Description:
BACKGROUND AND MATERIAL DISCLOSURE STATEMENT 
     This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a rotating polygon to a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to two point power control implemented by a microprocessor to digitally control the power from dual laser diodes. 
     In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets. 
     The light beam is reflected from a facet and thereafter focused to a “spot” on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements (“pixels”) of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. 
     A difficulty in the past, however, is that prior art techniques in power control of laser diodes has been done with thermoelectric (TE) coolers that regulate the temperature of the laser to minimize power variation. These (TE) coolers are expensive, bulky in size, and very inefficient to operate. More recent methods employ analog power controls which become increasingly difficult to implement when controlling the newest technology lasers with multiple beams in the same package. When using a dual laser diode for simultaneous imaging, it is very important to balance the power of the two beams to provide uniform exposure. In addition, in prior art machines, exposure control has often been set by a control knob implemented with analog signal wires sensitive to noise. 
     Thus it would be desirable to provide a power system control that overcomes many of these difficulties in the prior art. It is therefore an object of the present invention to overcome not only changing characteristics due to temperature, but also differences between dual lasers in providing the necessary power balance. It is another object of the present invention to provide a microprocessor based digital control with embedded intelligence and diagnostic capability in controlling laser power. Another object of the present invention is to vary exposure setpoints by serial download of digital information including functional parameter data such as control loop compensation data. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification. 
     SUMMARY OF THE INVENTION 
     This invention is a two point power control method implemented by a microprocessor to digitally control the power from a dual laser diode within a Raster Output Scanner (ROS) imager sub-system. The exposure power of each of two lasers is controlled for both the video ON exposure (Level) and the video OFF background (Bias) In particular, two different points on the laser diode characteristic curve are measured and each laser is controlled with two control loops, one for Bias and one for Level. The Bias control is done by indirect sensing method which also enables constant exposure power during Start Of Scan (SOS) detection. The Level control regulates the ON power for each of two beams to provide dual beam power balance with variable exposure as set by serial downloaded data. 
     For a better understanding of the present invention, reference may be had to the accompanying drawings wherein the same reference numerals have been applied to like parts and wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a typical ROS printing system incorporating dual beam laser power control in accordance with the present invention; 
     FIG. 2 is a general block diagram of a microprocessor two point power control for a dual laser diode in accordance with the present invention; 
     FIGS. 3 and 4 illustrate two concurrent control loops for Exposure On and Exposure Off of the dual laser diode of FIG. 2 in accordance with the sent invention; 
     FIG. 5 is a diagram of the laser Driver/Amplifier, Laser Diode, Feedback Photodiode, and Back Facet Amplifier in accordance with the present invention; 
     FIG. 6 illustrates a laser diode characteristic curve showing current components in accordance with the present invention; 
     FIG. 7 illustrates a laser diode characteristic curve defining a control range envelope in accordance with the present invention; 
     FIG. 8 illustrates a two point power control timing cycle in accordance with the present invention; and 
     FIG. 9 illustrates the laser power control states in accordance with the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     In FIG. 1 of the drawings, an embodiment of the present invention is incorporated in a multi-pass xerographic printing system depicted schematically and designated generally by reference numeral  10 . The system  10  includes a photoreceptive belt entrained about guide rollers  14  and  16 , at least one of which is driven to advance the belt  12  in a longitudinal direction of processing travel depicted by the arrow  18 . The length of the belt  12  is designed to accept an integral number of spaced image areas I 1- I n  represented by dashed line rectangles in FIG.  1 . As each of the image areas I 1- I n  reaches a transverse line of scan, represented at  20 , it is progressively exposed on closely spaced transverse raster lines  22  shown with exaggerated longitudinal spacing on the image area I 1  in FIG.  1 . 
     In the embodiment depicted in FIG. 1, the line  20  is scanned by a raster output scanner so that a modulated laser beam  24  is reflected to the line  20  by successive facets  25  on a rotatable polygon-shaped mirror  26  driven by motor  27  providing suitable feedback signals to control  30 . The beam  24 , illustrated in dotted lines is actually two beams, emitted by a laser device  28  such as a dual beam laser diode, operated by a laser drive module and power control forming part of a control processor generally designated by the reference numeral  30 . The processor  30  includes other not shown circuit or logic modules such as a scanner drive command circuit, by which operation of motor  27  for rotating the polygon mirror  26  is controlled. A start of scan(SOS) sensor, illustrated at  66  determines a start of scan reference point and also provides suitable feedback signals to control  30 . In addition, a laser power sensor  40 , also referred to as a back facet photodiode, senses a portion of the power of laser  28  to convey a power reading to control  30 . 
     In the operation of the system  10 , as thus far described, the control  30  responds to a video signal to expose each raster line  22  to a linear segment of the video signal image. In xerographic color systems, each image area I 1- I n , must be exposed in the same manner to four successive exposures, one for each of the three basic colors and black. In a multi-pass system such as the system  10 , where only one raster output scanner or head is used, complete exposure of each image area requires four revolutions of the belt  12 . It should also be noted that the present invention is equally applicable to black and white exposure systems. 
     The image areas I 1- I n  are successively exposed on successive raster lines  22  as each raster line registers with a transverse scan line  20  as a result of longitudinal movement of the belt  12 . It is to be noted that the length of the transverse scan line  20  in system  10  is longer than the transverse dimension of the image areas I. Scan line length, in this respect, is determined by the length of each mirror facet  25  and exceeds the length of the raster lines  22 . The length of each raster line is determined by the time during which the laser diode is active to reflect a modulated beam from each facet  25  on the rotating polygon  26  as determined by the laser drive module. Thus, the active portion of each transverse scan line may be shifted in a transverse direction by control of the laser drive module and the transverse position of the exposed raster lines  22 , and image areas I 1- I n , shifted in relation to the belt  12 . 
     Downstream from the exposure station, a development station (not shown) develops the latent image formed in the preceding image area. After the last color exposure, a fully developed color image is then transferred to an output sheet. An electronic Sub System (ESS)  32  contains the circuit and logic modules which respond to input video data signals and other control and timing signals, to drive the photoreceptor belt  17  synchronously with the image exposure and to control the rotation of the polygon by the motor. For further details, reference is made to U.S. Pat. Nos. 5,381,165 and 5,208,796 incorporated herein. As illustrated any suitable marker on the photoconductive surface or belt or any suitable hole such as T1, T2, and T3 provides a reference for each projected image on the belt surface. 
     In accordance with the present invention, a microprocessor controls a pair of dual beam lasers with a total of 4 control loops in a shared time slot multiplexed mode. There are two control loops per dual beam laser, a Bias and a Level Control loop. The same microcontroller is also shared with the Motor Polygon Assembly (MPA) speed control and all sub-system applications such as softstart ramping of lasers and diagnostics of laser failures with controlled ROS shutdowns. The microcontroller generates SAMPLE timing, logic sequencing of video overrides and Level control loop selections ONE ON, BEAM SELECT, HIGH, LOW in order to acquire the sampled power data for each of the four power control loops. A power sample is taken between each scan line during the rescan time (time laser jumps to next facet on polygon). This critical one sample per scan timing is generated by high speed capture and compare event timing also done by the microcontroller internally to implement an independent stand alone sub-system operation with simplified video interface that has been reduced to two channel video inputs and SOS pulse output. 
     The Bias control tracks the threshold knee of the laser, as shown in a typical laser power curve, as it changes with temperature to allow high speed modulation of laser and also minimize droop and crosstalk between dual lasers. The Bias current is not switched ON and OFF with video modulation whereas the Level current is switched On and OFF with video. The Bias is controlled indirectly at a fixed point above the threshold knee to overcome the problem of insufficient light for good feedback where Bias is actually set below threshold. The Bias current being controlled has a fixed LEVEL LOW offset current riding on top with video ON such that by regulating the power of the combination, the LEVEL LOW offset implements a “back off” when the video is actually OFF. 
     The Level control compensates for change in slope efficiency as it changes with temperature Unlike the Bias control that regulates the output power to an indirect fixed power point, the Level control directly regulates the exposure output to a variable reference byte that implements the ability to change exposure. 
     The result of the indirect regulation of the Bias at a fixed power point makes available a constant exposure level that is used during SOS detection This provides advantages in two ways. It enables use of lower cost SOS detector circuits that would otherwise need to be insensitive to change in laser power over the full exposure range during detection. It also positional repeatability of SOS detector in sensing the beam which becomes very important when applied to Image on Image (IOI) and micron level color registration especially in a multi-pass system where exposure is changed by large steps when switching colors. 
     Using a dual beam laser also leads to possible repeatability errors in SOS detection if exposing the SOS with both beams or worse yet alternating between beams. This system implements exposing the SOS with only one beam at the fixed low level provided by the indirect bias control. 
     With respect to FIG. 2, there is shown a general block diagram of a microprocessor based dual beam two point laser power control in accordance with the present invention, in particular, microcontroller  42  receives serial communications designated at  48  such as laser exposure references and control parameters. Microcontroller  42  also includes suitable digital to analog converters illustrated at  50 ,  52 ,  54 , and  56  providing control signals to dual laser driver  64  appropriate to level and bias control loops for beams A and B of a dual beam laser. In addition, microcontroller  42  includes analog to digital converters  58  and  60  receiving power A and power B sense signals from power sense amplifier  66 . Also, microcontroller provides suitable video override and timing control signals illustrated at  46 . 
     Dual laser driver  64  includes power sense amplifier  66  conveying signals from power sensor  40  to microcontroller  42 . The power sensor  40  alternately senses a portion of the laser output power for beam A shown at  72  and beam B shown at  74  to provide the appropriate measure of beam power to microcontroller  42 , in turn providing the appropriate level and bias control adjustments to dual driver laser  64 . 
     FIGS. 3 and 4 illustrate the bias and level control loops for a given laser beam. With reference to FIG. 3, for bias control laser  80  provides an output beam having a given power. A portion of the output beam, in one embodiment approximately one percent of the output beam, is sensed by photo diode sensor  40  to provide a measure of the output power of the beam. Photo diode sensor  40  conveys a signal representing output power to power sense amplifier  78 , in turn providing a signal to analog to digital converter  88 . A summing node  92  receives the output of the analog to digital converter  88  as well as a bias reference  90  to provide an error signal to controller compensator  94 . The bias reference  90  is an indirect fixed reference related to the OFF exposure of the laser beam. A digital to analog converter  96  converts the signal from the controller compensator  94  to control laser driver  82 , a voltage control current source. The output of the laser driver  82 , is conveyed to laser  80  with a level low fixed offset current  84  switched in as illustrated at node  86  to indirectly sense bias. This is the bias or OFF exposure control loop. 
     With reference to FIG. 4, there is shown the ON exposure or level control loop. In particular, photodiode sensor  40  provides a measure of the laser power output from laser  80  conveyed to power sense amplifier  78 , to analog to digital converter  106  to summing node  99 . A second input to summing node  99  is the exposure set point reference illustrated at  100 . Controller compensator  102  via digital to analog converter  104  provides a suitable signal to laser driver  82 . The output voltage of the laser driver  82  is responsive to digital to analog converter  104  and exposure current illustrated at  98  to drive laser  80 . It should be noted that in both FIGS. 3 and 4 the digital to analog and analog to digital converters, the controller compensators, the summing nodes, and bias control and level control reference signals are preferably included in microcontroller software. 
     The laser power control states are generally illustrated in FIG.  9 . In particular, the laser OFF standby state is shown at  210 , and the laser ON condition initiates a soft start ramp time out shown at  212  with a ramp time out fault illustrated at  214 . A ramp complete condition results in a bias control converge state  216  with a bias converge time out fault shown at  218 . The bias ready condition results in a level control converge state  220  with a level control time out fault shown at  222 . And finally, the level and biased ready condition results in the maintain laser ready to print state shown at  224  with a maintain ready fault shown at  226 , and a laser OFF resulting in a return to the laser off standby state  210 . 
     FIG. 5 illustrates the laser driver/amplifier, laser diode, feedback photodiode, and back facet amplifier portion of the control loops for dual lasers  120 A and  120 B with gain adjusts  121 A and  121 B. Photodiode  122  alternately samples the power from lasers  120 A and  120 B for feedback to a microprocessor or a digital controller on lines  126 A and  126 B through amplifiers  124 A and  124 B. Summing junctions  132 A and  132 B combine the adjusted bias voltages from the Bias controls  130 A and  130 B via lines  128 A and  128 B with the video modulator signals  136 A  136 B to drive the lasers  120 A and  120 B. Level A and level B signals  134 A,  134 B to voltage control current sources  135 A and  135 B provide input to the video modulators  136 A and  136 B along with the video signals video A and video B from the video override control  138 , in turn responsive to the override selection circuitry  140 ,  142 , and  144 . 
     FIG. 6 illustrates the use of a laser diode characteristic curve in accordance with present invention, in particular illustrating a curve of laser diode power in milliwatts as a function of the laser diode current in milliamps. Point A on the curve, bias control, illustrates the point on the curve of the level of bias current at a video off or background level. Point B, level control, illustrates a bias current reference, that is, an indirect measurement at a fixed power level, also used for start of scan exposure. Point C on the curve represents a power level for video on, in particular an exposure at five milliwatts as a default setting. As shown, there is an exposure level adjustment range from two to ten milliwatts. 
     The bias current is a variable current controlled by a bias control loop which has a fixed reference. This bias defines the OFF or background point to reside slightly below the lasing threshold knee. The off exposure is controlled by measuring the laser power resulting from both bias current and level low current. Level current is a current that is one of two sources selected by a HIGH, LOW signal on an analog multiplexer depending upon whether bias or level is being controlled. Level low current is a fixed back off current riding on top of BIAS current that implements the indirect measure of the OFF exposure point. High current is a variable current controlled by the LEVEL control loop which has a variable reference that is used to set to ON exposure point. 
     FIG. 7 illustrates a typical laser diode characteristic curve defining a control range. Again, laser diode output power in milliwatts is plotted verses laser diode current in milliamps. Curve  202  illustrates a maximum slope efficiency with minimum bias current at a temperature of about 15° centigrade. Curve  204  illustrates a minimum slope efficiency at maximum bias current at a maximum temperature of about 50° centigrade. A threshold knee range is shown between twenty and forty milliamps with a bias control range at 0 to 40 milliamps. FIG. 8 illustrates a time division multiplex of the control loops for a two point power control of dual beam laser power in accordance with the present invention. It also shows time slots for coordinated polygon speed control with the same microprocessor. 
     While the invention has been described with reference to the structure disclosed, it will be appreciated that numerous changes and modifications are likely to occur to those skilled in the art, and it is intended to cover all changes and modifications which fall within the true spirit and scope of the invention.