Method and apparatus for automatically controlling a power of a laser diode

A method and apparatus to automatically control an output power of a laser diode, include generating an error voltage between an output voltage of the laser diode sampled during an automatic power control period and a reference voltage, and performing proportional-integral processing on the error voltage to generate a compensated control voltage and applying the compensated control voltage to the laser diode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 2002-50525, filed on Aug. 26, 2002, which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic control of an output power of a laser diode, and more particularly, to a method and apparatus for automatically controlling an output power of a laser diode using a proportional-integral control.

2. Description of the Related Art

Generally, a laser printer is an apparatus to reproduce an image by writing a latent image on a photoreceptor drum according to a video signal of the image, using a laser beam emitted from a laser diode and transferring the latent image to a medium, such as paper. Here, an output power characteristic of the laser diode changes depending on an ambient temperature, which leads to a degradation of a quality of a printed result. Therefore, many methods to compensate for a variation of an output power according to a change in the ambient temperature have been studied.

FIG. 1is a schematic block diagram of a conventional apparatus to automatically control the output power of the laser diode. The conventional apparatus includes a sensor15, which senses the output power of a laser diode14positioned within a laser scanning unit (not shown); a sensed voltage input unit16, which converts the sensed output power of the laser diode14to an appropriate form and inputs a converted result to a printer controller11; and an automatic power controller13, which controls the output power of the laser diode14using an ON/OFF method under an authorization of the printer controller11receiving the sensed voltage from the sensed voltage input unit16.

In such a conventional apparatus to automatically control the output power of the laser diode, because the printer controller, the automatic power controller, and the sensed voltage input unit exist as separate blocks, a circuit occupies a wide area when implemented. Accordingly, it is difficult to manufacture a miniaturized, light, and inexpensive printer. Moreover, because the automatic power controller is realized as an analog circuit, the automatic power controller has low flexibility for feedback control. In addition, because in the ON/OFF method the amount of control performed until the output power of the laser diode has a target value varying in a range of 0-100%, the accuracy of control is low and the controlled output power of the laser diode continuously fluctuates near the target value.

SUMMARY OF THE INVENTION

The present invention provides a method of automatically controlling an output power of a laser diode to minutely approach a target value using proportional-integral control.

The present invention also provides an apparatus to automatically control an output power of a laser diode.

The present invention also provides a laser printer controller including an apparatus to automatically control the output power of the laser diode there within.

According to an aspect of the present invention, there is provided a method of automatically controlling an output power of a laser diode, the method comprising: generating an error voltage between an output voltage of the laser diode sampled during an automatic power control period and a reference voltage; and performing proportional-integral processing on the error voltage to generate a compensated control voltage and applying the compensated control voltage to the laser diode.

According to another aspect of the present invention, there is provided an apparatus to automatically control an output power of a laser diode, the apparatus including: an error voltage generation unit generating an error voltage between an output voltage of the laser diode sampled during an automatic power control period and a reference voltage; and a control voltage generation unit performing proportional-integral processing on the error voltage provided from the error voltage generation unit to generate an effective control voltage.

According to an aspect of the present invention, the error voltage generation unit includes: an analog-to-digital converter converting the output voltage of the laser diode from an analog form to a digital form; an effective output voltage extractor extracting an effective output voltage from the digital output voltage provided from the analog-to-digital converter; and a subtractor subtracting a reference voltage from the effective output voltage provided from the effective output voltage extractor to generate the error voltage.

In accordance with an aspect of the present invention, the effective output voltage extractor includes: a sampler sampling the digital output voltage provided from the analog-to-digital converter during the automatic power control period; a comparator comparing the sampled output voltage with a first maximum and a first minimum, determining whether the sampled output voltage exists within an effective range defined by the first maximum and the first minimum, and extracting the effective output voltage within the effective range; an accumulator accumulating the effective output voltage extracted by the comparator; and a divider dividing the accumulated effective output voltage by a number of accumulations to obtain an average effective output voltage.

According to still another aspect of the present invention, there is provided a laser printer controller including: an engine processor module controlling an entire operation of a printer engine; and an automatic power control module of a laser diode automatically controlling an output power of the laser diode positioned within a laser scanning unit by sampling an effective output voltage from an output power of the laser diode during a predetermined automatic power control period and performing proportional-integral processing on the effective output voltage, wherein the laser printer controller is structured in a single integrated circuit.

According to an aspect of the present invention, there is provided a printer controller controlling a laser diode and connected to a printer engine, including: an engine processor module controlling an operation of the printer engine; and an automatic power control module automatically controlling an output power of the laser diode by generating an error voltage between an output voltage of the laser diode sampled during an automatic power control period and a reference voltage, and performing proportional-integral processing on the error voltage to generate a compensated control voltage and applying the compensated control voltage to the laser diode.

According to an aspect of the present invention, there is provided a method or computer readable medium including a computer program of a printer controller controlling a laser diode and connected to a printer engine, including: controlling an operation of the printer engine; automatically controlling an output power of the laser diode by generating an error voltage between an output voltage of the laser diode sampled during an automatic power control period and a reference voltage; and performing proportional-integral processing on the error voltage to generate a compensated control voltage and applying the compensated control voltage to the laser diode.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present aspects of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The aspects are described below in order to explain the present invention by referring to the figures. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

FIG. 2is a block diagram showing a printer controller21including an automatic power control module23for a laser diode, according to an aspect of the present invention, and associated peripheral elements. The printer controller21is an integrated circuit including an engine processor module22and the automatic power control module23. The engine processor module22controls an entire operation of a printer engine24. The automatic power control module23automatically controls an output power of a laser diode25, which is sensed by a sensor26, to minutely approach a target value using proportional-integral control. The laser diode25is included within a laser scanning unit (not shown).

FIG. 3is a block diagram of the automatic power control module23shown inFIG. 2, according to an aspect of the present invention. The automatic power control module23includes an error voltage generation unit310and a control voltage generation unit320. The error voltage generation unit310includes an analog-to-digital converter311, an effective output voltage extractor312, and a subtractor318. The effective output voltage extractor312includes a sampler313, a first comparator314, an accumulator315, and a first divider316. The control voltage generation unit320includes a proportional-integral processor321, an effective control voltage extractor325, and a digital-to-analog converter329. The proportional-integral processor321includes a proportional section322, an integral section323, and an adder324. The effective control voltage extractor325includes a second comparator326. In the meantime, in order to simplify a decimal point calculation, the effective output voltage extractor312further includes a multiplier317, and the effective control voltage extractor325further includes a second divider327.

The error voltage generation unit310generates an error voltage between an output voltage of the laser diode25(FIG. 2), which is extracted for a duration of an automatic power control set to have a predetermined period, and a reference voltage.

More specifically, the analog-to-digital converter311converts a voltage of the output power of the laser diode25to a digital output voltage. The sampler313samples the digital output voltage received from the analog-to-digital converter311during an automatic power control period. In another aspect of the present invention, the sampler313can set the automatic power control period and control the analog-to-digital converter311to perform a conversion only during the automatic power control period. In addition, the sampler313can set a number of samplings during the automatic power control period. In this case, the sampler313performs the set number of samplings on the digital output voltage received from the analog-to-digital converter311.

The first comparator314previously sets a first maximum and a first minimum to define an effective range of the digital output voltage of the laser diode25, compares the first maximum and the first minimum with the sampled digital output voltage received from the sampler313, and determines whether the sampled digital output voltage exists within the effective range. As the result of the determination, the first comparator314outputs only an effective output voltage within the effective range to the accumulator315and increases a number of accumulations Cs by 1 whenever outputting the effective output voltage to the accumulator315. The first maximum and the first minimum are set in order to take only normal components from the output power of the laser diode25except for error components and obtained through experiments.

The accumulator315accumulates the effective output voltage provided from the first comparator314. The first divider316divides an accumulated result output from the accumulator315by a number of accumulations Cs to calculate an average effective output voltage. The multiplier317multiplies the average effective output voltage received from the first divider316by a predetermined multiplication constant Km in order to simplify a decimal point calculation in the proportional-integral processor321and outputs a multiplied result to the subtractor318. The subtractor318subtracts the reference voltage, i.e., a control target value for the laser diode, from the multiplied result received from the multiplier317to generate the error voltage.

Furthermore, the control voltage generation unit320performs a proportional-integral processing on the error voltage received from the error voltage generation unit310to generate a compensated control voltage and applies the compensated control voltage to the laser diode25.

More specifically, in the proportional-integral processor321, the proportional section322multiplies the error voltage by a proportional constant Kp to generate a proportional term. The integral section323accumulates the error voltage and multiplies the accumulated error voltage by an integral constant Ki to generate an integral term. The adder324adds up the proportional term and the integral term and outputs a result of the addition. The proportional constant Kp and the integral constant Ki are optimal values selected from the results of an actual control using a cut-and-try method. The proportional-integral processor321can add a single sign bit to the output of the subtractor318in order to simplify a proportional-integral processing because a negative value may be generated as a result of the subtraction from the subtractor318.

In the effective control voltage extractor325, the second comparator326previously sets a second maximum and a second minimum which define an effective range of a control voltage for the laser diode25, compares the proportional-integral processed error voltage, i.e., a control voltage received from the proportional-integral processor321with the second maximum and the second minimum, and determines whether the control voltage exists within the effective range. As a result of the determination, only an effective control voltage within the effective range is output to the second divider327. When the control voltage is beyond the effective range, the control voltage is ignored, and the second comparator326waits until another control voltage obtained during the next automatic power control period is received from the proportional-integral processor321.

The second divider327divides the effective control voltage received from the second comparator326by a division constant Kd and outputs the divided effective control voltage to the switch328. Here, the division constant Kd may be the same as the multiplication constant Km, in accordance with an aspect of the present invention. The division constant Kd and the multiplication constant Km can be obtained through experiments.

The switch328switches the output of the effective control voltage provided from the second divider327. When different types of automatic power control modules23, such as one using an ON/OFF method and one using a proportional-integral control method, are implemented in a single circuit, the switch328is operated according to a user's selection so as to provide the effective control voltage from the second divider327to the digital-to-analog converter329. The digital-to-analog converter329converts the effective control voltage provided from the switch328to an analog form and applies the converted effective control voltage to the laser diode25.

FIG. 4is a flowchart of a method of automatically controlling the output power of the laser diode25, according to an aspect of the present invention. Referring toFIG. 4, at operation41, the automatic power control period for the laser diode25is set. At operation43, the output voltage of the laser diode25is converted from an analog form to a digital form.

At operation45, the digital output voltage of the laser diode25is sampled during the automatic power control period, and the error voltage between the effective output voltage extracted from sampled digital output voltages and the reference voltage is generated. Operation45will be described in detail with reference toFIG. 5.

Referring toFIG. 5, the number of samplings or a sampling rate during the automatic power control period of operation41is set in operation51. According to an aspect of the present invention, operation51may be selectively performed when necessary.

At operation52, the set number of samplings is performed on the digital output voltage obtained in operation43during the automatic power control period. Each sampled digital output voltage is compared with the first maximum and the first minimum, which are the upper and lower limits of an effective range previously set, and, at operation53, the digital output voltage within the effective range is extracted as the effective output voltage.

At operation54, the effective output voltage is accumulated. At operation55, the average effective output voltage is calculated. At operation57, the error voltage between the average effective output voltage and the predetermined reference voltage is generated. Here, at operation56, in order to simplify decimal point calculation, the average effective output voltage may be multiplied by a predetermined multiplication constant Km.

Referring back toFIG. 4, at operation47, the proportional-integral processing is performed on the error voltage generated in operation45to generate the compensated control voltage, and the effective control voltage is generated from the compensated control voltage. Operation47will be described in detail with reference toFIG. 6.

Referring toFIG. 6, at operation61, the proportional-integral processing is performed on the error voltage generated in operation45using the proportional constant Kp and the integral constant Ki to generate a compensated control voltage.

The compensated control voltage is compared with the second maximum and the second minimum, which are the upper and lower limit of the predetermined effective range, to determine, at operation62, whether the compensated control voltage exists within the effective range. If the method determines that the compensated control voltage does not exist within the effective range, the method returns back to operation41to be performed with respect to the next automatic power control period.

Conversely, if the method determines that the compensated control voltage exists within the effective range, at operation64, the compensated control voltage is generated as an effective control voltage. In the meantime, at operation65, in the case where the multiplication constant Km is used during the generation of the error voltage, the effective control voltage is divided by a division constant Kd.

Referring back toFIG. 4again, at operation49, the effective control voltage is converted from the digital form to the analog form and then applied to the laser diode25.

The above-described invention may be embodied as a computer readable program code or may be embodied in a general purpose digital computer by running such a program from a computer readable medium, including but not limited to storage media such as magnetic storage media (e.g., ROM's, floppy disks, hard disks, etc.), optically readable media (e.g., CD-ROMs, DVDs, etc.) and carrier waves (e.g., transmissions over the Internet).

As described above, according to an aspect of the present invention, among digital output voltages of a laser diode which are sampled during a predetermined automatic power control period, a digital output voltage within an effective range is extracted, and an error voltage between the extracted digital output voltage and a reference voltage is obtained. Next, among compensated control voltages obtained by performing proportional-integral processing on the error voltage, an effective control voltage within an effective range is extracted and used to control the output power of the laser diode. Therefore, according to an aspect of the present invention, the present invention guarantees that the output power of the laser diode is quickly stabilized at a target value even if an ambient temperature of the laser diode increases. In addition, according to an aspect of the present invention, an automatic power control module for the laser diode can be realized in a digital form and integrated into a printer controller, and a miniaturized, light, and inexpensive laser printers can be manufactured.