Light quantity control device

Abstract: The light quantity control device is suitable for use in a laser beam printer. A controller (ll0, l02, l03, l04, l05, l06) regulates the quantity of light emitted by a semiconductor laser (l08) so as to maintain a constant light quantity. The regulation of the light quantity is according to the output of a monitor (l09) for detecting the light quantity from the semiconductor laser (l08).

Description: In the field of beam recording apparatus equipped with a light quantity control device, a laser beam printer is known, in which a photosensitive member is scanned with a laser beam modulated according to input information to form an electrostatic latent image, and said latent image is rendered visible with magnetic developing powder, called toner, allowing it to be transferred onto a recording material, such as paper. Figure 12 illustrates an example of such a conventional laser beam printer in which a photosensitive drum 1, having a semiconductor surface layer of selenium or cadmium sulphide, is rotatably supported in a housing and rotated at a constant speed in a direction A. A semiconductor laser 2 and a control circuit 2A are also provided for intensity and on-off control of the laser beam from the laser 2. The laser beam L emitted by the semiconductor laser 2 is expanded to a predetermined diameter by a beam expander 3, and then enters a polygon mirror 4 having plural mirror faces and rotated at a fixed speed by a scanner motor 5, whereby said laser beam is deflected by said polygon mirror to perform a scanning motion in the substantially horizontal direction. Then the beam is focused, by an imaging lens 6 of f- characteristic, as a light spot on the photosensitive drum 1 previously charged to a predetermined polarity by a charger 13. The laser beam reflected by a mirror 8 is detected by a detector 7, of which detection signal determines the timing of modulation of the semiconductor laser 2 for obtaining desired light information on the photosensitive drum 1. On said drum 1, an electrostatic latent image is formed by the scanning laser beam, according to the above-mentioned input information. Said latent image is rendered visible by toner deposition in a developer unit 9, and the visible image thus obtained is transferred onto a recording material, usually paper, fed from a cassette 10 or 11. The recording material then passes a fixing unit 12 for fixing said image to the material, and is discharged to an unrepresented tray. The semiconductor laser usually employed in such laser beam printer has an I - ℓ characteristic, or the relation of the laser driving current and the emitted light quantity, as shown in Fig. 13. The semiconductor laser does not emit a beam until the current reaches a threshold value (I th ), but emits the laser beam when said threshold value is exceeded. In such beam emission state, the light quantity ℓ shows a certain inclination α as a function of the laser driving current, and said α is called slope efficiency. In such laser beam printer, the control circuit 2A controls the light quantity of the semiconductor laser 2 and determines the laser current I T so as to obtain a predetermined light quantity ℓ T , and this light quantity is maintained constant by the supply of constant laser current I T . However the I - ℓ characteristic of the semiconductor laser, which has an initial form A as shown in Fig. 14, during a constant current drive with a laser current I T for obtaining a predetermined light quantity ℓ TA , may vary to a form B or C for example due to an increase in the chip temperature caused by the current in the semiconductor laser. Though a normal operation can be achieved while the photosensitive drum is scanned with the necessary predetermined light quantity ℓ TA , the above-explained latent image may not be formed if the I - ℓ characteristic changes to B shown in Fig. 14 due to the increase in temperature so that the light quantity of the laser decreases to ℓ TB despite that the laser current is maintained at I T . On the other hand the laser chip may be destructed if the light quantity increases to ℓ TC as indicated by C. The present applicant already proposed light quantity controlling devices in the US Patents 4,201,994 and 4,443,695 but sufficient effect could not be achieved by the processes disclosed in these patent applications. The present invention is concerned with reducing the above-mentioned drawbacks. A light quantity control device according to the preamble of claim 1 is known from EP-A-0 061 034. According to an aspect of the present invention, there is provided a light quantity control device as set out in claim 1. The invention will now be described by way of example only; with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a laser drive current control device embodying the present invention; Fig. 2 is a chart showing the function of the circuit shown in Fig. 1; Fig. 3 is a flow chart showing the function of a laser light quantity comparator/controller shown in Fig. 1; Figs. 4A and 4B are charts showing the relation between the bias count (or light-emission count) and the bias current (or light-emission current) in an embodiment of the present invention; Fig. 5 is a chart showing the relation between the laser light quantity and the monitor voltage in an embodiment of the present invention; Fig. 6 is a circuit diagram showing an example of the laser light quantity setting circuit shown in Fig. 1; Figs. 7A and 7B are charts showing the function of the laser light quantity comparator/controller shown in Fig. 1; Fig. 8 is a block diagram showing other circuit of the embodiment of the present invention; Fig. 9 is a timing chart showing the output of the circuit shown in Fig. 8; Fig. 10 is a flow chart showing the function of a central processing unit shown in Fig. 8; Fig. 11 is a timing chart showing the output signal of the circuit shown in Fig. 8; Fig. 12 is a perspective view showing a conventional laser recording apparatus; Fig. 13 is a chart showing the relation between current and light quantity of a semiconductor laser; Fig. 14 is a chart showing the relation between light quantity and current of a semiconductor laser; Figs. 15A and 15B are charts showing the characteristic of the embodiment of the present invention in a high-speed high-precision control operation; and Fig. 16 is a flow chart showing a high-speed high-precision control operation of the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. Fig. 1 is a block diagram of the principal part of an embodiment of the present invention. In Fig. 1 there are shown a central processing unit (CPU) 101 for controlling the entire information recording apparatus; a laser light quantity comparator/controller l02 composed of a one-chip microcomputer incorporating an analog-to-digital (A/D) converter for effecting light quantity control as shown in Fig. 2 according to a control sequence as shown in Fig. 3; a bias current control circuit 103 having a digital-to-analog (D/A) converter connected to outputs Bl - Bn of the light quantity comparator/controller 102; a bias current regulator 104 receiving the bias current I B and controlled by the circuit 103; a light emission current control circuit 105 having a D/A converter connected to other outputs Dl - Dn of the light quantity comparator/controller 102; and a light emission current regulator 106 controlled by said circuit 105 and receiving the light emission current I D through a light emission current switching circuit 107. There are further provided a semiconductor laser 108; a photodiode 109 for receiving the light beam from said laser 108; a light quantity monitor circuit 110 for receiving a detection signal from the photodiode 109 and supplying the light quantity comparator/controller with a voltage V M corresponding to the detected light quantity; a light quantity setting circuit 111 for setting the light quantity of the laser; an AND gate 121; and an OR gate 122. Now reference is made to a flow chart shown in Fig. 3 for explaining the function of the light quantity comparator/controller 102 explained above. When an auto light quantity control start signal (APCST) 113 is sent from the central processing unit (CPU) 101 (step 201), the light quantity comparator/controller 102 clears, in a step 202, all the input signals Dl - Dn, Bl - Bn for the D/A converter of the bias current control circuit 103 and the light emission current control circuit 105 thereby sufficiently reducing the laser current I ℓ (light emission current I D + bias current I B ) in the semiconductor laser 108 (for example setting a laser current I ℓ in the laser unit) through the light emission current regulator 106 and the bias current regulator 104. A next step 203 shifts the video (image) signal LON 116 to a true level, to open the gate of the switching circuit 107, thereby supplying the semiconductor laser 108 with a light emission current. As explained above, the light emission current control circuit 105 and the bias current control circuit 103 are provided with digital-to-analog converting circuits, such as the D/A converters, and count up or down the digital output signals Dl - Dn and Bl - Bn from the light quantity comparator/controller 102, thereby varying the laser current I in the laser by an analog value corresponding to said up-count or down-count, through the light emission current regualtor 106 or the bias current regulator 104. In the present embodiment, the bias count X B counted by the bias current control circuit 103 is represented by an n-bit binary number having the output Bl of the light quantity comparator/controller 102 as the least significant bit (LSB) and the output Bn as the most significant bit (MSB), and is explained as being counted up or down. Fig. 4A shows a state of counting up the bias count X B wherein "0" indicates a low or false level, while "1" indicates a high or true level. As shown in Fig. 4B, it is assumed that the bias current I B increases proportionally with the increase of the bias count X B . A similar relation exists between the light emission count X D and the light emission current I D . The light emitted by the semiconductor laser 108 is photoelectrically converted by the photodiode 109 in the laser unit 112 and is processed by the light quantity monitor circuit 110 to obtain a monitor voltage V M , as shown in Fig. 5, for feedback to the light quantity comparator/controller 102. After the bias count X B and the light emission count X D are both brought to "0" in the above-explained step 202, a bias current control is conducted in a sequence starting from a step 203. In said control, as represented by a region A in Figs. 2A and 2B, the bias current count X B is counted up stepwise while the light emission count X D is maintained at "0" (step 204), whereby a laser current corresponding to thus increasing count X B is supplied to the semiconductor laser 108 through the bias current regulator 104. The detected value of the light emitted by said laser current is fed back, through the light quantity monitor circuit 110, to the light quantity comparator/controller 102. The semiconductor laser 108 emits a beam when the laser current exceeds a threshold current I th . Then the bias current count X B is increased to increase the laser current in the semiconductor laser 108. The up-counting of the bias current count X B is terminated when the monitor voltage V M reaches a bias current specifying voltage V B0 (step 205). The bias current count in this state is defined as the bias current specifying count X B0 . In a states X B = X B0 , the laser 108 emits a laser beam with an intensity which may be enough for forming a latent image on the photosensitive drum 1. Consequently the bias current actually employed is defined by a count X BT obtained by multiplying a constant value (for example 80%) on said specifying count X B0 (step 206). The current in the laser 108, corresponding to the bias current setting count X BT , is definded as the set bias current I BT . The bias voltage V M is confirmed while the set bias current I BT is supplied to the semiconductor laser 108, and the absolute value of the difference of the monitor voltage V M and the bias current specifying voltage V B0 exceeds the voltage V B0 multiplied by a constant a (step 207), the sequence returns to the step 202, in order to reset the bias current count X B to "0", thereby terminating the laser current, and to repeat the bias current control from the beginning. When a condition |V B0 - V M | ≦ V B0 x α is satisfied, the sequence proceeds from the steps 207 to 208. In a sequence starting from the step 208, the light emission current is controlled while said set bias current I BT is supplied to the semiconductor laser 108. In this state the video signal is shifted to the true level, in order to open the gate in the switch circuit 107, thereby enabling the supply of the light emission current I D to the semiconductor laser 108. As represented by the region B in Figs. 2A and 2B, the control of the light emission current is conducted by maintaining the bias current count X B at the bias current setting count X BT thereby supplying the semiconductor laser 108 with the set bias current I BT . The light emission count X D is increased stepwise from "0" (step 208). A current I D corresponding to the thus increasing count X D is added, by the light emission current regulator 106, to the set bias current I BT . Consequently the laser current Iℓ is represented by the sum of the sel bias current I BT and the light emission current I D , that is, Iℓ = I BT + I D . The intensity of the laser beam caused by the laser current Iℓ is photoelectrically converted by the photodiode 109 incorporated in the semiconductor laser unit 112 and is fed back, by the light quantity monitor circuit 110, to the light quantity comparator/controller 102. In response to the up-counting of the light emission count X D by one, the light emission current I D increases, thereby increasing the intensity of the laser beam. The increase in the light quantity is detected by the increase in the monitor voltage V M . The light emission count X D is counted up to increase the current in the laser 108, and the up-counting of the light emission count X D is terminated when the monitor voltage V M reaches the light quantity specifying voltage V D (step 209). The light emission current count in this state is defined as the light emission current setting count X DT . The current in the semiconductor laser 108 in this state, corresponding to the light emission current setting count X DT , is defined as the set light emission current I DT . The monitor voltage V M is confirmed while said set light emission current is supplied to the semiconductor laser 108. If the monitor voltage V M is not within a predetermined range (for example ± 5%) with respect to the light quantity specifying voltage V D (step 210), the light emission current count X D is cleared (step 211) to terminate the light emission current I D , and the sequence returns to the step 208 to repeat the light emission current control. On the other hand, if the monitor voltage V M is within said range with respect to the light quantity specifying voltage V D and the APCST signal from the central processing unit 101 is in the false level, the light emission current setting count X DT and the bias current setting count X BT are retained but the gate of the switching circuit 107 is closed to limit the current in the laser 108 only to the bias setting current I BT . Also in case said APCST signal is in the true level, the gate of the switching circuit 107 is maintained open, and is closed when the APCST signal is shifted to the false level. The light quantity specifying voltage V D mentioned above is determined in the following manner. At first the light quantity setting voltage V 0 is determined by the light quantity setting circuit 111. Said voltage V 0 may be obtained by a voltage division with resistors as shown in Fig. 6. Said voltage V 0 supplied to the light quantity comparator/controller 102 is corrected by the sensitivity of the photosensitive drum 1, supplied by signals 114 (CSEN1, CSEN2) from the central processing unit l. The correcting value for V 0 is represented for example as +10% or -10% with respect to the central value V 0 , and thus corrected value is defined as the light quantity specifying voltage V D . As shown by the regions A and B in the flow chart in Fig. 3, the bias count X B and the light emission count X D are respectively increased by one step until the monitor voltage V M reaches the bias current specifying voltage V B0 or the light quantity specifying voltage V D (steps 204, 208 in Fig. 3). Though such up-counting enables precise control of the light quantity, a longer time will be required if a higher precision is intended by an increase in the number of bits of the count X B or X D . The time available for automatic light quantity control is limited since this operation is usually conducted in the interval between sheet feedings. For this reason the present embodiment employs a high-speed and high-precision light quantity control as shown in Figs. 15 and 16. This control operation will be explained in the following according to the flow chart shown in Fig. 16. In this operation the sequences of the regions A and B in Fig. 3 are replaced by those in Fig. 16. Thus, after the steps 203 and 207 shown in Fig. 3, there is executed a step 301 shown in Fig. 16 to discriminate whether the monitor voltage V M has reached a value V B0β (or V Dβ ) representing a predetermined proportion of B B0 (or V D ), and, if V M is lower than V B0β (or V Dβ ), a next step 302 increases the bias count X B (or the light emission count X D ) by 3 counts at a time. Then, when the monitor voltage V M exceeds V B0β (or V Dβ ), a step 303 compares the voltage V M with another proportion V B0α (or V Dα ) of the voltage V B0 (or V D ) and, if the monitor voltage V M is smaller, a step 304 increases the count X B (or X D ) by 2 counts at a time. Then, when the monitor voltage V M exceeds V B0α (or V Dα ), a step 305 compares the monitor voltage V M with the bias specifying voltage V B0 (or the light emission specifying voltage V D ), and, if the monitor voltage V M is smaller, the count X B (or X D ) is increased by one count at a time until the monitor voltage V M reaches V B0 (or V D ). When V M reaches V B0 (or V D ), the sequence proceeds to the step 206 or 210 shown in Fig. 3. The above-explained sequence enables automatic light quantity control of high precision within a short time as shown in Fig. 15. In the foregoing explanation, the parenthesized portions correspond to the region B in Fig. 3. Then, as shown in Figs. 7A and 7B, the laser current Iℓ is increased by the up-counting of the bias count X B , and, if the monitor voltage V M does not reach the bias current specifying voltage V B0 even at a time T B where the bias count X B is at the maximum value, the bias current is determined as the current I BMAX at said maximum bias count, or a predetermined proportion of said current I BMAX . The control of the light emission current is conducted under the presence of a predetermined bias current. The laser current is increased by up-counting of the light emission count X D , and, if the monitor voltage V M does not reach the light quantity specifying voltage V D at a time T D when the light emission count X D reaches maximum, the control of the light emission current is terminated in the presence of a light emission current I DMAX at said maximum light emission count. Fig. 8 shows the circuit structure other than the laser controller 50l detailedly shown in Fig. 1. In Fig. 8 there are shown a scanner motor control circuit 503; a photosensitive drum rotation control circuit 504; and a photosensitive drum detecting circuit 505. A central processing unit 101 controls the scanner motor control circuit 503 and the rotation control circuit 504, controlled for example through known integrated circuits for PLL control, and the detecting circuit 505 composed of microswitches or photointerruptors. The CPU 101 also receives a print control signal (PRINT) 506 or a video signal 507 from an operation panel or a reader unit, and a test signal (MLON) 508 from an external control unit, and controls the laser control circuit 50l shown in Fig. 1 in response to these signals. Now reference is made to a timing chart shown in Fig. 9 and a flow chart shown in Fig. 10 for explaining the control sequence of the CPU 101. In response to the entry of the PRINT signal 506 from the external control apparatus in a step 601, a step 602 shifts a drum drive signal (DRMD) 513 and a scanner motor drive signal (SCNON) 509 to the true level. Then a step 603 confirms a SCNRDY signal 510 indicating that the scanner motor is ready, and a step 604 shifts the auto light quantity control start signal (APCST) 113 to the true level. In response, in a step 605, the light quantity comparator/controller 102 of the laser controller 501 effects the automatic light quantity control of the semiconductor laser 108. Then, when the drum drive signal (DRMD) 513 or the scanner motor rotation ready signal (SCNRY) 510 is shifted to the false state in a step 606, a next step 607 clears the bias count X B and the light emission count X D through the light quantity comparator/controller 102, thereby sufficiently reducing the current in the semiconductor laser 108. The operation of the step 607 may be executed when the drum drive signal (DRMD) 513 is shifted to the false level, or within a certain period before or after such shifting to the flase level. Also the APCST signal 113 assumes the true level, in case of a continuous printing operation, between successive printing operations, i.e. in the non-image area between successive recording sheets as shown in Fig. 11, and the automatic light quantity control is completed within said non-image area. Also if the APCST signal 113 is not generated over a predetermined period T APC , the automatic light quantity control is conducted after said period T APC . A TOPER signal 601 shown in Fig. 11 indicates the above mentioned non-image area.

Claims: A light quantity control device comprising: a semiconductor laser (108); monitoring means (109, 110) for providing a monitor voltage (V M ) in accordance with the detected light quantity of a beam generated by said semiconductor laser (108); and control means for controlling the light quantity of the beam generated by said semiconductor laser (108) in response to said monitor voltage (V M ) and a setting voltage (V 0 ) from a light quantity setting circuit, said control means comprising: a comparator/controller (102) incorporating analog-to-digital conversion means (A/D) for converting said monitor voltage (V M ); a first current control circuit (103), having a digital-to-analog converter connected to first outputs (B 1 -B n ) of said comparator/controller (102), for controlling a first current (I B ) supplied to said semiconductor laser (108); characterised in that: said control means further comprises a second current control circuit (105), having a digital-to-analog converter connected to second outputs (D 1 -D n ) of said comparator/controller (102), for controlling a second current (I D ) supplied to said semiconductor laser (108) in addition to said first current (I B ), in that said comparator/controller (102) is operative sequentially to update the digital value of said first outputs (B 1 -B n ) until the monitor voltage (V M ) reaches a predetermined voltage (V B0 ) which exceeds a threshold value of said semiconductor laser, and thereafter sequentially to update the digital value on said second outputs (D 1 -D n ) until the monitor voltage (V M ) reaches a light quantity specifying voltage (V D ) derived from said setting voltage (V 0 ), and in that the rate at which the first current (I B ) is changed when the first outputs (B 1 -B n ) are updated is greater than the rate at which the second current (I D ) is changed when the second outputs (D 1 -D n ) are being updated. An apparatus according to claim 1, wherein the comparator/controller (102) increments the first and second outputs thereof (B 1 -B n ;D 1 -D n ) in unit step sizes. An apparatus according to claim 1, wherein the comparator/controller (102) increments the first and second outputs thereof (B 1 -B n ;D 1 -D n ) by different step sizes depending on the value of the monitored voltage (V M ). A device according to claim 1, 2 or 3, wherein the semiconductor laser and the detector element (109) of said monitoring means are incorporated in the same unit (112). Use of a device according to any preceding claim to effect beam intensity control of a semiconductor laser used in a printing operation. Use of a device according to any of claims 1 to 4 to effect beam intensity control of a semiconductor laser used in a continuous printing operation, wherein beam intensity control is actuated by said current control circuits (103, 105) between successive printing operations.

Inventors: INUYAMA TOSHIHIKO, KIMIZUKA JUNICHI, KUSANO AKIHISA, SATA KAORU, SOYA TAKASHI NO, INUYAMA, TOSHIHIKO, KIMIZUKA, JUNICHI, KUSANO, AKIHISA, SATO, KAORU, SOYA, TAKASHI, SATA, KAORU, SOYA, TAKASHI NO. 102
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: H01S3
IPC codes: H01S5, G06K15